Effect of an in-Situ H2 Plasma Pretreatment on the Minority Carrier lifetime of a-Si:H(i) Passivated Crystalline Silicon

Abstract

In the future, thin (<100um) crystalline silicon (c-Si) solar cells based on amorphous heterostructures (a-Si:H/c-Si) are likely to become an essential branch of wafer-based photovoltaic industry. Within this thin technology, the surface passivation will have a larger influence on effective minority charge carrier lifetime, therefore an accurate analysis of the a-Si:H/c-Si interface is paramount to achieve high efficiencies. In this paper we studied an in-situ hydrogen (H2) plasma pretreatment before a-Si:H deposition, obtaining a surface recombination velocity (Srec) of 3.8±1.0cm/s at 1x1015 minority charge carriers/cm3 on moderately-doped n-type <100> silicon wafers and, therefore, we show a significant drop compared to the reference value (6.9±1.4cm/s). The trend is related to a change in surface hydrogen configuration toward lower hydrides (from SiH3 to SiH2 and SiH) and, thus, it indicates an atomic reconstruction more suited for the growth of a-Si:H(i) passivating layer. However, if the treatment is extended above an optimum time, the Srec increases abruptly above 10cm/s which suggests the introduction of defects. These effects are explained by hydrogen-induced reactions on the c-Si surface during the H2 pretreatment. The exposure to H2 plasma improves surface passivation without relevant modifications in the a-Si:H deposition process itself: hence, it represents an interesting option to be implemented in future heterojunction solar cells.