Mixed halide lead perovskites such as methylammonium lead iodide bromides MAPb(Br<sub>x</sub>I<sub>1-x</sub>) have emerged as one of the most promising absorber materials of future solar cells, offering high power conversion efficiencies and bandgap tunability. Among other factors the reversible phase segregation under even low light intensities is still limiting their potential use. During this process the material segregates locally into an iodide-rich and a bromide-rich phase, lowering the effective bandgap energy. While several studies have been done to illuminate the mechanism and suppression of phase segregation, fundamental aspects still remain unclear. Phase compositions after segregation vary strongly between different studies and the exact amounts often remain unmentioned. For iodide-rich phases end-point composition of around x=0.2 are widely accepted but the proportion is difficult to measure. In our research we observe the phase segregation using spectroscopic ellipsometry, a powerful, nondestructive technique that has been employed in the study of other degradation mechanisms of lead halide perovskites before. We took ellipsometric measurements from mixed halide lead perovskite thin films protected by a polystyrene layer under green laser light. Changes in the bandgap region can be correlated to the changes in composition caused by phase segregation. In addition to the composition of a sample, thickness and optical properties can be extracted from the measurement.

Spectroscopic ellipsometry measurements on phase segregation of mixed halide lead perovskites

ACS Spring 2022

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.