Improvement of μc-Si:H n–i–p cell efficiency with an i-layer made by hot-wire CVD by reverse H2-profiling

Abstract

The technique of maintaining a proper crystalline ratio in microcrystalline silicon (μc-Si:H) layers along the thickness direction by decreasing the H2 dilution ratio during deposition (H2 profiling) was introduced by several laboratories while optimizing either n–i–p or p–i–n μc-Si:H cells made by PECVD. With this technique a great increase in the energy conversion efficiency was obtained. Compared to the PECVD technique, the unique characteristics of HWCVD, such as the catalytic reactions, the absence of ion bombardment, the substrate heating by the filaments and filament aging effects, necessitate a different strategy for device optimization. We report in this paper the result of our method of using a reverse H2 profiling technique, i.e. increasing the H2 dilution ratio instead of decreasing it, to improve the performance of μc-Si:H n–i–p cells with an i-layer made by HWCVD. The principle behind this technique is thought to be a compensation effect for the influence of progressing silicidation of the filaments during the growth of μc-Si:H, if the filament current is held constant during growth. The dependence of the material crystallinity on thickness with and without H2 profiling is discussed and solar cell J–V parameters are presented. Thus far, the best efficiency of μc-Si:H n–i–p cells made on a stainless steel substrate with an Ag/ZnO textured back reflector made in house has been improved to 8.5%, which is the highest known efficiency obtained for n–i–p cells with a hot-wire μc-Si:H i-layer.