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 nonuniform minority-carrier lifetime τ and carrier density NA on device performance. In a uniformly doped device, maximum open-circuit voltage V_oc is obtained for high τ and high NA. The fill-factor (FF) is mainly dependent on the lifetime. When the lifetime is low, and NA is high, the FF suffers losses due to voltage-dependant carrier collection. For a low carrier density and low lifetime, the electric field strength is low, recombination is a competitive process to drift, and the FF is reduced. Simulations predict that it might be possible to increase the device efficiency with lower carrier density, 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, while keeping the space-charge region wide. In addition, a device with such a profile is very tolerant toward lifetime variations of the highly doped layer. With our simulation parameters, when the absorber properties are uniform, efficiencies >;18% require experimentally unrealistic doping and lifetime values. If the back of the absorber is doped significantly higher than the rest, such efficiencies can be achieved with realistic values of doping and lifetime.