Exotic materials and innovative concepts for photovoltaics

Resume : Solar energy is the most abundant source of clean and renewable energy being photovoltaics the most promising technology for the coming decades. Conventional solar cells based on p-n junctions and other new generations are good light harvesters but are limited by the Shockley-Queisser limit as carriers are separated by the internal electric field at the p-n junction. This is not the case for ferroelectric materials, which present a spontaneous electric polarization providing an alternative way to separate excited carriers. Ferroelectric oxides, as perovskites, are stable in a wide range of mechanical, chemical and thermal conditions and can be synthesized using low-cost techniques. On the other hand, these oxides usually present band gaps located higher in energy that the visible solar spectrum. New promising perovskites based on metal oxide materials absorbing in the solar spectrum have been designed using doping techniques or applying strain. Here, we present a high-throughput combinatorial approach to systematically study the effects of doping in the structural, electronic and optical properties of ferroelectric oxides such as KNbO3. To demonstrate the applicability of our framework, the 0.5(KNbO3)0.5(BaNi0.5Nb0.5O3-x) systems is used as benchmark in this work. As oxygen vacancies, Ov, are present in this material, we start by generating all inequivalent Ov distributions. In two consecutive steps, the Ba and Ni dopants distributions are also generated, obtaining all symmetry inequivalent final structures and their degeneracy. Applying high level DFT calculations, relative stabilities for all generated structures are obtained. Combining these data with their degeneracies, we are able to statistically predict the electronic and optical properties of the material at different temperatures. We believe that this systematic approach can be extended to other systems to explore and to rationalize in an automatic fashion the potential use of these doped systems as solar cells components.

Resume : State-of-the-art (semi-)transparent photovoltaics (TPV) are mainly focused on the partial absorption of visible light and/or multiple conversion mechanisms of UV or IR spectra. By selectively absorbing UV and NIR light the Shockley–Queisser limit for the efficiency of a single junction with 100% AVT is 20.6%. Turning the focus into the UV-selective harvesting, the field is still to be developed, showing promising results in the last couple years. Herein, we report on the synthesis of all-oxide transparent devices based on ZnO/NiO, using transparent conductive oxides as electrical contacts. NiO has been synthesized by thermal and e-beam evaporation and ZnO by DC-pulsed magnetron sputtering. Additionally, post-deposition annealing treatments and O3 treatments have been explored in order to improve the optoelectronic properties. Characterization by Raman, UV-VIS, XRD, SE and SEM have been done. Devices have been fabricated with thickness in the range of 10-50 nm for NiO and 100-400 nm for ZnO. We have obtained rectifying diodes that are sensitive to UV light and act as photodetectors. Ongoing experiments are being done with a UV-enhanced solar simulator to evaluate the PV functionality of these devices. Results of the best devices will be presented as well as a discussion on the need of a specialized solar simulator for PV characterization in this topic. This work has received funding from the European Union H2020 Framework Programme under Grant Agreement no. 826002 (Tech4Win).

Resume : Our work investigates the structural, morphological, optical, and electrical properties of thin films of CuO deposited on soda-lime glass by spin-coating. The molar concentration of the precursor solutions, number of successive layers (from 1 to 3), drying time after the deposition of each layer (at 220°C in a furnace under air) and final annealing (at 550°C under oxygen for 1 hour), all appear crucial to achieve both light absorption and electrical conduction properties suitable for photovoltaic applications. The elementary compositions of the solutions are characterized by conductivity and ICP spectrometry. The films deposited from a 1 molar concentration solution, with a drying time of 15 min, features the better reproducibility of the measurements and the highest absorption coefficient (> 10^5 cm^(-1)). Structural analysis by XRD shows that all samples are polycrystalline with a monoclinic crystal structure. When we increase the number of layers, profilometer measurements show an increase of thickness about 160 nm per layer, SEM images show an increased average grain size, optical UV-Vis characterization shows a decrease in the transmission and an increase in the absorption coefficient. The optical band gap changes from 1.76 to 2.61 eV, while the carrier concentration and mobility (measured by Hall effect) vary respectively from 1.36 x 10^(15) to 5.84 x 10^(13) cm^(-3) and from 23.62 to 1.35 x 10^(3) cm^(2)/Vs.

Resume : TiO2 and SnO2 are popular metal oxide semiconductors (MOS) that are used for many applications due to their unique properties. This study reconstructs the importance of the combination of MOS as electron extraction layers for photovoltaic applications. Double metal oxide layer TiO2-SnO2 (DMOTS) can provide that, compared to their individual film, an improved optical and electrical properties. The morphological, physicochemical properties of the DMOTS was compared on the glass, FTO, ITO and SiO2/Si substrates. Surface facile photochemical treatment of DMOTS, such as UV–ozone (UVO) treatment, was applied to SnO2-TiO2 films deposited on the four substrates. Optical properties (PL and TRPL) showed to enhance the charge extraction and reduce charge recombination of DMOTS due to UV-sintering. Spectroscopic and microscopic techniques were conducted to investigate the physicochemical properties of the samples. The band structure, surface potential and carrier transport mechanism were explained by Kelvin probe force microscopy and ultraviolet photoelectron spectroscopy. Higher carrier concentration and lower resistivity of DMOTS was displayed by hall measurements when compared with TiO2. The results suggest that the DMOTS can serve as a better electron extraction layer to promote charge separation and suppress charge recombination. Therefore, these are beneficial for reducing the recombination of carriers, enhancing of perovskite solar cell performance.

Resume : The symmetry lies at the heart of the law of nature and forms the basis for modern physics. Breaking of inversion symmetry by external stimuli would enable in certain materials exotic phenomena which are otherwise are prohibited. Here, we would like to introduce a new physical concept to harvest solar energy into electricity by inversion asymmetry engineering, i.e. the flexo-photovoltaic effect. The flexo-photovoltaic effect is the generalized version of the well-known bulk photovoltaic effect induced by the strain gradient. The strain gradient, namely inhomogeneous strains, induces electric polarization in materials of all symmetry and in the same time enables manifestation of the bulk photovoltaic without any symmetry limitation. The non-equilibrium charge separation mechanism therein is distinctive from the conventional photovoltaic effect based on built-in field (such as p-n junctions). Breaking the inversion symmetry induces an asymmetric distribution of the non-equilibrium photo-generated carriers in momentum space. This flex-o-photovoltaic effect not only that provides new insights into understanding the structure-property correlations in photo-active materials, but since it can be induced in the present solar cell technologies it might provide a route to boost the energy conversion efficiency of a wide pool of established semiconductors.

Resume : Abstract: We have fabricated a heterojunction for wavelength selective photoconductivity based on n-IGZO material (bandgap 3.1 eV) [1] for UV and visible absorption and p-GeSe material (bandgap 1.1 eV) [2] for NIR photon absorption. The device exhibits photoresponse without external bias voltage under NIR light illumination (wavelength ~850 nm), UV light (wavelength ~360 nm) and visible photon (wavelength ~ 530 nm). The I-V characteristics of the device explicitly show the variation of the open-circuit voltage for dark (0 V), visible (0.28 V), UV (0.4 V) and NIR (-0.29 V). The time-dependent photoresponses of the device were carried out at zero bias under laser irradiation UV, visible and NIR, the photocurrent values corresponding to light on a state for UV and visible light were noticeably negative while for NIR light on the state were obviously positive. The rise time (τr time taken by device to reach 90% from 10%) and fall time (τf, time is taken by device to decay from 90% to 10%) [3] were examined, rise and fall time corresponding to under illumination of NIR of 0.32s and 0.34s respectively. This sensitivity changes under the wavelength of the selected light conditions, the effect is strictly depending on optical absorption properties and band gap of the material. The photogenerated by polarity flipping is useful for promising selective photodetection. References: [1]Yu, Jingjing, et al. "High-Performance Visible-Blind Ultraviolet Photodetector Based on IGZO TFT Coupled with p–n Heterojunction." ACS applied materials & interfaces 10.9 (2018): 8102-8109. [2] Liu S-C et al 2017 Investigation of physical and electronic properties of GeSe for photovoltaic applications Adv.Electron Mater.31700141 [3] Xie, C.; Yan, F. Flexible Photodetectors Based on Novel Functional Materials. Small 2017, 13, 1701822.

Resume : Transparent conducting materials (TCM) that are used as transparent electrodes in solar cells constitute a key component. The currently used TCM are indium tin oxide (ITO) or other transparent conducting oxides, TCO, such as aluminium-doped zinc oxide (AZO), or fluor-doped tin oxide (FTO). However indium scarcity and the lack of flexibility of TCO in general have prompted the search for alternative low cost and flexible TCM which can exhibit promising properties (such as transparency in the NIR) while containing no critical raw materials. The main properties concern not only the optical transparency, the sheet resistance but also the haziness for instance, or the roughness of the layers. There has been lately a growing interest for several material technologies. For instance metallic nanowire networks and metallic grids appear promising since they exhibit high optical transparency, very good electrical properties and mechanical flexibility; while they show promising integration results. Conductive polymers and carbon based TCM (graphene or carbon nanotubes), can also be potentially interesting. The association between these different TCM technologies is currently the subject of intensive research with the aim of optimizing these nanocomposites. Finally we will describe what appear to be the future most promising and innovative research directions in the field of emerging TCM for integration into solar cells, as well with the main associated future challenges.

Resume : Transparent Conducting Oxides TCOs, combine a wide bandgap with good conductivity properties, such an unusual combination can be obtained by doping (n-type or p-type) a wide band gap oxide. These material have critical role in industrial applications such as sensors, touchscreens, solar cells.. etc. p-type conductors are lagging behind in their performance because of the low hole mobility due to the presence of localized O 2p orbitals in the valence band. Breaking this localization and finding a p-type material with good conductivity as the one of n-type TCOs will make a revolution in the industry and will flip the type of materials that are used in some industrial applications such as solar cells. In a recent paper, Ba2BiTaO6 was reported as remarkable high mobility p-type TCOs though its conductivity is limited by charge compensation[1]. Here, we used first-principles computations to investigate the reasons behind such a low conductivity from a defects physics standpoint. The calculated defect formation energies confirm that K is an adequate p-type shallow extrinsic dopant but that high p-type doping is prevented by the presence of compensating defects. Our work stresses the inherent difficulty in doping Ba2BiTaO6 , we also highlight the potential directions for future improvements in its conductivity. [1] A. Bhatia et al., Chem. Mater. 28, 30-34 (2016)

Resume : Combinatorial materials science (CMS) is a promising method for fast discovery of new functional materials. CMS has been used to form thin films with composition and thickness gradients, thus, synthesizing on a single substrate a range of samples with systematically varying properties, which is a first step in finding new materials and device structures. Apart from the composition or thickness, film morphology can be especially important for an assortment of applications, especially in photovoltaics, as morphology governs the chemical reactivity, determines the surface area, and is important for charge mobility and recombination processes. However, to date CMS has mostly ignored film morphology as a study parameter. Here we describe how to vary morphology and material composition at the same time using an adapted shadow growth method based on glancing angle deposition (GLAD). In a one-step, well-controlled growth we quickly obtain a large number of nano columnar structures, including rods, helices, and zigzags with varying material compositions. GLAD also eliminates the commonly used wet chemical steps for nanostructure synthesis. Adapting GLAD for CMS, with accompanying high-throughput characterization, constitutes an integrated approach for discovering new materials and structures for a multitude of applications in many scientific fields. We use this method to fabricate a multi-component composite photovoltaic absorber and study the compositional and structural variations.

Resume : Over the past few years, organic-inorganic hybrid perovskites have emerged as a new class of solution processable semiconductor for many optoelectronic applications, such as solar cells and light emitting devices (LEDs). Their electronic, electrical and optical properties can be controlled by tuning their compositions and crystal structures. In this talk, I will discuss how to lean on the experience in interface engineering for organic solar cells and design new electron[1-3] and hole[4-5] transport conjugated materials with proper interfacial properties to improve the charge collection efficiency of perovskite solar cells. In the second part, I will also discuss how to control the dimension and nanostructure of perovskites by introducing small molecules and polymers with tailored functional groups that can strongly interact with the perovskite crystals. Using such strategy, we have developed very stable Sn-based low bandgap perovskite solar cells with much improved stability and efficiency[6] as well as highly efficient red[7] and blue[8,9] emitting perovskite LEDs. Reference: (1) S. Chen, H.-L. Yip, M. Wang, F. Huang, et al, Adv. Energy Mater., 2016, 6, 1501534. (2) L. Yan, Q. Xue, H.-L. Yip, et al, Adv. Mater. 2018, 30, 1802509 (3) J. Tian, Q. Xue, H.-L. Yip, et al, Adv. Mater. 2019, 31, 1901152 (4) Q. Xue, B. Zhang, H.-L. Yip, et al, Adv. Energy Mater., 2016, 6, 1502021. (5) Q. Xue, Y. Li, H.-L. Yip, et al, Adv. Funct. Mater., 2018, 28, 1707444 (6) Z. Chen, H.-L. Yip, et al, iScience 2018, 9, 337-346. (7) W. Cai, H.-L. Yip, et al. ACS Appl. Mater. Interfaces 2018, 10, 42564-42572. (8) Z. Chen, H.-L. Yip, et al Adv. Mater. 2017, 29, 1603157. (9) Z. Li, H.-L. Yip, et al Nat. Commun. 2019, 10, 1027

Resume : Analyzing the photoluminescence (PL) of semiconductors complementarily in time and wavelength allows to derive their key optoelectronic and transport properties. Up to now, separate acquisitions have to be performed. We developed a 4D imaging set-up that allows the acquisition of spectral and temporal luminescence intensity with micrometric spatial resolution. Very notably, this dataset is acquired simultaneously and hence with exact same experimental conditions. This novel set-up relies on single pixel imaging technology, an approach that enables the reconstruction of the spatial information recorded from a higher resolution non-imaging detector. The sample PL signal is spatially modulated with different patterns by a digital micro-mirror device1. We make use of this technique for the first time with a streak camera as a detector, allowing to record the PL intensity decays and spectrum for each pixel with very high temporal (<100ps) and spectral resolutions (< 1nm). A patent application has been filled. We already performed successful measurements and 4D numerical reconstructions and we demonstrate the use of this setup by characterizing hybrid mixed halide perovskite samples. We extract and analyze the material properties (gap) at different time scales as well as the carrier recombination (lifetime, radiative coefficient) and transport (diffusion coefficient) properties with a single experiment. 1 Duarte et al, IEEE Signal Process. Mag. 25, 83 (2008).

Resume : The spectacular rise of the photovoltaic power conversion efficiency of hybrid organic-inorganic perovskites to over 25% in just a decade has conferred them a promising position in the field of photovoltaics1. This has urged an extensive worldwide research interest in solar energy solutions based on the so-called MAPbI3 (MA = CH3NH3) material and its mixed cations/anions variants. The excellent photovoltaic performance of MAPbI3 is partly attributed to its interfacial properties with the proper choice of charge selective layers and doping/alloying. Among the possible contact materials, nickel oxide (NiO) is commonly used as a hole transporting layer in thin-film optoelectronic technologies based on organic or hybrid materials. Here, we computationally scrutinize the interfacial properties of the prototype MAPbI3/NiO heterostructure, which has shown excellent photovoltaic performance and in particular a large open-circuit voltage2,3. We study the valence band energy level alignment between MAPbI3 and NiO considering i) the defect-free system ii) the role of defects and iii) doping. We show that the defect-free ideal NiO interface with MAPbI3 is not in favor of enhanced hole collection and largely points to a valence band offset that seemingly contradicts the experimentally reported values. Our results indicate that the presence of native Ni-vacancies in deposited NiO is at the origin of near ideal valence band energy level alignment between MAPbI3 and NiO. We further demonstrate the improvement of hole transport across the interface due to NiO doping with Li. Our Subsequently, we propose alternative dopants. Our results suggest the high tunability of NiO interfaces for the design of optimized optoelectronic devices. More, the developed methodology is suitable to inspect the properties of other interfaces4 and devices far beyond halide perovskites. References 1. https://www.nrel.gov/pv/cell-efficiency.html, Accessed: 10-01-2020. 2. Traore et al., “Importance of Vacancies and Doping in Hole Transporting Nickel Oxide Interface with Halide Perovskites”, ACS Appl. Mater. Interfaces, 2019, in press 3. Nie et al., “Critical Role of Interface and Crystallinity on the Performance and Photostability of Perovskite Solar Cell on Nickel Oxide”, Adv. Mater., 30, 1703879, 2018 4. Devesa et al., “Halide Perovskite High-k Field Effect Transistors with Dynamically Reconfigurable Ambipolarity”, ACS Materials Lett., 1, 633-640, 2019

Resume : Perovskite solar cells (PSCs) with transparent electrodes can be integrated with existing solar panels in tandem configurations to increase the power conversion efficiency. A critical layer in semi-transparent PSCs is the inorganic buffer layer, which protects the PSC against damage when the transparent electrode is sputtered on top. The development of n-i-p structured semi-transparent PSCs has been hampered by the lack of suitable p-type buffer layers. In this presentation we develop a p-type CuOx buffer layer, which can be grown uniformly over the perovskite device without damaging the perovskite or organic charge transport layers, can be grown using industrial scalable techniques and has high hole mobility (4.3 ± 2 cm2 V-1 s-1), high transmittance (>95%), and a suitable valance band maximum for hole extraction (5.3 ± 0.2 eV). Semi-transparent PSCs with efficiencies up to 16.7% are achieved using the CuOx buffer layer. Our work provides a new approach to integrate the most efficient architecture for PSCs for tandems, as well as other applications requiring semi-transparent devices, such as bias-free solar fuel production, transistors with transparent electrodes, and spectroscopy on devices under operation.

Resume : Metal oxide charge carriers transporting materials have been explored in numerous ways to fabricate perovskite solar cells (PSCs) due to its excellent chemical stability, wide bandgap, high mobility as well as low-cost. In this study, we report a low-temperature solution-processed facile intercalation method to allow metal oxide to behave bipolar transporting capability for optoelectronic applications. A p-type nickel oxide (NiO) surface-intercalated with n-type cesium carbonate (Cs2CO3) was used as hole and electron transport layers applicable in both inverted (p-i-n) and planar (n-i-p) type of PSCs. As a result, the champion power conversion efficiency for the inverted and planar-based PSCs has been reached up to 12.08, and 13.98%, respectively, which indicated that this type low-temperature solution-processed metal oxide is very promising for optoelectronics applications especially used as an interconnecting layer in tandem photovoltaics.

Resume : An emerging variant of metal halide perovskites, the 2D perovskites are gaining popularity due to their excellent stability. This arises due to the presence of bulky organic spacer cations which can block moisture and offer spatial confinement to the diffusion of ions through the inorganic [PbI6]4- octahedral network. This feature comes with a price as the bulky cations result in reduced and anisotropic charge transport in solar cells made of 2D perovskites, making them less efficient than their 3D counterparts. Standard formulation developed in our lab involves Phenyl Ethyl Ammonium Iodide (PEAI) as large cation, Methyl ammonium iodide and Lead Iodide in a 2:4:5 molar ratio forming the perovskite layer (n = 5), together with the use of coordinating additives like NH4SCN (TC). This formulation gives us devices in the efficiency range 9 -11% with low hysteresis. However, short circuit current density (Jsc) in the range of 11 – 12 mA/cm2 and Fill factor (FF) in the range 65% – 70% strongly indicate problems related to charge carrier transport across the solar cell. To achieve seamless carrier transport in 2D perovskites a perfect vertical orientation of the [PbI6]4- octahedral cages with respect to the electrodes is desired. We engineered the additive, by replacing TC with Thiosemicarbazide (TSC), due to its high coordination strength to lead, coming from the S-donor (which acts as a strong Lewis base). We replaced PEAI with 4F-PEAI (fluorine atom substituted at the para position of the phenyl ring), due to enhanced orbital interactions between inorganic layers. Both replacements are expected to help in templating preferred orientation of the perovskite sheets. The resulting solar cells outperformed our standard devices with 25% higher gains in efficiency. The Jsc is in the range 14.8 – 16.4 mA/cm2, to go with a FF of 73% -78%, a significant leap from our standard devices. Transient Absorption Spectroscopy (TAS) revealed a coexistence of 2D and 3D perovskite phases with a higher contribution of 2D phases (n = 2,3,4) close to the substrate and a higher contribution of the 3D phase (n = ꝏ) close to the surface. The non-encapsulated solar cells show excellent storage efficiencies for several months and also very stable performance under accelerated test conditions for 400 hours. These results can be attributed to the specific combination of both the large cation and the additive.

Resume : Bismuth-based compound have recently gained increasing attention as air-stable, defect tolerant and non-toxic alternatives to lead-halide perovskites. We are interested in highly stable Bi-based inorganic compounds. Bismuth oxyiodide (BiOI) with an electronic structure replicating that of lead halide perovskites, has emerged as a promising contender. It is predicted to show tolerance towards anti-site and vacancy defects. Notably, BiOI thin films have been found to be at least two orders of magnitude more air-stable than methylammonium lead iodide. We experimentally investigate the tolerance of BiOI to Bi, I and O vacancy and anti-site defects by vacuum annealing BiOI films at low temperature 100°C under vacuum. Despite large defect concentrations, the electronic and optoelectronic properties of BiOI remain unaltered which experimentally validates the calculations and proves the defect tolerance of the phase and robustness of the compound. Furthermore, through single crystal growth experiments, we show the relation between carrier lifetimes and the nature of the defects.

Resume : Bismuth oxyiodide (BiOI) has emerged as a promising contender amongst non-toxic alternatives to lead-halide perovskites. This is due to BiOI demonstrating air-stability over 3 months and compelling early-stage photovoltaic performance. Notably, BiOI is predicted from calculations to tolerate vacancy and anti-site defects. In this work, we experimentally investigate the tolerance of BiOI to point defects. BiOI thin films are annealed at a low temperature of 100 °C under vacuum. We observe a relative reduction in the surface atomic fraction of iodine by over 40%, reduction in the surface bismuth fraction by over 5%, and an increase in the surface oxygen fraction by over 45%. Remarkably, the Bi 4f7/2 core level position, Fermi level position and valence band density of states of BiOI are not significantly changed, nor are the charge-carrier lifetimes and photovoltaic performance. Our data demonstrates BiOI to be electronically and optoelectronically robust to percent-level changes in surface composition. However, from photo-induced current transient spectroscopy measurements, we find that the as-grown BiOI films have deep traps located approximately 0.3 to 0.6 eV from the band-edge. These are three-four orders of magnitude lower in concentration than the defects we induce through vacuum annealing and have a different origin. Future improvements in the performance of BiOI photovoltaics will need to focus on identifying how these arise. Nevertheless, our work shows BiOI to be robust against processing conditions that lead to percent-level iodine-, bismuth- and oxygen-related surface defects which is crucial for reductions in the cost of fabricating BiOI-based electronic devices.

Resume : Zinc tin nitride (ZnSnN2) is a new semiconductor material with earth-abundant elements and low-cost production. It has a tunable direct band gap (1.0–2.2 eV) that may be due to cation disorder [1]. The band gap also can be tuned by changing the atomic ratio between Zn and Mg [2]. It has recently attracted considerable interest for solar energy applications and high speed electronics to replace the III-V materials historically used in optoelectronics. However, few works have been devoted to MgSnN2 [2,3]. This work presents the results of growth of ZnxMg1-xSnN2 thin films by reactive co-sputtering using zinc, magnesium and tin metallic targets. The stoichiometry of the films was controlled by optimizing operating parameters such as the target current and power. By changing the stoichiometry the morphology and the texture of the films can be tuned. We grew ZnSnN2 thin films with different stoichiometries. The structure of the films was studied by X-ray diffraction and the morphology by scanning electron microscopy. The microstructure was observed by transmission electron microscopy. More detailed information about the chemical environment of tin atoms has been obtained using 119Sn conversion electron Mössbauer spectroscopy. The optical band gap has been deduced from UV-Visible spectroscopy and ellipsometry measurements. The electrical resistivity and free-electron concentration was studied by Hall-effect measurement at room temperature. References [1] A. N. Fioretti, et al. « Effects of low temperature annealing on the transport properties of zinc tin nitride », in 2015 IEEE 42nd Photovoltaic Specialist Conference (PVSC), 2015, p. 1‑5, doi: 10.1109/PVSC.2015.7355694. [2] R. A. Makin et al., « Alloy-Free Band Gap Tuning across the Visible Spectrum », Phys. Rev. Lett., vol. 122, no 25, p. 256403, juin 2019, doi: 10.1103/PhysRevLett.122.256403. [3] F. Kawamura, et al., « Synthesis of a Novel Rocksalt-Type Ternary Nitride Semiconductor MgSnN2 Using the Metathesis Reaction Under High Pressure », Eur. J. Inorg. Chem., vol. n/a, no n/a, doi: 10.1002/ejic.201901059.

Resume : The rapid development of the photovoltaic and optoelectronic industries and rising demand for cheap and efficient products are straining the supply of rare elements often required for the fabrication of the most cost-effective devices. Zinc phosphide (Zn3P2) solar cells are promising earth-abundant alternatives and could lead to widespread deployment of sustainable thin-film solar technology. The 1.5 eV direct bandgap of this semiconductor, close to optimal for photovoltaics, makes it a strong contender for this application. The micrometer-range minority carrier diffusion lengths [1] and the strong visible light absorption in the phosphide material further justify its investigation. Despite knowing the direct bandgap energy of our material with good accuracy, there remains a lot of controversy regarding the details of its band and defect structure. An indirect bandgap smaller than the direct bandgap is sometimes measured [1], but not predicted by ab initio calculations [2]. In this work, we shed light on the optical and electronic properties of zinc phosphide mono- and polycrystalline thin films grown by molecular beam epitaxy on indium phosphide. Using steady-state photoluminescence (PL), time-resolved PL and cathodoluminescence (CL), we have identified the different emissions and studied the effects of sample size and crystallinity. Additionally, we determined the optical properties of monocrystalline thin films using ellipsometry, with a robust first model for the material. Finally, we used CL to measure the diffusion length of the carriers in thin films. [1] G. M. Kimball, et al., Appl. Phys. Lett., 95(11), 112103 (2009) [2] J. Andrzejewski and J. Misiewicz, Phys. Status Solidi B, 227(2), 515-540 (2001)

Resume : Silicon-based solar cells are of the greatest interest for terrestrial photovoltaic because Si is relatively low cost. However, Si solar cells with efficiency exceeding 26% approached closely to the theoretical limit of single junction solar cells leaving almost no possibility for further improvement. The most efficient way to overcome this theoretical limit related to thermalization losses is to use multijunction solar cells. According to theoretical estimations a combination of top subcell with a band gap of 1.6–1.7 eV and Si (1.12 eV) bottom subcell could give an efficiency of more than 35% for dual junction solar cells. Low-dimensional structures provide possibility to achieve desired band gap by varying the width of the wells in superlattice-type structures. Thus GaP/Si superlattice could be efficiently used as an active material for top subcell of Si based photovoltaic devices. Previously n-doped GaP was obtained by PE-ALD at the temperature of 380°C. In this work using PE-ALD p-type GaP layer was obtained by introducing Zn dopant during the growth. For the first time plasma deposited p-i-n structures with GaP n- and p-type doped layers and an undoped region based on GaP/Si superlattices on a Si substrate showed a substantial internal quantum efficiency in the spectral range of 300-750 nm and almost complete absence of absorption at longer wavelengths, which indicates their promise for use in solar cells as the top-junction.

Resume : Van der Waals heterojunctions based on atomically thin two-dimensional (2D) materials have attracted numerous attention for their special scientific research value and promising applications as a rectifier and photodiodes. In this research, photovoltaic properties of p-GeSe/n-MoTe2 van der waals heterojunctions were studied. It has been reported that GeSe has high photoresponsivity along the a3 (perpendicular to the plane) direction. The device was fabricated by mechanical exfoliation of GeSe and MoTe2. In order to reduce the schottky effect, Pd/Au contacts were fabricated with GeSe and Cr/Au contacts were fabricated with MoTe2. Pd/Au and Cr/Au contacts results in the ohmic contact with GeSe and Mote2 respectively. The devices were then subjected to NIR light illumination at a wavelength of 850 nm. GeSe/MoTe2 vdW heterojunction p-n diode showed excellent response to NIR laser. The diode exhibits excellent response at Vds = 0V and Vbg = 0V (self-powered). The rise and decay time are important factors in photodiode characterization. The rise and decay time of GeSe/MoTe2 p-n diode are 54ms and 55ms respectively. Excellent photoresponse, rise and decay time will assist in the development of efficient and high-performance photodetectors.

Resume : Two-dimensional (2D) layered perovskites are highly attractive materials for fundamental research and practical applications due to their natural inherent quantum well[1], improved stability [2], and the possibility of mechanical modulation of the photoluminescence[3] and exfoliation of single flakes[4]. The vibrational modes are of fundamental importance for their optoelectronic properties, as optical relaxation channels and heat dispersion depend on the vibrational mode spectrum and on the electron-phonon coupling.[5] We present an ultralow frequency Raman study of the vibrational modes of 2D layered perovskite materials in unprecedented detail. Our measurements on single flakes with controlled orientation of the linear polarization direction of the incident light to the 2D perovskite lattice allow to distinguish the geometrical properties of the lattice vibrations. Here the choice of the organic moieties can be used to tailor the in-plane anisotropy of the vibrational modes and their inter-layer coupling. Such insight provides a new toolbox for tailoring the optoelectronic properties of 2D layered perovskite structures and will boost their impact in optical and optomechanic applications. References: 1. L. Dou et al., Science 2015, 349, 1518. 2. H. Tsai et al., Nature 2016, 536, 312. 3. A. Castelli et al., Adv. Mater. 2018, 1805608. 4. B. Dhanabalan et al., Nanoscale 2019, 11, 8334. 5. X. Gong et al., Nat. Mater. 2018, 17, 550; F. Thouin et al., Nat.Mater. 2019, 18, 349.

Resume : We recently reported photovoltaic properties of Cu2O Schottky-type heterojunction solar cells fabricated using Al-doped ZnO (AZO) transparent conducting thin films by a low-damage magnetron sputtering (MSD) method. These properties were higher than those of the pulsed laser deposition (PLD) method.1) In this paper, we describe photovoltaic properties of AZO/n-type oxide semiconductor thin film/Cu2O heterojunction solar cells using various multi-component n-type oxide semiconductor thin film layers, e.g., Zn-Ge-O, Zn-Ga-O, formed by a low-damage MSD method. The obtained photovoltaic properties of Cu2O heterojunction solar cells strongly depend on the sputter deposition conditions of the n-type oxide semiconductor layer. For example, in AZO/n-Zn-Ge-O thin film/p-Cu2O sheet heterojunction solar cells, an AZO thin film is laminated at DC 75 W on a Zn-Ge-O thin film, which is a multicomponent oxide semiconductor formed by the above MSD in a pure Ar+H2 gas atmosphere. The highest conversion efficiency of 3.74% was achieved. Because Cu2O sheets were vertically arranged using the rf power imposed dc-MSD method and hydrogen gas was used to form a Zn-Ge-O multi-component n-type oxide semiconductor thin film, a high-quality Zn-Ge-O multi-component n-type oxide semiconductor thin film/Cu2O interface could be produced with less damage due to excessive oxidation of the Cu2O surface. In conclusion, AZO/n-Zn-Ge-O thin film /p-Cu2O sheet heterojunction solar cells fabricated by vertically arranging Cu2O sheets using the rf+dc-MSD method and introducing hydrogen gas achieved a conversion efficiency drastically improved over that of AZO thin film/p-Cu2O solar cells. [1] T. Miyata et al., Journal of Semiconductors. 40, 032701 (2019).

Resume : Visible-light-transparent solar cells are innovative devices that are used in various applications. Our group has produced visible-light-transparent solar cells that combine p-type nickel oxide (NiO) and n-type zinc oxide (ZnO), which are transparent oxide semiconductors (TOSs) and have been proposed as visible-light-transparent devices in the Internet of Things. In addition to the high transparency feature, TOSs are materials with high environmental resistance because of their large bandgap energy. In order to investigate the environmental resistance of visible-light-transparent solar cells, degradation properties of the devices in the harsh environment (such as radiation, thermal cycles, and ultraviolet ray’s) have been evaluated. In this study, the degradation phenomena of NiO-related visible-light-transparent solar cells was investigated. NiO-related visible-light-transparent solar cells were fabricated using reactive radio frequency magnetron sputtering because they are most suitable for economically depositing large-area films. Various resistance tests were carried out. The results reveal that NiO-related visible-light-transparent solar cells exhibit high resistance properties to proton irradiation compared to conventional space solar cells by approximately 10000 times. This result demonstrates the high environmental resistance of NiO-related visible-light-transparent solar cells and the possibility of their practical use as novel space- and ground-use solar cells.

Resume : Nickel-Oxide (NiO) is an exotic material for visible-light-transparent photovoltaic applications that exhibits only p-type conductivity with a wide bandgap (4.0 eV). In addition, the carrier density of NiO is easily controlled by doping impurities like Li or by introducing intrinsic defects like Ni vacancies and/or interstitial O. Therefore, NiO has been used as both an insulator and a semiconductor with a carrier density in the order of 10^13 - 10^21 cm^-3. However, its defect properties have not been fully evaluated. In addition, very high-quality crystal films are needed for evaluating the intrinsic defect properties. Therefore, thin films of epitaxial NiO are expected to be used for defect properties evaluation. Recently, NiO films have been deposited using different methods, with sputtering being the most suitable for economically depositing large surface films with well-controlled compositions. However, the epitaxial growth mechanism of sputter-grown NiO has not been clarified because of the few existing studies. In this study, the crystal structure of thin epitaxial NiO films grown by RF reactive magnetron sputtering is investigated. Epitaxial NiO thin films were grown by RF reactive magnetron sputtering on (0001) Al2O3 and (100) MgO substrates under various substrate temperatures, RF power, and O2 partial pressure. The results show that on the sapphire substrate, the epitaxial thin NiO films have a different crystal structure between the interface and the surface.

Resume : Over the last two decades, cadmium sulphide CdS synthesited by chemical bath deposition (CBD) was the most used buffer layer for thin film solar cells. However, the replacement of CdS in solar cells by other possible buffer layers is of high interest for several reasons. Indeed, CdS contains the toxic metal Cd which represents a threat to the human health and the environment and has a relatively low band gap of 2.4 eV which limits the performance and the efficiency of the cells. Zn(O,S) and (Zn,Sn)O oxide semiconductors are among the most promising Cd-free materials for replacing the CdS as earth abundant and non-toxic buffer layer in solar cells. In this work, we present experimental studies of both materials. (Zn,Sn)O and Zn(S,O) thin films were prepared by two steps method: The deposition of metalic precursors by thermal evaporation followed by an annealing in air or sulfur atmospheres. Using different characterization techniques such as UV-Vis-NIR spectroscopy, X-ray diffraction, atomic force microscopy, we demonstrate that both materials exhibit structural, morphological and optical properties that are suitable for photovoltaic applications.

Resume : Transparent conducting oxide (TCO) films play an essential role for various optoelectronic devices. Unfortunately, it is very difficult, if not impossible, to obtain high performance TCO films with both of high optical transparency and high electrical conductivity. In this work, properties of ZnO-based multilayer films, with high conductivity, high transmission, and large energy bandgaps, and their device performances have been investigated. It was observed that the GaZnO/InCdO/GaZnO multilayer films showed excellent properties of ~90% average transmittance over visible/IR range and 7.5×10-5 Ω·cm resistivity. Efficiency of the CuInGaSe solar cells using this multilayer structure as the front electrode shows much higher efficiency (8.5%) than that (4.7%) of the devices with GaZnO electrodes. The ZnMgBeO/Cu/Ag/ZnMgBeO multilayer structure also showed excellent properties, with ~ 90% average transmission, 1.7×10-4 Ω·cm resistivity, and 6.0 eV energy bandgap. Results of the solar cell fabrication, however, indicated somewhat deteriorated performance, probably due to the high “vertical” resistivity. Further discussions on the detailed observations, as well as results on the oxide films with larger energy bandgaps, will be discussed during the presentation.

Resume : Organo-lead halide perovskite materials have been considered for the next generation solar cells thanks to superior absorption properties in the visible light range, which were originated from their inherent bandgap of about 1.55 eV. However, this point serves as the biggest limitation for improving the power conversion efficiency (PCE) of perovskite solar cells (PSCs), meaning that the PSCs rarely utilize the non-visible sunlight. To overcome this limitation, many researchers have incorporated the up-conversion nanoparticles (UCNPs) into PSCs. However, because the UCNPs were generally contained and dispersed in solutions (de-ionized water; D.W. etc.), there was a limitation to transfer the UCNPs into PSCs due to its vulnerable nature to H2O. Moreover, a low quantum yield of up-conversion efficiency (UCE) of UCNPs (~1 %) was an obstacle to obtain dramatically enhanced PCE. In this work, we applied a simple dry transfer method to transfer the UCNPs within PSCs. Since no solvent such as D.W. was needed during the transfer of UCNPs within PSCs, UCNPs were transferred without any damage to perovskite film. After that, gold (Au) top electrode was deposited on UCNPs/HTL. Herein, the Au was acting as not only top electrodes, but also the plasmonic substances. Surface plasmonic resonance (SPR) was induced by using Au top electrode, which led to an increase in the UCE of UCNPs. Therefore, an average PCE of 15.56 % was obtained in that case, which was enhanced by 8.4 % compared to reference cell. This improvement was attributed to NIR light utilization due to the plasmon-enhanced UCNPs originating from the Au top electrode.

Resume : Luminescent solar concentrators (LSC) photovoltaic concept attracts lately a lot of attention [1]. In view of recently developed cost competitive low-temperature sol-gel fused quartz synthesis with simultaneous quantum dots (QD) luminophore doping, we first critically revisited available experimental results for main types of LSC waveguide matrices. To maximize the photoconversion efficiency, we carried out their parameters comparison to formulate the basic requirements for transparent matrix materials, including, e.g., high transparency for sunlight (not less than 98%), particularly in the spectral range of the luminophores’ emission, and sufficiently high refractive index (above 1.5). We established spectral range for these materials that minimize light absorption. To ensure LSC stable characteristics for 20-30 years, chemical and temperature stability, mechanical hardness are also required. Even though PMMA matrix currently dominates in LSCs (due to well-established luminophore doping and luminescence quantum yield close to unity), fused quartz wide 0.25-2.5 μm transparency range is advantageous as compared to only 0.4-0.85 μm range of PMMA. Considering higher mechanical and chemical stability, and lesser toxicity of fused quartz, we simulated its LSC matrix, including role of its refractive index, the critical angle, luminescent photons losses due to escape-cone and other important parameters. [1] R. Mazzaro, et al. Advanced Energy Materials 8, 1801903 (2018)

Resume : Cu2O is a potential absorber material for solar photovoltaic and water-splitting due to its direct band gap of 2.0 eV, high absorption coefficient (10^4 cm^-1), and preferable conduction band alignment. The inevitable formation of Cu-vacancy (V_Cu) and its complex intrinsically make it a p-type semiconductor. However, these defects also act as trap centres which impede charge carrier transport by affecting hole mobility. Maximum efficiency (8.1 %) for Cu2O solar cells was based on thermally oxidized Cu2O, which shows the least acceptor defect density (10^14 cm^-3). To further decrease the acceptor defect density and enhance charge transport, we propose passivation of V_Cu by Cu-doping. The resultant lower free carrier density is not of much concern here as enough carriers get generated upon illumination. Cu2O, prepared by Cu-foil oxidation at 1000 ⁰C in air is studied using temperature dependent Hall measurement. Cu is doped at different temperatures ranging from 300 - 800 ⁰C in Ar ambient (1 to 10^-5 bar). It is observed that passivation of V_Cu by Cu-atom is limited by the low formation energy of Cu-interstitial (Cu_i) at process temperatures. Also, increased concentration of Cu at interstitial favours the formation of another defect complex, which is observed experimentally for the first time. Formation of this complex along with Cu_i further tends to decrease the hole mobility from 80 to 30 cm^2/V-s. This suggests that doping possibility of cations needs to be further studied.

Resume : Copper oxides are a p-type semiconductor materials with a band gap energy in the range from 1.2 – 2.1 eV and great potential for use in thin-film solar cells. Common methods of CuO copper oxides manufacturing are magnetron sputtering, chemical vapor deposition or thermal oxidation. However, their main problem is providing a non-single phase layer which is usually a mixture of tenorite (CuO) and cuprite (Cu2O). Therefore, in this presentation, we propose a paste sintering method for pure CuO layer manufacturing. The CuO paste used for the experiment was a commercial CuO component in an organic medium. The paste was deposited on a substrate by using screen printing method and subsequently fired in a resistance furnace at the temperature of 850 - 1000°C in air. At temperatures of 850 and 900°C for 30 min., no crystallization of the layer occurs. What is more, the lower the firing temperature was, the worse layer adhesion to the substrate was reported. When burning at 950°C within 30 minutes, the layer is almost crystallized, although a numerous of unfilled spaces between the grains could be seen. At 1000°C for 10 minutes resulted in a lack of structure continuity and large crystallites which diameter is even 6 m. Shortening the firing time at 1000°C to 1 minute allowed to obtain a fully compact structure without any holes. X-ray analysis indicated that the layer produced is a single-phase CuO. Optical and material parameters of CuO layers were also obtained as a result of spectrophotometric analysis. The resistance measurement confirmed high resistivity of the formed oxide layers, which increases with firing temperature increasing.

Resume : Indium-phosphide (InP) is a widely studied III-V semiconductor material with adequate properties (such as a direct bandgap close to the optimum for solar energy conversion and high crystallinity) to be used as absorber material in photovoltaic cells. This work assesses the possibility of increasing the photovoltaic efficiency of InP semiconductor through hyperdoping with transition metals (TM = Ti, V, Cr, Mn). For this, electronic band and optical absorption features of TM-hyperdoped InP (TM@InP), with general formula TMxIn1-xP, have been deeply studied by using accurate ab-initio electronic structure calculations. TM replacement does not lead to an important crystal structure distortion, while the energetic of the hyperdoping process evidence the possibility of obtaining TM@InP materials. The analysis of the electronic structure shows that TM 3d-orbitales induce new states in the host semiconductor bandgap, leading to improved absorption features that cover the whole range of sunlight. The best results are obtained for Cr@InP, which is an excellent candidate as in-gap-bap (IGB) absorber materials. Thus, the sunlight absorption in considerably improved through new-sub-bandgap transitions across de IGB. Our results provide a perspective on the effects of transition metal hyperdoping into the exploitation of new semiconductor materials with improved sunlight absorption performance.

Resume : In the present work will be exposes the development of a designed device that carry out thin film growth processes, using chemical bath deposition technique (CBD). The device has four reactors for the location of different solutions, each reservoir has an agitation control to prevent precursors precipitation in the system, a temperature control and monitoring up to 60 ° C, using power resistors and a fiberglass coating as a thermal conductor, additionally its structure allows the movement and immersion of the substrate through the different reservoirs in a fully automated way. Through the graphical interface, it is possible to monitor the entire process, in addition to configuring the order and immersion time of the substrate and at the same time controls the temperature and agitation of each solution. For the validation of the device, copper sulphide (Cu2S) growth tests were performed, for this purpose a solution consisting of 25ml of copper chloride (CuCl2) 0.1M, 2.5ml of triethanolamine (TEA - C6H15NO3) and 5ml of thiourea (CH4N2S) 1M were used. Subsequently the temperature of the first container was set with the solution at 50 ° C, during 25 min (immersion time) and a moderate agitation, during the immersion the solution was dosed with ammonia (NH3) until reaching a total of 50ml. The acquired thin films were analyzed by scanning electron microscope SEM-EDS technique, where the nano-zise crystals around 100 nm of Cu2S are evident. The calculated band-gap using UV-Vis spectroscopy suggests adequate optoelectronic properties to be used nano-size Cu2S as absorber layer in thin film solar cell devices.

Resume : The multijunction solar cells promise to reduce the cost of solar photovoltaic energy production significantly through high power conversion efficiencies. Nevertheless, monolithic multijunction solar cells often have efficiency losses due to current‐mismatching tolerated in the interests of lattice matching. Therefore, new approaches should be developed to reduce these losses. One of the proposed techniques for enhancing the power conversion efficiency is to split sunlight into suitable spectral bands for a set of solar cells with appropriate energy band gaps. In this paper, metal/Oxide/metal meshing is used as a new concept to enhance the light splitting properties. Finite difference time domain (3D-FDTD) modeling is carried out to investigate the impact of engineered layers on the spectral beam splitter performances. Our investigation demonstrates that metal layer and Oxide material are responsible for light splitting effects, where their geometry and position play an important role in modulating the spectral beam splitter optical properties. Besides, an enhanced optical performance over the conventional spectral splitter is recorded by developing a new hybrid modeling approach based on 3D-FDTD supported by Genetic Algorithm (GA) optimization to identify the intermediate metallic layer and oxide films. Therefore, the proposed approach can offer a novel systematic strategy for designing high-performance spectral beam splitter coating layers for multijunction photovoltaic applications.

Resume : Transparent Conductive Oxides (TCO) in solar cells are used as transparent electrodes. They must necessarily have a high optical transmission in order to allow efficient transport of photons to the active layer and also good electrical conductivity which is required to obtain the least losses of transport of photo-generated charges. Aluminum, Magnesium and Tin doped ZnO thin films are some of these. In this contribution, the TCO materials formed of Al, Mg and Sn doped ZnO thin films (AZO, MZO, TZO) were synthesized by spin coating method. The structural, optical and electrical properties of these films are investigated as a function of dopant concentration. X-ray diffraction patterns show that all AZO, MZO and TZO films exhibit the hexagonal würtzite structure with a preferential orientation along [002] direction with no additional peak corresponding to secondary phases. The surface morphologies show that AZO and MZO films are homogeneous and dense while TZO films showed different surface morphologies going from homogeneous to porous structure depending on Sn concentration. An enhancement of optical transmittance of about 95% with a blue shift of the gap energy was found for both Al and Mg doped ZnO layers against 70% of average level transmittance and a red shift of optical window for Sn doped ZnO films. Photoluminescence (PL) measurements at room temperature shows excitonic peaks corresponding to the AZO, MZO and TZO films with a low defects level for 1% Al doped ZnO compared to MZO and TZO films that showing a large intensive band defects centred around 514 nm characterizing the substitution defects of the zinc atoms by oxygen atoms. Electrical measurements show that all the films present an Ohmic comportment with a best electrical conductivity of 2.45 S.cm-1 obtained for 1% AZO in accordance with the photoluminescence analysis. The reduced electrical conductivity shown in our films is attributed to the low mobility of charge carriers due to their diffusion by the grain boundaries.

Resume : The concept of intermediate band solar cell (IBSC) was introduced 20 years ago as an innovative way to overcome the celebrated Shockley Queisser limit by harvesting low energy photons [1]. Since then, IBSC have received much attention both from a theoretical and experimental perspective. Yet, despite these efforts, no working proof of concept could be achieved, notably due to the important voltage loss induced by recombination. It was recently suggested that a slight energy shift in the intermediate transition – an electronic ratchet – could increase the efficiency of IBSC [2]. We have shown that this feature is not a mere improvement of the IBSC concept, but is actually mandatory to outmatch the Shockley Queisser limit [3] as soon as non-idealities such as non radiative recombinations and narrow transitions were taken into account [4]. Furthermore, we offer a physical explanation of the influence of the ratchet by performing an analytical optimization [5]. Using Lagrange multipliers, we show that the ratchet allows the system to match photo-currents to- and from the intermediate band while preventing voltage degradation. By lifting a constraint set on the energy gaps, the ratchet thus allows the system to take the best of both worlds: the open circuit voltage and fill factor from a single junction, but the current increase of an IBSC. [1] Luque A., Marti A., Phys Rev Lett, 78, 26 (1997) [2] M. Yoshida et al., Appl. Phys. Lett. 100, 263902 (2012) [3] A. Delamarre, D. Suchet, N. Cavassilas et al., IEEE JPV, 8, 6 (2018) [4] N. Cavassilas, D. Suchet, A. Delamarre et al., EPJ Photovoltaics 9, 11 (2018) [5] D. Suchet, A. Delamarre, N. Cavassilas et al., Prog. in PV, 26, 10, pp. 800 (2018)

Resume : Recently, perovskite solar cells have triggered a huge scientific interest in the photovoltaic field due to the outstanding performance of lead-based hybrid perovskite-based devices. This has led the researchers to develop cesium tin halide based perovskites such as Cs2SnI6 owing to its near ideal optoelectronic properties towards the development of stable, non- toxic and high efficient solar cells. In the present work, we report fabrication of p-n junction solar cells using spray coated Cs2SnI6-nano Si hybrid films as absorber and chemically deposited CdS as window layers. The Si-nanocolloids were formed by pulsed laser ablation in using silicon target in DMF. The best device showed a Voc higher than 0.8 V, Jsc of 1.5 mA/cm2 and FF of 0.5. Detailed studies on the changes in the crystalline structure, morphology, photophysical properties of Cs2SnI6 films due to incorporation of Si nano particles were done. A comparative analysis of X-ray diffraction patterns showed that the crystal structure remained cubic (Fm3m group) and air-stable. Nevertheless, the electrical conductivity and the photoconductivity were improved by incorporating the nanoparticles. An in-depth analysis of the devices formed at different conditions will be presented. This study provides very useful insights into the development of stable inorganic lead-free perovskites films by spray deposition, and also the in situ incorporation of nanoparticles to improve their photovoltaic properties.

Resume : Transparent Conductive Materials (TCM) are of great importance for many opto-electrical devices and energy applications. Tin-doped indium oxide (ITO) is the most used TCM due to its high transparency (~ 80%) and low resistivity (10^(-2)-10^(-3) Ωcm). However, its large-scale usage faces the issue of scarcity, price and toxicity of In. Doping indium oxide with a transition metal (Mo, Zr, Hf, Ta) has been proposed as potential strategy to improve conductivity and transparency in ultra-thin TCM and, at the same time, reduce the amount of In. In this work, we show a highly conductive and transparent TCM based on Mo-doped ITO (ITMO). Thin films (~100nm) of ITMO with different concentrations of Mo dopant were deposited by RF magnetron co-sputtering from ITO and Mo targets. Post-deposition thermal annealing, up to 400°C, was performed to improve optical and electrical properties. Samples have been optically and electrically characterized using UV-VIS-NIR spectrophotometry and 4-points electrical probe. Our ITMO shows resistivity as low as 10^(-4) Ωcm and high wide range transparency of ~ 80% in the 400-2000 nm spectral range, after a thermal process at T lower than those reported for Indium Molybdenum Oxide (IMO). Rutherford-backscattering spectrometry and X-Ray photoelectron spectroscopy allowed us to clarify the role of Mo as effective dopant in ITMO films. These results will be discussed and help to shed light on new strategies to improve the properties of next generation TCM.

Resume : The development of cost-effective hole-transporting materials (HTM) that are capable of providing stable solar cells under variable or even extreme environmental conditions is of paramount importance to prospect an effective commercialization of emerging third-generation photovoltaic technologies.[1] We study here an organic-inorganic composite HTM based on a semiconducting poly(3-hexylthiophene) (P3HT) matrix used to disperse copper thiocyanate nanoplatelets. The choice of this particular materials combination comes from rational design of the HTM, based on exploiting the ease of processing a polymer as P3HT and the electron-blocking ability of a high-band gap semiconductor as CuSCN. Such composite HTM is tested in a standard architecture perovskite solar cell (PSC) maintained for one month in a water-saturated atmosphere, ensuring the maintenance of a good power conversion efficiency of more than 9%. This remarkable stability in conditions which would normally cause a fast deterioration of the perovskite layer, is interpreted as a CuSCN-mediated in-situ p-doping of the P3HT HTM, happening in the presence of water molecules permeated within the layer, that are locally converted into molecular oxygen. References [1] a) F. Lamberti, T. Gatti, E. Cescon, R. Sorrentino, A. Rizzo, E. Menna, G. Meneghesso, M. Meneghetti, A. Petrozza, L. Franco Chem 2019, 5, 1806-1817. b) T. Gatti, F. Lamberti, P. Topolovsek, M. Abdu-Aguye, R. Sorrentino, L. Perino, M. Salerno, L. Girardi, C. Marega, G. A. Rizzi, M. A. Loi, A. Petrozza, E. Menna Solar RRL, 2018, 2, 1800013.

Resume : All-oxide photovoltaics is a new and very promising technology for low cost and efficient solar cells using earth-abundant and environmentally compatible materials. The main materials developed in this technology are cuprous oxide (Cu2O) and related compounds. Minami’s group [1] used a combination of costly Pulsed Laser Deposition (PLD) and thermal oxidation techniques to elaborate these promising materials and developed a solar cell with 8.1 % of photovoltaic efficiency. This efficiency, high for a proof-of-concept, was obtained with a ZnxGe1-xO buffer layer on Cu2O absorber. The germanium composition in ZnxGe1-xO can be engineered to obtain band alignment with the absorber. The complex and costly laboratory techniques used by the Minami's group permitted to show the feasibility and pave the way toward efficient all-oxide solar cells. Such techniques can however not be used in the solar cell industry where the main key is the efficiency to cost ratio. In this work, we used the low-cost ultrasonic spray pyrolysis technique (no vacuum, low temperature, large area deposition) to elaborate the high-potential oxide materials based on these compounds which can be easily transferable to industry. These materials properties were investigated, with respect to the elaboration parameters, using morphological characterization (profilometry, AFM), Raman spectroscopy and electrical transport measurements (Hall mobility, carrier concentration). The main growth parameters conditions investigated in this work are the temperature, pressure, deposition speed and flux. In addition, the impact of these parameters was evaluated in relationship to the photovoltaic application. [1] Minami et al., Applied Physics Express 9, 052301 (2016), http://doi.org/10.7567/APEX.9.052301

Resume : Atomic Layer Deposition (ALD) method found new applications in the field of photovoltaics. First of all, ALD deposited layers are used as passivating layers for silicon-based solar cells, as anti-reflective layers, and, as we show, as transparent conductive electrodes (TCO layers). Other possible applications include use of ZnO as a buffer layer and/or as a n-type emitter in modified Si-based solar cells. The latter application we demonstrated recently [1-3]. In our work, we show solar cells based on a n-type ZnO structures (layers and nanorods). ZnO/p-type Si photovoltaic cells were constructed on silicon wafers with thicknesses of 170 μm and 50 μm, using developed by us 3D electrode based on ZnO nanorods. For 170 μm thick solar cells, we received light conversion efficiency up to 14%. For thinner silicon wafers (50 μm) the Jsc value decreases from 38 mA/cm2 to 30 mA/cm2, whereas the Voc remains similar, of about 500 mV. The solar cells efficiency was ~10%. Further modification includes passivation of the rear contact (like in standard PERC solar cells). Thin aluminum oxide (Al2O3) layer is deposited between Al-contact and back side of silicon. Using laser cutter, small dot patterns are cut in the Al2O3 before deposition of Al contact. The results for these modified cells will be presented. This work was partially supported by the National Centre for Research and Development TECHMATSTRATEG1/347431/14/NCBR/2018. 1. R. Pietruszka, et al., Solar Energy 155 (2017) 1282-1288. 2. R. Pietruszka, et al., Solar Energy Materials & Solar Cells 147, (2016) 164-170. 3. R. Pietruszka, et al., Solar Energy Materials & Solar Cells 143, (2015) 99–104.

Resume : Molybdenum oxides (MoOx) are widely used materials in electronics, sensors, electrochromic devices, photovoltaics, organic LEDs and other applications, due to their adaptable electronic and optical properties arising from the existence of various oxide stoichiometries, such as MoO3 and MoO2. In the present study, thin MoOx films were sputter-deposited from an electrically conductive ceramic target using different oxygen partial pressures in the plasma, leading to different film stoichiometries. The films’ optical, electronic and structural properties were investigated and correlated to their stability in water that is particularly important for further applications. We show that layers with higher oxygen content have increased solubility in water. More specifically, the stability of the thin films was measured in an ICP-MS set-up, allowing to monitor the dissolution of the film in water over time, as well as the totally dissolved material. While the film’s stability increases with decreasing oxygen content, the film’s transparency rapidly decreases. Amongst others, this trade-off is unfavorable for the application of MoOx as a transparent selective contact in devices such as organic and hybrid solar cells. To this end, we show that molybdenum oxide compounds alloyed with additional elements lead to a significant increase in water stability, while maintaining favorable optical properties for the implementation in such devices.

Resume : III-V semi-conductors are recognized as high-performance materials for the fabrication of (opto)electronics devices such as avalanche photodiodes, light emitting diodes and lasers. In this case, advanced contactless photoluminescence characterization is known as a powerful approach to determine carriers’ dynamics and recombination mechanisms. One classical method called Time Resolved Photoluminescence (TRPL) has been commonly used for probing fast recombination mechanisms under the nanosecond regime in thin film semi-conductors. The interpretation of the TRPL temporal decays may be difficult when presenting non mono-exponential behavior. Even a mono exponential decay is not necessarily an indication of a simple recombination mechanism, as it has been already observed during our work. In order to provide complementary information, another approach, which is called High-Frequency Modulated Photoluminescence (HF-MPL) can also be used to investigate carrier lifetime by using sinusoidal illumination variation and recording the phase shift between the excitation signal and emitted signal. We upgraded this method by extending the modulation frequency range up to 100 MHz, which is the fastest MPL setup to our knowledge and suitable for probing III-V materials. HF-MPL can provide different excitation conditions and enable to focus on slow mechanisms such as carrier trapping. Measurements have been performed on bulk InP as well as AlGaAs layers and GaAs photodiodes, showing carrier trapping features invisible to TRPL. Considering the complexity of the studied systems, we firstly focused on data interpretation for InP. Other III-V data are currently under study.

Resume : The electrical performance of the solar cell in an outdoor condition deteriorates due to various environmental factors and internal losses like parasitic absorption and carrier recombination. Among them, the only temperature can be reduced using active or passive cooling techniques. Due to the low heat transfer coefficient of EVA and Tedlar, the heat generated in the solar cell does not evacuate easily and that further raises the temperature of the solar module. Radiative cooling is one of the best approaches to cool down the solar cell temperature along with front and back surface convective heat transfer. With the modified double-layer ARC have properties to improve the current density and radiative emission in the range of 4-25µm wavelength. The current density of the solar cell has been improved by 1.9 % with respect to a single layer ARC (commercial) solar cell. The double-layer ARC has also improved the emissivity in the range of 4-25µm and that has reduced the temperature of the solar cell by 1-2 ºC. This temperature reduction also improves the performance of the solar cell. Due to the temperature reduction, the life of the commercial solar cell will improve.

Resume : Al2O3/SiNx stack passivation layers are among the most popular layers used for commercial silicon solar cells. In particular, aluminum oxide has a high negative charge, while the SiNx film is known to supply hydrogen as well as impart antireflective properties. Although there are many experimental results that show that the passivation characteristics are lowered by using the stack passivation layer, the cause of the passivation is not yet understood. In this study, we investigated the passivation characteristics of Al2O3/SiNx stack layers. To identify the hydrogenation effect, we analyzed the hydrogen migration with atom probe tomography by comparing the pre-annealing and post-annealing treatments. For chemical passivation, capacitance-voltage measurements were used to confirm the negative fixed charge density due to heat treatment. Moreover, the field-effect passivation was understood by confirming changes in the Al2O3 structure using electron energy-loss spectroscopy.

Resume : Very high conversion efficiency is reached using the concept of selective contacts in silicon based heterojunction solar cells. These use a stack consisting of a very thin layer of undoped hydrogenated amorphous silicon (a-Si:H) ensuring a good chemical passivation of the surface of the c-Si, and a layer of doped a-Si:H, n-type or p-type, for contact selectivity to extract electrons or holes, respectively. A new path, based on phosphide-type materials deposited by a low temperature plasma technique is explored in this paper. Gallium (GaP) and boron (BP) phosphides have a band gap significantly larger than c-Si and even than a-Si:H, thus limiting the absorption of the solar spectrum in these layers, which augurs well for a gain on the short-circuit current compared to the use of a-Si:H. Preliminary work has shown that GaP can be highly n-type doped, and has a significant valence band offset with c-Si, making it an excellent candidate as a selective electron contact, without requiring an additional ITO layer. Similarly, BP could be p-type doped, thus providing a selective contact for the holes. Phosphide layers with a bandgap around 2 eV were successfully grown by PE-ALD and PE-CVD processes. The material and heterojunction properties will be analyzed by combining several types of characterization techniques together with modeling studies.

Resume : When light transmits into the solar cell, reflection occurs at the interface between air and an absorptive medium lowers solar cell efficiency. In this study, we present micro-dome structured anti-reflection (AR) films fabricated via simple imprinting technique. The functionality of the film in photovoltaic devices is evaluated, revealing reduced reflection caused by gradient refractive index effect and omnidirectional antireflection behavior, which lead to enhance the photovoltaic performance under both outdoor and indoor lighting. In addition, the AR film does not require any manipulation of solar cells as using a simple strategy of attaching the film on the surface, and its hydrophobic property affords self-cleaning effect, which should be beneficial for building integrated photovoltaics. Combined with these properties, our detachable AR films provide a practical guideline to boost solar cell performance.

Resume : Aim and approach: High efficiency silicon heterojunction (SHJ) solar cells require well-performing transparent (TCO) contact materials. To be suitable as antireflection and carrier transportation layer, the material must exhibit very low absorption and sufficiently high conductivity at the same time. In this work, we aim at optimizing optoelectronic properties of direct current (DC) and radio frequency (RF) sputtered Titanium oxide doped Indium oxide (ITiO) with substrate temperatures ≤ 200 oC, and compare the difference between these two sputter methods in obtaining high quality ITiO films and the application into SHJ solar cells. Our study will be divided in two parts, one part is optimization of the sputtering process for high quality ITiO films on corning glass substrates; the other part is the application of sputtered ITiO on SHJ solar cells and investigating the influence of the sputter conditions on the cell performance. The contact resistance between ITiO layer and silicon thin films is especially studied. 100 nm-thick ITiO films are deposited on corning glass substrates and the varied process parameters are heater temperature, sputtering pressure, power density and Ar/O2 ratio. The influence of post annealing process is also taken into consideration. Transmittance and reflectance are measured by a Perkin Elmer LAMBDA 950 spectrophotometer. Carrier density and Hall mobility are characterized by Hall Effect measurements. For cell preparation, hydrogenated intrinsic amorphous silicon layers are deposited on both sides of 170 µm-thick n-type double side textured Czochralski wafers, followed by the deposition of n-type a-Si:H on the front and p-type a-Si:H layers on the rear side by plasma-enhanced chemical vapor deposition (PECVD). 70 nm-thick ITiO films are then deposited on both sides of the samples using the same sputtering conditions. In the end, Ag electrodes are screen printed on both sides of the cells. The cell aperture area is 3.6 cm2. Solar cell performances are characterized under standard test condition. Scientific innovation and relevance: ITiO is a high performance transparent conducting oxide because of the expectionally high carrier mobility > 60 cm²V-1s-1.[1] Previous studies have employed soft deposition methods for example RF sputtering, ultrasonic spray pyrolysis and high process temperatures to fabricate high mobility ITiO.[2][3][4] In the industrial fabrication of TCOs the most established deposition method is DC mode sputtering which is a hard deposition method because of the high kinetic energy of the sputtering species. So far, the influence of DC mode sputtering on the quality of ITiO films and the cell performance of SHJ solar cells has not been investigated. Our study aims at closing this gap and presents the first results on DC mode sputtered ITiO on SHJ solar cells. Additionally, we show that this material can be processed at low temperature (≤ 200 oC) and high throughput which are two beneficial factors for the industrial fabrication of high efficiency SHJ solar cells. Preliminary results and conclusions: The electrical and optical properties of RF sputtered ITiO using different deposition conditions are presented. The Hall mobility of ITiO increases with temperature and reaches its maximum value of 53.5 cm2V-1s-1 at a substrate temperature of 200 °C. The films processed at 200 °C have the lowest resistivity of 2.16 × 10-4 Ω ·cm. With increasing O2 concentration in the plasma, the carrier density in the ITiO films decrease. The absorption of the ITiO layers increases in the near infrared (NIR) range with decreasing O2 concentration and substrate temperature because the increased carrier density induces free carrier absorption. When we apply the optimized RF sputtered ITiO films on SHJ solar cells we see that higher O2 concentrations increase the current density and series resistance. This is because of the lower absorption in the NIR range and higher sheet resistance of the ITiO layers. In comparison to SHJ solar cells with Sn-doped Indium oxide (ITO), the standard TCO used in our R&D baseline at the moment, the solar cells with RF sputtered ITiO exhibit a higher fill factor of 82.7 % (81.8 % with ITO). In this study, the champion cell with RF sputtered ITiO reaches a conversion efficiency of 23.8%. The study on DC sputtered ITiO films is ongoing and solar cell results will be presented at the conference. In addition, we will compare the difference from DC and RF sputtered ITiO and discuss the observations. Acknowledgements: This work has been supported by the HEMF (Helmholtz Energy Materials Foundry) infrastructure funded by the Helmholtz Association (HGF) and Street project (No. DB001618). The author Zhirong Yao thanks for the financial support from China Scholarship Council (CSC, 201806380137). Reference [1] R. Hashimoto, Y. Abe, and T. Nakada, “High mobility titanium-doped In2O3 thin films prepared by sputtering/post-annealing technique,” Appl. Phys. Express, vol. 1, no. 1, p. 015002, 2008, doi: 10.1143/APEX.1.015002. [2] L. T. Yan, J. K. Rath, and R. E. I. Schropp, “Electrical properties of vacuum-annealed titanium-doped indium oxide films,” Appl. Surf. Sci., vol. 257, no. 22, pp. 9461–9465, 2011, doi: 10.1016/j.apsusc.2011.06.035. [3] L. T. Yan and R. E. I. Schropp, “Changes in the structural and electrical properties of vacuum post-annealed tungsten- and titanium-doped indium oxide films deposited by radio frequency magnetron sputtering,” Thin Solid Films, vol. 520, no. 6, pp. 2096–2101, 2012, doi: 10.1016/j.tsf.2011.08.060. [4] S. Panyata, S. Eitssayeam, G. Rujijanagul, T. Tunkasiri, S. Sirisoonthorn, and K. Pengpat, “Preparation of titanium-doped indium oxide films by ultrasonic spray pyrolysis method,” Ferroelectrics, vol. 454, no. 1, pp. 63–69, 2013, doi: 10.1080/00150193.2013.842791.

Resume : The approach to adapt upconversion nanoparticles to plasmonic structures has been studied to improve their inherent low quantum efficiency. However, the metal-based plasmonic structures have limits in extending their applications since they are rigid and opaque. In this study, we demonstrate a plasmonic film composed of an array of microbeads, which provides transferability and flexibility as well as distinctive upconversion luminescence enhancement. Our proposed structure comprises plasmonic silver nanoparticle-decorated silica microbeads on the dielectric-metal substrate, thereby providing remarkable improvements in absorption efficiency of near-infrared (NIR) light, and emission of visible (Vis) light via resonance mode excitation. As a result, the observed upconversion luminescence intensity exhibited three-order enhancement compared to reference under low power excitation. In addition, transparency and wide viewing angle property of emitted light for our film further extend its applicability for various fields such as absorption enhancement of photovotaics through visible light conversion in NIR and attachable NIR detection film.

Resume : Single walled carbon nanotubes (SWCNT) are characterized by high conductivity and transparency. In addition, SWCNT have a wide direct band gap, significant photoabsorption and high mobility of charge carriers contributing to their efficient transport. All of this makes SWCNT an ideal photovoltaic material compatible with GaAs planar semiconductor structures. So SWCNT can act as a good alternative to metal contacts in a GaAs-based solar cell (SC). The use of SWCNT can reduce ohmic losses and shading of the active region in the SC. This work is devoted to fabrication and characterization of a GaAs solar cell with a SWCNT top contact. The characteristics of a conventional solar cell with a metal contact grid and solar cell with SWCNT electrode were compared to demonstrate the benefits of a SWCNT based approach. The photoelectric properties of the solar were are studied by measuring the IV characteristics and external quantum efficiency. The contact between the SWCNT and SC was examined by means of the optical and electron beam induced current techniques. SC with SWCNT contact has shown slight increase in the short circuit current density from 16.9 to 17.9 mA/cm2 and the EQE leading to 0.9% efficiency rise compared to SC with the conventional metal grid contact. Demonstrated technology of SWCNT electrodes fabrication can be applied to a wide range of semiconductor photovoltaic devices, including flexible thin-film solar cells and solar cells based on nanowires.

Resume : Thanks to the abundancy of zinc and phosphorous, zinc phosphide has great potential for large-scale thin film photovoltaics. In particular, it exhibits high carrier mobility, diffusion length, absorption coefficient, and furthermore, its direct bandgap of 1.5 eV is close to optimal value of the Shockley-Queisser limit. Separate source Molecular Beam Epitaxy (MBE) is an ideal platform for optimizing the growth of thin films as it provides precise control over the molecular/atomic fluxes and substrate temperature. Here, we present the growth of zinc phosphide thin films on InP(100) substrates. While there had been previous attempts to grow this material using compound and separate source MBE, none of them had been carried out of this substrate. We provide an overview of the thin film morphology and crystalline quality as a function of the zinc and phosphorous fluxes as well as V/II ratio. The temperature window resulting in high crystalline quality is assessed by X-ray diffraction, transmission electron microscopy (TEM) and Raman spectroscopy. The figure below provides an overview of a typical Raman spectrum of a high quality film. The observed peaks are analysed in terms of crystalline quality and crystalline texture. The functionality of the materials is further evaluated by photoluminescence (PL). This study provides information about the growth of zinc phosphide thin films that could be valuable for implementation of this material for photovoltaics applications.

Resume : The search for new semiconductor materials and the improvement of the existing materials represent a major focus for innovative photovoltaic applications and device integration. Doping is the usual approach for modifying and widening physical and chemical semiconductor properties, while multinary alloys allow for the simultaneous adjustment of both the band gap and lattice constant, allowing for an increase in a radiant efficiency at a broader range of wavelengths. In this study, a novel approach of deposition by close-spaced sublimation of thin-films of the Sb2Se3-SnS chalcogenide alloy is investigated. Sb2Se3-SnS thin films are obtained by in-situ addition of SnS into Sb2Se3 with concentration varying from 0.1 to 5 at%. SnS introduced p-type doping in Sb2Se3, decreases the resistivity, increases the dark-to-light resistance ration and enhances the carrier density. No evident changes in structure and morphology compared to Sb2Se3 films are observed. Sb2Se3-SnS films shows basic orthorhombic structure but with little variations in the lattice parameter and band gap are observed for high extent Sb2Se3-SnS alloying. Thin-film solar cells with Sb2Se3-SnS absorber are fabricated. Admittance spectroscopy and low-temperature photoluminescence measurements are used for defect studies and insights into the physico-chemistry of the material and devices are provided.

Resume : This article presents the implementation of an automatic spray equipment for the creation of multilayer materials, which allow to growth materials for manufacturing Grätzel solar cells devices; uses of spray coating open atmosphere techniques causes different changes in physical and chemical thin film technology (TFT) properties mainly in morphology, mechanical, compositional, magnetic and optical ones, improving at the same time nanoparticle adhesion phenomena. Through the Conceive-Design-Implement-Operate or CDIO method, a system was designed to control spray type, movement, cycles, surrounding conditions and temperature sensing, and this device was developed to apply the most efficient coating technique when performing production TFT processes. Due to the development of this device were obtained significant results in the fabrication of solar cells using graphene-like material, TiO2, molecules for sensitizing semiconducting materials on substrate slides glass with accuracy thickness in the order of nm. Above materials were used to performed photovoltaic Grätzel cell devices and was found a maximum electrical potential between 120 mV and 520 mV, in a 7.0 cm2 on standard illumination conditions, these results depend of fabrication process and molecules used on.

Resume : Intermediate band solar absorber materials based on well-known chalcopyrite type semiconductors can be a low cost and efficient alternative in solar energy production [1]. A comprehensive analysis of the structural incorporation of manganese in the wide band gap CuGaS2 chalcopyrite type semiconductor is demonstrated by means of X-ray diffraction, neutron diffraction and photoluminescence spectroscopy. In this systematic study powder samples were prepared by solid state reaction of pure elements, following the pseudo-binary section of CuGa1-xMnxS2 and (CuGaS2)1-x(2MnS)x, respectively. The alloyed ternary phase consequently crystallizes in the chalcopyrite type structure with a maximum manganese content of 0.091 mol-%. It is shown that the actual incorporation of manganese follows rather the pseudo-binary section of a coupled substitution ((CuGaS2)1-x(2MnS)x) than of a unilateral substitution (CuGa1-xMnXS2) independent of their initial composition [2]. The increasing manganese content results in a continuous increase of the lattice parameters a and c, as respective marker for the incorporation into the crystal structure. From analysis of the average neutron scattering length of the cation sites a manganese incorporation is observed for both cation sites of the chalcopyrite host phase. The occurring extrinsic and intrinsic point defects,deduced from neutron diffraction data, were related to the photoluminescence spectra and correlated with the predictions given by density functional theory (DFT) [3]. [1] Marti et al., JAP 103, 073706 (2008) [2] Marquardt et al. Phys. Stat. Sol. A (2019) 1800882 [3] Zhao & Zunger, Phys. Rev. B 69 (2004)

Resume : Hot carrier solar cells promise high solar energy conversion efficiencies far beyond the Shockley-Queisser limit, yet no practical improvements have been achieved. Opto-electronic reciprocity relations have proven very useful in improving the efficiency of single junction solar cells. Here, we derive opto-electronic reciprocity relations for hot carrier cells by analysing their behaviour in the dark. We find wildly differing predictions by using the two prevalent models for hot carrier solar cells, namely the particle conservation model with a finite quasi-Fermi level separation in the absorber and the purely thermal carrier model. Assuming ideally energy selective contacts, a purely thermal electron distribution in the absorber results in a temperature of the absorber that is defined by the ratio between applied bias and contact energy, independent of cooling rates. The dark I-V does not resemble a diode. In contrast, the particle conservation model predicts that the temperature in the absorber depends on the cooling rate and contact energy and is independent of applied bias in a large bias regime. The applied bias changes the quasi-Fermi level separation in the absorber resulting in diode-like behaviour. The reciprocity relations relate the magnitude of the current to the cooling rate in case of thermal carrier distributions and the temperature as function of bias in case of carrier distributions that follow the particle conservation model.

Resume : The volatility of organic cations in hybrid perovskites based on lead (Pb), and the possibility of lead contamination are the two main subjects of concern in the manufacture of optoelectronic devices based on this type of perovskites. A promising new family of hybrid metal halide perovskites based on Titanium Cs2TiX6 (X = Cl, Br) non-toxic, abundant in earth will be an alternative to these lead (Pb)-based perovskites. In this article, we have performed calculations to reveal the optoelectronic and transport properties for this new material. The results of the calculations are in good agreement with the experimental ones available. The Gap energy calculated in the case of Cs2TiCl6 is reduced when "Cl" is replaced by "Br". Studies of optical absorption spectra carried out in the energy range from 0 to 5 eV, confirm the efficient use of these compounds in solar cells and other optoelectronic applications. In addition, calculations of transport properties made using the semi-classical Boltzmann theory show thermoelectric power near the ambient temperature range, which admits the possible use of these compounds as thermoelectric materials at low temperature.

Resume : Recent computational design strategies towards designing new materials that do not contain Pb have been remarkably successful, and lead to the discovery of a series of novel compounds within the so-called halide double perovskites family. [1] Today there are five inorganic crystals that have been synthesized and characterized; Cs2BiAgCl6, Cs2BiAgBr6, Cs2SbAgCl6, Cs2SbAgBr6, and Cs2InAgCl6. Among these, Cs2BiAgBr6 has the narrower indirect band gap of 1.9 eV, and Cs2InAgCl6 is the only direct band gap semiconductor, yet with a large gap of 3.3 eV. All of them exhibit low carrier effective masses and consequently, are prominent candidates for opto-electronic applications including photovoltaics. Here, we probe oxide perovskites to look for potential promising oxide perovskites following our substitution-based computational design approach. We identify a new oxide double perovskite semiconductor: Ba2AgIO6 (BAIO), and report on its synthesis by low-temp solution process. BAIO has a band gap in the visible range (i.e. 1.9 eV), and a broad PL spectrum. Surprisingly, BAIO has an electronic band structure remarkably similar to that of our recently discovered perovskite Cs2AgInCl6. This allows us to further develop a universal analogy concept that can be used to identify links between oxide and halide perovskites. As such example, we show how BAIO and Cs2AgInCl6 are both analogs of the well-known transparent conductor BaSnO3, but the significantly lower band-gap of BAIO makes this new compound more promising for oxide-based optoelectronics and for novel monolithic halide/oxide devices. [1] Patent WO 2017/037448 Al (2015); JPCL 7 1254 (2016); JPCL 8 772 (2017); APL 112 243901 (2018) [2] JPCL 8 3917 (2017) [3] Patent Application 19386013.7 (2019) ; JPCL 10 1722 (2019)

Resume : Recent advances in lanthanide doping of CsPbX3 (X = Cl, Br, I) perovskites have opened new opportunities for their application as spectral conversion phosphors. This talk will describe recent research into understanding and controlling the photophysical process of quantum-cutting in ytterbium-doped CsPb(Cl1-xBrx)3 for photon management in solar spectral-conversion applications. A combination of variable-temperature and time-resolved spectroscopy provide insight into the quantum-cutting process that produces remarkably high photoluminescence quantum yields in these materials (>100%). The application of these materials for solar photon management in next-generation photovoltaics will also be described.

Resume : Silicon solar cells dominate the photovoltaic (PV) market. Their high efficiencies, along with their relatively low cost, make them the most valuable solution nowadays. However, it is well known that silicon solar cells are extremely sensitive to temperature, and they can lose up to 15 – 20% of their room temperature efficiency under normal operating conditions. In this perspective, less sensitive solar cells such as amorphous silicon (a-Si), Gallium Indium Phosphide (GaInP), and Perovskites, have been proposed as valuable alternative solutions. However, the room temperature efficiencies of these candidate materials are still lower than those of silicon devices, making them not competitive enough. Hybrid Thermoelectric-Photovoltaic (HTEPV) systems, which recover solar cell heat losses to produce an additional power output, are a suitable option to enhance the competitiveness of these kind of solar cells. In this communication we report on the development and the characterization of bismuth telluride thermoelectric generators with a layout optimized to hybridize aSi, GaInP or Perovskites solar cells. The results showed in all three cases, efficiency enhancements up to 3.5% depending on the solar cell and on the irradiation level. Efficiency improvements are in excellent accordance with theoretical estimations. For Perovskites solar cells the efficiency of the hybrid device was found to be higher than that of typical silicon solar cells. This experimental evaluation demonstrated in an accurate fashion the real potential of thermoelectric hybridization of solar cells.