Amorphous silicon based tandem solar cells entirely fabricated by hot-wire CVD

For the first time, we report on a-Si:H/a-Si:H tandem solar cells, entirely fabricated by hot-wire CVD. Until now, our effort to develop a-Si:H/a-Si:H tandem cells has resulted in a 7.0% initial efficiency. Two types of tunnel junction, n⁺-μc-Si:H/p⁺-a-SiC:H and n⁺-μc-Si:H/p⁺-μcSi:H, have been developed which yield low series resistance (<25 Ωcm²). Systematic variation of top i-layer thickness was carried out to optimize the current matching, which otherwise appeared to be difficult since band gap tailoring of top and bottom junction could not yet be accomplished in our hot wire CVD process. For a-Si:H/μc-Si:H tandem structure, an intrinsic microcrystalline Si material with high photosensitivity (≈10³) has been developed with a growth rate, 10 Å/s by controlling its microstructure. The problems encountered in incorporating this material into a-Si:H/μc-Si:H tandem solar cells are discussed