Effect of an in-situ H2 plasma pretreatment on the minority carrier lifetime of a-Si:H(i) passivated crystalline silicon

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 (H-2) plasma pretreatment before a-Si:H deposition, obtaining a surface recombination velocity (S-rec) of 3.8 +/- 1.0cm/s at 1x10(15) minority charge carriers/cm(3) 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 S-rec 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 H-2 pretreatment. The exposure to H-2 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. (C) 2012 Published by Elsevier Ltd. Selection and peer-review under responsibility of the scientific committee of the SiliconPV 2012 conference.