Organic photovoltaics is a promising emerging technology for solar energy conversion due to low production cost, large area roll-to-roll production possibilities, flexibility, lightweight, and semitransparency.[1-2] Most organic solar cells contain non-fullerene acceptors with large fused-ring systems. However, the production of these acceptors is due to their synthetic complexity costly and often involves toxic materials and solvents. Non-fused electron acceptors are an alternative, since they require fewer synthesis steps, are cheaper to produce, and still have the required planarity of the molecule.

In our work, we have successfully synthesized derivatives of the COTIC-4F acceptor comprising a cyclopentadithiophene core, thiophene linkers and fluorinated IC end groups. The molecules B1 and B2 differ from the COTIC-4F molecule in their side chains (hexyl instead of 2-ethylhexyloxy) attached to the thiophene linker. In B2, we additionally changed the position of the hexyl chain at the thiophene ring from 3 to 4. By this modification, we expected an increased photostability due to structural confinement mitigating photoisomerization and photooxidation of the molecule.[3] The NFAs B1 and B2 reveal higher optical band gaps (1.31 eV) than COTIC-4F (1.1 eV).[4] In bulk heterojunction organic solar cells in conventional architecture, power conversion efficiencies of 9.59% were obtained for solar cells with the PTB7-Th:B1 absorbers and 9.94% for PTB7-Th:B2 based absorber layers. The higher performance of the B2 based solar cells is most presumably due to a more balanced hole and electron mobility and higher exciton dissociation probabilities.

In addition, we investigated the stability of these materials and solar cells in detail. For instance, we studied the change of absorption properties of B1 and B2 films under continuous solar simulator illumination in ambient conditions and obtained a significantly decreased fading in absorption intensity of the B2 film. Moreover, PTB7-Th:B1 and PTB7-Th:B2 based solar cells were tested under various conditions (shelf life at room temperature and 65 °C, continuous illumination) and found that the PTB7-Th:B2 solar cells show a significantly higher photostability (t₈₀: ~600 h) as well as higher thermal stability (t₈₀: ~600 h) compared to the B1 based solar cells, which reveal t₈₀-times of only ~125 h under both testing conditions. This work reveals that the investigated modification in the NFA design does not only lead to improved photostability but also to increased stability at elevated temperatures, which we will investigate in more detail also for other non-fused NFA structures in the near future.