Front contact optimization for rear-junction SHJ solar cells with ultra-thin n-type nanocrystalline silicon oxide

In this work, ultra-thin n-type hydrogenated nanocrystalline silicon oxide [(nc-SiOx:H (n)] film was used to replace amorphous silicon [a-Si:H (n)] as electron transport layer (ETL) in rear-junction silicon heterojunction (SHJ) solar cell to reduce front parasitic absorption. The contact resistivity between the transparent conductive oxide (TCO) and ultra-thin ETL interface plays an important role on the cell performance. A nanocrystalline silicon (nc-Si:H) contact or seed layer was introduced in the solar cell with ultra-thin nc-SiOx:H and the impact of the nc-Si:H thickness on the cell performance was investigated. To demonstrate scalability, bifacial solar cells with 10 nm ETL were fabricated on the M2 (244 cm2) wafer. The best cell performance is obtained by the solar cell with 5 nm nc-SiOx:H (n) and 5 nm nc-Si:H (n) contact layer and it exhibits open-circuit voltage (Voc) of 738 mV, fill factor (FF) of 80.4%, short-circuit current density (Jsc) of 39.0 mA/cm2 and power conversion efficiency (η) of 23.1% on M2 wafer. Compared to the one with nc-SiOx:H (n), an increase of 3%abs of FF and 0.5%abs of η and lower front contact resistivity is demonstrated for the solar cells with nc-Si:H (n) / nc-SiOx:H (n) double layer, which is caused by the lower energy barrier for electrons, according to the band diagram calculated by the AFORS-HET simulator. A simulation on the solar cell optical and electrical losses was done by the Quokka 3 simulator and shows much lower electrical transport loss and a bit higher front surface transmission loss for the one with double layer than nc-SiOx:H (n) single layer.