Role of Electronic Relaxation in the Injection Process of Organic Push-Pull Dyes in Complete Dye-Sensitized Solar Cells.

There is a growing consensus that the charge separation taking place in dye-sensitized solar cells is a multiscale process occurring on a times scale from a few to hundreds of picoseconds. We studied the excited-state dynamics of the robust and efficient push-pull dye RK1 in solution, on mesoporous films and in complete photovoltaics cells by femtosecond fluorescence upconversion and transient absorption. In a polar environment and cells, the dynamics at early times are dominated by an intramolecular electronic relaxation, while electron injection is predominant on thin films only. In cells, the electron injection process becomes visible at a later stage, from tens to hundreds of picoseconds. Our study shows that it is crucial to record and analyze full time-resolved fluorescence spectra in order to obtain wavelength-independent dynamics and get a correct description of the nature and the population of the excited state.