# The impact of the halide cage on the electronic properties of fully inorganic caesium lead halide perovskites

## TL;DR

This study accurately measures key electronic parameters of fully inorganic CsPbX3 perovskites, revealing their stable band gap evolution and dielectric properties, which are crucial for optimizing perovskite solar cell performance.

## Contribution

It provides the first precise measurements of exciton binding energy and reduced mass in fully inorganic CsPbX3 perovskites, enhancing understanding of their electronic properties.

## Key findings

- Exciton binding energies determined for CsPbI3, CsPbBr3, and their alloy.
- No structural phase transitions observed between 4-270 K.
- Dielectric screening dominated by atomic motion within the lead-halide cage.

## Abstract

Perovskite solar cells with record power conversion efficiency are fabricated by alloying both hybrid and fully inorganic compounds. While the basic electronic properties of the hybrid perovskites are now well understood, key electronic parameters for solar cell performance, such as the exciton binding energy of fully inorganic perovskites, are still unknown. By performing magneto transmission measurements, we determine with high accuracy the exciton binding energy and reduced mass of fully inorganic CsPbX$_3$ perovskites (X=I, Br, and an alloy of these). The well behaved (continuous) evolution of the band gap with temperature in the range $4-270$\,K suggests that fully inorganic perovskites do not undergo structural phase transitions like their hybrid counterparts. The experimentally determined dielectric constants indicate that at low temperature, when the motion of the organic cation is frozen, the dielectric screening mechanism is essentially the same both for hybrid and inorganic perovskites, and is dominated by the relative motion of atoms within the lead-halide cage.