Effect of Applied Potential on Photoinduced Charge Separation at Metal Complex Thin-Film Electrodes

Abstract

The effect of applied potentials on photocurrents at metal complex thin-film electrodes was, studied. Four different kinds of metalloporphyrins (Fig.1) and tris(2, 2'-bipyridine)ruthenium(II) complex were used.1x10¹⁵ molecules of each metal complex was coated on a Pt electrode (0.5 cm² in area). The methods of thin-film attachment were twofold and were described elsewhere. No redox. agents were added in sample solutions except for water and supporting electrolytes. The focusedi light of 400-800 nm range was applied. The photocurrent I, versus applied potential relationship I_p=f(E) was divided into three groups: Exponential, linear and others. The results were interpreted in terms of the film resistivity (≈10⁶Ω) which is uncompensated with the potentiostat and is changed with illumination. The second part of the present work is the comparison between a three electrode potentiostatic condition and a two electrode short circuited condition with no external load. Fig.6shows that the photocurrents are greater for the two electrode, system than those of three electrode system in both sides of extreme pH regions. This result suggests that the function of a potentiostat affects the photoinduced charge separation process in the metal complex thinfilm.. The above statement is further supported by the observation as following: At a tris(2, 2'bipyridine)ruthenium(II) thin-film electrode, a three electrode potentiostatic condition gave only 470 nm peak in the photocurrent action spectrum, whereas an additional 520 nm shoulder peak corresponding to a singlet-triplet direct transition was observed with a two electrode measuring system.