ZnO Heterojunction Research Articles

Network Structured SnO2/ZnO Heterojunction Nanocatalyst with High Photocatalytic Activity

A network-structured SnO(2)/ZnO heterojunction nanocatalyst with high photocatalytic activity was successfully synthesized through a simple two-step solvothermal method. The as-synthesized samples are characterized by X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy, scanning electron microscopy, N(2) physical adsorption, and UV-vis spectroscopy. The results show that the SnO(2)/ZnO sample with a molar ratio of Sn/Zn = 1 is a mesoporous composite material composed of SnO(2) and ZnO. The photocatalytic activity of SnO(2)/ZnO heterojunction nanocatalysts for the degradation of methyl orange is much higher than those of solvothermally synthesized SnO(2) and ZnO samples, which can be attributed to the SnO(2)-ZnO heterojunction, the pore structure, and higher Brunauer-Emmett-Teller (BET) surface area of the sample: (1) The SnO(2)-ZnO heterojunction improves the separation of photogenerated electron-hole pairs due to the potential energy differences between SnO(2) and ZnO, thus enhancing the photocatalytic activity. (2) The SnO(2)/ZnO sample might possess more surface reaction sites and adsorb and transport more dye molecules due to the higher BET surface area and many pore channels, also leading to higher photocatalytic activity.

Inverted photovoltaic device based on ZnO and organic small molecule heterojunction

The solar cells with an inverted structure of indium tin oxide (ITO)/ZnO/copper-phthalocyanine (CuPc):fullerene (C60)/CuPc/poly(3,4-oxyethyleneoxythiophene):poly(styrene sulfonate) (PEDOT:PSS)/Ag were fabricated. The current density–voltage curve indicates that ZnO was a highly effective electron-selective contact and plays important roles on the rectification behavior and the photovoltaic performance improvement. The incident photo to current conversion efficiency combined with the absorption spectrum indicates that most of the photocurrent generation was attributed to the excitation of CuPc. The shelf lifetime of unencapsulated devices was over 900 h in air due to the inverted structure which gives a promising way for fabrication of solar cells with improved stability.

Electroluminescence from ZnMgO/ZnO Heterojunctions

The p-type Zn0.8Mg0.2O/n-type ZnO heterojunctions have been fabricated by a pulsed laser deposition (PLD) technique and the relations between the I-V and the electroluminescence (EL) characteristics were investigated. The ZnMgO film shows the best p-type characteristic for the 10% Sb concentration. The different Ga concentrations in n-type ZnO layers show different I-V and EL characteristics for the ZnMgO/ZnO heterojunctions. The 0.1% of Ga in the ZnO film shows the lowest resistivity (0.048 ohm·cm) and the highest electron mobility (59 cm2/Vs), however, the ZnMgO/ZnO junction shows only an Ohmic contact and no EL is observed. The heterostructure using the ZnO layer with 5.0% Ga shows the best rectifying behavior, however, no EL is observed. The 1.0% of Ga in the ZnO film shows the middle resistivity (0.51 ohm·cm) and the middle electron mobility (27 cm2/Vs) and the ZnMgO/ZnO heterojunction shows only a weak rectifying behavior but clear EL is observed. The threshold voltage of the EL is about 10V and the EL intensity is increased with the forward biases up to 20 V. These results indicate that the EL from ZnMgO/ZnO junctions does not necessarily require good ideality factors of the diodes. [DOI: 10.1380/ejssnt.2009.366]

Field Emission Properties of Hybrid Carbon Nanotube−ZnO Nanoparticles

ZnO nanoparticles are uniformly coated on the walls of carbon nanotubes (CNTs) via a straightforward process, and the particle size and interparticle distance can be controlled by coating time. The appropriate amount of coated nanoparticles effectively reduces the formation of various structural defects induced by oxygen or hydrogen atoms on the walls of CNTs, which can be evaluated through a decrease in the intensity ratio of disorder graphitic band (D peak) over graphitic C−C stretching band (G peak) in the Raman spectrum. An overincrease in coating time simultaneously causes an increase in interstitial zinc and oxygen vacancies in ZnO. The high local electric field around the ZnO particle on the walls of CNTs can increase the tunneling probability at CNTs−ZnO heterojunction, significantly enhancing the field emission property for CNTs.

Plasma-Enhanced Chemical Vapor Deposited SiNx and SiO2 Passivation Effects on ZnO Heterojunction Light Emitting Diodes

ZnO heterojunction light emitting diodes (LEDs) were passivated by plasma-enhanced chemical vapor-deposited SiO2 and SiNx. Post-dielectric deposition annealing was critical in obtaining good LED electrical and optical characteristics. No diode characteristics or light emission were observed unless the structures were annealed at 350 {degree sign}C after fabrication. Annealed diodes showed band-edge electroluminescence (385 nm) and a broad defect band with a peak at 930 nm at room temperature. The SiO2 and SiNx had very different passivation effects in terms of the electrical and electro-luminescence characteristics of the LEDs. After annealing, the SiO2 passivated ZnO LEDs showed diode IV characteristics and emitted light. However, the annealed SiNx-passivated ZnO LEDs showed leaky diode characteristics and no light emission. We attribute these differences to the role of hydrogen on the LEDs.

Epitaxial growth and luminescence properties of ZnO-based heterojunction light-emitting diode on Si(1 1 1) substrate by pulsed-laser deposition

An epitaxial ZnO heterojunction light-emitting diode with an n-ZnO/MgO/TiN/n+-Si structure is produced by pulsed-laser deposition. By introducing a thin MgO/TiN buffer and a low temperature (LT) ZnO buffer, layer-by-layer growth of high quality ZnO epi-layer on Si(1 1 1) has been realized, which was confirmed by in situ reflection high-energy electron diffraction (RHEED), transmission electron microscopy, high-resolution x-ray diffraction, resonant Raman spectra and photoluminescence spectroscopy. Combining in situ RHEED with Phi-scan XRD analysis, the in-plane epitaxial growth of ZnO[1 1 0]||MgO[1 0 ]||Si[1 0 ] has been demonstrated. The strong room temperature electroluminescence (EL) with a broad emission band ranging from 1.46 to 3.5 eV and centred at 2.31 eV could be observed from the diode under relative low injection current. Furthermore, the EL output light intensity is enhanced obviously by improving the ZnO crystal quality via inserting a ZnO LT buffer layer.

Plasma-Enhanced Chemical Vapor-Deposited SiO2 and SiNx Passivation Effects on ZnO Heterojunction Light Emitting Diodes

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Dielectric passivation effects on ZnO light emitting diodes

Plasma-enhanced chemical vapor-deposited SiO2 and SiNx were used to passivate ZnO heterojunction light emitting diodes (LEDs). Postdielectric deposition annealing was critical in obtaining good LED electrical and optical characteristics. No diode characteristics or light emission was observed unless the structures were annealed at 350°C after fabrication. Annealed diodes showed a band-edge electroluminescence (EL) (385nm) and a broad defect band with a peak at 930nm at room temperature. The SiO2 and SiNx had very different passivation effects in terms of the electrical and EL characteristics of the LEDs. After annealing, the SiO2 passivated ZnO LEDs showed diode I-V characteristics and emitted light. However, the annealed SiNx-passivated ZnO LEDs showed leaky diode characteristics and no light emission. We attribute these differences to the role of hydrogen on the LEDs.

Field emission from ordered carbon nanotube-ZnO heterojunction arrays

Low operating electric field and stable emission current have been achieved on ordered carbon nanotube (CNT)-ZnO heterojunction arrays. Pyramid-like micropatterns were firstly prepared via capillarity-driven self-assembly from aligned CNT arrays, ZnO nanowires were subsequently grown via vapor-phase transport (VPT) method. Based on the effect of geometrical structure, low turn-on and threshold fields, and stable emission current were obtained. The high aspect ratio of ZnO nanowires leads to the high field enhancement factor. The optimal density of ZnO nanowires and the excellent interconnection between ZnO and CNT are beneficial to avoid the screening effect and improve the current stability.

Selective formation of Ohmic junctions and Schottky barriers with electrodeposited ZnO

Constant-potential electrochemical synthesis of ZnO on metal substrates enables selective formation of either Ohmic or Schottky-barrier contacts. Using a mildly acidic nitrate-based aqueous electrolyte, there is a substrate-dependent deposition potential below which electrodeposited ZnO heterojunctions display Schottky response with high contact resistances (∼105Ω) and above which Ohmic behavior and low contact resistances (∼1Ω) occur. Voltammetric evidence for Zn metal deposition, in conjunction with Schottky-barrier heights that are consistent with values expected for a ZnO–Zn junction, suggests that more negative deposition potentials create a Zn-based interface between the substrate and ZnO that leads to rectifying behavior.

Determination of the valence band offset of wurtzite InN∕ZnO heterojunction by x-ray photoelectron spectroscopy

The valence band offset (VBO) of the wurtzite InN∕ZnO heterojunction is directly determined by x-ray photoelectron spectroscopy to be 0.82±0.23eV. The conduction band offset is deduced from the known VBO value to be 1.85∓0.23eV, which indicates a type-I band alignment for InN∕ZnO heterojunction.

Resistive Losses at c-Si/a-Si:H/ZnO Contacts for Heterojunction Solar Cells

AbstractWe study resistive losses at (p)c-Si/(p)Si:H/(n)ZnO heterojunction back contacts for high efficiency silicon solar cells. We find that a low tunnelling resistance for the (p)a-Si:H/(n)ZnO part of the junction requires deposition of Si:H with a high hydrogen dilution RH > 40 resulting in a highly doped μc-Si:H layer. Such a μc-Si:H layer if deposited directly on a Si wafer yields a surface recombination velocity of S  180 cm/s. Using the same layer as part of a (p)c-Si/(p)Si:H/(n)ZnO back contact in a solar cell results in an open circuit voltage Voc = 640 mV and a fill factor FF = 80 %. Insertion of an (i)a-Si-layer between the μc-Si:H and the wafer leads to a further decrease of S and, for the solar cells to an increase of VOC. However, if the thickness of this intrinsic layer exceeds a threshold of 3 nm, resistive losses lead to a degradation of the fill factor of the solar cells. These resistive losses result from a valence band offset δEV between a-Si:H and c-Si of about 600 meV. The fill factor losses overcompensate the VOC gain such that there is no benefit of the (i)a-Si:H interlayer for the overall solar cell performance when using an (i)a-Si:H/(p)uc-Si:H double layer.

Water‐Assisted Growth of Aligned Carbon Nanotube–ZnO Heterojunction Arrays

Using water-assisted chemical vapor deposition, aligned CNT–ZnO (CNT: carbon nanotube) heterojunctions are formed on a Zn substrate. The aligned CNT heterojunctions with intimately connected ZnO particles/rods show interesting optoelectronic properties of practical significance. This cost-effective and efficient approach can be scaled up for the production of large-area heterojunction arrays (see figure).

P -type behavior from Sb-doped ZnO heterojunction photodiodes

Antimony (Sb) doping was used to realize p-type ZnO films on n-Si (100) substrates by molecular beam epitaxy. These samples were fabricated into p-n heterojunction diodes. p-type behavior of Sb-doped ZnO was studied by carrying out I-V and capacitance-voltage (C-V) measurements. I-V curves showed rectifying behavior similar to a p-type Schottky diode with a turn-on voltage around 2.4V, which is consistent with the Schottky barrier of about 2.2V obtained from C-V characterization. Good photoresponse in the UV region was obtained, which further proved that Sb doping could be used to fabricate p-type ZnO for photodetector and other optoelectronic applications.

Design and fabrication of ZnO light-emitting devices using filtered cathodic vacuum arc technique

Recent progress in the fabrication of ultraviolet (UV) ZnO light-emitting devices using filtered cathodic vacuum arc (FCVA) technique is reviewed. The design and fabrication of ZnO optical waveguides are investigated. The phenomenon of random laser action in ZnO suggests efficient UV lasing from the ZnO waveguide under optical excitation can be achieved. The methods of eliminating the undesired optical characteristics of random laser action such as high scattering loss, non-axial radiation and multiple wavelength emission are also studied. Furthermore, the fabrication of UV ZnO heterojunction light-emitting diodes is described.

Measurement of Zn0.95Cd0.05O∕ZnO (0001) heterojunction band offsets by x-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy was used to measure the energy discontinuity in the valence band (ΔEv) of Zn0.95Cd0.05O∕ZnO heterostructures grown by rf plasma-enhanced molecular-beam epitaxy. A value of ΔEv=0.17±0.03eV was obtained by using the Zn 2p energy level as a reference. Given the experimental band gap of 2.9 eV for the Zn0.95Cd0.05O, this would indicate a conduction band offset ΔEC of 0.30 eV in this system.

Cu 2ZnSnS 4 thin film solar cells

Aiming to develop the solar cells free from the environmental contaminants, promising solar cell of a thin film type could be produced by using Cu 2ZnSnS 4 (CZTS) film as the absorber. The CZTS film possesses promising characteristic optical properties; band gap energy of about 1.5 eV and large absorption coefficient in the order of 10 4 cm −1, which means large possibility of commercial production of the most suitable absorber by using the CZTS film. In addition, as the CZTS film contains neither rare metals nor toxic materials, combining this film with Cd-free buffer layer, we can expect coming solar cells with nontoxic thin films in the near future. CZTS thin films were able to be produced successfully in our previous work by vapor phase sulfurization of the stacked precursors that were prepared by sequential vacuum evaporation of Cu, Sn, and ZnS. The best conversion efficiency with the heterojunction of ZnO:Al/CdS and CZTS was 5.45%, which showed the possibility of a very low cost solar cell. It has been, however, quite difficult to prepare CZTS film with high reproducibility. This paper showed the photovoltaic properties of CZTS-based thin film solar cells fabricated by using three types of precursors: (1) the conventional precursor with stacked layers of Cu/Sn/ZnS, (2) the modified precursor with the stacking order of Sn/Cu/ZnS, and (3) the precursor with five periods of Cu/SnS 2/ZnS, which was produced to increase the amount of S and to enhance the interdiffusion in precursor.

Sensing properties of CuO–ZnO heterojunction gas sensors

CuO–ZnO heterojunction gas sensor is a new type gas sensor and has many advantages, such as low cost, simple processing, and convenient testing, etc. In this work, the sensing properties of these sensors to gases were studied on the condition of different ratios of ZnO to CuO, including the resistance-temperature properties of various samples and their sensitivities and response behavior at different temperatures and in different gases. The obtained results are analyzed and discussed in this paper.

Copper indium diselenide thin film solar cells fabricated on flexible foil substrates

Flexible thin film solar cells have many promising applications in space and terrestrial photovoltaic power systems. In this work polycrystalline thin films of copper indium diselenide (CuInSe2) were deposited on flexible metal foil substrates using the selenization technique. In this method E-beam evaporated CuIn precursor layers were reacted with an atmosphere containing H2Se gas at around 400°C to obtain CuInSe2. Mo, Ti and Al foils were evaluated as possible substrate materials for CuInSe2 solar cells. Flexible devices of Mo/CuInSe2/CdS/ZnO heterojunction structure were fabricated on Mo foil substrates with a conversion efficiency of above 8%.