Interplay between Localized and Free Charge Carriers Can Explain Hot Fluorescence in the CH₃NH₃PbBr₃ Perovskite: Time-Domain Ab Initio Analysis

Abstract

Delayed high-energy fluorescence observed experimentally in methylammonium lead bromine CHNHPbBr (MAPbBr) demonstrates long-lived energetic charge carriers with extremely high mobilities that can be used to enhance photon-to-electron conversion efficiency of perovskite solar cells. It has been suggested that hot fluorescence is associated with reorientational motions of the MA molecules. We support this hypothesis by time-domain ab initio quantum dynamics calculations showing that reorientation of the MA molecules can affect strongly the perovskite emission energy and lifetime. We demonstrate MAPbBr structures differing in the MA orientations and exhibiting the same emission properties as in the experiments. The higher bandgap structures responsible for hot fluorescence support delocalized wave functions that can be interpreted as free charge carriers. The lower energy structures exhibit localized polaron-like electrons and holes, and a significantly longer electron-hole recombination time, in agreement with experiment. The fluorescence lifetimes differ owing to variation in the nonadiabatic coupling between the emitting and ground states, stemming from charge carrier localization. Loss of coherence due to elastic electron-phonon scattering is similar in the two cases. The simulations provide a detailed atomistic understanding of excited-state dynamics in MAPbBr and show how structural transformations can rationalize the experimentally reported hot fluorescence in MAPbBr. Other localized structures involving inorganic lattice distortions, defects, domain boundaries, ion diffusion, electric ordering, etc., can be invoked with the proposed two-emitter interpretation of hot and regular luminescence.

Dynamic Disorder Dominates Delocalization, Transport, and Recombination in Halide Perovskites

“…This carrier localization and self-trapping into large polarons occurred preferentially at elevated temperatures due to the strongly anharmonic potential of the halide perovskite lattice. Fortunately, the formation of large polarons enhanced the charge recombination lifetime by nearly an order of magnitude through a combination of a small reduction in mobility, 16,36,37 reduction of wavefunction overlap, 66 and introduction of energetic barriers to electron-hole recombination. 67 Our findings reveal the influence that atomic-scale fluctuations can have on the interplay between carrier transport and recombination in halide perovskites.…”

Conformational disorder of organic cations tunes the charge carrier mobility in two-dimensional organic-inorganic perovskites

“…Owing to the unique structure of 2D OIHPs, involving polar organic molecules with many degrees of freedom embedded within a rather rigid inorganic Pb-I framework, motions of the organic component can greatly affect the perovskite photovoltaic properties such as conductivity and mobility 53 . For instance, rotation and other rearrangement of the highly anisotropic and polar organic molecules can “solvate” the charge carriers, and thus screen the interaction of charge carriers with longitudinal optical (LO) phonons, which is accompanied by deformation of the inorganic framework 54 , 55 . Unfortunately, molecular structures of [PbI 6 ] 4− -bound organic molecules are not well understood.…”

Hot carrier solar cells and the potential of perovskites for breaking the Shockley–Queisser limit

“…106 Zhang et al supported the proposition of the reorientational motion of MA molecules to lead to the pronounced hot fluorescence in MAPbBr 3 . 107 In a recently published time domain ab initio study they showed that depending on the MA orientation, low energy structures give rise to polaron-like electrons and holes, whilst the higher energy structures lead to more delocalised wavefunctions -interpreted as free carriers. They hence propose a two-emitter system of the MAPbBr 3 perovskite with hot and regular luminescence components.…”