Solar Water Splitting Cells Research Articles

In situ growth of monocrystal p-CuGaO2 nanosheet as a hole transfer layer in a photoelectrode for solar hydrogen production

Hole transporting layers are very important for solar energy conversion devices, such as perovskite solar cells, organic photovoltaic devices and solar water splitting cells. Recently, CuGaO2 as a hole transporting layer has attracted intense interest due to its high hole mobility and wide band gap. However, there is still a challenge to prepare a semitransparent CuGaO2 film. It is desirable to develop a facile method to prepare a CuGaO2 film with good electronic contact and semi-transparency for application in solar energy conversion devices. Herein, for the first time, a one-step hydrothermal method is developed to directly grow semi-transparency p-CuGaO2 films on FTO substrates. CuGaO2 single crystals with much larger grain sizes are obtained, which are important for the applications as hole transporting layers. We find that Br− in the precursor solution plays a key role in obtaining a semitransparent CuGaO2 film. The effects of Br− on the growth of CuGaO2 are also investigated in details. As an example, a CuGaO2 film is used as a hole transfer layer to improve photoelectrochemical performance of a CuInS2 photocathode for solar water splitting.

Three-dimensional Cu2O nanorods modified by hydrogen treated Ti3C2TX MXene with enriched oxygen vacancies as a photocathode and a tandem cell for unassisted solar water splitting

Cuprous oxide (Cu2O) has been envisioned as a promising photocathode material for photoelectrochemical (PEC) water reduction due to its abundance, scalability and nontoxicity. Herein, a three-dimensional (3D) Cu2O nanorods (NRs) photocathode is prepared by in-situ growing Cu2O NRs on a copper foam substrate. The bare 3D Cu2O NRs photocathode exhibits a high photocurrent density of −4.02 mA cm−2 at 0 V versus the reversible hydrogen electrode (RHE) under AM 1.5 G simulated sunlight. The oxygen vacancies enriched Ti3C2TX flakes are obtained by reducing Ti3C2TX with H2/Ar gas (denoted as H:Ti3C2TX). The H:Ti3C2TX/Cu2O, Ti3C2TX/Cu2O as well as Au/Cu2O composite photocathodes are fabricated through a facile dip-coating method. The H:Ti3C2TX/Cu2O and Ti3C2TX/Cu2O yield a photocurrent density of −5.41 and −4.45 mA cm−2 respectively, which is higher than the bare Cu2O and Au/Cu2O (−4.31 mA cm−2). The results suggest that the Ti3C2TX is more effective than noble metal Au for improving the PEC performance of Cu2O photocathode. The superior performance of H:Ti3C2TX/Cu2O is attributed to enriched oxygen vacancies in the H:Ti3C2TX which can enhance the conductivity, light-harvesting efficiency and charge-transfer ability of fabricated photocathode. A PEC tandem cell with H:Ti3C2TX/Cu2O as the photocathode and BiVO4 as the photoanode is designed for unassisted solar water splitting. Due to the high porosity and optical transmittance of the 3D copper foam substrate, we illuminate the cell from the H:Ti3C2TX/Cu2O side rather than from the BiVO4 side in usual fashion. The present configuration exhibits a 1.6 times higher solar-to-hydrogen (STH) conversion efficiency of 0.55% than the conventional design with a STH conversion efficiency of 0.34%. The enhanced STH conversion efficiency in our configuration is due to the higher light-harvesting efficiency.

Photoelectrochemical Driving and Simultaneous Synthesis of 3-pyridinecarboxylic Acid and Hydrogen in WO3 Photoanode-Based Cell

In conventional photoelectrochemical (PEC) cell for solar water splitting, its solar-to-hydrogen efficiency is limited by the sluggish oxygen evolution on its photoanode. 3-pyridinecarboxylic acid (3-PA) is an important organic in industry, but there are several disadvantages in its conventional synthesis approaches. Here, we reported that the oxidation of 3-methylpyridine (3-MP) into 3-PA as an alternative to the oxygen evolution on WO3 photoanode for improving the hydrogen-evolution efficiency and the product added-value of PEC cell. In a WO3 film photoanode-based dual-chamber PEC cell, 3-PA and hydrogen were simultaneously synthetized. Since the oxidation of 3-MP into 3-PA is more thermodynamically and kinetically favorable than water oxidation, higher efficiency of hydrogen evolution was achieved in the PEC cell with function of 3-PA synthesis relative to the conventional PEC cell for water splitting. Our investigations indicated that the oxidation of 3-MP into 3-PA on WO3 photoanode is co-initiated by the photogenerated holes and the H2O2 intermediate that originated from two-holes pathway of solar water oxidation on WO3. Additionally, the Cr2O72−/Cr3+ redox pair could mediate and promote the oxidation of 3-MP into 3-PA on WO3 photoanode, and thus the PEC efficiency of 3-PA synthesis can be improved by adding Cr3+ into the H2SO4-3-MP electrolyte.

Progress on ternary oxide-based photoanodes for use in photoelectrochemical cells for solar water splitting

Solar water splitting using photoelectrochemical cells (PECs) has emerged as one of the most promising routes to produce hydrogen as a clean and renewable fuel source. Among various semiconductors that have been considered as photoelectrodes for use in PECs, oxide-based photoanodes are particularly attractive because of their stability in aqueous media in addition to inexpensive and facile processing compared to other types of semiconductors. However, they typically suffer from poor charge carrier separation and transport. In the past few years, there has been tremendous progress in developing ternary oxide-based photoelectrodes, specifically, photoanodes. The use of ternary oxides provides more opportunities to tune the composition and electronic structure of the photoelectrode compared to binary oxides, thus providing more freedom to tune the photoelectrochemical properties. In this article, we outline the important characteristics to analyze when evaluating photoanodes and review the major recent progress made on the development of ternary oxide-based photoanodes. For each system, we highlight the favorable and unfavorable features and summarize the strategies utilized to address the challenges associated with each material. Finally, by combining our analyses of all the photoanodes surveyed in this review, we provide possible future research directions for each compound and an outlook for constructing more efficient oxide-based PECs. Overall, this review will provide a critical overview of current ternary oxide-based photoanodes and will serve as a platform for the design of future oxide-based PECs.

Correlated Single-Molecule Reaction Imaging and Photocurrent Measurements Reveal Underlying Rate Processes in Photoelectrochemical Water Splitting

Understanding reaction processes at the semiconductor/electrolyte interface is critical to developing (photo)electrochemical energy conversion systems. For the water oxidation reaction at a photoanode in a solar water splitting cell, there are multiple reaction steps that produce surface adsorbed reaction intermediates, such as OH• and H2O2. Little is known about how the reaction intermediate surface concentrations evolve during the water oxidation reaction. Here we use single-molecule fluorescence microscopy to track the time-dependent surface concentration of Ti-OH• species on TiO2 photoanodes under chopped light illumination. We correlate the reaction intermediate surface concentration with the net rate of photoelectrochemical water oxidation (i.e., the photocurrent). The reaction intermediate dynamics follow different time scales in its temporal evolution from the photocurrent dynamics. By fitting the temporal evolutions of Ti-OH• species and photocurrent over a range of light on/off conditions and applied potentials, we observed that the rate constants for interfacial hole transfer and O-O bond formation depend on the applied potential under illumination. The variation in the rate constants could be attributed to the presence of surface states and/or a change in the chemical nature of Ti-OH• species on the photoanode surface. Our findings provide insight into water oxidation kinetics under intermittent solar irradiation conditions.

Unassisted solar water splitting with 9.8% efficiency and over 100 h stability based on Si solar cells and photoelectrodes catalyzed by bifunctional Ni–Mo/Ni

Designing a highly efficient and stable photoelectrochemical (PEC) tandem cell for unassisted solar water splitting is considered a promising approach for large-scale solar energy storage.

Reprint of “Photoelectrocatalytic behavior of electrodeposited zinc ferrite films with varying Zn:Fe ratio”

We report on stoichiometrically varied zinc ferrite films prepared by electrochemical deposition which can potentially be used as photoanodes in solar water splitting cells. The ferrite films are prepared from an electrodeposition bath containing iron and zinc ions in the presence of triethanolamine as a complexing agent. The ratio of Zn to Fe is controlled by fine tuning of the deposition potentials and the concentrations of the two metal ions. The film thickness is monitored by the time of deposition or the charge passing during the deposition process. The deposited films are converted to highly crystalline oxides by post thermal treatment at 600 °C. Structural and morphological characterization was performed by transmission electron microscopy and X–ray diffraction. Raman spectroscopy was used to locally identify the phase compositions. The surface composition and the chemical state of the species were investigated by X-ray photoelectron spectroscopy which shows that the surface composition of some films is different from the bulk. Photoelectrochemical measurements show that the ferrites are n-type semiconductors and the onset and the magnitude of the photocurrent is strongly dependent on the ratios of Zn:Fe at the surface. Photoelectrodes containing understoichiometric amount of zinc at the surface (Zn:Fe ratio = 0.25–0.40) exhibit higher photocurrents than those being stoichiometric or slightly excessive in zinc.

(Invited) Metal-Organic Framework-Metal Oxide Composite for Solar Water Splitting

The metal-organic framework (MOF) has been deposited on metal oxide surface layer-by-layer. The synthesis of MOF-inorganic oxide composite has been optimized. The optimized MOF-oxide composite has been used in a photoelectrochemical cell for solar water splitting. The composite shows much higher photocurrent density than the metal oxide alone.

Photoelectrocatalytic behavior of electrodeposited zinc ferrite films with varying Zn:Fe ratio

We report on stoichiometrically varied zinc ferrite films prepared by electrochemical deposition which can potentially be used as photoanodes in solar water splitting cells. The ferrite films are prepared from an electrodeposition bath containing iron and zinc ions in the presence of triethanolamine as a complexing agent. The ratio of Zn to Fe is controlled by fine tuning of the deposition potentials and the concentrations of the two metal ions. The film thickness is monitored by the time of deposition or the charge passing during the deposition process. The deposited films are amorphous and converted to highly crystalline oxides by post thermal treatment at 600 °C. Structural and morphological characterization was performed by transmission electron microscopy and X–ray diffraction. Raman spectroscopy was used to locally identify the phase compositions. The surface composition and the chemical state of the species were investigated by X-ray photoelectron spectroscopy which shows that the surface composition of some films is different from the bulk. Photoelectrochemical measurements show that the ferrites are n-type semiconductors and the onset and the magnitude of the photocurrent is strongly dependent on the ratio Zn:Fe at the surface. Photoelectrodes containing understoichiometric amount of zinc at the surface (Zn:Fe ratio = 0.25–0.40) exhibit higher photocurrents than those being stoichiometric or slightly excessive in zinc.

Understanding and Tailoring the Performance of Transition Metal Oxides for the Oxygen Evolution Reaction

The oxygen evolution reaction (OER) is the bottleneck in direct solar and electrocatalytic water splitting cells, and rechargeable aqueous metal-air batteries. Improving the cost-efficiency of these devices requires development of efficient and cheap oxygen evolving catalysts. A good catalyst material should fulfil several important criteria: the composition and structure should be stable at conditions of interest, it should be able to conduct electrons from the active site, and it should be sufficiently active to catalyze water oxidation to oxygen. We explore three different oxide classes comprising 1st row transition metals 1) pristine and doped Co and Ni oxyhydroxides (ox-hy), 2) pristine and doped perovskite oxides ABO3 (A is the earth-alkali or lanthanide metal and B the 1st row transition metal) and 3) pristine and doped αMnO2 as catalysts for the oxygen evolution reaction in neutral to alkaline pH. The emphasis in the analyses is placed on the three most important performance parameters: structural stability, catalytic activity and electronic conductivity. For the ox-hy class we systematically investigate the influence of the nano-scale structure going from 3D catalysts to 2D nanosheets; we examine different edge and terrace terminations, as well as the effect of all stable metal dopants on the performance at pertinent potentials. We identify Fe doped Co, and V, Fe, Ru, Ir and Rh doped Ni ox-hys, as the best catalyst materials, out of which Rh doped Ni ox-hys exhibits the highest activity. Furthermore, we find that the electronic conductivity changes with the nature and valence of the dopant atom, which in turn depends on potential. For the perovskite oxides, we identify Fe4+, Co3+(IS), Ni3+ and Mn3+/Mn4+ pairs as electronically conductive species and distinguish among three different conduction types: intrinsic conductance (Fe4+, Co3+(IS) and Ni3+), electron polaron hoping in Mn compounds with mixed valences and conduction via oxygen 2p and metal 3d holes in the valence band. We investigate whether the performance of αMnO2, which according to many studies is the most active MnO2 polymorph, can be further improved by means of doping. Through a systematic approach we identify Pd doped αMnO2 to be a stable and highly active catalyst for the oxygen evolution reaction. This work was supported by the Horizon 2020 framework, grant number 646186.

Porous multi-junction thin-film silicon solar cells for scalable solar water splitting

Monolithic solar water splitting devices implemented in an integrated design approach, i.e. submerged in the electrolyte, pose a significant limitation when it comes to up-scaling. The ion transport distances around the monolith are long and consequently, the ionic Ohmic losses become high. This fact turns out to be a bottleneck for reaching high device efficiency and maintaining optimum performance upon up-scaling. In this paper, we propose a new device design for integrated monolithic solar water splitting based on porous multi-junction silicon solar cells. Simulation results highlight that porous monoliths can benefit from lower ionic Ohmic losses compared to dense monoliths for various pore geometries and monolith thicknesses. In particular, we show how micrometer scale pore dimensions could greatly reduce Ohmic losses, thereby minimizing overpotentials. A square array of holes with a diameter of 20 µm and a period of 100 µm was fabricated on single-junction and multi-junction amorphous and microcrystalline silicon solar cells. A small impact on the open circuit voltage (Voc) and short circuit current density (Jsc) was obtained, with porous triple junction solar cells reaching Voc values up to 1.98 V. A novel device design is proposed based on porous triple-junction silicon-based solar cells.

Visible Light-Driven Water Oxidation on an In Situ N2 -Intercalated WO3 Nanorod Photoanode Synthesized by a Dual-Functional Structure-Directing Agent.

With a view to developing a photoanode for visible light-driven water oxidation in solar water splitting cells, pure-monoclinic WO3 nanorod crystals with N2 intercalated into the lattice were synthesized by using hydrazine with a dual functional role-as an N atom source for the in situ N2 intercalation and as a structure-directing agent for the nanorod architecture-to gain higher incident photon-to-current conversion efficiency at 420 nm than with most previously reported WO3 electrodes.

A precious metal-free solar water splitting cell with a bifunctional cobalt phosphide electrocatalyst and doubly promoted bismuth vanadate photoanode

A bifunctional cobalt phosphide (CoP) electrocatalyst is applied to a doubly promoted BiVO4 photoanode as an oxygen evolution as well as to a cathode as a hydrogen evolution reaction (HER) catalyst to establish a photoelectrochemical (PEC) water splitting cell made of only earth abundant elements without any precious metals.

Optimized photoelectrochemical tandem cell for solar water splitting

In the field of photoelectrochemical (PEC) hydrogen production most studies focus on developing efficient nanostructured materials and less attention is given to the optimization of PEC devices. However, the reactor design greatly influences the overall performance of these devices and it is a key element to turn PEC hydrogen generation competitive and marketable.In this work, a 50cm2 tandem PEC-PV cell, named CoolPEC cell (compact, optimized, open light PEC cell), was designed and optimized for continuous operation comprising improved key features: i) an open path for the sunlight to reach a PV cell placed at the back of the PEC cell; ii) a photoelectrode that simultaneously is one of the windows of the cell; iii) an integrated manifold to feed the electrolyte. CFD simulations were performed to improve the reactor design assuring good heat dissipation and an efficient collection of evolved oxygen and hydrogen in separated compartments.The CoolPEC cell presented a stable performance over 42 days (1008h) in a continuous operation mode. The 50cm2 hematite photoelectrode produced ca. 0.45mAcm−2 at a bias potential of 1.6V provided by two silicon heterojunction (SHJ) solar cells, under 1000Wm−2 and with constant electrolyte feeding at 45°C. SEM, EDS and ICP analyses confirmed that the hematite photoelectrode was resistant to the accelerated corrosion promoted by the operating conditions.

Highly Efficient Photocatalytic Water Splitting over Edge-Modified Phosphorene Nanoribbons.

Two-dimensional phosphorene with desirable optoelectronic properties (ideal band gap, high carrier mobility, and strong visible light absorption) is a promising metal-free photocatalyst for water splitting. However, the band edge positions of the valence band maximum (VBM) and conduction band maximum (CBM) of phosphorene are higher than the redox potentials in photocatalytic water splitting reactions. Thus, phosphorene can only be used as the photocathode for hydrogen evolution reaction as a low-efficiency visible-light-driven photocatalyst for hydrogen production in solar water splitting cells. Here, we propose a new mechanism to improve the photocatalytic efficiency of phosphorene nanoribbons (PNRs) by modifying their edges for full reactions in photocatalytic water splitting. By employing first-principles density functional theory calculations, we find that pseudohalogen (CN and OCN) passivated PNRs not only show desired VBM and CBM band edge positions induced by edge electric dipole layer, but also possess intrinsic optoelectronic properties of phosphorene, for both water oxidation and hydrogen reduction in photocatalytic water splitting without using extra energy. Furthermore, our calculations also predict that the maximum energy conversion efficiency of heterojunction solar cells consisting of different edge-modified PNRs can be as high as 20% for photocatalytic water splitting.

Enhancing Mo:BiVO4 Solar Water Splitting with Patterned Au Nanospheres by Plasmon‐Induced Energy Transfer

AbstractPlasmonic metal nanostructures have been extensively investigated to improve the performance of metal oxide photoanodes for photoelectrochemical (PEC) solar water splitting cells. Most of these studies have focused on the effects of those metal nanostructures on enhancing light absorption and enabling direct energy transfer via hot electrons. However, several recent studies have shown that plasmonic metal nanostructures can improve the PEC performance of metal oxide photoanodes via another mechanism known as plasmon‐induced resonant energy transfer (PIRET). However, this PIRET effect has not yet been tested for the molybdenum‐doped bismuth vanadium oxide (Mo:BiVO4), regarded as one of the best metal oxide photoanode candidates. Here, this study constructs a hybrid Au nanosphere/Mo:BiVO4 photoanode interwoven in a hexagonal pattern to investigate the PIRET effect on the PEC performance of Mo:BiVO4. This study finds that the Au nanosphere array not only increases light absorption of the photoanode as expected, but also improves both its charge transport and charge transfer efficiencies via PIRET, as confirmed by time‐correlated single photon counting and transient absorption studies. As a result, incorporating the Au nanosphere array increases the photocurrent density of Mo:BiVO4 at 1.23 V versus RHE by ≈2.2‐fold (2.83 mA cm−2).

A low-cost photoelectrochemical tandem cell for highly-stable and efficient solar water splitting

The stability and cost of solar-driven water splitting system are still the main bottlenecks for its large-scale implementation. Here, an ingeniously designed low-cost photoelectrochemical tandem cell possessing three performance-superior components, i.e., a front novel photoanode of WO3 nanoplates epitaxially coated with [001]-oriented TiO2 nanoprickles, and a rear Si photovoltaic cell, and a counter-cathode of interlayer-expanded MoS2 nanostructures, is reported for highly-stable and efficient solar water splitting. The tandem cell shows a close-to-unity Coulomb efficiency for water splitting, and a high and sustained-stable solar-to-hydrogen efficiency in a 35 days’ test. The result also reveals that it is versatile in solar-driven wastewater treatment. Most importantly, the composite photoanode that, for the first time, is proposed using epitaxial-growth based on double-match, i.e. lattice match and band match, shows dramatical improvement in stability and photoelectrochemical performance, in which the photocurrent of the WO3/ TiO2 photoanode is improved by 80% without any obvious decay after 100h of continuous testing compared with a 92% decay in photocurrent for the WO3 photoanode. The epitaxial TiO2 layer facilitates interfacial charge transportations and provides a complete sealing of WO3 surfaces at an atomic level. Such efficient interfacial design and system-level integration of high-performance nanomaterials shed a unique light on the potential large-scale implementation of solar water splitting for renewable hydrogen fuel production.

Enhanced photoelectrochemical water splitting with TiO2@Ag2O nanowire arrays via p-n heterojunction formation

Nanostructured TiO2@Ag2O p-n heterojunction arrays were fabricated by combining the hydrothermal method with the chemical-bath method, and they were used as a photoelectrode in photoelectrochemical (PEC) cell for high-performance solar water splitting. To investigate the morphology, microstructure and composition, the pristine TiO2 nanowire arrays (TiO2 NWAs) and the TiO2 nanowires decorated with Ag2O nanoparticles (TiO2@Ag2O NWAs) were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and electrochemical impedance spectra (EIS). Nanostructured TiO2 nanowire arrays were assembled with Ag2O nanoparticles, producing a large number of nano p-n heterojunctions. The TiO2@Ag2O p-n heterojunction arrays showed a photocurrent density of 1.61mA/cm2 at 1.23V vs. a reversible hydrogen electrode (RHE) in 0.5M Na2SO4 electrolyte under simulated AM 1.5 illumination (100mW/cm2), two times higher than that of the pristine TiO2 NWAs. The enhancement was attributed to the efficient electron-hole separation because of the p-n heterojunction interface between p-Ag2O and n-TiO2, which were revealed by the electrochemical impedance spectra including both Mott-Schottky and Nyquist plots with or without illumination. TiO2@Ag2O p-n heterojunction arrays were expected to have the considerably potential applications in solar water splitting.

Understanding and Tailoring the Performance of Transition Metal Oxides for the Oxygen Evolution Reaction

The oxygen evolution reaction (OER) is a bottleneck in direct solar and electrocatalytic water splitting cells, and rechargeable aqueous metal-air batteries. Improving the cost-efficiency of these devices requires development of efficient and cheap oxygen evolving catalysts. A good catalyst material should fulfil several important criteria: the composition and structure should be stable at conditions of interest, it should be able to conduct electrons from the active site, and it should be sufficiently active to catalyze water oxidation to oxygen. Furthermore, the catalyst should be cheap and non-toxic. We explore two different oxide classes: pristine and doped perovskite oxides (ABO3, where A is the alkaline earth or lanthanide metal and B the 1st row transition metal) and several MnO2polymorphs. Stability and the electronic conductivity of doped materials are assessed and compared to pristine catalysts by computing formation energies and analyzing the electronic structure of bulk crystals, respectively. After selecting stable dopants, we calculate the relevant surface termination and deduce the reaction overpotential from the energetically most favorable reaction mechanism. For the perovskite oxides, we identify Fe4+, Co3+(IS), Ni3+ and Mn3+/Mn4+ pairs as electronically conductive species and distinguish among three different electron conduction types: intrinsic conductance (Fe4+ and Ni3+), electron polaron hoping along the Metal-Oxygen-Metal chains (Mn3+/Mn4+) and conduction via oxygen holes in the valence band. Although, the intrinsic stabilities of La perovskites are rather low, they are likely to be the most stable catalysts in open systems because they do not form carbonates. From a combinatorial analysis on La perovskites, we identify the most promising oxygen evolving catalysts. We find that the OER activity of MnO2 polymorphs reduces in the αMnO2 > βMnO2 > γMnO2 sequence. We pinpoint αMnO2 as the most active catalyst and subsequently investigate whether its performance can be furthered by doping. The electronic conductance is greatly improved by creating Mn3+ sites, i.e. Mn3+/Mn4+ pairs, which is accomplished by intercalating electrolyte ions (e.g. Na+ or K+) inside αMnO2 voids. From catalytic analysis, we identify Pd and Co doped αMnO2 as the most active catalyst for oxygen evolution. This work was supported by the Horizon 2020 framework, grant number 646186 Figure 1

Low temperature dynamics of surface and bulk electronic structure of quantum dots

Absolute energies of band edges have proven to be very important for various applications like hydrogen generation, solar water splitting and solar cell optimization. Energy differences as small as 50–100 meV have been shown to largely affect device efficiencies. Device operational temperature can vary largely and temperature dependence of band gap is well known in bulk semiconductor literature. However, there are only a few studies on variation of band gap in quantum dots and none of them characterize the relative energy variation of band edges in spite of their importance in various applications. This is mainly due to the absence of an internal standard that can be used to study the variation of band edges. Here, in this paper, we introduce a technique wherein we utilize Cu dopant emission as an internal probe. Using this technique, we report the variation of band gap, conduction band and valence band edges of CdS and CdSe quantum dots as a function of temperature and size. We found that band gap variation is similar to that of bulk but with a higher average phonon energy. The band edge variation is characterized by a dominant conduction band shift for larger sizes with decreasing temperature while the smaller size QDs show the variation in both conduction band and valence band. Further, we have also utilized this method to study the binding energy of the trap states as a function of temperature using Cu photoluminescence quantum yield and average lifetime of Cu photoluminescence.