Screening and design of high-performance indoline-based dyes for DSSCs

The molecular structures and photophysical properties of three D-A-pi-A dyes (WS-2, WS-92 and WS-95) were calculated based on density functional theory (DFT) and time-dependent DFT (TD-DFT). The results show that different donor (D) groups and pi-spacer bridges affect the photophysical properties. WS-95 presents the highest light harvesting efficiency (LHE), maximum absorption peak, dipole moment (mu(normal)), electron affinity (EA), electrophilicity (omega) and electron accepting power (omega(+)), as well as a small natural bond orbital (NBO) charge of the electron acceptor, ionization potential (IP) and chemical hardness (h). These critical parameters have a close relationship with the open-circuit photo-voltage (V-OC) and the short-circuit current density (J(SC)), and lead to WS-95 exhibiting higher efficiency. In order to obtain an efficient dye, we designed a series of dyes based on WS-95 and analyzed their optical and electronic properties. It was found that upon introducing a CN group into WS-95 (as in the designed molecules 1, 2, 3, 7, 8 and 9), the band gap energies of the dyes were decreased, leading to an absorption peak red-shift compared to WS-95. The structural modifications also result in an increase in the electrophilicity and electron accepting power, and a decrease in the chemical hardness. When -NH2 is introduced into WS-95 (4, 5, 6, 10, 11 and 12), the opposite trend is observed compared to dyes 1, 2, 3, 7, 8 and 9. We hope that these results will be helpful for experiments to synthesize new and highly efficient dyes.