Design of donor-acceptor-donor (D-A-D) type small molecule donor materials with efficient photovoltaic parameters

Four Donor-Acceptor-Donor (D-A-D) type of donor molecules (M1-M4) with triphenylamine (TPA) as donor moiety, thiophene as bridge, and thiazolothiazole as acceptor unit were designed and its photovoltaic parameters were equated with reference molecule R. DFT functional CAM-B3LYP/6-31G (d,p) was found best for geometry optimization and TD-CAM-B3LYP/6-31G (d,p) was found suitable for excited state calculations. Among designed donor molecules, M4 manifests suitable lowest band gap of 4.73 eV, frontier molecular orbital energy levels as well as distinctive broad absorption of 455.3 nm due to the stronger electron withdrawing group. The electron-withdrawing substituents contribute to red shifts of absorption spectra and better stabilities for designed molecules. The theoretically determined reorganization energies of designed donor molecules suggested excellent charge mobility property. The lower (e) values in comparison with (h) illustrated that these four donor materials would be ideal for electron transfer and M4 would be best amongst the investigated molecules with lowest (e) of 0.0177. Furthermore, the calculated Voc of M4 is 2.04 V with respect to PC60BM (phenyl-C61-butyric acid methyl ester). This study revealed that the designed donor materials are suitable and recommended for high performance organic solar cell devices.