# Non-equilibrium thermodynamics of charge separation in organic solar cells

## TL;DR

This paper develops a theoretical framework using stochastic thermodynamics to analyze charge separation in organic solar cells, highlighting the roles of disorder, delocalization, and entropy in efficiency.

## Contribution

It introduces a novel non-equilibrium thermodynamic model for charge separation in OSCs, linking microscopic dynamics with thermodynamic quantities and transient behaviors.

## Key findings

- High disorder leads to equilibrium-like site populations.
- Delocalized pairs show non-equilibrium behavior at low disorder.
- Large Gibbs entropy and initial separation are key for efficiency.

## Abstract

This work presents a novel theoretical description of the non-equilibrium thermodynamics of charge separation process in organic solar cells (OSCs). Using the theory of stochastic thermodynamics, we connect the phonon-assisted dynamics and recombination of electron-hole pairs within a photo-excited organic bilayer with the thermodynamic free energy. We analyze the impact of energetic disorder and delocalization on the free energy, average energy and entropy. For high energetic disorder, the site population is well described by equilibrium. We observe significant deviations from equilibrium for delocalized electron-hole pairs at small energetic disorder, representing efficient OSCs. Our results emphasize that both a large Gibbs entropy and large initial separation are required to achieve efficient charge separation. A decrease in free energy barrier with increased distances between charges does not necessarily correlate with the separation yield. Our presented framework can further shed light on the transient thermodynamic free energy, allowing previously inaccessible insight into the individual thermodynamic contributions from energy and entropy on sub-ns timescales. Transient simulations reveal large Gibbs entropy on ps-timescales for even highest disorder, which may explain the efficient separation of "hot" CT states.