Excitonic Properties versus Structure Stability Trade‐Off in Halide Perovskite Photovoltaics Caused by van der Waals Interactions

Lead halide perovskites, and their derivatives, are among the most promising photovoltaic materials for third generation solar cells. Despite the large number of available works on some of these materials, excitonic properties whose assessment has been challenging are less investigated. These include quantitative measures of excitonic properties variations with van der Waals (vdW) interactions. Consistent comparisons of how vdW interactions affect phononic and optical properties are also desirable. This work focuses on cubic phases of with X = Cl, Br, I, and MA = methylammonium, using density functional theory simulations including vdW interactions. These cause 30%–38% increase of absolute cohesive energies and 15%–37% reduction of ionic/vibrational contributions to static dielectric constants, along with 10%–29% reduction of exciton Bohr radii and 29%–107% increase of exciton binding energies. The effects on band gaps, frequency‐dependent dielectric functions, and exciton effective masses are less pronounced. Within the Mott–Wannier exciton model, the results suggest a trade‐off between photovoltaic performance and structure stability. The results can help assess stability, feasibility, and performance of hybrid photovoltaic materials.