Carrier-selective contacts for Si solar cells

Abstract

Carrier-selective contacts (i.e., minority carrier mirrors) are one of the last remaining obstacles to approaching the theoretical efficiency limit of silicon solar cells. In the 1980s, it was already demonstrated that n-type polysilicon and semi-insulating polycrystalline silicon emitters form carrier-selective emitters which enabled open-circuit voltages (Voc) of up to 720 mV. Albeit promising, to date a polysilicon emitter solar cell having a high fill factor (FF) has not been demonstrated yet. In this work, we report a polysilicon emitter related solar cell achieving both a high Voc = 694 mV and FF = 81%. The passivation mechanism of these so-called tunnel oxide passivated contacts will be outlined and the impact of TCO (transparent conductive oxide) deposition on the injection-dependent lifetime characteristic of the emitter as well as its implications on FF will be discussed. Finally, possible transport paths across the tunnel oxide barrier will be discussed and it will be shown that the passivating oxide layer does not lead to a relevant resistive loss and thus does not limit the solar cell's carrier transport. Contrary to amorphous silicon-based heterojunction solar cells, this structure also shows a good thermal stability and, thus, could be a very appealing option for next generation high-efficiency silicon solar cells.