Improvement of the SRH bulk lifetime upon formation of n-type POLO junctions for 25% efficient Si solar cells

- Improvement of bulk lifetime upon n-type POLO junction formation.
- Gettering of metal impurities by a combination of poly-Si and high P doping.
- Implementation into a solar cell process flow resulting in 25% efficient cells.

Carrier-selective contact schemes, like polysilicon on oxide (POLO), provide low contact resistivities while preserving an excellent passivation quality. These junctions offer an important additional feature compared to a-Si/c-Si heterojunctions. We find that the formation of n-type POLO junctions lead to a huge increase of the Shockley-Read-Hall (SRH) lifetime of the substrate, a prerequisite for highly efficient solar cells. The SRH lifetime improvement can be observed for both bulk polarities and for a variety of bulk resistivities. Thus we suggest that the highly doped POLO junction getters impurities that have more or less symmetric SRH capture cross sections. We are able to achieve SRH lifetimes of > 50 ms. By applying POLO junctions to interdigitated back contact cells, we achieve cells with an efficiency of 25%.