Kinetics and mechanism of electron injection and charge recombination in dye-sensitized nanocrystalline semiconductors

Abstract

Our recent experimental and theoretical work on the kinetics and mechanism of electron injection and charge recombination in dye-sensitized nanocrystalline semiconductors is reviewed. In our experimental studies of electron injection, nanocrystalline ZnO films were chosen as the semiconductor. In order to reveal the kinetics and mechanism of electron injection we have developed several types of transient absorption spectrometers which enable us to observe the time profiles of the absorption spectra of the oxidized form of dyes and conducting electrons with high sensitivity over a wavelength range from near IR to visible and over a time range from femtoseconds to submicroseconds. For N3 dye/ZnO system, the aggregation of N3 dyes and its effect on electron injection have been clarified spectroscopically. The electron injection process has been measured by a femtosecond pump-probe method and it has been found that a fraction of electron injection occurs via an intermediate state. The absolute efficiency of electron injection has been measured and a new theoretical model has been developed for electron injection to explain the dependence of the efficiency of electron injection on the free energy change for injection. Concerning charge recombination a consistent theoretical model has been developed which explains not only the observed highly dispersive kinetics of charge recombination but also the effects of the light intensity, the applied bias and the dye structure on the kinetics.