Structural engineering of small molecule-based narrow bandgap dithieno-3,2-b:2′,3′-dlpyrrole-based non-fullerene electron acceptors for efficient organic solar cells

Abstract

Non-fullerene small molecular acceptors (NF-SMAs) attract huge attention in preparing efficient organic solar cells (OSCs). Herein, we engineered eight new NF-SMAs (ABG1-ABG8) with a narrower band gap and improved intramolecular charge transfer (ICT). A systematic study has examined the structure-property relationship, optical, optoelectronics, and photovoltaic characteristics of these NF-SMAs using density functional theory (DFT) and time-dependent (TD-DFT). The photophysical parameters of the designed (ABG1-ABG8) materials were examined and compared with the standard molecule (ABGR). Further, we estimated frontier molecular orbitals, electrostatic potential, density of states, transition density matrix, and reorganization energy analysis. Compared to the synthetic reference molecule ABGR (2.03 eV), the designed acceptor series presented a smaller bandgap (E_g = 1.85–1.98 eV) and displayed a red shift in absorption maxima (λ_max 761.99–864.00 nm), respectively. The minimum value of the E_g for ABG3 and the maximum λ_max was for ABG6. Furthermore, the ABG1-ABG8 designed series shows a lower value of excitation energy (E_x) 1.43eV for ABG6 compared to the reference (E_x 1.72 eV). Additionally, these designed materials presented more efficient photovoltaic characteristics than reference ABGR molecule. The value of open-circuit voltage (V_oc) is maximum for ABG8 (1.55eV) while for all proposed molecules the value of (V_oc) is in the range of 1.19–1.55 V. Improved V_oc and fill factor values further enhance the overall power conversion efficiency of designed molecules compared to the ABGR molecule. The maximum value of PCE was for ABG8 among all newly designed molecules. To determine the charge-accepting ability of the designed molecules, we performed the donor:acceptor complex study using polymer donor PTB7-Th with the designed ABG6 molecule. Our findings demonstrate that newly developed non-fullerene-based materials (ABG1-ABG8) exhibit improved photovoltaic and optoelectronic features. Therefore, we recommend these engineered materials for next-generation efficient and cost-effective organic photovoltaics.