High efficiency amorphous and nanocrystalline silicon solar cells

This paper reviews our progress of using nc-Si:H as a low bandgap absorber material to substitute for a-SiGe:H alloys in multi-junction solar cells. We have focused on three topics: (I) high deposition rate, (2) large area uniformity of thickness and material properties, (3) high solar cell and module efficiencies. Initially, we investigated various deposition methods, including RF, VHF, and microwave glow discharges. After several years of systematic studies, we have been convinced that VHF glow discharge is an applicable method to attain high rate and large-area uniform nc-Si:H depositions. We also studied the stability of nc-Si:H solar cells and observed various metastability phenomena in nc-Si:H solar cells. We have reported an initial active-area cell efficiency of 15.4% using an a-Si:H/a-SiGe:H/nc-Si:H triple-junction structure. Subsequently, we have increased the deposition rates to around 1.0-1.5 nm/s and achieved an initial active-area efficiency of 13.4% using an a-Si:H/nc-Si:H/nc-Si:H triple-junction structure. Recently, a stable total-area efficiency of 12.5% was measured at NREL on our a-Si:H/nc-Si:H/nc-Si:H triple-junction solar cell. We have also developed large-area VHF deposition systems and demonstrated encouraging module efficiencies using a-Si:H/nc-Si:H/nc-Si:H triple-junction structures. Initial and stable aperture-area (400 cm(2)) efficiencies of 11.0 and 10.1% have been achieved with fully encapsulated modules with a-Si:H/ncSi:H/nc-Si:H triple-junction structures. In this paper, various aspects of nc-Si:H solar cells are discussed, including the material structures, device design, light trapping, metastability, high efficiency solar cell optimization, and large-area deposition. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim