3D/2D Passivation as a Secret to Success for Polycrystalline Thin-Film Solar Cells

Polycrystalline photovoltaic materials offer low costs and good scalability; however, grain boundaries in these materials are extended defects, which can drastically increase carrier recombination. Interestingly, three leading polycrystalline thin-film technologies—cadmium telluride (CdTe), CuIn_1-xGa_xSe₂ (CIGS), and perovskite solar cells (PSCs)—passivate absorber surfaces in the same way: via formation of low-dimensional, typically two-dimensional (2D), van der Waals materials. This has primarily occurred serendipitously through process optimization, but in some cases, 2D capping layers are intentionally incorporated to improve device performance. Here, evidence compiled from the literature, supplemented with new data where necessary, is presented to illustrate the existence of 3D/2D interfaces in CdTe, CIGS, and PSCs, and their correlation with improved passivation and device performance. This suggests that 3D/2D passivation might be a heretofore unappreciated key to successful polycrystalline thin-film photovoltaics. Finally, the desired attributes of successful low-dimensional layers are presented with rational design strategies for next-generation polycrystalline solar cells.