An Effect of Electrode Property-Control on Electrochemical Spectral Sensitization An Optimization of Donor Density of Semiconductor Substrates

Abstract

Electrochemical spectral sensitization has been attracting interest in connection with light energy conversion as well as the photosynthetic reaction in vivo. Recently we have found that the quantum efficiency of the photocurrent (q) for spectral sensitization depends largely upon the donor density (N_d) of the semiconductor substrate in SnO₂-xanthene dye systems. This phenomenon may provide us a key to understand the dynamics of spectralsensitization and to construct effective electrode systems for it. In this paper we tried to analyze the N_d dependence of q and the supersensitizing action of hydroquinone (HQ) (Fig.7) quantitat ively. A model for the reaction schemen was introduced as shown in Fig.9. Rate equations were derived (Fig.3) assuming that the electron, injected into the semiconductor surface from the excited dye, was trapped in the vicinity of the oxidized dye molecule. The solution for q was revealed to explain the experimental results as shown in Fig.11. The rate of electron re-injection (k₁ in Fig.9) and that of tunneling (k₃ in Fig.9) increases as Nd increases. The Nd dependence of q may be interpreted by a drift mechanism (Eq. (17)) when N_d is relatively small, and by field emission (Eq. (18)) or resonance tunneling when N_d exc eeds 10²⁰cm-3 (Fig.12). The process II in Fig.9 is deactivation of the injected electron, and its rate was estimated to be 3×10⁷s^-1. The value of q is determined by the competition between I and II in Fig.9 when N_d is small (Eq. (15)); q depends weakly in [HQ]. The pronounced dependence of q on [HQ] for high N_d's is explained by the suppression of the process III which being relatively slow (Eq. (13), Figs.7, 10 and 11).