Precise positioning of acceptor benzothiadiazole in D-(A)3 push-pull system to tailor the optoelectronic and photovoltaic performance: A DFT and TD-DFT study

Abstract

Push-pull π-conjugated small molecules have found extensive application in organic solar cells. The development of such molecular systems has been considered in a wide variety of pathways by altering the donor/acceptor units and π-linkers. In this work, three series of D-(A)₃ benzothiadiazole (BT) coupled push-pull systems; Series 1 (BT1-BT3), Series 2 (BT4-BT6), and Series 3 (BT7-BT9) were designed by precise positioning of the acceptor benzothiadiazole unit around the triphenylamine core, and systematically explored using density functional theory (DFT) to establish structure–property relationships. The impact of benzothiadiazole acceptor positioning across various permutations on the three phenyl arms of the triphenylamine was investigated in terms of structural and optoelectronic properties, as well as intramolecular charge-transfer behavior. In all three series, the acceptor benzothiadiazoles at the single arm of the triphenyamine (TPA) show remarkably reduced HOMO-LUMO energy gap, improved λ_max values, and high oscillator strength (f). Hole-electron localization and electronic excitation processes in the excited state were examined using Transition density matrix maps. The efficiency of these materials was determined by evaluating the photovoltaic parameters (V_OC, J_SC, and PCE). The photovoltaic parameters for devices with PC₇₁BM as acceptor in the device structure show significantly improved values for molecular systems in Series 3 compared to Series 1 and Series 2. It is observed that positioning multiple BT/CNBT acceptors on the triphenylamine arms, supplemented with π-spacer and CN-acceptor groups, is undoubtedly a potential strategy to enhance the performance of donor materials for OSC applications. The Series 3 system exhibits a strong push-pull mechanism, leading to a small optical band gap (1.24 eV), strong intramolecular charge transfer (ICT), and low reorganization energies. These properties contribute to an enhanced short-circuit current density (J_SC) ∼ 21.97 mA/cm², an open-circuit voltage (V_OC) ∼ 1.04 V, minimal energy loss (0.53 eV), and theoretical power conversion efficiency (PCE) up to ~15.66%.