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Boostandoost M.,TU Berlin | Friedrich F.,PVcomB | Kerst U.,TU Berlin | Boit C.,TU Berlin | And 3 more authors.
Journal of Materials Science: Materials in Electronics | Year: 2011

Three different optical interaction techniques have been employed to characterise the electrical and material parameters of polycrystalline silicon (poly-Si) thin-film solar cells with an interdigitated mesa structure. First, Light Beam Induced Current (LBIC) in the infrared range was used to locally analyse the light collection properties. Second, electroluminescence in forward bias (EL) yielding information on band to band recombination was performed. Third, electroluminescence in reverse bias (ELR) was utilized to gain information on the intraband relaxation. The EL and ELR measurements were performed using cooled Si-CCD (Silicon-based Charge Coupled Device) and InGaAs (Indium Gallium Arsenide) detectors. The high resolution IR-LBIC measurement equipped with a 1,064 nm wavelength laser has been applied to investigate the grain boundary characteristics in the absorber layer. Additionally, the local electrical characteristics of the absorber layer (diffusion length, doping concentration and built-in potential) have been extracted by performing a bias-dependent IR-LBIC measurement based on a simple theoretical model with the assumption of relatively small diffusion length compared to the absorber layer thickness. The local/spatial distribution of the diffusion length in the absorber layer of the thin-film solar cell has been extracted. Furthermore, the temperature dependence of the photocurrent of thin-film solar cells in a temperature range of -25 to +70 °C has been locally investigated using IR-LBIC. Additionally, the temperature dependence of the reverse bias characteristics of the poly-Si thin-film solar cell is analysed and compared with that of monocrystalline Si solar cell. For the EL and ELR measurements a spectral analysis of the emitted light has been performed. From the EL results material properties like diffusion length and process induced defects have been deduced and insights on the quality of production processes like metallization and etching were gained. The complementary information from the ELR measurements provides access to additional types of defects resulting from generation centres, such as lattice disorder, crystal defects and charged coulomb centers. © 2011 Springer Science+Business Media, LLC.

News Article | November 12, 2015
Site: cleantechnica.com

“Teams from the Helmholtz-Zentrum Berlin and École Polytechnique Fédérale de Lausanne, Switzerland, have been the first to successfully combine a silicon heterojunction solar cell with a perovskite solar cell monolithically into a tandem device.” THZB reports this hybrid tandem cell has shown an efficiency of 18%, and while not the highest solar cell efficiency rating, this is the highest reported value for this particular  type of cell architecture. Organic-inorganic perovskite materials are said to represent a great step forward in ongoing solar cell research. Increases in efficiency continue to be reported for perovskite solar cells, which can be manufactured from solution and cost-effectively printed on large surface areas. Perovskite layers are known to efficiently absorb light in the blue region of the spectrum. But combining these with silicon layers, which convert long-wavelength red and near-infrared light, has proven to be difficult. HZB writes, “This is because for high-efficiency perovskite cells, it is usually required to coat the perovskite onto titanium dioxide layers that must be previously sintered at about 500 degrees Celsius. However, at such high temperatures, the amorphous silicon layers that cover the crystalline silicon wafer in silicon heterojunction degrades.” The team behind this project was led by Professor Bernd Rech and Dr. Lars Korte from the HZB Institute for Silicon Photovoltaics, working in cooperation with HZB’s PVcomB and a group headed by Professor Michael Graetzel at the École Polytechnique Fédérale de Lausanne (EPFL). This group is the first to have fabricated this kind of monolithic tandem cell,  depositing a layer of tin dioxide at low temperatures — to replace the standard titanium dioxide. A thin layer of perovskite was then spin-coated onto this intermediate layer and covered with hole-conductor material. In addition, a crucial element in the device architecture is the transparent top contact. Typically,  metal oxides are deposited by sputtering, but this would destroy the sensitive perovskite layer, as well as the hole-conductor material. Therefore, the team from HZB modified the fabrication process and incorporated a transparent protective layer. A number of chemical researchers believe perovskites open the door to a new era of high-efficiency, low-cost solar cells. If this is true, the economic impact on the photovoltaics market and the solar industry will be felt worldwide. As CleanTechnica’s Tina Casey puts it: “Perovskites are easily synthesized, and their distinctive crystalline structure makes them a perfect match for the development of efficient solar cells that can beat the current gold standard, which is silicon.” According to the research, this tandem cell attained an 18% efficiency level — nearly 20% higher than the efficiency of individual cells. The open-circuit voltage is 1.78 volts. “At that voltage level, this combination of materials could even be used for the generation of hydrogen from sunlight,” said Dr. Steve Albrecht, lead author of the paper that has now appeared in the journal Energy & Environmental Science. Right now the perovskite-silicon tandem cell is being fabricated on a polished silicon wafer. Efficiencies may change dramatically, states Dr. Lars Korte, head of the silicon heterojunction solar cell group at the Institute for Silicon Photovoltaics, if the wafer surface is textured using light-trapping features, like random pyramids. “The efficiency might be increased further to 25 or even 30%,” she added. More important than maximizing efficiencies will be how this technology is integrated into existing technologies, pointed out Professor Bernd Rech; “Silicon technology currently dominates 90% of the market, which means there are many established production facilities for silicon cells. The perovskite layers could considerably increase the efficiency level. To achieve this, the fabrication techniques only need to be supplemented with a few more production steps. For that reason, our work is also extremely interesting for industry. However, the problems of long-term stability and the lead content of perovskite solar cells still need to be solved in future research.” Images via HZB    Get CleanTechnica’s 1st (completely free) electric car report → “Electric Cars: What Early Adopters & First Followers Want.”   Come attend CleanTechnica’s 1st “Cleantech Revolution Tour” event → in Berlin, Germany, April 9–10.   Keep up to date with all the hottest cleantech news by subscribing to our (free) cleantech newsletter, or keep an eye on sector-specific news by getting our (also free) solar energy newsletter, electric vehicle newsletter, or wind energy newsletter.  

Hammerschmidt M.,Zuse Institute Berlin | Lockau D.,Zuse Institute Berlin | Lockau D.,Helmholtz Center Berlin | Burger S.,Zuse Institute Berlin | And 8 more authors.
Solid-State and Organic Lighting, SOLED 2012 | Year: 2014

We present a FEM based simulator for 3D rigorous optical modeling of a-Si/mc-Si tandem thin-film solar cells with randomly textured layer interfaces. Our focus lies on a detailed analysis of the numerical error. © 2012 Optical Society of America.

Hammerschmidt M.,Konrad Zuse Zentrum fur Informationstechnik Berlin | Lockau D.,Konrad Zuse Zentrum fur Informationstechnik Berlin | Burger S.,Konrad Zuse Zentrum fur Informationstechnik Berlin | Burger S.,JCMwave GmbH | And 7 more authors.
Proceedings of SPIE - The International Society for Optical Engineering | Year: 2013

Light trapping techniques are one of the key research areas in thin film silicon photovoltaics. Since the 1980s randomly rough textured front transparent oxides (TCOs) have been the methods of choice as light trapping strategies for thin-film devices. Light-trapping efficiency can be optimized by means of optical simulations of nano-structured solar cells. We present a FEM based simulator for 3D rigorous optical modeling of amorphous silicon/microcrystalline silicon tandem thin-film solar cells with randomly textured layer interfaces. We focus strongly on an error analysis study for the presented simulator to demonstrate the numerical convergence of the method and investigate grid and finite element degree refinement strategies in order to obtain reliable simulation results. © 2013 Copyright SPIE.

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