Bilateral energy conferenceY
Advanced materials and characterization techniques for solar cells II
The symposium will focus on advances in materials science, processing, and device issues aimed to achieve a cost reduction and an efficiency increase of future solar cells. As a continuation of the last edition, this symposium will provide a large platform for discussion on key aspects of solar cell research and fabrication.
The expected huge increase in the world energy consumption linked to the Earth-scale environmental problems, and the remarkable growth of photovoltaics industry generated a great interest in the scientific community and increasing investments in research and development of materials and concepts useful for future solar cells. Although silicon is the dominant material used in solar cells today, alternative materials are needed to meet long-term clean energy economic goals. Moreover, novel concepts and device ideas can be actually manufactured using present nanotechnology. Advanced analytic tools are also a key factor for achieving efficiency improvement and reducing cost.
The focus of the symposium is on areas of material growth techniques, modeling and advanced characterizations of component materials and individual solar cells. Special emphasis will be placed on the effect of the material properties on the device efficiency with particular interest on cells manufacturing, thin films, nanostructures, phenomena at interfaces, structural defects, bulk and surface properties, carriers transport properties, light trapping, etc. A fruitful mixing of materials science, nanotechnology, device engineering, industry related issues, produced by the presence of excellent invited speakers, solid selection of contributions, relaxed atmosphere and synergy with other symposia present at the conference, will ensure the success of this symposium. All colleagues interested in the recent progresses and future challenges of photovoltaics are invited to participate and encouraged to submit their contributions for oral and poster presentation.
Hot topics to be covered by the symposium:
- Novel concepts for photovoltaics applications
- Advanced synthesis and application of nanostructures for sunlight-energy conversion
- Innovative multi-junction cells
- Multiple exciton generation, carrier multiplication
- Intermediate Band solar cells
- Up- and down- converters
- Light trapping and plasmonic effects
- Photogenerated carriers transport and modeling
- Si-based and thin film solar cells
- Surface and interface issues in solar cell design
- Innovative materials for transparent contacts
- Advanced glass and flexible substrates
- Manufacturing issues and In-line characterization of solar cells
- compound thin film systems (CIGS, CdTe, etc.)
- organic and dye-sensitized solar cell systems
Tentative list of invited speakers:
- Harry Atwater (CalTech, USA) Light trapping in printable resonant dielectric nanosphere arrays
- Christophe Ballif (EPFL, Switzerland) Silicon filaments for next generation Photovoltaics
- Christiane Becker (HZB, Germany) Nanostructured thin film crystalline silicon for PV applications
- Kylie Catchpole (Australian Natl Univ) Plasmonics and nanophotonics for photovoltaics
- Andreas Gerber (IEK5 Forschungscentrum Julich, Germany) - Advanced large area characterization of thin film solar modules
- David Ginley (National Renewable Energy Lab., USA) Enhanced Efficiency in Organic Solar Cells
- Thomas Krauss (York University, UK) Engineering gratings for light trapping in photovoltaics
- Seigo ITO (Univ. Hyogo, Japan) Inorganic printed solar cells
- Antonio Luque (Univ. Madrid, Spain) Utmost efficiency in photovoltaics
- Susanne Siebentritt (Luxembourg university) Kesterites: a challenging material for solar cells
- Marko Topic (University of Ljubljana) Approaches and challenges in optical modeling and optical characterization of thin-film solar cells
Tentative list of scientific committee members:
- Atilla Aydinly (Bilkent Univ., Turkey)
- Isodiana Crupi (CNR-IMM, Italy)
- Guglielmo Fortunato (CNR-IMM, Italy)
- Blas Garrido (UB, Spain)
- Karl-Heinz Heinig (Helmholtz Center Dresden-Rossendorf, Germany)
- Andrej Kuznetsov (Centre for Material Science and Nanotechnology, Oslo, Norway)
- Arne Nylandsted Larsen (University of Aarhus, Denmark)
- Antonio Martì (Univ. Madrid, Spain)
- Ivan Pelant (IP AVCR, Prague, Czechia)
- Jef Poortmans (IMEC, Belgium)
- Yosef Shacham (Tel Aviv Univ., Israel)
- Abdelilah Slaoui (CNRS, France)
- Baojie Yan (United Solar Ovonic, USA)
- Margit Zacharias (Freiburg University, Germany)
- Miro Zeman (Delft Univ., The Netherlands)
Via Santa Sofia, 64
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Phone: +420 2 21911272
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Dep. Phys. METU 06531
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Fax: +90 312 2105099
Pasadena, CA 91125
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Authors : S. Mirabella
Affiliations : CNR-IMM
Resume : introduction
Authors : Ivan Gordon
Affiliations : IMEC
Resume : TOP CHALLENGES IN PHOTOVOLTAICS
Authors : Lucio Claudio Andreani, Angelo Bozzola, Piotr Kowalczewski, Marco Liscidini
Affiliations : Physics Department, University of Pavia, Italy
Resume : Light trapping is crucial to increase efficiency and to reduce the cost of thin-film solar cells. It is especially important to enhance light absorption at low energy, and to approach the ultimate or Lambertian limit to absorption. In this work we report a theoretical study of thin-film silicon solar cells with various types of photonic structures that are either fully ordered, or randomly rough, or with a combination of order and disorder[1,2]. Ordered photonic lattices like 1D or 2D photonic crystals allow achieving a substantial improvement of the short-circuit current, but Jsc is still far from the Lambertian limit. Photonic lattices with size or position disorder yield a further improvement of Jsc and can still be realized with lithography and etching. Randomly rough surfaces are described by a Gaussian disorder. This model describes very well the scattering properties of actual rough substrates, and optimization of the disorder parameters allows to reach the Lambertian limit for absorption. However, this requires values of the roughness that are too high for growth of good-quality material. Thus we introduce hybrid structures that combine moderate Gaussian disorder with a photonic crystal: such structures have still a Jsc close to the Lambertian limit.  A. Bozzola et al, Opt. Express 20, A224 (2012); Progr. Photov. Res. Appl. (2013), DOI: 10.1002/pip.2385  P. Kowalczewski et al, Opt. Lett. 37, 4868 (2012); Opt. Express 21, A808 (2013).
Authors : Paola Lova 1,2, Valentina Robbiano 3, Franco Cacialli 3, Davide Comoretto 4, and Cesare Soci 2.
Affiliations : 1 Energy Institute at NTU (ERI@N), Interdisciplinary Graduate School, Nanyang Technological University, 50 Nanyang Avenue, 639798, Singapore; 2 School of Physical and Mathematical Sciences, Division of Physics and Applied Physics, Nanyang technological University, 21 Nanyang Link, 637371, Singapore; 3 Department of Physics and Astronomy and London Centre for Nanotechnology, University College London, Gower Street, London WC1E 6BT, United Kingdom; 4 Dipartimento di Chimica e Chimica Industriale, Università degli studi di Genova, via Dodecaneso 31, 16146 Genoa, Italy
Resume : The growing demand for photovoltaic energy generation technologies has fostered research on highly absorbing, low-bandgap semiconductors to allow thinner photoactive layers with reduced cost, and on light management strategies to increase solar cell power conversion efficiency. Here we report a simple and scalable process for the production of black GaAs, where remarkable antireflective properties are obtained roughening the GaAs wafer surface by Metal Assisted Chemical Etching (MACE). In this process, anisotropic etching of the wafer surface along the <111> and <311> planes is mediated by metal nanoparticles. Similar to black silicon, where high aspect ratio nanowires arrays are used to enhance light harvesting in the photoactive layer, microstructured GaAs shows surface reflectance as low as 10% throughout the visible to near-infrared spectral range. MACE could be used in conjunction with high-efficiency, thin film GaAs solar cells to increase both photon absorption and emission1, as well as to produce high aspect ratio GaAs structures to be used as additives in hybrid organic-inorganic photovoltaic devices. 1 O. D. Miller, E. Yablonovitch and S. R. Kurtz, IEEE Journal of Photovoltaics 2 (3), 303-311 (2012).
Light management II : T. Krauss
Authors : T. Gregorkiewicz
Affiliations : Van der Waals - Zeeman Institute, University of Amsterdam
Resume : The most important limitation for efficiency of photovoltaic energy conversion appears due to the enormous mismatch between the broad-band character of the solar radiation and the discrete operation mode of solar devices, as determined by the specific bandgap energy of the active medium. One remedy for that would be the spectral transformation of the solar radiation before it enters a photovoltaic device. Specifically, such a solar shaper should split the large energy photons into smaller ones, whose conversion efficiency is optimal. In my presentation, I will discuss how shaping of the solar spectrum can be achieved using layers of Si nanocrystals with and without Er doping. In such systems efficient photon transformation can be realized by quantum cutting  and/or emission from Er3+ ions sensitized by Si nanocrystals . I will present evaluation of external quantum efficiency of emission from thin layers of Si nanocrystals in SiO2 and its evolution upon introduction of Er3+ ions. Using differently prepared materials I will evaluate how this can be maximized. I will also discuss energy efficiency of solar shapers based on Si nanocrystals and Er3+ ions. In a separate part, I will discuss also strategies how Si nanocrystals can be employed for quantum pasting by which low-energy photons could be used in photovoltaic conversion by a standard Si solar cell.  M.T. Trinh et al., Nature Photonics 6, 316-321 (2012).  S. Saeed et al., under consideration
Authors : E. M. L. D. de Jong and T. Gregorkiewicz
Affiliations : Van der Waals-Zeeman Institute, University of Amsterdam, Netherlands
Resume : Si-based solar cells feature very good efficiency in the visible regime, but significant losses occur in the infrared. These low-energy photons could be used through up-conversion, taking place before they enter Si cell. In this study, we investigate how this can be achieved with Si nanocrystals (SiNCs), which we have shown to be attractive for photovoltaic applications1,2, among others. We explore how hot carriers can be generated in SiNCs by low-energy excitation making use of multi-photon absorption, Auger recombination of multiple excitons, and free carrier absorption. We use Er3+ ions to detect the hot carriers, and in that way reveal the up-conversion process. In the investigated material, SiO2 doped with SiNCs and Er3+ ions, hot carriers enable very characteristic ultrafast excitation into the 4I13/2 state responsible for the 0.8eV photoluminescence. We monitor dynamics of this emission band and from these data we extract information on hot carrier generation. The results obtained under various excitation modes corresponding to the above mentioned scenarios for up-conversion allow us to investigate their feasibility and efficiency. The investigated mechanisms will be relevant for future photovoltaics, where a more efficient use of low-energy photons will be possible. 1 M.T. Trinh et al., Nat. Photon. 6, (2012) 2 F. Priolo et al., Nat. Nanotechnol. 9, (2014)
Authors : N.X. Chung, R. Limpens, B. Bruhn, T. Gregorkiewicz
Affiliations : Van der Waals-Zeeman Institute, University of Amsterdam, Science Park 904, 1098XH Amsterdam, the Netherlands
Resume : Photon down-conversion and also efficient multiple exciton generation (MEG) can be achieved with solid-state dispersions of Si nanocrystals (SiNCs) in an SiO2 matrix. Past research has shown that in order to maximize efficiency of the photovoltaic conversion, the photon conversion energy should be around 0.8 eV. This cannot be achieved with SiNC whose bandgap always exceeds that of bulk Si. Here we report on investigations how the photon conversion at the optimal energy can be achieved with SiNCs co-doped with boron and phosphor. In this case (radiative) recombination takes place between impurity levels in the bandgap, and therefore lower energy photons are generated. We investigate the absolute external quantum yield (AQY) of this emission as function of excitation energy. Materials with different doping characteristics are used and the results are compared with those for undoped NCs of the similar size. MEGs visualized by a gradual QY increase towards higher energies is concluded, with the threshold energy marking the onset of the multiple exciton generation. We demonstrate that the threshold energy downshifts upon co-doping. Finally. we discuss the application potential of doped Si NCs for solar shaping in photovoltaics.
Authors : K. Bouras1*, J.- L. Rehspringer2, G. Schmerber2, G. Ferblantier1, S. Colis2, A. Dinia2 and A. Slaoui1
Affiliations : 1ICube, CNRS-Université de Strasbourg, 23 rue du Loess, BP 20 CR, 67037 Strasbourg Cedex 2, France 2IPCMS, CNRS-Université de Strasbourg, UMR7504, 23 rue du Loess, BP 43, 67034 Strasbourg Cedex 2, France
Resume : Tin dioxide is one of the most attractive materials studied in the last decade due to its several applications such as optoelectronic devices, gas sensors, and solar cells. It is n- type semiconductor with a wide band gap (around 3.6 eV for bulk material at 300 K). Doping SnO2 films with rare earth (RE) elements (such as Yb, Pr, Tb, Nd) can enhance their photoluminescence properties thanks to the optical transitions involving the 4f shell of the dopant. However, the transparency and structural properties of the RE doped SnO2 films should not be affected. In this work, we have synthesized undoped SnO2 and Nd doped SnO2 (Nd:SnO2) powders and thin films by sol gel method in order to understand the insertion process of Nd in SnO2 matrix and to produce luminescent materials for photon shifting purpose. The crystallinity, structure and particles size for powder and thin films were determined by XRD measurements. We show that the tetragonal crystalline phase is achieved for the Nd:SnO2 structure at an annealing temperature of 700°C. Furthermore, a slight diffraction angle shift toward lower angles is observed with increasing Nd content, indicating Nd incorporation. No secondary phases like Nd oxide are detected. A change of optical band gap is also observed from UV-Visible spectroscopy confirming XRD results. The intensity of Raman peaks decreases gradually as the Nd content increases. SEM micrographs show that the grain size is homogenous for Nd:SnO2 powders and the surface of spin-coated films is uniform. Finally, photoluminescence measurements are carried out to check if an electronic transfer from SnO2 to Nd+3 ions is achieved
Authors : P. Morvillo, A. De Girolamo Del Mauro, R. Ricciardi, G. Nenna, R. Diana, C. Minarini
Affiliations : ENEA, P.le E. Fermi, 1, 80055 Portici (NA), Italy
Resume : In this work we improved the performance of ITO-free polymer solar cells (PSCs) by incorporating silver nanoparticles (AgNPs) in the highly conductive PEDOT:PSS anode. The AgNPs were synthetized in-situ in the PEDOT:PSS water dispersion. This anode was used to realize PSCs with the following geometry: glass/HC-PEDOT:PSS)/PBDTTT-C:PCBM/Ca/Al. All the devices were characterized by UV-VIS spectroscopy, impedance spectroscopy, IV light, IV dark and quantum efficiency measurements. We made a comparative study of the electrical behavior of different PSCs in order to investigate the influence of AgNPs concentration and size. The best device reached a power conversion efficiency greater than 6%. The presence of AgNPs in the HC-PEDOT:PSS anode contributes to improve the absorption of the photoactive layer (plasmonic effect) and to lower the resistivity of the anode. The impedance spectroscopy was used to derive the circuit elements of the device in order to explain the change in the photovoltaic performances of the realized PSCs.
Authors : M.A. Jafarov, E.F. Nasirov
Affiliations : Baku State University, Baku 1045, Z.Khalilov st.23, Azerbaijan
Resume : The intensive development of solid-state electronics stimulates the complex physical and technological investigations of the semiconductive films with a complex chemical composition. A simple and a low-cast technology high reproducibility and modification of their physical properties characterize the films obtained by method of chemical deposition from the aqueous solution. The investigation of Cd1-xZnxS films deposited from the solution showed that their high sensitivity could be realized on a wide spectral range (0.3-1.2m). The Cd1-xZnxS (0x0.6) films of 8-10m thickness were deposited on glass ceramic substrates from aqueous solution by the method described in3. We have created and investigated some properties of the thin-film solar cells with heterojunctions CuInGaSe2-Cd1-xZnxS(Se). In this case the studied heterojunctions (HJ) obtained by precipitation from solution in a single process cycle. CuInGaSe2 film deposited from an aqueous solution of CuCl2 InCl3 GaCl3 HCl. The Cd1-xZnxS films composition (0x1) was changed by partial substitution of the thiourea and was controlled by chemical, spectral and X-ray phase analyses. A part of films was subjected to heat-treatment in the air at temperatures of 400-5000C for 0-30 min. In recent years, resumed intensive studies of heterojunctions, in connection with the possibility of using them on the most economical and relatively efficient photodetectors. In this economy is determined by the structure of production technology and the value of the source material. Thus there is a substitution reaction on the surface of the films Cd1-xZnxS formed a second layer of CuInGaSe2 whose thickness is determined by the time of deposition. When light 1,45104 lux photocells studied CuInGaSe2-Cd1-xZnxS(Se)generated voltage 0,580,65V and in 0,50-0,58V, current Jsc 18 22 mA/sm2 and 15-18mA/cm2 and efficiency were = 11%. and 8%. respectively. It is established that with increasing content of Zn in the base material-circuit voltage Uos photocell increases and short-circuit current decreases. Using as a base material of solid solutions of Cd1-xZnxS and Cd1-xZnxSe causes an increase in the potential barrier at the contact. On the other hand, the discrepancy decreases the lattice constants of contacting materials, which leads to a decrease in the density of states at the interface of HJ, as well as the rate of degradation. The prospects of practical application of non-ideal heterojunction is related primarily to the more cost-effective technologies to create polycrystalline heterostructures in comparison with single crystal. One of the directions in the study of nonideal heterojunctions is the possibility of applying the criteria developed in the classical photographic sensitometry, the optical image converters into an electrical signal based on heterojunction CuInGaSe2-Cd1-xZnxS(Se). Consider the possibility of such a system in relation to the registration of optical images of different spectral composition. The maximum effect is achieved at 445 nm (absorption edge of x=0,6). More shortwave light is strongly absorbed in the base layer of cadmium sulfide, so the concentration of photoexcited holes in the vicinity of space-charge region determined by the thickness of the layer of cadmium sulfide and the diffusion length of holes in this material. AES not reach all the photogenerated holes, which leads to a decrease in short-stimulation. The sharp drop in sensitivity of the sample in the short-range due to the fact that the generated charge carriers recombine in the volume of the layer of Cd1-xZnxS, without having to reach the space charge region, absorption occurs in the surface layer of Cd1-xZnxS. Decline of sensitivity in the wavelength region shows the decrease of the coefficient of HJ Cd1-xZnxS- CuInGaSe2 the presence of the impurity centers in Cd1-xZnxS, involved in the processes of generation of carriers. To increase the sensitivity should either reduce the thickness of the base layer, or create an optical image from a thin layer of CuInGaSe2 layer. Thickness reduction of the CuInGaSe2absorber layers would not only be useful to reduce the material cost in the production process but could also lead to better solar cell properties by reducing recombination losses in the bulk. However, control of the film growth and recrystalsation due to post-deposition treatment is necessary to obtain thin films which are compact and free of pinholes. Employing the CSS technique; this is the best technique of deposition the CuInGaSe2 layer as it provides high rates and good film quality while the material yield is very high. CSS- CuInGaSe2 films can consist of large grains being useful for the electronic transport properties. However, the actual film thickness needed may be larger than the absorption depth due to the possibility of increased shunting in thinner films. Thus, considering all these consequences the CuInGaSe2 layer thickness was selected at 1 μm for this investigation. However there are possibilities of increasing SR and cell conversion efficiency further more if the ZnxCd1-xS window layer thickness (100 nm) can be reduced with proper buffer layer insertion in between front contact (ITO) and ZnxCd1-xS layer. When the ZnxCd1-xS layer thickness was reduced, the absorption loss in the blue region due to thin ZnxCd1-xS layer also reduces, which improves mainly Jsc and consequently the cell conversion efficiency. As a result, cell conversion efficiency was increased with reduced ZnxCd1-xS layer. The SR of ZnxCd1-xS layer thickness variation from 20 nm to 0.2 μm was investigated through AMPS simulation and it was noticed when the wavelength was in between 400-540 nm the quantum efficiency (QE) is much affected with the increasing ZnxCd1-xS layer thickness which influences the cell conversion efficiency. However, in considering fabrication challenges and to fabricate good quality cells we have selected the ZnxCd1-xS film thickness of 60 nm with conversion efficiency of 12%. The improvement in conversion efficiency was achieved due to the improvement of Jsc with reduced ZnxCd1-xS layer. However, this reduced ZnxCd1-xS layer might allow forward leakage current to front contact through possible pinholes of ultra thin ZnxCd1-xS layer although the pinholes is greatly reduced using ZnxCd1-xS window layer instead of CdS in our proposed solar cell. In order to prevent this unwanted forward leakage current a high resistive buffer layer of suitable material must be inserted in between front contact and ZnxCd1-xS layer. Further numerical analysis was done aiming to improve the conversion efficiency of the ultra thin CuInGaSe2-Cd1-xZnxS(Se) cell by inserting a suitable buffer layer. It is possible to take advantage of the different properties of two TCOs by forming a buffer layer. High-efficiency CIGS devices are generally fabricated with such buffer layer structures. The modified proposed cell structure consists of a highly conducting layer (ITO) for low-resistance due to contact and lateral current collection and a much thinner high-resistivity layer (called buffer layer) of a suitable material. By incorporating a very thin resistive buffer layer, the ZnxCd1-xS layer thickness can be reduced to 60 nm, which significantly improves the blue response, ZnxCd1-xS film morphology and conversion efficiency of the CuInGaSe2- Cd1-xZnxS(Se) devices.
Authors : Vanessa L. Pool, M. Imteyaz Ahmad, C. Jackson Stolle, Taylor B. Harvey, Brian A. Korgel, Michael F. Toney
Affiliations : 1 The University of Texas at Austin, McKetta Department of Chemical Engineering, Texas Materials Institute, Center for Nano- and Molecular Science, Austin, TX 2SSRL, Materials Science Division, SLAC National Accelerator Laboratory, Menlo Park, CA
Resume : Cu(In,Ga)(S,Se)2 (CIGS) is a promising absorber material for photovoltaic devices. Vacuum based processing of CIGS films has yielded solar cells having efficiencies greater than 20%, however, the vapor deposition technique is not a cost or energy effective option. High throughput solution based processing has been shown to be cost effective and can deposit films of uniform thicknesses over relatively large areas. This solution-based processing involves deposition of films using a suspension of CIGS nanoparticles. In order to achieve greater efficiency, the nanoparticles must be sintered to micron-sized grains without leaving any inter-granular impurities. Efficiencies up to 12% have been achieved by sintering at 500 ⁰C in a selenium rich atmosphere, and the processing plays an important role in achieving higher-efficiency devices. However, the effect of these processing parameters on the phase evolution and grain growth kinetics is scarcely understood, limiting progress in achieving higher efficiencies. In-situ characterization of CIGS film formation using x-ray diffraction and florescence with a time resolution up to 100 ms allows for a deeper insight into the reaction pathways during the thermal processing. Our in-situ measurements of the grain growth and phase evolution of CIGS nanoparticle films, deposited on molybdenum coated glass substrates, will be presented.
Authors : M. Theelen, H. Steijvers, Z. Vroon, N. Barreau, M. Zeman
Affiliations : M. Theelen1,2,3; H. Steijvers1; Z. Vroon1; N. Barreau4; M. Zeman2 1 TNO, dept. Thin Film Technology - De Rondom 1, 5612 AP, Eindhoven, The Netherlands 2 Delft University of Technology, Photovoltaic Materials and Devices, Mekelweg 4, 2628 CD Delft, The Netherlands 3 Materials innovation institute (M2i), Mekelweg 2 2628 CD DELFT, The Netherlands 4 Institut des Matériaux Jean Rouxel (IMN)UMR 6502, Université de Nantes, CNRS, 2 rue de la Houssinière B.P. 32229, 44322 Nantes Cedex 3 France
Resume : The reliability of thin film Copper Indium Gallium Selenide (CIGS) solar cells is currently tested according to IEC standards, which do not give a complete picture of the degradation mechanisms. We have designed and built an hybrid degradation setup, in which humidity, temperature and illumination are all used as loads in order to accelerate degradation of CIGS. The setup consists of a climate chamber, which can vary the temperature and humidity. Furthermore, an area of 80x80 cm2 is illuminated, which allows both the study of light induced degradation and the in-situ measurement of the IV curve of the cell during the test, so the degradation behaviour can be observed in time. The IV output is automatically logged and can be converted to current, voltage and resistance characteristics. The 40x40 cm2 in the center of the illuminated area was calibrated BAA according to IEC norm 60904-9. Twelve cells or minimodules can be degraded and measured in-situ at the same time. Extra external loads can be applied to make sure that the cells operate at the maximal powerpoint to optimally simulate field operation. The continuous in-situ IV measurements allowed us, for example, to observe reversible degradation of the Voc and the Rsh, which would not be visible in regular damp heat tests. It also allowed us to very accurately determine the temperature dependency and the temperature hysteresis of our cells by slow heating and cooling.
Authors : David A Keller, Klimentiy Shimanovich, Hannah-Noa Barad, Assaf Y Anderson, Yaniv Bouhadana, Adam Ginsburg, Eli Rosh-Hodesh, Koushik Majhi, David Sriker, Sven Rühle, Arie Zaban
Affiliations : Bar-Ilan University
Resume : All-oxide photovoltaic devices are a new and emerging type of solar cells. The all-oxide solar cells are based solely on semiconducting metal oxides that act as the absorber, the electron conductor, and as selective back contacts. Fe2O3 has been extensively investigated for its photocatalytic properties as it is highly abundant and has a high thermodynamic stability. Most of the research on Fe2O3 has focused on doping Fe2O3 nano-particles and thin films with various dopants, since Fe2O3 has poor electrical properties, such as very high resistivity and very short charge carrier diffusion lengths. These properties have prevented wide exploration of Fe2O3 as a light-absorber for photovoltaic applications. In order to overcome these obstacles, a combinatorial material science approach was selected. A thin film, between 100-300 nm thick, with continuous compositional spread of Fe-Nb-O was created using Fe2O3 and Nb2O5 as starting materials, and deposited by pulsed laser deposition (PLD). Electrical, optical, and structural high-throughput measurements were performed in order to characterize the different mixing ratios between Fe, Nb, and O. These measurements were conducted using home built scanners such as sheet resistance, optical spectroscopy, etc. Specific mixing ratios between Fe, Nb, and O showed much lower resistivity than the pure Fe2O3 or Nb2O5. Further improvement in Fe2O3 properties will allow its future use as the main light absorber in all-oxide PV devices.
Authors : Kuo-Chin Wang, Po-Shen, Shena, Peter Chen
Affiliations : Department of Photonics, National Cheng Kung University, Tainan, Taiwan 701
Resume : We present a new paradigm for organometallic hybrid perovskite solar cell using NiO inorganic metal oxide nanocrystalline as electrode material and realized the first mesoscopic NiO/perovskite/PCBM heterojunction photovoltaic device. The photo-induced absorption spectroscopy results verified that the architecture is an effective sensitized junction which is the first inorganic p-type metal oxide contact material for perovskite-based solar cell. Power conversion efficiency of 9.51 % was achieved under AM 1.5 G illumination, which significantly surpassed the reported conventional p-type sensitization dye solar cells. The replacement of organic hole transport materials by metal oxide is advantageous for providing robust device architecture and the development of fully inorganic perovskite-based thin film solar cells. We believe this significant advance will stimulate further contributions for novel configurations that could facilitate the future design of low-cost fully inorganic thin film and tandem photovoltaics.
Authors : Artem A. Bakulin, Huib J. Bakker, Zhenhua Sun, Zhuoying Chen
Affiliations : FOM institute AMOLF, Science Park 104, Amsterdam 1098 XG, The Netherlands; Laboratoire de Physique et d'Etude des Matériaux, ESPCI/CNRS/UPMC UMR 8213, 10 rue Vauquelin, 75005 Paris, France;
Resume : Solution-processed organic-inorganic hybrid materials like colloidal quantum dots (QD) or perovskites hold promise for cost-effective thin-film solar cells. The carrier dynamics in these materials is determined by the conduction and valence band structure, distribution of trapping states and the pathways for carrier relaxation. In current contribution, we apply a set of novel ultrafast electro-optical techniques, including Vis-pump IR-probe, pump-push photocurrent, and 3-pulse transient absorption spectroscopies to elucidate the carrier relaxation and trapping dynamics in PbS QD and methylammonium lead halide perovskite films. The material properties were controlled by the QD ligand-exchange and perovskite surface passivation. We show that the carrier relaxation in the studied systems can occur on a variety of timescales form 200 fs to longer than 10 ps. The particular charge relaxation and recombination rates strongly depend on the presence of defect states and the strength of electron-hole coupling. In QD photovoltaic device we observed that some photogenerated electron-hole pairs are intrinsically weakly bound. At the same time, charge diffusion in the QD films leads to substantial charge trapping on the ~ns timescale. Our study provides important information for the development and implementation of hot-electron extraction and carrier multiplication in third-generation photovoltaic devices.
Authors : Hasan Huseyin Gullu 1-3, Emre Coskun 1-2-3, Ozge Bayrakli 1-3, Idris Candan 1-3, Mehmet Parlak 1-3
Affiliations : 1-Department of Physics, Middle East Technical University (METU), Ankara 06800, Turkey 2-Department of Physics, Çanakkale Onsekiz Mart University, Çanakkale 17100 Turkey 3-Center for Solar Energy Research and Applications (GÜNAM), METU, Ankara 06800, Turkey
Resume : In this study, the CZST thin films were prepared by sequential-sputtering from compound targets of Cu, SnTe and ZnTe in an argon atmosphere by using three magnetron DC/RF sputtering system. During the deposition process, the substrate temperature was kept at about 200°C. The composition of the films was determined by EDXA and the percentage of atomic ratios were found as about Cu:Zn:Sn:Te = 13:10:21:56 %. XRD measurements of the thin films showed the main orientation along (112) plane direction. All of the as-grown films were in polycrystalline structure, with the mixed phases of CZST, ZnTe and SnTe. The thicknesses of the films were found as about 210 nm with ±1 nm uncertainty. Optical measurements showed that the thin films have direct energy gap of about 1.5 eV. The measurement of the electrical properties by the four-point probe method at room temperature showed that it had a sheet resistance of about 106 Ω/sq. Moreover, all films were determined to be p-type by Hall-effect measurements with ~2.3 x 1018 cm-3 carrier concentration and ~18.5 cm2/V.s mobility. A detailed device characterization of the Ag/n-Si/p-CZST/In sandwich structure was performed as a function of sample temperature. The temperature dependent current-voltage (I-V) characteristics of the sample were studied to determine the possible conduction mechanisms and device parameters. They showed a good diode behavior with about 2 order rectification factor. The ideality factor n and the barrier height values were determined. The spectral photo-response measurements were also performed in order to see the effects and contributions of n-Si and p-CZST in the device structure.
Authors : E. Dumiszewska (1), P. Knyps (1), M. Wesolowski (1), A. Krajewska (1), W. Strupinski (1), J. Kalbarczyk (1)
Affiliations : (1) Institue of Electronic Materials Technology, Warsaw, Poland
Resume : Epitaxial lift-off (ELO) is a process which enables the removal of solar cell structures (one junction GaAs, two junction GaAs/InGaP or three junction GaAs/InGaAs/InGaP) from the substrate on which they are grown and their transfer onto lightweight carriers such as metal or polymeric insulator films. Such solar cells exhibit superior power conversion efficiency compared with alternative single-junction photovoltaic cell designs such as those based on crystalline Si, copper indium gallium sulfide (CIGS) or CdTe. The major advantage of ELO solar cells is the potential for wafer reuse, which can enable significant manufacturing cost reduction by minimizing the consumption of expensive wafers. Here in this work we have grown one junction GaAs solar cells on GaAs (100) substrates. A 10 nm thick AlAs layer has been used as a release layer, which has been selectively etched in HF solution. We have investigated different methods of transferring thin films onto polymer and copper foils, including the usage of temporary mounting adhesives and electro-conductive pastes. We have also analyzed techniques for reusing GaAs substrates and their repolishing aimed at obtaining the epi-ready quality wafers to be used in next epitaxial growths. We have achieved a thin film solar cell without any defects and cracks on 3-inch substrates. Lift-off has been demonstrated to be a very promising technique for producing affordable solar cells with a very high efficiency of up to 30%.
Authors : Yoon-Jung Na, Oh Young Kim, Seok-Ho Hwang*
Affiliations : Department of Polymer Science & Engineering, Dankook University
Resume : It is well established that polymer absorbers containing a long, planar, π-conjugated backbone can offer broad overlap of their absorption with the solar spectrum and often demonstrate high charge-carrier mobility due to π-π stacking interactions. In this presentation, we designed a series of semi-random conjugated copolymers composed of a benzothiadiazole and 2,6-dibromo-4,8-bis(2-ethylhexyloxy) benzo[1,2-b:4,5-b']dithiophene. The carbazole moiety in this study is used as bridge molecule to combine two different donor and acceptor units. First of all, 3-bromo-9H-carbazole is coupled with 4,7-dibromobenzo[c][1,2,5]thiadiazole via Ullmann reaction in the presence of copper iodide and K2CO3 to form the 3-bromo-9-(4-(3-bromo-9H-carbazol-9-yl)benzo[c][1,2,5]thiadiazol-7-yl)-9H-carbazole (A). And then boronic ester were introduced to the A by Miyaura borylation reaction with bis(pinacolate)diboron to obtain 3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9-(4-(3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9H-carbazol-9-yl)benzo[c][1,2,5]thiadiazol-7-yl)-9H-carbazole. The copolymerization of two different monomers was conducted by Suzuki-Miyaura cross-coupling reaction in the presence of Pd(PPh3)4 as a catalyst. Solar cells were produced from blends of synthesized copolymer with the fullerene acceptor [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). In the following, the synthesis and optical, electrochemical and morphological properties are presented and the performance of organic solar cell devices under an illumination of AM1.5G, 100 mW cm−2 will be discussed.
Authors : G. Sarau1, B. Hoffmann1, P. Klement2, S. Geissendoerfer2, S. Christiansen1
Affiliations : 1. Max Planck Institute for the Science of Light, Günther-Scharowsky-Str. 1, 91058 Erlangen, Germany; 2. NEXT ENERGY, EWE Research Centre for Energy Technology, Carl-von-Ossietzky-Str. 15, 26129 Oldenburg, Germany
Resume : The main challenge to a detailed in-depth characterization of thin films for solar cells is their intrinsic small thickness up to a few micrometers. For this purpose, two methods are used to virtually increase the thickness and thus the number of data points: mechanical polishing or chemical etching to form wedges along the entire film depth or successive surface removal and probing of the remaining planar film. Here, we employed the focused gallium ion beam (FIB) technique in a SEM to prepare wedges in a-Si:H/µc-Si:H by PECVD and mc-Si by laser crystallization and solid phase epitaxy (LC-SPC) thin film solar cells at selected positions without damaging and contaminating the entire sample (not provided by the other approaches). The implantation of Ga atoms and the local heating that may lead to in-depth mechanical damage and amorphization of the thin films was found negligible when using low ion energy and current values of 10 keV and 100 pA, respectively. This was confirmed on 4o wedges with widths between 10140 µm and stretching factors up to 14.3 by the micro-Raman mapping of the crystallinity in the a-Si:H/µc-Si:H cells and by the EBSD mapping of the grain structure in LC-SPC cells. Furthermore, we were able to study the interface between the layers themselves and the substrate. Our work opens the way towards advanced depth-resolved spectroscopy and microscopy investigations of bulk and interface properties of thin film solar materials at pre-characterized positions.
Authors : Orman Gref1, Jon Sandström1, Moshe Weizman2, Holger Rhein3, Stefan Gall3, Rutger Schlatmann2, Christian Boit1, Felice Friedrich1
Affiliations : 1 Technische Universität Berlin, Semiconductor Devices Division / PVcomB, Sekr. E4, Einsteinufer 19, 10587 Berlin, Germany; 2 University of Applied Sciences (HTW) Berlin / PVcomB, Schwarzschildstr. 3, 12489 Berlin, Germany; 3 Helmholtz-Zentrum Berlin für Materialien und Energie (HZB), Silicon Photovoltaics / PVcomB, Kekulestr. 5, 12489 Berlin, Germany
Resume : Rear side contacting is a promising route for wafer as well as thin-film silicon solar cells. Recently, it was shown that for crystalline silicon on glass solar cells, featuring a rear side point contacting scheme, an initial solar cell record efficiency of 11.7 % could be achieved. These solar cells, however, suffered from an efficiency degradation which was shown to be related to the aluminum-silicon absorber contact. A few months ago, it was reported that a subsequent laser firing of these contacts could improve the series resistance and helped to stabilize the cell efficiency. In this current study, such laser-fired contacts were investigated by conductive probe atomic force microscope (c-AFM) in order to detect, with an adequate spatial resolution, the local improvement in conductivity. By using the advantage of simultaneously mapping the topography and current distribution, a drastic increase in the local conductivity within the laser-fired region over a spot of about 30 µm in diameter was observed. Current profiling of the laser treated contact with the c-AFM technique thus provides the means to systematically optimize the laser focusing and energy fluence.
Authors : Jan Valenta
Affiliations : Charles University in Prague, Department of Chemical Physics & Optics, Faculty of Mathematics & Physics, Prague, Czechia
Resume : Luminescence spectroscopy is an important method for characterization of solar cells (SC) as it reveals radiative recombination of photo-generated carriers the process concurrent to generation of electrical current but unavoidably present in highly-efficient SC devices. Especially electroluminescence is applied to characterization of SCs in both the development and production. For technical difficulties, most of luminescence measurements are done as relative experiments, it means without absolute (energetic or quantum) scale of luminescence intensity. However, the absolute calibration should enable to get much more insight into the processes taking place in a studied SC. Here we describe our imaging-micro-spectroscopy set-up with two parallel detection channels for visible and near-infrared regions. This apparatus is absolutely calibrated using special thin-layers of luminescing Si nanocrystals with the luminescence quantum yield determined in a separate apparatus. The capabilities of our methods are demonstrated by studies of several model materials.
Authors : N. Segercrantz, D. Vial, F. Tuomisto, J. Slotte, J. Puustinen and M. Guina
Affiliations : Department of Applied Physics, Aalto University, P.O. Box 14100, FI-00076 Aalto, Finland; Department of Applied Physics, Aalto University, P.O. Box 14100, FI-00076 Aalto, Finland; Department of Applied Physics, Aalto University, P.O. Box 14100, FI-00076 Aalto, Finland; Department of Applied Physics, Aalto University, P.O. Box 14100, FI-00076 Aalto, Finland; Optoelectronics Research Centre, Tampere University of Technology, P.O. Box 692, FI-33101, Tampere, Finland; Optoelectronics Research Centre, Tampere University of Technology, P.O. Box 692, FI-33101, Tampere, Finland
Resume : Due to its many possible technical applications, GaAsBi has attracted increasing interest during the last decade. Properties such as large spin-orbital splitting, large band gap reduction and relatively weak temperature dependence makes GaAsBi a promising candidate for both long wavelength optoelectronic as well as spintronic related devices. Using positron annihilation spectroscopy (PAS) in Doppler broadening mode, we have studied MBE-grown epitaxial layers of GaAsBi on SI-GaAs. The samples had a constant layer thickness of 250 nm and Bi concentration of 1.5 or 1.6 % Bi. The growth temperature was 220 °C or 315 °C and the As/Ga flux ratio 1.5 or 1.6, respectively. As-grown samples as well as samples treated in a rapid thermal annealing (RTA) furnace at temperatures 500-700 °C for 60 s were studied. From the results, a clear difference between the as-grown samples and the RTA-threated samples can be seen. For the samples with 1.6 % Bi in the epitaxial layers, annealing at 500 °C led to a decrease in the S parameter and a lowered monovacancy concentration compared to the as-grown sample as well as the sample annealed at 600 °C. The W(S)-curve indicates that larger open volume defects are generated when the samples are annealed at 700 °C. Comparatively, the S parameter decreased compared to the as-grown sample when the sample with 1.5 % Bi was annealed at 600 °C.
Authors : Y. Ohshita1), T. Nishi1), D. Kodera1), K. Ikeda1), K. Shimomura1), H. Suzuki2), T. Sasaki3), I. Kamiya1) and M. Takahasi3)
Affiliations : (1)Toyota Technological Institute (2)University of Miyazaki (3)Japan Atomic Energy Agency
Resume : Dislocation-mediated strain relaxation during lattice-mismatched InGaAs/GaAs(001) heteroepitaxy was studied through in-situ x-ray reciprocal space mapping (in-situ RSM). Tandem type solar cell composed of lattice-mismatched III-V compound semiconductors is considered one of the candidates for realizing the super high conversion efficiency. However, the high density of threading dislocations in active layers grown on buffer layers causes the short minority carrier lifetime, which deteriorates the solar cell performance. Therefore, understanding the strain relaxation processes during the crystal growth is important to realize the expected high efficiency. The in-situ observation experiments were performed at the synchrotron radiation beamline 11XU at SPring-8, Japan, where a molecular beam epitaxy (MBE) X-ray diffraction (XRD) system was used. RSM of 022 reflections were obtained from the InxGa1-xAs growing layer on a GaAs(001) substrate. To understand the strain relaxation kinetics, the effect of growth interruptions on the strain relaxation behavior was studied. During the growth interruption, the relaxation ratio quickly increased and reached a maximum. The relaxation rate at the initial stage after the interruption increased as the growth temperature increased. On the other hand, the maximum values were constant independent of the temperature. The relaxation during the interruptions along the  reached saturation more readily than along [1-10] direction. This result suggested that the blocking process with orthogonally aligned α dislocations limited β dislocation motions.
Authors : R. Pietruszka1, G. Luka1, B. S. Witkowski1, L. Wachnicki1, S. Gieraltowska1, E. Zielony2, P. Bieganski2, E. Płaczek-Popko2, M. Godlewski1,3
Affiliations : 1Institute of Physics, Polish Academy of Sciences, Warsaw, Poland 2Institute of Physics, Wroclaw University of Technology, Wroclaw, Poland; 3 Department of Mathematics and Natural Sciences College of Science, Cardinal Stefan Wyszynski University, Warsaw, Poland
Resume : ZnO, a wide band gap semiconductor with 3.3 eV band gap at room temperature, is intensively studied for photovoltaic (PV) applications mostly as a transparent conductive oxide (TCO) and n type partner for p-type substrates. In this work we study PV structures based on n type ZnO grown at low temperature either by atomic layer deposition (ALD) or hydrothermal method (nanorods). In the first approach, we deposited thin films of n-type ZnO on p-type silicon substrates. Such obtained PV structures of the II generation (ZnO:Al/ZnO/Si/Al) show efficiency of about 6%. In the second approach, we grown ZnO nanorods (ZnONR) on p-type Si. The growth process was initialed by 15 cycles of ALD process forming ZnO nanoseeds on a Si substrate. Then, the so-deposited ZnO nanoseeds nucleate growth of ZnO nanorods in a hydrothermal process. So obtained ZnONR were covered with n-type ZnO and ZnO:Al films by ALD process. The best PV efficiency for such IV generation structures (ZnO:Al/ZnO/ZnONR/Si/Al) was equal to 12.5%. This work was partially supported by the Innovative Economy grant (POIG.01.01.02-00-108/09, the National Centre for Research and Development grant (PBS1/A5/27/2012), and (E. Zielony, P. Biegański and E. Popko) by the National Laboratory of Quantum Technologies (POIG. 02.02.00-00-003/08-00).
Authors : S. Polivtseva1, I. Oja Acik1, A. Mere1, V. Mikli2, M. Krunks1
Affiliations : 1 Tallinn University of Technology, Department of Materials Science, Laboratory of Thin Film Chemical Technologies, 19086 Tallinn, Estonia; 2 Tallinn University of Technology, Department of Materials Science, Chair of Semiconductor Materials Technology, 19086 Tallinn, Estonia
Resume : SnxSy films as were grown by the chemical spray pyrolysis method. Aqueous solutions consisted of tin chloride (SnCl2) and thiourea (SC(NH2)2) at molar ratio of Sn:S=1:1, 1:2 and 1:4 were deposited onto preheated glass substrates at temperatures Ts=200-400°C in air. The films were characterized by X-ray diffraction (XRD), UV-VIS spectroscopy and scanning electron microscopy (SEM). According to XRD, the films deposited at Ts=200-320°C were composed of polycrystalline SnS as main crystalline phase independent of the Sn:S ratio in the spray solution. By increasing the Ts from 200 to 320°C, the film thickness decreases and the mean crystallite size of SnS decreases from 29 to 14 nm and from 17 to 7 nm while spraying 1:4 and 1:1 solutions, respectively. At similar growth temperatures, the thickness of films from 1:4 solutions was approximately twice higher than from 1:1 solutions. The bandgap of the films grown at Ts270°C from 1:4, 1:2 and 1:1 solutions was 2.1, 2.2 and 2.6 eV, respectively. According to XRD, films obtained at Ts370C from 1:4 solutions still contained of SnS as the main phase with addition of SnS2 as a secondary phase, while films from 1:1 solutions were composed of SnS2 and SnO2 phases. Characterisation of the films by XPS and EDX techniques is in progress. In this study we showed that the precursors molar ratio in the solution is an important parameter controlling the properties of sprayed Sn-S films.
Authors : M.V. Yakushev1,2; P. Maiello3; T. Raadik4; M.J Shaw1; P.R. Edwards1; J. Krustok4; A.V. Mudryi1,4; I. Forbes3; and R.W. Martin1
Affiliations : 1Department of Physics, SUPA, University of Strathclyde, Glasgow, G4 0NG, UK; 2Academy of Science of Russia and URFU, Ekaterinburg, Russia; 3Northumbria Photovoltaics Applications Centre, Northumbria University, Newcastle upon Tyne, UK; 4Tallinn University of Technology, Tallinn, Estonia; 5Scientific-Practical Material Research Centre of the National Academy of Science of Belarus, Minsk, Belarus
Resume : Cu3BiS3 is a semiconductor with high potential for the absorber layer in sustainable thin film solar cells. Thin films of p-type material were synthesised by heating metallic Cu-Bi precursors with a layer of thermo-evaporated S in a tube furnace. The metal multilayer precursors were magnetron sputtered onto Mo/glass substrates. The elemental composition, structural and electronic properties have been studied. The Raman spectrum shows four modes with the dominant peak at 292 cm-1. Photoreflectance spectra demonstrate two band gaps EgX and EgY and their evolution with temperature. These bands are associated with the X and Y valence sub-bands split due to the influence of the crystal field and spin orbital coupling. Analysis of the temperature dependencies of the band-gaps gives values of 1.24 and 1.53 eV for the X and Y band gaps at 0 K as well as the average phonon energy. Low temperature photoluminescence spectra reveal two broad emission bands at 0.84 and 0.99 eV, which simultaneously quench with anactivation energy of 40 meV. The photocurrent excitation measurements demonstrate a photoresponse suggesting a direct allowed nature of the X band gap.
Authors : A.I. Baranov, A.S. Gudovskikh, K.S. Zelentsov, E.V. Nikitina, A.Yu. Egorov
Affiliations : St Petersburg Academic University-Nanotechnology Research and Education Centre of Russian Academy of Sciences, Hlopina str. 8/3, 194021, St.-Petersburg, Russia
Resume : GaPNAs with concentration of nitrogen above 0.5% is a direct band material, which can be lattice-matched with silicon wafers in range of bandgaps (1.5-2.0 eV). According to theoretical estimations lattice-matched GaPNAs/Si triple-junction solar cell efficiency can reach 51.2%. However GaPNAs is relatively new material and its electrical properties are not studied enough. In this paper we present the defect study of GaPNAs layers grown on GaP and Si wafers. Silicon doped n-GaPNAs and undoped GaPNAs layers were studied by admittance spectroscopy in p-n and p-i-n solar cell heterostructures, respectively. For doped n-GaP layer on GaP substrate point defect with activation energy of Ea=0.22 eV(T1) was found. It is attributed with SiGa+VP system similar to GaP:N material. For undoped GaPNAs on GaP substrate two types of defects with Ea=0.18 eV(B) and Ea=0.24 eV(B2) were obtained. Defect B2 disappeared after annealing at 500 °C, defect with Ea=0.18 eV(B) was not changed, while another defect with Ea=0.31 eV(C1) was detected. Also we studied undoped GaPNAs layers in double-junction solar cells on Si wafer. In result, only defects with Ea=0.24 eV were detected, but after annealing at 500 °C no defect was detected with admittance spectroscopy. Further defect study is expected using DLTS technique.
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Light management III : T. Gregorkiewicz
Authors : Kylie Catchpole
Affiliations : Australian National University
Resume : Light trapping is of fundamental importance in many types of solar cells to allow maximum efficiencies, and hence lowest costs, to be reached. We show that that light trapping can lead to substantial efficiency increases using rear surface scattering, near-field enhancement and for tandem solar cells. A doubling of the photocurrent due to light trapping is demonstrated by the combination of silver nanoparticles with a highly reflective back scatterer on the rear of a silicon thin film solar cell. We also propose a planar ultra-thin absorber concept exploiting plasmonic resonance absorption enhancement. We calculate a maximum absorption of 90% for TM polarized normally incident light in a 5 nm thin-film absorber with a single-pass absorption of only 1.7%. Broadband and wide-angle absorption is demonstrated. Tandem solar cells based on crystalline silicon present a practical route toward low-cost cells with efficiencies above 30%. We evaluate inorganic thin-film top cells in a tandem stack with a high-efficiency c-Si bottom cell. We show when light trapping is incorporated, even relatively low quality earth-abundant semiconductor materials with luminescence efficiencies of 10-5 and diffusion lengths below 100nm are compatible with tandem cell efficiencies above 30%
Authors : Seweryn Morawiec1,2, Manuel J. Mendes1, Sergej A. Filonovich3, Tiago Mateus3, Salvatore Mirabella1, Hugo Águas3, Isabel Ferreira3, Francesca Simone2, Elvira Fortunato3, Rodrigo Martins3, Francesco Priolo1,2,4, and Isodiana Crupi1
Affiliations : 1 MATIS IMM-CNR, via S. Soﬁa 64, I-95123 Catania, Italy; 2 Dipartimento di Fisica e Astronomia, Università di Catania, via S. Soﬁa 64, I-95123 Catania, Italy; 3 CENIMAT/I3N, Departamento de Ciência dos Materiais, and CEMOP/UNINOVA, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal; 4 Scuola Superiore di Catania, Università di Catania, Via Valdisavoia 9, 95123 Catania, Italy
Resume : Plasmonic light trapping is nowadays considered a promising solution for retaining high efficiency in thin film silicon solar cells while reducing their volume of semiconductor material. However, to produce significant efficiency enhancement, it is essential to achieve broadband light scattering from subwavelength metallic nanoparticles (NPs) together with suppression of the parasitic absorption in the particles. Here we study the performance of a-Si:H solar cells, with a substrate n-i-p configuration, using self-assembled silver NPs incorporated in the cells rear contact, forming a plasmonic back reflector (PBR). The optical properties of the PBRs are optimized according to the morphology of the nanostructures, which can be tuned by the parameters of the solid state dewetting process . A broadband photocurrent enhancement of 22.3% is achieved and attributed to both the plasmon-assisted light scattering from the NPs and the front surface texture originated from the conformal growth of the cell material over the particles. Based on the analysis of more than 40 cells built on distinct PBRs we demonstrate a linear relation between the PBRs diffuse reflection and the photocurrent enhancement in the a-Si:H light trapping window (600800 nm), which reaches values as high as 60%. Additionally, remarkably high values of Jsc and Voc are achieved, relative to those reported in the literature for the same type of devices.  S. Morawiec et. al., Nanotechnology, 24 (2013) 26560
Authors : .J.N. Hart, M. Cutini, N. L. Allan
Affiliations : School of Materials Science and Engineering, UNSW Australia, UNSW, NSW, Australia; School of Chemistry, University of Bristol, Bristol, United Kingdom; School of Chemistry, University of Bristol, Bristol, United Kingdom
Resume : Many potential semiconductors for photocatalysis of water splitting have band gaps that are too large for absorption of visible light and hence give low efficiencies under sunlight. Two such examples are ZnO and ZnS. Solid solution formation is one approach to reducing band gaps and enhancing visible-light absorption. For example, it has been shown that GaN-ZnO solid solutions are promising visible-light photocatalysts . In this work, we show based on density functional theory calculations that both ZnO-AlN and ZnS-GaP solid solutions are promising photocatalysts. The solid solutions have small energies of mixing from the two constituents (lower than ZnO-GaN in the case of ZnO-AlN). The solid solutions have tunable band gaps that depend on both composition and atomic ordering. Addition of only a small amount of AlN to ZnO and GaP to ZnS reduces the band gap into the correct energy range for absorption and emission of visible light and close to the optimum for photocatalysis of water splitting under sunlight . Significantly, the band gaps of the solid solutions are smaller than those of either constituent on their own. Furthermore, addition of a small amount of ZnS to GaP produces a direct band gap semiconductor, in contrast to pure GaP which has an indirect band gap, and this should therefore increase the efficiency of light absorption.  K. Maeda, et al., Nature, 440 (2006) 295.  J. N. Hart, N. L. Allan, Advanced Materials, 25 (2013) 2989.
Advanced concepts II : T. Buonassisi
Authors : Susanne Siebentritt
Affiliations : University of Luxembourg
Resume : Kesterites (Cu2ZnSn(S,Se)4) are compound semiconductors which do not make use of any rare metals, have band gaps suitable for the use in solar cells and a crystal structure similar to the successful Cu(In,Ga)Se2 compounds. Therefore theses materials are worldwide intensely studied as solar cell absorbers. Efficiencies up to 12% have been reached by IBM, and around 10% by a number of labs worldwide. However, to be competitive the efficiencies have to improved to a level comparable to CIGS and CdTe. The presentation will summarise the current state of the art of kesterite solar cells and the challenges on the path to higher efficiencies.
Authors : Antonio Abate, Henry J. Snaith
Affiliations : Clarendon Laboratory, Department of Physics, University of Oxford, Parks Road, Oxford, OX1 3PU, United Kingdom
Resume : Organometal trihalide perovskites, such as CH3NH3 Pb X3 (X = I-, Br-, Cl-), are attracting growing interest to prepare low cost solar cells that are capable of converting sun light to electricity at the highest efficiencies. Despite negligible effort on enhancing materials purity or passivation of surfaces, high efficiencies have already been achieved. However, we show that trap states at the perovskite surface generate charge accumulation and consequent recombination losses in working solar cells. We identify that under-coordinated iodine ions within the perovskite structure are responsible and establish a supramolecular strategy to successfully passivate these sites. Following this strategy we demonstrate solar cells with stabilized power conversion efficiency over 15% under simulated full sun light.
Authors : L. Dumont*, P. Benzo*, J. Cardin*, C. Labbé*, I-S YuϮ, F. Gourbilleau*
Affiliations : * CIMAP CNRS/CEA/ENSICAEN/UCBN, 6 Boulevard Maréchal Juin, 14050 Caen Cedex 4, France Ϯ Department of Materials Science and Engineering, National Dong Hwa University, Da Hsueh Rd, Shoufeng, Hualien 97401,Taiwan
Resume : Management of the solar spectrum is one of the key issues to improve the solar cell efficiency. Among the existing solutions, one consists in a layer absorbing energetic UV photons and converting them into two IR photons that match the absorption band of classical Si solar cells. This mechanism called frequency- or down- conversion process may increase the cell efficiency by almost doubling the number of IR photons absorbed as well as reducing the thermalisation losses. In this paper, we propose an innovative approach using a co-doped SiN matrix that presents the advantages of being compatible with Si solar cells fabrication process. Layers were deposited on Si substrates by reactive magnetron co-sputtering of Si, Tb, and Yb targets under a nitrogen-rich plasma. The microstructure of the grown layers were investigated using TEM, EDX and Glow Discharge Mass Spectroscopy while their optical properties and excitation mechanisms were analysed by means of ellipsometry, photoluminescence, and photoluminescence excitation spectroscopies. The quantum efficiency of the system has been studied through time-resolved photoluminescence experiments. Finally, optimized layers have been deposited on a solar cell to measure the carrier lifetime as well as the external quantum efficiency under AMG 1.5 illumination.
Authors : Umme Aiman Mahmood, Keivan Sedhigi, Benjamin Riedmueller, Ulrich Herr
Affiliations : Institute for Micro- and Nanomaterials, Ulm University, D-89081 Ulm, Germany
Resume : ZnO is a promising material for photovoltaic and photocatalytic applications since its band gap is in the near UV. It is already applied in large quantities for UV protection. We report about a study of nanocrystalline commercial ZnO which has been subjected to an annealing treatment in reducing atmosphere. Intensive blue-green luminescence is found after that treatment, which is attributed to defects. The material has been characterized in detail using photoluminescence spectroscopy, and subsequently been combined with a commercial Si solar cell. A quantitative model has been developed to compare the experimental results with theoretical predictions. For this purpose, we have determined the photoluminescence quantum efficiency of the powders. In addition, we have determined the transmission spectrum of glass slides coated with PDMS films containing varying amounts of the luminescent powder. By combining the measured data with spectrally resolved photocurrent measurements of plain and ZnO-covered Si cells, we are able to quantitatively model the behavior of the system. I t is found that under the present conditions the strong back-scattering from the ZnO layer leads to a reduction of the overall efficiency of the system. However, in the UV region we find an enhancement of the photocurrent. The model allows us to predict the conditions for improving cell efficiency by this method and amount of overall increase of the cell efficiency which can be reached.
Authors : I. Crupi 1, S. Morawiec 1, M. Müller 2, K. Ganzerová 2, J. Holovský 2, A. Vetushka 2, M. Ledinský 2, M. J. Mendes 1, S. Mirabella 1, F. Priolo 1,3, A. Fejfar 2
Affiliations : 1) MATIS CNR-IMM and Dipartimento di Fisica e Astronomia, Università di Catania, via S. Soﬁa 64, 95123 Catania, Italy 2) Institute of Physics, Academy of Sciences of the Czech Republic, Cukrovarnicka 10, Prague, Czech Republic 3) Scuola Superiore di Catania, Università di Catania, via Valdisavoia 9, 95123 Catania, Italy
Resume : Metallic nanoparticles (MNPs) are gaining importance for application in silicon solar cells as an efficient method to enhance the light absorption in the active layer due to their pronounced scattering properties at the surface plasmon resonance. It has been recently demonstrated that the preferential location for the MNPs in solar cells is in the rear contact, embedded in the transparent conductive oxide (TCO) layer which separates the back mirror from the absorber, forming what is known as a plasmonic back reflector (PBR) . Here we study how light trapping in silicon thin films is influenced by the distinct layers composing the PBR structures. The PBRs were fabricated by sequential depositions of silver (Ag) and aluminium doped zinc oxide (AZO) and the MNPs were formed in a self-assembly manner by solid-state dewetting of thin Ag films. A 1 m thick hydrogenated nanocrystalline silicon (nc-Si:H) layer was deposited on top of the PBR structures at different stages of completion. The obtained surface topographies were investigated by atomic force microscopy (AFM) and scanning electron microscopy (SEM). The light trapping induced by the MNPs is evidenced by the enhancement of the Raman signal (acquired with weakly absorbed excitation laser excitation at 785 nm) in the nc-Si:H film and spectrally analyzed by diffused and total reflectance in the UV-Vis-NIR wavelength range.  S. Morawiec et. al., Nanotechnology, 24 (2013) 265601
Authors : Dongsheng Li, Meng Yuan, and Deren Yang
Affiliations : State Key Laboratory of Silicon Materials and Department of Materials Science and Engineering, Zhejiang University
Resume : Changing the size and morphology of silver nanoparticles leads to kinds of distinctive scattering properties and localized surface plasmon resonant effect, which has demonstrated a huge potential in photovoltaic applications. We have studied lumpy silver particles as rear located strengthen materials for silicon thin-film solar cell. Though theoretical simulations, we find that the large-size lumpy silver particle has a more advantageous property of scattering incident light back than the spherical particle in a broad wavelength range. This kind of large-size silver particles can be used as rear-position strengthening materials for silicon thin-film solar cells. We demonstrate that when the Ag particles' coverage density is 10%, the light absorption enhancement is optimal.
Authors : A. Čiegis, . Mekinis, A. Vasiliauskas, K. lapikas, R. Gudaitis, S. Tamulevičius
Affiliations : Institute of Materials Science of Kaunas University of Technology, Savanorių 271, 50131 Kaunas, Lithuania
Resume : In last few years group I metal (Ag, Au, Cu) nanoparticle based plasmonic layers received considerable interest due to the increased light trapping in the underlying semiconducting absorber layer. Environmental stability of these nanoparticles can be increased by coating them with ultrathin protective dielectric (or semiconductor) layer. In addition such layer can be used for additional control of the position of the surface plasmon resonance by changing refractive index of the media surrounding nanoparticle. Along with the coated core shell nanoparticles, metal and dielectric nanocomposites can be used for such a purpose. Diamond like carbon is prospective matrix material of such a nanocomposite due to the high optical transmittance, high hardness, corrosion resistance as well as possibility control refractive index and electrical properties in wide range. In present study diamond like carbon films containing Ag nanoclusters (DLC:Ag) were deposited by reactive unbalanced magnetron sputtering. Optical properties of synthesized nanocomposite films were investigated in 180-1100 nm range. Structure of the films was studied by Raman spectroscopy, x-ray diffractometry and atomic force microscopy. Effects of the annealing on optical and electrical properties as well as structure of DLC:Ag films were studied. Significant changes of the position, shape and width of the plasmon resonance peak were observed.
Authors : Kudryashov Dmitry, Gudovskikh Alexander, Morozov Ivan
Affiliations : SPbAU RAS - St. Petersburg Academic University
Resume : There are currently many investigations in the field of photovoltaics making current technology solar cells more efficient and cheaper. Multijunction solar cells based on GaPNAs/Si lattice matched heterostructures are very promising. However GaPNAs grown to-date appears to have very short diffusion lengths. Thus it is very hard to provide current matching of GaPNAs- and Si-subcells. This paper presents a 2D-simulation of new design two junction nanowire (NW) solar cells based on GaPNAs/Si lattice matched heterostructures. The effects of NW's height and distance on the electrical characteristics were investigated. The efficiency of NW solar cells increases with increasing of NW height due to collecting more carriers and reaches a plateau at 4 microns. Decreasing the distance between NWs leads to increase of solar cells efficiency due to increase of current spreading. Also it was shown that NW solar cells with lower thickness of GaPNAs layer are able to reach higher efficiency compared to its flat design.
Authors : Aijaz Ahmad, Akshay Balgarkashi, S. Sengupta and S.Chakrabarti*
Affiliations : Department of Electrical Engineering, Indian Institute of Technology Bombay, Mumbai-400076, Maharashtra, India
Resume : A comprehensive study on the impact of GaAs and InGaAs capping layers on the carrier transport in quantum dots (QDs) for infrared detectors and solar cell application is carried out. The effect of variation in dot monolayer coverage has been studied. For GaAs capped QDs, the activation energies obtained from temperature-dependent I-V measurements is found to increase with monolayer(ML) coverage upto 3.0 ML. This can be attributed to increase in width of the potential well and corresponding increase in bound-to-continuum energy gap. The spectral peaks were observed to be decreasing from 8.37μm to 7.38μm which affirms the above trend. For InGaAs capped QDs, the corresponding activation energies with increase in ML coverage were found to be much higher than those for the GaAs capped structures. Due to higher lattice mismatch in GaAs capped QDs, strain enhancement at apex of dots leads to significant In adatom migration reducing the height of QDs. InGaAs capping having smaller lattice mismatch with InAs QDs, the In atom migration is hindered and the QD size remains significantly unchanged which justifies the higher activation energies in these structures. The spectral peaks obtained here at lower wavelengths complements the above results. A decreasing trend in activation energy is observed for InGaAs capped structures. This can be ascribed to the increase in defect states arising out of higher strain induced due to higher ML coverage. Riber, France and DST, India is acknowledged.
Authors : Akshay Balgarkashi, Aijaz Ahmad, S. Sengupta and S.Chakrabarti*
Affiliations : Department of Electrical Engineering, Indian Institute of Technology Bombay, Mumbai-400076, Maharashtra, India
Resume : We have studied the effect of post-growth rapid thermal annealing (RTA) on the stability of InGaAs capped quantum dots (QDs) for infrared photodetectors and solar cell applications. For comparison, GaAs capped quantum dots heterostructures have also been investigated. QDs with different monolayer (ML) coverage were grown using molecular beam epitaxy (MBE). It is observed that with increasing monolayer coverage the shift in the photoluminescence (PL) peak with RTA is suppressed. This is attributed to higher stability attained due to larger size of dots. Higher ML coverage also prevents the interdiffusion of In and Ga, which results in a reduced blueshift even at temperatures upto 7500C. A temperature of 8000C is, however, sufficient to elevate the interdiffusion enhancing the blueshift. The activation energies calculated from temperature-dependent PL measurements show minor variation with annealing upto 7500C for higher monolayer dots which is in excellent agreement with the above discussion. With the InGaAs capping, the PL peak shift is further reduced in contrast to the GaAs capped dots. The capping layer prevents In interdiffusion to a higher extent consequently reducing shift in the PL peaks. The restrained shift in luminescence even at higher annealing temperatures indicates good thermal stability which is preferable for QD device applications. Riber, France and DST, India are acknowledged.
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Authors : O. Durand1, S. Almosni1, P. Rale2,3,4, J. Rodi?re2,3,4, Y. PingWang1, A. Letoublon1, H. Folliot1, A. Le Corre1, C. Cornet1, J. Even1, A. Ponchet5, L. Lombez2,3,4, J.-F. Guillemoles2,3,4
Affiliations : 1Universite Europeenne de Bretagne, INSA, FOTON-OHM, UMR 6082, 35708 Rennes, France. 2EDF R&D, Institute of Research and Development on Photovoltaic Energy (IRDEP), Chatou, France 3CNRS, IRDEP, UMR 7174, 78401 Chatou, France 4Chimie ParisTech, IRDEP, 75005 Paris, France 5CEMES-CNRS, Universit? de Toulouse, 29 rue Jeanne Marvig, BP94347 Toulouse cedex 04, France
Resume : By their potential to overcome the Shockley?Queisser limit of a single junction solar cell, the third generation solar cells would enable a lower material usage and/or installation cost share. To this end, a solution consists in the MBE-epitaxial growth of III-V heterostructures towards the elaboration of innovative concepts of solar cells. First, this paper describes our approach to obtain high efficiencies tandem solar cells on Si substrates. GaAsPN diluted nitride alloy is studied as the top junction material due to its perfect lattice matching with the Si substrate and due to its ideal bandgap energy for current matching with the Si bottom cell. Through structural analyses (XRD and TEM), we have developed a growth strategy which dramatically reduces the III-V/Si defects interface. An optimisation of the GaAsPN absorber using a post-growth annealing treatment and first results on p-i-n junctions grown onto GaP substrates are also reported. A second approach consists in the study of InGaAsP/InP MQWs, towards the elaboration of Hot-Carrier Solar Cells, which aim to reduce both the non-absorption of the incoming light (small bandgap) and the thermalization process (electron-phonons interactions). Using different PL techniques, the quasi Fermi level splitting Δμ is estimated as a function of the excitation power. High value of Δμ is found as well as high carrier temperature. These results are compared to electrical measurements. Support: ANR project MENHIRS 2011-PRGE-007-01.
Authors : Francesco Pastorelli1,2, Sebastien Bidault3, Pablo Romero Gomez2, Rafael Betancur4, Alberto Martinez-Otero2, Jordi Martorell2,5, Nicolas Bonod1
Affiliations : 1 Institut Fresnel, Aix-Marseille Université, CNRS, Domaine Universitaire de St Jérôme, Marseille 13397, France; 2 ICFO-Institut de Ciències Fotòniques, Mediterranean Technology Park, Castelldefels (Barcelona) 08860, Spain; 3 Institut Langevin, CNRS, ESPCI Paris Tech, 1 rue Jussieu, Paris 75005, France; 4 Centro de investigación, innovación y desarrollo de materiales - CIDEMAT Universidad de Antioquia, Medellín Colombia; 5 Departament de Fisica i Enginyeria Nuclear, Universitat Politecnica de Catalunya, Terrassa 08222, Spain;
Resume : The goal of our work is to increase light-matter interactions in organic photovoltaics. Our first design involves self-assembled gap antennas with interparticle distances of a few nanometers controlled by a molecular cross-linker (dithiothreitol). The spacing molecules ensure a minimum distance that plays a fundamental role in the formation of intensity hot spots in the nanogap as well as large and red-shifted scattering peaks. This device exhibited an efficiency 14% higher than the reference one showing a relevant enhancement in the red part of the EQE measurements. As second design we build up a photonic crystal and a metal cavity around a transparent organic solar cell. We enclosed the active material in between two metal electrodes forming an optical cavity designed to optimize photon trapping inside the cell. To increase near IR light trapping, while maintaining transparency in the visible, an anti-reflection coating was deposited on top of the front metal contact while a non-periodic multi-layer was inserted in between the back metal contact and the substrate. The cavity configuration was designed specifically for the cell architecture used and we achieved semi-transparent cells with 5.3% PCE, corresponding to 90% the PCE of the opaque cell.
Advanced characterization : M. Topic
Authors : Andreas Gerber
Affiliations : IEK5 Forschungscentrum Julich, Germany
Resume : tba
Authors : Jean-Paul Kleider, José Alvarez, Aurore Brézard-Oudot, Marie-Estelle Gueunier-Farret, Olga Maslova
Affiliations : LGEP; CNRS UMR8507; SUPELEC ; Univ Paris-Sud ; UPMC Univ Paris 06 ; 11 rue Joliot-Curie, Plateau de Moulon, F-91192 Gif-sur-Yvette Cedex, France
Resume : Capacitance techniques have been widely applied to crystalline semiconductors homojunctions in order to study defect levels or doping profiles. The theory was also extended to crystalline semiconductors heterojunctions, where for instance it was proposed how to use the bias dependence to determine band offsets. In amorphous semiconductors, the theory was modified in order to take into account the predominant role of the continuum of defects in the band gap. In this presentation, we will briefly review the basic concepts of capacitance techniques and the peculiarities related to amorphous semiconductors, paying tribute to J. D. Cohen and to his pioneering work. Then, we will extend the discussion to very high efficiency silicon heterojunction (SiHET) solar cells that gather the ingredients of both amorphous semiconductors and heterojunctions. By presenting both modeling and experimental results, we demonstrate that the conventional theory of heterojunction capacitance based on the depletion approximation, cannot reproduce the measured temperature and bias dependence, and leads to strong errors if applied to the determination of band offsets. We show that this is not related to the amorphous nature of a-Si:H, but to the existence of a strongly inverted c-Si surface layer. SiHET solar cells thus offer a unique opportunity to reconsider the theory of junction capacitance and to explain how, in this case, capacitance measurements can bring insight into the device physics.
Authors : W. Soppe, M. D?renk?mper, D. Zhang, C. van der Werf and R. Schropp
Affiliations : ECN-Solliance, High Tech Campus 5, 5656 AE Eindhoven, the Netherlands
Resume : Micro-crystalline silicon (uc-Si) has already been discovered in 1968 by Veprek and Maracek, but it is still an interesting and promising material for solar cells, because of the low processing temperatures and the abundance of the feedstock material. uc-Si, however, also has some drawbacks, and one of them is that the material consists of small grains, requiring excellent passivation of the grain boundaries. The latter is accomplished by tuning the conditions of the PECVD process such that the material contains a certain fraction of hydrogenated amorphous silicon (a-Si:H). a-Si:H is capable of passivating the grain boundaries of uc-Si very well, but in practice an over-proportional fraction of around 30 % a-Si:H in the layers is required to obtain reasonable open-circuit voltages (Voc). This high fraction of a-Si:H not only leads to a non-negligible light induced degradation of the solar cells, but also to a certain loss in short-circuit current (Jsc): charge carriers generated in the a-Si:H tissue have little chance to contribute to the current of the solar cell. In order to reduce the a-Si:H fraction in the uc-Si layers, but maintaining the grain boundary passivation, we have conducted post-deposition hydrogenation experiments. We made uc-Si layers with a relatively high crystalline fraction and subsequently hydrogenated these layers by two different methods: microwave (MW)-PECVD and hot wire(HW)-CVD. Both methods resulted in a significant improvement of the ratio of the photo to dark conductivity. Before hydrogenation this ratio was about 50 and after hydrogenation this ratio improved to a value of 400 for HW-CVD and to 775 for MW-PECVD. Cell fabrication is ongoing and the results of these experiments will be reported at the conference.
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Authors : A. Slaoui1, S. Roques1, O. Lunder2, A. Ulyashin3, O. Mahboub4, Z. Sekkat4,
Affiliations : 1 ICube, University of Strasbourg-CNRS, 23 rue du Loess, B.P.20, F-67037 Strasbourg, France 2 SINTEF, Material and Chemistry, Høgskoleringen 5, NO- 034 ,Trondheim, Norway 3 SINTEF, Material and Chemistry, Forskningsveien 1, NO-0314, Oslo, Norway 4 MAScIR, MAScIR, Rue Mohamed Al Jazouli Madinat Al Irfane Rabat 10 100 Morocco
Resume : Thin film silicon solar cells on low cost foreign substrates could be attractive for highly efficient and low cost production of photovoltaic electricity. This work aims at the synthesis of high-quality continuous polycrystalline silicon (pc-Si) layers on flexible aluminium based substrates using direct crystallization (DC) or through the aluminium induced crystallization (AIC) procès of amorphous silicon. Pure aluminium (Al) or anodic alumina (Al2O3/Al structures) were used as substrates. Amorphous silicon films with thicknesses ranging from 200 to 1000 nm were deposited by ECR-PECVD and PVD on the substrates The direct crystallization using a conventional furnace was carried out at temperatures ranging from 450 to 550°C and durations from 1h to 5h. Rapid thermal annealing using halogen lamps at températures of 700-800°C and durations lower than 15min were attempted. For the AIC process, a 200nm aluminium layer was first evaporated on the substrates prior to depositing amorphous silicon. Similar thermal treatments than above were performed. The resulting crystallized layers were characterized by Raman, UV reflectance spectroscopy as well as by XRD analysis. It was found that the complete crystallisation of the a-Si layers is strongly dependent on the initial layer thickness and the thermal budget. On the other hand, the as-grown AIC pc-Si films were found to be continuous and densely packed without amorphous phase. The migration of impurities from the substrate to the pc- Si films were studied systematically in terms of chemical and stress level analysis, which are the important aspects to be considered when metallic foils are used as substrates. Complementary experiments and analysis are currently carried out to improve the quality of DC and AIC pc-Si films.
Nanostructures I : H. Stiebig
Authors : A. Fejfar, M. Hývl, A. Vetushka, M. Ledinský, S. Misra, M. Foldyna, Linwei Yu, P. Roca i Cabarrocas
Affiliations : a) Institute of Physics, Academy of Sciences of the Czech Republic, Cukrovarnická 10, 162 00 Prague 6, Czech Republic b) Laboratoire de Physique des Interfaces et des Couches Minces (LPICM), Ecole Polytechnique, CNRS, F-91128 Palaiseau, France
Resume : Radial junctions based on silicon nanowires (SiNWs) are an example of nanostructured design of solar cells with excellent light trapping and efficient photogenerated charge collection. A single pump-down process used to prepare a randomly grown matrix of SiNWs and conformal p-i-n radial junctions led to cells with efficiencies over 8% . Considerable influence of irregularities in SiNWs lengths, orientations, shapes and mutual interactions on the photovoltaic action can be expected. Direct measurement of these effects requires microscopic measurements of photoresponse. This is possible using atomic force microscopy (AFM) with conductive cantilever which serves as a contact to individual radial junctions . At the same time the cantilever can measure the local nanomechanical properties, including local stiffness of the wires, which can only sustain contact forces up to ~1 nN. Resulting conductivity maps show substantial variation of the local electronic properties. The AFM tip cannot reach deeper into the SiNWs matrix and correlation with scanning electron microscopy of the identical nanowires was sought in order to identify the reason for conductivity variations. The results are discussed in terms of random photodiode arrays connected in parallel with overall performance limited by weak diodes.  S. Misra et al., Sol. Energy Mat. Sol Cells. 118 (2013) 9095.  A. Fejfar et al., Sol. Energy Mat. Sol. Cells. (2013) 228234.
Authors : S. Gaiaschi, E.V. Johnson, M-E. Gueunier-Farret, C. Longeaud, P. Chapon, J-P. Kleider
Affiliations : LGEPCNRS/SUPELEC, 11 rue Joliot Curie - Plateau de Moulon, 91192 Gif sur Yvette, FRANCE LPICM-CNRS, Ecole Polytechnique, 91128 Palaiseau, FRANCE; LPICM-CNRS, Ecole Polytechnique, 91128 Palaiseau, FRANCE; LGEPCNRS/SUPELEC, 11 rue Joliot Curie - Plateau de Moulon, 91192 Gif sur Yvette, FRANCE; LGEPCNRS/SUPELEC, 11 rue Joliot Curie - Plateau de Moulon, 91192 Gif sur Yvette, FRANCE; HORIBA Jobin Yvon, 16-18, rue du Canal, 91165 Longjumeau CEDEX, FRANCE; LGEPCNRS/SUPELEC, 11 rue Joliot Curie - Plateau de Moulon, 91192 Gif sur Yvette, FRANCE
Resume : Hydrogenated microcrystalline silicon-carbon alloys (µc-Si1-xCx:H) can be expected to be an alternative to the unstable hydrogenated amorphous silicon (a-Si:H) in tandem solar cells. These thin films can be obtained by standard Radio Frequency Plasma Enhanced Chemical Vapour Deposition (RF-PECVD). Their energy gap depends on the carbon incorporation and values close to the band gap value of a-Si:H can be obtained. Actually, according to the binary Si-C phase diagram, silicon carbide is the only stable compound and alloys with small carbon content are thermodynamically metastable. They are characterized by silicon crystallites embedded in an a-Si1-xCx:H matrix. We studied µc-Si1-xCx:H samples deposited by RF-PECVD at 175 °C from a silane and methane gas mixture highly diluted in hydrogen. The material quality was assessed through Raman spectroscopy, X-ray diffraction and transport property measurements, as a function of the carbon content determined from radio frequency glow discharge optical emission spectroscopy measurements. P-I-N junction photodiodes were made using these alloys as the absorber layer. The influences of RF power and carbon content on the diode characteristics under dark and on the solar cell performance were studied, finding a good agreement between material and device properties.
Nanostructures II : A. Fejfar
Authors : S. Cosentino1, S. Mirabella1, M. Miritello1, I. Crupi1, P. Liu2, A. Zaslavsky2, D. Pacifici2, A. Terrasi1
Affiliations : 1 MATIS IMM-CNR and Dipartimento di Fisica e Astronomia, Università di Catania, via S. Sofia 64, 95123 Catania, ITALY; 2 School of Engineering, Brown University, Providence, RI 02912, USA
Resume : Quantum confinement effect (QCE) can tune many properties relevant to light harvesting, such as optical bandgap, efficiency of luminescence and oscillator strength of the optical transition. Since Ge has a quite large Bohr exciton radius (24 nm), the quantum confinement regime in Ge nanostructures (NS) is relatively easy to get. In addition, Ge shows an absorption coefficient more than 10 times higher with respect to Si in the solar energy range. Thus, Ge NS are really attractive as active absorber for efficient light harvester, solar cells and novel optoelectronic devices. This work aims to show how and to what extent the QCE modifies the optical behavior of a variety of Ge NS confined in SiO2. Materials and devices, prepared by sputtering deposition of Ge-rich SiO2 thin films in single or multiple layer configuration, comprise: Ge quantum dots (QD, 2-10 nm in size) with QD-QD distance accurately fixed between 3 and 20 nm, and single Ge quantum well (2-30 nm in thickness). Light absorption spectroscopy with effective mass simulation evidenced the relationships among synthesis parameters and response to light absorption in prototypal devices, getting light detection with photoconductive gain up to 1500%. Our data fix the NS size as a critical parameters for QCE, but also demonstrate that proximity and interaction among NS play key roles in the efficiency of photon absorption process. Ref. S. Cosentino et al. Nanoscale Res. Lett. 8, 128 (2013) S. Mirabella et al. Appl. Phys. Lett. 102, 193105 (2013) S. Cosentino et al. Appl. Phys. Lett. 98, 221107 (2011)
Authors : E. G. Barbagiovanni (a), S. Cosentino (a), A. Terrasi (a), S. Mirabella (a), D. J. Lockwood (b), R. N. Costa Filho (c)
Affiliations : (a) MATIS IMM-CNR and Dipartimento di Fisica e Astronomia, Universita di Catania, Catania 95123, Italy (b) Measurement Science and Standards, National Research Council, Ottawa, Ontario K1A 0R6, Canada (c) Departamento de Fisica, Universidade Federal do Ceara, Caixa Postal 6030, Campus do Pici, 60455-760 Fortaleza, Ceara, Brazil
Resume : There is renewed interest in Ge nanostructures (NSs) for photovoltaic and optoelectronic applications due to recent fabrication advances. Here, we investigate the structural and optical properties of Ge quantum dots (QDs) formed in different matrices. High quality Ge QDs were prepared by sputtering, plasma enhanced chemical vapour deposition (PECVD) and ion implantation embedded in either a SiO2 or Si3N4 matrix . Optical absorption measurements indicate that Ge QDs in a Si3N4 matrix have a higher absorption efficiency compared to a SiO2 matrix, due to a higher areal density. On the other hand, Ge QDs embedded in the SiO2 matrix exhibit a larger increase in the optical energy gap (EG) with decreasing QD diameter (D) compared to the Si3N4 matrix. This result is theoretically modelled using a finite confinement potential as a function of the lowered inter-facial EG between Ge/Si3N4 compared to Ge/SiO2 . Regardless of the preparation method, Ge QDs exhibit a larger confinement effect compared to their Si counterpart. To explain this observation, we have developed a new theoretical model under the effective mass approximation (EMA) and a spatially dependent effective mass (SPDEM) . Within this model, we calculated a reduction in the NS effective mass (EM) from the bulk value, which increases EG. Further differences in the confinement energy due to the matrix arise as a function of the interface defect states. Furthermore, our results indicate that Ge NSs experience an additional confinement mechanism due to the dimensionally dependent EM, which modifies the dispersion relation and can be exploited for device fabrication.  S. Cosentino et al., submitted to J. Appl. Phys. (2014)  E. G. Barbagiovanni et al., accepted J. Appl. Phys. (2014)
Authors : Julie Goffard1-2, Patrice Miska*2, Davy Gérard1, Michel Vergnat2 and Jérôme Plain1
Affiliations : 1) Université de Technologie de Troyes-12 rue Marie Curie 10000 Troyes (France) 2) Institut Jean Lamour UMR CNRS 7198 Nancy Université UPV Metz , Boulevard des Aiguillettes, 54500 Vandoeuvre-les-Nancy France
Resume : The use of silicon nanocrystals (SiNC) in optoelectronic devices has risen from a decade thanks to the discovery of photoluminescence in porous silicon in 1991 . However the SiNCs exhibit a low quantum yield, which prevent from currently using them in optoelectronic devices. This problem can be bypassed by using localized surface plasmons (LSP). Indeed LSPs can modify emitters photoluminescence by changing the optical local density of states and/or increasing the local excitation field. Numerous studies have been performed with different emitters, but the analyze of LSP coupled to SiNC is scarce. Seminal studies by Biteen and coworkers [2,3] have paved the way for increasing SiNCs performances with LSPs. In these preliminary works only the SiNCs emission wavelength was coupled to LSPs. In this work, we first present a study where LSPs resonances of gold nanodisks (GNDs) are coupled to SiNCs . Then, we show results concerning the coupling of both the absorption and emission wavelengths of SiNCs to gold nanorods. In both cases, the use of an original fabrication method gives us the control all the geometrical parameters that modify the SiNCs LSPs coupling. In the first study, we use GNDs with different diameters and located at different distances to the SiNCs. We then evaluate the maximum quantum yield enhancement for the sample with the optimized geometry. The use of nanorods with different shapes and metallic materials allows us to obtain in the same structure two surface plasmon modes exhibiting different polarizations. To understand them, we studied the coupling of LSP to the SiNCs photoluminescence intensity, polarization and spatial redirection.  A. G. Cullis, L. T. Canham, Nature 1991, 353, 335338.  J. S. Biteen, D. Pacifici, N. S. Lewis, H. A. Atwater, Nano Lett. 2005, 5, 17681773.  J. S. Biteen, N. S. Lewis, H. A. Atwater, H. Mertens, A. Polman, Appl. Phys. Lett. 2006, 88, 131109.  J. Goffard, D. Gérard, P. Miska, A.-L. Baudrion, R. Deturche, and J. Plain, Scientific Reports 3, 2672 (2013).
Authors : C. Weiss, M. Schnabel, P. Löper, S. Janz
Affiliations : Fraunhofer Institute for Solar Energy Systems, Heidenhofstr. 2, 79110 Freiburg, Germany; Fraunhofer Institute for Solar Energy Systems, Heidenhofstr. 2, 79110 Freiburg, Germany; École Polytechnique Fédérale de Lausanne, Rue de la Maladière 71b, CP 526, CH-2002 Neuchâtel 2, Switzerland; Fraunhofer Institute for Solar Energy Systems, Heidenhofstr. 2, 79110 Freiburg, Germany
Resume : Please delete this abstract!
Authors : K. Kusova, P. Hapala, P. Jelinek, J. Valenta, L. Ondic, O. Cibulka and I. Pelant
Affiliations : K. Kusova; P. Hapala; P. Jelinek; L. Ondic; O. Cibulka and I. Pelant Institute of Physics ASCR, Prague, Czech Republic J. Valenta Faculty of Mathematics and Physics, Charles University, Prague, Czech Republic
Resume : Bulk silicon is notoriously known as an inefficient light emitter due to its indirect bandgap, which is usually perceived as an obstacle towards the utilization of this material in optoelectronics and bioimaging, despite the evident advantages of this approach. Some alleviation of the poor light emission performance of bulk silicon can be achieved by switching to silicon nanocrystals, whose light-emission efficiency can be reasonably high, but whose photon emission rates are still usually low due to the remaining character of the indirect badstructure. In this contribution, we show that the bandstructure of silicon nanocrystals can be strain-engineered to achieve fundamental direct bandgap, leading to a 10 000x increase in the photon emission rate. We will first briefly discuss the validity of the bandstructure concept on the nanoscale  and then present both theoretical and experimental evidence that the joint action of quantum confinement and tensile strain induced by surface capping lead to indirect-to-direct bandgap cross-over .  P. Hapala et al.: `Theoretical analysis of electronic bandstructure of 2-to-3-nm Si nanocrystals,' Phys. Rev. B 2013, 87(19) 195420.  K. Kusova et al.: `Direct Bandgap Silicon: Tensile-Strained Silicon Nanocrystals,' Adv. Mater. Int. 2014, in press, DOI: 10.1002/admi.201300042.
Authors : K. Kacha1, F. Djeffal1, T. Bentrcia2 and I. Berbezier3
Affiliations : 1) LEA, Department of Electronics, University of Batna, Batna 05000, Algeria. 2) Department of Physics, University of Batna,Batna 05000, Algeria. 3) IM2NP Aix-Marseille Universités, UMR CNRS n°7334, Faculté des Sciences St-Jérôme - Case 142, 13397 Marseille Cedex 20 France. E-mail: email@example.com, firstname.lastname@example.org, Tel/Fax: 0021333805494
Resume : Nowadays, solar cell technology attracts much attention in reliable and high efficiency photovoltaic applications. Thin-film SiGe solar cells have important advantages such as high photocurrent and their compatibility with the process developed for pure Si cells. In order to improve the electrical efficiency performances of the conventional SiGe solar cell, we have introduced a new multi-trench technique. In the proposed method, the multi-trench is created in the silicon layer and filled with n-type doped SiGe. The p-type trenches under the SiGe layer improve the electrical performance of the proposed design. By 2-D numerical simulation, we have investigated the electrical performances of the proposed design and compared with it a conventional thin-film SiGe solar cell. The proposed accurate numerical models have been used as objective function to optimize the electrical performance of the SiGe-based solar cell. The obtained results show that the proposed design can be considered as a potential candidate for high performance photovoltaic applications.
Authors : Jeeyoung Lee, Jisuk Park, Junyeon Heo, Hyungseok Min, Myeongkyu Lee
Affiliations : Department of Materials Science and Engineering, Yonsei University, Seoul, Korea
Resume : The light harvest in a solar cell can be greatly improved if the sunlight is made obliquely incident into the absorption layer. Not only enhancing light absorption by increasing the optical path length, this configuration is also favorable for the extraction of charges as they are generated closer to the collecting electrode. Despite many advantages, the deflection of incident light has not been employed for dye-sensitized solar cell (DSSC) because of the inherent difficulty of incorporating a relevant structure into DSSC. Here we report that a refractive-index grating can be embedded into the TiO2 electrode and this architecture highly increases light absorption in DSSC by diffracting the incident light. A thin TiO2 film spin-coated on conducting glass was molded by imprinting and treated with a higher TiCl4 concentration than the later-coated thicker TiO2 absorption layer. Due to the refractive index difference between two layers, the incident light was widely diffracted, giving diffraction efficiencies over 70% in the visible range. As a result, the IPCE and current density of the cells were much improved. This facile approach can be effectively utilized to increase the light harvest of DSSC and its energy conversion efficiency.
Authors : Rosa Chierchia, Alberto Mittiga, Enrico Salza, Luca Serenelli, Mario Tucci
Affiliations : Enea Italian National Agency for New Technologies, Energy and Sustainable Economic Development
Resume : The Al doped Zinc Oxide (ZnO:Al) is a transparent and conductive oxide used in alternative to the Indium Tin Oxide (ITO) as contact and antireflection layer in amorphous silicon heterojunction and chalcogenide based solar cell. Generally good film quality can be obtained by low cost magnetron sputtering, even though it is difficult to obtain a resistivity below 10-3 ohm cm when grown at the temperature below 200°C commonly used in solar cells manufacturing to avoid thermal stress. Following the recent theoretical and experimental confirmations of the Hydrogen doping property in Al:ZnO films, in this work the Hydrogen effect on the Al:ZnO film as optical, electrical and structural properties is investigated. Two different growth techniques have been considered: DC and Pulsed DC magnetron sputtering. A clear improvement in the electrical properties of the layer has been noted in particular when films have been grown by DC pulsed magnetron sputtering. Resistivity of 6.7x10-4 ohm cm, carrier concentration of 5.4x1020 cm-3 and electrical mobility of 17 cm2/Vs have been obtained even on film thinner than 100nm. While the low mobility value suggests that the film grain size still needs to be increased, XRD measurements show better quality material. Finally the stability of the hydrogen doped ZnO:Al film properties is evaluated when the film is subjected to thermal stress as in solar cell manufacturing applications.
Authors : S. Boscarino1,2, G. Torrisi3, I. Crupi2, A. Alberti4, S. Mirabella2, F. Ruffino1,2, F. Simone1, A. Terrasi1,2
Affiliations : 1Dipartimento di Fisica e Astronomia, Università di Catania, via S. Sofia 64, 95123 Catania, Italy; 2MATIS IMM-CNR, via S. Sofia 64, 95123 Catania, Italy; 3Distretto Tecnologico Sicilia Micro e Nanosistemi, via Strada VIII 5, 95121 Catania, Italy; 4CNR-IMM, via Strada VIII 5, 95121 Catania, Italy
Resume : The combination of low electrical resistivity with high optical transparency make Al-doped ZnO (AZO) films widely used as transparent electrodes in solar cells. In order to improve conductivity and transmittance of these films, annealing treatments are normally employed during or after the growth process. We report on the increase of the conductivity of ultra-thin AZO films upon irradiation with high energy O2+ ions. This improvement is linked to the modifications of both optical and structural properties of the material. The AZO thin films (60 nm) have been grown by RF-magnetron sputtering at room temperature, then irradiated with 350 MeV passing O2+ ions at fluencies of 3x1015, 1x1016 and 3x1016 ions/cm2. By X-ray diffraction analysis, all films have shown hexagonal wurtzite structure, improving the crystalline quality as a function of the ions fluencies. AFM analysis for the surface morphology was also performed. In particular, we observed an enhancement of the electrical conductivity of more than two orders of magnitude, along with grain size increasing and in-plane strain change from compressive to tensile values. The optical bandgap increases with the ion doses, partly due to the Burstein-Moss effect connected with an improved crystalline quality. Annealing effects, up to 400°C, are also reported and compared with the irradiation ones. Our results indicate that the structural quality and conductivity of the AZO thin films can be improved by specific ion implantation processes.
Authors : L. Serenelli 1, R. Chierchia 1, M. Izzi 1, M. Tucci 1, L. Martini 2 D. Caputo 2, R. Asquini 2, G. de Cesare 2
Affiliations : 1 ENEA, Research Centre Casaccia, via Anguillarese, 301 Rome, Italy 00123; 2 Dept. of Information, Electronic and Telecommunication Engineering, Sapienza Univ. of Rome, via Eudossiana, 18 Rome, Italy 00184;
Resume : The most attracting way to fabricate high efficiency solar cells is the amorphous/crystalline silicon technology, because of the high Voc obtainable as a consequence of excellent c-Si surface passivation offered by a-Si:H films. This passivation is obtained by saturation of silicon dangling bonds at c-Si/a-Si interface and can be influenced by the hydrogen inclusion within the a-Si:H layer. In this work we propose the use of hydrogen plasma treatments subsequently performed on the thin amorphous silicon layer deposited to passivate the c-Si surface. We compare the hydrogen effect on the interface with that of thermal annealing of the interface to identify how the hydrogen evolves inside the a-Si:H network modifying the defect density and the a-Si:H/c-Si interface. To monitor the hydrogen effect on the heterointerface we propose the use of surface photovoltage technique as a contact-less tool for the evaluation of the energetic distribution of the state density at amorphous/crystalline silicon interface. This technique results to be very sensitive to the different experimental treatments, and therefore it can be considered a precious tool to monitor and improve the interface electronic quality. These characteristics are compared with FTIR spectra measurements performed on both hydrogen plasma post-treated and thermal annealed a-Si:H/c-Si samples. We find out that hydrogen plasma post treatment is effective to stably reduce interface state density.
Authors : Y. Vygranenko, M. Fernandes, M. Vieira, A. Khosropour, A. Sazonov
Affiliations : Electronics, Telecommunications and Computer Engineering Department, ISEL, Lisbon, Portugal; CTS-UNINOVA, 2829-516 Caparica, Portugal; Electrical and Computer Engineering Department, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada
Resume : Solar cells on lightweight and flexible substrates have advantages over the glass- or wafer-based photovoltaic devices in both terrestrial and space applications. Here, we report on amorphous silicon solar cells on plastic foils in the substrate configuration having a front metal grid. A two-dimensional distributed circuit model of the photovoltaic cell has been developed for performance analysis and device design optimization. The circuit simulator SPICE is used to calculate current and potential distributions in a network of sub-cell circuits. The equivalent circuit of each sub-cell includes a diode, a current source representing current generation in the n-i-p structure, and resistors representing the p-layer, transparent-conducting oxide (TCO) electrode, top metallization and shunting resistances. The input parameters also include performance characteristics of the n-i-p structure, which can be determined experimentally. This approach enables a realistic device model that predicts output current-voltage characteristics and maps Joule losses in the TCO electrode and the metal grid. As an example of usage the optimization of contact grid geometry at various TCO sheet resistances has been performed. These results are used to determine the optimal thicknesses for the ZnO:Al electrode and top metallization in the developed photovoltaic module thus boosting its power conversion efficiency.
Authors : P J M Isherwood, J W Bowers, J M Walls
Affiliations : CREST, Holywell Park, School of Electrical, Electronic and Systems Engineering, Loughborough University, LE11 3TU, UK
Resume : Metal oxide semiconductors are widely used as transparent contact materials for solar cells. However the vast majority are n-type. This is because of the typical band structure of metal oxides and the high electronegativity of oxygen. The development of transparent conducting p-type oxides could be of use in a range of PV applications such as bi-facial and multijunction cells, as well as possible high work-function back contact materials for organic or CdTe cells. Cupric oxide (CuO) is a p-type material with an indirect band gap of 1.2 eV. This study examines the effects of doping on the resistivity of sputtered CuO, and aims to increase the material band gap by co-sputtering with SnO2. It was found that a target doped with 2% Na produced films with resistivities of four orders of magnitude lower than equivalent undoped films. Addition of oxygen to the films was found to reduce the resistivity further. The best films were found to have resistivities of 4.3x10-2Ω.cm, which is an order of magnitude better than has been reported for other p-type oxides. Addition of SnO2 was found to increase the band gap significantly, although it also caused an increase in the resistivity. Despite not being as conductive as n-type oxides such as ITO or AZO, this study demonstrates that cupric oxide-based materials have potential as effective p-type transparent conductors.
Authors : Giovanni Carapezza, Giuseppe Fisicaro, Cosimo Gerardi, Giuseppe Nicosia, Antonino La Magna, Salvatore Lombardo, Giovanni Mannino, Andrea Patanè, Vittorio Romano, Andrea Santoro
Affiliations : CNR-IMM VIII Strada 5, Catania (Italy); Dipartimento di Matematica Università di Catania, Via Santa Sofia 64, Catania (Italy)
Resume : We present a composite design methodology for the simulation and optimization of the solar cell performances. Our method is based on the synergic use of different simulation techniques and it is especially designed for the thin-film cell technology. In particular, we aim to efficiently simulate light trapping and plasmonic effects to enhance the light harvesting of the cell. Our method is based on the sequential application of a hierarchy of approaches: a) full Maxwell simulations are applied to study of the light scattering in systems presenting textured interfaces; b) calibrated Photonic Monte Carlo is used in conjunction with the scattering matrixes method to evaluate coherent and scattered phonon absorption in the full cell architectures; c) the results of these advanced optical simulations are used as the pair generation terms in model implemented in a conventional TCAD tool for the derivation of the cell performances; d) genetic-type algorithms are applied to the automatic optimization procedure and the cell design. The material calibration includes optical and electrical properties of common substances used in thin film solar cells, while both full Maxwell simulations and experimental measurements are used to calibrate the angular scattering probability used in the Photonic Monte Carlo method. We discuss the application of our methodology to the study and the optimization of different typologies of solar cell.
Authors : A. Sytchkova*, A. Ulyashin**
Affiliations : *ENEA, Rome, Italy **SINTEF Materials and Chemistry, Oslo, Norway
Resume : Si based heterojunction solar cell structures with ITO and a-Si:H layers have been analysed by spectroscopic variable angle ellipsometry in order to trace dependence of free carriers distribution along the film depth as a function of film thickness as well as its change upon annealing. ITO layers were deposited at RT or 200°C preheated silicon substrates. Properties of HJ based solar cell structures have been compared with the ITO film optical constants inhomogeneity, i.e. material properties along the film thickness. The obtained results show that optical as well as electrical properties of thin ITO films prepared by pulsed DC sputtering are depth dependent. In particular, the deposition conditions used ensure a well-determined reproducible non-uniform distribution of free carriers within the film thickness. It has been found that distribution of free carriers in annealed around 300 °C samples is qualitatively different from that of as-deposited layers. It is shown that such annealing leads to an improvement of the HJ solar cell efficiencies. It is concluded that in-depth variations of the optical function of ITO layers responsible for the improvement of Si-based heterojunction ITO/a-Si:H/Si structures upon anneals.
Authors : P. Prepelita, V. Craciun
Affiliations : National Institute for Lasers, Plasma and Radiation Physics, 409 Atomistilor Street, PO Box MG-36, Magurele 077125, Ilfov, Romania
Resume : Transparent contact electrodes are fascinating oxides with interesting physical properties. AZO, ITO and SnO2 were tested for the development of photovoltaic devices, such as solar cells. They have good electrical properties and are usually used as transparent contact electrodes in such devices. In the present paper we discuss the application of RF magnetron deposited AZO, ITO and SnO2 thin films as transparent contact electrodes in high quality chalchogenide solar cells. Therefore, a reliable assessment of their crystallographic and optical properties is essential. The influence of growth parameters (i.e. oxygen pressure, flow rates, substrate type) on the morpho-structural characteristics of the thin films were also investigated. AZO, ITO and SnO2 films with a thickness of ~ 500nm were deposited by RF magnetron sputtering onto glass substrate, using a constant deposition rate of 6 Å/s. Morphological features were investigated by Atomic Force Microscopy and Scanning Electron Microscopy, which showed profiles that are densely packed and well adherent to the surface of the substrate. The X-ray diffraction analyzes revealed that all films are polycrystalline and show a strong orientation after the main planes perpendicular to the substrate. It was highlighted that ITO and SnO2 have good conductivity and are transparent (90-94%) materials, whereas AZO depends on the Al (3%) doping to exhibit improved properties.
Authors : P. Prathap*, R. Nautiyal, M. Srivastava, Vandana, S.K. Srivastava, R.K. Singh, C.M.S Rauthan, P.K. Singh
Affiliations : Silicon Solar Cell Group, CSIR-Network of Institutes for Solar Energy, CSIR-National Physical Laboratory, Dr. K.S. Krishnan Marg, New Delhi-110012, India
Resume : Nitric acid oxidation of silicon (NAOSi) is an attractive method to oxidize silicon to form ultra-thin SiO2 layers with good interface quality at lower temperatures. In the present investigation, (100) n-type Si wafers were subjected to two-step nitric acid oxidation, involving a treatment in 40 % HNO3 for 10 min followed by 68 % HNO3 treatment at 121 oC for several time periods. The grown samples were annealed in different ambients. SiO2 thickness practically stopped after attaining a critical thickness. Quasi-steady state photoconductivity measurements on the oxidized samples indicated improvement in minority carrier lifetime after annealing at 400 oC for 30 min in forming gas. The passivation observed in n-type silicon surface was attributed to chemical passivation. Capacitance Voltage characteristics revealed that SiO2 layers formed using NAOSi route has positive fixed charges. A short high-temperature annealing affected the local bonding configuration and enhanced the positive charge density of SiO2 layers after hydrogen annealing. The results indicate that NAOSi grown oxide layers may find application in silicon solar cells as passivating layers alone or as buffer layer in conjunction with other surface passivation layers such as Al2O3 or Si3N4.
Authors : P Prathap1, D Praveen Kumar2, G. Sumana1, M V Shankar2
Affiliations : 1CSIR-National Physical Laboratory, Dr. K.S. Krishnan Marg, New Delhi-110012, India 2Nanocatalysis Research Lab, Department of Materials Science & Nanotechnology, Yogi Vemana University, Kadapa ? 516 003, India
Resume : Nitric acid oxidation of silicon (NAOSi) is an attractive method to oxidize silicon to form ultra-thin SiO2 layers with good interface quality at lower temperatures. In the present investigation, (100) n-type Si wafers were subjected to two-step nitric acid oxidation, involving a treatment in 40 % HNO3 for 10 min followed by 68 % HNO3 treatment at 121 oC for several time periods. The grown samples were annealed in different ambients. SiO2 thickness practically stopped after attaining a critical thickness. Quasi-steady state photoconductivity measurements on the oxidized samples indicated improvement in minority carrier lifetime after annealing at 400 oC for 30 min in forming gas. The passivation observed in n-type silicon surface was attributed to chemical passivation. Capacitance ? Voltage characteristics revealed that SiO2 layers formed using NAOSi route has positive fixed charges. A short high-temperature annealing affected the local bonding configuration and enhanced the positive charge density of SiO2 layers after hydrogen annealing. The results indicate that NAOSi grown oxide layers may find application in silicon solar cells as passivating layers alone or as buffer layer in conjunction with other surface passivation layers such as Al2O3 or Si3N4.
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Authors : Andreas Gerl*, Moses Richter**, Gebhard Matt**, Heiko B. Weber* and Christoph J. Brabec**
Affiliations : ** Department of Materials for Electronics and Energy Technology, Friedrich-Alexander University Erlangen-Nürnberg, Martensstraße 7, 91058 Erlangen; * Department for Applied Physics, Friedrich-Alexander University Erlangen-Nürnberg, Staudtstr. 7, 91058 Erlangen;
Resume : In this work we present a model which describes the current-voltage characteristics of an organic solar cell under illumination with 0.25 2 sun equivalents. The IV characteristics of an ideal solar cell, under illumination, are described by the sum of the dark current (J_dark) and photo-generated current (J_photo). In a practical solar cell it is also necessary to consider the recombination rate (R) of the photo-generated charge carriers, which in fact reduces the power output of the solar cell (i.e. decreased fill factor). Practically it is a challenging task to determine the recombination rate. Here we present a novel approach based on charge extraction (CE) and transient photo-voltage measurements (TPV) to calculate R. For the calculation of R, the voltage dependent carrier density n(V) must be known as well as the recombination order, often described by the recombination coefficient α. Latter is derived from the charge carrier lifetime dependence on the charge density. Both parameters are accessed by TPV and CE measurements. Based on this, the reconstructed IV characteristic is in excellent agreement with the measured data. It is noted, that the herein introduced model does not need any free fit parameters and allows a consistent recreation of the IV characteristics of an organic solar cell.
Authors : T. Sameshima, S. Yoshidomi, and M. Hasumi
Affiliations : Tokyo University of Agriculture and Technology
Resume : We report the mechanical stacking technology used for fabricating large size multi-junction solar cells. 1) We developed transparent conductive adhesive and stacking technology. 20 micrometer diameter ITO conductive particles were dispersed in epoxy-type transparent adhesive jell named Cemedine. We also developed a gas press method for stacking semiconductor substrates up to 8 inch size. Two low-resistivity silicon substrates were pasted together by the Cemedine adhesive including 6 wt% ITO particles as an intermediate layer. Then, the sample was placed in a pressure proof chamber. Air gas was introduced in the camber at 0.7 MPa for 2 h to hardening the intermediate Cemedine adhesive at room temperature. Current voltage measurement of the sample with the structure Si/adhesive/Si revealed a good ohmic characteristic with a connecting resistivity of about 1 ohm cm2, which was enough low for fabricating multi-junction solar cells. 2) We experimentally demonstrate multi-junction solar cells using our mechanical stacking technique as described above. Commercial GaInP/GaAs/Ge three junction solar cells with a size of 8x4 cm2 were prepared. The initial solar cell characteristics were first measured under AM1.5 light at 100 mW/cm2. The open circuit voltage (Voc), fill factor (FF), and efficiency were 2.51V, 0.85, 31.6%, respectively. Then, the rear metal electrodes were removed and Ge rear surfaces were pasted to a low resistivity silicon substrates using the Cemedine adhesive including 6 wt% ITO particles. Measurement of solar cell characteristics under AM1.5 light at 100 mW/cm2 resulted in Voc, FF, and efficiency as 2.53V, 0.84, 31.4%. The solar cell performances were almost the same as the initial ones. Those results show that our transparent conductive adhesive has a capability of fabrication of large size multi-junction solar cells. In the conference, we will also report solar cells fabricated by stacking GaInP/GaAs epitaxial thin solar cells on Ge cells and stacking amorphous silicon thin film solar cells on crystalline silicon solar cells using our transparent conductive adhesive.
Authors : Giovanna Pellegrino1, Guglielmo Guido Condorelli2, Francesca De Rossi3, Thomas M. Brown3, Francesco Giovenale4 , Corrado Bongiorno1 and Alessandra Alberti1*.
Affiliations : 1 IMM-CNR, Zona Industriale, VIII Strada 5, Catania; 2 Università degli Studi di Catania and INSTM UdR Catania; Viale Andrea Doria 6, Catania; 3 CHOSE (Centre for Hybrid and Organic Solar Energy), Department of Electronic Engineering, University of RomeTor Vergata, via del Politecnico 1, I-00133, Rome, Italy; 4 Hamamatsu Photonics Europe, Arzbergerstr. 10, D-82211 Herrsching am Ammersee, Germany
Resume : Nano-sized TiO2 films have been grown by DC-Reactive Sputtering at ~150 °C on  ZnO:Al substrates to be applied in Dye Sensitized Solar Cells. The effects of post-deposition thermal treatment at 500°C on the optical and structural (density, grain size) properties of the TiO2/ZnO:Al bi-layers have been investigated in details. The effects of such modifications on the surface properties, have been evaluated by dye-sensitization with 5,10,15,20 Tetrakis(4-carboxyphenyl) porphyrin (TCPP). Different interactions of the probe-molecules with the as grown and annealed TiO2 surfaces have been observed. In particular, Quantum Yield (QY) analysis of TCPP-sensitized films showed that the emitted/absorbed photons ratio in the as grown layers is a factor of ~3 lower than in the annealed layers. Based on simplified test solar cells, the electron injection is thought to be the most appropriate mechanism to explain the QY results. Our approach gives particular emphasis to the role of the surfaces and the interfaces involved in the TCPP/ TiO2/AZO multilayer structures with the intent of proposing the as sputtered TiO2 layer as blocking layer in complete architecture for low thermal budget solutions (<200°C).