Compositional flexibility in halide perovskite is a key advantage for their exploitation in energy related applications. By tuning the halide component, from pure iodine-, bromine-, chlorine-, throughout intermediate compositions, it is possible to shift the material band gap within all the visible range, which is a fundamental prerequisite for color-management in Light Emitting Diodes (LEDs) and light-absorption management in photovoltacs.¹ In particular, perovskites with mixed iodide-bromide compositions have been successfully used as wide gap component in tandem solar cells. Interestingly these materials showed excellent photovoltaic performances,² which is indicative of excellent native electronic properties, as further observed from Urbach tail analysis.³

This fact, in turn raises an important fundamental question, on how such apparently resilient electronic structure can co-exist with the inherent compositional inhomogeneity for the halide component. Otherwise stated, is it possible that the inherent chemical disorder stemming from the inhomogeneous halide composition does not negatively impact on the electronic properties of halide perovskites and on the performances of related devices?

To clarify this point, we performed electronic structure simulations of pure- as well as mixed-halide iodide/bromide lead-based perovskites, aiming at discriminating the impact from different sources of disorder. Analyses performed by neglecting thermal disorder show that the electronic properties of mixed halide perovskites are not negatively influenced by the compositional disorder, intrinsically. No intragap trap states associated with atomic substitutional defects of a narrow gap component into a wide gap one is observed, as in the case of one iodine substitution into a lead-bromine lattice. Differences associated with the inhomogeneous local potential felt by electrons and holes in the material gets equalized by electron delocalization, as described by hopping the integrals parameters in a tight-binding Hamiltonian representation. Halide mixing is negative instead when phase segregation kicks in. In  that case, the wide gap component may act as a potential barrier for transport, especially for hole diffusion.⁴ Additional analyses performed including thermal disorder, included via ab-initio molecular dynamics, point out towards electronic properties for finely intermixed halide phases that closely resemble those of their a pure-halide analogues. In turn, halide segregated domains feature both enhanced disorder from inhomogeneous source, potentially associated with interfacial states at the domain boundaries.⁵ All in all, compositional disorder in the halide component does not negatively impact the electronic properties of the mixed-halide perovskites, with only caveat of controlling halide distribution over the time. Mixed halide perovskites-based devices have therefore no inherent limitations, compared with their pure halide-based analogues.

1. J. H. Noh, S. H. Im, J. H. Heo, T. N. Mandal, S. I. Seok, Nano Lett. 13, 1764, 2013.
2. K. O. Brinkmann, et al., Nature, 604, 280, 2022.
3. A. Sadhanala, et al., J. Phys. Chem. Lett. 5, 2501, 2014.
4. V. Diez, J. Even, D. Beljonne, C. Quarti, Adv. Optical Mater. 9, 2001832, 2021.
5.  V. Diez, S. Giannini, D. Beljonne, C. Quarti, Adv. Optical Mater. 12, 2301105, 2024.