Electrodeposited ZnO/Cu2O heterojunction solar cells

In this paper the fabrication and the characterization of heterojunction solar cells based on electrodeposited ZnO and Cu₂O is described. The effect of the electrodeposition conditions (pH and temperature) on the cell performance has been investigated. The cells made with a Cu₂O layer deposited at high pH (12) and moderate temperature (50 °C) have shown conversion efficiency as high as 0.41%.     

Effect of Al doped ZnO on optical and photovoltaic properties of the p-Cu2O/n-AZO solar cells

Metal oxide thin films have fared so well in the semiconductor industry because of their superior physical, electrical, and optical properties. The applications of these materials in solar cells, biosensors, biomedicine, supercapacitors, photocatalysis, luminous materials, and laser systems are becoming increasingly popular. In this study, the influence of Al concentration on Cu₂O/AZO heterojunction thin films was examined systematically. First, arrays of n-ZnO and AZO rods were produced on an ITO substrate using a hydrothermal technique at 140 °C. Then, using an alkaline cupric lactate solution, a thin films of p-Cu₂O were electrodeposited at 60 °C onto the ZnO arrays. The structure and morphology of the produced materials and the solar cells were studied using X-ray diffraction and scanning electron microscopy. The optical measurements demonstrate a shift in the absorption edge with increasing Al content. Solar cells have been created with a device structure of ITO/ZnO/Cu₂O/Al and ITO/Al-doped ZnO/Cu₂O/Al configurations. The power conversion efficiency (?) of the inorganic solar cell with 6% Al-doped ZnO is ? = 0.282%, which is greater than the ? of the ZnO-based solar cell (? = 0.17%).     

Potentiostatic electrodeposition of cuprous oxide thin films for photovoltaic applications

Potentiostatic deposition of Cu₂O thin films on glass substrates coated with F-doped SnO₂ from an alkaline electrolyte solution (pH 12.5) containing copper (II) sulfate and lactic acid was studied for fabrication of a Cu₂O/Al-doped ZnO (AZO) heterojunction solar cell. The band gap of the electrodeposited Cu₂O films was determined by photoelectrochemical measurements to be around 1.9 eV irrespective of the applied potentials. The solar cells with a glass/FTO/Cu₂O/AZO structure were fabricated by sputtering an AZO film onto the Cu₂O film followed by deposition of an Al contact by vacuum evaporation. The highest efficiency of 0.603% was obtained with a Cu₂O film deposited at −0.6 V (vs. Ag/AgCl). This was attributed to better compactness and purity of the Cu₂O film than those of the Cu₂O films deposited at other potentials.     

Hydrothermal fabrication of p-Cu2O−n-ZnO films and their properties for photodegradation and ultraviolet sensors

This article reports spin coating and hydrothermal approaches to the synthesis of Cu₂O seed layer−ZnO and Cu₂O film−ZnO heterojunction films on fluorine-doped tin oxide substrates. Cu₂O seed layers and an ethylene glycol (EG) reducing agent were employed to obtain pure, uniform, and adhesive Cu₂O films on the substrate. Transmission electron microscopy validated the heterojunctions with clear interfaces between each component on the p-Cu₂O film−n-ZnO (with EG) sample, the conductive types of which were determined through Mott−Schottky measurements. Constructed energy band diagrams supported the Mott−Schottky result, manifesting favorable conduction band positions for the generation of •O₂⁻ radicals for all constituent materials and indicating smooth charge carrier transport for the p-Cu₂O film−n-ZnO (with EG) sample. Furthermore, abundant p−n junction interfaces synergistically enabled the sample to exhibit the most satisfactory photodegradation capability (rate constant ≈ 8.9 × 10⁻³ min⁻¹), which was attributable to the predominance of •OH radicals. The sample's rectifying (diode) behavior with a ratio of the current density (J) at +3 V (forward bias) to that at −3 V (reverse bias) of approximately 27 was observed without ultraviolet illumination. Moreover, the J at −3 V is under illumination approximately 80 times that without illumination, implying the suitability of the sample for UV detectability.     

p-Type and n-type Cu2O semiconductor thin films: Controllable preparation by simple solvothermal method and photoelectrochemical properties

p-Type and n-type Cu₂O thin films were controllably prepared using a simple solvothermal method by adjusting pH value of the copper (II) acetate aqueous solution. Photoelectrochemical experiments show that the Cu₂O thin films synthesized in acid and alkaline (or neutral) media present n-type and p-type semiconductor character, respectively. Moreover, the films prepared at pH 5 have the best photoelectrochemical properties. The mechanism for the formation of these p-type and n-type Cu₂O films is discussed. The Cu₂O p–n homojunction fabricated in this study shows typical p–n junction character. This facile preparation method may be a promising way to prepare p–n homojunctions for semiconductor devices.     

Performance of Cu2O/ZnO Solar Cell Prepared By Two-Step Electrodeposition

Cu₂O/ZnO solar cells with improved performance were fabricated by an inexpensive two-step process. The process involves potentiostatic deposition of ZnO on NESA glass (tin-oxide-coated glass) followed by galvanostatic deposition of Cu₂O to form Cu₂O/ZnO/NESA solar cells with a short-circuit photocurrent density of 2.08 mA cm⁻²,an open-circuit voltage of 0.19 V, a fill factor of 0.295 and conversion efficiency of 0.117%. The performance of the solar cells thus prepared is discussed in terms of the laminated structure, construction of the heterojunction, and the crystallinity and optical properties of each semiconductor.     

Synthesis of pure Cu2O thin layers controlled by in-situ conductivity measurements

An easy synthesis route for cuprous oxide (Cu₂O) nanoparticles is reported via thermal annealing improved and controlled by in-situ conductivity measurements. The crystalline structure, phase transition, surface morphology and particle size/shape, were investigated through X-ray diffraction, a conductivity setup and scanning electron microscopy, respectively. X-ray diffraction patterns revealed that initial metallic Cu nanoparticles were transformed to Cu₂O nanoparticles with high purity, under specific conditions critically dependent on the temperature and annealing duration. This transformation was also dependent on the film thickness and atmospheric composition in the test chamber during the annealing process.     

Synthesis and magnetic properties of Cu-doped ZnO nanowire arrays

Arrays of Cu-doped ZnO nanowires were successfully fabricated by electrodeposition of Zn²⁺ and Cu²⁺ into anodic aluminum oxide template and post-oxidation annealing in air atmosphere. The transmission electron microscopy result shows that the nanowires are uniform, about 100 nm in diameter and with the aspect ratio of up to 40. Selected area electron diffraction and X-ray diffraction results indicate that the nanowires are in hexagonal wurtzite structure. Magnetization measurements show that the Zn_1−xCu_xO (x = 0.07 and 0.11) nanowires exhibit room-temperature ferromagnetism and the enhancement of the ferromagnetism is revealed for the Zn_0.93Cu_0.07O nanowires annealed in vacuum.     

A general deposition method for ZnO porous films: Occlusion electrosynthesis

In this paper, a modified electrodeposition method, occlusion electrosynthesis (OE), was used to prepare ZnO porous films. The processes of OE were similar to those of electrodeposition except the addition of ZnO nanoparticles in electrolyte. The ZnO porous film prepared by OE (OE-ZnO) was highly porous with considerable thickness (55 μm). The quantum dots-sensitized solar cell based on OE-ZnO porous film showed superior photoelectrochemical performance to that based on the ZnO porous film prepared by doctor-blade (DB) method at the suitable concentrations of ZnO nanoparticles in electrolyte, about 16–32 g/L. In addition to the ZnO porous film constructed with ZnO nanoparticles, the ZnO porous film constructed with ZnO nanorods, ZnO/multi-walls carbon nanotubes and ZnO/TiO₂ composite porous films have also been successfully synthesized by OE, which were expected to be widely applied in various fields. The low temperature (60 °C) processes without post treatment made OE more promising for preparing ZnO porous films than DB, especially the ZnO porous films for flexible devices.     

Preparation, characterization and activity evaluation of p–n junction photocatalyst p-CaFe2O4/n-ZnO

p–n junction photocatalyst p-CaFe₂O₄/n-ZnO was prepared by ball milling of ZnO in H₂O doped with p-type CaFe₂O₄. The structural and optical properties of the p–n junction photocatalyst p-CaFe₂O₄/n-ZnO were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV–vis diffuse reflection spectrum (DRS) and fluorescence emission spectra. The photocatalytic activity of the photocatalyst was evaluated by photocatalytic degradation of methylene blue (MB). The results showed that the photocatalytic activity of the p-CaFe₂O₄/n-ZnO was higher than that of ZnO. When the amounts of doped p-CaFe₂O₄ were 0.0 wt.% and 1.0 wt.%, the photocatalytic degradation efficiencies were 50.1 and 73.4%, respectively. Effect of ball milling time on the photocatalytic activity of the photocatalyst was also investigated. The mechanisms of influence on the photocatalytic activity were also discussed by the p–n junction principle.     

Electrodeposition of Cu2O nanocrystalline on TiO2 nanosheet arrays by chronopotentiometry for improvement of photoelectrochemical properties

A nano-sized Cu₂O/TiO₂ nanosheet arrays (TiO₂ NSAs) heterojunction film was fabricating by electrodepositing Cu₂O onto TiO₂ NSAs using chronopotentiometry. The cyclic alternating current on working electrode is the main contribution to nano-sized Cu₂O, which in turn improve the photoelectrochemical properties of the Cu₂O/TiO₂ NSAs heterojunction film because nano-sized Cu₂O, with better electron transfer ability and smaller change transfer resistance in electrolyte, increases the separation and transportation efficiencies of photo-induced electron-hole pairs. Furthermore, electrolyte pH value and deposition current density have significant influence on Cu₂O particles size. The results show that the sample prepared with pH?=?8 and current density j?=?0.25?mA/cm² exhibits best photoelectrochemical properties because of the smaller sized Cu₂O and uniform distribution on TiO₂ NSAs.     

Tuning of defects in ZnO nanorod arrays used in bulk heterojunction solar cells

With particular focus on bulk heterojunction solar cells incorporating ZnO nanorods, we study how different annealing environments (air or Zn environment) and temperatures impact on the photoluminescence response. Our work gives new insight into the complex defect landscape in ZnO, and it also shows how the different defect types can be manipulated. We have determined the emission wavelengths for the two main defects which make up the visible band, the oxygen vacancy emission wavelength at approximately 530 nm and the zinc vacancy emission wavelength at approximately 630 nm. The precise nature of the defect landscape in the bulk of the nanorods is found to be unimportant to photovoltaic cell performance although the surface structure is more critical. Annealing of the nanorods is optimum at 300°C as this is a sufficiently high temperature to decompose Zn(OH)₂ formed at the surface of the nanorods during electrodeposition and sufficiently low to prevent ITO degradation.     

A new synergistic effect in one step sputtered ZnO/Zn2SnO4 heterojunction films for H2 sensing related to crystal structure and film compactness

In this paper, ZnO/Zn₂SnO₄ heterojunction films were one step fabricated by magnetron sputtering and the dependence of crystal structures, film compactness and H₂ sensing properties on annealing process were investigated and discussed. The results showed that three typical surface morphologies can be controlled by adjusting annealing temperatures and periods. The films annealed at the temperature of 550 °C for 6 h showed the best H₂ sensing properties. It exhibited a response (R_a/R_g) of 28.3–100 ppm H₂ at the temperature of 230 °C and the detection limit is 30.2 ppb. Meanwhile, it also showed a good selectivity and long-term stability to H₂. The H₂ sensing mechanism is attributed to the synergistic effect between ZnO (0001) signal crystal facets and ZnO/Zn₂SnO₄ heterojunction structures which enhanced the gas reactivity and resistance modulation range. On the contrary, insufficient annealing restricts the film crystallinity and the growth of hexagonal ZnO while undue annealing destroys the compactness of the films, leading to poor H₂ sensing properties.     

Photocatalysis performance enhancement of Ag2O/Al-doped ZnO heterojunction by introducing ZnO nanorod array

Fabric-based nano semiconductor photocatalysts have attracted much attention in recent years. In this study, polyester fibers were modified by Ag₂O/ZnO nanorod (NR)/Al-ZnO p-type/high resistivity/n-type (P-HR-N) heterojunction arrays. Al-doped ZnO (Al-ZnO) seed layer was reactively sputtered on polyester fibers. Al doping was used to change the work function (WF) of the ZnO film. The ZnO NR arrays were hydrothermally grown on the Al-ZnO seed layer and modified with Ag₂O film. The work functions (WFs) of Ag₂O and Al-ZnO layers were measured, resulting in $5.53$ and $4.81\mathrm{e}\mathrm{V}$, respectively. The WF difference between the two layers was $0.72\mathrm{e}\mathrm{V}$. The electron-hole separation of the P-HR-N heterojunction array was characterized by surface photovoltage (SPV) spectra. The photocatalytic activities of the prepared samples were investigated by removing Rhodamine B (RB) under UV irradiation. The P-HR-N heterojunction sample displayed a better photocatalytic performance than the Ag₂O/Al-doped ZnO PN heterojunction sample and ZnO NR/Al-ZnO array-coated sample. The enhanced photocatalytic performance of the P-HR-N heterojunction based sample was mainly attributed to the high and widened built-in electric field (BEF) in the multilayer film.     

Electrochemical study of one-step electrodeposition of copper–indium–gallium alloys in acidic conditions as precursor layers for Cu(In,Ga)Se2 thin film solar cells

This paper investigates one step electrodeposition of copper indium gallium metallic precursor layers for preparing CuIn_1−xGa_xSe₂ (CIGS) absorber layers in thin film solar cells (0 ≤ x ≤ 1). Electrodeposition was carried out in acidic aqueous solutions at about pH 2. At first partial single or binary electrodeposition systems Cu, In, Ga, Cu–Ga, Cu–In were investigated by cyclic voltammetry. Then ternary Cu–In–Ga electrodeposition system was studied. The nature of the supporting electrolyte (sodium sulfate vs. sodium chloride) and the influence of sodium citrate were more specifically investigated. The applied potential, the pH and the nature of the electrolyte were optimized to obtain x values around 0.3 needed for high efficiency devices. Depositions were carried out under potentiostatic conditions in a paddle cell configuration. The electrodeposited Cu–In–Ga alloys were annealed under Se atmosphere at temperatures between 400 and 600 °C to produce CIGS absorbers. Films were characterized by XRF, SEM and XRD analysis. Device efficiencies up to 9.3% are achieved for optimal gallium content.     

Strong photo-response in a flip-chip nanowire p-Cu2O/n-ZnO junction

Cu(2)O nanoneedles are synthesized on a copper substrate by a simple anodization and reducing ambient annealing protocol. ZnO nanorods are grown on ITO coated glass by a low temperature chemical route. The electronic and photo-response properties of the p-Cu(2)O/n-ZnO flip-chip heterojunction are then studied and analyzed. We show that the I-V characteristic is rectifying and the junction exhibits a good photoresponse (～120% under 1 V reverse bias) under AM 1.5 (1 Sun) illumination. This nano-heterojunction photo-response is far stronger as compared to that of a pulsed laser deposited thin film p-Cu(2)O/n-ZnO heterojunction, which can be attributed to higher junction area in the former case.     

Characterization of electrodeposited CuInSe2 (CIS) film

CuInSe₂ thin film was grown by one-step cathodic electrodeposition on Pt-coated glass using amperometry mode (fixed potential), in a three-electrode potentiostatic system containing the precursor solutions, 10 mM CuSO₄, 50 mM InSO₄, 30 mM SeO₂, and 0.1 M K₂SO₄ as a supporting electrolyte, at a pH of 1.5 (±0.1) adjusted with 0.1 M H₂SO₄. The structure, chemical composition, morphology, optical properties, and uniformity of the electrodeposited CuInSe₂ film were characterized by X-ray diffraction (XRD), electron probe micro analysis (EPMA), UV-vis-NIR spectrophotometry, field-emission scanning electron microscopy (FE-SEM), and Auger electron spectroscopy (AES), respectively. Several experiments were conducted in which the deposition voltage and post-annealing conditions were varied. As the applied deposition voltage was increased from −0.6 V to −0.7 V versus Ag/AgCl, a CuInSe₂ thin film with a chalcopyrite structure came to be predominantly formed, in accordance with the chemical composition, while the formation of the binary compounds (CuO and Cu_xSe) which influence the degradation of the performance in the application of CIS-based solar cells, rapidly decreased. The optimum conditions consisted of an annealing temperature of about 350 °C for 30 min under nitrogen ambient. At this temperature, a CuInSe₂ thin film, in the form of ternary compound, is formed with the required conditions, namely good crystallinity, good stoichiometry, a suitable bandgap, and depth uniformity.     

Preparation and application of (Cu2O-Ag)@TA composite nanomaterials with enhanced stability and photocatalytic antibacterial activity

Food spoilage caused by bacterial growth is a serious threat to human health, so food preservation technology with long-acting antibacterial effect has been widely studied. In this work, (Cu₂O-Ag)@TA heterojunction composite antibacterial material with a core-shell structure was synthesized to improve the stability of Cu₂O. Morphological characterization shows that silver nanoparticles are uniformly distributed on the surface of Cu₂O microspheres, (Cu₂O-Ag)@TA has a core-shell structure, the particle size is about 300 nm, and the thickness of the tannic acid shell is about 6 nm. In addition, (Cu₂O-Ag)@TA exhibits excellent dispersibility and stability, it can stably release Cu²⁺ within 14 days and has enhanced reactive oxygen species release performance, thus achieving excellent antibacterial properties, with a minimum inhibitory concentrations of 96 μg ml⁻¹ against Escherichia coli and Staphylococcus aureus. In addition, adding (Cu₂O-Ag)@TA into the LbL-PVA/CS/FA composite film can improve the mechanical properties and antibacterial ability of the composite film, providing a feasible solution for new biodegradable antibacterial packaging materials.     

Electrochemistry of V2ON with lithium

V₂ON thin film has been successfully fabricated by reactive dc sputtering method and annealing process and was investigated for its electrochemistry with lithium. The reversible discharge capacities of V₂ON/Li cells cycled between 0.01 and 4.0 V were found in the range of 803–915 mAh g⁻¹ during the first 50 cycles. By using ex situ scanning electron microscopy, transmission electron microscopy, selected-area electron diffraction and X-ray photoelectron spectroscopy measurements, the reversible transformation between nanocrystalline V₂ON and well dispersed V, Li₂O, Li₃N nano-composites were revealed in the lithium electrochemical reaction. V₂ON thin film exhibits high reversible capacity and good cycle performance with remarkable lower polarization than VN thin film.     

Investigation of bulk hybrid heterojunction solar cells based on Cu(In,Ga)Se2 nanocrystals

This work presents the systematic studies of bulk hybrid heterojunction solar cells based on Cu(In, Ga)Se₂ (CIGS) nanocrystals (NCs) embedded in poly(3-hexylthiophene) matrix. The CIGS NCs of approximately 17 nm in diameter were homogeneously blended with P3HT layer to form an active layer of a photovoltaic device. The blend ratios of CIGS NCs to P3HT, solvent effects on thin film morphologies, interface between P3HT/CIGS NCs and post-production annealing of devices were investigated, and the best performance of photovoltaic devices was measured under AM 1.5 simulated solar illumination (100 mW/cm²).