Impact of the transparent conductive oxide work function on injection dependent a Si H c Si band bending and solar cell parameters

An analysis of the contact formation between degenerated n type transparent conductive oxide TCO and p type amorphous silicon a Si H as it is used for front side contacts in high efficiency a Si H crystalline silicon c Si heterojunction solar cells is presented. It is shown that the deposition of a TCO on a p a Si H emitter layer causes a reduction of charge carrier lifetime in low injection levels which leads to a lowering of the implied fill factor. Simulation based analysis of charge carrier lifetime and direct measurements by surface photovoltage reveals that TCO deposition induces a change of the c Si band bending. The magnitude of this change depends on the p a Si H doping level. Both observations are explained by the impact of the TCO a Si H work function difference on the c Si band bending. Based on numerical simulations, the reduced injection dependent band bending is identified as the reason for the reduced fill factor of final solar cell