Hydrogen production via electrochemical water splitting is a promising approach for storing solar energy. For this technology to be economically competitive, it is critical to develop water splitting systems with high solar-to-hydrogen (STH) efficiencies. Here we report a photovoltaic-electrolysis system with the highest STH efficiency for any water splitting technology to date, to the best of our knowledge. Our system consists of two polymer electrolyte membrane electrolysers in series with one InGaP/GaAs/GaInNAsSb triple-junction solar cell, which produces a large-enough voltage to drive both electrolysers with no additional energy input. The solar concentration is adjusted such that the maximum power point of the photovoltaic is well matched to the operating capacity of the electrolysers to optimize the system efficiency. The system achieves a 48-h average STH efficiency of 30%. These results demonstrate the potential of photovoltaic-electrolysis systems for cost-effective solar energy storage.

High quality dilute nitride subcells for multijunction solar cells are achieved using GaInNAsSb. The effects on device performance of Sb composition, strain and purity of the GaInNAsSb material are discussed. New world records in efficiency have been set with lattice-matched InGaP/GaAs/GaInNAsSb triple junction solar cells and a roadmap to 50% efficiency with lattice-matched multijunction solar cells using GaInNAsSb is shown.

The system presented in this work shows a good repeatability (0.5% ) and high accuracy (6. 3%) in the computer controlled spectral characterization of photovoltaic devices by the determination of internal quantum efficiency over an extended wavelength range. Biasing conditions (for both light and voltage) are fully controlled by computer as well. A corresponding I-V curve can be obtained in the same location without moving the sample on a different set-up. Scanning images (maps) of internal quantum efficiency (IOE) and of external quantum efficiency (OE) can be obtained with a spatial resolution better than 10 microns. Parameters extracted from I-V curves can be mapped with the same spatial resolution. Because IOE curves and I-V curves are obtained in the same location and using same optics, a full characterization of the photovoltaic device under test is possible in a single run. By providing the high accuracy and extended versatility, this system will have wide-range of application in photovoltaics both research and production.

Luminescent coupling effects are considered crucial for the performance of multijunction solar cells. We report a novel approach based on small signal measurement, which can directly measure the luminescent coupling efficiency of a multijunction solar cell with different voltage bias. In addition, this method demonstrated the light and voltage dependence of the coupling efficiency, and can potentially lead to a deeper understanding of luminescent coupling effects as well as more effective design of multijunction solar cells.

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