Dual-channel D-(π-A)_2 phenoxazine/phenothiazine dyes with an auxiliary N-alkoxy benzoic acid anchor for fabrication of dye-sensitized solar cells

摘要

Research and exploitation of metal free organic photosensitizers with multi-electron transfer channels have attracted extensive attention in recent decade because of these chromophores possess obvious merits comparing to the single electron transfer channel analogues. Herein, four dual-channel D-(pi-A)(2) dyes (OT6, OT12, TT6, and TP6) with an auxiliary N-alkoxy benzoic acid anchor in side chain have been synthesized and characterized. The variation of structural segments such as donor phenoxazine/phenothiazine, n-bridge thiophene/benzene, and the different length of alkyl branch chain n-hexyl/n-dodecyl embedded in the skeleton of dyes exerts a conspicuous influence on the photophysical, electrochemical, and photoelectrochemical properties of the pristine dyes. The rich electron-donating unit of phenoxazine or thiophene employed as donor or n-bridge is beneficial to broaden the absorption spectra, increase the light-harvesting efficiency, reduce the electron injection voltage loss, and diminish the torsion dihedral angle between donor and pi-bridge. These aforesaid factors are conducive to generate a relatively superior short-circuit current for the corresponding devices. Meantime, the longer alkyl side chain of n-dodecyl amalgamated with the auxiliary benzoic acid anchor helps in dominating the adsorption of dyes and hindering the charge recombination processes at the TiO2 interface and thus to achieve the higher open circuit-voltage which derived from the longer excited electron lifetime of the cells. Consequently, a maximal power conversion efficiency of 5.74% for OT12 is attributed to the comparatively higher short-circuit current and open circuit-voltage of the cells in liquid iodide/triiodide redox couple.