Hybrid organic-inorganic perovskite materials have emerged over the past five years as absorber layers for new high-efficiency yet low-cost solar cells that combine the advantages of organic and inorganic semiconductors. Despite this sky rocketing evolution, the physics behind the electronic transport in these materials is still poorly understood.

Here, employing the linear response (DFPT) approach of Density Functional Theory (DFT) and frozen phonon calculations, we reveal strong anharmonic effects in the inorganic CsPbI₃ perovskite structure and found a double-well instability at the center of the Brillouin zone for both cubic and orthorhombic phases. We show that previously reported¹ soft modes are stabilized at the actual lower symmetry equilibrium structure, which occurs in a very flat energy landscape. Our results highlight the anharmonic behavior of CsPbI₃, showing that this perovskite structure can oscillate between two equilibrium states at room temperature, and allow us to give an anharmonicity-corrected value for its band gap. These results are coherent with previsouly reported low energies of the whole accoustic phonon at 80 ^◦C and unusually large Debye-Waller factors. If further taken into account into the models used for electron-phonon interactions² and band gap calculations³, these anharmonic effects could lead to a better understanding of the electrical transport properties of perovskite solar cells (PSCs).

As for hybrid perovskites, the PSC community has for some time had trouble agreeing on CH₃NH₃PbI₃ (MAPbI₃)’s exciton binding energy, and on whether it behaves more like organic compounds (high exciton binding energy, low dielectric constant) or vice-versa like inorganic compounds. Using the phonon spectrum obtained for the pseudocubic phase of MAPbI₃ and performing ellipsometry measurements, we here confirm that MAPbI3, similar to inorganic semiconductors, has a rather high relative dielectric constant (≈ 18) at low frequencies, suggesting a low exciton binding energy and confirming the electrical behavior of hybrid perovskites as free-carrier devices^4-5-6.

[1] H. Kawai et al., Nano Letters, vol. 15, pp. 3103-3108, 2015.
[2] L. D. Whalley et al., Phys. Rev. B, vol 94, 220301, 2016.
[3] C. E. Patrick et al., Phys. Rev. B, vol 92, 201205, 2015.
[4] J. Even et al., J. Phys. Chem. C, vol. 119, pp. 10161-10177, 2015.
[5] A. Miyata et al., Nature Physics, vol. 11, pp. 582-587, 2015.
[6] M. Shirayama et al., Phys. Rev. Applied, vol 5 (1), 014012, 2016.