Scientific work: The thesis will take place in the framework of the ORGANIST research project (ORGANIc/Si 3-terminal Tandem solar cells) supported by the National Research Agency (ANR) in partnership with two other French laboratories (GeePs and LPICM). In this context, organic materials are very promising regarding the large choice of materials to develop either low or high band gap devices, their high absorption and the recent impressive progress in the power conversion efficiently of organic solar cells. Silicon is an appropriate low bandgap (1.12 eV) bottom cell in a tandem configuration and there is great research activity investigating on larger bandgap semiconductors for the top cell. The dominant multijunction design today is the tandem solar cell which is achieved by combining two different bandgap materials, allowing a reduction of thermalization losses in comparison with a single junction solar cell. The development of new concepts to surpass the SQ limit is therefore a hot topic in PV research and multijunction devices are the most promising solution. A record efficiency of 26.7% has been reached for the silicon single junction solar cell and only marginal improvements in performance remain possible. Solar cell efficiency is getting closer and closer to the single-junction theoretical Shockley-Queisser (SQ) efficiency limit, specifically the Auger limit of 29.4% for crystalline silicon (c-Si. Gerald Siefer at the Fraunhofer Institute used three independent lamps of high intensity.Context : Photovoltaics (PV) has today become a mature industry, dominated by silicon technology (more than 90% of the PV market). The first of these problems has been addressed in two ways. There have been two aspects of testing these cells that have proven difficult, 1) calibration of input i llumination power, and 2) accurate curve tracing under flash illumination. Therefore, the testing of silicon concentrator cells had to be somewhat re-developed to accommodate only Amonix as we are the only company currently working on high concentration silicon solar cells. Also, as multi-junction concentrator cells are the flavor of the month, most concentrator cell testing efforts have been in that direction. However, it has been a difficult task to get reliable independent measurement of cell efficiency as the high lifetime back-contact solar cell is very difficult to use on a flash testing system. The other important aspect of development is device performance. The only other piece of equipment that was purchased new was the silicon etching equipment as Amonix is the only customer of the semiconductor foundry that etches silicon (surface texturing). Results of the lifetime / contamination have been reported previously. However, after having used this tool for the last 3 years (during development) the semiconductor manufacturing facility now measures the lifetime of float zone wafers for all 40 diffusion furnaces to monitor contamination. This has been a very valuable tool as the semiconductor foundries have little or no use for measuring minority carrier lifetime. One particular piece of new equipment was the Ron Sinton lif etime tester. An example of this is that the entire amount of money spent on new equipment for the 10 MW facility is approximately $50,000 although a lot more was spent on used equipment. This has the obvious advantage that the costs can be quite low and the technology is constantly improving without any cost to Amonix. Amonix uses only existing (primarily used) equipment from the semiconductor industry. The uniqueness of the challenge is unparallel in PV today where every technology uses either custom made equipment often costing millions of dollars or equipment dedicated to the manufacture and / or deposition of a thin film technology. To be able to meet the cost target of $0.20 / Watt for the solar cell a very low cost (relative to other semiconductor products) process must be used. The essential aspect to the solar cell development has been production worthiness – wafer yield, electronic yield, process simplicity, process control, consistency and most all cost.
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