Organic semiconductors for energy and electronics: from fundamental properties to devices

Resume : The emerging research field of organic bioelectronics has developed rapidly over the last few years and elegant examples of biomedically important applications including for example in-vivo drug delivery and neural interfacing have been demonstrated. The organic electrochemical transistor (OECT), capable of transducing small ionic fluxes into electronic signals in an aqueous environment, is an ideal device to utilise in bioelectronic applications. To date, nearly all OECTs have been fabricated with commercially available PEDOT:PSS, heavily limiting the variability in performance. We have previously shown that tailor-made semiconducting polymers are fully capable of matching the performance of PEDOT:PSS. To capitalise on this discovery and the versatility of the organic chemistry toolbox, further materials development is needed. In my talk I will discuss our recent work in this area covering examples of both molecular and polymeric semiconducting materials and their performance in bioelectronic devices.

Resume : Organic semiconductors (OSCs) promise to deliver next‐generation electronic and energy devices that are flexible, scalable and printable. Unfortunately, realizing this opportunity is hampered by increasing concerns about the use of volatile organic compounds (VOCs), particularly toxic halogenated solvents that are detrimental to the environment and human health. Here, a cradle‐to‐grave process is reported to achieve high performance p‐ and n‐type OSC devices based on indacenodithiophene and diketopyrrolopyrrole semiconducting polymers that utilizes aqueous‐processes, fewer steps, lower reaction temperatures, a significant reduction in VOCs (>99%) and avoids all halogenated solvents. The process involves an aqueous mini‐emulsion polymerization that generates a surfactant‐stabilized aqueous dispersion of OSC nanoparticles at sufficient concentration to permit direct aqueous processing into thin films for use in organic field‐effect transistors. Promisingly, the performance of these devices is comparable to those prepared using conventional synthesis and processing procedures optimized for large amounts of VOCs and halogenated solvents. Ultimately, the holistic approach reported addresses the environmental issues and enables a viable guideline for the delivery of future OSC devices using only aqueous media for synthesis, purification and thin‐film processing. (A. Rahmanudin, et al. Adv. Sci. 2020, 7, 2002010.)

Resume : The power conversion efficiency and open-circuit voltage of organic photovoltaic devices is determined by the recombination of photo-generated charge carriers. In this process, charge transfer (CT) states at the interface between the electron-donating and electron-accepting materials comprising the photo-active layer, play a crucial role.[1] These electronic states are characterized by absorption and emission bands within the optical gap of the interfacing materials. Depending on the used donor and acceptor materials, CT states can be very emissive and generate free carriers at high yield.[2] In this talk, I will discuss the fundamental properties of CT states, with focus on high and low frequency vibrational modes, and link them to organic opto-electronic device performance. Furthermore, using a new device architecture, we introduce strong light-matter coupling, resulting in redshifted and steepened absorption edges, as well as reduced energy losses in organic photovoltaic devices.[3] [1] Nature Energy 2, 17053 (2017) [2] Nature Materials 18, 459 (2019) [3] Nature Communications 10, 3706 (2019)

Resume : Infrared (IR) photodetection is critical to modern technologies, enabling applications such as medical imaging, environmental sensing, and night vision. These technologies remain dominated by inorganic semiconductors that require complex manufacturing, high costs, and require cooling that limit their applications within emerging technologies. Conjugated polymers offer a promising alternative offering low-cost and scalable fabrication, monolithic integration, room temperature operation, and many other attributes that are not available using current technologies. Here, we demonstrate how precise control of the structural and electronic properties of donor–acceptor conjugated polymers with narrow bandgaps, spanning the near- and short-wavelength infrared (NIR-SWIR: 0.9 < λ < 3 µm), enables efficient charge photogeneration and high-performing devices that operate within these spectral regions. A key feature of these materials is charge generation assisted by polymer aggregation, which is essential to compensate for the energy gap law. Fundamental investigations of polymer and device physics have resulted in correlations that connect intrinsic molecular features and thermal noise with device performance, resulting in photodiodes with performance that rivals commercial inorganic systems in the SWIR. Collectively, these investigations have enabled new soft matter systems and device paradigms enabling optical to electrical transduction of IR light.

Resume : High-energy radiation detectors based on organic semiconductor materials have already demonstrated to be a promising candidate in order to achieve some of the major quests for large-area and flexible sensors such as portability, mechanical conformability, low-cost, real-time response and human-tissue equivalence [1-3]. All these requirements are essential in several fields of application for instance in nuclear waste management, citizens security and finally for radiotherapy monitoring or personal protection devices. Here we present a deep investigation regarding the mechanism of detection in this class of materials, showing the results obtained employing bis-(triisopropylsilylethynyl)-pentacene thin-films deposited by bar-assisted meniscus shearing [4]. In order to control and enhance the sensitivity of the detectors we report two independent strategies. On one side, varying the deposition speed of the organic layer we could tune the morphology of the thin film and control the density of the trap states responsible for the inner amplification of the sensor signal. On the other side, the use of bis-(triisopropylsilylethynyl)-pentacene:polystyrene blends allowed to improve the transport properties of the organic device (i.e. the holes mobility) maximizing the charge collection efficiency and photoconductive gain factor of the active layer of the sensor. In fact, by adding polystyrene to the semiconductor solution, we reduce the interface hole trap density and consequently the charge carrier mobility is enhanced, as well as the device sensitivity. The deep study of the physical processes behind X-ray photoconversion in this class of detectors and the employment of these two strategies brought us to obtain a record sensitivity for organic-based direct X-ray detectors: 1.3 · 104 µC Gy-1·cm-2). Finally, such a sensor shows a very low minimum detectable dose rate (35 µGy s-1) making itself a promising candidate for medical applications. [5] Thus, the employment of organic large-area direct detectors for X-ray radiation in real-life applications can be foreseen. [1] A. Ciavatti et al. Adv. Mater., vol. 27, pp. 7213?7220, (2015). [2] Basiricò, L. et al. Nat. Commun. 7, 13063 (2016). [3] Thirimanne, H. M. et al. Nat. Commun. 9, 2926 (2018). [4] Temiño, I. et al. Adv. Mater. Technol. 1, 1600090 (2016). [5] I. Temiño et al. Nat. Commun., vol. 11, 2136, (2020).

Resume : Over recent years several new classes of conjugated polymers have shown promise as materials with high carrier mobilities in both field-effect transistors but also in high bulk-doped films. Many of the recently discovered high mobility polymers, in particular donor-acceptor copolymers, owe their excellent charge transport properties to a low degree of energetic disorder associated with a well-defined backbone conformation with small variations in torsion angles. In this presentation we will present our current understanding of the transport physics of these materials and focus in particular on the relationship between molecular structure, thin film processing and charge transport and thermoelectric properties of these materials.

Resume : The ‘phonon-glass electron-crystal’ concept has triggered most of the progress that has been achieved in inorganic thermoelectrics in the past two decades. Organic thermoelectric materials, unlike their inorganic counterparts, exhibit molecular diversity, flexible mechanical properties and easy fabrication, and are mostly ‘phonon glasses’. However, the thermoelectric performances of these organic materials are largely limited by low molecular order and they are therefore far from being ‘electron crystals’. Here, we report a molecularly n-doped fullerene derivative with meticulous design of the side chain that approaches an organic ‘PGEC’ thermoelectric material [1]. This thermoelectric material exhibits an excellent electrical conductivity of >10 S/cm and an ultralow thermal conductivity of <0.1 W/mK, leading to the best figure of merit ZT = 0.34 (at 120 °C) among all reported single-host n-type organic thermoelectric materials. The key factor to achieving the record performance is to use ‘arm-shaped’ double-triethylene-glycol-type side chains, which not only offer excellent doping efficiency (~60%) but also induce a disorder-to-order transition upon thermal annealing. This study illustrates the vast potential of organic semiconductors as thermoelectric materials. [1] J. Liu, B. van der Zee, R. Alessandri, S. Sami, J. Dong, M.I. Nugraha, A.J. Barker, S. Rousseva, L. Qiu, X. Qiu, N. Klasen, R.C. Chiechi, D. Baran, M. Caironi, G. Portale, R.W. A. Havenith, S.J. Marrink, J.C. Hummelen, and L.J.A. Koster, Nature Comm. 11, 5694 (2020).

Resume : Thermoelectricity depends mainly on a temperature difference between two materials which results in energy production. Such control of a temperature difference is rather difficult with nanometer thick samples (thin films) which are the main studied structure in this field. Here, we present a bulk-size alternative architecture for conducting organic materials in order to create thermoelectric generator prototypes. Inspired by the highly porous silicon aerogels,[1] nanostructured polymer dried gels[2] make excellent candidates as good thermal insulators. Hence, combining the low thermal conductivity of porous nanostructures with the good Seebeck and electrical conductivity of organic conductors can be an efficient strategy to enhance the thermoelectric properties of bulk size organic materials. Three different conductive porous architectures based on poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) were designed and characterized by Scanning Electron Microscopy (SEM) and spectroscopy techniques. PEDOT:PSS firstly forms a fiber-like 3D network through acid treatment.[3] Then, the polymer network is dried either by supercritical drying (aerogel), freeze-drying (cryogel) or ambient air drying (xerogel) techniques. Xerogels porous structure is uncontrolled and heterogeneous while aerogels conserve the 3D nanofibrillar structure of the wet material. Cryogels honeycomb-like structure can be tuned by the control of solvent crystallisation during freezing process prior to sublimation in freeze-drying technique. We demonstrated that the pore structure and size can thus be tailored from 30 to 300 µm, influencing thermoelectric properties. We were able to produce few mm-thick samples with very high porosity up to 97% and very low estimated thermal conductivity ? 3.10-2 W.m-1.K-1 while keeping a reasonable power factor > 10-2 µW.m-1.K-2. Here, we aim at discussing the limits and/or the key parameters of the drying techniques to control the porous structure and their impact on the TE properties of the PEDOT:PSS dried gels. [1] N. Hüsing, U. Schubert, Angew. Chemie - Int. Ed. 2007, 37, 22. [2] M. Noroozi, M. Panahi-Sarmad, M. Abrisham, A. Amirkiai, N. Asghari, H. Golbaten-Mofrad, N. Karimpour-Motlagh, V. Goodarzi, A. R. Bahramian, B. Zahiri, ACS Appl. Energy Mater. 2019, 2, 5319. [3] B. Yao, H. Wang, Q. Zhou, M. Wu, M. Zhang, C. Li, G. Shi, Adv. Mater. 2017, 29, 1700974.

Resume : Perovskites have been intensively investigated for their use in solar cells and light-emitting diodes. However, research on their applications in thin-film transistors (TFTs) has drawn less attention despite their high intrinsic charge carrier mobility. In this study, we report the universal approaches for high-performance and reliable p-channel lead-free phenethylammonium tin iodide TFTs. These include self-passivation for grain boundary by excess phenethylammonium iodide, grain crystallisation control by adduct, and iodide vacancy passivation through oxygen treatment. We found that the grain boundary passivation can increase TFT reproducibility and reliability, and the grain size enlargement can hike the TFT performance; thus, enabling the first perovskite-based complementary inverter demonstration with n-channel indium gallium zinc oxide (IGZO) TFTs. The inverter exhibits a high gain over 30 with an excellent noise margin. This work aims to provide widely applicable and repeatable methods to make the gate more open for intensive efforts towards high-performance printed perovskite TFTs.

Resume : In organic heterojunction devices, current generation results from the sequence of photon absorption, charge separation, and charge collection in competition with recombination. To understand and design organic PV devices, we need models of these processes that incoporate both the device architecture and the molecular nature of the materials. Device models work fairly well in describing charge collection and recombination, and resulting curent-voltage curves, but usually with some empirical form for the charge generation efficiency and recombination coefficients. A full description of microscopic processes such as interfacial charge transfer requires molecular scale models. For design purposes, we would like to be able to predict device behaviour from the properties of the molecular components, but it is challenging to combine these aspects in a single model. In this talk we will discuss the degree to which molecular level models and time-resolved device models can explain measurements both of charge carrier dynamics, and of overall device behaviour. We will then address the challenges in bringing the two approaches together into a single framework.

Resume : In bulk heterojunction (BHJ) solar cells, the heterojunction interface between electron donor and acceptor drives the exciton-to-charge conversion, yet it also adds to energy and carrier losses. In principle, in low-bandgap non-fullerene acceptor (NFA) BHJs both electron affinity (EA) and ionization energy (IE) offsets should equally control the internal quantum efficiency (IQE). Allegedly, exciton-to-charge conversion is efficient even for close-to-zero offsets. Here, we rebut both notions and demonstrate that counterintuitively, the charge transfer from the exciton rather than the further charge separation is the limiting step controlled by the IE offset and secondly, that sizeable IE offsets are required to reach high exciton-to charge conversion efficiency. We find that efficient Förster Resonant Energy Transfer to the low bandgap acceptor precedes the charge transfer, which thus always occurs via hole transfer from the acceptor, hence the unimportance of the EA offset. We discuss the reasons for the threshold IE offset in terms of interface energetics and find that two physical parameters are sufficient to describe the evolution of the IQE with IE offset on a very large range of material systems. Our model also explain other experimental observations such as the difficulty of observing CT states emission and absorption in NFA based systems.

Resume : Most efficient OPV cells comprise a donor:acceptor blend arranged in a so-called bulk heterojunction (BHJ) architecture. The BHJ morphology is complex; it is now, for instance, well accepted that most BHJ blends used in OPVs are at least to a certain extent finely intermixed, rarely featuring entirely phase-pure domains. This renders elucidating the nature of molecularly mixed phases, and how these relate to device characteristics, a major challenge in the field. Ade et al. have recently introduced the so-called domain-composition-variation concept, which accounts for the relative purity of domains as compared to a reference sample. This is a great advance because this approach allows for a phenomenological correlation between the domain purity and device characteristics for each specific donor:acceptor material system. However, the lack of quantitative information about the absolute domain composition in real devices is still hindering a more refined understanding of how mixed phases impact the device function. Here, we present a new methodology based on ultrafast calorimetry that allows to readily determine the absolute composition of intermixed domains in BHJs processed using the same parameters as those employed for device fabrication (e.g. spin-casting a sub-200-nm thin film on PEDOT:PSS). The method exploits the fact that the glass transition of a finely intermixed glassy blend is coherently affected by the relative amount of donor and acceptor molecules in the blend. We demonstrate the applicability of our method for various donor:acceptor systems and discuss the impact of the knowledge gained on materials selection, device functioning and long-term stability.

Resume : Light-harvesting layers fabricated from aqueous or alcoholic nanoparticle dispersions promote the eco-friendly production of organic solar cells. Typically, either miniemulsion routes or nanoprecipitation are used to synthesize batches of nanoparticle dispersions. In this work, microfluidic solvent handling is explored for the rapid, precise and high-throughput precipitation of organic semiconductor nanoparticles. Microfluidic systems enable the rapid mixing of small amounts of fluids in channels with tailored dimensions below 500 µm. The mixing and hence the precipitation process can be controlled precisely, rendering this process highly reproducible. After initial investigation of the process parameters, a reliable and reproducible process for the fabrication of organic semiconductor nanoparticle dispersions from blend solutions is reported, featuring the organic semiconductors poly(3-hexylthiophene-2,5-diyl) (P3HT) and the fullerene indene-C60 bisadduct (ICBA), i.e. the fruit-flies of organic light-harvesting semiconductors, for best comparability with the scientific literature. The initial concentration of the semiconductor solution is critical to the nanoparticle formation as well as several environmental conditions. The respective organic solar cells with light-harvesting layers from organic nanoparticle dispersions exhibit power conversion efficiencies of 4 %, which compares with solar cells made of batch-processed organic nanoparticles and even with solar cells processed from blend solutions. The continuous-flow synthesis of organic semiconductor dispersions yields highly reproducible nanoparticle diameters as well as excellent dispersion stability for more than one year. This concept is an important leap towards a scalable production of eco-friendly semiconductor inks for the large-area fabrication of organic solar cells. Larger amounts of dispersion can readily be fabricated by employing continuous pumping systems and by parallelizing several microfluidic setups, without affecting the process outcome or the nanoparticle quality.

Resume : Triazine derivatives have been established as versatile tools and as building blocks for the synthesis of multifunctional materials [1]. Covalent triazine frameworks are synthesized through a nucleophilic substitution of a triazine core with aromatic networks of diamines under heating [2], [3]. Depending on the reaction conditions, π-conjugated polymeric materials are obtained, that can be further modified through exfoliation in water and rearrangement in organic solvents. A first emphasis is given on the redox states, the optical fingerprint, and the response towards varying stimulus of these colored semiconducting oligomers [4] at solid and liquid state. Extensive fluorescence measurements verify that the emission of the covalent triazine frameworks can be tuned by altering mildly their surrounding environment, offering them sensing properties. A more detailed insight into the excited state dynamics will be presented by a time-resolved photoluminescence study from 5 to 300 K. [1] N. Wu, L. Ma, S. Zhao, and D. Xiao, Solar Energy Mater. Solar Cells, vol. 195, pp. 114–121, Jun. 2019. [2] P. Dallas et al., Polymer, vol. 49, no. 5, pp. 1137–1144, Mar. 2008. [3] A. B. Bourlinos, P. Dallas, Y. Sanakis, D. Stamopoulos, C. Trapalis, and D. Niarchos, Eur. Polymer J., vol. 42, no. 11, pp. 2940–2948, Nov. 2006. [4] J.-K. Sun, Y.-J. Zhang, G.-P. Yu, J. Zhang, M. Antonietti, and J. Yuan, J.Mater Chem C, vol. 6, no. 34, pp. 9065–9070, 2018.

Resume : We have computed the optical properties of a large set of molecular crystals (~2200 structures) composed of molecules whose lowest excited states are strongly coupled and generate wide excitonic bands. Such bands are classified in terms of their dimensionality (1-, 2-, 3-dimensional), the position of the optically allowed state in relation with the excitonic density of states, and the presence of Davydov splitting. Our analysis confirms that simple one-dimensional aggregates are rare in molecular crystals highlighting the need to go beyond the simple Kasha’s theory. Furthermore, we have used this large set of data to search for technologically interesting and less common properties. For instance, now we are able to answer "what is the largest plausible excitonic bandwidth" and what are the characteristics of materials showing such large bands. We have also explored this database for technologically interesting yet less common properties. As such, we have identified materials with strong super-radiant states and have explored the possibility that strong excitonic coupling can be used to generate emissive states in the near-infrared in materials formed by molecules with bright visible absorption. We beleive these insights provide practical guidelines for designing materials with interesting optical properties.

Resume : Light-emitting electrochemical cells (LECs) are highly efficient, air-stable, low-cost, and single-layered lighting sources fabricated using up-scalable and sustainable solution-based techniques.[1] They consist of two electrodes sandwiching a thin film of electroluminescent material doped with ionic electrolyte, that allows for charge transport, recombination, and light-emission, which are necessary to the functioning of the device. In this context, white LECs (WLECs) are particularly important for indoor lighting applications; however, their achievement still represents a challenge. Commonly, the emissive material is constituted by ionic transition metal complexes (iTMCs), conjugated polymers, or small molecules. The search for sustainable emitters such as cheap metal based iTMCs and small molecules has attracted the attention of numerous research groups; however, the achievement of WLECs based on i) Cu-based iTMCs, and ii) small-molecules have only sporadically been reported. Herein, the most recent advances in our group are described. Firstly, deep-red emitting [Cu(N^N)(P^P)]+-based iTMCs, where N^N is 4,4’-diethylester-2,2’-biquinoline (dcbq) and P^P is bis-triphenylphosphine, bis[2-(diphenylphosphino)phenyl)ether] (POP) or 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene (Xantphos) were reported. They featured high irradiances (>100 μW/cm2, long stabilities of > 20 h and excellent color stability (x/y 0.66/0.32).[2] In particular, their performances highly depend on the P^P ligand type, with the figures-of merit going hand-in-hand with enhanced rigidity of the P^P ligand. By combining the best performing Cu-iTMC, featuring Xantphos as P^P ligand, with the high-bandgap hole transport material 4,4′-Bis(9-carbazolyl)-1,1′-biphenyl (CBP), proof-of-concept WLECs with X/Y CIE of 0.32/0.31, high CRI of 92 and luminances of 5 cd/m2 were achieved. These results open the way to fabricate all-Cu(I) based WLECs. Secondly, we reported on a BN-doped nanographene featuring efficient blue fluorescence in thin-films. Its implementation in devices led to efficient and stable over days WLECs with a broad white spectrum with peaks at 435, 505, 550, 695, and 750 nm – i.e., x/y CIE color coordinates of 0.28-0.31/0.31-0.38 and average CRI = 87.[3] These were easily achieved regardless of active layer composition – i.e., type of ion-doped matrix and its interaction with the device architecture and operation – i.e., thickness, electrodes, and driving conditions. The origin of this exclusive white electroluminescent response is a very rare multi-emissive mechanism involving a ternary emission that includes fluorescence and a thermally activated dual phosphorescence. This multi-emissive mechanism was found to be temperature dependent and is an intrinsic feature of the emitter as it is operative for both photo- and electro-luminescence processes. This represents the first example of ternary emission activated in lighting devices. [1] E. Fresta, R. D. Costa, J. Mater. Chem. C 2017, 5, 5643. [2] E. Fresta, M. D. Weber, J. Fernández-Cestau, R. D. Costa, Adv. Opt. Mater. 2019, 7, 1900830. [3] E. Fresta, J. Dosso, J. Cabanillas-Gonzalez, D. Bonifazi, R. D. Costa, Adv. Funct. Mater. 2020, 30, 1906830.

Resume : Thin-film organic photovoltaics (OPVs) are expected as one of low-cost power generation devices for the next generation. To improve the efficiency of OPVs, the use of moth eye surface, which is a periodic array of cones with nanometer-order size, is a promising technique for enhancing light trapping in the thin active layer. Nanoimprint lithography (NIL) is a practical and widely applied method to fabricate nanotextured surfaces. Although NIL is suitable for fabricating large-area nanotextures cost effectively, the moth eye array produced by NIL necessarily contains a spatial clearance between adjacent cones, which could decrease the light trapping performance. Here, we first analyze the effects of the geometry of moth eye surface on the OPV performance using optical finite-difference time-domain simulation. The simulation shows that the photocurrent level decreases significantly and superlinearly with increasing the size of the spatial clearance within the cone array. Based on the simulation results, we experimentally fabricate the near-optimized moth eye structure, which has reduced clearance size, by using UV NIL. The experiment demonstrates that the application of moth eye surfaces produces a significant increase in the efficiency of OPVs, which is nearly comparable to the numerical prediction. The present results provide a practical design and fabrication method of nanostructured surfaces to realize highly efficient light trapping for OPVs [1]. References: [1] Kubota et al., J Photopolym Sci Technol, 33: 103-109, 2020.

Resume : Near infrared photodetectors are a widespread and fundamental technology in many disciplines, from astronomy and telecommunications to medical sciences. Current technologies are striving to include interesting innovative features in this field, such as wearability, flexibility and tunability. Organic photodetectors easily offer many of those advantages, but their relatively high bandgaps restrict NIR operation. In this work, we demonstrate solution processed organic photodetectors with improved NIR response thanks to a nanostructured active layer in the shape of a photonic crystal. The latter strongly increases the charge transfer state absorption, which is normally weak but broadband, increasing the optical path of light and resulting in remarkable photoresponse significantly below the band gap of the blend. We show responsivities up to 50 mA W-1 at 900 nm for PBTTT:PC70BM based photodetectors. On top of that, by varying the lattice parameter of the photonic crystal structure, the spectral response of the photodetectors can be tuned beyond 1000 nm. Furthermore, our photonic structure can be easily implemented in the device in a single nanoimprinting step, with minimal disruption on the fabrication process, which makes this approach very promising for upscaling.

Resume : New method for single crystal growth of tris(8-oxiquinoline)aluminum (Alq3) by direct crystallization of melt under gradient of partial pressures of the ligand forming compound (8-hydroxiqionoline) and constant temperature has been developed. The method allows growing a certain polymorph depending on the chosen temperature and the p(8-Hq) range. The growth parameters have been determined through the investigation of p(8-Hq)-T diagram in the temperature range from RT to the Alq3 maximum melting temperature. Different single crystals of Alq3 polymorphs have been grown using 99.999 wt% raw materials. The luminescent properties of the grown crystals have been studied. The research was supported by Russian Science Foundation by the grant 19-79-10003.

Resume : Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), known as useful conducting polymer, is a material commonly used for hole injection layer (HIL) in quantum-dot light-emitting diodes (QLEDs), but several drawbacks remain to be solved. An organic-inorganic hybrid HIL can be easily fabricated with a single layer while improving the disadvantages of the organic layer. In this study, we introduced a hybrid HIL using vanadium oxide (V2O5) blended PEDOT:PSS for efficient and stable QLED. The effective hole injection ability of V2O5 mixed PEDOT:PSS HIL was confirmed through the hole-only-devices (HODs). The device with PEDOT:PSS:V2O5 (10:1 V/V) showed the maximum luminance and current efficiency of 36,198 cd m−2 and 13.9 cd A−1, respectively, and the operating lifetime of more than 300 h at an initial luminance of 100 cd m−2. The enhancement of hole injection was analyzed using photoelectron spectroscopy (XPS and UPS). We noticed the formation of V4+ oxidation state and an increase in density of state near the Fermi energy level. It is expected that transition metal oxide mixed PEDOT:PSS is an alternative to organic HIL for high-performance QLEDs.

Resume : In an effort to gain a comprehensive picture for the interfacial states in bulk heterojunction (BHJ) solar cells, we provide a comprehensive microscopic model for the analysis of the energetics and dynamics of the first electronic CT state in donor:acceptor (D:A) BHJ blends, showing wide optical bandgaps and large frontier orbital energy offset. Specifically, we focus on N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB) : 1,4,5,8,9,11-hexaazatriphenylenehexacarbonitrile (HAT-CN) blends, an interesting case study for resolving the existing dilemmas about the nature and fate of interfacial charge-transfer (CT) states, as these can be probed over an unusually large energy window. By varying the blend composition and temperature, we unravel the static and dynamic contributions to the disordered density of states (DOS) of the CT states manifold, and assess their recombination to the ground state. Our accurate computational protocol entails a combination of molecular dynamics (MD) simulations, density functional theory (DFT) and time-dependent (TD) DFT calculations, and a microelectrostatic (ME) model, specifically designed to include environmental effects. We observe that the CT DOS is broadened mostly because of both conformational disorder associated with the NPB flexibility and electrostatic disorder due to the HAT-CN quadrupolar layout, and is essentially static on the timescale of charge separation/recombination. We also predict fast (~ps) non-radiative decay from the lowest CT states that should overcome charge separation hence rendering these tail states EQE silent. Therefore, the tail of CT DOS does not contribute to charge generation but rather to recombination This is fully confirmed by comparison to experimental sensitive absorption versus external quantum efficiency (EQE) measurements showing that the CT DOS explored by EQE is cropped at low energy. In such a case, the standard procedure of fitting the tail of the EQE signal with a Gaussian function to extract static and dynamic disorder becomes unreliable.

Resume : Porphyrins are ubiquitous organic molecules having a pivotal role in biological processes and as active layers in thin-film based advanced technological devices. Free-base porphyrins and porphyrinic compounds are subjected to tautomerism, i.e. the reversible prototropic formation of two isomers differing only in the position of the H-atoms bonded to the pyrrole N-atoms inside the molecular cavity. Different tautomers may have substantially different molecular conformation, conferring an anisotropic character to the system. Tautomerism characterizes the isolated porphyrin molecules and is generally also active in solid state aggregates [1]. The accomplishment of a frozen tautomeric form in monomolecular aggregates of porphyrins represented the basis for the exploitation of an intra-molecular mechanism for the development of single-molecule optoelectronic devices. Here, we show the results of growth methods, which ensure at room temperature: i) the nucleation of a pure porphyrin 2D phase on a substrate without the simultaneous growth of 3D crystals; ii) the selection of a single tautomeric form; iii) the prediction of the tautomer orientation of the deposited molecules; iv) the generation of an electronically correlated 2D layer of molecules giving rise to two stable states (the two orientations of the tautomers) to be exploited in logic devices. The successful iso-orientation at room temperature of tautomers in monomolecular domains of free-base tetraphenyl porphyrin (H2TPP) was accomplished onto exfoliated HOPG(0001) [2,3]. However, the six-fold symmetry of HOPG(0001) generates an overall isotropic distribution of such domains, hindering the possibility to scale the physical properties from multidomains of the molecular film down to the single molecule. Moreover, HOPG is a conductive substrate, which represents an obstacle for the integration of the assembly into devices. These issues were recently solved by using as a substrate the triclinic and chiral cleavage surface of the mixed crystal of fumaric acid and glycine anhydride. These characteristics allow the growth of uniaxially oriented domains of frozen tautomers of H2TPP [4,5]. Free-base porphyrins are also bases able to react with Brønsted acids (e.g. HCl) reversibly, by adding up to two protons to their inner core; this removes the anisotropic character of the molecule. The possibility to control the protonation and deprotonation of up to two H-atoms, in solid state, of frozen oriented porphyrin tautomers represents a fundamental milestone for the comprehension of some biomolecular phenomena and for their exploitation for fabricating electronic devices and sensors on the molecular scale. [1] Braun et al., J. Am. Chem. Soc. 1994, 116, 6593-6604 [2] Bussetti et al. Adv. Funct. Mater. 2014, 24, 958-963 [3] Bussetti et al. J. Phys. Chem. C 2014, 118, 15649-15655 [4] Campione et al. Nano Letters 2019, 19, 5537-5543 [5] Campione et al. Crystal Growth & Des 2020, 11, 7450-7459

Resume : Organic-inorganic hybrid semiconductors are one of the most rapidly advancing novel materials for X-ray detection[1–3]. This can be ascribed to several promising characteristics such as high broadband sensitivity, low power functionality, and conformable operation. However, existing hybrid detector concepts display dark currents that are three to four orders of magnitude higher than the industry standard of 10 pA mm-2, thereby hampering mass scale commercialization. Herein, we introduce a new approach to realise ultra-low dark currents below 10 pA mm-2 even at applied electric fields as high as ~4 V μm-1 for hybrid detectors fabricated by incorporating X-ray attenuating bismuth oxide nanoparticles into an organic bulk heterojunction consisting of p-type Poly(3-hexylthiophene-2,5-diyl) (P3HT) and n-type [6,6]-Phenyl C71 butyric acid methyl ester (PC70BM). We show that the miscibility of the above components results in a vertical phase segregation induced enrichment of the p-type hole selective polymer near the anode contact, leading to excellent charge blocking characteristics and therefore, ultra-low dark currents. These detectors display broadband performance with a record high sensitivity of ~1.5 mC Gy-1 cm-2 and less than 6% deviation in angular dependence response when irradiated under 6 MV X-ray radiation from a clinal linear accelerator. The above-mentioned characteristics combined with excellent dose and dose rate linearity, reproducibility, and long-term detector stability suggests the potential of this hybrid detector concept to perform on par with the state-of-the-art detectors, with added benefits such as low power operation and conformability. These characteristics also enhance their suitability to other applications such as dosimetry and imaging in medical and industrial applications. References [1] M. P. A. Nanayakkara, L. Matjačić, S. Wood, F. Richheimer, F. A. Castro, S. Jenatsch, S. Züfle, R. Kilbride, A. J. Parnell, M. G. Masteghin, H. M. Thirimanne, A. Nisbet, K. D. G. I. Jayawardena, S. R. P. Silva, Adv. Funct. Mater. 2020, 2008482. [2] H. M. Thirimanne, K. D. G. I. Jayawardena, A. J. Parnell, R. M. I. Bandara, A. Karalasingam, S. Pani, J. E. Huerdler, D. G. Lidzey, S. F. Tedde, A. Nisbet, C. A. Mills, & S. R. P. Silva, Nat. Commun. 2018, 9, 2926. [3] K. D. G. I. Jayawardena, H. M. Thirimanne, S. F. Tedde, J. E. Huerdler, A. J. Parnell, R. M. I. Bandara, C. A. Mills, S. R. P. Silva, ACS Nano 2019, 13, 6973.

Resume : The low-frequency lattice dynamics of small-molecule organic semiconductors have a significant impact on their transport properties. Many current studies do not take into account the possible strong anharmonic behavior of the lattice dynamics in these materials. In my presentation, I will show that similar molecules can have significantly different degrees and expressions of anharmonic lattice dynamics. I will do that by presenting the lattice dynamics of BTBT, ditBu-BTBT, diC8-BTBT, diPh-BTBT, and DNTT single crystals measured by polarization-orientation (PO) Raman combined with temperature-dependent Raman in the low-frequency range. I will show an expression of strongly anharmonic lattice dynamics in BTBT indicated by the change of the PO Raman dependence of the lattice vibrations as the temperature is increased. This anharmonic expression is almost completely suppressed in the other crystals. I will also show other expressions of anharmonicity in the form of an order-to-disorder phase transition in ditBu-BTBT and a polymorphic phase transition in diC8-BTBT. I will discuss the physical origin of these anharmonic expressions, addressing their large-amplitude motion and their anharmonic potential energy surface. I will argue that these expressions of anharmonicity are not captured within the quasi-harmonic framework. I will also talk about the effect of these anharmonic expressions on the nonlocal electron-phonon interactions, and their decoupled nature.

Resume : Organic and organic-inorganic hybrid thermoelectric (TE) devices are currently emerging energy harvesting systems for near-ambient applications (< 200 °C). In this work, a series of solution-processed colloidal plasmonic gold nanoparticles (Au NPs) were applied into organic TE generators to achieve a solution-processed, flexible, organic-inorganic hybrid nanogenerators for solar-thermoelectric energy harvesting. Two different Au NP inks were formulated, capable to absorb either the visible portion of solar spectrum (from  = 500 nm to 850 nm) or the near-infrared (NIR) spectrum (from  = 800 nm to 1800 nm). Hybrid NP-organic TE generators were fabricated by incorporating these NPs into wrist-band type PEDOT:PSS-based TE modules. A strong boosting of the solar TE energy harvest was observed on hybrid generators for either the visible or the NIR spectrum depending on the NP ink applied. In an optimized condition, by comparison to the control device without NPs, this leads to a more than 300% higher temperature difference measured across the device and a more than 13-fold increase of power output achieved on the hybrid TE generator.

Resume : Fullerene, one of the most vital molecules, has been discovered and proposed to be a good candidate for capturing individual atoms and small fragments resulting in new exciting electronic properties. Later, these materials were incorporated into real electronic and photovoltaic devices with not very good efficiency. However, fullerenes remain extremely interesting and promising materials for possible applications in nanotechnology. One of the most significant achievements of this area in recent years is discovery of nearly free electron (NFE) states in empty states of C60 that are proposed to enhance electronic transport. Using density functional theory (DFT) calculations we demonstrate the spatial and energetic distribution of the delocalized superatom molecular orbitals (SAMO) of Li @C60 and their evolution depending on Li position inside the fullerene cage and coupling with the Cu(111) surface. The theoretical results are discussed in terms of theoretical STM images and directly compared to real STM images exhibiting isotropic and delocalized states in Li @C60 islands.

Resume : During the last decade, there has been a surge of interest in hybrid thermoelectric (TE) materials consisting of inorganic nanostructures embedded in conducting polymer matrices, making it possible to realize flexible and light weigh energy conversion devices. Specifically, TE materials based on poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) have shown high thermoelectric performance at room temperature. One example that shows the versatilities of hybrid TE materials consists of tellurium nanowires (Te NWs) coated with PEDOT:PSS, where it has been demonstrated that the composite materials outperform individual materials and that it is also possible to perform the rational design of the inorganic component, allowing the controlled growth of heterostructures based on Te/PEDOT:PSS hybrid materials. In this work, we investigate the synthesis of AgxTe/PEDOT:PSS hybrid materials with controlled stoichiometry via topotactic chemical transformation processes. This synthetic route allows us to have precise control over the metal chalcogenide nanocrystal stoichiometry and the properties associated with the polymer/inorganic interface and thus its thermoelectric performance. To follow and demonstrate the chemical transformation process, the synthesized materials were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The thermoelectric characterization was performed on thin-film architectures. The Te/PEDOT:PSS parent material presents a Seebeck coefficient of 88.9 µV/K and an electrical conductivity of 29.4 S/cm at room temperature. After the chemical transformation process, the Ag2Te/PEDOT:PSS hybrid material presents a Seebeck coefficient of -56.7 µV/K and an electrical conductivity of 27.6 S/cm. Additionally, when the Ag2Te/PEDOT:PSS thin-films are subject to a de-doping post-treatment, the Seebeck coefficient and electrical conductivity show a 20 % increase. These results demonstrate that we were able to perform a controlled transformation of the Te/PEDOT:PSS parent material into Ag2Te/PEDOT:PSS material through a simple synthetic route that enables the production of both efficient p-type and n-type hybrid thermoelectric materials.

Resume : Solution processing of organic and inorganic semiconductors has opened novel pathways to fabricating advanced thin film devices such as flexible and tandem solar cells, with simple procedures promising compatibility with large scale roll-to-roll and sheet-to-sheet manufacturing methods. Furthermore, from the perspective of both researchers developing new thin film devices, as well as industry establishing large scale production lines, there is a need to demonstrate the scalability of novel processes beyond fundamental research and small areas. There are a variety of solution processing techniques available to modern printed solar cell researchers (most notably spin coating, blade coating, inkjet printing, and slot-die coating), with many literature examples of devices produced by each technique. However, the rationale behind choosing one production method over another at the research stage is sometimes overlooked due to limited comparisons of the realistic capabilities of these technologies in lab-scale research and production settings. Hence, there is a need for discussion around optimal techniques to employ for high impact thin film solar device R&D. Concurrently, the tools employed at the research stage must afford high quality data, flexible parameter control for experimentation and rapid iteration, and be compatible with a typical lab-scale environment and workflow. This presentation is intended to provide a brief overview of advantages and disadvantages of typical lab-scale solution processing techniques, with additional emphasis on FOM Technologies efforts to introduce the robust technical capabilities of the slot-die coating process to the modern materials research environment.

Resume : The optical properties of poly(3-hexylthiophene-2,5-diyl) (P3HT) can be modified by processing the polymer in the form of nanoparticles dispersed in aqueous mediums. Colloids of P3HT were prepared by two different methods: flash –or reprecipitation- and miniemulsion precipitation technique, which result in the latter case in a polymer-water system whose stability is mediated by an ionic surfactant. Both systems yield different color dispersions, thus different absorption spectra, that is explained to be caused by the influence of the surfactant on the structure of P3HT. Differential scanning calorimetry and synchrotron radiation X-ray diffraction experiments reveal the origin of these optical differences, showing evidences of the formation of a hybrid structure formed due to the interaction between P3HT and the ionic surfactant. This work evidences the potential use of techniques for preparing nanoparticles in water dispersions as a tool to manipulate the structure and thus, the optical properties of these materials.

Resume : The performance of organic field-effect transistors (OFETs) strongly depends on the crystal structure of organic semiconductors. Hence, their structural anisotropy has a clear impact on the charge transport. The crystal packing of organic semiconductor structures as well as their thin film morphologies can be controlled by the modification of the deposition parameters and ink formulation [1]. In particular, in the preparation of thin films prepared by solution-shearing, the screening of the shearing speed, temperature and ink formulation is crucial to optimize and understand the overall device performance [1,2,3] Here, we report on the charge mobility and transport anisotropy of 2-Decyl-7-phenyl[1]benzothieno[3,2-b][1]benzothiophene (Ph-BTBT-10) solution-sheared thin film OFETs, in which the thin film microstructure is controlled by tuning the solution processing conditions. We found that the shearing speed and ink formulation during the deposition have a key role on the thermodynamics and kinetics of the crystallization process. First, among the different formulations tested, we observed that the best mobilities are found in devices based on blends of Ph-BTBT-10 and polystyrene (PS). Further, we experimentally explore the differences in charge transport anisotropy between Ph-BTBT-10 deposited at 10 and 1 mm·s-1. At 1 mm·s-1 we achieved an aligned polycrystalline film of Ph-BTBT-10/PS exhibiting a mobility anisotropy (μ∥/μ⊥) of ∼5 and a maximum mobility of ∼0.5 cm2 V−1 s−1 along the coating direction. On the other hand, at 10 mm·s-1, Ph-BTBT-10/PS films show an order of magnitude higher in mobility (∼1.3 cm2 V−1 s−1) but reveal a much lower mobility anisotropy of ∼1. These results are rationalized by analyzing how the crystals are oriented in the thin films and the difference in the molecular electronic overlap in the herringbone crystal structure of Ph-BTBT-10. Therefore, through precise control of the molecular alignment, the impact of crystal orientation and thin film homogeneity on charge transport is demonstrated.[4] [1] S. Galindo et al, Adv. Funct. Mater. 27, 1700526 (2017) [2] I. Temiño et al, Adv. Mater. Technol. 1, 1600090 (2016) [3] F. G. del Pozo et al, Adv. Funct. Mater. 26, 2379 (2016) [4] A. Tamayo et al. submitted.

Resume : Y6 and derivatives are currently the benchmark non-fullerene acceptor molecules for organic solar cells (OSCs), though the interest of these molecules may go well beyond the field of OSCs. In this work, we show electronic mobility values up to 2.4 cm2·V-1·s-1 in properly crystallized spun-cast Y6 thin films, thereby comparable to the highest values achieved in n-type small-molecule semiconductors. We establish that these outstanding charge transport properties are associated to a polymorph showing 3D structural packing, edge-on molecular orientation and micrometric smooth platelets, developed via thermal annealing between 210°C and 220°C. Interestingly, our results prove that the high-mobility phase is generated from glassy Y6, and it cannot be obtained from the other polymorphs identified in this work, which show significantly lower mobility values. The straightforward fabrication of a stable Y6 crystalline phase with levels of charge mobility in the order of amorphous silicon clearly highlights its potential application in organic electronic devices.

Resume : Organic Solar Cells (OSCs) have attracted the attention of the scientific community due to the various advantages that make them suitable to replace the expensive silicon in conventional photovoltaic: they are lightweight, flexible, and have a low production cost. Tin oxide films (SnO2) are commonly used as cathode buffer layer in OSCs. Although these films present a high visible light transmittance, near-infrared light reflectivity, and excellent electrical properties, pure SnO2 has a low electrical conductivity. The aim of this work is to improve the SnO2 properties with doping. Undoped and antimony (Sb), indium (In) and germanium (Ge) doped tin oxide films (SnO2 : Sb or SnO2 : In or SnO2 : Ge) have been prepared by solgel technique. The effects of the doping on the structure and photoelectric properties films were investigated using different characterization techniques such as, X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), UV-Visible spectroscopy (UV) and Hall Effect measurement. The XRD analysis confirm the formation of tetragonal structure and the presence of preferential orientation along the (110) direction. The films are homogenous and continuous with small grains. The optical transmittance of the doped and undoped SnO2 films is over 85%. From the Hall Effect measurement, it was clearly observed the n-type nature of the films with an electrical resistivity of about 10−2 Ω.cm. Then, we studied the inverted bulk heterojunction (BHJ) solar cells using poly(3-hexylthiophene-2,5-diyl): [6,6]-phenyl-C60-butryric acid methyl ester (P3HT:PC60 BM) as active layer, where the doped and undoped SnO2 films are used as a cathode buffer layer. The device’s PV properties are investigated through the measurement of the I/V characteristic and external quantum efficiency (EQE).

Resume : In recent years, organic field-effect transistors (OFETs) has taken immense attention due to their applications in various field of flexible and wearable electronics such as display technology, radio frequency identification tag (RFID), electronic skin, and the broad range of various wearable sensors. The design simplicity, biocompatibility, cost-effectiveness, large areal integration to the integrated circuit, and amplification capability of inferior signal make OFETs more engaging for sensing applications. In this work, we have presented a flexible low power consumption (~10 µW) with high-performance OFETs using gelatin as a moisture-induced ionic gate dielectric system. Gelatin is a well-known biopolymer consisting of polar groups, such as –COOH and –NH2, absorbs water molecules from ambient humidity and produces protons inside it. It converts gelatin as a solid ionic dielectric film where the concentration of the ions depends on the relative humidity (RH). The devices were shown a highly sensitive humidity sensing behavior. About three orders of magnitude variation in device current have been observed on changes in humidity level from RH10% to RH80%. The sensitivity of the device at different relative humidity conditions has been studied at different gate biases to find out the effect of the electric field on it. Nonlinear as well as linear behavior has been observed in sensitivity depending on the applied gate bias (VGS). At VGS = –2 V (for a constant VDS of –2 V) sensitivity of the device shows a linear behavior that is suitable for humidity sensors. The highest sensitivity of the device is about 2185, which is greater than any reported OFET based flexible humidity sensor-operated within 2 V. Though, the highest sensitivity obtained for linear dependent variation with humidity is 910. The devices show a fast response with a response and recovery time of ~ 100 ms and ~110 ms, respectively. The devices are flexible up to a 5 mm bending radius. It has been used to monitor the respiratory rate of human beings to detect different sleep disorders like Sleep apnea, COPD and asthma, etc. Keywords: Organic field-effect transistors (OFETs), Biopolymers, Flexible device, Humidity sensor, and Mobility overestimation. Reference: S. Mandal et al., ACS Appl. Mater. Interfaces 2020, 12, 17, 19727–19736 .

Resume : Providing energy to offshore floating equipment (e.g., sensors and LEDs aboard marine buoys) is a challenging task, given their typical remote location at sea. Solar panels and small wind turbines, coupled with batteries, have been used as energy supply sources for such purpose. Yet, they face various limitations, such as resource intermittency. In contrast, wave energy is a highly dense and abundant resource that can be exploited, although its irregular nature negatively influences the performance and efficiency of wave energy conversion devices. Triboelectric nanogenerator (TENG) is a promising and efficient energy harvesting technology able to harvest and convert random and regular/irregular external mechanical energy into electrical power based on the contact electrification and electrostatic induction effects. Here, three TENGs based on rolling-spheres were developed and their performance compared in both a “dry” bench testing system under rotating motions, and in a large-scale wave basin under realistic sea-states installed within a scaled navigation buoy. TENGs consisted of Nylon 6,6 and PTFE films with electrodes made of conductive silver thin films fabricated using screen printing. The periodic contact-separation of the two tribo-materials is promote by the free movement of stainless steel spheres on top of them, generating electrical energy. Under the excitation of waves, Nylon and PTFE will come into contact, becoming positive and negatively charged, respectively, due to contact electrification. While the spheres role, this will induce opposite charges on the two electrodes, which will result in an alternating current flowing between them. The experiments showed that the electrical outputs of these TENGs tend to rise with increasing pitch amplitudes and decreasing period due to the increase of the spheres’ velocity. The capability of these TENGs to harvest energy from ocean waves when incorporated into a navigational buoy was demonstrated under realistic sea states. The voltage generated by the TENGs achieves maximum values for periods close to the natural period of the scaled buoy (~ 0.92 s). The wave basin tests clearly demonstrated a significant dependency of the electrical outputs on the pitch degree of freedom and the need to take into account the full dynamics of the buoy, and not only that of TENGs, when subjected to the excitations of waves. The advanced fabrication and internal compartimentation of buoys allows the integration of sensors and electronic systems to support the long-term operation of maritime sensors in remote oceanic or coastal locations. As a result, the encapsulation of the TENGs within the floating buoy can be an effective approach to isolate the device from external environmental factors. This provides a more stable and reliable output even in harsh working conditions and ensures protection of vital electronic components from the surrounding ocean environment and inherent detrimental conditions.

Resume : Thin-film organic field-effect transistors (TF-OFETs) are extremely promising as gas sensors due to their outstanding sensitivity and low production cost provided by scalable solution processing. The sorption of polar gases on the active layer affects the electrical properties of the devices. Monomolecular films can increase the sensitivity and decrease the response time, which is actively used for various gases detection [1]. At the same time, there is still no theoretical model describing the sensory properties in connection with the electrical characteristics of individual OFETs, such as operating currents, charge carrier mobility, and threshold voltage or kinetic parameters associated with their degradation over the time. The literature shows only a few correlations of the OFETs electrical properties with their sensory sensitivity, which are valid for strictly defined materials and device architectures [2] and cannot describe more general cases. In this work, we aim to reveal the correlation of the main electrical characteristics of a field effect transistor, such as carrier mobility, threshold voltage, subthreshold slope, hysteresis, and bias stress characteristic time with sensory properties of OFET-based gas sensors, such as detection limit and sensitivity. Since literature data claim that there are charge traps in the transistor channel, which determine both the electrical and sensory properties of the OFET [3]; therefore, we analyze each of these parameters in connection with the density of the trap states in order to develop an operation model of the OFET-based sensors. We analyze over 60 field-effect transistors based on monolayer films of the D2-Und-BTBT-Hex semiconductor fabricated by the Langmuir-Schaeffer technique on the top of various dielectrics having different densities of surface charge traps. This approach preserves dielectric surface due to lack of solvent during film deposition and allows to describe its intrinsic electrical properties. We have fully characterized electrical properties of sensors and their response to nitrogen dioxide and examined the sensitivity correlations with individual electrical parameters. It was shown that trap density derived from bias stress kinetic shows the highest correlation with sensitivity which is consistent with key role of deep charge traps in gas detection mechanism. This work takes the first steps towards predicting of OFETs sensory properties on the base of their electrical properties which can be measured much faster and paves the way to propose a novel model of OFET-based gas sensors sensitivity. This work was supported by the Russian Foundation for Basic Research project #20-03-00810. 1. Sizov A., et al., ACS applied materials & interfaces, 2018, 10, 50, 43831–43841 2. Duarte D., et al., Journal of Applied Physics, 2012. 111(4): p. 044509. 3. Huang W., et al., Advanced Materials, 2017. 29(31): p. 1701706.

Resume : Photoactive organic semiconductors are envisioned as a novel class of materials able to transduce light into stimulating signals inside biological cells or tissue [1]. The direct interface between the semiconductor and the electrolyte gives rise to different, competing optoelectronic transduction mechanisms. A detailed understanding of such chemical and photo-induced interactions is necessary to develop and optimize future devices. In the case of organic semiconductors, without suitable cocatalysts like Platinum, Hydrogen evolution has proven to be inefficient, while Oxygen reduction reactions have been discovered to be highly favored. Specific concentrations of Reactive Oxygen Species and more specifically of Hydrogen Peroxide can act as a signal for the cells and trigger different responses [2]. At the same time, electrogenic cells will also respond to a sufficiently high electric field induced by a photocapacitive stimulus [3]. It is thus fundamental to identify the underlying electrical behavior of the interface and to understand the properties of the organic semiconductor that impact on capacitive or faradic response. Here we address the problem in organic photoelectrodes considering both a single organic polymeric film and a planar p/n junction [4]. The single polymers studied are P3HT and PBDB-T, two well characterized tiophene-based donors in OPV, while the p/n junction featured Phthalocyanine as donor and PTCDI as acceptor. We propose a novel technique to acquire photovoltage spectra and transients when the system is in a quasi-floating state, either in contact with air or with electrolyte. Spectroscopic photovoltage and current measurements provide insight into the energetics of the involved processes while transient measurement allow to identify the kinetics of light stimulated charge transfer processes across the electrolyte. We show that by combining these techniques with impedance spectroscopy and kinetic modelling, one can identify the role of interfacial energy levels in the photoactivated processes and distinguish photocapacitive and photofaradaic contributions. The findings pave the way towards a rational design of specific and efficient organic photoelectrodes for optically controlled biotransducers. [1] J. Hopkins, L. Travaglini, A. Lauto, T. Cramer, B. Fraboni, J. Seidel and D. Mawad, Adv. Mater. Technol. 4, 1 (2019) [2] F. Lodola, V. Rosti, G. Tulii, A. Desii, L. Tapella, P. Catarsi, D. Lim, F. Moccia and M.R. Antognazza, Sci. Adv. 5 (2019) [3] D. Rand, M. Jakešová, G. Lubin, I. Vebraite, M, David-Pur, V. Đerek, T. Cramer, N.S. Sariciftci, Y.Hanein and E.D. Głowacki, Adv. Mater. 1707292, 1 (2018) [4] T. Paltrinieri, L. Bondi, V. Đerek, B. Fraboni, E.D. Głowacki and T. Cramer, Adv. Funct. Mater. 2010116, (2021)

Resume : The continuous advancements in the field of electronics have paved the way to the development of new applications, such as wearable or tattoo electronics, where the employment of ultra-flexible and ultra-conformable devices is required. One of the main strategies to improve the device mechanical flexibility and conformability is the reduction of its total thickness, of which the substrate usually represents the biggest contribution. To this end, organic materials can be considered enablers, owing to their advantageous mechanical characteristics, not achievable with inorganic counterparts, and to the possibility to obtain films with thicknesses at the nanometric scale at low temperature even from solution. Yet, with available processes, challenges are faced in obtaining operating devices when scaling thickness below a few hundreds of nanometers, apparently setting a limit. Moreover, most methods require physical deposition techniques. In this work we have successfully fabricated ultra-flexible, 143 nm-thick all-organic field-effect transistors, based on an ultra-thin solution processable dielectric bi-layer, adopting only solution based techniques, such as spin-coating and inkjet printing. The devices can operate at low voltages and, thanks to their very low thickness, can conform to human skin in an imperceptible way and sustain bending radii as low as 0.7 μm, without significant variations in their electrical performance, thus setting a new limit in thickness and flexibility. Moreover, this is the first demonstration of ultra-thin (< 1 μm) fully solution processable transistors, including the substrate. The electrical characterization showed a very good reproducibility, demonstrating the solidity of the proposed fabrication process, and the stability test in air make the devices in principle compatible with single-day, disposable usage. Moreover, the employed materials and their very low thickness guarantee a high transparency of the device. Besides providing an alternative and valid technological solution for the cost-effective integration of ultra-thin devices in more complex systems, the proposed approach can be a candidate for a plethora of possible use ranging from wearable to tattoo/epidermal electronics for sensing, healthcare or biomedical applications where conformability, ultra-flexibility and transparency are necessary.

Resume : Organic (opto)electronics has played a major role in academic and industrial research in the last 30 years, eventually reaching market maturity and holding promises for sizeable further growth. Materials are constantly developed possessing better performances and higher stability. Comparatively less efforts are dedicated to the development of sustainable synthetic strategies for their preparation. Protocols are tortuous, inefficient and heavily relying in flammable and toxic organic solvents. According to the Green Chemistry prescriptions, processes should be developed minimizing the use of solvents other than water, the use of heavy metal catalysts, the number of steps and the amount of wastes produced. As the vast majority of organic semiconductors are water insoluble, the development of sustainable synthetic routes for their preparation requires dedicated efforts. The use of specifically devised surfactants along with water and water insoluble reagents, leads to the formation of a range of interface rich systems the like of micellar solutions, emulsions, microemulsions and dispersions. Within such microheterogenous environment, a wide variety of C-C and C-N forming reactions become accessible at room temperature, low catalyst loading and at high yield with no need for inert atmosphere. In this contribution we will show how this kind of “benchtop” chemistry enables the efficient preparation of notable small molecule as well as polymeric organic semiconductors.

Resume : Isoindigo, an isomer of the natural dye indigo, is a natural pigment that can be found in various plant sources, such as the leaves of the plant Isatis tinctoria. Isoindigo received great attention as an efficient electron acceptor unit in organic semiconductors, leading to over 8% photoelectric conversion efficiency in organic photovoltaics (OPV) and mobility of over 3 cm2 V−1 s−1 in organic field effect transistors (OFET).[1] Despite the excellent performance, intrinsic limitations such as its low molecular planarity and weak light absorption in the visible region inhibited further developments in these areas. In this work, we explored the application of isoidigo-based copolymers as active materials for accumulation-mode organic electrochemical transistors (OECTs).[2] OECTs are electronic devices that use the mixed electronic/ionic conductivity of conjugated polymers to transduce electronic to ionic signals and vice versa.[3] Due to this peculiar property, OECTs found wide application in bioelectronics, where they have been used to amplify biological signals at physiologically relevant conditions.[4] However, to date the pool of materials that have been tested for OECTs remains limited. Herein, we investigated the effect of the donor unit and side chains on OECTs operation and stability. First, we established bis-3,4-ethylenedioxythiophene (bis-EDOT) as a suitable donor unit to produce copolymers exhibiting low oxidation potential in contact with an aqueous electrolyte. Then, we investigated the effect of the side chain architecture on the microstructure of the films and OECT performance. Our data indicate that hybrid alkyl-ethylene glycol side chains provide a better choice in terms of OECT performance with respect to alkyl-only or ethylene glycol-only side chains, leading to devices that are stable (over 6 hours ON/OFF pulses of 6 seconds each) and robust (1.5 hours sonication). [1] J.-L. Li, J.-J. Cao, L.-L. Duan, H.-L. Zhang, Asian J. Org. Chem. 2018, 7, 2147. [2] Y. Wang, E. Zeglio, H. Liao, J. Xu, F. Liu, Z. Li, I. P. Maria, D. Mawad, A. Herland, I. Mcculloch, W. Yue, Chem. Mater. 2019, 31, 9797. [3] E. Zeglio, O. Inganäs, Adv. Mater. 2018, 30, 1. [4] E. Zeglio, A. L. Rutz, T. E. Winkler, G. G. Malliaras, A. Herland, Adv. Mater. 2019, 31, 1806712.

Resume : Polar polythiophenes with oligoethylene glycol side chains are exceedingly soft materials. A low glass transition temperature and low degree of crystallinity prevent their use as a bulk material. We report the synthesis of a copolymer comprising (1) soft polythiophene blocks with tetraethylene glycol side chains, and (2) hard urethane segments. Our molecular design is contrary to that of other semiconductor-insulator copolymers, which typically combine a soft non-conjugated spacer with hard conjugated segments. Copolymerization of polar polythiophenes and urethane segments results in a ductile material that can be used as a free-standing solid. The copolymer displays a storage modulus of 25 MPa at room temperature and elongation at break of 95 %. Both chemical doping and electrochemical oxidation reveal that the introduction of urethane segments does not unduly reduce the hole charge-carrier mobility and ability to take up charge. Further, we observe stable operation when the copolymer is used as the active layer of organic electrochemical transistors.

Resume : Optoelectronic materials based on organic compounds bring key advantages to integration in devices, including chemical diversity, low-cost processing and compatibility with large-area flexible and bendable substrates. One of the great challenges is to minimize the number of processing steps in device fabrication protocols. This can be achieved by integrating different materials with properties that can be independently modified by applying an external stimulus or by exploiting the interfacial interactions between them. In this presentation I will discuss several types of interactions between the electrode surface and the organic semiconductor and provide examples on how the emerging phenomena related to these interactions were exploited to create electronic devices with tunable properties. In molecular rectifiers, we found that the molecule/electron coupling resulting from the delocalization of electrons lone pairs controls the position of the molecular orbitals participating in transport, making it efficient for one bias polarity and inefficient for the opposite polarity. In the case of a strong coupling, this led to rectification ratios greater than 5000. This performance was obtained in molecular diodes fabricated on silicon and may yield hybrid systems that can expand the use of silicon toward novel functionalities governed by the molecular species grafted onto its surface. In organic field-effect transistors, control of the interfacial interactions allowed microstructure control at different length scales within the organic semiconductor layer and resulted in an aggressive reduction in the contact resistance in both small molecule and polymeric devices. As a result, charge carrier mobilities as high as 20 cm2/Vs in devices with near-ideal current-voltage characteristics were demonstrated.

Resume : It is widely recognized that charge transport in organic semiconductors is extremely sensitive to the presence of disorder, both intternal and external (with the gate dielectric), especially for n-type materials. Intrinsic dynamic disorder stems from large thermal fluctuations both in intermolecular transfer integrals and (molecular) site energies in weakly interacting van der Waals solids, and leads to transient localization of the charge carriers. The molecular vibrations that drive transient localization typically operate at low-frequency (< a-few-hundred cm-1), which renders it difficult to assess them experimentally. This has so far prevented the identification of clear molecular design rules to control and reduce internal dynamic disorder. In addition, the disorder can also be external, being controlled by the gate insulator dielectric properties. We have made on a comprehensive study of charge transport in two closely related n-type molecular organic semiconductors using a combination of temperature-dependent inelastic neutron scattering and photoelectron spectroscopy corroborated by electrical measurements, theory and simulations. We provide unambiguous evidence [1] that ad hoc molecular design enables to free the electron charge carriers from both internal and external disorder to ultimately reach band-like electron transport. [] M.-A. Stoeckel et al, Adv Mater 2021 in press

Resume : Owing to the advancement in new material design over the past few years, the power conversion efficiencies (PCE) of polymer solar cells have now reached over 15%[1] and 17%[2] for single-junction and multi-junction devices, respectively. However, in order the meet the commercialization requirement, further improvement in device stability and the exploration of niche applications of polymer solar cells are urgently needed. In this talk, I will discuss how to utilize an integrated strategy combining material design, interface engineering and optical management to tackle the efficiency and stability[3] challenges of polymer solar cells. The molecular structures, crystal structures, thin film nanostructures, interface and device structures and their relationship to the overall properties of polymer solar cells will be discussed. I will particularly highlight our work on using advanced optical management as a powerful means to enhance the performance of polymer solar cells by maximizing the light harvesting property of the devices. The capability to use optical model to precisely predict the light propagation property and charge generation rate within the devices allows us to design optimal device architectures with improved performance and stability.[4] I will also discuss how to apply high throughput optical model to rapidly screen more than 10 million device structures in order to identify the very best device design for extremely high performance tandem[2, 5, 6] and semitransparent polymer solar cells (ST-PSCs).[7,8] In addition, I will highlight how to engineer the optical property of ST-PSC for smart greenhouse applications,[9] as well as the design of multiple-function ST-PSC with both heat insulation and power generation properties[10]. References 1. J. Yuan, Y. Li, H.-L. Yip, Y. Zou, et al, Joule, 2019, 3, 1140. 2. M. Li, X. Wan, L. Ding, H.-L. Yip, Y. Cao, Y. Chen, et al, Science, 2018, 361, 1094 3. X. Xu, J. Xiao, G. Zhang, H.-L. Yip, et al, Science Bulletin, https://doi.org/10.1016/j.scib.2019.10.019 4. G. Zhang, R. Xia, H.-L. Yip, Y. Cao, et al, Adv. Energy Mater., 2018, 8, 1801609. 5. K. Zhang, F. Huang, X. Peng, L. Ding, H.-L. Yip, Y. Cao, et al, Adv. Mater., 2016, 28, 4817. 6. M. Li, X. Wan, H.-L. Yip, X. Peng, Y. Cao, Y. Chen et al, Nature Photonics, 2017, 11, 85. 7. R. Xia, C. Brabec, H.-L. Yip, Y. Cao, Joule, 2019, 3, 2241. 8. H. Shi, R. Xia, C. Sun, H.-L. Yip, et al, Adv. Energy Mater., 2017, 7, 1701121. 9. H. Shi, R. Xia, H.-L. Yip, et al, Adv. Energy Mater., 2018, 8, 1803438 10. C. Sun, R. Xia, H. Shi, F. Huang, H.-L. Yip, et al, Joule, 2018, 2, 1816

Resume : Organic solar cells have gained renewed interest due to recent progress reducing the efficiency gap towards inorganic and hybrid photovoltaic technologies. The separation of photogenerated excitons into free charge carriers is considered to be most efficient from charge transfer (CT) states, which emerge at heterojunctions between electron-donating and -accepting materials. Currently discussed models describe CT states as intermolecular states incorporating reorganization and energetic disorder, whose respective impact on device parameters is yet to be understood in detail. We investigated temperature-dependent CT state absorption and emission – both of which are connected by the electro-optical reciprocity relations – in solar organic cell devices with sensitive external quantum efficiency and electroluminescence spectroscopy in the sub-bandgap region. From the temperature characteristics of the spectral linewidths, and the validation of the device's emission temperature, we found a fundamental asymmetry between CT absorption and emission spectra in low-donor content, small molecule solar cells. Thorough modelling of the spectral data reveals the dominant role of dynamic reorganization, and a negligable influence of static energetic disorder for the nature of intermolecular CT states in donor-acceptor blends in organic solar cells.

Resume : The introduction of non-fullerene acceptors (NFAs) has led to a rapid improvement of the performance of organic solar cells [1]. NFAs are far more emissive than their fullerene-based counterparts, asking for a detailed understanding on the involvement of NFA excitons in the charge carrier dynamics [2]. Here, we present results from an ongoing study of the spectral properties of photocurrent generation and recombination of the blend of the donor polymer PM6 with the NFA acceptor Y6 [3]. This blend has become the fruit fly of research on NFA-based solar cells [4,5]. We show that the radiative recombination of free charges proceeds almost entirely through the re-occupation and decay of the Y6 singlet, but that this pathway contributes to less than 1 % of the total recombination current. As such, the open-circuit voltage of the PM6:Y6 blend is almost entirely determined by the energetics and kinetics of the charge transfer (CT) state, irrespective of the position and emission properties of the singlet state. We find that no information on the energetics of the CT state manifold can be gained from the low energy tail of the photovoltaic external quantum efficiency spectrum, which is dominated by the excitation spectrum of the Y6 exciton. We, finally, estimate the charge separated state to lie only 120 meV below the Y6 singlet exciton energy, meaning that this blend indeed represents a high efficiency system with a low energetic offset. [1] Green, M. A.; Dunlop, E. D.; Hohl‐Ebinger, J.; Yoshita, M.; Kopidakis, N.; Hao, X. Prog. Photovoltaics Res. Appl. 2020, 28, 629. [2] Qian, D.; Zheng, Z.; Yao, H.; Tress, W.; Hopper, T. R.; Chen, S.; Li, S.; Liu, J.; Chen, S.; Zhang, J.; et al. Nat. Mater. 2018, 17, 703. [3] Perdigón-Toro, L.; Phuong, L. Q.; Zeiske, S.; Vandewal, K.; Armin, A.; Shoaee, S.; Neher, D. ACS Energy Lett. 2021, 6, 557. [4] Yuan, J.; Zhang, Y.; Zhou, L.; Zhang, G.; Yip, H.-L.; Lau, T.-K.; Lu, X.; Zhu, C.; Peng, H.; Johnson, P. A.; Leclerc, M.; Cao, Y.; Ulanski, J.; Li, Y.; Zou, Y. Joule 2019, 3, 1140. [5] Perdigón‐Toro, L.; Zhang, H.; Markina, A.; Yuan, J.; Hosseini, S. M.; Wolff, C. M.; Zuo, G.; Stolterfoht, M.; Zou, Y.; Gao, F.; Andrienko, D.; Shoaee, S.; Neher, D. Adv. Mater. 2020, 32, 1906763.

Resume : In the field of organic solar cells the role of excess photon energy in photogeneration is still under debate. We explored its implications for two solar cell systems consisting of the polymer donor PTB7-Th and two non-fullerene acceptors, ITIC and h-ITIC, the latter designed as the dipolar analogue of ITIC. Using the time delayed collection field (TDCF) technique, we have determined that in the PTB7-Th:ITIC blend photogeneration proceeds through nonrelaxed charge transfer states as excess energy drives delocalisation of electron-hole pairs at the donor-acceptor interface. In PTB7-Th:h-ITIC, on the other hand, the charge separation efficiency is independent of excitation energy. Our results suggest that the activation energy for free charge formation is reduced due to the large molecular quadrupole moment of the A-D-A type acceptor molecule ITIC. As a consequence, the observed yield of free charge carrier generation in the PTB7-Th:ITIC blend is higher than in PTB7-Th:h-ITIC, as consistently shown by transient absorption, TDCF and sensitive EQE measurements.

Resume : Hybrid heterojunctions comprising both organic and inorganic materials are interesting alternatives to organic heterojunctions in optoelectronic devices, as they offer the possibility to combine the advantageous properties of both types of materials. In particular, such hybrid heterojunctions enable the combination of light absorbing organic semiconductors, which tend to have narrow-bandwidth absorption coefficients that can be spectrally tuned to suit specific applications, with high dielectric constant, crystalline inorganic semiconductors that facilitate charge-separation. In practice, however, hybrid heterojunction optoelectronic have generally not reached the theoretically expected performance, especially compared to their all-organic heterojunction counterparts, with a major limitation being poor photoexcited charge transfer and separation efficiency from the organic to the inorganic material in most instances [1][2]. We have previously demonstrated that hybrid pseudo-bulk-heterojunctions devices made from the low-cost, wide-band gap, p-type inorganic semiconductor copper thiocyanate (CuSCN) and the methanofullerene PC70BM, yield photovoltaic (PV) power conversion efficiencies greater than 5%, as a result of efficient hole-transfer from the HOMO of PC70BM to the valence band (VB) of CuSCN. [3]. Here, we demonstrate the universality of this heterojunction, through showing that efficient charge-generation can be achieved in hybrid heterojunctions of CuSCN and a variety of fullerene (C70, C60) and non-fullerene (IT-4F, Y6, ITIC) molecular acceptors. Interestingly, we show that charge-generation proceeds via the formation of a charge-transfer state that can be efficiently dissociated, which enables the fabrication of photovoltaic (PV) devices with non-radiative voltage losses as low as 0.21 ± 0.02 in CuSCN/Y6 devices, which is comparable to losses in the best organic PV devices. Uniquely, due to the transparency of CuSCN in the visible spectrum, the absorption properties of such devices can be tuned solely by the choice of the organic semiconductor, allowing the facile fabrication of colour-tuneable and semi-transparent optoelectronic devices. Using a novel joint electronic-optical model, we demonstrate the potential of such devices through the modelling of PV-photoelectrochemical (PEC) tandem cells, in which the PV and PEC (WO3/BiVO4) share the solar spectrum. In a configuration in which the PV is placed behind the PEC, we show that solar-to-hydrogen (STH) efficiencies of around 4% can be achieved, similar to what is possible using amorphous silicon PV tandem cells. In addition, the possibility to fabricate highly semi-transparent solar cells means extra versatility in the tandem device architecture can be achieved by placing the PV in front of the PEC, with only a small decrease in the STH efficiency. This work thus re-opens the door to exploring hybrid heterojunctions for low-cost optoelectronic devices that may rival those based on all-organic or all-inorganic heterojunctions. [1] [Cappel et al., The Journal of Physical Chemistry Letters 2013, 4 (24), 4253-4257. [2] Yunus et al., Advanced Materials Interfaces 2016, 3 (7), 1500616. [3] Wai‐Yu et al., Advanced Science 2018, 5 (4), 1700980.

Resume : The field of organic photovoltaics (PV) is experiencing a renaissance with remarkable advances in power conversion efficiencies recently reported for both single and double junction devices. Despite these major breakthroughs many aspects of device physics of organic PVs, in particular those with low energy offsets, remain unknown. One of the most common aspects of the device, routinely used for device physics interpretation is the energy level diagram (energetic landscape) of the solar cell, with such diagrams being ubiquitous in literature, appearing in almost every publication. Despite the importance of energy level diagrams in determining the elementary processes taking place in the device (e.g. charge generation, transport and extraction), accurately determining these diagrams is extremely challenging, especially for solution-processed systems. Most commonly, these diagrams are constructed by combining energy values for the individual components as obtained by different methods, resulting in a large scatter of reported values even for the same material systems. In addition, this approach neglects to account for interfacial effects such as formation of dipoles or band bending. In this talk, I will present a new method that can directly measure the vertical energetic landscape of solution-processed organic photovoltaic systems. Our methodology is based on UPS depth profiling, made possible by the use of a gas-cluster ion beam that allows essentially damage-free sputtering of organic materials. First, we demonstrate the efficacy of our technique for both bi-layer and bulk heterojunction layers of a model PV system and establish its ability to accurately probe interfacial effects such as dipoles and band bending. Next, we apply our method to the study of a range of high performance material systems with either fullerene or non-fullerene acceptors and demonstrate an excellent agreement between the measured photovoltaic gaps, the CT state energies and corresponding open-circuit voltages of the devices. Finally, I will present three examples in which we apply UPS depth profiling to study vertical stratification in sequentially deposited organic solar cells, effects of degradation in polymer and small molecule based-devices and the energetic alignment in ternary blends consisting of fullerene and non-fullerene acceptors.

Resume : The combination of automated high throughput laboratory equipment with machine learning approaches holds the potential to revolutionize materials science. Advancements in the understanding of the requirements for organic semiconductors for photovoltaic applications, together with ever better simulation capabilities to identify promising molecules, has led to a steady stream of new materials and has driven the efficiency of OPV beyond 18% in the past few years. The screening of these materials, both for efficiency and stability, is becoming more and more of a bottleneck, especially as the complexity of devices due to ternary and quarternary material combinations is increasing. In our contribution we discuss the application of machine learning and automated equipment to the screening of new materials for OPV. We show that various properties of OPV blends can already be optimized by carefully utilizing the absorption spectra of blends. With a high throughput approach quarternary blends can be optimized for light stability, while a bayesian optimizer driven search for the most stable blends needs ca. 30x less samples to identify an optimum than a classical grid search. Using our automated manufacturing line we produced 100 cells with varying process conditions from the materials system PM6:Y6 and investigated both efficiency and stability. Using Gaussian Process Regression to analyze features of the absorption spectra of the produced films we demonstrate that there are good correlations between these features and Voc, efficiency and even stability of these films. The optimization of OPV-architectures in a 6-dimensional space, spanned by blend composition and process conditions, utilizing bayesian optimization and other algorithms is under current investigation. The effective use of machine learning to solve materials research problems is an active field of research. With our contribution we demonstrate how organic photovoltaics can benefit through the use of these techniques.

Resume : A significant obstacle for the industrial development of organic electronic devices is the lack of a patterning technology having the disruptive power that photolithography exerted in traditional microelectronics. Here we present a new scalable patterning technology for organic semiconductors that takes advantage of the existing photolithography infrastructure and is compatible with digital direct-write patterning and sequential roll-to-roll (R2R) solution coating. The Moule group works on a series of solubility control techniques including the use of marginal solvents and polymer doping, that reduce the solubility of polymers at room temperature, but allow patterning at elevated temperatures. Using these techniques, we are able to vertically stack and laterally pattern mutually soluble polymer layers, which are vital processing steps needed to expand the use of organic semiconductors in device applications. Optimization of these techniques has yielded diffraction limited film patterning with regular features of 200-300 nm with only solution processing steps and direct write laser patterning. Here we present the power dependence and quantitative etching rate for both doped and undoped conjugated polymer films. We demonstrate a pattern write speed of up to 1 microsecond per pixel. We also demonstrate photomask (many pixels simultaneously) writing over 1 mm^2. Further, we demonstrate sub-micrometer patterning of several different conjugated polymers into extended wires. Characterization of the wires shows that they can be doped, de-doped, and re-doped from solution. As-cast and dedoped wires show identical properties as do doped and re-doped wires, demonstrating perfect reversibility of doping with no electrochemistry needed.

Resume : Colloidal quantum dots (QDs) and corresponding light emitting diodes (QLEDs) are generally considered as a promising candidate for displays because of their unique optical properties, including color tunability, narrow emission bandwidth and high device efficiency. Despite the rapid performance development in QLEDs, an intrinsic problem of electron-hole imbalance still exists. Many research groups attempt to improve charge balance with different solutions, such as reduction of electron mobility by using Mg-doped ZnO nanoparticles (ZnO NPs) or addition of an electron-blocking layer between QDs and electron transport layer (ETL). In this research, we intend to improve device performance of QLEDs by adding an additive BYK-P105 into PEDOT:PSS as the hole transport layer. BYK-P105 can be purchased from commercial sources and has a large acid value of 365 mg KOH/g that leads to a strong induced effect on hole injection and transport. In addition, the polyethyleneimine ethoxylated (PEIE)-modified ZnO NPs as the ETL was firstly applied in regular QLEDs for achieving charge balance. A very high brightness of 139,909 nits and current efficiency of 27.2 cd/A were obtained from the optimized device with the configuration of ITO/PEDOT:PSS BYK-P105/PVK/CdSe QDs/ PEIE-modified ZnO NPs/LiF/Al, showing promising use in light-emitting applications.

Resume : Self-assembly in organic semiconductors can efficiently be guided by hydrogen bonds (H-bonds).[1] Our group has used diketopyrrolopyrrole (DPP) as a model system, since they are powerful electroactive segments with interesting optoelectronic properties. DPPs are considered H-bonded pigments due to the presence of unsubstituted amides in their structure, which are normally alkylated to provide solubility. Nevertheless, H-bonding groups can be attached in different parts of the core, as in the periphery of the aromatic rings linked to the central core or as part of the solubilizing tails pending from the lactam rings.[2,3] We show chiral and achiral H-bonded DPP derivatives displaying different aggregation states. The addition of chiral groups next to the H-bonding moieties, results in great changes in their optical properties and photoconductivity. Strong J-type aggregates were observed for the chiral derivative together with intense circular dichroism signals at the aggregates wavelength. These results together with the different types of helical morphologies found are correlated to their optoelectronic properties and photoconductivity. Very interesting electronic and spintronic properties can be achieved with the presence of chiral centers, opening up great alternatives for these molecules as energy harvesting materials. [1] Chem. Mater. 2015, 27, 1201–1209 [2] ChemNanoMat 2018, 4, 790–795 [3] J. Mater. Chem. A 2019, 7, 23451–23475

Resume : The 2,7‐dioctyloxy[1]benzothieno[3,2‐b]benzothiophene (C8OBTBT) molecule, one of the most promising organic semiconductors, presents two different crystalline phases: a co-facial structure (bulk phase) and a herringbone structure also defined surface-induced phase (SIP phase). It has been reported that the SIP phase with aging or after solvent vapor annealing transforms to the thermodynamically stable bulk structure. In this work we report the stabilization of the SIP phase by blending the C8OBTBT with an insulating polymer and preparing the thin film transistors (TFTs) by the bar-assisted meniscus shearing (BAMS) technique. All the TFTs based on C8OBTBT with polystyrene (PS) of low molecular weight (i.e., 3K) and high molecular weight (i.e., 100K) present the SIP structure and display good electrical properties with a field-effect mobility close to 1 cm^2/V·s, a threshold voltage around 0 V and an on/off current ratio of 10^7-10^8. However, after 3 months in ambient conditions the C8OBTBT:PS100K film retain the same morphology, while the film based on the lower molecular weight PS presents the formation of different crystal domains on top of the pristine layer. The structural inhomogeneity of the C8OBTBT:PS3K was investigated by Lattice phonon Raman spectroscopy. DFT calculated charge transfer integrals show higher values for the SIP phase confirming its higher performance compared with the bulk structure based devices.

Resume : Polymer-fullerene solar cells have active layer in form of bulk heterojunction - a nanoscale blend of polymer (donor) and fullerene (acceptor) materials. The anode and cathode materials decide on junction polarization. While the most of the published research is dedicated to donor and acceptor materials, our goal was to increase the cell efficiency by investigation of metallic cathodes. Here we report about influence of chemical character and work function of metal on open circuit voltage and efficiency of PEDOT:PSS/PTB7:PC70BM/Metal and PEDOT:PSS/P3HT:PCBM/Metal solar cells. Along with standard cathodes like Al and Ag we checked 11 more metals and metal bilayers: In, In/Al, 3004 Al alloy, Ca/Al, Mg/Al, Zn, Zn+Al, Sn, Ti, Au, Au+Ti, and Ni. We have found that although the photo-voltage is generated mainly in the active layer, the work function of cathode materials also affects the open circuit voltage and overall efficiency. The cells with different cathodes were examined by current-voltage characteristics, external quantum efficiency spectra and ageing measurements (10 months). We have found, for example, that In/Al cathode increase efficiency of cell up to 4.9% by enhancement of short circuit current and decrease of serial resistance. Additionally cells with In/Al cathode are ageing over time about 2 times slower then cells with conventional electrode. The research was financed by the National Centre for Research and Development (Poland) under the project TECHMATSTRATEG1/347431/14/NCBR/2018.

Resume : The organic electrochemical transistor (OECT) is an important component in bioelectronic circuits designed to interface with tissue. Desirable features, e.g., low power consumption, can be obtained by building complementary circuits featuring two or more OECTs. A complementary circuit normally combines a p- and n-type transistor in series. However, n-type OECTs are less common mainly due to poor stability of the n-type channel material in aqueous electrolyte. Further n-type organics often suffer from poor electrical performance relative to the p-type organics. We present a complementary circuit made using a pair of nominally identical OECTs with a polyaniline (PANI) channel [1]. PANI was chosen due to an interesting peak in current versus gate voltage that allows us to obtain both p-type and n-type gate slopes in the same device under appropriate bias conditions. Our PANI-OECT inverter circuit gives a gain of ~ 7 and can be transferred to a flexible biocompatible chitosan substrate with demonstrated operation in aqueous electrolyte, paving the way towards the realisation of flexible complementary circuits for bioelectronic applications. [1] Travaglini et al., Adv. Funct. Mater. 2007205 (2020).

Resume : The microstructure of donor: acceptor blend photoactive layers in organic solar cells directly influences the properties of interfacial charge-transfer (CT) states, and thereby influences the minimum value of the energy loss that can be achieved in the photovoltaic device. However, relatively few studies are able to relate blend phase behaviour directly to device energy losses. In this work, we study the correlation between the phase behaviour of two poly-3-hexylthiophene (P3HT): acceptor blends made using different non-fullerene acceptors (NFAs), namely IDTBR and IDFBR, and the energy losses of the blend devices. Whilst the two acceptors are chemically very similar, small differences in molecular structure result in large differences in the degree of crystallisation and mixing in the blend films. In particular, P3HT:IDTBR blends shows a higher degree of crystallinity and more phase separated domains, whereas P3HT:IDFBR blends show lower crystallinity and more mixing of polymer and acceptor [1]. The different blends exhibit different trends in terms of energy loss as a function of blend composition, indicating disparate mechanisms on energy losses. To understand this, I will firstly quantify the radiative and nonradiative component of energy losses based on the concept of reciprocity relation between photon emission and absorption [2,3], and secondly extend the model of interfacial charge recombination [4,5] to include the effects of energetic disorder of charge transfer states as shown in Figure 1. Then I will apply this new model to the studied system, and finally relate the measured energy losses to the recombination dynamics in the different blends and finally to the blend microstructure. I will bring the observations together to discuss the relationship between phase behaviour and recombination losses using the P3HT:NFA systems as a model.

Resume : The past decade has witnessed a significant increase in the demand for X-ray radiation detectors in a wide variety of applications ranging from medical diagnostics, industrial inspection, scientific research, to cultural heritage preservation. The object of interest in most of the situations has a complex geometry and shape, while the detectors used to analyse them are of rigid and flat in nature, thus giving rise to an uncertainly in the accuracy of the measurement. Such issues can be mitigated through the use of conformable detectors. Organic-inorganic hybrid semiconductors are a promising class of novel materials, that is often based on a combination of organic and inorganic semiconductors, which potentially allows conformable and flexible detectors to be fabricated over large areas using roll-to-roll or sheet-to-sheet printing techniques. Among the material combinations reported so far, the incorporation of X-ray absorbing bismuth oxide (Bi2O3) nanoparticles (NPs) into an organic bulk heterojunction (BHJ) consisting of p-type polymer Poly(3-hexylthiophene-2,5-diyl) (P3HT) and the n-type [6,6]-Phenyl C71 butyric acid methyl ester (PC70BM) has proven to be highly promising, owing to their unique properties such as high sensitivity, ultra-low dark currents, and low voltage operation[1,2]. However, for conformable detectors, it is imperative to maintain high levels of bendability for the relatively thick P3HT: Bi2O3: PC70BM without mechanical failure while maintaining optimum detector performance. In this study, we evaluate the key influence of the P3HT molecular weight (Mw) on the bendability of hybrid X-ray detectors. Through careful screening of P3HT Mw in the range of 20 – 70 kDa, we show that there is a strong correlation between the P3HT Mw, the P3HT crystallinity and the stiffness of the resulting hybrid layer. In particular, we identify higher Mw to be more appropriate for conformable or flexible detector architectures, which can be traced back to the ability of higher Mw polymers to form crystalline regions which are connected by bridging polymer chains, therefore leading to much better mechanical cohesion. This is demonstrated through the characterisation of X-ray detectors fabricated on flexible substrates with tailored thicknesses, under mechanical deformation. Our work identifies design guidelines, an aspect that has not been discussed so far within the community working on flexible X-ray detectors. References [1] M. P. A. Nanayakkara, L. Matjačić, S. Wood, F. Richheimer, F. A. Castro, S. Jenatsch, S. Züfle, R. Kilbride, A. J. Parnell, M. G. Masteghin, H. M. Thirimanne, A. Nisbet, K. D. G. I. Jayawardena, S. R. P. Silva, Adv. Funct. Mater. 2020, 2008482. [2] H. M. Thirimanne, K. D. G. I. Jayawardena, A. J. Parnell, R. M. I. Bandara, A. Karalasingam, S. Pani, J. E. Huerdler, D. G. Lidzey, S. F. Tedde, A. Nisbet, C. A. Mills, & S. R. P. Silva, Nat. Commun. 2018, 9, 2926.

Resume : In the field of organic semiconductors, monolayers are well known as a way to modify the interfacial properties of the oxide semiconductor. However, most of them have a rather „passive“ role, focusing on the change of the surface energy and/or modification of the energetics of the surface. Therefore, the number of applications, where monolayers are used as the functional layer, is rather limited. The one important exception is dye-sensitized solar cells (DSSCs), where in a combination with the mesoporous TiO2, monolayers are able to provide sufficient absorption of the solar radiation. Unfortunately, the efficiency of DSCCs is still behind that of the conventional solar cells, and more recently they were outperformed by perovskite and organic solar cells. Nevertheless, the example of DSSCs shows the great potential of the monolayers to serve as one of the major components of the device. Use of the monolayer provides several main advantages, in comparison to the conventional solution- (or vacuum-) processed organic films, i.e.: low consumption of materials; resistance to the further processing steps; conformal coverage of the rough surfaces. In addition, due to the ultimately low thickness of the film, it has virtually no parasitic absorption and does not introduce resistive losses. In our work, we had the aim to get a „perfect“ hole-transporting material for the application in tandem solar cells. To do so, we decided to exploit the above-mentioned advantages of the monolayers, and have developed a family of novel materials. As a main building block 9H-carbazole, which is one of the most well-known hole-selective chromophores, was used. It was further functionalized with the phosphonic acid anchoring group, which provides fast adsorption on the surface of the ITO. Overall, several materials with different substitutions of carbazole were synthesized and used for the fabrication of the solar cells. The use of the Me-4PACz has led to the record-breaking efficiency of 29.2% in silicon/perovskite two-terminal solar cell.[A. Al-Ashouri et al. Science, 2020] Moreover, the advantage of the conformal coverage was used to deposit monolayer on top of the rough CIGS solar cell, achieving another record of 24.2% for CIGS/perovskite two-terminal tandem solar cell. In addition, the advantage of the use of monolayers was demonstrated in another emerging photovoltaic technology. 2PACz was used as the hole-selective material for the fabrication of the organic solar cell, leading to the 18% efficiency and improved stability of the device.[Y. Lin et al. ACS Energy Lett, 2020] Upon further optimization of the molecular structure, it is expected that even higher efficiency can be achieved. Overall, we believe, that these results are demonstrating that monolayers are a good choice for the formation of the selective layer in solar cells, and it can be expected that their more detailed study could contribute to the faster development of the field of organic electronics.

Resume : The performance of solar cells based on molecular electronic materials has historically been limited by relatively high non-radiative voltage losses. With the advent of non-fullerene acceptors, non-radiative voltage losses as low as 0.2V have been achieved by reducing the energetic offset between the initial photo-excited singlet state and charge-transfer (CT) state in the bulk-heterojunction. However, the devices with the lowest voltage losses appear to be limited instead by a low Fill Factor (FF), for reasons which are not yet fully understood, which significantly reduces their power conversion efficiency. To understand the correlation between the energy offset and the device performance, we developed a multi-scale device model that accounts for the transfer, dissociation and recombination of the exciton and CT excited states, and the transport and collection of the free charge carriers. We then apply the model to devices based on high performing donor polymer: non-fullerene acceptor (C8-ITIC) blends where the energy offset is modulated via fluorination of the dithienobenzene-thiophene polymer. We have previously reported that the non-radiative voltage losses in the blend with the lowest offset was as low as 0.23V, however the FF of these devices was limited to 55% whereas the devices based on the largest offset system had FF around 70%. Using time resolved spectroscopy to study the impact of the energy offset on the early time evolution of the excited states, we find that the CT state dissociation into free charges is slower for the low offset system. We then measure the free charge carrier lifetime using a newly developed optoelectronic technique, and show that the device with lowest offset has a smaller charge carrier lifetime. Our model can reproduce the different experimental results by taking account of the effect of exciton-CT state energy offset on: 1) charge recombination rate; and 2) exciton dissociation rate constant; 3) CT to free charge dissociation rate; and 4) the back-formation rate constant from free charges to CT state. These effects are sufficient to reproduce the trends in photoluminescence and electroluminescence, picosecond excited state dynamics, and intensity-dependence of device performance. Therefore, the model offers a new and valuable approach to analysing and interpreting experimental results of organic solar cells.

Resume : In certain conditions, conjugated polymers may exhibit both electronic and ionic conductivity, giving rise to numerous applications, such as batteries, bioelectronic devices and neuromorphic systems. For instance, in the field of bioelectronics, the organic electrochemical transistor (OECT) relies on the injection of ions from an electrolyte into a film. The ions interact with the polymer chains creating charged species, such as polarons and bipolarons, tuning the conductivity of the film. Due to the growing number of applications that depend on this electrochemical doping process, there has been a great interest in understanding its mechanisms and characteristics. However, even though the formation of polarons and its impact in morphology, for example, has been demonstrated using different techniques such as Raman spectroscopy and x-ray scattering, little is debated about the formation of bipolarons. [1], [2] To this end, we have characterised the electrochemical doping and dedoping of poly(3-hexylthiophene) (P3HT) thin films using in-situ time-resolved Vis-NIR spectroscopy. We are able to follow the doping evolution with a great resolution (17 ms) and, by decomposing the spectra, to identify the different species involved in the process and their concentrations. Furthermore, to describe the interconversion between the species, we built a kinetic model that provided us with means of determining the overall reaction pathway including its intermediates and rate constants. Additionally, by changing the degree of crystallinity of the P3HT, our kinetic model allowed us to identify the morphology impact in the reaction speed. [1] Nightingale, J.; Wade, J.; Moia, D.; Nelson, J.; Kim, J. S., J. Phys. Chem. C. 2018, 122, 29129-29140 [2] Thomas, E. M..; Brady, A. M.; Nakayama, H.; Popere, B. C.; Segalman, R. A.; Chabinyc, M. L.; Adv. Fun. Mat. 2018, 28, 1803687

Resume : The replacement of fullerenes with non-fullerene acceptors (NFAs) in the fabrication of organic solar cell (OSC) devices in recent years has paved the way to achieve power conversion efficiencies (PCE) approaching 18% for solution processed OSCs. These developments are critical for the field of OSC, as PCE is one of the key metrics deciding on the eventual electricity cost and such high PCEs take the technology many steps closer to successful large-scale commercialisation. Despite the clear importance of NFAs in the future of OSC research, there are few studies at this moment on vacuum evaporated NFA OSCs, i.e. the device technology that is currently dominating the field of organic light emitting diodes and is realised in roll-to-roll production of OSC as well. It has been widely shown in literature that control over the morphology of organic thin films is a key lever to tune the efficiency and behaviour of organic devices. In this work we probe the morphology of vacuum deposited NFA thin films to investigate the mixing behaviour of several donor:NFA blends in thin film. On the donor side, we plan to use both amorphous and semi-crystalline molecules, e.g. TAPC and oligothiophene derivatives. On the NFA side, we are looking into small molecules with the typical A-D-A structure as seen in e.g. IDIC. The main techniques we use to understand the morphology include Grazing Incidence Wide Angle X-ray Scattering (GIWAXS), X-ray Reflectometry (XRR), Atomic Force Microscopy (AFM), and Differential Scanning Calorimetry (DSC). We look at the impact of this morphology on the optical properties of the films using Spectroscopic Ellipsometry (SE) as well as the impact of morphological changes on the device performance using JV measurements. It is our hope that this work will contribute to the current understanding of the behaviour and role of NFAs in OSCs as well as allow for the fabrication of more efficient vacuum deposited OSCs.

Resume : The ability of numerous organic molecules to adopt different crystal structures without changing their chemical structure is called polymorphism and gained interest in recent years due to the influence it has on the solid-state properties of organic materials e.g., the modulation of charge carrier mobility in organic semiconductors (OSCs) by several orders of magnitude. Especially the stabilization of meta-stable polymorphs at room temperature via different processing or post-processing methods has been the subject of recent investigations. Suitable methods include for example meniscus-guided coating with different processing parameters or solvent-vapor/thermal annealing of thin films. Especially solution processing parameters like temperature, solvent or processing speed differ drastically between methods, materials and desired film morphology. Processing temperatures over 100 °C are common for recent high performing but low solubilizing materials. Here we present the investigation of a previously unknown polymorphic transition of the state-of-the-art organic small molecule semiconductor 2,9‐didecyldinaphtho[2,3‐b:2′,3′‐f]thieno[3,2‐b]thiophene (C10-DNTT) in thin film form between a low temperature and a meta-stable high temperature phase. We investigate the two phases and their transition for vacuum-deposited and solution processed thin films via in situ heating grazing-incidence wide angle X-ray scattering (GIWAXS), in situ heating cross-polarized optical microscopy, AFM, and DFT calculations. From refinement calculations on the basis of the experimentally observed scattering intensities, we derive detailed packing information for both phases in thin film form. Additionally, we show the unit cell expansion and change with temperature and link the anisotropy of the in-plane expansion to differences in electronic coupling strengths for different directions in the unit cell, which we obtained by charge transfer integral calculations for both crystal structures. The finding of a polymorph of C10-DNTT poses an important step towards fully understanding film formation, degradation and the resulting performance fluctuations of devices employing this and similar materials. Due to the existence of an easily accessible high temperature polymorph, C10-DNTT is a good candidate for further investigations regarding polymorphic transitions in its materials class. The detailed analysis of the thermal behavior of the thin films will help to improve the thermal stability of devices in order to make organic devices even more stable and reliable in the future.

Resume : The structure-based tuneability of the electronic and optical properties of conjugated polymers have enabled their application across a range of fields, including energy harvesting, photovoltaics, and medical imaging. However, in the context of biological sensing, the use of conjugated polymers has thus far incorporated little specificity in terms of covalent modification. Developing robust, highly selective, biologically compatible sensing platforms is of critical importance because the measurement of analyte concentrations in biological samples is crucial for the management or detection of many diseases. For example, fluctuations above or below the optimal range of sodium ion concentration in many bodily fluids can impact blood pressure, nerve and muscle function, while glucose concentration in the blood must be monitored constantly in diabetes. Currently, commercial glucose sensors and many proposed alternatives rely on enzymes, which are expensive, unstable and subject to temperature and pH sensitivity. Meanwhile, other devices are based on complex composite designs featuring separate components to impart conductivity, analyte binding, or selective response. Synthetic strategy with conjugated polymers, especially using the technique of molecular imprinting, enables the efficient combination of analyte specificity, biological interfacing and electroactive or optical functionality into a single multipurpose material. In addition, these entirely organic systems offer affordability, stability, biocompatibility, and simple design and fabrication. However, due to the well-known difficulties associated with the covalent modification of one of the most effective polymers for this application, poly(3,4-ethylenedioxythiophene) (PEDOT), one of the main challenges in this area thus far has been the attainment of specificity at a molecular level. I will present my work on the concept, design and synthesis of novel electroactive polymers for biological sensing. This includes the creation of a sodium-chelating electrochromic polymer, and a glucose-binding electroactive molecularly imprinted polymer, both based on covalently modified PEDOT. I will discuss the selectivity and sensing behaviour of these materials in response to the respective analytes, their optical and electrical properties, and how this highly adaptable approach is being applied to create a range of materials for many applications across the field of biological sensing.

Resume : Long-range electrostatic phenomena profoundly affect the energy landscape of charge and energy carriers in molecular solids, films and interfaces, offering challenges and opportunities for the optimization of materials and devices for organic electronics. [1] This talk will cover two recent studies, in which theoretical calculations made possible by a precise knowledge of the supramolecular organization in complex molecular architectures allowed to make sense of puzzling photoemission and optical spectra, shedding light on fundamental mechanisms of great potential for applications. First, I will discuss the efficient charge generation in a single-component solar cell whose active layer is composed by a polycrystalline sexithiophene film. [2] Our embedded many-body ab initio simulations show that the electrostatic landscape at the boundary between crystalline domains of different orientation determine an energy offset similar to that of donor/acceptor heterojunctions, resulting in resonating charge-transfer and molecular excitations, and leading to remarkable external quantum efficiency (44%) and open-circuit voltage (1.6 V). I will then report on the latest chapter of the saga of the photoemission spectra of pentacene (PEN) and perfluoro-PEN (PFP) films, which had a foundational role for the acknowledgement of electrostatic phenomena. Similar to pristine films, also PEN-PFP 1:1 co-crystals show a marked dependence on molecular orientations on different substrates, with the UPS spectra of films of lying molecules on graphite offering a very challenging scenario, characterized by several bands and an ultra-low ionization potential. [3] The rationalization of this intricate puzzle testifies the advanced level of understanding of electrostatic phenomena in molecular heterostructures of increasing complexity, paving the way for the engineering of these subtle interactions in organic electronics devices. [1] G. D’Avino, L. Muccioli, F. Castet, C. Poelking, D. Andrienko, Z. G. Soos, J. Cornil, D. Beljonne, J. Phys. Condens. Matter 28, 433002 (2016) [2] Y. Dong, V. C Nikolis, F. Talnack, Y.-C. Chin, J. Benduhn, G. Londi, J. Kublitski, X. Zheng, S. C.B. Mannsfeld, D. Spoltore, L. Muccioli, J. Li, X. Blase, D. Beljonne, J.-S. Kim, A. A Bakulin, G. D’Avino, J. R. Durrant, K. Vandewal, Nature Commun. 11, 4617 (2020) [3] G. D’Avino, S. Duhm, R. G. Della Valle, G. Heimel, M. Oehzelt, S. Kera, N. Ueno, D. Beljonne, I. Salzmann, Chemistry of Materials 32, 1261 (2020)

Resume : Charge carrier mobility is a central property that determines the performance of organic semiconductors in many devices and has been shown to be highly dependent on molecular structure and the related thin-film nanostructure [1]. Planar molecules are expected to enhance intermolecular interactions and facilitate charge transport along with the stacking directions. However, it often remains difficult to design highly planar molecules that are both soluble and have the desired opto-electronic properties. In this work, we investigate a donor-acceptor-donor small-molecule based on a TAT-TPD-TAT structure, where TAT represents a planar, highly soluble, functionalized electron donor triazatruxene moiety and TPD an electron acceptor thiophene-thienopyrroledione-thiophene unit. These molecules led recently to promising results in solar cells [2], which were partly attributed to a relatively high SCLC hole mobility of 2.10-3 cm2/V*s in as-deposited films. Here, an in-depth investigation of the SCLC mobility as a function of annealing treatment is presented. It is shown that the mobility increases by two orders of magnitude when changing from the as-deposited to the crystalline state. A remarkably high SCLC mobility of 0.2 cm2/V*s could be achieved in 30 µm thick devices. [1] Y. Tsutsui, Y. H. Geerts et al. Adv. Mater., 2016, 28(33): 7106-7114. [2] T. Han, T. Heiser et al. J. Mater. Chem. C, 2017, 5(41), 10794-10800.

Resume : Perovskite solar cells (PSCs) require high efficiency and good long-term stability that must come at low costs to achieve the commercial viability. Counting more than 20 years, spiro-OMeTAD now dominates the field of PSCs and is the hole-transporting material routinely employed as highly efficient reference material for the research interests despite its high price (~250 $/g). Although PSCs have recently achieved certified power conversion efficiency (PCE) exceeding 25%, one major bottleneck of stabilizing the performance is the lack of stable HTMs, which extract positive charges from the active perovskite light absorber and transmit them to the electrode. Here, four HTMs containing well-established spirobifluorene core with introduced enamine arms were synthesized by a simple condensation reaction that does not require expensive catalysts, inert reaction conditions and time-consuming sophisticated product purification. Therefore, it may result in significantly reduced synthesis costs. To investigate photoelectrical properties of synthesized HTMs hole drift mobility and ionization potential were measured. Solid-state ionization potential was measured using the electron photoemission in the air (PESA) to study the HOMO energy level of spiro-enamine HTMs. Ionization potential values of V1305, V1306 that have one and two enamine arms, respectively, tetra-substituted V1307 and trans-configurated V1308 were found to be 5.33, 5.37, 5.46, and 5.46 eV, respectively, which is significantly stabilized comparing to that of spiro-OMeTAD (5.00 eV). Based on the solid-state optical gap and ionization potential values electron affinities were calculated to be 2.43, 2.53, 2.63, and 2.68 eV for V1305, V1306, V1307 and V1308, respectively. It is important that the electron affinities are smaller comparing with the conduction band (CB) energy of the perovskite (-4.10 eV), therefore effective electron blocking from the perovskite to the electrode should be ensured. Xerographic time of flight (XTOF) technique was used to determine the charge mobility of investigated HTMs layers. V1308 and V1307 exhibited the highest zero-field hole drift mobility among the series, both outperforming that of spiro-OMeTAD. n-i-p solar cells were fabricated to test novel materials as HTMs. The most efficient devices containing V1305, V1307 and V1308 presented similar power conversion efficiency values of 19.0, 19.2 and 19.1%, respectively. Data analysis confirms the correlation between hole drift mobility and device performance, being most efficient the devices containing the HTM with higher hole mobility values following the order: V1306˂V1305˂V1308˂=V1307. This work shows that simple enamine condensation protocol could be used as universal approach achieving HTMs for highly efficient and stable PSCs.

Resume : Organic photovoltaics (OPVs), due to the advent of non-fullerene acceptors, now exceed 18% power conversion efficiency at 1 sun illumination, a laudable achievement. However, efficiencies still lag behind silicon and perovskite solar cells which limits the potential market for OPVs. One promising route to market for OPVs is for targeted low-light and indoor applications where illuminance is orders of magnitude smaller than 1 sun. Under low-light OPVs perform significantly better than silicon solar cells, showing only small losses in VOC and improved FF compared to 1 sun performance. However, devices considered in the literature so far employ materials or fabrication methods which are not suitable for large scale-up, these include, for example, evaporated interlayers, inflexible substrates, or solvent vapour annealing. In this work, we demonstrate the commercial viability of OPVs for low-light applications by utilizing a fully scalable device architecture with currently upscaled OPV materials. We show superior device performance to silicon and discuss important parameters to consider for low-light device optimisation. In particular, we identify and characterise a critical light-soaking (LS) effect that is ruinous for low-light performance. This LS effect results from poor electron extraction at the electron transport layer (ETL)/photoactive layer interface due to a high density of mid-gap states in the ZnO ETL and poor energetic alignment. We effectively remove this LS effect by employing SnO2 nanoparticles as ETL, which we then use to demonstrate scale-up potential by fabricating a 21.6 cm2 module, achieving a maximum power output of 17.2 µW cm-2 at 1000 lx. [1] J. Luke, L. Corrêa, J. Rodrigues, J. Martins, M. Daboczi, D. Bagnis and J. S. Kim, Adv. Energy Mater., 2021, 2003405. DOI: 10.1002/aenm.202003405

Resume : Perovskite solar cells (PSCs) represent now rapidly emerging and highly promising photovoltaic technology with the best laboratory device efficiencies surpassing 25% threshold. [1] While the efficiencies of PSCs come close to that of crystalline silicon solar cells, there is a severe gap in operation stability of these two types of photovoltaic devices. Silicon solar cells can operate for 25-40 years, while most of perovskite solar cells degrade within few thousand hours under continuous illumination. [2] There is a growing evidence that stability of PSCs is strongly impacted by interfacial charge transport layers. [3] In particular, hole transport layer (HTL) is responsible for multiple degradation pathways discovered for PSCs. State-of-art HTLs require additional p-doping with oxygen and hydroscopic Li-salts, which impairs ambient stability of PSCs. Moreover, doped HTLs are intrinsically unstable in combination with complex lead halides since cationic species tend to oxidize I- to molecular iodine. Therefore, designing new p-type materials enabling decent transport of positive charge carriers without additional doping is crucially important. In this work, we present a systematic study of a big series of p-type organic small-molecular semiconductors as HTL materials in PSCs. These materials represented triazatruxene-, triphenylamine- and dithiophenosilole- derivatives with various solubilizing chains. Highest occupied molecular orbital (HOMO) energies of these materials varied from -5.06 to –6.00 eV, thus providing excellent screening library for matching the perovskite valence band. All materials were evaluated in PSCs with n-i-p configuration and HTL film thickness and deposition conditions were thoroughly optimized. The best devices showed power conversion efficiencies approaching 20% thus suggesting efficient hole extraction from the device active layer. Most importantly, we revealed a relationship between HOMO energies of HTL materials and open circuit voltage (and also efficiency) of perovskite solar cells. The obtained results establish guidelines for designing new dopant-free HTL materials for efficient perovskite solar cells. References 1. https://www.nrel.gov/pv/assets/pdfs/best-research-cell-efficiencies.20190923.pdf cited on 14.10.2019 2. Han, Y., Meyer, S., Dkhissi, Y., Weber, K., Pringle, J. M., Bach, U., Cheng, Y.-B. Journal of Materials Chemistry A, 2015, 3, 8139–8147. 3. Akbulatov, A. F., Frolova, L. A., Griffin, M. P., Gearba, I. R., Dolocan, A., Vanden Bout, D. A., Tsarev, S., Katz, E. A., Shestakov, A. F., Stevenson, K. J., Troshin, P. A. Adv. Energy Mater. 2017, 7, 1700476.

Resume : Beyond solar cells using hybrid halide perovskite CH3NH3PbX3 (X =Cl, Br, I), broad applications for optoelectronics are being cultivated owing to high absorption coefficient, direct bandgap, long carrier lifetime, high balanced hole and electron mobility. Most of reported perovskite devices depending on polycrystalline thin films immensely suffer from poor stability and high trap density created by grain boundaries, which limits the performance in device applications. Recently, single crystals of perovskite have been investigated for the realization of stable device with low trap density compared to thin film counterpart. In the study, new growth technique is applied to inverse temperature crystallization (ITC) of CH3NH3PbBr3 perovskite single crystal using low dissolution temperature and heating rate control with the first demonstration of low temperature inverting solubility of CH3NH3PbBr3. We compared the crystallinity and charge transport of perovskite single crystal stuructures as a function of dissolution temperature dependence. New growth technique can provide quick process and high yield of large scaled single crystal growth with high quality. Our advanced growth technique and resulting single crystal structures will contribute to demonstrating its potential in optoelectronic applications.

Resume : In this work we study the optical and thermoelectric properties of ZnO/PEDOT:PSS hybrid thin films. First, we focus on the influence of the thickness of the PEDOT:PSS thin films on their thermal and optical properties. We address the problem in a comprehensive manner based on the concept of thickness gradients, i.e. we study a single sample deposited through blade coating on a commercial glass microscope slide, where the thickness of the PEDOT:PSS thin film changes along the deposition direction. The thickness at each position on the samples was determined using optical ellipsometry and ranges between 10 nm and 250 nm. The key advantages of this approach are: (i) the high throughput of experimental results it provides within one sample, and (ii) the obtained results provide and excellent relative determination of the influence of the thickness on investigated particular property (thermal conductivity and Raman in this case), since a single fabrication step is used making each measurement consistently comparable. Furthermore, we also investigate the influence of “doping” the PEDOT:PSS thin films with ZnO nanocrystals. In particular, we address the influence of the concentration of ZnO nanocrystals on the optical and thermoelectric properties of the films. As a novel finding, we observed that as the ZnO concentration increases, the thermal conductivity exhibits up to a two-fold increase.

Resume : Solution-processable organic field effect transistors (OFETs) based on conjugated polymers are emerging as promising building blocks for displays, sensors, etc. Improvement of the charge carrier mobility (µ) has been one of the key objectives in the field of organic electronics, and several polymer semiconductors with µ >10 cm2/Vs have been reported. However, significant overestimations of the µ values based on non-ideal device characteristics have been often noted. Effective mobility (µeff) has been proposed for OFETs based on a reliability factor (r) to avoid overestimations of the saturation mobility (µsat). One of the reasons for these non-idealities that limit the charge mobility in OFETs is the charge carrier trapping. Molecular doping has emerged as one of strategies for mitigation of these trap states. However, the current tool-box of molecular dopants is limited and the stability of doped device, uniformity of doping, energy levels matching, so that the practical application of the doping strategy in OFET manufacturing is still under question. We introduce nitrofluorene (NFs) derivatives as novel p-dopants for polymer OFETs and show their superior air stability, solubility in polar and non-polar solvents and easy tunability of their electron affinity (3 to 4.8 eV). Adding NFs to DPP-DTT films significantly improves the charge mobility and suppresses the parasitic effects (contact resistance, charge carrier trapping, gate dependence of the mobility). A 6-fold enhancement in µeff (maximum mobility of ~8 cm2/Vs) is realized in ambient air operation. The EPR and Vis-NIR spectroscopy, AFM and XRD analyses, and electrical characterization elucidate the role of NF dopants in mitigating charge-carrier traps, lowering the contact resistance, and maximizing the order of the polymer films. Variable temperature mobility measurements show a ~2-fold reduction of the transport activation energy due to reduced energetic disorder. We believe this work opens new opportunities for realizing stable polymer OFET devices for soft electronics.

Resume : Recent years have witnessed great progress of both conjugated polymers in terms of structural diversity, processing, morphology analysis and application in all sub-areas of organic electronics. However, with step-growth polycondensation still being the dominant reaction mechanism, molecular weight (MW) control remains a major drawback, which complicates comparability of individual studies. Another issue pertains to the role of prevalent chemical defects that eventually form during direct arylation polycondensation (DAP) of aromatic monomers. Here we present examples of high performance copolymers that can be made via DAP with a minimum of reaction steps and controlled MW weight to address the influence of main chain defects and MW on properties usually observed for short chain systems. Selected examples include copolymers based on naphthalene diimide, diketopyrrolopyrrole and benzothiadiazole copolymers, which are used for applications in all-polymer organic photovoltaics, n-channel organic field-effect transistors and organic thermoelectrics.

Resume : During the last decades, the performance and environmental stability of organic transistors have been improved significantly. However, a remaining challenge for the employment of organic transistors in industrial applications such as flexible displays, is their operational stability. A widely studied topic is the on-state bias stress stability of (organic) transistors: During operation trapped charges screen the applied gate field and lead to undesired threshold voltage shifts. In displays for example, this instability in transistor performance would lead to varying brightness of the respective pixels. One unanimously reported cause of such operational instabilities are water induced traps that affect the performance of various devices, such as organic thin-film transistors and diodes [M. Nikolka, H. Sirringhaus et al. Nature Mater., 2016, 16, 356; M. Kettner, R. T. Weitz et al. ACS Appl. Mater. Interfaces, 2018, 10, 35449; P. A. Bobbert, D.M. de Leeuw, Adv. Mater, 2012, 24, 1146.]. Furthermore, water and oxygen can not only impact the on-state, but also the off-state bias stress stability (non-conductive stress) of oxide TFTs. While the on-state bias stress is widely studied, the off-state bias stress is mostly overlooked in organic TFTs, even though equally important for the stable operation of displays as well as sensors or logic circuits. Recent findings show that water-induces traps in polymer devices can be passivated by the addition of particular (non-doping) molecular additives, leading to significant improvements in the device performance and the on-state bias stress stability of polymer TFTs (thin-film transistors). In this work, we close the gap and focus on the off-state bias stress stability of p-type polymer transistors, show that threshold voltage shifts can be remarkably reduced and discuss the underlying mechanisms with the aid of temperature-depend bias stress measurements.

Resume : Recent progress in smart phone, tablet PC and smart sensor technologies have intensified research and development of flexible, thinner and wider display screens at reduced cost. Compared to their inorganic counterparts such as LTPS TFTs and oxide TFTs, organic thin film transistors (OTFTs) are promising for developing flexible, large area application at low temperature and low-cost processing [1-3]. Low field effect mobility is a critical limiting factor in advancement of OTFTs. However, in recent years the advancement of solution‐processed small molecule organic semiconductors (OSCs) demonstrating crystalline nature have drawn substantial attention for their potential in the low cost, flexible, printed OTFT with high mobility. In this regard solution processed 2D materials such as dinaphtho[2,1-b:2′,1′-f ]thieno[3,2-b]thiophene (DNTT), benzothieno[3,2-b][1]benzothiophene (BTBT) and their derivatives have shown high field effect mobility ≥ 10 cm2 V−1 s−1 [3,4]. In this work, series of 6,6′ bis (trans-4-n alkyl cyclohexyl)-dinaphtho[2,1-b:2′,1′-f ]thieno[3,2-b]thiophene (nH-21DNTT) (Ushio Chemix Corporation) were studied and characterized by XRD analysis and further evaluation of OTFT operation using solution processable thin films of 3H-21DNTT, 5H-21DNTT and 7H-21DNTT was performed. On comparison with related DNTT-based organic semiconductors 5H-21DNTT exhibited superior OTFT performance. We demonstrated that using 5H-21DNTT as an active layer a bottom-gate, bottom-contact (BGBC) OTFT exhibiting mobility ≥ 10 cm2 V−1 s−1 and on/off ratio of 10^4 was successfully fabricated. 5H-21DNTT film was fabricated via a drop-casting deposition, with its crystallinity and morphology studied under polarization camera, XRD and AFM analysis. In particular, for 5H-21DNTT based BGBC OTFT with Parylene C as organic gate dielectric maximum mobility over 14 cm2 V−1 s−1 for devices with 50 μm channel length was achieved. A solution‐processed all organic OTFT array used as a switching circuit to drive a 7-segment inorganic electroluminescence display was fabricated using 5H-21DNTT as semiconductor films and Parylene C as gate dielectric. This work establishes a simple yet effective approach for fabricating high-performance and low-cost solution processable OTFT. References [1] Xiaotao Zhang, Wenping Hu et al, Chem. Soc. Rev., 2019, 48, 1492. [2] M. Mizukami, et al., IEEE Electron Device Letters, 2018, 39, 1, 39. [3] L. Chan et al, Adv. Sci., 2019, 6, 1900775. [4] J. Takeya et al, Appl. Phys. Lett., 2012, 101, 223304.

Resume : The terms “epidermal electronics”, refer to a class of electronic devices that due to their flexibility and/or stretchability are able to be intimately coupled with the complex features of the epidermis. Electronic devices conformally mounted directly on the skin can provide a versatile means to acquire information about the body through the monitoring of biologically relevant chemical and physical variables, which could be tracked for continuous health monitoring, as well as for robotic feedback and control, prosthetics and rehabilitation, and human/computer interfaces. Freestanding nanofilms are large area (square cm) ultra-thin (tens-hundreds of nm) polymeric films, showing as an intrinsic property the capability to adhere and conform spontaneously to surfaces due to physical interactions (i.e. Van Der Waals), making possible the intimate contact between such films and virtually any surface, including skin. Nanofilm technology results particularly promising for the assembly and development of conformable electronic (passive and active) and circuits, when polymers with specific electrical properties are used (conjugated polymer films, ultrathin dielectric films). In this talk the last achievements achieved by our group in this field will be reviewed, with particular focus on possible applicative scenarios.

Resume : Organic electrochemical transistors (OECTs) are highly sensitive sensors used in increasingly challenging biologic applications such as wearable textiles with integrated biosensors and in vivo recording of brain activity.[1,2] They can be described as an ionic circuit embedded with an electronic circuit. Upon a voltage bias, ions penetrate an organic channel (ionic circuit) and change its conductivity (electronic circuit) in both the macro and microscale. During the last years, investigations on such devices were focusing on the evolution of the channel macroscopic conductivity (superior to few hundred of nanometers) upon voltage bias modulation.[3] In this work, we propose an innovative in operando bottom-up approach that correlates the doping extent of the channel with its microscopic conductivity (few nanometers). To access the doping extent, we use in operando UV-vis-NIR absorption spectroelectrochemistry and a multivariance curve resolution (MCR) analysis[4] to extract the neutral, polaron and bipolaron populations. The nanoscale conductivity is probed via THz spectroscopy[5], which represents the first measurement of this kind on OECTs. Overall, this new versatile approach provides unique information on two intriguing aspects: 1) The intrinsic conductivity of different doping states of the organic channel (neutral, polaronic and bipolaronic) 2) Combined with macroscopic conductivity, it allows to disentangle the underlying mechanisms limiting the charge transport in OECTs. Results on the archetypal PEDOT:PSS-based OECTs, will be discussed. As well as the perspectives of this new approach on the developpement of OECTs in general. [1] Gualandi I., Marzocchi M., Achilli A. et al. Textile Organic Electrochemical Transistors as a Platform for Wearable Biosensors. Sci Rep 6, 33637 (2016). [2] Khodagholy D., Doublet T., Quilichini P. et al. In vivo recording of brain activity using organic transistors. Nat Commun 4, 1575 (2013). [3] Rivnay J., Inal S., Salleo A. et al. Organic electrochemical transistors. Nat Rev Mater 3, 17086, (2018) [4] De Juan A., Jaumot J., Tauler R. Multivariate Curve Resolution (MCR). Solving the mixture analysis problem. Anal. Methodes 6, 4964-4976 (2014) [5] Unuma T., Yamada N., Nakamura A. et al. Direct observation of carrier delocalization in highly conducting polyaniline. Appl Phys Lett 103, 053303 (2013)

Resume : In the view of a rapid increase in efficiency of organic solar cells, reaching their long-term operational stability represents one of the main challenges to be addressed on the way toward commercialization of this photovoltaic technology. However, intrinsic degradation pathways occurring in organic solar cells under realistic operational conditions remain poorly understood. The light-induced dimerization of fullerene-based acceptor materials discovered recently is considered to be one of the main causes for the burn-in degradation of organic solar cells. In this work, we reveal the mechanism of the light-induced dimerization of the fullerene derivatives and establish important correlations with their molecular structure and electronic properties. We also show that conjugated polymers and small molecules undergo similar light-induced crosslinking regardless of their chemical composition and structure. In the case of conjugated polymers, crosslinking leads to a rapid increase in their molecular weight and consequent loss of solubility, which can be revealed straightforwardly by gel permeation chromatography analysis via a reduction/loss of signal and/or smaller retention times. Our results, thus, shift the paradigm of research in the field toward designing a new generation of organic absorbers with enhanced intrinsic photochemical stability to reach practically useful operation lifetimes required for successful commercialization of organic photovoltaics. More details can be found in our publication: O. R. Yamilova, I. V. Martynov, N. Stingelin, P. A. Troshin et al., Adv. Energy Mater., 2020, 10, 1903163

Resume : In the last few years a significant evolution in the field of organic photovoltaics (OPVs) was achieved thanks to the synthesis of new generation donor-acceptor polymers with reported power conversion efficiencies (PCE) above 10%. This study focuses on one of those polymers known as PCE11 (poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)-alt-(3,3´´´-di(2-octyldodecyl) 2,2´;5´,2´´;5´´,2´´´-quaterthiophen-5,5´´´-diyl)]). PCE11 is a polymer that shows highly tunable optical properties under various processing conditions and has a reported PCE of 11.5%.[1] We employ temperature dependent Resonance Raman Spectroscopy (RRS) to develop a basic understanding on how temperature affects the polymer conformation and thus its optoelectronic properties. Moreover, by combining experimental data obtained by steady state RRS and Absorption it is possible to extract additional information regarding the excited-state structure and dynamics. In particular, the shape of the absorption spectra can be determined by the displacements between the ground and excited state potential energy surface minima in each mode, which are directly related to the intensities of RR bands. In the present study, we use a computational method called Resonance Raman Intensity Analysis (RRIA) that provides access to a quantitative picture of the excited state, such as the change in the molecular geometry, the initial excited-state dynamics, as well as the nature of that state, gaining thus insights that aid in the interpretation of the photophysical behaviour of this donor-acceptor polymer.

Resume : The recent increase in power conversion efficiency (PCE) of bulk heterojunction solar cells has enlarged the prospective for this technology to contribute to large scale renewable energy production. However, for a sustainable scale-up of solution-processed organic photovoltaic modules, the replacement of toxic solvents, generally used at laboratory scale, by alternative “green” solvents with a reduced impact on the environment and human health is a critical pre-requisite. Only few examples of donor: acceptor blends that can be processed from less toxic non-halogenated solvents without loss of performances have been reported so far. Searching for new solvents is indeed a complex problem, which requires taking into account multiple properties, such as Hansen solubility parameters, boiling temperature, toxicity …Due to this complex relationship between solvent properties and device performance, the selection of alternative solvents has so far relied primarily on time-consuming and costly trial-and-error approaches. In this contribution, we will present a new way to achieve a more efficient and less empirical selection of sustainable solvents. Our method involves numerical modelling of molecular properties and reverse design of solvent molecules with target properties using the recently developed IBSS®CAMD software.1 The methodology is applied to various donor/acceptor blends including PF2 or P3HT electron donor polymers and PC71BM or EH-IDTBR electron acceptors. This allows us to establish lists of alternative solvents ranked quantitatively by a global performance function encompassing a given number of desired properties. The actual performances of the selected solvents are evaluated by elaborating photovoltaic devices and comparing the power conversion efficiencies with those obtained with devices processed from halogenated solutions. By defining appropriate target values for eleven solvent properties, five sustainable solvents were found suitable to be used for processing OPV devices based on either P3HT:PC71BM or P3HT:EH-IDTBR blends. In particular, an average PCE of 3.26% was obtained with P3HT:PC71BM processed from p-cymene, while for P3HT:EH-IDTBR, anisole led to an average PCE of 4.55%. Similarly, in the case of PF2:PC71BM blends, p-xylene was identified to be the best choice as sustainable solvent, leading to an average PCE of 9%. For each blend, the performances of the devices obtained with the alternative solvents were similar or superior to those of standard devices processed from halogenated solvents, corroborating the relevance of the proposed method. 1 I. Rodriguez-Donis, S. Thiebaud-Roux, S. Lavoine and V. Gerbaud, Comptes Rendus Chim., 2018, 21, 606–621.

Resume : Non-fullerene acceptors (NFAs) for organic solar cells (OSCs) have significantly developed over the past five years with continuous improvements in efficiency now over 18%. However, a key challenge still remains in order to fully realize their commercialization potential: the need to extend device lifetime and to control degradation mechanisms. Herein, we investigate the effect of two different molecular engineering routes on the widely utilized ITIC NFA, to tune its optoelectronic properties and interactions with the donor polymer in photoactive blends. Heavier selenium (Se) atoms substitute sulfur (S) atoms in the NFA core in either outer or inner positions, and methyl chains are attached to the end groups. By investigating the effects of these structural modifications on the long-term operational stability of bulk-heterojunction OSC devices, we identify outer selenation as a powerful strategy to significantly increase device lifetime compared to ITIC. Combining outer selenation and methylation results in an impressive 95% of the initial OSC efficiency being retained after 450 h under operating conditions, with an exceptionally long projected half-lifetime of 5600 h compared to 400 h for ITIC. We find that the heavier and larger Se atoms at outer-core positions rigidify the molecular structure to form highly crystalline films with low conformational energetic disorder. It further enhances charge delocalization over the molecule, promoting strong intermolecular interactions among acceptor molecules. Upon methylation, this strong intermolecular interaction stabilizes acceptor domains in blends to be resilient to light-induced morphological changes, thereby leading to superior device stability. Our results highlight the crucial role of NFA molecular structure for OSC operational stability and provide important NFA design rules via heteroatom position and end-group control. Ref. C. Labanti, J.-S. Kim et al. ACS Nano 15, 4 (2021), 7700-7712.

Resume : Solution‐processed polymer bulk heterojunction organic photovoltaic (BHJ-OPV) devices have gained serious attention during the last decade. They are one of the leading next-generation photovoltaic technologies for low-cost power production1. The active layer of a polymer solar cell consists of a thin solid film of an electron donor blended with an electron acceptor. The morphology of the active layer is one of the important factors for solar cell performance. To control the morphology, one needs to understand the morphology formation on a molecular level. It is known that molecular interactions govern morphology formation and purity of mixed domains of conjugated polymers and small-molecule2. Therefore, understanding these interactions relations would give us insight into the challenges of coating OPV from green solvent to achieve the required nanostructured interpenetrating network of donor and acceptor domains based on a rational choice of solvent3. In this work, we first calculate the Hansen solubility parameters (HSP)3,4 of the donor and acceptor materials, followed by careful choice of solvents with selective relative solubilities for the two materials. After the theoretical screening procedure, solubility tests were performed to determine the HSP parameters relevant to the systems. Then finally, thin films were prepared by spin-coating from the solvent screened using this method. Our results show that the blend film morphology prepared in this way is similar to those prepared from the commonly used halogenated solvents. References: 1. Brabec, C. J. et al. Polymer-Fullerene Bulk-Heterojunction Solar Cells. Adv. Mater. 22, 3839–3856 (2010). 2. Ye, L. et al. Quantitative relations between interaction parameter, miscibility, and function in organic solar cells. Nat. Mater. 17, 253–260 (2018). 3. Holmes, N. P. et al. Unravelling donor-acceptor film morphology formation for environmentally-friendly OPV ink formulations. Green Chem. 21, (2019). 4. Jalan, I., Lundin, L. & van Stam, J. Using Solubility Parameters to Model More Environmentally Friendly Solvent Blends for Organic Solar Cell Active Layers. Materials (Basel). 12, (2019).

Resume : Pi-conjugated polymers are being used in the fabrication of a wide variety of organic electronic devices such as organic field-effect transistors (OFETs), organic photovoltaic (OPV) devices, and organic light-emitting diodes (OLEDs). Since the seminal work on the conductivity of polyacetylene by Heeger, MacDiarmid, and Shirakawa was published in 1970s, the field of organic electronics has grown exponentially. Our group has been studying and developing techniques to grow semiconducting polymers using a living polymerization method and more recently using direct arylation polymerization. This has allowed us to synthesize polymer architectures that we haven’t been able to access till now including polythiophene brushes, star-shaped P3HT, as well as hyperbranched P3HT. It also allows us to accurately control the molecular weights of P3HT and produce materials with a narrow molecular weight distribution. Our unique synthetic capabilities allows us to specifically control defects in these polymers. Our work in controlling polymer defects and their effect on microstructure and thus optoelectronic properties will be presented. More recently, we have begun to study the mechanical properties of semiconducting polymers as well as the development of mixed ionic/electronic conductors. As the polymers’ practical applications have extended into the health and life sciences areas (e.g., electronic skins and artificial muscles), the mechanical compliance (i.e., low stiffness and high ductility) and the interplay between ionic and electronic conductivities has become increasingly important. This in turn requires one to establish an understanding of the relationship between polymer structure and their mechanical properties as well as their (opto)electronic properties. In this presentation, the synthesis of a series of semiconducting polymers will be discussed along with the feasibility of using these polymers in stretchable and mixed conduction devices.

Resume : Direct printing of thin-film transistors has enormous potential for ubiquitous and lightweight wearable electronic applications. This talk presents our recent advances in flexible and printed integrated circuits and active-matrix sensor arrays. First, A 3D integration approach is introduced to achieve technology scaling in printed transistor density, analogous to Moore’s law driven by lithography. We demonstrate the scalable 3D integration of single- and dual-gate organic transistors on plastic foil by printing with high yield, uniformity, and year-long stability. The vertical stacking of three complementary transistors enables us to propose a programmable 3D logic array as a new route to design printed flexible digital circuitry essential for the emerging applications. Second, we use our robust printing process for TFTs to design and fabricate wearable active-matrix sensor arrays. An active-matrix TFT array printed on ultra-thin polymer film is Integrated with a microdome-structured pressure sensor film through via holes. We demonstrate that the pressure sensor array enables monitoring and mapping of heart rate on a wrist with high-accuracy and low-power consumption.

Resume : The diffusion of portable and wearable technologies results in a growing interest in electronic devices having features such as flexibility, lightness-in-weight and wireless operation. Organic electronics was proposed decades ago as a potential candidate to fulfill such need, in particular targeting pervasive Radio-Frequency (RF) applications. Still, limitations in terms of device performances at RF, particularly severe when large-area and scalable fabrication techniques are employed. In this work we demonstrated that rectification of an electromagnetic wave at 13.56 MHz thanks to a fully inkjet printed polymer diode is possible. The rectifier, a key enabling component of future pervasive wireless systems, is fabricated through scalable large-area methods on plastic. Thanks to fine tuning of the barriers at the semiconductor-metal interfaces, the diode has a rectification ratio higher than 10^6: at the best of our knowledge this is the first time that a fully inkjet printed diode with these performances has been obtained. When integrated in an organic rectifier, the excellent dynamic properties of the diode allow to rectify sinusoidal signals above 20 MHz, well covering the near-field communication range. As demonstrated in this work, our fully printed polymer diode opens the possibility to couple power to a plastic foil, or in future other non-rigid substrates such as paper, by rectifying a 13.56 MHz AC electromagnetic wave, as requested in near-field RFID applications.

Resume : The turn of the 21st century heralded in the semiconductor age alongside the Anthropocene epoch, characterized by increasing human impacts on the environment. The environmental impact of semiconductor chip manufacturing is one of the highest within the electronics industry, currently relying upon large amounts of toxic chemicals. The development and production of electronic goods are continually growing; thus, it is imperative that the environmental sustainability concerns are addressed, and accurate information is available concerning the life cycle of these products. Current work in polymer deposition methodologies such as Area selective Deposition or Directed self-assembly of Block copolymers on silicon wafers offer a new route to self-alignment and patterning of materials on substrates to facilitate metal ion inclusion. It is believed that this technique promises greater efficiency, is economically favorable, and reduces toxic waste when compared with standard subtractive lithography processes in the semiconductor industry. A life cycle assessment evaluating the environmental impact of new polymer lithography techniques versus conventional lithography is required to prove that polymer deposition strategies can replace conventional deposition techniques in industry as a more environmentally friendly option. At present there is no published life cycle assessment (LCA) of this Nature. This research addresses the gap in the literature and outlines a novel framework for evaluating LCA of polymer deposition strategies in the semiconductor industry. Using an LCA framework to evaluate environmental sustainability will form a vital tool in allowing researchers to direct future green chemistry transitions in the semiconductor production. This is of particular importance at the research and design phase where such transitions can mitigate severe ecological impacts, reduce waste generation, energy costs and the hazardous impacts of novel materials such as nanoparticles.

Resume : In this presentation we report the development of novel semiconductors for mechanically flexible and more ductile transistors and circuits. Particularly, we show that ?ultra-soft? polymers comprising naphthalenediimides (NDI) units co-polymerized with ?rigid? and ?flexible? organic units can change how charge transport is affected by mechanical stress, demonstrating that polymer backbone composition is more important that film degree of texturing. This strategy enables to reduce the elastic modulus of the semiconducting film by >2-4x while retaining charge transport characteristics. In addition, by fabricating polymer/polymer blends by shear techniques, it provides a new avenue to enhance charge transport and achieve excellent mechanical robustness, which is further increased by modification of the film morphology. Thus, we demonstrate that these materials can enable TFT-based circuits for ultra-flexible displays and sensors on plastics. Finally, new polycrystalline molecular semiconductor films are plasticized by using an innovative polymer additive design strategy enabling localization at the fragile grain boundaries. This approach greatly enhances film bendability and charge transport stability upon multiple bending (> 1000 x vs < 10 for the pristine molecular film).

Resume : Even though significant breakthroughs with over 18% power conversion efficiencies (PCEs) in polymer:NFA bulk heterojunction organic solar cell (OSC) devices have been achieved, a comprehensive understanding of the underlying factors governing these devices is needed to further improve the PCEs. However, it can be rather challenging to delineate device photophysics in polymer:NFA blends comprehensively, and even more complex to trace the origins of the differences in device photophysics to the subtle differences in energetics and morphology. In this work, varying the molecular weight fractions of the donor polymer was used as a tool to exert control over the interfacial and bulk morphology in high performing PM6:Y6 OSCs. A drop in PCEs from ~15% to ~5% was observed when the concentration of low molecular weight fractions (LMWFs) of the PM6 polymer was increased from 1% to 52%. The drop in PCEs is found to be due to the lowering of the short-circuit current density (JSC) and fill-factor (FF) values as a result of compromised charge generation efficiencies, increased bulk trap densities, reduced charge transport, and inefficient charge extraction. The origins of the high device performance in the 1% LMWF blend is rationalized by the favorable bulk and interfacial morphological features, resolved from four techniques at sub-nanometer to sub-micrometer length scales: solid-state NMR spectroscopy, GIWAXS, photo-conductive AFM, and resonant soft x-ray scattering (RSoXS) techniques. The closer D:A interactions, smaller D and A domains, and increased D:A interfacial area facilitate ultrafast electron and hole transfer at the D:A interface. Furthermore, the better long-range ordering and optimal phase separation of the donor:acceptor (D:A) regions lead to superior charge transport and extraction. This study provides insight into the detailed bulk and interfacial morphological features that are critical in achieving high PCEs of over 15% in polymer:NFA OSCs.