Role of extended end-capped acceptors in non-fullerene based compounds towards photovoltaic properties

Abstract

Fullerene free organic solar cells (OSCs) have found at the forefront in the photovoltaic sector due to effective optical and electronic characteristics. The quantum chemical investigation focused on the designing of pentacyclic aromatic bislactam based chromophores for highly efficient OSCs. A series of eight molecules (PCLMD1–PCLMD8) was designed from reference compound (PCLMR), with A2–π–A1–π–D–π–A1–π–A2 configuration via end group redistribution with benzothiophene based acceptors. Through benchmark study of UV–Vis between simulated and experimental values of PCLMR, MPW1PW916-31G(d,p) functional was selected of DFT approach. Various analyses such as absorption properties (UV–Vis), frontier molecular orbitals analysis (FMOs), transition density matrix (TDM), open circuit voltage (Voc) and density of state (DOS) were performed in order to explore the photovoltaic properties of above-mentioned chromophores. Modification of peripheral acceptors brought about significant changes in their charge-transfer properties. These alterations resulted in reduced exciton binding energy (Eb) ranging from 2.277 to 2.087 eV, a narrower bandgap (Eg) ranging from 1.746 to 1.868 eV and an increased maximum absorption wavelength (λmax) ranging from 829 to 882 nm in the solvent phase. These values were observed in comparison to PCLMR, which had an Eb of 2.443 eV, an Eg of 1.895 eV and λmax at 813 nm. The FMO analysis indicated a significant charge distribution across HOMO-LUMO in the designed chromophores. Furthermore, the rates of electron and hole mobility as well as the open-circuit voltage (Voc) were higher for all the benzothiophene acceptor-based molecules (PCLMD1–PCLMD8) than that of PCLMR. The analysis showed that the integration of benzothiophene acceptors with electron-withdrawing groups amplifies charge transfer towards acceptor components, leading to heightened JSC and Voc values in organic solar cells (OSCs). This phenomenon is attributed to the decrease in the energy level of the lowest unoccupied molecular orbital (LUMO), while the highest occupied molecular orbital (HOMO) remains mostly unchanged, along with the effective broadening of the absorption spectrum. Hence, it can be concluded that by employing molecular engineering with benzothiophene acceptor moieties, the efficiency of photovoltaic materials based on NF compounds could be improved.