Optimizing CdTe solar cell performance; Impact of variations in minority carrier lifetime and carrier density profile

Using numerical simulations, we study the combined effects of minority carrier lifetime and carrier density variation, that is observed in actual CdTe solar cells, on device performance. High open-circuit voltage (Voc) requires high built in potential as well as low recombination rate in the space-charge region (SCR). Therefore, in a uniformly doped device, maximum open-circuit voltage will be obtained for high carrier density and high lifetime. The fill-factor (FF) is mainly dependent on the minority carrier lifetime. In a case of a highly defective absorber with high carrier density, the SCR is narrow in forward bias and some of the photogeneration occurs outside of the SCR. When the diffusion length is short, these carriers are likely to recombine before being collected, thus creating voltage dependent collection, and FF decrease. For a low carrier density, on the other hand, the electric field strength in the SCR is low enough, that for low lifetimes recombination is a competitive process to drift. This again reduces the FF for low lifetimes. Simulations predict that it is possible to increase the device efficiency for lower carrier density, 1013–1014 cm−3, if the back of the absorber is highly doped. This configuration increases the built in potential and the electric field close to the junction region. In addition, a device with such a doping profile is very tolerant towards the minority carrier lifetime of the highly doped layer. Our simulations show that efficiencies greater than 18% are possible using high doping at the back of the absorber. In contrast, when the absorber properties are uniform, efficiencies >18% require unrealistic doping and lifetime values.