Tailoring electronic structure and charge transport in carbazole-based small donors: Bi-functional acceptor strategy for efficient bulk heterojunction organic solar cells

Abstract

Organic solar cells (OSCs) featuring a bulk heterojunction active layer have received substantial attention in the academic and industrial communities due to their lightweight nature, high versatility, low cost, mechanical flexibility, compatibility, and transparency with solution-based fabrication. In this study, five small molecule-based donors (SMDs) with A–D–A structure, namely CTPT, CTPS, CTQTD, CTQT, and CTQDT, have been designed for OSCs. Using density functional theory (DFT) and time-dependent DFT (TD-DFT) simulations, the electronic and charge-transporting properties, absorption profile, stability, electronic excitation analyses, solubility, open-circuit voltage, and energy loss ability of engineered SMDs and a reference SMD (PCz(DPP)₂) are investigated. The results showed that the engineered SMDs have low bandgaps (1.73 to 2.25 eV), low energy losses (0.24 to 0.81 eV), high light-harvesting efficiency (0.0450 to 0.8095), high absorption (extending to the near-infrared (NIR) region), superior solubility (except CTQT SMD), and low exciton binding energy (except CTQT and CTQDT SMDs) with comparable stability than PCz(DPP)₂ SMD. Analyses of the transition density matrix, hole electron distribution, and inter-fragment charge transfer demonstrated that engineered SMDs (except CTQT SMD) indicated effective transfer of excited electrons from the donor to the acceptor portions, stronger exciton dissociation, minimal recombination losses, and high charge transfer compared to the PCz(DPP)₂ SMD. Moreover, the results of hole hopping rate (3.023 × 10¹³ to 7.172 × 10¹⁴ s⁻¹), total amount of charge transfer (2.15 to 2.71 e), hole transfer integral (0.0709 to 0.2883 eV), and hole reorganization energy (0.1443 to 0.1906 eV) indicated that the engineered SMDs exhibited high-charge transport properties for high-efficiency OSCs. Therefore, these newly tailored SMDs are expected to significantly enhance the performance of OSCs in the future.