DFT and TD-DFT study of Optical and Electronic Properties of new donor-acceptor-donor (D-A-D') monomers for polymer solar cells

The Density Functional Theory (DFT) and time-dependent-DFT method with Becke's three-parameter Lee-Yang-Parr functional approach at a basis set of 6-311G was used to analyze the ground state and excited state properties of newly designed donor-acceptor-donor (D-A-D') donor molecules based on triphenylamine and carbazole as donor units and benzothiadiazole and its derivatives as acceptor units to make a total of nine potential monomers. The energies associated with highest occupied molecular orbital, lowest occupied molecular orbital, energy gap (Eg), electron excitation (Eopt), exciton binding (Eb) and open-circuit voltage (Voc) were calculated, and the simulated absorption spectra in both gas and chlorobenzene solvent were plotted. The outcomes of replacing the acceptor building unit and substituting the donor units to tailor the optoelectronic properties of the designed monomers were discussed. The monomer molecules A7, A8 and A9 are suitable for [6,6]-phenyl-C61-butyric acid methyl ester because of their small Eg, Eopt, Eb and, more importantly, large Voc values. Suggesting changing the acceptor unit and substituting the donor units of the D-A-D' seem to be an excellent approach to tailoring the optoelectronic properties of the molecules.