Thermal annealing effect on non-fused ring acceptor based bulk heterojunction investigated by transient absorption spectroscopy

Abstract

Organic solar cells based on non-fused ring fullerene acceptors have more potential to realize low-cost fabrication and commercialization. However, the power conversion efficiencies (PCEs) are much lower than those of the fused-ring non-fullerene acceptors. Thermal annealing treatment as a performance optimization way is performed on PBDB-T blend film based on fully non-fused ring acceptor, o-4TBC-2F, and the PCE is enhanced from 7.43% to 10.26%. Herein, transient absorption spectroscopy measurement is conducted to unveil the thermal annealing effect in the perspective of ultrafast dynamics. Two types of J-aggregations are present in the o-4TBC-2F film. Thermal annealing treatment induces more o-4TBC-2F molecules in the form of strong intermolecular coupling J-aggregation with long-range order. The aggregation is less heterogeneous, leading to a slow yet efficient hole transfer process, and the generated hole polarons are delocalized and long-lived. The dynamics results well explained the improved performance after thermal annealing treatment. The observations also show that the aggregation in blends can optimize the charge generation, which can be beneficial for designing highly efficient non-fused ring acceptor-based organic solar cells.