End-capped group manipulation of non-fullerene acceptors for efficient organic photovoltaic solar cells: a DFT study

Abstract

A series of acceptors, S1–S5, has been designed based on the acceptor–π–donor–π–acceptor (A–π–D–π–A) architecture by incorporating a phenothiazine unit as the central donor unit. Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods have been employed to study the effect of various end-capped groups on the geometric, electronic, optical and charge transport properties of the designed acceptor molecules. The results reveal that on increasing the electron-withdrawing nature of the end-capped groups, the performance of the acceptor molecules increases. It is also observed that on increasing the flexibility of the end-capped groups, the planarity of the molecules gets destroyed and, as a result, the performance of the acceptor molecules decreases. The investigated molecules exhibit high electron affinity (EA) and low reorganization energy for electrons (λ₋), indicating the electron acceptor nature of the designed molecules. The absorption properties of the molecules manifest that compounds S2–S4 possess high values of the maximum wavelength (λ_max) of absorption. We have also studied the properties of a D/A active layer by considering PffBT4T-2OD as the electron donor and arranging PffBT4T-2OD/S1–S5 molecules in a face to face manner. Properties of the D/A blend indicate that molecules S2–S4 have capacity to promote charge carrier separation at the D/A active layer. Our results provide guidelines for further designing of acceptors to enhance the performance of organic solar cells (OSCs).