Kayo Koike

A 24.4% solar to hydrogen energy conversion efficiency by combining concentrator photovoltaic modules and electrochemical cells

The highest efficiency of 24.4% for the solar-to-hydrogen (STH) energy conversion was obtained in an outdoor field test by combining concentrator photovoltaic (CPV) modules with InGaP/GaAs/Ge three-junction cells and polymer-electrolyte electrochemical (EC) cells. The high efficiency was obtained by using the high-efficiency CPV modules (~31% under the present operation conditions) and the direct connection between the CPV modules and the EC cells with an almost optimized number of elements in series. The STH efficiency bottleneck was clarified to be the efficiency of the CPV modules, the over-potential of the EC cells, and matching of the operation point to the maximal-power point of the CPV modules.

Improvement of Light Extraction Efficiency and Reduction of Leakage Current in GaN-Based LED Via V-Pit Formation

Four types of GaN-based light-emitting diodes (LEDs) with V-pits formed in different regions were grown by metal-organic chemical vapor deposition. The position of the V-pits embedded in the layers of the LED structures was controlled by varying the growth temperature. We achieved the highest output power and lowest leakage current values with the LED structures comprising V-pits embedded in active regions and the p-GaN textured surface. The V-pit formation enhances the light output power and reverse voltage values by 1.3 times the values of the conventional LED owing to the enhancement of the light scattering probability and the effective filtering of threading dislocations.

Band Alignment at n-GaN/Electrolyte Interface Explored by Photo-Induced Offset of Open-Circuit Potential for Efficient Water Splitting

The band alignment at the interface between an electrolyte and n-GaN photoanode was investigated by observing open-circuit potential (OCP) of the GaN as a function of irradiated Xe-lamp intensity. The OCP drifted to negative values almost linearly with respect to the logarithm of light intensity, similarly to the behavior of open-circuit voltage in solar cells. For the smaller light intensity than 10-2 mW/cm2, OCP value was less dependent on light intensity and it was sensitive to both the surface treatment of GaN and the surface damage which was introduced intentionally prior to the photoelectrochemical analysis by the irradiation of accelerated Ar atoms, high-energy electrons and γ-ray. The relationship between OCP and light intensity can be obtained without modifying the surface by photocurrent and it would be a good indicator how the surface treatment of a photoelectrode affects the photocatalytic activity.