The Potential of Geminate Pairs in Lead Halide Perovskite revealed via Time-resolved Photoluminescence

TL;DR

This study uses time-resolved photoluminescence to investigate geminate pairs in lead halide perovskites, revealing energy losses and potential efficiency improvements in solar cells.

Contribution

It introduces new PL-based concepts to quantify transient electron-hole pair properties and identifies a significant unutilized energy in perovskites.

Findings

Hot charge carrier separation occurs within ~100 nm.

Geminate pairs exhibit a correlation energy up to 90 meV.

Energy loss due to geminate pairs is often overlooked in efficiency models.

Abstract

Photoluminescence (PL) under continuous illumination is commonly employed to assess voltage losses in solar energy conversion materials. However, the early temporal evolution of these losses remains poorly understood. Therefore, we extend the methodology to time-resolved PL, introducing the concepts of geminate PL, doping PL, and sibling PL to quantify the transient chemical potential of photogenerated electron-hole pairs and key optoelectronic properties. Analyzing the initial PL amplitudes reveals hot charge carrier separation for around 100 nm and is likely limited by the grain size of the triple cation perovskite. The following PL decay is caused by the diffusive separation of non-excitonic geminate pairs and time-resolves a fundamental yet often overlooked energy loss by increasing entropy. For triple-cation halide perovskite, we measure a "geminate correlation energy" of up to 90 meV, persisting for ~ten nanoseconds. This energy is unutilized in standard solar cells and is considered lost in the Shockley-Queisser model. Therefore, this geminate energy could substantially enhance the device's efficiency, particularly under maximum power point and low-illumination conditions.