End group modulation of A–D–A type small donor molecules for DTP based organic photovoltaic solar cells: a DFT approach

Here, five new acceptor–donor–acceptor (A–D–A) type small donor molecules C1–C5, have been designed based on the central D unit, dithieno[3,2-b:2′,3′-d]pyrrole (DTP). Besides, five different A units, viz. 1,1-dicyano-methylene-5,6-dimethyl-3-indanone, 1,1-dicyano-methylene-5,6-difluoro-3-indanone, 1,1-dicyano-methylene-5,6-dichloro-3-indanone, 1,1-dicyano-methylene-5-nitro-3-indanone, and 1,1-dicyano-methylene-5,6-diamino-3-indanone are selected for these designed compounds C1–C5, respectively. Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods have been employed to study the influence of different A units on the geometric, electronic, optical, charge transport and photovoltaic properties of the designed donor molecules. The results reveal that the performance of the designed donor molecules have been improved on attachment of the strong electron withdrawing A units. The observed reorganization energy (λ) values infer the electron donating nature of the designed compounds. Moreover, the absorption properties of the designed compounds manifest that compound C4 possesses the high values of maximum wavelength (λ(max)) in both gas and solvent phases. The properties of the D/A blends reveal that all designed blends C1–C5/C(60)–CN, have the capacity to promote charge carrier separation at the D/A interface. Further, the photovoltaic performance of the D/A complexes also reveal that complex C4/C(60)–CN, with a theoretical PCE of 18%, can be considered as the most promising candidate for application in OSCs.