Sensitization of p-GaP with Monocationic Dyes: The Effect of Dye Excited-State Lifetime on Hole Injection Efficiencies

The sensitized hole injection to the p-type gallium phosphide (p-GaP) electrode was evaluated for several triarylmethane (6O + ), acridine (4O +, 2O +, Me 2 N-Acr +, T-Acr + ), and flavin dyes (Et-Fl + ). Thermodynamics for sensitized charge injection were evaluated using steady-state ultraviolet–visible absorption and cyclic voltammetry experiments. All dyes (except 4O + ) had strong absorption at wavelengths above 550 nm (absorption cutoff for GaP), and their highest occupied molecular orbital energies were below the valence band of GaP (+1.20 V vs normal hydrogen electrode), indicating that the electron transfer from p-GaP electrode to the excited dye molecules was thermodynamically favorable. Photoelectrochemical measurement conducted on p-GaP electrodes immersed in aqueous electrolytes and dye showed sensitization for only two dyes (2O + and Et-Fl + ), and the sensitization efficiencies were found to depend on the chemical nature of differently prepared p-GaP electrodes. Femtosecond pump–probe measurements revealed that the “inefficient” dyes had short-lived excited states (few picoseconds), preventing the successful charge transfer into p-GaP surface. Collectively, this work provides insight on time scales of hole-injection rates during dye-sensitization processes.