Low-mobility solar cells: a device physics primer with application to amorphous silicon

Abstract

The properties of pin solar cells based on photogeneration of charge carriers into lowmobility materials were calculated for two models. Ideal p-and n-type electrode layers were assumed in both cases. The first, elementary case involves only band mobilities and direct electron-hole recombination. An analytical approximation indicates that the power in thick cells rises as the 1 4 power of the lower band mobility, which reflects the buildup of space-charge under illumination. The approximation agrees well with computer simulation. The second model includes exponential bandtail trapping, which is commonly invoked to account for very low hole drift mobilities in amorphous silicon and other amorphous semiconductors. The two models have similar qualitative behavior. Predictions for the solar conversion efficiency of amorphous silicon-based cells that are limited by valence bandtail trapping are presented. The predictions account adequately for the efficiencies of present a-Si : H cells in their ''asprepared'' state (without light-soaking), and indicate the improvement that may be expected if hole drift mobilities (and valence bandtail widths) can be improved. r

Carrier drift-mobilities and solar cell models for amorphous and nanocrystalline silicon

“…We shall refer to this model as the "5-parameter" model because the larger of the carrier mobilities usually has no significant effect on its predictions. This model has been developed at some length elsewhere [2,5,6], and may be directly useful in some materials [7]. For a-Si:H, we think that two additional parameters are required to give a good account of as-deposited cells (before lightsoaking is significant).…”

“…The 7-parameter model is motivated by general arguments [2,5,6] that the spacecharge of slowly drifting holes is likely to Table 1. Based on ref.…”

“…The approach we have taken is to assume that the valence bandedge E V lies within the exponential bandtail, in which case one can derive the equation relating it to the effective density-of-states N V [2]:…”

“…In c-Si the space-charge layer is the depletion region, whose width is determined by the dopant density. In a-Si:H and in other lowmobility, highly insulating materials, the width of the space-charge layer L SC is determined directly by the carrier mobilities and the photocarrier generation rate G [2]. Holes drift so slowly in such materials that the space-charge from slowly drifting holes screens the built-in potential, thus limiting the width of the region from which holes can be collected.…”

“…Holes drift so slowly in such materials that the space-charge from slowly drifting holes screens the built-in potential, thus limiting the width of the region from which holes can be collected. Denoting the limiting drift mobility as µ p , the expression for the width of this region is [2]: ( )…”