Analysis of effects of dangling-bond defects in doped a-Si:H layers in heterojunction silicon solar cells with different electron affinities of ITO contacts

Abstract

The effects of dangling-bond defects in doped hydrogenated amorphous-silicon layers (p-a-Si:H and n-a-Si:H) in heterojunction silicon (SHJ) solar cells are studied in relation to applied Indium-Tin-Oxide (ITO) contacts with different electron affinities. A state-of-the-art numerical model of the SHJ solar cell was employed, including ITO contacts as full, volumetric semiconductor layers, applying the trap-assisted, band-to-band and direct tunnelling mechanisms at heterointerfaces in the device. The levels of dangling bond defect concentrations were varied in both p-a-Si:H and n-a-Si:H layers and ITOs with two different electron affinities were considered at both sides of the device. We show that the effects of the defects on the short-circuit current density, open-circuit voltage, fill factor and conversion efficiency of the device become more pronounced if ITOs with non-optimal electron affinities are used. Possibility to reach higher doping levels of the doped a-Si:H layers would mitigate the effects of its dangling bond states, which becomes more important if ITO electron affinity is not optimized to the doped a-Si:H layers. We demonstrate that the reduced efficiency due to the increase in dangling-bond density originates from the decrease of the fill-factor and open-circuit voltage, whereas the short-circuit current density has a small effect on efficiency for the chosen variation span. The reduction of the fill-factor is further explained by a drop in maximum-power-point voltage, which is more pronounced if optimization of ITO electron affinity is not taken into account.