Abstract
Among the parameters determining the efficiency of an organic solar cell, the open-circuit voltage (VOC) is the one with most room for improvement. Existing models for the description of VOC assume that photogenerated charge carriers are thermalized. Here, we demonstrate that quasi-equilibrium concepts cannot fully describe VOC of disordered organic devices. For two representative donor:acceptor blends, it is shown that VOC 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 VOC.
Highlights
Among the parameters determining the efficiency of an organic solar cell, the open-circuit voltage (VOC) is the one with most room for improvement
Organic photovoltaics (OPVs) achieve quantum yields[1] and fill factors (FFs)[2,3] that are competitive with established technologies such as crystalline Si and GaAs
We will first focus on TQ1:PC71BM blends, for which the importance of nonequilibrium effects is well documented.[19,20,22]
Summary
Among the parameters determining the efficiency of an organic solar cell, the open-circuit voltage (VOC) is the one with most room for improvement. The experimental VOC is 0.1−0.2 V higher than predicted by quasi-equilibrium device simulations and by eq 1 with input parameters from the reciprocity analysis.[8,23] Instead, using an experimentally calibrated kinetic Monte Carlo (KMC) model[24] gives a good description of the device VOC as well as its dependence on thickness and temperature.
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