Why does the open-circuit voltage in a micro-crystalline silicon PIN solar cell decrease with increasing crystalline volume fraction?

In hydrogenated micro-crystalline silicon (μc-Si:H) thin film solar cells, the open-circuit voltage (VOC) shows a decline when the crystalline volume fraction (Fc) of the intrinsic μc-Si:H layer increases from 60% to over 90%. In this article we have simulated the experimental characteristics of solar cells, having intrinsic layers of different degrees of crystallinity to understand the reasons why. In order to model all aspects of the characteristics, we had to assume (a) wider band tails, (b) a higher mid-gap defect density and (c) a lower band gap for the more crystallized material. Modeling reveals that all three factors lower the field in the volume of the device and hence VOC, due to higher photo-generated hole-trapping close to the P/I interface. The third factor brings the quasi-Fermi levels closer to the band-edges, resulting in higher free and trapped carrier densities throughout the device, with the trapped hole population particularly high at the P/I interface. We further show that VOC is higher in a crystalline silicon PN cell, in spite of a sharply reduced band gap, because the lower effective density of states at the band-edges and sharply reduced band gap defect density overcome the effect of the smaller band gap.