Development of fullerene free acceptors molecules for organic solar cells: A step way forward toward efficient organic solar cells

Continuous strides are being made to explore non-fullerene acceptors for organic solar cells. Here, optoelectronic properties of four new acceptor-donor-acceptor (A-D-A) type non-fullerene acceptor molecules are evaluated for their potential use in organic solar cells. The designed molecules contain indacenodithiophene (IDT) donor core connected with various acceptor groups through benzothiadiazole (BT) bridge unit. The designed molecules differ from each other in end-capped acceptor groups. The end-capped acceptors are 2-methylene-3-oxo-2,3-dihydroinden-1-ylidene) malononitrile (M1), 2,5,6-difluoro-2-methylene-3-oxo-2,3-dihydroinden-1-ylidene) malononitrile (M2), 2-methylene-2-H-indene-1,3-dione (M3), and 2,5-methylene-6-oxo-5,6-dihydrocyclopenta-c-thiphen-4-ylidene-malononitrile (M4). The optoelectronic properties are evaluated in comparison with the recently reported R which is structurally similar to the designed molecules. The reference compound R contains indacenodithiophene (IDT) donor core unit with 3-ethyl-d-ethylidene-2-thioxothiazolidin-4-one end capped acceptor group. The A-D-A acceptor molecules evaluated here exhibit proper frontier molecular orbital diagram to facilitate the charge mobility. M2, containing 2-(5,6-difluoro-2-methylene-3-oxo-2,3-dihydroinden-1-ylidene) malononitrile end-capped acceptor group, exhibits maximum absorption at 805 nm which is attributed to extended conjugation with the BT unit. Results of transition density matrix (TDM) show clear effect of end caped acceptor group on electro-hole exciton. Small coupling of electron and hole in M4 illustrates easy dissociation of exciton as compared with other molecules. Among all, M4 shows the highest hole transfer rate as revealed from low hole reorganization energy (lambda(h)). The open circuit voltage (V-oc) of reference R is 1.96 V with donor polymer PTB7-Th while the designed molecule M3 exhibits the V-oc of 2.02 V. Hence, the designed fullerene free acceptor molecules are suitable for transparent organic solar cells.