INFLUENCES OF ASYMMETRICALLY DISTRIBUTED DEFECT STATES AT REAR c-Si/a-Si:H INTERFACE ON PERFORMANCES OF SILICON HETERO-JUNCTION SOLAR CELLS

Abstract

The performances of silicon double hetero-junction solar cells are investigated by the numerical simulator "Automat For Simulation of Hetero-structures (AFORS-HET)". Asymmetrical distribution of acceptor and donor defect states at the rear hetero-junction interface is introduced in both a-Si:H(n)/c-Si(p)/a-Si:H(p+) and a-Si:H(p)/c-Si(n)/a-Si:H(n+) structures in our study. The influences of total defect density and rear emitter parameters (doping concentration and band discontinuity to silicon substrate) on performances of the solar cells are analyzed. The simulated results indicate that the npp + cells with acceptor dominated defect states at the rear interface can obtain better performances, and the same for the pnn + cells with donor dominated defect states. Moreover, the npp + cells are more sensitive to the total interface defect density and its distribution mode due to the unfavorable band offset between the absorber and the rear emitter. But when total defect density is small, the npp + cells show more sensitivity to change of the doping concentration at the rear emitter than the pnn + cells, and conversely, the pnn + cells show more sensitivity to the distribution mode of defect states at the rear emitter than the npp + cells. Due to weaker inherent back diffusion ability of minority carrier, the pnn + cells with moderate band offset (0.25 ~ 0.37 eV ) at the rear hetero-junction are more sensitive to the incentive effect of localized charges at the interface formed by asymmetrically distributed defect states, and the interface with donor dominated defect states is a better option for the pnn + cells when the total defect density is relatively small.