Assessing the Validity and Accuracy of Effective Electronic Materials: Can 1D Simulations Predict Polycrystalline Device Performance?

General rules to reduce models of polycrystalline photovoltaic materials such as amorphous silicon (a-Si), cadmium telluride (CdTe), and Cu(In,Ga)Se
₂
(CIGS) to 1D have been sought for decades. However, a key limitation associated with the 1D simulation of thin-film photovoltaics is the absence of grain boundaries (GBs), which are commonly found in polycrystalline solar cells. Depending on the orientation of growth, GBs are categorized as columnar GBs (roughly perpendicular to incident sunlight) and horizontal GBs (perpendicular to incident sunlight). In this work, we investigate whether polycrystalline grain materials can be appropriately treated as an effective medium, using 1D and 2D simulations of CdTe solar cells that are CSS grown and Br:MeOH+CdCl
₂
treated. It is found that the grain boundary interface recombination has a marked effect in degrading Voc, which is consistently difficult to capture in a 1D model of an effective medium. In particular, we show that if the initial I-V curve of polycrystalline CdTe does not exhibit a low Schottky barrier, then Br:MeOH-etching the back will not necessarily improve device performance.