Phase Stability of MAxFA1−xPbI3 at elevated temperatures and its effects on photoluminescence

Abstract

Halide perovskites are a promising class of solution-processable materials with useful applications for photovoltaics. Methylammonium / formamidinium lead iodide (MAxFA1−xPbI3) in particular has a suitable bandgap and good charge-transport properties for photovoltaic applications. However, achieving stability of the film structure in the face of elevated temperatures and humidity is a challenge. In this paper, I examine the structure of polycrystalline MAxFA1−xPbI3 thin films for x = 0,0.20.4,0.6,0.8, and 1 from room temperature up to 200◦ C or until degradation occurred. I found that the films remained in the cubic phase from room temperature until degradation for FA cation concentrations between 20% and 80%. Phase transitions from tetragonal to cubic occurred for the composition range 0.9 ≤ x ≤ 1 at temperatures from 35◦ C at a 90% concentration of MA cations to 57◦ C for a pure MAPbI3 film. Co-existences of the cubic perovskite and the hexagonal non-perovskite phase occurred for the composition window 0 ≤ x ≤ 0.1. I then examined the relationship between photoluminescence (PL), which provides information about bandgap, and temperature to determine if phase changes would impact this relationship. The PL peak energy is not recoverable in MAPbI3 upon heating and cooling, and the film’s peak PL energy exhibits a sharper increase with temperature across the tetragonal-cubic transition than when the film is entirely in the cubic phase. However, this behavior seems to be unique to the MAPbI3 film, and is not observed in films containing FA cations. This indicates that the FA cations help stabilize the temperature dependence of bandgap when the film is exposed to thermal stresses.