Optimized molecular aggregation and photophysical process synergistically promoted photovoltaic performance in low-regularity benzo[c][1,2,5]thiadiazole-based medium-bandgap copolymers via modulating π bridges

Abstract

π-Bridges of D–π–A conjugated copolymers (CPs) not only determine the optical absorption, bandgap, energy levels, miscibility, molecular configuration, and aggregation but also affect the dipole moment, electrostatic potential, and photophysical process. Herein, we judiciously regulated the octylthienyl π bridge on both sides of electron-deficient benzo[c][1,2,5]thiadiazole (BT) core, and three medium band gap copolymers, irregular PClBDT-TBT, regular PClBDT-BT, and PClBDT-DTBT were synthesized. DFT calculations demonstrated that the irregular models (ClBDT-TBT-1)₃ and (ClBDT-TBT-2)₃ had larger dipole moment (Δμ) and reduced exciton binding energy (E_b), but the irregular models (ClBDT-TBT-3)₃ and (ClBDT-TBT-4)₃ possessed decreased Δμ and elevated E_b. Irregular PClBDT-TBT not only effectively depressed excessive aggregation and altered molecular orientation but also regulated the molecular crystallinity to improve the charge mobility. Moreover, femtosecond transient absorption (fs-TA) spectroscopic analyses suggested that low-regularity PClBDT-TBT:Y6 film had a prolonged exciton diffusion time so as to suppress recombination and ameliorate the charge transfer. Consequently, the low-regularity PClBDT-TBT-based device yielded excellent power conversion efficiency (PCE) as high as 13.04% when paired with Y6. Moreover, regular PClBDT-BT- and PClBDT-DTBT-based devices obtained PCEs of 5.54% and 11.12%, respectively, under identical testing conditions. Our work demonstrates that regulating π bridges in D–π–A type CPs is an easy and effective strategy aimed at further boosting the device performance by tuning the molecular aggregation and photophysical processes.