Impact of threading dislocations on both n/p and p/n single junction GaAs cells grown on Ge/SiGe/Si substrates

Single junction GaAs solar cells having an n/p polarity were grown on p-type Ge/SiGe/Si substrates for the first time. The cell performance and material properties of these n/p cells were compared with p/n cells grown on n-type Ge/SiGe/Si substrates for which record high minority carrier hole lifetimes of 10 ns and open circuit voltages (V_oc) greater than 980 mV (AM0) were achieved. The initial n/p experimental results and correlations with theoretical predictions have indicated that for comparable threading dislocation densities (TDD), n/p cells have longer minority carrier diffusion lengths, but reduced minority carrier lifetimes for electrons in the p-type GaAs base layers. This suggests that a lower TDD tolerance exists for n/p cells compared to p/n cells, which has implications for the optimization of n/p high efficiency cell designs using alternative substrates.