A computational study on the design of donor - acceptor copolymers for organic photovoltaic materials

Abstract

Organic solar cells are one kind of renewable energy. In this study, 50 different donor-acceptor oligomers, which were used for photovoltaic materials, were investigated by using 5 different acceptors, which are cyclopentadiphenyl (CPDP), benzodithiophene isomer 1 (BDT1), benzodithiophene isomer 2 (BDT2), cyclopentadithiophene (CPTz), dithienopyridine (DTPn), and 5 different donors, which are benzo[c][1,2,5]thiadiazole (BT), benzo[c][1,2,5]oxadiazole (BX), hydrogentriazole (HTAZ), benzo[c][1,2,5]selenadiazole (BSe), quinoxaline (Qx). Density functional theory (DFT) and time-dependent DFT (TD-DFT) at the B3LYP/6-311G* level have been employed for the calculations. The number of donors increased from 5 to10 by using the heteroatom substitution on the donors. Thus, the effect of heteroatom substitution on optical and geometrical properties of molecules was investigated. Benchmark studies on the methodology were performed to determine HOMO, LUMO energies and optical band gaps of the oligomers. Transition dipole moments, HOMO, LUMO energies and band gaps were calculated as a function of the chain length so, tetramers were found as suitable materials to ensure the energy saturation. Tetramers, which were the optimal chain length, have been used to evaluate the optical and geometrical properties like reorganization energies, distortion energies, frontier molecular orbitals, bond length alternations and excited-state vertical transition energies of oligomers. In conclusion, suitable photovoltaic materials were designed from a class of oligomers by taking 4 important factors into consideration. The choice of oligomers was based on the following criteria: low reorganization energy, LUMO energy higher than -3.8 eV, HOMO energy range between -5.27 eV and -5.7 eV and the optical band gap between 1.4 eV to 1.9 eV. Overall,16 out of 50 tetramers were found as suitable materials which can be used in solar cells.