Modeling of reverse bias dark currents in pin structures using amorphous and polymorphous silicon

The reverse bias dark currents in pin solar cells having standard amorphous silicon (a-Si:H) or polymorphous silicon (pm-Si:H) as the intrinsic layer have been studied using experiments and modeling. Our electrical-optical model, takes into account high field enhancement of thermal generation via the Poole-Frenkel effect. Polymorphous silicon films have a higher hole mobility and a lower bulk density of states (DOS) than standard a-Si:H. However this does not result in lower dark leakage currents, except in thick (∼2.5 μm) pin diodes. For the latter, we have measured dark leakage currents as low as ∼3 pAcm ^-2 at -3 V, which to our knowledge is the lowest reported for such devices. Modeling reveals that the dark leakage current is controlled by thermal generation through the mid-gap defects in the intrinsic layer for a-Si:H cells, while it is dominated by the p/i interface defects in pm-Si:H cells. We conclude by estimating from the model, the dark leakage current possible in pm-Si:H diodes, if their p/i interface DOS were to be as in standard a-Si:H ones.