Thickness Of ZnO Layer Research Articles

Effect of ZnO layer thickness upon optoelectrical properties of NiO/ ZnO heterojunction prepared at room temperature

In this work, p-NiO/n-ZnO heterostructures were successfully prepared at room temperature using RF sputtering technique. The influence of ZnO layer thickness on the performance of the heterojunction was investigated. The deposited ZnO layers have a hexagonal Wurtzite structure with preferable growth orientations along (002) and (103) for thinner films. Increasing the thickness results in more crystallographic orientation randomness. The current–voltage measurements of the realized heterojunctions showed a clear rectifying behavior. The measured ideality factor varies from 2.5 to 1.6 according to the thickness of ZnO layer. The series resistance of the device is enlarged with increasing ZnO thickness. The deduced parameters from the I–V characteristics suggest that 200 nm is the optimal thickness of the ZnO layer according to our experimental conditions. We attribute the relatively better performance of this thickness to achieving reasonable compensation between serial resistance and ideality factor. The best heterojunction was tested and successfully used as a UV detector.

New Process Concepts Towards Area-Selective Atomic Layer Deposition and Atomic Layer Etching of Zinc Oxide

Area-Selective Atomic Layer Deposition, AS-ALD, can be initiated by local area-activation1,2 or -deactivation.3 We studied the maskless growth of ZnO from diethyl zinc and water. The method is based on the different nucleation delays that arise from precursor adsorption on OH-terminated versus that on H-terminated Si-substrate. TEOS (tetraethyl orthosilicate) and H2O were used to deposit a SiO2 seed layer by EBID (e-beam-induced deposition) in a pattern of 500x500 nm2 dots on H-terminated a-Si:H substrate. After this local activation the substrate was coated with a ZnO film in a conventional ALD reactor using 80 cycles of diethyl zinc and water vapor. In situ spectroscopic ellipsometry (Fig. 1), SEM, cross-sectional TEM and energy dispersive X-ray spectroscopy (EDX) showed good areal selectivity with ZnO growth only occurring on the SiO2 dots. This observation was corroborated by Density functional theory calculations suggesting a kinetically hindered surface reaction between diethyl zinc and H-terminated Si. Atomic Layer Etching, ALE, was also tested on a ZnO case. Current ALE technology is emerging in both thermal, isotropic and plasma-based, anisotropic approaches.4 In this presentation we will demonstrate a plasma-assisted ALE-process driven by radicals, and therefore being isotropic. The ALE-process was tested at temperatures between 150 and 250 °C. We used alternating doses of acetylacetone (Hacac) and O2-plasma intermitted by Ar-purging. The ZnO-layer thickness as measured by spectroscopic ellipsometry decreased linearly with the number of cycles. In a synergy test, we proved that only the alternated dosing of Hacac and O2-plasma caused ZnO etching, whereas Hacac and O2-plasma alone did not (Fig. 2). Preliminary infrared studies suggest that Hacac forms volatile complexes by metal oxide surface chelation (e.g. Zn(acac)2), whereas the O2-plasma step removes non-reactive Hacac fragments to refresh the surface for the next etching cycle.5 TEM- and XPS-inspection indicated no damage of the ZnO surface, good preservation of the ZnO-stoichiometry throughout the etching process, and no distinct contamination. Furthermore, we demonstrated this ALE-process to be selective over SiO2. We believe that this novel plasma-assisted ALE-concept can be extended to several other materials. [1] A. J. M. Mackus et al., Nanoscale, 4, 4477 (2012). [2] R. Chen et al., Adv. Mater., 18, 1086 (2006). [3] A. Mameli et al., Chem. Mater. 29, 921 (2017). [4] D. R. Zywotko, et al., Chem. Mater. 29, 1183 (2017). [5] M. A. George, et al., J. Electrochem. Soc., 143, 3257 (1996). Figure 1

Response Surface Methodology Optimization for Photodegradation of Methylene Blue in a ZnO Coated Flat Plate Continuous Photoreactor

Abstract In this paper, a continuous flat plate photoreactor with ZnO coating was studied in the photodegradation of methylene blue. The structural properties of catalyst were characterized by means of X-ray diffraction, Field emission scanning electron microscopy (FESEM), and energy dispersive X-ray (EDX). The XRD results indicate that high crystalline ZnO particles with average size of 13.5 nm were coated on the glass plate. The thickness of ZnO layer was 39.67 μm and the coating was uniform and crack free. The EDX showed clear border between glass and ZnO layer which confirmed no material transfer between glass and ZnO layer during thermal treatment. The influence of reactor parameters such as the slope of the glass plate, number of UV lamps, distance between lamp and ZnO coated glass plate and flow rate of wastewater was investigated using optimal custom design which is a subset of response surface methodology (RSM). The results indicated that the maximum photodegradation of methylene blue was achieved under the following conditions: plate slope of 9, 3 UVA lamps, 12 ml/s wastewater flow rate and 10 cm distance between lamp and glass plate. The response of surface methodology at optimum conditions was 65.05% while experimental value was 64.66%, showing good agreement between the experimental values and those predicted by the models, with relatively small errors which were only 0.64. The kinetic study was also performed for methylene blue photodegradation at optimum conditions.

Enhanced Power Conversion Efficiency of PbS QD/ZnO Solar Cells By Thermal Annealing

【Introduction】 Colloidal quantum dots (CQDs) are useful absorbers and transporter for a next-generation solar cell because of size-dependent bandgap tunability and because CQD-based active layers can be formed by solution processes. In spite of recent progress in PbS CQD solar cells1, there are still many challenges left to further improve solar cell performance. Elongation of the carrier diffusion length of the CQD solid layers is a challenge. This is because the typical carrier diffusion length of the layers deposited by a conventional solid-phase ligand exchange is approximately 200 nm, which makes it difficult to achieve thick active layers capable of sufficient light absorption, particularly in the infrared region. In this study, we employed a liquid-phase ligand exchange method to deposit PbS CQD active layers2, and treated them at different temperatures, with the aim of increasing carrier diffusion lengths. The PbS QD/ZnO solar cells fabricated with the active layers that had been treated at 80 °C gave a power conversion efficiency of 10.3%. 【Experimental Methods】 We used PbS CQDs capped with oleic acid (OA) ligands and giving the first exciton absorption peak at 935 nm. The 100 nm thick ZnO layers were first formed on FTO glass substrates by a spin-coating method 3. A liquid-phase ligand exchanged PbS-I CQD ink was deposited on the ZnO layer by a single-step spin-coating method 2. The PbS-I CQD layers were then annealed at five different temperatures (60, 80, 100, 120, 140 ℃) for 30 mins each. Subsequently PbS COD were deposited on top of the PbS-I layers, and treated with 1,2-ethanedithiol (EDT) to replace OA ligands with EDT ligands. Au electrodes were then thermally evaporated onto the layers to complete solar cell fabrication. Photocurrent density vs voltage (J-V) characteristics of the solar cells were measured under one-sun illumination to obtain the short-circuit density (Jsc ), open circuit voltage (Voc ), fill factor (FF), and power-conversion efficiency (PCE). The defect density (Nt ) of the PbS-I layers was estimated from the Mott-Schottoky plots obtained by measuring the built-in potential and the frequency dependence (0.1 MHz - 20 MHz) of the impedance spectra of the solar cells. 【Results and Discussion】 Figure 1(a) shows the active layer thickness dependence of solar cell performance. All the solar cell performance values (Jsc , Voc , FF, and PCE) increased as the layer thickness increases. The Jsc monotonously increases with increasing thickness, and showed no sign of Jsc 's decreasing even at a thickness of approximately 300 nm. This indicates that the carrier diffusion length becomes longer by the liquid-phase ligand exchange. As a result of the increased carrier diffusion length, we succeeded in improving the EQE of the solar cells in the infrared region. The resulting PCE reached 9.41%. We annealed active layers at different temperatures, aiming to increase PbS QD coupling and decrease the defect density in the active layers. Figure 1(b) shows the annealing temperature dependence of the solar cell performance and defect density. The defect density of the active layers gives the lowest value when annealed at 80 ℃, thereby resulting in a maximum power conversion efficiency of 10.3%. 【References】 1) G. H. Carey, A. L. Abdelhady, Z. Ning, S. M. Thon, O. M. Bakr, E. H. Sargent, Chem. Rev. 115 , 12732 (2015). 2) M. Liu, O. Voznyy, R. Sabatini, F. P. Garcia de Arquer, R. Munir, A. H. Balawi, X. Lan, F. Fan, G. Walters, A. R. Kirmani, S. Hoogland, F. Laquai, A. Amassian, E. H. Sargent, Nat. Mater., 16, 258 (2017). 3) H. Wang, V. Gonzalez-Pedro, T. Kubo, F. Fabregat-Santiago, J. Bisquert, Y. Sanehira, J. Nakazaki, H. Segawa, J. Phys. Chem. C, 119, 27265 (2015). Figure 1

Three-dimensional finite element simulation of Love mode surface acoustic wave in layered structures including ZnO piezoelectric film and diamond substrate

Three-dimensional (3D) finite element method (FEM) was used to simulate the surface acoustic wave (SAW) propagation properties in layered structures including ZnO piezoelectric film and diamond substrate. The excitation conditions of Love mode and Rayleigh mode SAWs are discussed considering the ZnO crystal orientation and the SAW propagation direction which are specified using Euler angles (α, β, γ). The phase velocity and electromechanical coupling coefficient K2 of Love mode SAW in the ZnO/diamond structure are provided as functions of the normalized ZnO layer thickness hZnO/λ and the Euler angles (α, β, γ). The results show that the maximum K2 of 4.26% is reached at hZnO/λ = 0.3 and (α, β, γ) = (0°, 90°, 0°) for Love mode SAW; the associated phase velocity is 3436 m/s and the temperature coefficient of frequency (TCF) value is −26.5 ppm/°C. The TCF properties are greatly improved by adding a SiO2 layer between the ZnO film and the diamond substrate. A zero TCF can be obtained when the normalized SiO2 thickness hSiO2/λ is 0.25 and the hZnO/λ is 0.2, where the corresponding K2 is 3.26% and the phase velocity is 3126 m/s.

Solution-processed ZnO thin-film transistors codoped with Na and F

Na and F codoping was utilized to realize high-performance ZnO thin-film transistors derived from sol–gel solution process. The effect of NaF doping concentration on the electrical properties of ZnO thin-film transistors annealed at 350 °C was investigated. Films with 10 at.% doping exhibited excellent performance with an average mobility of 70.98 cm2 V−1 s−1 and a high Ion/Ioff ratio on the order of 106. The X-ray photon spectroscopy exhibited oxygen bound with oxide lattice with and without vacancy related peak position shifted towards lower energy side when doped at 10 at.% and did not show the hydroxyl group on the NaF doped ZnO surface which indicates the improved performance of the thin-film transistors devices. The electrical performance of the devices also changed considerably as the thickness of the NaF-doped ZnO layer increased. Increasing the grain size and realizing a smooth surface morphology by varying the NaF concentration were found to enhance the thin-film transistors mobility.

Influence of P+-ZnTe back surface contact on photovoltaic performance of ZnTe based solar cells

In order to improve photovoltaic performance of solar cells based on ZnTe thin films two device structures have been proposed and its photovoltaic parameters have been numerically simulated using Solar Cell Capacitance Simulator software. The first one is the ZnO/CdS/ZnTe conventional structure and the second one is the ZnO/CdS/ZnTe/P+-ZnTe structure with a P+-ZnTe layer inserted at the back surface of ZnTe active layer to produce a back surface field effect which could reduce back carrier recombination and thus increase the photovoltaic conversion efficiency of cells. The effect of ZnO, CdS and ZnTe layer thicknesses and the P+-ZnTe added layer and its thickness have been optimized for producing maximum working parameters such as: open-circuit voltage Voc, short-circuit current density Jsc, fill factor FF, photovoltaic conversion efficiency η. The solar cell with ZnTe/P+-ZnTe junction showed remarkably higher conversion efficiency over the conventional solar cell based on ZnTe layer and the conversion efficiency of the ZnO/CdS/ZnTe/P+-ZnTe solar cell was found to be dependent on ZnTe and P+-ZnTe layer thicknesses. The optimization of ZnTe, CdS and ZnTe layers and the inserting of P+-ZnTe back surface layer results in an enhancement of the energy conversion efficiency since its maximum has increased from 10% for ZnO, CdS and ZnTe layer thicknesses of 0.05, 0.08 and 2 µm, respectively to 13.37% when ZnO, CdS, ZnTe and P+-ZnTe layer thicknesses are closed to 0.03, 0.03, 0.5 and 0.1 µm, respectively. Furthermore, the highest calculated output parameters have been Jsc = 9.35 mA/cm2, Voc = 1.81 V, η = 13.37% and FF = 79.05% achieved with ZnO, CdS, ZnTe, and P+-ZnTe layer thicknesses about 0.03, 0.03, 0.5 and 0.1 µm, respectively. Finally, the spectral response in the long-wavelength region for ZnO/CdS/ZnTe solar cells has decreased at the increase of back surface recombination velocity. However, it has exhibited a red shift and showed no dependence of back surface recombination velocity for ZnO/CdS/ZnTe/P + -ZnTe solar cells.

Enhanced resolution imaging of ultrathin ZnO layers on Ag(111) by multiple hydrogen molecules in a scanning tunneling microscope junction

Molecular hydrogen in a scanning tunneling microscope (STM) junction has been found to enhance the lateral spatial resolution of the STM imaging, referred to as scanning tunneling hydrogen microscopy (STHM). Here we report atomic resolution imaging of 2- and 3-monolayer (ML) thick ZnO layers epitaxially grown on Ag(111) using STHM. The enhanced resolution can be obtained at a relatively large tip to surface distance and resolves a more defective structure exhibiting dislocation defects for 3-ML-thick ZnO than for 2 ML. In order to elucidate the enhanced imaging mechanism, the electric and mechanical properties of the hydrogen molecular junction (HMJ) are investigated by a combination of STM and atomic force microscopy. It is found that the HMJ shows multiple kinklike features in the tip to surface distance dependence of the conductance and frequency shift curves, which are absent in a hydrogen-free junction. Based on a simple modeling, we propose that the junction contains several hydrogen molecules and sequential squeezing of the molecules out of the junction results in the kinklike features in the conductance and frequency shift curves. The model also qualitatively reproduces the enhanced resolution image of the ZnO films.

Effective atomic interface engineering in Bi2Te2.7Se0.3 thermoelectric material by atomic-layer-deposition approach

Grain boundaries play a critical role in the carrier/phonon transport in thermoelectric materials. It remains a big challenge to control over both chemical composition and dimension of grain boundary precisely by traditional approaches. Herein, an bottom-up grain boundary engineering strategy based on atomic layer deposition (ALD) is first introduced to atomically control and modify the grain boundary of Bi2Te3-based thermoelectric materials. To demonstrate the effect of this strategy, ultrathin ZnO interlayer is deposited on the Bi2Te2.7Se0.3 (BTS) grain boundaries to optimize of the carrier/phonon transport for achieving high thermoelectric performance. In situ TEM experiments upon heating reveals that the ZnO interlayer will give rise to the precipitation of Te nanodot at ZnO/BTS interface, which can be atomically controlled by adjusting the thickness of ZnO layer. Benefited from the atomically precise modified grain boundary, a maximum ZT of 0.85 is obtained, approximately 1.8 times higher than that of the pure BTS. As a powerful interfacial modification strategy, ALD-based approach can be extended to other thermoelectric material system simply, which may contribute to the development of high performance thermoelectric material of great significance.

Thickness‐dependent optical and structural properties of polytetrafluoroethylene/zinc oxide films by radio frequency magnetron sputtering

AbstractPTFE/ZnO films with varying ZnO thickness were prepared by RF magnetron sputtering under high vacuum (100W RF power). The PTFE/ZnO films show slight enhancement in transmission in the visible range as compared to bare ZnO films. There is a clear UV light cutoff (rapid fall below 380 nm with negligible transmission below ~300 nm) after 10‐min deposition of ZnO, which remains unaffected by further depositing PTFE. Transmission data reveal that absorption edge falls in the range of 300–380 nm, below which, the transmission reduced to only ~0.02% giving an optical density of 4 (for the film deposited for 20 min [total thickness ~415 nm]) in the 200–250 nm range. As the ZnO layer thickness increases, the polycrystallinity increases and simultaneously, roughness increases. It leads to a change in crystallite orientations and also in the distribution of grains on film surface. Corresponding AFM image shows island‐like growth, leading to an intermixed ZnO/PTFE layer. The roughness of surface in lower as well as higher thickness range is found to be high (~8–9 nm) except the film in which continuous layer seems to be grown. Based on this, a layer model was developed that consists of glass substrate, native oxide SiO2 layer on this substrate, then a dense ZnO layer followed by mixed PTFE/ZnO layer which is then covered by top PTFE as evident from the ellipsometry fitting. Overall results suggest that by choosing proper thicknesses and number of layers, such thin films might be used in antireflection coatings with UV protection below 400 nm. Based on these experimental results, an optimized simulated layer model consisting of suitable layer thicknesses of ZnO and PTFE has been suggested to achieve high transmission in the visible wavelength region.

High Voltage Flexible ZnO Solar Cells Employing Bulky Organic Dye and [Co(bpy)3]2+/3+ Redox Electrolyte

We present fabrication and characterization of flexible high voltage dye-sensitized solar cells (DSSCs) by screen printing of porous ZnO photoelectrode to power up IoT (Internet of Things) nodes to substitute batteries. Combination of triphenylamine based organic dye, D35, and [Co(bpy)3]2+/3+ (bpy = 2,2’-bipyridine) redox electrolyte achieved high voltages approaching 1 V, owing to strong rectification of charge transfer processes by passivation of the ZnO surface by the adsorbed D35 layer. The system, however, suffered from limited diffusion of the Co complex redox couple within the porous photoelectrode, causing saturation of photocurrent and poor fill factor to limit the overall conversion efficiencies under high light illumination. The current/voltage linearity was improved by optimizing the thickness of porous ZnO layer and by use of low viscosity electrolyte solvent. The highest conversion efficiencies we obtained were 3.73 and 2.63% for the cells employing F-doped SnO2 (FTO) coated glass and indium tin oxide (ITO) coated poly-ethylenenaphthalate (PEN) film substrates, respectively.

Spectral Response of CuGaSe2/Cu(In,Ga)Se2 Monolithic Tandem Solar Cell With Open-Circuit Voltage Over 1 V

Thin-film multijunction solar cells are considered to be the most promising structure for next-generation photovoltaic devices. We fabricated CuGaSe2 (CGS)/Cu(In,Ga)Se2 (CIGS) monolithic tandem solar cells. The intermediate AZO film was used as a recombination layer between the top cell and the bottom cell, and its thickness was varied from 50 to 200 nm. The best tandem cell parameters with a 50 nm thick Al-doped ZnO (AZO) layer showed a V OC = 1.03 V, J SC = 10.24 mA/cm2, FF = 41.5%, and efficiency = 4.32%. We showed the V OC of monolithic tandem cell to be over 1 V under illumination. We also observed the current continuity between the CGS cell and the CIGS cell which were connected in series as subcells. As the AZO thickness increased, the spectral response of the top cell decreased and the bottom cell was not completely saturated. The 50 nm thick AZO layer leads the CIGS bottom cell to be current-limiting, whereas the 200 nm thick AZO layer shifts the limitation to the CGS top cell. The results also showed that the In element diffusion into the CGS top absorber enhanced the electrical and optical properties of the top cell, whereas the Zn element diffusion into CIGS bottom absorber tended to degrade the bottom cell simultaneously.

Influence of a ZnO layer on Seebeck coefficients in asymmetric double-barrier tunnel junctions

The Seebeck coefficients are studied in asymmetric double-barrier magnetic tunnel junctions (DBMTJs) with the structures as CoFeB/MgO/ZnO/MgO/LSMO. In the linear response regime, oscillatory behavior is observed with change in the ZnO layer thickness because of the resonant tunneling in the DBMTJs. Effects of average temperature and magnetizations orientation are also discussed. It is found that the Seebeck coefficients are greater in the DBMTJs than asymmetric single-barrier MTJs (SBMTJs). Therefore, it is to be achieved the greater Seebeck coefficients by choosing a proper thickness for the ZnO layer in the proposed asymmetric DBMTJs.

Corrigendum: Chemical Activity of Thin Oxide Layers: Strong Interactions with the Support Yield a New Thin-Film Phase of ZnO.

Small Cu particles supported on and most likely activated by a ZnO substrate are the active component in the industrial catalyst used to convert syngas (H2, CO, CO2) into methanol, the third most important chemical product worldwide. Although a topic of intense research, the nature of the active site is still under debate. Recently, it has been pointed out that Zn atoms present at the surfaces of the Cu particles exhibit pronounced chemical activity and could explain some of the experimental findings. Another interesting suggestion is the presence of a thin layer of ZnOx species which forms on the surface of the Cu particles under reaction conditions. The importance of such thin oxide layers on the surface of metals under reaction conditions has already been pointed out in other contexts, where it was found that their chemical properties may differ substantially from those of the corresponding bulk oxides. In the case of ZnO this question is particularly interesting, since strong interactions between ZnO and the supporting metal have been reported for ZnO/ Cu and in recent work thin layers of ZnO have been shown to adopt a depolarized, graphitic structure, ZnO(gr), different from the wurtzite-type bulk. The properties of oxide thin films supported on metal substrates have been successfully studied in a number of cases, for example, for thin aluminum oxide films grown by oxidation of Ni/Al alloys. In contrast, the chemical activity of ZnO thin films supported on Cu single crystals has been investigated in a few cases only. Maroie et al. have investigated the adsorption and oxidation processes for single-crystal brass(110), brass(100), and brass(111) surfaces by X-ray photoelectron spectroscopy (XPS). Brass(110) and brass(111) show the same behavior with regard to the interaction with oxygen: the dissociative adsorption of oxygen on the surface is followed by the growth of thin ZnO layers. Wiame et al. reported that after oxidation of (111)-oriented Cu0.7Zn0.3 samples at room temperature the surface is covered by ZnO islands; it was suggested that these islands have (0001) and (0001) surface terminations. A more detailed characterization of the chemical properties of the thin ZnO layers was not carried out in this early work. In the present paper we report a detailed multitechnique investigation of a brass(111) single-crystal substrate (Cu/Zn ratio 9:1) subjected to different oxidation procedures using scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FTIR) under ultrahigh vacuum (UHV) conditions. The experimental findings are then interpreted by comparison with the results of a rather extensive set of density functional theory (DFT) calculations. Our results reveal the growth of thin ZnO adlayers with chemical properties that are markedly different from those of normal, wurtzite-type ZnO substrates. XPS data recorded for the brass(111) surface before and after different oxidation procedures for two different exit angles of the photoelectrons are shown in Figure 1. Since it is difficult to discriminate between Cu and Cu on the basis of XPS data, also the corresponding results from Auger electron spectroscopy (AES) are shown. For the clean brass(111) substrate the data indicate a Zn atom concentration of 5%, clearly lower than the 10% expected based on the bulk Cu/Zn ratio. Upon oxidation, the XPS data reveal an increase of the surface Zn concentration. Since it is a crucial question whether, in addition to Zn, also Cu or Cu is present, we have carefully analyzed the XPS and AES data. Neither in the Cu2p XPS data nor in the Cu L3M45M45 Auger data were the characteristic signatures of Cu or Cu species resulting from an oxidation of copper atoms detected. Cu exhibits a L3M45M45 peak at electron kinetic energies of 915 eV– 917 eV, which is clearly absent in the present data (see Figure 1). This observation, which agrees with the conclusions presented in a previous study by Rameshan et al., is expected, since in the presence of the less noble Zn one would expect the formation of ZnO to precede that of CuxO. Oxidation at elevated temperatures results in a substantial increase of the Zn signal, revealing the formation of thicker ZnO adlayers. The thickness of these thin ZnO layers was determined from the intensity of the Zn2p3/2 and Cu2p3/2 XPS signals. Exposure of the samples to 500 L of O2 at room temperature yields a ZnO adlayer with an average thickness of about 1.7 , consistent with the presence of a monolayer. More extended exposures to oxygen at room temperature did not result in a significant further increase of the thickness of the ZnO layer. Even the oxidation of the brass substrate at [*] Dr. V. Schott, Dr. A. Birkner, Dr. M. Xu, Dr. Y. Wang Chair of Physical Chemistry Ruhr-University Bochum (Germany)

Effect of metal-fingers/doped-ZnO transparent electrode on performance of GaN/InGaN solar cell

The effect of doped-ZnO transparent conductive oxide (TCO) with metal (Ag)-fingers contact on GaN/InGaN solar cell is investigated through numerical simulations. An optical and electrical analysis of different dopant elements (such as B, Al, Ga, In and Sn) with ZnO as a top TCO layer is studied. A comparative analysis of metal square pad electrode, metal grid pattern electrode and metal-finger/ZnO type electrodes are taken into consideration to ensure the effect of anti-reflectivity by ZnO. The effect of thickness of ZnO and i-InGaN layer on performance of solar cell is also studied in detail. The proposed solar cell structure with Ag-fingers/ZnO:Al as top contact electrode shows interesting device characteristics compared to other dopants and metal top electrodes. The device achieves open circuit voltage ∼2.525 V, short circuit current ∼4.256 mA/cm2, fill factor ∼87.86% and efficiency ∼9.22% under 1 Sun, air mass 1.5 global illumination.

Luminescence related to two-dimensional gas in ZnO/ZnMgO heterostructures

In c-oriented (001) wurtzite ZnMgO/ZnO heterostructures the polarization induced electric field is the driving force for the formation of two-dimensional electron gas (2DEG) at the ZnMgO/ZnO interfaces. In this work we report on our results of photoluminescence (PL) studies of single ZnMgO/ZnO heterostructures with different thicknesses of top ZnO layers grown on a-plane sapphire by molecular beam epitaxy. We show that the PL spectrum of ZnO layers depends on their thicknesses. In particular, in thin ~5–8nm ZnO layers a broad emission band is observed at 3.1–3.3eV, which shifts to higher energies with increasing temperature and excitation power. We suggest that this PL involves 2D electrons confined in the triangular potential well at the ZnMgO/ZnO interface. We also observe an emission near 3.4eV, which we interpret as emission involving 2D electrons and holes from the ZnMgO barrier. The dependence of the PL on the thickness of the ZnO layer is discussed and we suggest that emission involving two dimensional 2D electrons reflects occupation of states in the triangular potential at the interface.

ITO-Free Transparent Organic Solar Cell with Distributed Bragg Reflector for Solar Harvesting Windows

We demonstrated an indium tin oxide (ITO)-free, highly transparent organic solar cell with the potential to be integrated into window panes for energy harvesting purposes. A transparent, conductive ZnO/Ag/ZnO multilayer electrode and a Ag:Ca thin film electrode were used in this transparent device as the bottom and top electrode, respectively. To further improve the transmittance of the solar cell, the thickness of the top ZnO layer was investigated both experimentally and with simulations. An average visible transmittance of >60% was reached, with a maximum transmittance of 73% at 556 nm. Both top and bottom illumination of the solar cell generated comparable power conversion efficiencies, which indicates the wide application of this solar cell structure. In addition, we fabricated distributed Bragg reflector mirrors with sputtered SiO2 and TiO2, which efficiently increased the power conversion efficiency over 20% for the solar cells on glass and poly(ethylene terephthalate) (PET) substrates.

LWIR all-atomic layer deposition ZnO bilayer microbolometer for thermal imaging

We propose an all-ZnO bilayer microbolometer, operating in the long-wave infrared regime that can be implemented by consecutive atomic layer deposition growth steps. Bilayer design of the bolometer provides very high absorption coefficients compared to the same thickness of a single ZnO layer. High absorptivity of the bilayer structure enables higher performance (lower noise equivalent temperature difference and time constant values) compared to single-layer structure. We observe these results computationally by conducting both optical and thermal simulations.

Luminescence of Nanoporous Si and ALD-Deposited ZnO on Nanoporous Si Substrate

This paper reports the attempt at synthesizing nanoporous silicon (Si) with a dendritic-like structure and atomic layer deposition (ALD) of ZnO on nanoporous Si to control light emission intensity and emission center by applying an optimum voltage, etching time and thickness of ZnO layer. The dendritic-like structure of nanoporous Si was formed with low etching voltages of 5–10 V. Fourier transform infrared absorption spectra of the nanoporous Si reveals that the intensities of hydride stretching, SiH2 scissor mode and Si-O-Si vibration peak increase with the increasing of etching time. The formation of a thick dendritic-like structure with an increasing SiH2 bond resulted in significant enhancement of luminescence. In addition, the ALD-deposited ZnO layer on nanoporous Si resulted in light emission from both ZnO and nanoporous Si under a single excitation source. These results suggest the potential application of an ALD-deposited ZnO layer on nanoporous Si in designing materials for advanced optoelectronics.

Investigations of Sol-Gel ZnO Films Nanostructured by Reactive Ion Beam Etching for Broadband Anti-Reflection

A novel ZnO dry etching approach is introduced using reactive ion beam etching of thick sol-gel ZnO layers for controlled nanodisk/nanocone array fabrication. In this approach the same system can be used for the colloidal lithography mask (silica particles) size reduction by a fluorine-based chemistry and etching of the ZnO nanostructures by a CH4/H2/Ar chemistry. This resulted in a ZnO:SiO2 etch selectivity of ∼3.4 and etch rate of ∼56 nm/min. Thick sol-gel ZnO layers, nanodisk arrays and (truncated) nanocone arrays were fabricated and their optical properties analyzed by finite-difference time-domain simulations and spectrally-resolved total/specular reflectivity measurements. The demonstrated broadband omnidirectional anti-reflection, controlled nanostructure period/geometry and low absorption in the visible-NIR spectrum makes these sol-gel ZnO nanostructures very interesting for many optoelectronic applications, including photovoltaics.