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 sigma-pi 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. The A-cation influence on the mechanism of slow hot carrier cooling in perovskites is controversial. Here, Man et al. resolve a debated issue regarding A-cation influence on the electronic structure of lead halide perovskites.

A detailed examination of the electronic structures of methylammonium lead triiodide (MAPI) and methylammonium iodide (MAI) is performed with ab initio molecular dynamics (AIMD) simulations based on density functional theory, and the theoretical results are compared to experimental probes. The occupied valence bands of a MAPI single crystal and MAI powder are probed with X-ray photoelectron spectroscopy, and the conduction bands are probed from the perspective of nitrogen K-edge X-ray absorption spectroscopy. Combined, the theoretical simulations and the two experimental techniques allow for a dissection of the electronic structure unveiling the nature of chemical bonding in MAPI and MAI. Here, we show that the difference in band gap between MAPI and MAI is caused chiefly by interactions between iodine and lead but also weaker interactions with the MA⁺ counterions. Spatial decomposition of the iodine p levels allows for analysis of Pb–I σ bonds and π interactions, which contribute to this effect with the involvement of the Pb 6p levels. Differences in hydrogen bonding between the two materials, seen in the AIMD simulations, are reflected in nitrogen valence orbital composition and in nitrogen K-edge X-ray absorption spectra.

A detailed understanding of the surface and interface properties of lead halide perovskites is of interest for several applications, in which these materials may be used. To develop this understanding, the study of clean crystalline surfaces can be an important stepping stone. In this work, the surface properties and electronic structure of two different perovskite single crystal compositions (MAPbI₃ and Cs_xFA_1–xPbI₃) are investigated using synchrotron-based soft X-ray photoelectron spectroscopy (PES), molecular dynamics simulations, and density functional theory. The use of synchrotron-based soft X-ray PES enables high surface sensitivity and nondestructive depth-profiling. Core level and valence band spectra of the single crystals are presented. The authors find two carbon 1s contributions at the surface of MAPbI₃ and assign these to MA⁺ ions in an MAI-terminated surface and to MA⁺ ions below the surface. It is estimated that the surface is predominantly MAI-terminated but up to 30% of the surface can be PbI₂-terminated. The results presented here can serve as reference spectra for photoelectron spectroscopy investigations of technologically relevant polycrystalline thin films, and the findings can be utilized to further optimize the design of device interfaces.

Using x-ray photoelectron spectroscopy of the oxygen 1s core level, the ratio between intact (D2O) and dissociated (OD) water in the hydrated stoichiometric TiO2 (110) surface is determined at varying coverage and temperature. In the submonolayer regime, both the D2O:OD ratio and the core-level binding energy of D2O (Delta BE) decrease with temperature. The observed variations in Delta BE are shown with density functional theory to be governed crucially and solely by the local hydrogen bonding environment, revealing a generally applicable classification and details about adsorption motifs.

The electronic structures of four precursors for organic–inorganic hybrid perovskites, namely, methylammonium chloride and iodide, as well as formamidinium bromide and iodide, are investigated by X-ray emission (XE) spectroscopy at the carbon and nitrogen K-edges. The XE spectra are analyzed based on density functional theory calculations. We simulate the XE spectra at the Kohn–Sham level for ground-state geometries and carry out detailed analyses of the molecular orbitals and the electronic density of states to give a thorough understanding of the spectra. Major parts of the spectra can be described by the model of the corresponding isolated organic cation, whereas high-emission energy peaks in the nitrogen K-edge XE spectra arise from electronic transitions involving hybrids of the molecular and atomic orbitals of the cations and halides, respectively. We find that the interaction of the methylammonium cation is stronger with the chlorine than with the iodine anion. Furthermore, our detailed theoretical analysis highlights the strong influence of ultrafast proton dynamics in the core-excited states, which is an intrinsic effect of the XE process. The inclusion of this effect is necessary for an accurate description of the experimental nitrogen K-edge X-ray emission spectra and gives information on the hydrogen-bonding strengths in the different precursor materials.

Organic-inorganic halide perovskites have been intensively reinvestigated due to their applications, yet the optoelectronic function of the organic cation remains unclear. Through organic-selective resonant Auger electron spectroscopy measurements on well-defined single-crystal surfaces, we find evidence for electronic coupling in the unoccupied states between the organic and inorganic sublattices of the prototypical hybrid perovskite, which is contrary to the notion based on previous studies that the organic cation is electronically inert. The coupling is relevant for electron dynamics in the material and for understanding optoelectronic functionality.

Lead-free double perovskites, A₂M⁺M^′3+X₆, are considered as promising alternatives to lead-halide perovskites, in optoelectronics applications. Although iodide (I) and bromide (Br) mixing is a versatile tool for bandgap tuning in lead perovskites, similar mixed I/Br double perovskite films have not been reported in double perovskites, which may be due to the large activation energy for ion migration. In this work, mixed Br/I double perovskites were realized utilizing an anion exchange method starting from Cs₂AgBiBr₆ solid thin-films with large grain-size. The optical and structural properties were studied experimentally and theoretically. Importantly, the halide exchange mechanism was investigated. Hydroiodic acid was the key factor to facilitate the halide exchange reaction, through a dissolution–recrystallization process. In addition, the common organic iodide salts could successfully perform halide-exchange while retaining high mixed-halide phase stability and strong light absorption capability.

The performance of hybrid perovskite materials in solar cells crucially depends on their electronic properties, and it is important to investigate contributions to the total electronic structure from specific components in the material. In a combined theoretical and experimental study of CH3NH3PbI3-methylammonium lead triiodide (MAPI)-and its bromide cousin CH3NH3PbBr3 (MAPB), we analyze nitrogen K-edge (N Is-to-2p*) X-ray absorption (XA) spectra measured in MAPI and MAPB single crystals. This permits comparison of spectral features to the local character of unoccupied molecular orbitals on the CH3NH3+ (MA(+)) counterions and allows us to investigate how thermal fluctuations, hydrogen bonding, and halide-ion substitution influence the XA spectra as a measure of the local electronic structure. In agreement with the experiment, the simulated spectra for MAPI and MAPB show close similarity, except that the MAPB spectral features are blue-shifted by +0.31 eV. The shift is shown to arise from the intrinsic difference in the electronic structure of the two halide atoms rather than from structural differences between the materials. In addition, from the spectral sampling analysis of molecular dynamics simulations, clear correlations between geometric descriptors(N-C, N-H, and H center dot center dot center dot I/Br distances) and spectral features are identified and used to explain the spectral shapes.

Silver bismuth iodide (Ag–Bi–I) light absorbers are interesting candidates as lead-free and low-toxic metal-halide materials for solar cell applications. In this work, the partial exchange of bismuth, Bi, with antimony, Sb, is investigated in samples prepared from a solution targeting stoichiometry AgBi₂I₇. Samples with a gradually increased exchange of Bi by Sb are prepared and light absorption measurements show that the absorption edge is gradually blue-shifted with increasing the amount of Sb. This trend in the shift in combination with the X-ray diffraction and X-ray photoelectron spectroscopy measurements, suggest that new materials with a mixture of Sb and Bi are formed. The density functional theory based electronic structure calculations reproduce the trend observed in the experiments when including spin–orbit coupling, which indicates the importance of relativistic effects in these materials. X-ray photoelectron spectroscopy is used to characterize the materials, and confirms the exchange of Bi to Sb in the samples. When Sb is included in the material, the grain size changes between 50 and 200 nm and the solar cell performance also changes. An optimal power conversion efficiency with excellent reproducibility and stability is obtained for a solar cell with the ratio of Sb/Bi equal to 3.