Atomic layer deposited gallium oxide buffer layer enables 1.2 V open-circuit voltage in cuprous oxide solar cells

Phase-Dependent Phonon Heat Transport in Nanoscale Gallium Oxide Thin Films

Different phases of Ga2O3 have been regarded as superior platforms for making new-generation high-performance electronic devices. However, understanding of thermal transport in different phases of nanoscale Ga2O3 thin-films remains challenging, owing to the lack of phonon transport models and systematic experimental investigations. Here, thermal conductivity (TC) and thermal boundary conductance (TBC) of the( 1 ¯ 010 ) $( {\bar 1010} )$ α-,( 2 ¯ 01 ) $( {\bar 201} )\;$ β-, and (001) κ-Ga2O3 thin films on sapphire are investigated. At ≈80 nm, the measured TC of α (8.8 W m-1 K-1) is ≈1.8 times and ≈3.0 times larger than that of β and κ, respectively, consistent with model based on density functional theory (DFT), whereas the model reveals a similar TC for the bulk α- and β-Ga2O3. The observed phase- and size-dependence of TC is discussed thoroughly with phonon transport properties such as phonon mean free path and group velocity. The measured TBC at Ga2O3/sapphire interface is analyzed with diffuse mismatch model using DFT-derived full phonon dispersion relation. Phonon spectral distribution of density of states, transmission coefficients, and group velocity are studied to understand the phase-dependence of TBC. This study provides insight into the fundamental phonon transport mechanism in Ga2O3 thin films and paves the way for improved thermal management of high-power Ga2O3-based devices.

Machine Learning-Enabled Optical Architecture Design of Perovskite Solar Cells

Perovskite solar cells, emerging as a cutting-edge solar energy technology, have demonstrated a power conversion efficiency (PCE) of >26%, which is below the theoretical limit of 33%. This study, employing a combination of neural network models and high-throughput simulation calculations, taking the single-junction FAPbI3 cell as an illustrative example, indicates that a pyramid structure, in comparison of a planar one, can increase the highest Jsc to 27.4 mA/cm2 and the PCE to 28.4%. Both Jsc and PCE surpass the currently reported experimental results. The optimized periodicity and tilt angle of the pyramid structure align with the textured structure of crystalline silicon solar cells. These results underscore the substantial development potential of neural network inverse design based on high-throughput calculations in the field of optoelectronic devices and provide theoretical guidance for the design of monolithic perovskite-silicon tandem solar cells.

Heterojunction Devices Fabricated from Sprayed n-Type Ga2O3, Combined with Sputtered p-Type NiO and Cu2O

This work reports on the properties of heterojunctions consisting of n-type Ga2O3 layers, deposited using ultrasonic spray pyrolysis at high temperature from water-based solution, combined with p-type NiO and Cu2O counterparts, deposited by radio frequency and reactive, direct-current magnetron sputtering, respectively. After a comprehensive investigation of the properties of the single layers, the fabricated junctions on indium tin oxide (ITO)-coated glass showed high rectification, with an open circuit voltage of 940 mV for Ga2O3/Cu2O and 220 mV for Ga2O3/NiO under simulated solar illumination. This demonstrates in praxis the favorable band alignment between the sprayed Ga2O3 and Cu2O, with small conduction band offset, and the large offsets anticipated for both energy bands in the case of Ga2O3/NiO. Large differences in the ideality factors between the two types of heterojunctions were observed, suggestive of distinctive properties of the heterointerface. Further, it is shown that the interface between the high-temperature-deposited Ga2O3 and the ITO contact does not impede electron transport, opening new possibilities for the design of solar cell and optoelectronic device architectures.

Characteristics of tunable aluminum-doped Ga2O3thin films and photodetectors

Aluminum-doped Ga2O3(AGO) thin films were prepared by plasma-enhanced atomic layer deposition (PE-ALD). The growth mechanism, surface morphology, chemical composition, and optical properties of AGO films were systematically investigated. The bandgap of AGO films can be theoretically set between 4.65 and 6.8 eV. Based on typical AGO films, metal-semiconductor-metal photodetectors (PDs) were created, and their photoelectric response was examined. The preliminary results show that PE-ALD grown AGO films have high quality and tunable bandgap, and AGO PDs possess superior characterizations to undoped films. The AGO realized using PE-ALD is expected to be an important route for the development of a new generation of gallium oxide-based photodetectors into the deep-ultraviolet.

Improving the photovoltage of Cu2O photocathodes with dual buffer layers

Cuprous oxide (Cu2O) is a promising oxide material for photoelectrochemical water splitting (PEC), and increasing its photovoltage is the key to creating efficient overall PEC water-splitting devices. Previous reports are mostly focused on optimizing the energy band alignment between Cu2O and the n-type buffer layer to improve the photovoltage of Cu2O photocathodes. However, the band alignment between the n-type buffer layer and the protective layer is often ignored. In this work, Cu2O photocathodes with a single buffer layer (Ga2O3) and dual buffer layers (Ga2O3/ZnGeOx) are fabricated, and their PEC performances are compared. Results show that after inserting the second buffer layer (ZnGeOx), the onset potential of the Cu2O photocathode increases by 0.16 V. Operando electrochemical impedance spectroscopy measurements and analysis of the energy-level diagrams of each layer show that an energy level gradient between Ga2O3 and TiO2 is created when ZnGeOx is introduced, which eliminates the potential barrier at the interface of Ga2O3/TiO2 and improves the photovoltage of the Cu2O photocathode. Our work provides an effective approach to improve the photovoltage of photoelectrodes for solar water splitting by introducing dual buffer layers.

Formation and Characterization of Stable TiO2/CuxO-Based Solar Cells

According to increasing demand for energy, PV cells seem to be one of the best answers for human needs. Considering features such as availability, low production costs, high stability, etc., metal oxide semiconductors (MOS) are a focus of attention for many scientists. Amongst MOS, TiO2 and CuxO seem to be promising materials for obtaining an effective photoconversion effect. In this paper, specific investigation, aimed at the manufacturing of the complete photovoltaic structure based on this concept is described in detail. A set of samples manufactured by DC magnetron sputtering, with various process parameters, is characterized by morphology comparison, layer structure and material composition investigation, and finally by the obtained photovoltaic parameters. Based on SEM studies, it was established that the films are deposited uniformly and complete their formation; without clearly defined faces, the conglomerates of the film grow individually. These are areas with a uniform structure and orientation of atoms. The sizes of conglomerates are in a normal direction range from 20 to 530 nm and increase with film thickness. The film thickness was in the range from 318 to 1654 nm, respectively. The I-V study confirms the photovoltaic behavior of thin film solar cells. The open-circuit voltage (Voc) and short-circuit current density (Jsc) values of the photovoltaic devices ranged from 1.5 to 300 mV and from 0.45 to 7.26 µA/cm3, respectively, which corresponds to the maximum efficiency at the level of 0.01%. Specific analysis of the junction operation on the basis of characteristics flow, Rs, and Rsh values is delivered.

Crystal orientation-dependent etching and trapping in thermally-oxidised Cu2O photocathodes for water splitting

Ammonia solution etching was carried out on thermally-oxidised cuprous oxide (TO-Cu₂O) in photocathode devices for water splitting. The etched devices showed increased photoelectrochemical (PEC) performance compared to the unetched ones as well as improved reproducibility. -8.6 mA cm^-2 and -7 mA cm^-2 photocurrent density were achieved at 0 V and 0.5 V versus the reversible hydrogen electrode (V_RHE), respectively, in the champion sample with an onset potential of 0.92 V_RHE and a fill factor of 44%. An applied bias photon-to-current efficiency of 3.6% at 0.56 V_RHE was obtained, which represents a new record for Cu₂O-based photocathode systems. Capacitance-based profiling studies showed a strong pinning effect from interfacial traps in the as-grown device, and these traps were removed by ammonia solution etching. Moreover, the etching procedure gave rise to a diverse morphology of Cu₂O crystals based on the different crystallographic orientations. The distribution of crystallographic orientations and the relationship between the crystal orientation and the morphology after etching were examined by electron backscatter diffraction (EBSD) and scanning electron microscopy (SEM). The high-index crystal group showed a statistically higher PEC performance than the low-index group. X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) revealed metallic copper at the Cu₂O/Ga₂O₃ interface, which we attribute as the dominant trap that limits the PEC performance. It is concluded that the metallic copper originates from the reduction of the CuO impurity layer on the as-grown Cu₂O sample during the ALD process, while the reduction from Cu₂O to Cu is not favourable.

Highly Transparent Bidirectional Transparent Photovoltaics for On-Site Power Generators

If we can transparently produce energy, we may apply invisible power generators to residential architectures to supply energy without losing visibility. Transparent photovoltaic cells (TPVs) are a transparent solar technology that transmits visible light while absorbing the invisible short wavelengths, such as ultraviolet. Installing TPVs in buildings provides an on-site energy supply platform as a window-embedded power generator or color-matched solar cell installation on a building surface. The record-high power generation (10.82 mW) and photocurrent value (68.25 mA) were achieved from large-scale TPVs (25 cm²). The metal oxide heterojunction is the fundamental TPV structure. The high-performance TPVs were achieved by adopting a thin Si film between ZnO and NiO as a functional light-absorbing layer. Based on the large energy band gap of metal oxides, TPVs have a clear transmittance (43%) and good color coordinates, which ensure degrees of freedom to adopt TPV power generators in various colored structures or transparent power windows. The bidirectional feature of TPVs is ultimately desirable to maximize light utilization. TPVs can generate electric power from sunlight during the day and can also work from artificial light sources at night. In the near future, humans will acquire electric power without losing visibility with on-site energy supply platforms.

Defective Indium Tin Oxide Forms an Ohmic Back Contact to an n-Type Cu2O Photoanode to Accelerate Charge-Transfer Kinetics for Enhanced Low-Bias Photoelectrochemical Water Splitting

In significant contrast to the tremendous research efforts mostly geared to addressing the severe hole accumulation at the back contact of a p-type Cu₂O photocathode with a fluorine-doped tin oxide (FTO) substrate, sluggish electron transfer from an n-type Cu₂O photoanode to a tin-doped indium oxide (ITO) substrate has been largely overlooked. To tackle this issue that has been reported to largely limit the photoelectrochemical performance of n-type Cu₂O photoanodes at a low bias, the present contribution puts forward a strategy to introduce oxygen vacancies into the ITO substrate via an unprecedented yet facile electrochemical approach. Such defect engineering turns out to decrease the work function of the ITO substrate, which in turn approaches the conduction band extremum of n-Cu₂O to highly efficiently extract the photoexcited electrons therein. Moreover, the dendritic growth of n-Cu₂O is, in the meantime, interfered by the oxygen vacancy manifested as pinholes distributed over the ITO substrate, which is thereby crystallized into several small grains with augmented surface roughness that is in favor of the injection of the photoexcited hole into the electrolyte. Such facile interfacial charge-transfer kinetics leads to a significant cathodic shift amounting to 200 mV of the onset potential to 0 V_Ag/AgCl, whereat the n-Cu₂O photoanode deposited on the defective ITO substrate delivers the maximum photocurrent density reaching 2 mA cm^-2 and, more significantly, its applied bias photon-to-current efficiency (ABPE) reaches 1.1%, which is among the highest performance reported to date for a variety of state-of-the-art metal oxide-based photoanodes in the literature.

Low Thermal Budget Heteroepitaxial Gallium Oxide Thin Films Enabled by Atomic Layer Deposition

This work explores the applicability of atomic layer deposition (ALD) in producing highly oriented crystalline gallium oxide films on foreign substrates at low thermal budgets. The effects of substrate, deposition temperature, and annealing process on formation of crystalline gallium oxide are discussed. The Ga₂O₃ films exhibited a strong preferred orientation on the c-plane sapphire substrate. The onset of formation of crystalline gallium oxide is determined, at which only two sets of planes, i.e., α-Ga₂O₃ (006) and β-Ga₂O₃ (4̅02), are present parallel to the surface. More specifically, this work reports, for the first time, that epitaxial gallium oxide films on sapphire start to form at deposition temperatures ≥ 190 °C by using an optimized plasma-enhanced ALD process such that α-Ga₂O₃ (006)∥α-Al₂O₃ (006) and β-Ga₂O₃ (2̅01)∥α-Al₂O₃ (006). Both α-Ga₂O₃ (006) and β-Ga₂O₃ (2̅01) planes are polar planes (i.e., consisting of only one type of atom, either Ga or O) and, therefore, favorable to form by ALD at such low deposition temperatures. Ellipsometry and van der Pauw measurements confirmed that the crystalline films have optical and electrical properties close to bulk gallium oxide. The film grown at 277 °C was determined to have superior properties among as-deposited films. Using TEM to locate α-Ga₂O₃ and β-Ga₂O₃ domains in the as-deposited crystalline films, we proposed a short annealing scheme to limit the development of α-Ga₂O₃ domains in the film and produce pure β-Ga₂O₃ films via an energy-efficient process. A pure β-Ga₂O₃ phase on sapphire with β-Ga₂O₃ (2̅01)∥α-Al₂O₃ (006) was successfully achieved by using the proposed process at the low annealing temperature of 550 °C preceded by the low deposition temperature of 190 °C. The results of this work enable epitaxial growth of gallium oxide thin films, with superior material properties offered by ALD, not only with potential applications as a high-performance material in reducing global energy consumption but also with an energy-efficient fabrication process.

Possible top cells for next-generation Si-based tandem solar cells

Si-based solar cells, which have the advantages of high efficiency, low manufacturing costs, and outstanding stability, are dominant in the photovoltaic market. Currently, state-of-the-art Si-based solar cells are approaching the practical limit of efficiency. Constructing Si-based tandem solar cells is one available pathway to break the theoretical efficiency limit of single-junction silicon solar cells. Various top cells have been explored recently in the construction of Si-based tandem devices. Nevertheless, many challenges still stand in the way of extensive commercial application of Si-based tandem solar cells. Herein, we summarize the recent progress of representative Si-based tandem solar cells with different top cells, such as III-V solar cells, wide-bandgap perovskite solar cells, cadmium telluride (CdTe)-related solar cells, Cu(In,Ga)(Se,S)₂ (CIGS)-related solar cells, and amorphous silicon (a-Si) solar cells, and we analyze the main bottlenecks for their next steps of development. Subsequently, we suggest several potential candidate top cells for Si-based tandem devices, such as Sb₂S₃, Se, CdSe, and Cu₂O. These materials have great potential for the development of high-performance and low-cost Si-based tandem solar cells in the future.

Influence of surface defect density on the ultrafast hot carrier relaxation and transport in [Formula: see text] photoelectrodes

Cuprous oxide ([Formula: see text]) is a promising material for photoelectrochemical energy conversion due to its small direct band gap, high absorbance, and its Earth-abundant constituents. High conversion efficiencies require transport of photoexcited charges to the interface without energy loss. We studied the electron dynamics in [Formula: see text](111) by time-resolved two-photon photoemission for different surface defect densities in order to elucidate the influence on charge carrier transport. On the pristine bulk terminated surface, the principal conduction bands could be resolved, and ultrafast, elastic transport of electrons to the surface was observed. On a reconstructed surface the carrier transport is strongly suppressed and defect states dominate the spectra. Evidence for surface oxygen vacancies acting as efficient carrier traps is provided, what is important for further engineering of [Formula: see text] based photoelectrodes.

Lead-Free Perovskite Narrow-Bandgap Oxide Semiconductors of Rare-Earth Manganates

Tremendous success has been achieved in photovoltaic (PV) applications, but PV-generated electricity still cannot compete with traditional power in terms of price. Chemically stable and nontoxic all-oxide solar cells made from earth-abundant resources fulfill the requirements for low-cost manufacturing under ambient conditions and thus are promising as the next-generation approach to solar cells. However, the main obstacles to developing all-oxide solar cells are the spectral absorbers. Besides photovoltaics, novel chemically stable, nontoxic, and earth-abundant narrow-bandgap semiconductors are desired for photochemical applications in photodetectors, photoelectrodes, or photocatalysts. Herein, were report novel lead-free perovskite narrow-bandgap rare-earth semiconductors, YMnO3, HoMnO3, ErMnO3, and YbMnO3, which were identified by screening a family of perovskite rare-earth manganates, RMnO3 (R = Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, and Yb). The sharp edge observed in their absorption spectra indicates the existence of band gaps, further confirmed with laser Raman fluorescence spectra. Good periodic on-off photoelectronic response was observed in 8 of the 12 members (i.e., R = La, Pr, Nd, Sm, Gd, Tb, Dy, and Yb). Among them, YbMnO3 is approved as an n-type semiconductor with a direct band gap near 1.35 eV, whose theoretical Shockley-Queisser efficiency is approximately 33.7% for single-p-n-junction solar cells. This work sheds light on exploring stable oxide semiconductors with a narrow band gap for future applications.

Assessment of a W:BiVO4-CuBi2O4Tandem Photoelectrochemical Cell for Overall Solar Water Splitting

We assess a tandem photoelectrochemical cell consisting of a W:BiVO₄ photoanode top absorber and a CuBi₂O₄ photocathode bottom absorber for overall solar water splitting. We show that the W:BiVO₄ photoanode oxidizes water and produces oxygen at potentials ≥0.7 V vs RHE when CoPi is added as a cocatalyst. However, the CuBi₂O₄ photocathode does not produce a detectable amount of hydrogen from water reduction even when Pt or RuO_x is added as a cocatalyst because the photocurrent primarily goes toward photocorrosion of CuBi₂O₄ rather than proton reduction. Protecting the CuBi₂O₄ photocathode with a CdS/TiO₂ heterojunction and adding RuO_x as a cocatalyst prevents photocorrosion and allows for photoelectrochemical production of hydrogen at potentials ≤0.3 V vs RHE. A tandem photoelectrochemical cell composed of a W:BiVO₄/CoPi photoanode and a CuBi₂O₄/CdS/TiO₂/RuO_x photocathode produces hydrogen which can be detected under illumination at an applied bias of ≥0.4 V. Since the valence band of BiVO₄ and conduction band of CuBi₂O₄ are adequately positioned to oxidize water and reduce protons, we hypothesize that the applied bias is required to overcome the relatively low photovoltages of the photoelectrodes, that is, the relatively low quasi-Fermi level splitting within BiVO₄ and CuBi₂O₄. This work is the first experimental demonstration of hydrogen production from a BiVO₄-CuBi₂O₄-based tandem cell and it provides important insights into the significance of photovoltage in tandem devices for overall water splitting, especially for cells containing CuBi₂O₄ photocathodes.

Solution-Processed ZnxCd1-xS Buffer Layers for Vapor Transport-Deposited SnS Thin-Film Solar Cells: Achieving High Open-Circuit Voltage

As an alternative buffer material to CdS, Zn_xCd_1-xS buffer layers for vapor transport-deposited SnS thin-film solar cells (TFSCs) were fabricated using the successive ionic layer adsorption and reaction (SILAR) method. Varying the Zn-to-Cd ratio resulted in a series of Zn_xCd_1-xS thin films with controllable band gaps in the range of 2.40-3.65 eV. The influence of the Zn-to-Cd ratio on the cell performance was investigated in detail. The Zn_0.34Cd_0.66S buffer layer was found to be the optimal composition for SnS TFSCs, and a record open-circuit voltage (V_oc) of 0.405 V was achieved with an efficiency of 3.72%, whereas the SILAR-CdS buffer layer rendered a V_oc of 0.324 V. The improvement in V_oc when using the Zn_0.34Cd_0.66S buffer layer was corroborated by the spike-type conduction band offset of 0.35 eV with the SnS absorber, as revealed by the X-ray photoelectron spectroscopy analysis. In addition, minimized interfacial recombination at the SnS/Zn_0.34Cd_0.66S heterojunction was confirmed by the temperature-dependent V_oc analysis under illuminated conditions.

Supersonic Cold Spraying for Energy and Environmental Applications: One-Step Scalable Coating Technology for Advanced Micro- and Nanotextured Materials

Supersonic cold spraying is an emerging technique for rapid deposition of films of materials including micrometer-size and sub-micrometer metal particles, nanoscale ceramic particles, clays, polymers, hybrid materials composed of polymers and particulates, reduced graphene oxide (rGO), and metal-organic frameworks. In this method, particles are accelerated to a high velocity and then impact a substrate at near ambient temperature, where dissipation of their kinetic energy produces strong adhesion. Here, recent progress in fundamentals and applications of cold spraying is reviewed. High-velocity impact with the substrate results in significant deformation, which not only produces adhesion, but can change the particles' internal structure. Cold-sprayed coatings can also exhibit micro- and nanotextured morphologies not achievable by other means. Suspending micro- or nanoparticles in a liquid and cold-spraying the suspension produces fine atomization and even deposition of materials that could not otherwise be processed. The scalability and low cost of this method and its compatibility with roll-to-roll processing make it promising for many applications, including ultrathin flexible materials, solar cells, touch-screen panels, nanotextured surfaces for enhanced heat transfer, thermal and electrical insulation films, transparent conductive films, materials for energy storage (e.g., Li-ion battery electrodes), heaters, sensors, photoelectrodes for water splitting, water purification membranes, and self-cleaning films.

Solar-Blind UV Photodetector Based on Atomic Layer-Deposited Cu2O and Nanomembrane β-Ga2O3 pn Oxide Heterojunction

Herein, we present a solar-blind ultraviolet photodetector realized using atomic layer-deposited p-type cuprous oxide (Cu₂O) underneath a mechanically exfoliated n-type β-gallium oxide (β-Ga₂O₃) nanomembrane. The atomic layer deposition process of the Cu₂O film applies bis(N,N'-di-secbutylacetamidinato)dicopper(I) [Cu(⁵Bu-Me-amd)]₂ as a novel Cu precursor and water vapor as an oxidant. The exfoliated β-Ga₂O₃ nanomembrane was transferred to the top of the Cu₂O layer surface to realize a unique oxide pn heterojunction, which is not easy to realize by conventional oxide epitaxy techniques. The current-voltage (I-V) characteristics of the fabricated pn heterojunction diode show the typical rectifying behavior. The fabricated Cu₂O/β-Ga₂O₃ photodetector achieves sensitive detection of current at the picoampere scale in the reverse mode. This work provides a new approach to integrate all oxide heterojunctions using membrane transfer and bonding techniques, which goes beyond the limitation of conventional heteroepitaxy.

Directly Photoexcited Oxides for Photoelectrochemical Water Splitting

Artificial photosynthesis promises to become a sustainable way to harvest solar energy and store it in chemical fuels by means of photoelectrochemical (PEC) cells. Although it is intriguing to shift the fossil-fuel-based economy to a renewable carbon-neutral one, which will alleviate environmental problems, there is still a long way to go before it rivals traditional energy sources. Existing solar water-splitting devices can be sorted into three categories: photovoltaic-powered electrolysis, PEC water splitting, and photocatalysis (PC). PEC and PC systems hold the potential to further reduce the cost of devices due to their simple structures in which photoabsorbers and catalysts are closely integrated. PC is expected to be the least expensive approach; however, additional costs and concerns are brought about by the subsequent explosive gas separation. At the heart of all devices, semiconductor photoabsorbers should be efficient, robust, and cheap to satisfy the strict requirements on the market. Therefore, this Review intends to give readers an overview on PEC water splitting, with an emphasis on oxide material-based devices, which hold the potential to support global-scale production in the future.

Recent Advances in Earth-Abundant Photocathodes for Photoelectrochemical Water Splitting

The conversion of solar energy into hydrogen through photoelectrochemical (PEC) water splitting is an attractive way to store renewable energy. Despite the intriguing concept of solar hydrogen production, efficient PEC devices based on earth-abundant semiconductors should be realized to compete economically with conventional steam reforming processes. Herein, recent milestones in photocathode development for PEC water splitting, particularly in earth-abundant semiconductors, in terms of new techniques for enhancing performance, as well as theoretical aspects, are highlighted. In addition, recent research into newly emerging low-cost p-type semiconductors in the PEC field, such as Cu₂ BaSn(S,Se)₄ and Sb₂ Se₃ , are scrutinized and the advantages and disadvantages of each material assessed.

Femtosecond time-resolved two-photon photoemission studies of ultrafast carrier relaxation in Cu2O photoelectrodes

Cuprous oxide (Cu₂O) is a promising material for solar-driven water splitting to produce hydrogen. However, the relatively small accessible photovoltage limits the development of efficient Cu₂O based photocathodes. Here, femtosecond time-resolved two-photon photoemission spectroscopy has been used to probe the electronic structure and dynamics of photoexcited charge carriers at the Cu₂O surface as well as the interface between Cu₂O and a platinum (Pt) adlayer. By referencing ultrafast energy-resolved surface sensitive spectroscopy to bulk data we identify the full bulk to surface transport dynamics for excited electrons rapidly localized within an intrinsic deep continuous defect band ranging from the whole crystal volume to the surface. No evidence of bulk electrons reaching the surface at the conduction band level is found resulting into a substantial loss of their energy through ultrafast trapping. Our results uncover main factors limiting the energy conversion processes in Cu₂O and provide guidance for future material development.

While cuprous oxide is a promising solar-to-fuel conversion material, photoelectrochemical devices substantially underperform. Here, the authors use femtosecond time-resolved two-photon photoemission spectroscopy to correlate photoexcited electron energetics and dynamics with performance losses.

Giant Defect-Induced Effects on Nanoscale Charge Separation in Semiconductor Photocatalysts

Defects can markedly impact the performance of semiconductor-based photocatalysts, where the spatial separation of photogenerated charges is required for converting solar energy into fuels. However, understanding exactly how defects affect photogenerated charge separation at nanometer scale remains quite challenging. Here, using time- and space-resolved surface photovoltage approaches, we demonstrate that the distribution of surface photogenerated charges and the direction of photogenerated charge separation are determined by the defects distributed within a 100 nm surface region of a photocatalytic Cu₂O particle. This is enabled by the defect-induced charge separation process, arising from the trapping of electrons at the near-surface defect states and the accumulation of holes at the surface states. More importantly, the driving force for defect-induced charge separation is greater than 4.2 kV/cm and can be used to drive photocatalytic reactions. These findings highlight the importance of near-surface defect engineering in promoting photogenerated charge separation and manipulating surface photogenerated charges; further, they open up a powerful avenue for improving photocatalytic charge separation and solar energy conversion efficiency.

Strategies for enhancing the photocurrent, photovoltage, and stability of photoelectrodes for photoelectrochemical water splitting

In this review, we survey recent strategies for photoelectrode optimization and advanced characterization methods towards efficient water splitting cells via feedback from these characterization methods.

Improving the Stability and Efficiency of CuO Photocathodes for Solar Hydrogen Production through Modification with Iron

Cupric oxide (CuO) is considered as a promising photocathode material for photo(electro)chemical water splitting because of its suitable band gap, low cost related to copper earth abundancy, and straightforward fabrication. The main challenge for the development of practical CuO-based photocathodes for solar hydrogen evolution is to enhance its stability against photocorrosion. In this work, stable and efficient CuO photocathodes have been developed by using a simple and cost-effective methodology. CuO films, composed of nanowires and prepared by chemical oxidation of electrodeposited Cu, develop relatively high photocurrents in 1 M NaOH. However, this photocurrent appears to be partly associated with photocorrosion of CuO. It is significant though that, even unprotected, a faradaic efficiency for hydrogen evolution of ∼45% is attained. The incorporation of iron through an impregnation method, followed by a high-temperature thermal treatment for promoting the external phase transition of the nanowires from CuO to ternary copper iron oxide, was found to provide an improved stability at the expense of photocurrent, which decreases to about one-third of its initial value. In contrast, a faradaic efficiency for hydrogen evolution of ∼100% is achieved even in the absence of co-catalysts, which is ascribable to the favorable band positions of CuO and the iron copper ternary oxide in the core-shell structure of the nanowires.

Operando deconvolution of photovoltaic and electrocatalytic performance in ALD TiO2 protected water splitting photocathodes

In this work, we demonstrate that buried junction photocathodes featuring an ALD TiO2 protective overlayer can be readily characterized using a variation of the dual working electrode (DWE) technique, where the second working electrode (WE2) is spatially isolated from the hydrogen-evolving active area. The measurement of the surface potential during operation enables the operando deconvolution of the photovoltaic and electrocatalytic performance of these photocathodes, by reconstructing J-ΔV curves (reminiscent of photovoltaic J-V curves) from the 3-electrode water splitting data. Our method provides a clearer understanding of the photocathode degradation mechanism during stability tests, including loss of the catalyst from the surface, which is only possible in our isolated WE2 configuration. A pn+Si/TiO2 photocathode was first investigated as a well behaved model system, and then the technique was applied to an emerging material system based on Cu2O/Ga2O3, where we uncovered an intrinsic instability of the Cu2O/Ga2O3 junction (loss of photovoltage) during long term stability measurements.

Improving carrier transport in Cu2O thin films by rapid thermal annealing

Cuprous oxide (Cu2O) is a promising material for large scale photovoltaic applications. The efficiencies of thin film structures are, however, currently lower than those for structures based on Cu2O sheets, possibly due to their poorer transport properties. This study shows that post-deposition rapid thermal annealing (RTA) of Cu2O films is an effective approach for improving carrier transport in films prepared by reactive magnetron sputtering. The as-deposited Cu2O films were poly-crystalline, p-type, with weak near band edge (NBE) emission in photoluminescence spectra, a grain size of ~100 nm and a hole mobility of 2-18 cm2 V-1 s-1. Subsequent RTA (3 min) at a pressure of 50 Pa and temperatures of 600-1000 °C enhanced the NBE by 2-3 orders of magnitude, evidencing improved crystalline quality and reduction of non-radiative carrier recombination. Both grain size and hole mobility were increased considerably upon RTA, reaching values above 1 µm and up to 58 cm2 V-1 s-1, respectively, for films annealed at 900-1000 °C. These films also exhibited a resistivity of ~50-200 Ω cm, a hole concentration of ~1015 cm-3 at room temperature, and a transmittance above 80%.

Low temperature growth of gallium oxide thin films via plasma enhanced atomic layer deposition

Herein we describe an efficient low temperature (60-160 °C) plasma enhanced atomic layer deposition (PEALD) process for gallium oxide (Ga₂O₃) thin films using hexakis(dimethylamido)digallium [Ga(NMe₂)₃]₂ with oxygen (O₂) plasma on Si(100). The use of O₂ plasma was found to have a significant improvement on the growth rate and deposition temperature when compared to former Ga₂O₃ processes. The process yielded the second highest growth rates (1.5 Å per cycle) in terms of Ga₂O₃ ALD and the lowest temperature to date for the ALD growth of Ga₂O₃ and typical ALD characteristics were determined. From in situ quartz crystal microbalance (QCM) studies and ex situ ellipsometry measurements, it was deduced that the process is initially substrate-inhibited. Complementary analytical techniques were employed to investigate the crystallinity (grazing-incidence X-ray diffraction), composition (Rutherford backscattering analysis/nuclear reaction analysis/X-ray photoelectron spectroscopy), morphology (X-ray reflectivity/atomic force microscopy) which revealed the formation of amorphous, homogeneous and nearly stoichiometric Ga₂O₃ thin films of high purity (carbon and nitrogen <2 at.%) under optimised process conditions. Tauc plots obtained via UV-Vis spectroscopy yielded a band gap of 4.9 eV and the transmittance values were more than 80%. Upon annealing at 1000 °C, the transformation to oxygen rich polycrystalline β-gallium oxide took place, which also resulted in the densification and roughening of the layer, accompanied by a slight reduction in the band gap. This work outlines a fast and efficient method for the low temperature ALD growth of Ga₂O₃ thin films and provides the means to deposit Ga₂O₃ upon thermally sensitive polymers like polyethylene terephthalate.

Interfacial study of Cu2O/Ga2O3/AZO/TiO2 photocathode for water splitting fabricated by pulsed laser deposition

Increases in photovoltage and photocurrent were obtained in a photocathode with the structure of Cu₂O/Ga₂O₃/AZO/TiO₂ fabricated by pulsed laser deposition.

A simple route to prepare a Cu2O–CuO–GN nanohybrid for high-performance electrode materials

The prepared Cu₂O–CuO–GN nanohybrid shows a higher specific capacitance than that of pure GN.

Electrochemical Synthesis of Highly Oriented, Transparent, and Pinhole-Free ZnO and Al-Doped ZnO Films and Their Use in Heterojunction Solar Cells

Electrochemical synthesis conditions using nonaqueous solutions were developed to prepare highly transparent (T > 90%) and crystalline ZnO and Al-doped ZnO (AZO) films for use in solar energy conversion devices. A focused effort was made to produce pinhole-free films in a reproducible manner by identifying a key condition to prevent the formation of cracks during deposition. The polycrystalline domains in the resulting films had a uniform orientation (i.e., the c-axis perpendicular to the substrate), which enhanced the electron transport properties of the films. Furthermore, electrochemical Al doping of ZnO using nonaqueous media, which was demonstrated for the first time in this study, effectively increased the carrier density and raised the Fermi level of ZnO. These films were coupled with an electrodeposited p-type Cu₂O to construct p-n heterojunction solar cells to demonstrate the utilization of these films for solar energy conversion. The resulting n-ZnO/p-Cu₂O and n-AZO/p-Cu₂O cells showed excellent performance compared with previously reported n-ZnO/p-Cu₂O cells prepared by electrodeposition. In particular, replacing ZnO with AZO resulted in simultaneous enhancements in short circuit current and open circuit potential, and the n-AZO/p-Cu₂O cell achieved an average power conversion efficiency (η) of 0.92 ± 0.09%. The electrodeposition condition reported here will offer a practical and versatile way to produce ZnO or AZO films, which play key roles in various solar energy conversion devices, with qualities comparable to those prepared by vacuum-based techniques.

P-type transparent conducting oxides

Transparent conducting oxides constitute a unique class of materials combining properties of electrical conductivity and optical transparency in a single material. They are needed for a wide range of applications including solar cells, flat panel displays, touch screens, light emitting diodes and transparent electronics. Most of the commercially available TCOs are n-type, such as Sn doped In2O3, Al doped ZnO, and F doped SnO2. However, the development of efficient p-type TCOs remains an outstanding challenge. This challenge is thought to be due to the localized nature of the O 2p derived valence band which leads to difficulty in introducing shallow acceptors and large hole effective masses. In 1997 Hosono and co-workers (1997 Nature 389 939) proposed the concept of 'chemical modulation of the valence band' to mitigate this problem using hybridization of O 2p orbitals with close-shell Cu 3d (10) orbitals. This work has sparked tremendous interest in designing p-TCO materials together with deep understanding the underlying materials physics. In this article, we will provide a comprehensive review on traditional and recently emergent p-TCOs, including Cu(+)-based delafossites, layered oxychalcogenides, nd (6) spinel oxides, Cr(3+)-based oxides (3d (3)) and post-transition metal oxides with lone pair state (ns (2)). We will focus our discussions on the basic materials physics of these materials in terms of electronic structures, doping and defect properties for p-type conductivity and optical properties. Device applications based on p-TCOs for transparent p-n junctions will also be briefly discussed.

Improved Heterojunction Quality in Cu2O-based Solar Cells Through the Optimization of Atmospheric Pressure Spatial Atomic Layer Deposited Zn1-xMgxO

Atmospheric pressure spatial atomic layer deposition (AP-SALD) was used to deposit n-type ZnO and Zn_1-xMg_xO thin films onto p-type thermally oxidized Cu₂O substrates outside vacuum at low temperature. The performance of photovoltaic devices featuring atmospherically fabricated ZnO/Cu₂O heterojunction was dependent on the conditions of AP-SALD film deposition, namely, the substrate temperature and deposition time, as well as on the Cu₂O substrate exposure to oxidizing agents prior to and during the ZnO deposition. Superficial Cu₂O to CuO oxidation was identified as a limiting factor to heterojunction quality due to recombination at the ZnO/Cu₂O interface. Optimization of AP-SALD conditions as well as keeping Cu₂O away from air and moisture in order to minimize Cu₂O surface oxidation led to improved device performance. A three-fold increase in the open-circuit voltage (up to 0.65 V) and a two-fold increase in the short-circuit current density produced solar cells with a record 2.2% power conversion efficiency (PCE). This PCE is the highest reported for a Zn_1-xMg_xO/Cu₂O heterojunction formed outside vacuum, which highlights atmospheric pressure spatial ALD as a promising technique for inexpensive and scalable fabrication of Cu₂O-based photovoltaics.

Scalable Binder-Free Supersonic Cold Spraying of Nanotextured Cupric Oxide (CuO) Films as Efficient Photocathodes

We demonstrate production of nanotextured p-type cupric oxide (CuO) films via a low-cost scalable supersonic cold spray method in open air conditions. Simply sweeping the spray nozzle across a substrate produced a large-scale CuO film. When used as hydrogen evolution photocathodes, these films produced photocurrent densities (PCD) of up to 3.1 mA/cm(2) under AM1.5 illumination, without the use of a cocatalyst or any additional heterojunction layers. Cu2O particles were supersonically sprayed onto an indium tin oxide (ITO) coated soda lime glass (SLG) substrate, without any solvent or binder. Annealing in air converted the Cu2O films to CuO, with a corresponding decrease in the bandgap and increase in the fraction of the solar spectrum absorbed. Annealing at 600 °C maximized the PCD. Increasing the supersonic gas velocity from ∼450 to ∼700 m/s produced denser films with greater surface roughness, in turn producing higher PCD. The nanoscale texture of the films, which resembles the skin of a dinosaur, enhanced their performance, leading to one of the highest PCD values in the literature. We characterized the films by X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, atomic force microscopy, scanning electron microscopy, and transmission electron microscopy to elucidate the origins of their outstanding performance. This supersonic cold spraying deposition has the potential to be used on a commercial scale for low cost mass production.

Cu2O Nanowire Photocathodes for Efficient and Durable Solar Water Splitting

Due to its abundance, scalability, and nontoxicity, Cu2O has attracted extensive attention toward solar energy conversion, and it is the best performing metal oxide material. Until now, the high efficiency devices are all planar in structure, and their photocurrent densities still fall well below the theoretical value of 14.5 mA cm(-2) due to the incompatible light absorption and charge carrier diffusion lengths. Nanowire structures have been considered as a rational and promising approach to solve this issue, but due to various challenges, performance improvements through the use of nanowires have rarely been achieved. In this work, we develop a new synthetic method to grow Cu2O nanowire arrays on conductive fluorine-doped tin oxide substrates with well-controlled phase and excellent electronic and photonic properties. Also, we introduce an innovative blocking layer strategy to enable high performance. Further, through material engineering by combining a conformal nanoscale p-n junction, durable protective overlayer, and uniform catalyst decoration, we have successfully fabricated Cu2O nanowire array photocathodes for hydrogen generation from solar water splitting delivering unprecedentedly high photocurrent densities of 10 mA cm(-2) and stable operation beyond 50 h, establishing a new benchmark for metal oxide based photoelectrodes.

Nanocrystal Engineering of Sputter-Grown CuO Photocathode for Visible-Light-Driven Electrochemical Water Splitting

Cupric oxide (CuO) thin film was sputtered onto fluorine-doped tin oxide (FTO) coated glass substrate and incorporated into a photoelectrochemical (PEC) cell as a photocathode. Through in situ nanocrystal engineering, sputtered CuO film shows an improvement in its stability and photocurrent generation capability. For the same CuO film thickness (150 nm), films deposited at a sputtering power of 300 W exhibit a photocurrent of ∼0.92 mAcm(-2) (0 V vs RHE), which is significantly higher than those deposited at 30 W (∼0.58 mAcm(-2)). By increasing the film thickness to 500 nm, the photocurrent is further enhanced to 2.5 mAcm(-2), which represents a photocurrent conversion efficiency of 3.1%. Systematic characterization using Raman, XRD, and HR-TEM reveals that the high sputtering power results in an improvement in CuO film crystallinity, which enhances its charge transport property and, hence, its photocurrent generation capabilities.

Inorganic Solar Cells Based on Electrospun ZnO Nanofibrous Networks and Electrodeposited Cu2O

The nanostructured ZnO/copper oxide (Cu2O) photovoltaic devices based on electrospun ZnO nanofibrous network and electrodeposited Cu2O layer have been fabricated. The effects of the pH value of electrodeposition solution and the Cu2O layer thickness on the photovoltaic performances have been investigated. It is revealed that the pH value influences the morphology and structure of the Cu2O layer and thus the device performances. The Cu2O layer with an appropriate thickness benefits to charge transfer and light absorption. The device prepared at the optimal conditions shows the lowest recombination rate and exhibits a power conversion efficiency of ~0.77 %.

Modulating light propagation in ZnO–Cu2O-inverse opal solar cells for enhanced photocurrents

The advantages of employing an interconnected periodic ZnO morphology, i.e. an inverse opal structure, in electrodeposited ZnO/Cu₂O devices are presented.

Engineering Schottky contacts in open-air fabricated heterojunction solar cells to enable high performance and ohmic charge transport

The efficiencies of open-air processed Cu2O/Zn(1-x)Mg(x)O heterojunction solar cells are doubled by reducing the effect of the Schottky barrier between Zn(1-x)Mg(x)O and the indium tin oxide (ITO) top contact. By depositing Zn(1-x)Mg(x)O with a long band-tail, charge flows through the Zn(1-x)Mg(x)O/ITO Schottky barrier without rectification by hopping between the sub-bandgap states. High current densities are obtained by controlling the Zn(1-x)Mg(x)O thickness to ensure that the Schottky barrier is spatially removed from the p-n junction, allowing the full built-in potential to form, in addition to taking advantage of the increased electrical conductivity of the Zn(1-x)Mg(x)O films with increasing thickness. This work therefore shows that the Zn(1-x)Mg(x)O window layer sub-bandgap state density and thickness are critical parameters that can be engineered to minimize the effect of Schottky barriers on device performance. More generally, these findings show how to improve the performance of other photovoltaic system reliant on transparent top contacts, e.g., CZTS and CIGS.

Electrodeposited ZnO-nanowire/Cu₂O photovoltaic device with highly resistive ZnO intermediate layer

Cl-doped ZnO-nanowire (Cl:ZnO-nws)/Cu2O photovoltaic devices were prepared by electrodeposition in aqueous solutions, and the effects of the insertion of the highly resistive ZnO (i-ZnO) layer has been demonstrated by an improvement of the photovoltaic performance. The Cl:ZnO-nws and i-ZnO layer were prepared by electrodeposition in a zinc chloride aqueous solution with saturated molecular oxygen and simple zinc nitrate aqueous solution, respectively. The i-ZnO layer was directly deposited on the Cl:ZnO-nws and suppressed the electrodeposition of the Cu2O layer on the Cl:ZnO-nws. The insertion of the i-ZnO layer between the Cl:ZnO-nws and Cu2O layers induced an improvement in the photovoltaic performance from 0.40 to 1.26% with a 0.35 V open circuit voltage, 7.1 mA·cm(-2) short circuit current density, and 0.52 fill factor due to the reduction of the recombination loss.