Adv. Energy Mater. http://doi.org/f266gm (2015)
The realization of organic solar cells with high efficiency also hinges on an improved understanding of the processes that link charge transfer states, which correspond to electrons and holes photogenerated in an organic heterojunction and interacting at the donor/acceptor interface, with free charge carrier states. A model developed by Timothy Burke and colleagues now shows that the population of charges in the charge transfer states is in equilibrium with free carriers. This condition allows the researchers to formulate an equation that correlates the open-circuit voltage of the photovoltaic device with several parameters, such as the average energy, lifetime and number of the charge transfer states as well as the interfacial disorder and the extension of the mixed region between the donor and the acceptor, that can be extracted by direct characterization of the materials and the device. Thus, the model helps explain the relatively low open-circuit voltage experimentally observed in organic solar cells and suggests guidelines to improve it.