Integrating transparent conductive oxides to improve the infrared response of silicon solar cells with passivating rear contacts
This work addresses the development of a transparent conductive oxide (TCO)/metal stack for n-type Si solar cells featuring a tunnel oxide passivating rear contact (TOPCon). While poly-Si based passivating contacts contacted by local fire-through metallization currently show an increased recombination at the metal contacts and a poor infrared (IR) response, we aim to realize a full-area metallization which maintains the high level of surface passivation and avoids IR losses. Some research groups have reported that sputtering TCOs on poly-Si based passivating contacts degrades the surface passivation and unlike the SHJ cells this degradation cannot be cured completely at Tcure ∼ 200°C. However, the higher thermal stability of TOPCon allows for higher Tcure of up to 400°C, which can effectively restore the surface passivation. On the other hand, the contact resistivity (rc) of the TOPCon/ITO/metal contact increased by several orders of magnitude in our test structures during annealing at such high temperatures. Possible reasons like the formation of an interfacial oxide are currently under investigation. Increasing the poly-Si thickness and/or doping mitigated the effect of sputter damage, but this will come at the cost of more parasitic absorption. However, by adapting the sputter and the subsequent annealing process, we were able to realize low damage deposition of ITO (loss in implied Voc ∼ 7 mV) on thin, lowly doped poly-Si layers on textured wafers, yielding reasonable contact properties (rc ∼ 40 mOcm2; of the whole rear contact stack).