Unified model of ballistic and diffusive carrier transport: Application to photovoltaic materials

A unified model of one-dimensional ballistic and diffusive carrier transport in semiconductors, valid for arbitrary mean free path and arbitrary shape of the band edge profiles, is applied in a study of the effect of grain boundaries on the transport properties of photovoltaic materials. Adopting the trapping model to describe the grain boundaries, dark conductivities and photoconductivities are calculated as a function of donor density for samples consisting of chains of grains with length comparable to the mean free path. The results of the unified model are found to deviate substantially from those of the purely ballistic and purely diffusive limits.     

Enhanced photoelectrochemical hydrogen evolution at p-type CuBi2O4 photocathode through hypoxic calcination

Uniform p-type CuBi₂O₄ thin film was prepared through a spin coating method on fluorine-doped tin oxide (FTO) coated glass substrate, with a subsequent hypoxic post-annealing process under semi-sealed condition to enhance its photoelectrochemical efficiency for hydrogen evolution reaction. Compared to CuBi₂O₄ specimen annealed in open-air environment, the semi-sealed annealed CuBi₂O₄ photocathode presents a remarkable improvement in cathodic photocurrent, from 0.42 mA/cm² to 0.7 mA/cm² at 0.25 V_RHE. X-ray photoelectron spectroscopy study revealed that the electronic structure of CuBi₂O₄ film was significantly modified during the post-annealing process and higher carrier concentration was obtained through Mott-Schottky measurement on semi-annealed CuBi₂O₄. We also demonstrate that the synthesized CuBi₂O₄ film with a thin overlayer of sputtered TiO₂ can retain good stability and efficiency as a photocathode. This work provides insights into the mechanism of the high efficiency CuBi₂O₄ photocathode achieved from the unique post-annealing treatment.     

Earth abundant ZnO/CdS/CuSbS2 core-shell nanowire arrays as highly efficient photoanode for hydrogen evolution

CuSbS₂ is regarded as a promising photo-absorber for solar energy conversion due to its proper bandgap, high absorptivity and earth abundance. Herein, novel low-cost ZnO/CdS/CuSbS₂ core-shell nanowire arrays were constructed as photoanode for hydrogen evolution through non-aqueous electrodeposition and cation exchange. The nanowire demonstrates even and compact multilayer structure. The optimal ZnO/CdS/CuSbS₂ photoanode achieves a photocurrent of 6.48 mA/cm² at 0 V versus Ag/AgCl and the remarkable IPCE value with approximately 52% at 480 nm in an electrolyte solution containing 0.35 mol/L Na₂SO₃ and 0.25 mol/L Na₂S. It also exhibited a good stability, maintained 87.9% of the initial current after 1 h measurement. The high performance benefits from well-crystalline and compact multilayer structure, high absorptivity of CuSbS₂ and CdS, p-n junction formed between CdS and CuSbS₂ which promotes the electron-hole separation and ZnO nanowire array as three dimensional scaffolds for electron percolation pathway. This work suggests the potential applications of low-cost p-n junction core-shell nanowire arrays as a highly efficient photoanode for hydrogen evolution in oil-field waste water with reductive sulfide.     

Photoelectrochemical hydrogen generation with nanostructured CdS/Ti–Ni–O composite photoanode

Composite photocatalysts have aroused great interest due to combination of favorable electronic and optical properties. Herein, novel CdS/Ti–Ni–O composite photoanodes were constructed through anodic fabrication of nanostructured Ni-doped TiO₂ (Ti–Ni–O) oxide films and CdS deposition by successive ionic layer adsorption and reaction (SILAR). The morphology and composition evolution, optical properties and photoelectrochemical (PEC) performance of the photoanodes were investigated. The composite nanofilms mainly consisted of micropores and nanotubes. The CdS/Ti–Ni–O composite photoanode demonstrated remarkable PEC hydrogen generation properties with a high photocurrent density (6.72 mA·cm^?2 at 0 V vs Ag/AgCl) which was 18.2 times to that of the bare Ti–Ni–O photoanode. The synergy of Ni-doping and CdS-coupling on the enhancement of PEC performance offers useful ideas to the exploitation of effective photocatalysts and contributes to the development of solar-driven PEC hydrogen generation.     

The reactor design for photoelectrochemical hydrogen production

The heat transfer and flow characteristics of a photoelectrochemical (PEC) hydrogen generation reactor are investigated numerically. Four different reactor designs are considered in this study. The solar irradiation is separated into short and long wavelength parts depending on the energy band gap of the photoelectrode used. While short wavelength part is used to generate electron and hole pairs, the long wavelength part is used to heat the system. Because the energy required for splitting water decreases as temperature is increased, heating the reactor by using the long wave energy increases the system efficiency. Thus, how the long wavelength energy is absorbed by the reactor is very important.The results show that more long wavelength energy kept inside the reactor can increase the solar-to-hydrogen efficiency, η_SH. For Fe₂O₃ photoelectrode, careful reactor design can increase η_SH by 11.0%. For design D under 4000 W/m² irradiation and a quantum efficiency of 30%, η_SH is found to be 14.1% and the hydrogen volume production rate is 166 L/m² h for Fe₂O₃. Effects of several parameters on the PEC hydrogen reactor are also discussed.     

Photoelectrochemical properties of hematite films grown by plasma enhanced chemical vapor deposition

Nanostructured α-Fe₂O₃ thin films were grown by plasma-enhanced chemical vapor deposition (PE-CVD) using iron pentacarbonyl (Fe(CO)₅) as precursor. Influence of the plasma parameters on photoelectrochemical (PEC) properties of the resulting hematite thin films toward solar oxidation of water was investigated under one sun illumination in a basic (1 M NaOH) electrolyte. PEC data analyzed in conjunction with the data obtained by scanning electron microscopy, X-ray diffraction and Mott–Schottky analysis showed 100 W plasma power to be an optimal RF-power value for achieving a high photocurrent density of ∼1098 μA/cm² at 0.9 V/SCE external applied potential. The donor density, flat band potential, grain size and porosity of the films were observed to be highly affected by RF-power, which in turn resulted in enhanced photoresponse.     

Analysis of the interband transitions in porous silicon

Porous silicon (PS) presents interesting phenomena such as efficient luminescence and a peculiar transport of carriers. Due to its possible optoelectronic applications, it is important to calculate the dielectric function from interband optical transitions in PS to include quantum effects. In this work, we apply a supercell model for PS within an sp³s^* tight-binding technique, to analyze the effects of pores on the above-mentioned transitions. The polarized light absorption is studied by observing the oscillator strength behavior within two different schemes, which are applied and compared. We have found a significant enlargement of the optically active zone in the k-space, due to the localization of the wave function. The calculated dielectric functions for crystalline silicon and PS are compared with experimental results, giving the correct energy range and shape.     

Modeling effective carrier lifetimes of passivated macroporous silicon layers

We derive and apply a model that determines the effective minority carrier lifetime of macroporous crystalline silicon samples as a function of bulk lifetime, surface passivation and pore morphology. Two cases are considered: A layer of periodic macropores at the surface of a silicon wafer and a free standing macroporous silicon layer. We compare the model with experimental lifetime measurements for samples with randomly positioned macropores with a length of 10-40 μm. The pores have an average pore diameter of 2.4 μm and an average pore distance of 5.2 μm. The surface is passivated by thermal oxidation. The model agrees with the measurements if we assume an average surface recombination velocity S=24 cm/s at the pore surface.     

Optimization hydrogen production over visible light-driven titania-supported bimetallic photocatalyst from water photosplitting in tandem photoelectrochemical cell

Solar hydrogen production was investigated over a Cu-Ni doped TiO₂ photocatalyst from water photosplitting in a tandem photoelectrochemical cell, which was made up by connecting a modified photoelectrochemical cell to dye solar cell in a series. A mathematical representation for preparation parameters for hydrogen production was successfully generated. Optimization of hydrogen production was conducted with varying preparation parameters of Cu-Ni doped TiO₂ photocatalyst including molar ratios of water, acetic acid and Cu to titanium tetraisopropoxide. The optimum preparation parameters of photocatalyst was obtained at molar ratios of water, acetic acid and Cu to titanium tetraisopropoxide of 32, 4.9, and 5.9, respectively. Physical and photoelectrochemical characterization revealed that low content of water and Cu decreased the charge transfer resistance and charge carrier recombination rate on Cu-Ni/TiO₂ surface. This is attributed to the better crystallinity and less degree of agglomeration which led to obtain optimum particle size at this condition. Maximum hydrogen production rate of 2.12 mL/cm². h was achieved under the optimum condition using the tandem photoelectrochemical cell in the aqueous KOH and glycerol solution under visible light irradiation (λ > 400 nm).     

Plasma-chemical reduction of iron oxide photoanodes for efficient solar hydrogen production

We demonstrate the effect of hydrogen plasma treatment on hematite films as a simple and effective strategy for modifying the existing substrate to improve significantly the band edge positions and photoelectrochemical (PEC) performance. Plasma treated hematite films were consist of mixed phases (Fe₃O₄:α-Fe₂O₃) which was confirmed by XPS and Raman analysis, treated films also showed higher absorption cross-section and were found to be a promising photoelectrode material. The treated samples showed enhance photocurrent densities with maximum of 3.5 mA/cm² at 1.8 V/RHE and the photocurrent onset potentials were shifted from 1.68 V_RHE (untreated) to 1.28 V_RHE (treated). Hydrogen plasma treatment under non-equilibrium conditions induced a valence dynamics among Fe centers in the sub-surface region that was sustained by the incorporation of hydrogen in the hematite lattice as supported by the density functional theory calculations.     

Recent trends in development of hematite (α-Fe2O3) as an efficient photoanode for enhancement of photoelectrochemical hydrogen production by solar water splitting

Solar-assisted water splitting using photoelectrochemical (PEC) cell is an environmentally benign technology for the generation of hydrogen fuel. However, several limitations of the materials used in fabrication of PEC cell have considerably hindered its efficiency. Extensive efforts have been made to enhance the efficiency and reduce the hydrogen generation cost using PEC cells. Photoelectrodes that are stable, efficient and made of cost-effective materials with simple synthesizing methods are essential for commercially viable solar water splitting through PEC technology. To this end, hematite (α-Fe₂O₃) has been explored as an excellent photoanode material to be used in the application of PEC water oxidation owing to its suitable bandgap of 2.1 eV that can utilize almost 40% of the visible light. In this study, we have summarized the recent progress of α-Fe₂O₃ nanostructured thin films for improving the water oxidation. Strategic modifications of α-Fe₂O₃ photoanodes comprising nanostructuring, heterojunctions, surface treatment, elemental doping, and nanocomposites are highlighted and discussed. Some prospects related to the challenges and research in this innovative research area are also provided as a guiding layout in building design principles for the improvement of α-Fe₂O₃ photoanodes in photoelectrochemical water oxidation to solve the increasing environmental issues and energy crises.     

Temperature dependence of the conductivity in large-grained boron-doped ZnO films

The temperature dependence of the conductivity is investigated as a function of boron doping in large-grained, degenerate polycrystalline ZnO films prepared by low-pressure chemical vapor deposition. Carrier transport in undoped and lightly doped films is mainly controlled by the grain boundary; field emission through grain boundaries limits the conductivity below 90 K, while thermally activated thermoionic-field emission leads to an increase in the conductivity with the temperature near room temperature. In contrast, carrier transport in highly doped films is mainly governed by intra-grain scattering, which does not depend on the temperature for degenerate electron gases, limits the mobility below 120 K, whereas a metallic behavior (decrease in conductivity with increasing temperature) is observed at room temperature, which is linked to the ionized impurity scattering. The transition between the “semiconductor”-like and metallic-like behavior at room temperature takes place for a film with carrier concentration between 6×10¹⁹ and 9×10¹⁹ cm⁻³.     

Nanotube enhanced photoresponse of carbon modified (CM)-n-TiO2 for efficient water splitting

Carbon modified (CM)-n-TiO₂ nanotube arrays were successfully synthesized by anodization of Ti metal sheet in fluoride solution and subsequent annealing in air and natural gas flame oxidation. Both nanotube structure and carbon doping contributed to the enhancement of photoresponse of n-TiO₂. About two fold increase in photocurrent density was observed at undoped n-TiO₂ nanotube film compared to that at its undoped n-TiO₂ flat thin film. Also, about eight fold increase in photocurrent density was observed at carbon modified (CM)-n-TiO₂ nanotube film compared to that at undoped n-TiO₂ flat thin film. The sample prepared by anodization at 20 V cell voltage for 20 h followed by annealing in air at 500 °C for 1 h and natural gas flame oxidation at 820 °C for 18 min produced highest photocurrent density. It was found that the bandgap of n-TiO₂ was reduced to 2.84 eV and an additional intragap band was introduced in the gap at 1.30 eV above the valence band. The bandgap reduction and the new intragap band formation in CM-n-TiO₂ extended its utilization of solar energy up to the visible to infrared region.