Structure-property relationship of different electron donors: new organic sensitizers based on bithiazole moiety for high efficiency dye-sensitized solar cells

Three metal-free bithiazole organic dyes (BTT-I–III) based on D–A-π-A building blocks were designed and synthesized for dye-sensitized solar cells (DSSCs) to study the influence of different electron donors on photovoltaic properties, in which the electron donors of BTT-I–III were carbazole, triphenylamine and indoline moieties, respectively. The UV/Vis absorption spectra of BTT-III containing indoline as electron-donor displayed red-shifted absorption compared to the other two dyes with an onset close to 700 nm. The incident photon-to-current conversion efficiency (IPCE) spectra of BTT-III showed a wide region and kept a value higher than 10–15% during 580–650 nm. Electrochemical measurement data indicated that the HOMO and LUMO energy levels could be tuned through introducing different electron-donors in the dye molecule. It was found that the overall conversion efficiency of indoline donor based dye BTT-III showed the highest efficiency of 7.86% under AM 1.5 irradiation (100 mW cm−2). The electron lifetime calculated from electrochemical impedance spectroscopy (EIS) measurement demonstrated the reduced charge recombination and the higher open-circuit voltage.