Numerical simulations of cathodoluminescence measurements in thin-film solar cells

Cathodoluminescence (CL) measurements can be applied to assess grain-boundary (GB) and grain-interior (GI) recombination in thin-film solar cell materials and made quantitative if we can develop CL models that account for material and measurement complexities. Recently, we developed a three-dimensional numerical CL model, based in MATLAB, that simulates the GI CL intensity as a function of four parameters: grain size, GI lifetime, and GB and surface recombination velocities. The model assumes that GB electrostatic potentials are screened by the high excesscarrier densities used in the CL measurement such that transport is governed by ambipolar diffusion. Here, we develop models to address directly GB potentials and their effects on these measurements. First, we transfer the MATLAB-based model to COMSOL software, and then introduce shallow donors to the GBs to produce potentials. We also develop a two-dimensional model in COMSOL to simulate CL GB contrast with GB potentials. Simulations indicate that GB potentials can increase or decrease CL intensities relative to the zero-potential case. However, the high electron-beam currents typically applied in CL measurements minimize the impact of GB potentials.