Studies of New 2,7-Carbazole (CB) Based Donor-Acceptor-Donor (D-A-D) Monomers as Possible Electron Donors in Polymer Solar Cells by DFT and TD-DFT Methods.

The new donor-acceptor-donor (D-A-D) monomers have been studied using density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods to evaluate the optoelectronic and electronic properties for bulk heterojunction (BHJ) organic solar cells. The TD-DFT method is combined with a hybrid exchange-correlation functional using the B3LYP method in conjunction with a polarizable continuum model (PCM) and a 6-311G basis set to predict the excitation energies and absorption spectra of all monomers. The predicted bandgap (E ) of the monomers decreasing in the following order D1<D2<D3<D4<D5<D6<D7<D9<D8. Furthermore, open-circuit voltage (V ) estimates for monomers with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) acceptor. The V of the studied monomers ranges from 0.976 to 1.398 eV in the gas and from 1.109 to 1.470 eV in the solvent phase with PC71BM acceptor, which is sufficient for efficient electron injection into the acceptor's LUMO. The results show that theoretically, a maximum energy conversion efficiency of roughly 5 % for D8 and 5.8 5 % for D7.