Rotor Lattice Model of Ferroelectric Large Polarons

Hybrid perovskites are praised for their outstanding performance as a building block of solar cells. This stems from the long carrier lifetime and diffusion length. However, they are also notoriously complex in terms of their physical properties due to their soft (liquid-like) nature. Therefore, there is a need for tractable models able to provide a qualitative understanding of the physical processes in these materials. Here, we develop a one-dimensional rotor lattice model of charge carrier-transport, which provides an intuitive physical mechanism for the recently proposed formation of a large polaron at the ferroelectric domain boundaries of lead halide perovskites [1–3]. In particular we demonstrate that the rotor-carrier coupling introduces a local ferroelectric ordering of the rotors the range of which increases with decreasing rotational constant, implying a long-range polaron coherence at the relevant parameter regimes for metal halide perovskites. The formation of this ferroelectric domain strongly modifies the effective mass of the polaron resulting to a drastic reduction of carrier mobility consistent with experimental observations. Finally, by examining the dielectric function we identify signatures of Debye relaxation along with a series of librational resonances for the rotors in frequency regimes where their coupling to phonons might be trigger the formation of ferroelectric order [3]. Our findings provide a basic framework upon which more realistic effective models for describing the carrier dynamics in lead halide perovskites can be constructed.

[1] K. Miyata, D. Meggiolaro, M. T. Trinh, P. P. Joshi, E. Mosconi, S. C. Jones, F. De Angelis, and X.-Y. Zhu, Large polarons in lead halide perovskites, Sci. Adv. 3, e1701217 (2017).

[2] K. Miyata and X.-Y. Zhu, Ferroelectric large polarons, Nat. Mater. 17, 379 (2018).

[3] F. Wang, Y. Fu, M. E. Ziffer, Y. Dai, S. F. Maehrlein, and X.-Y. Zhu, Solvated electrons in solids—ferroelectric large polarons in lead halide perovskites, J. Am. Chem. Soc. 143, 5 (2020).