Polycrystalline silicon thin films by high rate electron beam evaporation for photovoltaic applications Influence of substrate texture and temperature

Polycrystalline silicon poly Si thin films have the potential to overcome the limits of today s silicon thin film solar cell technology because of the chance to grow high quality material by low cost deposition techniques. As poly Si thin film fabrication is independent of rather slow and costly PECVD processes, it is possible to switch to physical high rate deposition methods such as electron beam e beam evaporation exhibiting a strong cost saving potential. In this contribution, the challenges and opportunities of two different poly Si solar cell preparation techniques using e beam evaporated silicon are investigated Thermal solid phase crystallization SPC of initially amorphous silicon layers, and direct growth of poly Si films at elevated temperatures gt;500 C. For both approaches, attention is given to the interplay between substrate texture, structure of the grown silicon and the final solar cell performance. Poly Si thin film solar cells with 7.8 conversion efficiency were prepared on a smooth substrate in cooperation with CSG solar, demonstrating the equality of e beam evaporation as deposition technique compared to PECVD. However since e beam evaporation leads to a non conformal deposition of rough surfaces, the implementation of a textured substrate for light trapping is highly desired but still challenging. A different behavior is observed for poly Si thin films directly grown at higher temperatures. For the best photovoltaic performance, a certain substrate microstructure is even necessary and can be optimized by the use of glass coated by differently textured ZnO films as substrate