An overview on processes that are relevant in light-induced fuel generation, such as water photoelectrolysis or carbon dioxide reduction, is given. Considered processes encompass the photophysics of light absorption, excitation energy transfer to catalytically active sites and interfacial reactions at the catalyst/solution phase boundary. The two major routes envisaged for realization of photoelectrocatalytic systems, e.g. bio-inspired single photon catalysis and multiple photon inorganic or hybrid tandem cells, are outlined. For development of efficient tandem cell structures that are based on non-oxidic semiconductors, stabilization strategies are presented. Physical surface passivation is described using the recently introduced nanoemitter concept which is also applicable in photovoltaic (solid state or electrochemical) solar cells and first results with p-Si and p-InP thin films are presented. Solar-to-hydrogen efficiencies reach 12.1% for homoepitaxial InP thin films covered with Rh nanoislands. In the pursuit to develop biologically inspired systems, enzyme adsorption onto electrochemically nanostructured silicon surfaces is presented and tapping mode atomic force microscopy images of heterodimeric enzymes are shown. An outlook towards future envisaged systems is given.

Homoepitaxial p-InP(100) thin films prepared by MOVPE (metallorganic vapor phase epitaxy) were transformed into an InP/oxidephosphate/Rh heterostructure by photoelectrochemical conditioning. Surface sensitive synchrotron radiation photoelectron spectroscopy indicates the formation of a mixed oxide constituted by In(PO 3 ) 3 , InPO 4 and In 2 O 3 as nominal components during photo-electrochemical activation. The operation of these films as hydrogen evolving photocathode proved a light-to-chemical energy conversion efficiency of 14.5%. Surface activation arises from a shift of the semiconductor electron affinity by 0.44 eV by formation of In-Cl interfacial dipoles with a density of about 10 12 cm −2 . Predominant local In 2 O 3 -like structures in the oxide introduce resonance states near the semiconductor conduction band edge imparting electron conductivity to the phosphate matrix. Surface reflectance investigations indicate an enhanced light-coupling in the layered architecture. Solar hydrogen generation from water represents a viable route for establishing a carbon-neutral energy infrastructure based on renewable energy resources.1-4 To achieve this long-term objective, numerous approaches are currently being pursued comprising adapting systems derived from photosynthesis, 5-7 identification of appropriate catalysts, 8 development of transition metal oxide photoelectrodes 9,10,11 as well as devising efficient semiconductor tandem structures.12-14 Because biomimetic systems inspired by natural photosynthesis are characterized by rather low theoretical efficiencies, 6 the use of photoresponsive semiconductor materials for the splitting of water appears currently most promising. It can be shown that dual-bandgap systems reach theoretically efficiencies well above 40% at AM1.5. 15 The development horizon suggests therefore the use of technologically advanced semiconductor materials which would allow comparably fast technical realization. Further, the orthogonalization of charge carrier and photon pathways as well as an increased built-in potential by interfacial doping was recently exploited to achieve efficiencies near 10% using Si as substrate. 16 p-type InP is one of the most efficient photocathode materials for hydrogen evolution available. Heller and Vadimsky 17 have shown three decades ago that hydrogen evolution can occur with an efficiency of 12% if rhodium is deposited as catalytically active and optically transparent thin-film on top of an In 2 O 3 /InP structure. In this work, a new approach based on thin film photoelectrodes is presented. The photocathode is realized by homoepitaxial growth of thin films onto InP wafers which allows the use of liftoff procedures already known for photovoltaic systems. 18,19 The removal of the thin film photocathodes from the growth substrate is thereby possible after fabrication of the devices. This approach reduces production costs, which is a decisive factor for many III-V devices. In this report, we evaluate the applicability of homoepitaxial devices with emphasis in...

Among various types of solar cells, MOVPE-grown triple-junction III-V compound semiconductors are today's most efficient photovoltaic devices with conversion efficiencies exceeding 40%. A next-generation multijunction cell with four or more junctions and optimized band gaps is expected to break the present record efficiency surpassing the 50% mark. High band gap material combinations that are lattice matched to GaAs are already well established, but the required low band gap combinations containing a band gap around 1eV are still to be improved. For this purpose, we have developed a low band gap tandem (two-junction) solar cell lattice matched to InP. For the top and bottom subcells InGaAsP (Eg = 1.03 eV) and InGaAs (Eg = 0.73 eV) were utilized, respectively. A new interband tunnel junction was used to connect the subcells, including thin and highly doped layers of n-type InGaAs and p-type GaAsSb. The delicate MOVPE preparation of critical interfaces was monitored with in-situ reflectance anisotropy spectroscopy (RAS). After a contamination-free transfer, the RAS signals were then benchmarked in ultrahigh vacuum (UHV) with surface science techniques like low energy electron diffraction (LEED) and X-ray photoelectron spectroscopy (XPS). XPS measurements revealed that the sharpest InGaAs/GaAsSb interface was achieved when the GaAsSb layer in the tunnel junction of the solar cell was grown on III-rich (2×4)- or (4×2)-reconstructed InGaAs(100) surfaces. The improved interface preparation had a positive impact on the overall performance of the tandem cell, where slightly higher efficiencies were observed for the cells with the III-rich-prepared tunnel junction interfaces.

Atomic layer deposition-grown Al2O3 thin films are grown on n-type GaN and annealed at 300 or 500 °C in various atmospheres. Metal–insulator–semiconductor capacitors (MISCAPs) are used as simplified test structures for AlGaN/GaN heterostructure field effect transistors with an Al2O3 gate dielectric. Electrical characterization of the unannealed MISCAPs reveals a low leakage current density of ∼1.4 × 10−9 A/cm2 at −2 MV/cm. Annealing at 500 °C in N2 or a forming gas results in a degradation of this leakage level by more than one order of magnitude, whereas the leakage current of the Al2O3 films annealed at 500 °C in O2 is increased to ∼5.2 × 10−9 A/cm2 at −2 MV/cm. The photoassisted capacitance–voltage technique, the conductance method, and border trap analysis are used to study the influence of the annealing ambient atmosphere upon the Al2O3/GaN interface. For all atmospheres, thermal treatments at 500 °C marginally affects the border oxide trap density, but the forming gas anneal at 500 °C passivates the interface traps most efficiently. While the O2 thermal treatment reduces the interface trap density in the Al2O3/GaN system, the N2 anneal creates interface trap states, indicating the formation of an oxygen deficient defect level at the Al2O3/GaN interface during N2 annealing.

Experimental measurements on tandem GaInAsP/InGaAs concentrator solar cells are presented that demonstrate how the short-circuit current can shift from that of the higher current subcell to that of the lower current subcell as irradiance increases. Theoretical modeling illustrates how this can occur when the current-limiting subcell has a noticeably nonzero slope in its current-voltage curve near short-circuit, and should be general to all series-connected multijunction cells of this nature.