Photovoltaics, i.e. the direct conversion of sunlight into electricity, is a key technology for meeting the European Union's climate targets. One particularly promising and emerging approach are organic photovoltaics (OPVs). In contrast to conventional inorganic devices, OPVs are based on abundant materials such as polymers and small molecules that can be processed from solution. This makes OPVs not only attractive for cost-effective mass production, but also opens up new fields of application such as flexible and transparent solar cells. However, there is still considerable need for improvement in terms of efficiency and long-term stability, accompanied by a lack of fundamental understanding in key areas. In particular, it is still not well understood how the nanoscale morphology of the active layer, typically a bulk-heterojunction (BHJ) blend of an electron donor and an electron acceptor material, affects elementary processes such as charge transport and recombination.
ReMorphOPV aimed to contribute to the fundamental understanding by developing a realistic numerical device model. The model makes accurate assumptions about the complex BHJ morphology, while fully taking into account the hopping nature of charge transport and non-equilibrium effects. It has been calibrated and extensively tested using experimental data from a range of recent material systems. In particular, it was shown that the model can predict complete current-voltage characteristics of OPVs, and thereby predict all relevant performance parameters such as the power conversion efficiency. With this powerful tool in hand, general design rules for better performing OPVs were then established.
One main result of ReMorphOPV is that the presence of aggregates of high crystalline quality within the donor or acceptor phase are a key to reduce losses due to charge recombination. This demonstrates that a theoretical model of recombination in OPVs must make much more complex assumptions about morphology than the two-phase descriptions commonly used to date. In addition, elementary insights into the role of non-equilibrium effects on device performance were gained. It was shown that the open-circuit voltage of OPVs is in fact higher than would be expected according to prevailing equilibrium concepts. The reason for this is the slow relaxation of charge carriers in the disorder-broadened density of states. Harvesting these non-thermalized carriers opens up entirely new routes for device optimization of OPVs.
