Models and mechanisms of ternary organic solar cells

Ternary organic solar cells adhere to a simple device fabrication strategy and are among the highest performing organic solar cells to date. This Review examines the multiple models of operation that have emerged for ternary cells, highlighting new insights and still-existing gaps in knowledge. In ternary organic solar cells (TOSCs), three different components are mixed to form the photoactive layer, opening up opportunities to boost the power conversion efficiency - for example, by broadening the absorption range, improving the blend morphology or tuning the exciton splitting and charge extraction. Because of these possibilities, ternary systems are among the best performing OSCs and will have a crucial role in the future of organic photovoltaics. Owing to the interplay of three different components, the mechanisms in TOSCs are complex. Multiple models for those mechanisms currently exist, which differ mainly in the description of the composition dependence of the open-circuit voltage. However, these models are not defined precisely, they are based on narrow presuppositions and they frequently contradict each other. Moreover, although the state of knowledge has evolved since the development of models, new TOSCs are still assigned to them. This Review describes the existing models and concepts, highlights their inconsistencies and summarizes newer results on electronic and morphological properties of TOSCs. Subsequently, the conventional models are revisited in the light of these new insights, with the aim of pointing out existing gaps and providing the stimulus for challenging old concepts.