Efficient Photocathode of an Ultrathin Organic p-n Bilayer Comprising p-Type Zinc Phthalocyanine and n-Type Fullerene for Hydrogen Peroxide Production

Hydrogen peroxide (H2O2) is a clean and environmentally friendly oxidant. At present, as an alternative to the conventional industrial procedure, namely, the anthraquinone method, a clean H2O2 production method is desired. The construction of an artificial photosynthetic system in which H2O2 can ideally be prepared from water and dioxygen (O2) is a promising approach. In such a system, an organic p-n bilayer comprising zinc phthalocyanine (ZnPc, p-type) and fullerene (C60, n-type) acts as a photocathode capable of O2 reduction to H2O2, where loading gold (Au) onto the C60 surface is necessary to achieve the corresponding reaction. However, the enhancement of the photocathodic activity of the organic p-n bilayer for H2O2 formation remains a critical issue. In this study, the effect of the thickness of an organo-bilayer (organo-photocathode) on photocathodic activity for H2O2 production was investigated. When both ZnPc and C60 were thin (approximately 10 nm each in thickness), the photocathodic activity of the ZnPc/C60 organo-photocathode was approximately 3.4 times greater than that of the thick ZnPc/C60 bilayer (i.e., ZnPc = ca. 70 nm and C60 = ca. 120 nm). The thin ZnPc/C60 bilayer exhibited a built-in potential at the p-n interface, where efficient charge separation occurs, resulting in a high concentration of electrons available for O2 reduction.