Polycrystalline silicon thin films by high-rate electronbeam 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 >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.