Even a little delocalization produces large kinetic enhancements of charge-separation efficiency in organic photovoltaics

TL;DR

This study demonstrates through 3D simulations that even minimal delocalization in organic photovoltaics significantly boosts charge-separation efficiency by increasing electronic state overlap, not by reducing Coulomb attraction.

Contribution

First to perform 3D simulations of charge separation in disordered organic materials considering delocalization and polaron effects, revealing kinetic enhancement mechanisms.

Findings

Minimal delocalization greatly improves charge separation efficiency.

Delocalization increases electronic state overlap, not Coulomb attraction.

Efficiency enhancement occurs even from thermalized charge-transfer states.

Abstract

In organic photovoltaics, charges can separate efficiently even if their Coulomb attraction is an order of magnitude greater than the available thermal energy. Delocalization has been suggested to explain this fact, because it could increase the initial separation of charges in the charge-transfer (CT) state, reducing their attraction. However, understanding the mechanism requires a kinetic model of delocalized charge separation, which has proven difficult because it involves tracking the correlated quantum-mechanical motion of the electron and the hole in large simulation boxes required for disordered materials. Here, we report the first three-dimensional simulations of charge-separation dynamics in the presence of disorder, delocalization, and polaron formation, finding that even slight delocalization, across less than two molecules, can substantially enhance the charge-separation efficiency, even starting with thermalized CT states. Delocalization does not enhance efficiency by reducing the Coulomb attraction; instead, the enhancement is a kinetic effect produced by the increased overlap of electronic states.