Hot carriers boost open-circuit voltage of organic solar cells

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 photo-generated charge carriers are thermalised. 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 modelling 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 'hot' carrier devices, since part of the photon excess energy is harvested in the form of an enhanced $V_\text{OC}$.