Polaron transport in hybrid CH3NH3PbI3 perovskite thin films

Abstract

A comprehensive study of the transport properties of a prototypical CH₃NH₃PbI₃ thin film is presented. The polaron–longitudinal optical (LO) phonon scattering mechanism, based on Low-Pines's polaron mobility, was studied to elucidate the charge-carrier mobility. We found that the calculated mobilities showed very good quantitative agreement with the experimental data measured in thin film samples using photoconductivity techniques. In THz mobility, the calculated results yielded room-temperature (RT) mobilities of ∼650 cm² V⁻¹ s⁻¹ (single crystal) and ∼220 cm² V⁻¹ s⁻¹ (disordered thin film) at a low quantum yield (ϕ) and 32 cm² V⁻¹ s⁻¹ (high-quality thin film) at ϕ = 1. The dynamic disorder due to organic reorientation was included in the calculations. Its effect provided a power law mobility of μ ∝ T^m and satisfactorily supported temperature-dependent mobility over the temperature range of 80–370 K. In the orthorhombic and tetragonal phases, the charge-carrier mobilities with dynamic disorder were approximately 47% and 22% lower than those obtained from phases without dynamic disorder. The RT mobility was 26 cm² V⁻¹ s⁻¹ at ϕ = 1. In the low-temperature orthorhombic phase, the structural phase transition was considered. The mobility followed a power law with m = −1.7. In the tetragonal and cubic phases, the mobility also followed a power law, but with m = −1.1, which is an intermediate range in optical phonon scattering. When combined with recent theoretical analysis, we also found three limitations of power law mobility with exponents between −0.46 and −1.1 for polaron–LO phonon scattering, −1.2 and −1.6 for bare carrier–LO phonon scattering, and −1.7 and −2.0 for carrier scattering off optical phonons and lattice fluctuations. This work not only provides a description of temperature-dependent mobility in CH₃NH₃PbI₃ thin films, but also gives new insights into THz photoconductivity and the relationship between LO phonon scattering and power law mobility.