Effects of Internal Electron-Withdrawing Moieties in D-A-π-A Organic Sensitizers on Photophysical Properties for DSSCs: A Computational Study

D-A-pi-A dyes differ from the traditional D-pi-A framework having several merits in dye-sensitized solar cell (DSSC) applications. With regard to D-pi-A dyes, D-A-pi-A dyes red-shift absorption spectra and show particular photostability. Nevertheless, the effects of internal acceptor on the charge transfer (CT) probability are unclear. We employed density functional theory (DFT), time-dependent DFT (TD-DFT), and TD-DFT molecular dynamics (MD) simulations to investigate the effects of internal acceptor on the photophysical properties of D-A-pi-A dyes on DSSCs. Our calculations show the absorption bands of D-A-pi-A dyes with strong electron-withdrawing internal acceptors exhibiting significant characteristics of dual CT; the excited electron density is transferred to the internal and terminal acceptors simultaneously. Particularly, the internal acceptor traps a significant amount of electron density upon photoexcitation. The TD-DFT MD simulations at 300 K show that only a small amount of excited electron density is pushing and pulling between the internal acceptor and terminal acceptor moieties; the thermal energy is not high enough to drive the electron density from the internal acceptor to the terminal acceptor. Our study reveals the nature of CT bands of D-A-pi-A dyes providing a theoretical basis for further rational engineering.