Effects of adding cyanovinyl moiety on the photovoltaic DSSCs phosphonic acid based cells

Donor-acceptor-pi-acceptor (D-A-pi-A) dyes represent a promising alternative to their ancestor pull-push D-pi-A structures for use in dye-sensitized solar cell (DSSC) applications. D-A-pi-A dyes exhibit a red-shift and notable photostability in comparison with D-pi-A dyes. Nevertheless, the effects of the position of the cyano moiety in the terminal acceptor of monofunctionalized phosphonic acid on the charge transfer (CT) in DSSCs remain unclear. Density functional theory (DFT) and time-dependent DFT (TDDFT) are applied herein to study the effects of the insertion of an electron-withdrawing cyanovinyl moiety and the position of the cyanide moiety on the geometrical, photophysical, CT, and nonlinear optical properties and reorganization energy of the designed dyes. The calculations reveal that a position of the CN moiety in the cyanovinyl phosphonic acid close to the anchor (CCCN2) results in the narrowest energy gap, and highest extinction factor and red-shift compared with the other dyes as well as the highest polarizability and first hyperpolarizability.