  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 $V_\text{OC}$.
