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

Abstract

© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. Organic ternary heterojunction photovoltaic blends are sometimes observed to undergo a gradual evolution in open-circuit voltage (Voc) with increasing amounts of a second donor or an acceptor. The Voc 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 Voc in the presence of a wide range of interfacial electronic states is investigated. The key charge transfer state interfaces responsible for Voc 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 Voc 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. The open-circuit voltage evolution and charge transfer state interfaces in ternary organic photovoltaic blends are investigated using several model systems. The changes in subgap spectra from energetic disorder and increased population of higher energy states are analyzed and the lowest charge transfer state distribution is observed to shift due to local aggregation and delocalization effects.