PNNL

Abstract

Heterogeneity in wide-bandgap (1.8 - 2.1 eV) mixed-halide perovskites is a key bottleneck in the development of efficient solution-processed multijunction photovoltaics. For compositionally complex mixed-iodide-bromide materials, heterogeneous crystallization of different halide-rich phases leads to the formation of distinct morphological domains. Notably, mixed-cation (formamidinium-methylammonium) wide-bandgap perovskites are prone to form micrometer-scale wrinkles, peak- and valley-like features, on the film surface in methylammonium- and bromide-rich compositions, which can interfere with the smooth surfaces ideal for multijunction devices. In situ X ray scattering reveals that these wrinkles form immediately after antisolvent casting and result from different crystallization rates of iodide- and bromide-rich perovskite phases. Nanoscopic X-ray fluorescence and hyperspectral photoluminescence imaging show that the wrinkled domains correlate to an intrinsic spatial halide segregation which results in increased local bandgap disorder and Urbach energy. Morphological and compositional heterogeneity also aggravates the formation of sub-bandgap electronic defects reducing photostability and accelerating light-induced segregation of iodide and bromide ions in thin films and solar cells.