Understanding the Effect of Lead Iodide Excess on the Performance of Methylammonium Lead Iodide Perovskite Solar Cells

TL;DR

This study combines experimental and theoretical methods to understand how excess lead iodide improves methylammonium lead iodide perovskite solar cell performance by passivating defects and influencing charge dynamics.

Contribution

It reveals the mechanisms by which lead iodide excess affects surface passivation, defect formation, and charge recombination in perovskite solar cells.

Findings

Lead iodide captures holes and modulates defect formation.

Excess lead iodide reduces surface recombination velocity.

Type I and II band alignments influence passivation and hole injection.

Abstract

The presence of unreacted lead iodide in organic-inorganic lead halide perovskite solar cells is widely correlated with an increase in power conversion efficiency. We investigate the mechanism for this increase by identifying the role of surfaces and interfaces present between methylammonium lead iodide perovskite films and excess lead iodide. We show how type I and II band alignments arising under different conditions result in either passivation of surface defects or hole injection. Through first-principles simulations of solid-solid interfaces, we find that lead iodide captures holes from methylammonium lead iodide and modulates the formation of defects in the perovskite, affecting recombination. Using surface-sensitive optical spectroscopy techniques, such as transient reflectance and time-resolved photoluminescence, we show how excess lead iodide affects the diffusion and surface recombination velocity of charge carriers in methylammonium lead iodide films. Our coupled experimental and theoretical results elucidate the role of excess lead iodide in perovskite solar cells.