Absolute energy level positions in tin and lead based halide perovskites

TL;DR

This paper systematically characterizes the energy levels of tin and lead halide perovskites, revealing how atomic properties and lattice structure influence their electronic properties, aiding the design of better optoelectronic devices.

Contribution

It provides the first comprehensive analysis of ionization energy and electron affinity in tin and lead perovskites, linking energy levels to atomic and structural factors.

Findings

Energy levels are mainly determined by metal cation and halide anion atomic levels.

Cation-anion interaction strength affects secondary energy level variations.

Results enable rational design of perovskite-based optoelectronic devices.

Abstract

Metal-halide perovskites are promising materials for future optoelectronic applications. One intriguing property, important for many applications, is the tunability of the band gap via compositional engineering. While experimental reports on changes in absorption or photoluminescence show rather good agreement for wide variety of compounds, the physical origins of these changes, namely the variations in valence band and conduction band positions, are not well characterized. Knowledge of these band positions is of importance for optimizing the energy level alignment with charge extraction layers in optoelectronic devices. Here, we determine ionization energy and electron affinity values of all primary tin and lead based perovskites using photoelectron spectroscopy data, supported by first-principles calculations. Through analysis of the chemical bonding, we characterize the key energy levels and elucidate their trends via a tight-binding analysis. We demonstrate that energy level variations in perovskites are primarily determined by the relative positions of the atomic energy levels of metal cations and halide anions. Secondary changes in the perovskite energy levels result from the cation-anion interaction strength, which depends on the volume and structural distortions of the perovskite lattices. These results mark a significant step towards understanding the electronic structure of this material class and provides the basis for rational design rules regarding the energetics in perovskite optoelectronics.