Numerical Analysis of Selective ITO/a-Si:H Contacts in Heterojunction Silicon Solar Cells: Effect of Defect States in Doped a-Si:H Layers on Performance Parameters

The effects of defect states in doped thin-film amorphous-silicon layers (p-a-Si:H and n-a-Si:H) and at heterointerfaces of the passivated crystalline-silicon (c-Si) wafer on the performance of heterojunction silicon solar cells are investigated using optoelectrical simulations. We used a state-of-the-art numerical model of a silicon heterojunction solar cell, including indium-tin-oxide (ITO) contacts as semiconductor layers, applying the accompanying tunneling mechanisms. We show that increasing the dangling-bond density in the p-a-Si:H reduces the conversion efficiency of the device more than in the n-a-Si:H, independent of the doping type of the c-Si wafer and illumination side of the device. Simulations revealed that this is due to: a larger ITO work-function mismatch with the p-a-Si:H than the n-a-Si:H; a larger valence-band offset at i-a-Si:H/n-c-Si interface at the p-side compared to the conduction-band offset at the n-side i-a-Si:H/n-c-Si; and asymmetric distribution of defects in the doped a-Si:H layers. The effects of individual defect-state parameters in doped a-Si:H layers and c-Si heterointerfaces are shown. We demonstrate that the decrease in conversion efficiency due to increased dangling-bond states in doped layers is primarily a consequence of a reduced fill-factor and open-circuit voltage. We explain the phenomenon by charge-redistribution linked to increased dangling-bond density in p-a-Si:H, whereas the increased free-charge recombination has only little effect on the short-circuit-current density over a wide range of defect-state variation. Besides quantitative results of defect-state variation on solar cell performance, this article aims to offer a thorough understanding of the physical processes in the device, governing these effects.