Designing small organic non-fullerene acceptor molecules with diflorobenzene or quinoline core and dithiophene donor...

The non-fullerene acceptors A1-A5 with diflourobenzene or quinoline core (bridge) unit, donor cyclopenta[1,2-b:3,4-b']dithiophene unit and 2-(2-methylene-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile as acceptor unit with additional phenyl, fulvene or thieno[3,2-d]pyrimidinyl 5-oxide groups have been designed through DFT calculations. The optimization of molecular geometries were performed with density functional theory (DFT) at B3LYP 6-31G (d,p) level of theory. The frontier molecular orbital (FMO) energies, band gap energies and dipole moments (ground and excited state) have been calculated to probe the photovoltaic properties. The band gap (1.42-2.01 eV) and dipole moment values (5.5-18. Debye) showed that these designed acceptors are good candidates for organic solar cells. Time-Dependent Density Functional Theory (TD-DFT) results showed λ_max (wave length at maximum absorption) value (611-837 nm), oscillator strength (f) and excitation energies (1.50-2.02 eV) in gas phase and in CHCl₃ solvent (1.48-1.89 eV) using integral equation formalism variant (IEFPCM) model. The λ_max in CHCl₃ showed marginal red shift for all designed acceptors compared with gas phase absorption. The partial density of states (PDOS) has been plotted by using multiwfn which showed that all the designed molecules have more electronic distribution at the donor moiety and lowest at the central bridge. The reorganization energies of electron (λ_e) (0.0007 eV to 0.017 eV), and the hole reorganization energy values (0.0003 eV to - 0.0403 eV) were smaller which suggested that higher charged motilities. The blends of acceptors A1-A5 with donor polymer D1 provided open circuit voltage (V_oc) and ∆HOMO off-set of the HOMO of donor and acceptors. These blends showed 1.04 to 1.5 eV values of V_oc and 0 to 0.38 eV ∆HOMO off set values of the donor-acceptor bends which indicate improved performance of the cell. Finally, the blend of D1-A4 was used for the study of distribution of HOMO and LUMO. The HOMO were found distributed on the donor polymer (D1) while the A4 acceptor was found with LUMO distribution. Based on λ_max values, and band gap energies (E_g), excitation energies (E_x), reorganization energies; the A3 and A4 will prove good acceptor molecules for the development of organic solar cells.