Efficiencies of Electron Injection from Excited Sensitizer Dyes to Nanocrystalline ZnO Films as Studied by Near-IR Optical Absorption of Injected Electrons

Near-IR absorption spectra of electrons injected from excited sensitizer dyes to nanocrystalline ZnO films have been studied. The absorption steeply increases with increasing wavelength and can be assigned to the optical transition of conducting electrons to higher energy levels of the conduction band. From the measurements in various sensitizer dyes, the relative efficiencies of the electron injection can be obtained as a function of the free energy change (ΔG) for the electron injection process. Around ΔG = 0 eV, the efficiencies increase gradually with increasing −ΔG and they are constant at negative ΔG. We succeeded in explaining the tendency by considering the site heterogeneity of the surface. The relation between the injection efficiencies and the solar cell performance will also be discussed.