Recent rapid progress in efficiencies for solar water splitting by photoelectrochemical devices has enhanced its prospects to enable storable renewable energy. Efficient solar fuel generators all use tandem photoelectrode structures, and advanced integrated devices incorporate corrosion protection layers as well as heterogeneous catalysts. Realization of near thermodynamic limiting performance requires tailoring the energy band structure of the photoelectrode and also the optical and electronic properties of the surface layers exposed to the electrolyte. Here, we report a monolithic device architecture that exhibits reduced surface reflectivity in conjunction with metallic Rh nanoparticle catalyst layers that minimize parasitic light absorption. Additionally, the anatase TiO 2 protection layer on the photocathode creates a favorable internal band alignment for hydrogen evolution. An initial solar-to-hydrogen efficiency of 19.3 % is obtained in acidic electrolyte and an efficiency of 18.5 % is achieved at neutral pH condition (under simulated sunlight). Main TextAdvances in the field of artificial photosynthesis 1 have led to the development of functional prototypes for photoelectrochemical water splitting 2 , featuring improved photoelectrode stability through the use of corrosion protection layers 3 and the realization of systems for unassisted water splitting 4-6 in integrated monolithic devices. The requirement for the device operating voltage under illumination to exceed the thermodynamic potential difference for water dissociation of 1.23 V imposes constraints on the energy bandgaps for the photoelectrode absorber layers and their combined operating potential in a series-connected tandem configuration. Several strategies have been followed. Early prototypes used single absorber

No abstract

Polarity determination of a-plane GaN on r -plane sapphire and its effects on lateral overgrowth and heteroepitaxy J. Appl. Phys. 94, 942 (2003); 10.1063/1.1578530Morphological aspects of continuous and modulated epitaxial growth of (GaIn)P GaP-layers on Si(001) can serve as pseudo-substrates for a variety of novel optoelectronic devices. The quality of the GaP nucleation layer is a crucial parameter for the performance of such devices. Especially, anti-phase domains (APDs) evolving at mono-atomic steps on the Si-surface can affect the quality of a layer adversely. The size, shape, and possible charge of the APDs and their boundaries depend on the polarity of the surrounding crystal. The observed polarity of the GaP is caused by the A-type double step configuration of the Si-surface reconstruction prior to GaP growth and the prevalent binding of Ga to Si under optimized growth conditions. The polarity of the GaP-layer and hence the atomic configuration at the Si-III/V interface can be changed by altering the growth conditions. With this knowledge, defect-free GaP/Si(001) templates for III/V device integration on Si-substrates can be grown. V C 2012 American Institute of Physics.

For the implementation of optoelectronic devices on silicon, which could be realized by a combination of Si and direct-bandgap III/V semiconductors, a defect free nucleation layer of GaP on Si is essential. This paper summarizes the results of structural investigations carried out by transmission electron microscopy on defects, which can be observed in GaP films grown by metal organic vapor phase epitaxy on exactly oriented (001) Si substrates. Under optimized growth conditions the anti phase domains (APDs), which arise in the III/V semiconductor at the monoatomic steps on the silicon surface, show a specific typical shape. They self-annihilate on {112} planes in the GaP and can be observed in [110] cross-section, looking perpendicular to the steps on the Si surface. In contrast to that, the anti phase boundaries (APBs) lie on {110} GaP planes in the [−110] direction, parallel to the steps on the Si surface. From convergent beam electron diffraction one can show, that the GaP has Ga-polarity in the [110] direction, viewing perpendicular to the steps on the Si-surface. With the knowledge of the polarity and the shape of the APDs, we suggest a model for chemical composition of their boundaries. According to this model the APBs, which lie on {110} and {112} planes, consist of an equal amount of Ga-Ga and P-P bonds. Furthermore, when stacking faults and twins are observed, they only occur in the Ga-polar [110] GaP direction, and consequently lie on {111}A planes. With the knowledge of the structure of the defects that arise at the GaP/Si interface we suggest growth conditions and an optimum Si surface structure, which guarantee a defect-free GaP overgrowth layer after several 10 nms of III/V material, even on exact Si substrates.

Photoelectrochemical solar fuel generation is evolving steadily towards devices mature for applications, driven by the development of efficient multi-junction devices. The crucial characteristics deciding over feasibility of an application are efficiency and stability. Benchmarking and reporting routines for these characteristics are, however, not yet on a level of standardisation as in the photovoltaic community, mainly due to the intricacies of the photoelectrochemical dimension. We discuss best practice considerations for benchmarking and propose an alternative efficiency definition that includes stability. Furthermore, we analyse the effects of spectral shaping and anti-reflection properties introduced by catalyst nanoparticles and their impact on design criteria for direct solar fuel generation in monolithic devices