Effects of grain boundaries on conversion efficiencies of single-crystal-like GaAs thin-film solar cells on flexible metal tapes

Abstract

We present a numerical simulation study of the effects of low-angle grain boundaries (GBs) in a single-crystal-like GaAs thin-film material on its photovoltaic performance. Here, 1D and 2D modeling are employed to simulate a solar-cell (SC) device based on the properties of single-crystal-like GaAs thin films grown on metal tape. Properties include minority carriers’ mobility, carrier lifetimes, and diffusion length. The 1D model, by incorporating a uniform biaxially textured GaAs compared to single crystal GaAs, predicts an efficiency ∼21% for average carrier lifetimes of 1 ns. This result is not consistent with an experimental results showing an efficiency of 4.3%. 1D simulation generally works well for the prediction of conventional crystalline SC I-V characteristics, but it fails for newly-developed single-crystal-like GaAs SC devices with different material properties and non-vertical device geometry. Hence, we develop a 2D model to study the effect of localized recombination centers in the material by defining different regions of defective low-angle grain boundaries and single crystalline intra-grains. The 2D model is comparable to the experimental results mainly due to its capability to consider localized material inhomogeneity and lateral carriers dynamic. The effect of grain boundary density on SC performance is studied by 2D modeling. Increasing the grain size of GaAs from 2 μm to 50 μm can improve efficiency from 4.8% to 12.3%. The open-circuit voltage (Voc) shows more sensitivity to varying grain boundary densities than other SC characteristic factors. The 2D model is also employed to study bulk passivation of GBs and suggests that an efficiency of ∼19.5% is achievable in the single-crystal-like GaAs SCs even with a small grain size of 2 μm, if effective grain boundary passivation is applied.