On understanding bandgap bowing and optoelectronic quality in Pb-Sn alloy hybrid perovskites

High quality small-bandgap hybrid perovskites (AMX(3) with M = Pb1-xSnx) are pivotal for all-perovskite multi-junction photovoltaics. The bandgap of these alloys significantly deviates from the linear interpolation between the bandgaps of APbI(3) and ASnI(3) for all A-site cations examined thus far. This non-linearity of the bandgap with composition is referred to as bandgap bowing. Here, we explore a wide-range of A-site compositions to understand bandgap bowing and identify the optimal Pb-Sn alloy composition. Optical and structural investigations of different APb(1-x)Sn(x)I(3) alloys reveal that the bandgap bowing is correlated with the extent of microstrain in their respective APbI(3) compounds. We discover that bandgap bowing in APb(1-x)Sn(x)I(3) alloys is primarily due to local structural relaxation effects (changes in bond angles and lengths) that result from the size, shape, and charge distribution of the cations on the A-site, and that these effects are intimately coupled with chemical effects (intermixing of atomic orbitals) that result from changes in the M-site. The choice of X-site also impacts bandgap bowing because of the X-site anions' influence on local structural relaxation and chemical effects. Further, we extend these results to provide a general rationale for the origin and modulation of bandgap bowing in HP alloys. Subsequently, using high-throughput combinational spray coating and photoluminescence analysis, we find that ternary combinations of methylammonium (MA), formamidinium (FA), and cesium (Cs) are beneficial to improve the optoelectronic quality of APb(1-x)Sn(x)I(3) alloys. The optimal composition, (MA(0.24)FA(0.61)Cs(0.15))(Pb0.35Sn0.65I3)I-3 has a desirable low bandgap (1.23 eV) and high optoelectronic quality (achieving 86% of the detailed balance limit quasi-Fermi level splitting). This study provides valuable insights regarding bandgap evolution in HP alloys and the optimal small-bandgap absorber composition desired for next-generation HP tandems.