Observation of Multiexponential Pico- to Subnanosecond Electron Injection in Optimized Dye-Sensitized Solar Cells with Visible-Pump Mid-Infrared-Probe Transient Absorption Spectroscopy

We compare visible-pump mid-infrared-probe femtosecond measurement of electron injection on dyesensitized mesoporous TiO2 films immersed in neat solvent or in I-/I-3(-) electrolyte. Results are presented for the Ru-polypyridyl dyes N3 and N719 and the organic dye D149. Using N719 and the type of electrolyte required to make efficient and stable dye sensitized solar cells (DSSCs), we find multiple exponential injection dynamics, ranging from <300 fs to 0.5 ns for the ruthenium metal-centered dyes. The organic dye D149 shows double exponential <300 fs to 30 ps injection kinetics. We show that this result is not dependent on our pulse intensity, average intensity, or dye aggregates deposited during dyeing. We find the N3 dye immersed in neat solvent shows dominantly ultrafast, <300 fs, injection kinetics, as previously reported. Ultrafast kinetics were also observed for all samples when pulse intensity was increased above a given range. We discuss the implication of the results for rational design of future NIR absorbing dyes for DSSCs.