A meta-linked isomer of ITIC: influence of aggregation patterns on open-circuit voltage in organic solar cells

Improving the open-circuit voltage (VOC) of organic solar cells (OSCs) remains an important challenge. While it is known that the energy levels at the donor/acceptor (D/A) interface affect the VOC, the impact of aggregation patterns on the energy levels at the D/A interface has not been fully elucidated. Herein, we focus on ITIC, a widely used acceptor in OSCs, and designed a meta-linked isomer of ITIC (referred to as im-ITIC) to alter molecular symmetry and modify substitution arrangements. Concentration-dependent 1H NMR spectra revealed that im-ITIC shows stronger aggregation behavior in solution. Single-crystal X-ray analysis showed that im-ITIC forms both tail-to-tail (J-aggregation) and face-to-face (H-aggregation) stacking modes, whereas ITIC exclusively forms tail-to-tail stacking. OSCs based on PBDB-T:im-ITIC showed a high VOC value of 1.02 V, which is 0.12 V higher than that of those based on PBDB-T:ITIC. Time-resolved infrared measurements revealed the lifetime of free electrons for the pristine and blend films. The energy levels of the charge transfer state (ECT) for PBDB-T:im-ITIC- and PBDB-T:ITIC OSCs were determined to be 1.57 and 1.39 eV, respectively, correlating with the VOC values. Theoretical calculations indicated that pronounced H-aggregation in im-ITIC increases the ECT compared with J-aggregation, contributing to the improved VOC. This study underscores the critical impact of molecular aggregation patterns on energy alignment and VOC enhancement, offering insights into molecular design for achieving high VOC in OSCs.