A Theoretical Perspective on the Impact from Compositional Disorder on the Electronic Properties of Mixed Halide Perovskites

Halide mixing is a key strategy to tune the optical absorption of metal halide perovskites within the full visible spectrum.[1] Since the pivotal demonstration of the progressive and monotonic change in the optical absorption from 800 nm to 550 nm, going from MAPbI3 (MA=methylammonium) to pure bromine MAPbBr3, through mixed iodine/bromine compositions,[2] halide mixing has emerged as a very effective strategy to obtain color-tunable materials, with implications for Light-Emitting Diodes (LEDs),[3] as well for tandem solar cells.[4] In turn, such compositional inhomogeneity inherently represents a source of disorder. Therefore, within the general consensus that disorder is negative for semiconductor, a fundamental question raises “whether such disorder may negatively influence the intrinsic electronic properties of halide perovskites, and to which extent?”
Early optical spectroscopic investigations via Urbach tail analysis and photoluminescence linewidths highlighted that pure halide perovskites present incredibly low Urbach energies, further pinpointing their exceptional electronic properties. Furthermore, they showed that halide mixing is not accompanied by significant change in the optical absorption onset, at least for moderate mixing.[5] Still, complex phenomena as light-triggered ion diffusion and formation of halide segregated domains hinders the rationalization of the experimental findings, hence calling for complementary theoretical simulations.
Here we present the results from atomistic, parameter-free, periodic Density Functional Theory (DFT) simulations of mixed halide perovskites. Cutting edge hybrid DFT calculations including spin-orbit-coupling point out that halide substitution does not lead to formation of intragap trap states, both in the dilute limit and in the case of homogeneously distributed iodine/bromine ions. Only in the case of halide segregation, the valence band maximum localizes in the iodine-rich domain, suggesting that the large-band gap, bromine-segregated domains may act as a barriers to hole diffusion, ultimately leading to unbalanced electron/hole transport.[6] With experimental evidences showing the main role from homogeneous broadening on the evolution of the Urbach energy with the temperature, we also included thermal-driven vibrational motion, resorting to DFT-based molecular dynamics simulations.[7] This allows to grasp the influence on the electronic properties due to “static” compositional disorder from iodine/bromine mixing and “dynamic” structural disorder associated with phonons, at once. The band gap broadening resulting from real-time ion dynamics falls in reasonable agreement with linewidth from photoluminescence,[8] finite-temperature simulations showing very small inhomogeneous contribution to broadening, both for pure halide and homogeneously dispersed iodine/bromine compositions. The increased broadening in segregated phases, potentially associated with charge carrier recombination, also parallels experimental finding and is related to an enhanced, inhomogeneous, static contribution. This peculiar behavior is associate with lattice mismatch at the iodine-rich/bromine-rich interface. All in all, our theoretical analyses point out that halide mixing does not negatively influences the electronic properties of perovskite semiconductors, except in the case of halide segregation, where more complicated mechanisms appear.

[1] L. N. Quan, et al. Chem. Rev. 2019, 119, 7444-7477
[2] J. H. Noh, et al. Nano Lett. 2013, 13, 1764-1769
[3] B. Zhao, et al. Nat. Photonics 2018, 12, 783-789
[4] J. Liu, et al. Science 2022, 377, 302-306
[5] A. Sadhanala, et al., J. Phys. Chem. Lett., 2014, 5, 2501-2505
[6] V. Diez-Cabanes, et al., 2021, 9, 2001832/1-10
[7] V. Diez-Cabanes, et al., Adv. Optical Mater., 2024, 12, 2301105/1-13
[8] A. D. Wright, et al., Nat. Commun. 2016, 7, 11755/1-9