Current–voltage curve asymmetry in a-Si:H solar cells

Abstract

The J–V curve asymmetry of an a-Si:H solar cell, that is exhibited under carrier mobility interchange, has been investigated computationally. The electron- and hole mobilities ( μ n and μ p) considered here are 13.2 and 2.2 cm 2V −1 s −1. We use the symbol ( n, p) to denote the case μ n =13.2 cm 2 V −1 s −1, μ p =2.2 cm 2 V −1 s −1, and the symbol ( p, n) denotes the case μ p=13.2 cm 2 V −1 s −1, μ n =2.2 cm 2 V −1 s −1. The maximum power output P M for the case ( n, p) is some 153% greater than that for the case ( p, n). We have investigated the possibility that this asymmetry is due to the asymmetrical character of the material and structural properties of the device. For example, the p-layer is thinner than the n-layer (we regard this as a positional asymmetry) and the dangling-bond defects in the p-layer are closer to midgap than the dangling-bond defects in the n-layer (we regard this as an energetic asymmetry). Other asymmetries include the dopant parameters, intrinsic layer defects, tail states and optical electron–hole pair generation rate profile. We eliminate the various asymmetries to construct a symmetrical cell which we find exhibits no difference in P M for the two cases. We then set about introducing the various asymmetries into the symmetrical cell. We find that doped layer thickness asymmetry favours the case ( p, n), but with only a slight difference in P M of about 0.5%. Dopant parameter asymmetry does not cause P M to differ for either case. Asymmetries pertaining to defects in the doped layers favour the case ( p, n) with a P M difference of about 3%. Intrinsic layer defect asymmetry has a moderate effect (20%) and favours ( n, p). An effect of similar magnitude (34%) arises from tail state asymmetries, although in favour of ( p, n). However, the positional asymmetry of the optical generation rate profile causes P M for the case ( n, p) to be some 227% greater than for the case ( p, n). It is clear that the asymmetry in the actual cell (153%) must be dominated by the asymmetry in the optical generation rate profile, since this was the only asymmetry capable of producing an effect large enough to be consistent with that observed in the actual cell. We conclude that the asymmetry in the actual cell performance is due to the nett effect of the various material and structural asymmetries, but that the positional asymmetry of the optical generation rate profile is the dominant cause.