Ultrafast Charge Separation and Nongeminate Electron-Hole Recombination in Organic Photovoltaics

TL;DR

This paper extends the theory of ultrafast charge separation in organic photovoltaics to include polaron formation, explaining how delocalised transport enables rapid charge separation and reduces nongeminate recombination, thus improving device efficiency.

Contribution

It introduces a theoretical model that incorporates polaron formation into ultrafast charge separation, linking delocalised transport to reduced recombination in organic solar cells.

Findings

Delocalised transport explains ultrafast charge separation (<100 fs).

Polaron formation influences charge recombination dynamics.

The model accounts for suppression of nongeminate recombination.

Abstract

The mechanism of electron-hole separation in organic solar cells is currently hotly debated. Recent experimental work suggests that these charges can separate on extremely short timescales (<100 fs). This can be understood in terms of delocalised transport within fullerene aggregates, which is thought to emerge on short timescales before vibronic relaxation induces polaron formation. However, in the optimal heterojunction morphology, electrons and holes will often re-encounter each other before reaching the electrodes. If such charges trap and cannot separate, then device efficiency will suffer. Here we extend the theory of ultrafast charge separation to incorporate polaron formation, and find that the same delocalised transport used to explain ultrafast charge separation can account for the suppression of nongeminate recombination in the best devices.