Researchers from Uppsala University, Stockholm University, Deutsches Elektronen-Synchrotron DESY, KTH Royal Institute of Technology and Tata Institute of Fundamental Research have used the element selectivity of high-resolution X-ray spectroscopy and density functional theory to uncover a previously hidden feature in the conduction band states, the σ-π energy splitting, and found that it is strongly influenced by the strength of electronic coupling between the A-cation and bromide-lead sublattice.
These findings provide an alternative mechanism to the commonly discussed polaronic screening and hot phonon bottleneck carrier cooling mechanisms. The new work emphasizes the optoelectronic role of the A-cation, provides a comprehensive view of A-cation effects in the crystal and electronic structures, and outlines a broadly applicable spectroscopic approach for assessing the impact of chemical alterations of the A-cation on perovskite electronic structure.
The team reported a joint experimental and computational investigation of three prototypical lead bromide perovskites (APB): MAPB, formamidinium lead tribromide (FAPB) and cesium lead tribromide (CsPB), facilitated by single crystals which are durable under X-ray irradiation. Density functional theory (DFT) was employed for ab initio molecular dynamics simulations (AIMD) combined with ground-state PDOS and bromine K-edge XAS calculations. Through this joint experimental-computational investigation, the team showed that the energetic width of the conduction band depends on the strength of electronic coupling between the A-cation and the lead-bromide sublattice. The finding of the σ–π splitting in the conduction band provides an additional mechanism to consider, in the search for the origins of slow hot carrier cooling and polaronic transport. In both the occupied and unoccupied states, the researchers found a correlation between higher coupling strength and higher bromide-lead bond ionicity and a decreased energy offset between a given energy level and Br 1s, and discussed the implications for energy level matching at perovskite device interfaces. They foresee the spectroscopic approach they employed in this work being utilized to study A-cation effects in other halide perovskites and potentially non-halide perovskites as well, thus unraveling unexplored structure-property relationships in these materials and furthering the development of optoelectronic devices.
