Triphenylamine-Based Ionic Dyes with Simple Structures: Broad Photoresponse and Limitations on Open-Circuit Voltage in Dye-Sensitized Solar Cells

Metal-free ionic dyes possess attractive near-infrared light-capturing abilities but show generally low open-circuit voltage ( V OC ) in dye-sensitized solar cells (DSCs). As part of our systematic development of triphenylamine-based organic sensitizers, two ionic dyes (TI dyes) featuring a 1,1-diphenylvinyl auxiliary electron donor and N -alkyl indolium carboxylic acid acceptors have been engineered. With a very simple π-conjugated bridge (a C═C double bond), one of them has achieved broad photoresponse ranging from 400 to 800 nm. Characteristic of ionic dyes, TI dyes show much lower V OC values than those of nonionic triphenylamine dyes, suffering from serious interfacial charge recombination and downward shift of the TiO 2 conduction band. Through photovoltaic measurements, electrochemical impedance investigation, recombination reaction monitoring, electrostatic potential analysis, and dipole moment calculation, we conclude that the electronic distribution over ionic dye molecules is intrinsically not ideal for the construction of efficient sensitizers, although the introduction of a squaraine unit in the dye backbone can, to some extent, alleviate such a situation.