Development of Preparation Systems with K_2CsSb Photocathodes and Study on the Preparation Process

The next generation of advanced light sources requires photons with large average flux and high brightness,which needs advanced electron gun matched with excellent photocathode materials. K_2CsSb photocathode has the advantages of high quantum efficiency, long lifetime and instantaneous response. This study introduces the design of a set of K_2CsSb photocathode preparation systems and detailed preparation process of K_2CsSb photocathodes, including sequential deposition process and co-deposition process, and finally develops a K_2CsSb photocathode. The influence of laser power on the quantum efficiency is also investigated.

Recent progress and perspectives on Sb_(2)Se_(3)-based photocathodes for solar hydrogen production via photoelectrochemical water splitting

Photoelectrochemical(PEC) cells involved with semiconductor electrodes can simultaneously absorb solar energy and perform chemical reactions, which are considered as an attractive strategy to produce renewable and clean hydrogen energy. Sb_(2)Se_(3) has been widely investigated in constructing PEC photocathodes benefitting of its low toxicity, suitable band gap, superior optoelectronic properties, and outstanding photocorrosion stability. We first present a brief overview of basic concepts and principles of PEC water splitting as well as a comparison between Sb_(2)Se_(3) and other numerous candidates. Then the material characteristics and preparation methods of Sb_(2)Se_(3) are introduced. The development of Sb_(2)Se_(3)-based photocathodes in PEC water splitting with various architectures and engineering efforts(i.e., absorber engineering, interfaces engineering, co-catalyst engineering and tandem engineering) to improve solar-to-hydrogen(STH) efficiency are highlighted. Finally, we debate the possible future directions to further explore the researching fields of Sb_(2)Se_(3)-based photocathodes with a strongly positive outlook in PEC processed solar hydrogen production.

Comparison of blue–green response between transmission-mode GaAsP-and GaAs-based photocathodes grown by molecular beam epitaxy

In order to develop the photodetector for effective blue-green response, the 18-mm-diameter vacuum image tube combined with the transmission-mode Alo.7Gao.3Aso.9Po.1/GaAso.9Po.1 photocathode grown by molecular beam epitaxy is tentatively fabricated. A comparison of photoelectric property, spectral characteristic and performance parameter be- tween the transmission-mode GaAsP-based and blue-extended GaAs-based photocathodes shows that the GaAsP-based photocathode possesses better absorption and higher quantum efficiency in the blue-green waveband, combined with a larger surface electron escape probability. Especially, the quantum efficiency at 532 nm for the GaAsP-based photocathode achieves as high as 59%, nearly twice that for the blue-extended GaAs-based one, which would be more conducive to the underwater range-gated imaging based on laser illumination. Moreover, the simulation results show that the favorable blue-green response can be achieved by optimizing the emission-layer thickness in a range of 0.4 μm-0.6 μm.

High-Integrated-Photosensitivity Negative-Electron-Affinity GaAs Photocathodes with Multilayer Be-Doping Structures

The effect of changing Be doping concentration in GaAs layer on the integrated photosensitivity for nega- tive-electron-affinity GaAs photocathodes is investigated. Two GaAs samples with the monolayer structure and the muhilayer structure are grown by molecular beam epitaxy. The former has a constant Be concentration of 1 × 10^19 cm^-3, while the latter includes four layers with Be doping concentrations of 1 × 10^19, 7 × 10^18, 4 × 10^18, and 1 × 10^18 cm^-3 from the bottom to the surface. Negative-electron-affinity GaAs photocathodes are fabricated by exciting the sample surfaces with alternating input of Cs and O in the high vacuum system. The spectral response results measured by the on-line spectral response measurement system show that the integrated photosensitivity of the photocathode with the muhilayer structure enhanced by at least 50% as compared to that of the monolayer structure. This attributes to the improvement in the crystal quality and the increase in the surface escape probability. Different stress situations are observed on GaAs samples with monolayer structure and muhilayer structure, respectively.

Effect of Surface Potential Barrier on the Electron Energy Distribution of NEA Photocathodes

By calculating the energy distribution of electrons reaching the photocathode surface and solving the Schrodinger equation that describes the behavior of an electron tunneling through the surface potential barrier,we obtain an equation to calculate the emitted electron energy distribution of transmission-mode NEA GaAs photocathodes. Accord- ing to the equation,we study the effect of cathode surface potential barrier on the electron energy distribution and find a significant effect of the barrier-Ⅰ thickness or end height,especially the thickness,on the quantum efficiency of the cath- ode. Barrier Ⅱ has an effect on the electron energy spread, and an increase in the vacuum level will lead to a narrower electron energy spread while sacrificing a certain amount of cathode quantum efficiency. The equation is also used to fit the measured electron energy distribution curve of the transmission-mode cathode and the parameters of the surface barri- er are obtained from the fitting. The theoretical curve is in good agreement with the experimental curve.

Structural Design and Experiment of Narrow-Band Response GaAlAs Photocathodes

To obtain the peak response at 532nm, narrow-band response GaA1As photocathodes with two GaAIAs ac- tive layers of different aluminum compositions are designed in consideration of the maximum absorptivity and quantum efficiency. The transmission-mode and the corresponding reflective-mode photocathodes are grown by metalorganic chemical vapor deposition. The results indicate that the peak response and the cut-off wavelength occur at 532nm for the two kinds of photocathodes respectively. The response of the reflection-mode photoeath- ode is an order of magnitude higher than that of the transmission-mode photocathode, whereas the better growth quality and the thicker second GaAIAs active layer can improve the transmission-mode response.

PHOTOEMISSION STABILITY OF MULTIALKALI PHOTOCATHODES WITH NEGATIVE ELECTRON AFFINITY

In this paper a negative electron affinity （NEA） multialkali photocathode of （Na2KSb-Cs）-O-Cs structure is fabricated by new technology. It is found that its emission stability is much better than that of the NEA GaAs photocathode, but is inferior to that of the conventional Na2KSb（Cs）. After 70 hour performance in a pumping vacuum system, the emission sensitivity of the NEA （Na2KSb-Cs）-O-Cs photocathode drops only by 2.5%. The emission stability is closely related to the states of the activation cesium and oxygen during activation, best results being obtained with cesium ions and excited oxygen. Furthermore, better photoemission sensitivity and emission stability may be obtained if the cathode is illuminated by intense white light during the activation process. The performance of the NEA （Na2KSb-Cs）-O-Cs cathode which has not been illuminated by intense white light during activation may be improved by the illumination even during operation intermission.

Distribution of carriers in gradient-doping transmission-mode GaAs photocathodes grown by molecular beam epitaxy

The gradient-doping structure is first applied to prepare the transmission-mode GaAs photocathode and the integral sensitivity of the sealed image tube achieves 1420μA/lm. This paper studies the inner carrier concentration distribution of the gradient-doping transmission-mode GaAs photocathode after molecular beam epitaxy （MBE） growth using the electrochemical capacitance-voltage profiling. The results show that an ideal gradient-doping structure can be obtained by using MBE growth. The total band-bending energy in the gradient-doping GaAs active-layer with doping concentration ranging from 1×10^19 cm-3 to 1×1018 cm-3 is calculated to be 46.3 meV, which helps to improve the photoexcited electrons movement toward surface for the thin epilayer. In addition,by analysis of the band offsets, it is found that the worse carrier concentration discrepancy between GaAs and GaA1As causes a lower back interface electron potential barrier which decreases the amount of high-energy photoelectrons and affects the short-wave response.

Temporal Characteristics of GaAs NEA and Alkali Metal Photocathodes

The temporal characteristics of GaAs NEA and alkali metal photocathodes are studied using Monte Carlo simulation method. The electron transit time and its distribution functions in the photocathodes are calculated. Based on the results, the time modulation transfer functions and temporal resolutions of the photocathodes are obtained. The results show that the response time and temporal resolution of alkali metal photocathode is in femitosecond order and those of GaAs NEA photocathode are in picosecond order.

Comparative research on the transmission-mode GaAs photocathodes of exponential-doping structures

Early research has shown that the varied doping structures of the active layer of GaAs photocathodes have been proven to have a higher quantum efficiency than uniform doping structures. On the basis of our early research on the surface photovoltage of GaAs photocathodes, and comparative research before and after activation of reflection-mode GaAs photocathodes, we further the comparative research on transmission-mode GaAs photocathodes. An exponential doping structure is the typical varied doping structure that can form a uniform electric field in the active layer. By solving the one-dimensional diffusion equation for no equilibrium minority carriers of transmission-mode GaAs photocathodes of the exponential doping structure, we can obtain the equations for the surface photovoltage （SPV） curve before activation and the spectral response curve （SRC） after activation. Through experiments and fitting calculations for the designed material, the body-material parameters can be well fitted by the SPV before activation, and proven by the fitting calculation for SRC after activation. Through the comparative research before and after activation, the average surface escape probability （SEP） can also be well fitted. This comparative research method can measure the body parameters and the value of SEP for the transmission-mode GaAs photocathode more exactly than the early method, which only measures the body parameters by SRC after activation. It can also help us to deeply study and exactly measure the parameters of the varied doping structures for transmission-mode GaAs photocathodes, and optimize the Cs-O activation technique in the future.

Comparison of the photoemission behaviour between negative electron affinity GaAs and GaN photocathodes

In view of the important application of GaAs and GaN photocathodes in electron sources, differences in photoe- mission behaviour, namely the activation process and quantum yield decay, between the two typical types of III-V compound photocathodes have been investigated using a multi-information measurement system. The activation exper- iment shows that a surface negative electron affinity state for the GaAs photocathode can be achieved by the necessary Cs-O two-step activation and by Cs activation alone for the GaN photocathode. In addition, a quantum yield decay experiment shows that the GaN photocathode exhibits better stability and a longer lifetime in a demountable vacuum system than the GaAs photocathode. The results mean that GaN photocathodes are more promising candidates for electron source emitter use in comparison with GaAs photocathodes.

Spectral transmittance and module structure fitting for transmission-mode GaAs photocathodes

A transmission-mode GaAs photocathode includes four layers of glass, Si3N4, Gal-xAlxAs and GaAs. A gradientdoping photocathode sample was obtained by molecular beam epitaxy and its transmittance was measured by spectrophotometer from 600 nm to 1100 nm. The theoretical transmittance is derived and simulated based on the matrix formula for thin film optics. The simulation results indicate the influence of the transition layers and the three thin-film layers except glass on the transmittance spectra. In addition, a fitting coefficient needed for error modification enters into the fitted formula. The fitting results show that the relative error in the full spectrum reduces from 19.51% to 4.35% after the formula is modified. The coefficient and the thicknesses are gained corresponding to the minimum relative error, meanwhile each layer and total thin-film thickness deviation in the module can be controlled within 73. The presence of glass layer roughness, layer interface effects and surface oxides is interpreted on the modification.

Fabrication of CuO_x thin-film photocathodes by magnetron reactive sputtering for photoelectrochemical water reduction

The CuO_x thin film photocathodes were deposited on F-doped Sn O_2 (FTO)transparent conducting glasses by alternating current(AC)magnetron reactive sputtering under different Ar:O_2 ratios.The advantage of this deposited method is that it can deposit a CuO_x thin film uniformly and rapidly with large scale.From the photoelectrochemical(PEC)properties of these CuO_x photocathodes,it can be found that the CuO_x photocathode with Ar/O_2 30:7 provide a photocurrent density ofà3.2 m A cm^(à2)under a bias potentialà0.5 V(vs.Ag/Ag Cl),which was found to be twice higher than that of Ar/O_2 with 30:5.A detailed characterization on the structure,morphology and electrochemical properties of these CuO_x thin film photocathodes was carried out,and it is found that the improved PEC performance of CuO_x semiconductor photocathode with Ar/O_230:7 attributed to the less defects in it,indicating that this Ar/O_230:7 is an optimized condition for excellent CuO_x semiconductor photocathode fabrication.

Developing silicon-based photocathodes for CO_(2) conversion

Photoelectrochemical(PEC)conversion of CO_(2) presents a promising avenue for solar-driven chemical fuel production,with silicon emerging as a cost-effective and high-light-absorbing material pivotal to this technology.Aiming at exploring opportunities for industrializing PEC CO_(2) reduction(PEC-CO_(2)R)by minimizing reaction energy consumption,enhancing reaction efficiency and selectivity,this review summarizes recent advancements in developing Si-based photocathodes for PEC-CO_(2)R.It outlines the fundamental principles,advantages,and limitations of Si photocathodes with key performance metrics.Based on this understanding,the strategies to enhance the performance of the PEC-CO_(2)R system,including light absorption,charge separation,and catalytic reactions are categorized as the interfacial modification,active site decoration,and protective layer design.The design ideas of this advantageous three-layer structure in promoting the efficiency,stability,and selectivity have been clarified.Then,this review scrutinizes the influence of the photocathodic chemical environment.This review consolidates the mechanism insights and notable breakthroughs of various fuel generation processes within Si-based PEC-CO_(2)R systems.Providing this wealth of information offers an up-to-date perspective on the dynamic developments in silicon-based PEC-CO_(2) conversion and underscores the promising pathways toward the sustainable fuel synthesis from pollutant CO_(2).

GaN nanowires/Si photocathodes for CO_(2) reduction towards solar fuels and chemicals: advances, challenges, and prospects

Photoelectrocatalytic(PEC)production of fuels and chemicals by using solar energy,water,and CO_(2) paves a promising avenue toward carbon neutrality.Over the past decades,for accelerating this process,a variety of photocathodes have been explored.Among them,the hybrid of GaN nanowires(NWs)and planar silicon has appeared as a disruptive platform for this grand topic,owing to their distinctive structural,optoelectronic,and catalytic properties.This review illustrates the most recent advances in GaN NWs/Si-based photocathodes for CO_(2) reduction reactions powered by simulated sunlight,beginning with a discussion of the critical requirements and fundamental challenges of PEC CO_(2) reduction.The characteristics of GaN NWs/Si are then discussed,showing its great potential in precisely controlling the behavior of photons,charges,and chemical species.As the focus of this review,the progress on the PEC CO_(2) reduction reactions toward different products over GaN NWs/Si-based photocathodes is highlighted.In the end,the challenges and prospects of GaN NWs/Si-based photocathodes for the practical synthesis of solarfuels and chemicals are proposed.

Triggering heteroatomic interdiffusion in one-pot-oxidation synthesized NiO/CuFeO_(2) heterojunction photocathodes for efficient solar hydrogen production from water splitting

Delafossite CuFeO_(2) is a promising photocathode material for cost-efficiently photoelectrochemical(PEC)water splitting,but the unfavorable conductivity and fast recombination dynamics of photogenerated carriers limit its PEC activity for water reduction.Here,we developed a heterostructure photocathode consisting of the Cu-doped NiO(Cu:NiO)hole selective layer(HSL)and Ni-doped CuFeO_(2)(Ni:CuFeO_(2))active layer by simply annealing a homogeneous Cu-Fe oxalate layer grown on the Ni film deposited on the fluorine doped tin oxide(FTO)substrate.The obtained heterostructure of Cu:NiO/Ni:CuFeO_(2) with enhanced charge carrier transportability and high-quality interface greatly promotes the separation of photogenerated carriers.Accordingly,the Cu:NiO/Ni:CuFeO_(2) photocathode exhibits a high photocurrent density of~0.9 mA·cm^(-2 )at 0.2 V(vs.reversible hydrogen electrode,RHE),outperforming most of the reported bare CuFeO_(2) photocathodes in the literature.And the photocurrent density can be further improved to 1.2 mA·cm^(-2) after decorating NiSx cocatalyst.

Manipulating metal-oxygen local atomic structures in singlejunctional p-Si/WO_(3) photocathodes for efficient solar hydrogen generation

Self-passivation in aqueous solution and sluggish surface reaction kinetics significantly limit the photoelectrochemical(PEC)performances of silicon-based photoelectrodes.Herein,a WO_(3) thin layer is deposited on the p-Si substrate by pulsed laser deposition(PLD),acting as a photocathode for PEC hydrogen generation.Compared to bare p-Si,the single-junctional p-Si/WO_(3) photoelectrodes exhibit excellent and stable PEC performances with significantly increased cathodic photocurrent density and exceptional anodic shift in onset potential for water reduction.It is revealed that the WO_(3) layer could reduce the charge transfer resistance across the electrode/electrolyte interface by eliminating the effect of Fermi level pinning on the surface of p-Si.More importantly,by varying the oxygen pressures during PLD,the collaborative modulation of W–O bond covalency and WO6 octahedral structure symmetry contributes to the promoted charge carrier transport and separation.Meanwhile,a large band bending at the p-Si/WO_(3) junction,induced by the optimized O vacancy contents in WO_(3),could provide a photovoltage as high as~500 mV to efficiently drive charge transfer to overcome the water reduction overpotential.Synergistically,by manipulating W–O local atomic structures in the deposited WO_(3) layer,a great improvement in PEC performance could be achieved over the singlejunctional p-Si/WO_(3) photocathodes for solar hydrogen generation.

Enhancing photoelectrochemical activity with three-dimensional p-CuO/n-ZnO junction photocathodes

Designing photoelectrodes with particular nanostructure and composition has been regarded as a promising approach to improve the photoelectrochemical(PEC) water splitting efficiency. We report the design and synthesis of a three-dimensional(3D) nanostructure with CuO nanocones as backbones and ZnO nanorods as branches,using a facile water bath reaction process together with the atomic layer deposition(ALD) technology. As utilized in photocathodes, the optimized CuO/ZnO nanostructure, 37 cycles of ALD ZnO seedlayer and 55 min of water bath reaction of ZnO nanorods, demonstrate highly improved PEC performance. The ratio of photo to dark current density for the 3D CuO/ZnO is 6.4, much higher than the value of 2.7for the CuO electrode. The enhanced activity is attributed to the synergistic effects of effective carrier separation and collection, reduced charge recombination, and increased carrier lifetime in the CuO/ZnO heterojunction. This work demonstrates the feasibility of designing novel 3D nanostructures by ALD technology as efficient photoelectrodes.

Characterisation of Photocathodes Based on Pb Thin Film Deposited by UV Pulsed Laser Ablation

This work deals with the deposition of lead （Pb） thin films by the UV pulsed laser ablation technique, for their further use as photocathode devices in superconducting radio frequency guns. Scanning electron microscopy and atomic force microscopy analyses were performed to study the morphological features of Pb thin films deposited on Si （100） and Nb substrates. The films showed a granular structure with a nearly fully covered surface only for that one deposited on Nb substrate. X-ray diffraction measurements indicate the growth of polycrystalline Pb thin films with a preferential orientation along （111） planes. Results of the photoemission performance of Pb thin film deposited on Nb substrate showed a very encouraging average value of quantum efficiency of 6 x 10-5 through a single-photon absorption process, promoting further studies in the realisation of Pb photocathodes by this technique.

Research on surface photovoltage spectroscopy for GaAs photocathodes with Al_xGa_(1-x)As buffer layer

Surface photovoltage spectroscopy equations for cathode materials with an AlxGa1-xAs buffer layer are determined in order to effectively measure the body parameters for transmission-mode （t-mode） photocathode materials before Cs-O activation. Body parameters of cathode materials are well fitted through experiments and fitting calculations for the designed AlxGa1-xAs/GaAs structure material. This investigation examines photo-excited performance and measurements of body parameters for t-mode cathode materials of different doping structures. It also helps study various doping structures and optimize structure designs in the future.