Unlocking the effects of ancillary electron-donors on light absorption and charge recombination in phenanthrocarbazole dye-sensitized solar cells

Abstract

Consecutive explorations on polycyclic aromatic hydrocarbons (PAHs) as the central blocks of organic photosensitizers have very recently brought forth a new opportunity towards efficiency enhancement of dye-sensitized solar cells (DSCs). In this paper, to scrutinize the effects of ancillary electron-donors on light absorption and charge recombination in phenanthrocarbazole (PC) dye-sensitized solar cells, we synthesized three dyes with the respective pristine PC, fluorenyl functionalized PC, and carbazyl functionalized PC electron-donors in conjunction with the 4-(7-ethynylbenzo[c][1,2,5]thiadiazol-4-yl)benzoic acid electron-acceptor. With respect to the reference dye only using PC as the electron-donor, attaching an ancillary electron-donor, 2-hexyldecyl tethering fluorenyl or carbazyl, to the free end of PC can result in a destabilized highest occupied molecular orbital energy level and a red-shifted absorption peak as well as an enhanced maximum molar absorption coefficient. The end-capping of PC with fluorenyl or carbazyl can also slow down the kinetics of charge recombination of titania electrons with both cobalt(III) ions and photooxidized dye molecules, giving rise to an increased photovoltage and an improved power conversion efficiency. Moreover, time-resolved photophysical measurements disclose that these styles of reducing energy-gaps do not exert influence on the yields of electron and hole injections.