The efficiency of electron injection (inj) in dye-sensitized nanocrystalline films has been studied by means of transient absorption spectroscopy. We observed inj of nearly unity for N3 dye adsorbed on nanocrystalline TiO2 films (N3/TiO2). We examined the effects of various experimental conditions, such as light intensity, excitation wavelength, and presence of additives (4-tert- butylpyridine, tBP and Li ions), on inj. We also used various semiconductors and sensitizer dyes to study the effect of free energy change (G) on inj. These results give us new insights for developing high-performance solar cell devices.