Investigation of post-annealing enhancement effect of passivation quality of hydrogenated amorphous silicon

Abstract

The excellent surface passivation scheme for suppression of surface recombination is a basic prerequisite to obtain high efficiency solar cells. Particularly, the HIT (heterojunction with intrinsic thin-layer) solar cell, which possesses an abrupt discontinuity of the crystal network at an interface between the crystalline silicon (c-Si) surface and the hydrogenated amorphous silicon (a-Si:H) thin film, usually causes a large density of defects in the bandgap due to a high density of dangling bonds, so it is very important for high energy conversion efficiency to obtain millisecond (ms) range of minority carrier lifetime (i. e. 2 ms). The a-Si:H, due to its excellent passivation properties obtained at low deposition temperatures and also mature processing, is still the best candidate materials for silicon HIT solar cell. Deposition of a transparent conductive oxide (TCO), such as indium tin oxide (ITO), has to be used to improve the carrier transport, since the lateral conductivity of a-Si:H is very poor. Usually, ITO is deposited by magnetron sputtering, but damage of a-Si:H layers by sputtering-induced ion bombardment inevitably occurs, thus triggering the serious degradation of the minority carrier lifetime, i. e., a loss in wafer passivation. Fortunately, this damage can be often recovered by some post-annealing. In this paper, however, the situation is different, and it is found that the minority carrier lifetime of ITO/a-Si:H/c-Si/a-Si:H heterojunction has been drastically enhanced by post-annealing after sputtering ITO on a- Si:H/c-Si/a-Si:H heterojunction (from 1.7 ms to 4.0 ms), not just recovering. It is very important to investigate how post-annealing enhances the lifetime and its physics nature. Combining the two experimental ways of HF treatment and vacuum annealing, three possible reasons for this enhancement effect (the field effect at the ITO/a-Si:H interface, the surface reaction-layer resulting from annealing in air, and the optimization of a-Si:H material itself) have been studied, suggesting this is irrelevant to the first two. The influence of post-annealing on a-Si:H/c-Si/a-Si:H heterojunction deposited at different temperatures has also been investigated. It is found that the remarkable enhancement effect of post-annealing is for low growth temperature(175 ℃) and not for high growth temperature(200 ℃), with the confirmation of an effective way for high quality passivation using growth at low temperature and then annealed at high temperature. Moreover, the configuration of a-Si:H at different growth temperatures between afore and after annealing has been discussed by an application of Fourier transform infrared (FTIR) spectroscopy. It is shown that the large increase of the lifetime of the heterojunction after annealing results from the improvement of microstructure of a-Si:H itself, which is essentially a competitive balance of the dominant role of some micro-factors, including hydrogen content, hydrogen bonding and network disorder in amorphous silicon film determined by the optimized matching between the growth temperature of a-Si:H materials and the annealing temperature of the heterojunction. An optimum control for this balance point is the essential cause of lifetime enhancement.