A-site cation influence on the conduction band of lead bromide perovskites

Abstract

Hot carrier solar cells hold promise for exceeding the Shockley-Queisser limit. Slow hot carrier cooling is one of the most intriguing properties of lead halide perovskites and distinguishes this class of materials from competing materials used in solar cells. Here we use 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 find that it is strongly influenced by the strength of electronic coupling between the A-cation and bromide-lead sublattice. Our finding provides an alternative mechanism to the commonly discussed polaronic screening and hot phonon bottleneck carrier cooling mechanisms. Our 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.

Electronic Structure and Chemical Bonding in Methylammonium Lead Triiodide and Its Precursor Methylammonium Iodide

“…This is also seen in the peak at 9.7 eV. Close inspection of the three prominent conduction band peaks from 1 to −3 eV shows that p x and p y , representing π interactions, and the p z σ bond all contribute equally to the first peak, while the second and third peaks are predominantly from I p z sigma bonds, in agreement with previous research on the conduction band of bromide HOIPs . Moreover, the I p z levels match reasonably closely the lead levels in the valence band edge because of the σ character, while the p x and p y levels with their out-of-plane π character do not.…”

“…In both MAPI and MAI materials, the MA + ions can interact with the iodide ions, but MAPI additionally has an inorganic PbI 3 – framework, which is sensitive to the organic cations . In a recent study, we have shown that the valence and conduction bands of lead bromide perovskites (APbBr 3 ) are strongly influenced by interactions with the positive cations at the A-site …”

Spatial microheterogeneity in the valence band of mixed halide hybrid perovskite materials

“…The hydrogen bonding yields a slight nitrogen admixture in the upper valence band, which recently has been discussed in the context of nitrogen K-edge X-ray emission 29 and resonant Auger 30 spectroscopy. With variations of the A ions in lead bromide perovskites, it has been shown in measured and simulated bromine K-edge X-ray absorption spectra that hydrogen bonding strongly influences bromine σ * and π * bands in the conduction band 31 .…”

“…With variations of the A ions in lead bromide perovskites, it has been shown in measured and simulated bromine K-edge X-ray absorption spectra that hydrogen bonding strongly influences bromine σ* and π* bands in the conduction band. 31 …”

“…With variations of the A ions in lead bromide perovskites, it has been shown in measured and simulated bromine K-edge X-ray absorption spectra that hydrogen bonding strongly inuences bromine s* and p* bands in the conduction band. 31 In general, the halide X site can be occupied by I, Br, Cl, or in principle any combination of the three for the system of CH 3 -NH 3 Pb(I 1Àx Br x ) 3Ày Cl y . Variation in the halide composition has previously been demonstrated to strongly inuence both the band gap in the material and the PCE of the solar cell.…”