Interrogating the ultrafast dynamics of an efficient dye for sunlight conversion

Abstract

We report on studies of the recently synthesized compound (TPC1) with a promising potential use in dye-sensitized solar cells. We used steady-state as well as femtosecond (fs) to nanosecond (ns) time-resolved emission techniques to understand its behaviour under different conditions of solvation and light excitation. In polar solvents the equilibrium between TPC1 normal and anion structures was found to depend on solvent H-bond acceptor ability and concentration of the dye. We observed a correlation between the contribution of the normal form in the total absorption spectrum and solar energy conversion efficiency of the photovoltaic devices prepared in different baths, which are high in dichloromethane and low in tetrahydrofurane. Both forms exhibit a large charge transfer character in the excited state manifested by a large Stokes shift between absorption and emission maxima (up to 9000 cm⁻¹ in acetonitrile). The lifetime of the relaxed state of the normal structure varies significantly with the solvent polarity (from 80 ps in acetonitrile to 1.8 ns in n-hexane), and it is considerably shorter than that of the anion one (1.2–2.6 ns). The ultrafast relaxation processes are dominated by the solvation dynamics which is the fastest in acetonitrile (below 1 ps) and the slowest in ethanol (about 25 ps, the amplitude-averaged time). The results reported here should be relevant to a better understanding of the photobehaviour of metal-free dyes for solar cells and help in the design of new and more efficient dyes for conversion of light to electricity.