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

Improving the open-circuit voltage (V_OC) 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 V_OC, 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 V_OC 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 (E_CT) for PBDB-T:im-ITIC- and PBDB-T:ITIC OSCs were determined to be 1.57 and 1.39 eV, respectively, correlating with the V_OC values. Theoretical calculations indicated that pronounced H-aggregation in im-ITIC increases the E_CT compared with J-aggregation, contributing to the improved V_OC. This study underscores the critical impact of molecular aggregation patterns on energy alignment and V_OC enhancement, offering insights into molecular design for achieving high V_OC in OSCs.