The open-circuit photovoltage Vx of a photoelectrochemical cell equipped with a platinum-coated n-Si semiconductor electrode remarkably increased when the Pt layer was made microscopically discontinuous. A strikingly high Vx of 0.685 V has been obtained for a Pt-coated and alkali-etched n-Si electrode in which Pt existed in the form of islands 5-20 nm wide. The Vx is much higher than those for normal p-n-junction Si solid solar cells (about 0.59 V). This is an encouraging result, which possibly makes a novel approach to highly efficient solar cells. Theoretical considerations on the effect of the discontinuity of the metal overlayers in metal-coated electrodes have shown that high photovoltages can be generated in cases where the metal exists in the form of extremely small, sparsely scattered islands, say about 5 nm wide, separated by about 20 nm from each other, even though no increase in the intrinsic barrier height at the metal-island/semiconductor interface is assumed to occur. The effective barrier height at the semiconductor-liquid junction can increase to the equivalent of the band gap by choosing appropriate redox couples due to the rapid decay of the potential modulation near the metal-island/semiconductor interface. In silicon electrodes covered with minute metal islands, the naked parts of the silicon surface are covered with insulating thin oxide layers, and the metal-covered parts serve as gates for the carrier transport. Accordingly, the saturation current can become quite low because most of the silicon surface is covered with the oxide layer, producing only low densities of intragap states, and the surface recombination at these parts becomes negligibly slow.

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