Efficient photocatalytic water splitting requires effective generation, separation and transfer of photo-induced charge carriers that can hardly be achieved simultaneously in a single material. Here we show that the effectiveness of each process can be separately maximized in a nanostructured heterojunction with extremely thin absorber layer. We demonstrate this concept on WO3/BiVO4+CoPi core-shell nanostructured photoanode that achieves near theoretical water splitting efficiency. BiVO4 is characterized by a high recombination rate of photogenerated carriers that have much shorter diffusion length than the thickness required for sufficient light absorption. This issue can be resolved by the combination of BiVO4 with more conductive WO3 nanorods in a form of core-shell heterojunction, where the BiVO4 absorber layer is thinner than the carrier diffusion length while it’s optical thickness is reestablished by light trapping in high aspect ratio nanostructures. Our photoanode demonstrates ultimate water splitting photocurrent of 6.72 mA cm−2 under 1 sun illumination at 1.23 VRHE that corresponds to ~90% of the theoretically possible value for BiVO4. We also demonstrate a self-biased operation of the photoanode in tandem with a double-junction GaAs/InGaAsP photovoltaic cell with stable water splitting photocurrent of 6.56 mA cm−2 that corresponds to the solar to hydrogen generation efficiency of 8.1%.

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A detailed study on the application of Pd nanoparticle arrays, produced by self-assembled block copolymer templates, in bonding of III–V-based solar cell materials was carried out. The Pd nanoparticle array-mediated bonding (mechanical stacking) of GaAs-based thin-films (cells) was readily performed on the surface of GaAs or InP-based substrates (cells) to form multi-junction device architectures. Using the optimized Pd NP array, a 30.4%-efficiency four-junction two-terminal cell, consisting of an InGaP/GaAs top cell and an InGaAsP/InGaAs bottom cell, was achieved owing to the excellent electrical and optical bonding properties (bonding resistance, 1.81 Ω cm2; optical loss, 2.9%). Together with the verification of the long-term reliability of the Pd nanoparticle array-mediated bonding, our approach would become practically attractive for producing high-efficiency multi-junction solar cells.

Low-temperature cleaning of GaAs substrate by atomic hydrogen irradiation has been demonstrated. Atomic hydrogen was provided by dissociation of hydrogen gas, which was carried out in a simple cracking cell with a 1500°C tungsten filament. Auger electron spectroscopy showed that carbon was removed at 200°C and oxygen was removed at 400°C by 30-min atomic hydrogen irradiation. The surface cleaning of GaAs was confirmed by the change of RHEED pattern from halo to streak after the hydrogen irradiation.

Multijunction (MJ) solar cells achieve very high efficiencies by effectively utilizing the entire solar spectrum. Previously, we constructed a III‐V//Si MJ solar cell using the smart stack technology, a unique mechanical stacking technology with Pd nanoparticle array. In this study, we fabricated an InGaP/AlGaAs//Si three‐junction solar cell with an efficiency of 30.8% under AM 1.5G solar spectrum illumination. This efficiency is considerably higher than our previous result (25.1%). The superior performance was achieved by optimizing the structure of the upper GaAs‐based cell and employing a tunnel oxide passivated contact Si cell. Furthermore, we examined the low solar concentration performance of the device and obtained a maximum efficiency of 32.6% at 5.5 suns. This performance is sufficient for realistic low concentration photovoltaic applications (below 10 suns). In addition, we characterize the reliability of the InGaP/AlGaAs//Si three‐junction solar cell with a damp heat test (85 °C and 85% humidity for 1000 h). It was confirmed that our solar cells have high long‐term stability under severe conditions. The results demonstrate the potential of GaAs//Si MJ solar cells as next‐generation photovoltaic cells and the effectiveness of smart stack technology in fabricating multijunction cells.

We have theoretically investigated two-dimensional photonic crystal (2D PC) L1–L21 cavities with low-refractive-index (low-n) material cladding using the 3D finite-difference time domain method assisted by group theory in the time domain. We investigated various optical properties of 2D PC L-type cavities including resonant frequency, modal symmetry, Q factor, modal volume, real-space distribution, wavevector-space distribution and resonant wavevector condition. The resonant modes in 2D PC L-type cavities are Bloch-like resonant modes. As the size of the cavity increases, the fundamental resonant mode changes from the Fabry–Perot (FP)-like resonant mode to the distributed-feedback (DFB)-like resonant mode with the envelope of the half-cycle sine window. The DFB-like resonant mode can suppress the radiation loss in the vertical direction. The DFB-like resonant mode can achieve a high Q factor (Q > 105) in a small cavity (L < 10 µm). The DFB-like resonant mode is the intrinsic resonant mode in 2D PC L-type cavities, and it is naturally formed without fine tuning. These results show the high potential of 2D PC L-type cavities with low-n material cladding.

Single-crystal Cu(In,Ga)Se2 (CIGS) solar cells were produced with techniques developed for high-efficiency polycrystalline CIGS solar cells. The CIGS layers of a lattice match with GaAs were grown on GaAs(001) substrates by co-evaporation. The presence of a single-crystal CIGS layer without dislocations was confirmed by transmission electron microscopy. Alkaline metal incorporations were achieved by doping and postdeposition treatments. Ga grading structures were fabricated by two-layer deposition with different Ga contents. The Ga grading significantly increased the fill factor and open-circuit voltage. The best efficiency of 20% was achieved after heat–light soaking.

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