Optimization of PECVD-ONO rear surface passivation layer through improved electrical property and thermal stability

The surface passivation is one of the crucial steps to achieve high conversion efficiencies in c-Si solar cells. Though thermal silicon oxide (SiO2) and silicon nitride (SiNx) provides a good passivation layer the high temperature process of SiO2 and the fixed positive charge of SiNx on p-substrate cause detrimental effect in surface passivation. A thermally stable thin film with a negative charge (for p-substrate) passivation layer is required to develop a good rear passivation suitable for PERC cells processed with screen printing. In this paper, a PECVD stack layer system consisting of SiOx, SiNx, and SiOx was employed for rear side passivation. The optimal refractive index of silicon oxide and silicon nitride thin film was found by varying the silane, ammonia and nitrous oxide gas ratios. A low surface recombination velocity (SRV) was achieved using interface treatment. Carrier lifetime of 212 μs and interface trap density of 8.265 × 109 cm−2 eV−1 was obtained. With optimized PECVD-ONO layer, the solar cell parameters measured were VOC of 647 mV and efficiency of 19.3%, while those of the reference solar cell were VOC of 621 mV and efficiency of 18.6%.