Rigorous Optical Dispersion Data Analysis of Single Crystal MAPbBr₃ for Optimized Perovskite Solar Cell Active Layer Absorption

Lead halide perovskites have garnered significant attention for optoelectronic applications including photovoltaics, light-emitting diode, photodetector, and laser technologies. However, material quality and stability issues lead to a wide range of reported values for the electrical and optical properties which may complicate device design and optimization strategies for commercialization. Critical to the development of perovskite solar cells (PSC) are the optical properties (refractive index, n, and extinction coefficient, k), which describe the way perovskites interact with light and subsequently function as a photovoltaic material. Optimizing perovskite active layer absorption necessitates solar cell optical coating design iterations using material optical constants derived from rigorous optical dispersion data analysis. The refractive index and extinction coefficient for MAPbX₃ have been observed to significantly vary in the literature depending on the processing methodologies. If such optical dispersion variation is ignored in practice at the design stage, PSCs can exhibit parasitic device absorption by non-perovskite (non-active) layers resulting in variable optical transmission, reflectance, or haze-like characteristics within solar radiation bands of interest (300-2500 nm). To our knowledge, no comprehensive experimental optical dispersion data analysis to derive the optical constants of MAPbX₃ has been performed to include notable anomalous optical dispersion characteristics including the band edge transition (bandgap ~539 nm) and methylammonium spectral overtones below the band edge (observed within 1050-2050 nm). As such, the optical property determination of exemplary MAPbX₃ has been largely under approximated and, consequently, evokes optical property uncertainty with respect to prior reports. Proper fundamental derivation of MAPbX₃ optical constants is critical toward the development of high-performance electronic and optoelectronic perovskite devices. Here, we present a rigorous optical dispersion data analysis of single crystal MAPbBr₃ via variable angle spectroscopic ellipsometry appended with transmission intensity spectra to provide a robust derivation of the optical constants for normal and anomalous optical dispersion regimes. Using derived optical constants for our single crystal MAPbBr₃, exemplary modeled solar cell optical device designs are optimized for perovskite layer absorption and we compare our derived optical properties to illustrative MAPbBr₃ literature values reported to-date.