Microstructural and Electronic Origins of Open‐Circuit Voltage Tuning in Organic Solar Cells Based on Ternary Blends

Organic ternary heterojunction photovoltaic blends are sometimes observed to undergo a gradual evolution in open‐circuit voltage (V_oc) with increasing amounts of a second donor or an acceptor. The V_oc is strongly correlated with the energy of the charge transfer state in the blend, but this value depends on both local and mesoscopic orders. In this work, the behavior of V_oc in the presence of a wide range of interfacial electronic states is investigated. The key charge transfer state interfaces responsible for V_oc in several model systems with varying morphology are identified. Systems consisting of one donor with two fullerene molecules and of one acceptor with a donor polymer of varying regio‐regularity are used. The effects from the changing energetic disorder in the material and from the variation due to a law of simple mixtures are quantified. It has been found that populating the higher‐energy charge transfer states is not responsible for the observed change in V_oc upon the addition of a third component. Aggregating polymers and miscible fullerenes are compared, and it has been concluded that in both cases charge delocalization, aggregation, and local polarization effects shift the lowest‐energy charge transfer state distribution.