Direct water electrolysis was achieved with a novel, integrated, monolithic photoelectrochemical-photovoltaic design. This photoelectrochemical cell, which is voltage biased with an integrated photovoltaic device, splits water directly upon illumination; light is the only energy input. The hydrogen production efficiency of this system, based on the short-circuit current and the lower heating value of hydrogen, is 12.4 percent.

https://science.sciencemag.org/content/sci/280/5362/425.full.pdf

A Monolithic Photovoltaic-Photoelectrochemical Device for Hydrogen Production via Water Splitting

There is increased recognition by the world’s scientific, industrial, and political communities that the concentrations of greenhouse gases in the earth’s
atmosphere, particularly CO_2, are increasing. For
example, recent studies of Antarctic ice cores to
depths of over 3600 m, spanning over 420 000 years,
indicate an 80 ppm increase in atmospheric CO_2 in
the past 200 years (with most of this increase
occurring in the past 50 years) compared to the
previous 80 ppm increase that required 10 000 years.2
The 160 nation Framework Convention for Climate
Change (FCCC) in Kyoto focused world attention on
possible links between CO2 and future climate change
and active discussion of these issues continues.3 In
the United States, the PCAST report4 “Federal
Energy Research and Development for the Challenges
of the Twenty First Century” focused attention
on the growing worldwide demand for energy and the
need to move away from current fossil fuel utilization.
According to the U.S. DOE Energy Information
Administration,5 carbon emission from the transportation
(air, ground, sea), industrial (heavy manufacturing,
agriculture, construction, mining, chemicals,
petroleum), buildings (internal heating, cooling, lighting),
and electrical (power generation) sectors of the
World economy amounted to ca. 1823 million metric
tons (MMT) in 1990, with an estimated increase to
2466 MMT in 2008-2012 (Table 1).

Catalysis Research of Relevance to Carbon Management: Progress, Challenges, and Opportunities

The science describing semiconductor−liquid interfaces is highly interdisciplinary, broad in scope, interesting, and of importance to various emerging technologies. We present a review of the basic physicochemical principles of semiconductor−liquid interfaces, including their historical development, and describe the major technological applications that are based on these scientific principles.

Physical Chemistry of Semiconductor−Liquid Interfaces

A metamorphic Ga0.35In0.65P/Ga0.83In0.17As/Ge triple-junction solar cell is shown to provide current-matching of all three subcells and thus composes a device structure with virtually ideal band gap combination. We demonstrate that the key for the realization of this device is the improvement of material quality of the lattice-mismatched layers as well as the development of a highly relaxed Ga1−yInyAs buffer structure between the Ge substrate and the middle cell. This allows the metamorphic growth with low dislocation densities below 106 cm−2. The performance of the approach has been demonstrated by a conversion efficiency of 41.1% at 454 suns (454 kW/m2, AM1.5d ASTM G173–03).

Current-matched triple-junction solar cell reaching 41.1% conversion efficiency under concentrated sunlight

High-efficiency multijunction or tandem solar cells based on group III–V semiconductor alloys are applied in a rapidly expanding range of space and terrestrial programs. Resistance to high-energy radiation damage is an essential feature of such cells as they power most satellites, including those used for communications, defense, and scientific research. Recently we have shown that the energy gap of In1−xGaxN alloys potentially can be continuously varied from 0.7 to 3.4 eV, providing a full-solar-spectrum material system for multijunction solar cells. We find that the optical and electronic properties of these alloys exhibit a much higher resistance to high-energy (2 MeV) proton irradiation than the standard currently used photovoltaic materials such as GaAs and GaInP, and therefore offer great potential for radiation-hard high-efficiency solar cells for space applications. The observed insensitivity of the semiconductor characteristics to the radiation damage is explained by the location of the band edge...

Superior radiation resistance of In1-xGaxN alloys: Full-solar-spectrum photovoltaic material system

We report on multijunction GaInP/GaAs photovoltaic cells with efficiencies of 29.5% at 1‐sun concentration and air mass (AM) 1.5 global and 25.7% 1‐sun, AM0. These values represent the highest efficiencies achieved by any solar cell under these illumination conditions. Three key areas in this technology are identified and discussed; the grid design, front surface passivation of the top cell, and bottom surface passivation of both cells. Aspects of cell design related to its operation under concentration are also discussed.

29.5%‐efficient GaInP/GaAs tandem solar cells

A two‐terminal, monolithic cascade solar cell with an efficiency of 27.3% is reported. The device structure consists of a Ga0.5In0.5P homojunction grown epitaxially upon a GaAs homojunction, with a GaAs tunnel diode interconnect. The tandem combination of these two materials is lattice matched, and has a theoretical efficiency of 34%. The device was grown by metalorganic chemical vapor deposition at 700 °C, using trimethylgallium, trimethylindium, arsine, and phosphine as sources. The minority‐carrier transport properties of the Ga0.5In0.5P are shown to be relatively insensitive to variations of the growth temperature and phosphine overpressure. Other factors that affect the efficiency of the device are presented and discussed.

A 27.3% efficient Ga0.5In0.5P/GaAs tandem solar cell

We present the first experimental evidence for the spontaneous breaking of cubic symmetry in the band structure of films of ${\mathrm{Ga}}_{0.52}$${\mathrm{In}}_{0.48}$P grown by organometallic vapor-phase epitaxy on (100) GaAs substrates. We show how this effect is related to the spontaneous ordering of the alloy, and its correlation with the anomalous lowering of the band gap observed in these films.

Polarized band-edge photoluminescence and ordering in Ga0.52In0.48P.

The band gap of Ga0.5In0.5P is reported as a function of growth rate and growth temperature. The Ga0.5In0.5P is grown lattice matched to 2°‐off (100) GaAs substrates by atmospheric pressure organometallic chemical vapor deposition using an inlet group V/III ratio of 65. The variation of the band gap is surprisingly complex, taking five different functional forms within the two‐dimensional parameter space. These include regions in which the band gap (1) increases with growth rate, (2) decreases with growth rate, (3) is independent of both growth rate and temperature, (4) is independent of growth rate, but dependent on growth temperature, and (5) is not measurable since three‐dimensional, instead of two‐dimensional, growth is observed. The behavior can only be explained by a theory involving competing processes. One such theory is described.

Effect of growth rate on the band gap of ga0.5in0.5p

The Ga0.5In0.5P/GaAs monolithic two-terminal tandem cell has been adapted for concentrator operation. the adaptation involves changes in the grid design, top-cell base thickness and top-cell emitter thickness. the prototype concentrator device has demonstrated an efficiency exceeding 30% in the concentration range of 115-260 suns, making this the first two-terminal monolithic cell to exceed 30% efficiency.

A. E. Kibbler

Papers

29.5%‐efficient GaInP/GaAs tandem solar cells

A 27.3% efficient Ga0.5In0.5P/GaAs tandem solar cell

Polarized band-edge photoluminescence and ordering in Ga0.52In0.48P.

Effect of growth rate on the band gap of ga0.5in0.5p

Accelerated publication 30.2% efficient GaInP/GaAs monolithic two‐terminal tandem concentrator cell