Design of Very thin CdTe Solar Cells with High Efficiency

Abstract

After several years without change, First Solar announced in 2011 an efficiency record for CdS/CdTe solar cells set at 17.3%, as confirmed by NREL [1]. This is a significant milestone that demonstrates the ongoing potential of CdTe thin-film solar cells. Then, a question arises on the new limits likely to be achieved for the efficiency of this kind of solar cells. The author [2] had previously predicted that with current technology, the practical limit was around 17.5%, but below 18%. In this work, a systematic study of the expected efficiency evolution as a function of fabrication parameters is made with the help of a new solar cell simulator (wxAMPS) [3]. We start from known parameters, so that the simulated results coincide with a previous reported record efficiency (16.5% under AM1.5 G radiation). Then, we show that as the thickness of the CdTe layer is reduced the efficiency will be increased mostly due to a reduced series resistance. It will be shown that there is an optimum thickness for which the highest efficiency becomes 17.7%, confirming the preceding author's prediction. In addition, it will be shown that the introduction of a ZnTe layer at the back of the solar cell (i. e. with a new device structure: CdS/CdTe/ZnTe) will help reaching even higher efficiencies, for very thin solar cells, above 19.5%. These results should be useful for guiding experimentalists to make more efficient CdTe solar cells.