Slow Relaxation of Photogenerated Charge Carriers Boosts Open-Circuit Voltage of Organic Solar Cells

TL;DR

This paper reveals that organic solar cells operate as nonequilibrium devices, with slow relaxation of photogenerated charge carriers leading to a higher open-circuit voltage than predicted by equilibrium models.

Contribution

It demonstrates that incomplete relaxation in disordered organic materials causes an excess voltage, challenging existing equilibrium-based models for $V_{OC}$.

Findings

$V_{OC}$ is 0.1-0.2 V higher than equilibrium predictions.

Slow relaxation in disorder-broadened states causes excess energy.

Organic solar cells can harvest photon excess energy as increased $V_{OC}$.

Abstract

Among the parameters determining the efficiency of an organic solar cell, the open-circuit voltage ($V_\text{OC}$) is the one with most room for improvement. Existing models for the description of $V_\text{OC}$ assume that photogenerated charge carriers are thermalized. Here, we demonstrate that quasi-equilibrium concepts cannot fully describe $V_\text{OC}$ of disordered organic devices. For two representative donor:acceptor blends it is shown that $V_\text{OC}$ is actually 0.1-0.2 V higher than it would be if the system was in thermodynamic equilibrium. Extensive numerical modeling reveals that the excess energy is mainly due to incomplete relaxation in the disorder-broadened density of states. These findings indicate that organic solar cells work as nonequilibrium devices, in which part of the photon excess energy is harvested in the form of an enhanced $V_\text{OC}$.