Chemical and electronic interface structure of spray pyrolysis deposited undoped and Al-doped ZnO thin films on a commercial Cz-Si solar cell substrate

We have studied differences in the interface between undoped and Al-doped ZnO thin films deposited on commercial Si solar cell substrates. The undoped ZnO film is significantly thicker than the Al-doped film for the same deposition time. An extended silicate-like interface is present in both samples. Transmission electron microscopy (TEM) and photoelectron spectroscopy (PES) probe the presence of a zinc silicate and several Si oxides in both cases. Although Al doping improves the conductivity of ZnO, we present evidence for Al segregation at the interface during deposition on the Si substrate and suggest the presence of considerable fixed charge near the oxidized Si interface layer. The induced distortion in the valence band, compared to that of undoped ZnO, could be responsible for considerable reduction in the solar cell performance.     

ZnO thin films deposited by RF magnetron sputtering: Effects of the annealing and atmosphere conditions on the photocatalytic hydrogen production

This work studied the effect of different annealing conditions of ZnO thin films grown by RF magnetron sputtering and their application as photocatalysts for hydrogen production without any sacrificial agent or co-catalyst. ZnO films were annealed in air, nitrogen, and argon atmospheres to study the effect of their physical properties in the photocatalytic activity. ZnO films showed high crystallinity and optical transparence of around 75–90% after annealing. Changes in composition and optical properties of the ZnO films were studied by x-ray photoelectron spectroscopy (XPS) and ellipsometry spectroscopy (SE), and results were correlated with the photocatalytic performance in hydrogen production. The highest photocatalytic hydrogen production was obtained with the ZnO thin film annealed in an air atmosphere with a result of 76 μmol.     

Solid-phase crystallization of amorphous silicon on ZnO:Al for thin-film solar cells

The suitability of ZnO:Al thin films for polycrystalline silicon (poly-Si) thin-film solar cell fabrication was investigated. The electrical and optical properties of 700 -nm-thick ZnO:Al films on glass were analyzed after typical annealing steps occurring during poly-Si film preparation. If the ZnO:Al layer is covered by a 30 nm thin silicon film, the initial sheet resistance of ZnO:Al drops from 4.2 to 2.2 Ω after 22 h annealing at 600 °C and only slightly increases for a 200 s heat treatment at 900 °C. A thin-film solar cell concept consisting of poly-Si films on ZnO:Al coated glass is introduced. First solar cell results will be presented using absorber layers either prepared by solid-phase crystallization (SPC) or by direct deposition at 600 °C.     

Doping influence on intrinsic stress and carrier mobility of LP-MOCVD-deposited ZnO:B thin films

Textured boron-doped zinc oxide (ZnO:B) films, suitable as transparent and conductive layers in thin film silicon-based solar cells, have been obtained by low-pressure metalorganic chemical vapour deposition (LP-MOCVD) technique. The complex role of the boron doping in ZnO layers was examined and its influence on the morphological and structural properties was analysed. Furthermore, a correlation between such properties, intrinsic stress and carrier mobility in the film has been analysed. ZnO:B films have a polycrystalline structure with a columnar texture shape, they show a rough surface with pyramidal large grains. At the increase of the boron doping the pyramidal shape of the grains deteriorates and the average grain size reduces. Furthermore the a-lattice parameter decreases indicating that the boron incorporation introduces a deformation in the crystal. In addition to these large structural modifications, the decrease of the carrier mobility at the increase of the doping content is observed. At the same time, the boron content also plays a meaningful role on the intrinsic stress inside the film. Stress behaviour in ZnO films has been investigated by X-ray diffraction measurements using the sin² ψ method. Tensile stresses have been observed and the thermal and intrinsic components have been calculated. Respect to undoped ZnO films, boron incorporation on substitutional or interstitial sites increases the tensile stress by means of a lattice strain mechanism that reduces the d-spacing value.     

Enhanced photovoltaic performance of inverted organic solar cells with In-doped ZnO as an electron extraction layer

In the present work, a systematic study has been carried out to understand the effect of In doping on the various properties of the ZnO nanocrystalline thin films. In-doped ZnO nanocrystalline thin films with different indium concentrations (1.98%, 4.03%, 6.74%, 8.62% and 10.48% In) have been synthesized by sol–gel method. The grain size and surface roughness of the In-doped ZnO thin films are observed to be smaller than those of the ZnO thin films. 6.74% In-doped ZnO films with a low resistivity of 1.95 × 10⁻³ Ω cm and a high mobility of 2.19 cm² V⁻¹ S⁻¹ have been prepared under optimal deposition conditions. Inverted organic solar cells containing In-doped ZnO as an electron extraction layer with the structure indium tin oxide (ITO)/In-doped ZnO/poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT): [6,6]-phenyl C₇₁-butyric acid methyl ester (PC₇₁BM)/MoO₃/Al have been fabricated. The inverted organic solar cell with 6.74% In-doped ZnO exhibited a power conversion efficiency of 5.58%, which is the best efficiency reported so far for these type of solar cells. The device performance has been optimized by varying the indium doping concentration. The results clearly demonstrate that significant improvement in power conversion efficiency can be obtained by incorporating In into the ZnO films.     

Electrodeposited Cr-Doped α-Fe2O3 thin films active for photoelectrochemical water splitting

Polycrystalline hematite (α-Fe₂O₃) Chromium (Cr)-doped thin films were electrodeposited on fluorine-doped tin oxide-coated glass substrates. The electrodeposition bath comprised an aqueous solution containing FeCl₃·6H₂O, NaCl, and H₂O₂.Chromium was added to the electrolyte at such a proportion that the Cr/(Cr + Fe) ratio remained within the 2–8 at. % range. The as-deposited films were subsequently annealed in air at 650 °C for 2 h. The structure and morphological characteristics of the undoped and Cr-doped α-Fe₂O₃ thin films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and UV–Vis spectroscopy. Cr doping led the main XRD lines to shift to lower angles, which mostly resulted from substituting Fe³⁺ for Cr⁴⁺ ions that leads to α-Fe₂O₃ lattice contraction. The SEM observations showed that the roughness and aspect of surfaces changed with the Cr doping level. The photoelectrochemical (PEC) performance of the α-Fe₂O₃ films was examined by chronoamperometry and linear sweep voltammetry techniques. The Cr-doped films exhibited greater photoelectrochemical activity than the undoped α-Fe₂O₃ thin films. The highest photocurrent density was obtained for the 8% Cr-doped α-Fe₂O₃ films in 1 M NaOH electrolyte. All the samples achieved their best IPCE at 400 nm. The IPCE values for the 8 at.% Cr-doped hematite films were 20-fold higher than that of the undoped sample.This Cr-doped hematite films ‘excellent photoelectrochemical performance was mainly attributed to improved charge carrier properties. Such high photoactivity was attributed to the large active surface area and increased donor density caused by increasing the Cr doping in the α-Fe₂O₃ films.     

Investigation of the copper concentration on photocurrent collection of CuInSe2 solar cells

In this study, CuInSe₂ (CISe) thin films were prepared from thermally evaporated Cu/In precursors, having various Cu/In atomic ratio, under the same selenization conditions. The precursors were converted into CISe absorber by annealing in a quartz tube furnace in the selenium vapours at substrate temperature of 500 °C. We developed four CISe films with Cu/In atomic ratio of 0.81–1.19, denoted as Cu‐very rich, Cu‐rich, Cu‐poor, and Cu‐very poor CISe thin films respectively. The effects of Cu/In atomic ratio on grain size, surface morphology, micro‐structure and defect formation of the resulting CISe films were examined. It has been found that the photovoltaic properties were strongly related to Cu concentration, as well as carrier transport mechanism. Defects at the surface and in the bulk of CISe thin films were observed using X‐ray diffraction (XRD), X‐ray photoelectron spectroscopy, Raman spectroscopy, energy dispersive X‐ray spectroscopy and scanning electron microscopy. Moreover, XRD revealed that the CISe film surface had a preferred orientation along the (112) plane. The XRD intensity and full width at half maximum of the (112) plane of CISe varied according to the Cu/In atomic ratio. Our experimental results show that the Cu‐rich solar cell achieves conversion efficiency of 4.55% and exhibits an exceptional high short‐circuit current density. Copyright © 2016 John Wiley & Sons, Ltd.     

Photoelectrocatalytic activity of spray deposited ZnO thin films against E. coli Davis

Abstract

Zinc oxide (ZnO) thin films have been deposited onto fluorine doped tin oxide coated glass substrates by economical chemical spray pyrolysis technique. Films were deposited using various quantities of solution from 50 to 200 cc (substrate temperature 400°C, solution concentration 0·2M) in order to achieve different thicknesses. The films were characterised by X-ray diffraction, SEM, AFM and optical absorption techniques. The films exhibit a hexagonal wurtzite crystal structure with preferred (002) orientation. Morphological study showed a smooth and nanocrystalline surface of ZnO films. Direct optical band gap energy of ZnO thin films is found to be 3·24 eV. The average transmission is of the order of 87% in the visible region. The photoelectrocatalytic response of the film against Escherichia coli Davis is studied using a specially designed photoelectrochemical (PEC) reactor module. Thickness and UV light dependent photoelectrocatalytic bactericidal properties of ZnO thin films have been investigated. It shows that biased 1·1 μm thick ZnO thin films with 2 mW cm^?2 UV light intensity give better bactericidal response compared to others. The relative percentage of killing of bacteria is 19·81% due to UV illumination, 52·71% due to UV illumination and passing over ZnO surface and 95·03% due to UV illumination and passing through PEC reactor with ZnO thin film after 2·5 h. It can be concluded that the ZnO thin film with photochemical reactor can be used in a water purifier to get bacteria free drinking water.     

Influence of gas ambient on the synthesis of co-doped ZnO:(Al,N) films for photoelectrochemical water splitting

Al and N co-doped ZnO thin films, ZnO:(Al,N), are synthesized by radio-frequency magnetron sputtering in mixed Ar and N₂ and mixed O₂ and N₂ gas ambient at 100 °C. The ZnO:(Al,N) films deposited in mixed Ar and N₂ gas ambient did not incorporate N, whereas ZnO:(Al,N) films grown in mixed O₂ and N₂ gas ambient showed enhanced N incorporation and crystallinity as compared to ZnO:N thin films grown in the same gas ambient. As a result, ZnO:(Al,N) films grown in mixed O₂ and N₂ gas ambient showed higher photocurrents than the ZnO:(Al,N) thin films deposited in mixed Ar and N₂ gas ambient. Our results indicate that the gas ambient plays an important role in N incorporation and crystallinity control in Al and N co-doped ZnO thin films.     

Improved performance of a dye-sensitized solar cell using a TiO2/ZnO/Eosin Y electrode

TiO₂/ZnO/Eosin Y structure films were prepared by a one-step cathodic electrodeposition method and used as a photoanode in a dye-sensitized solar cell (DSSC). Using this TiO₂/ZnO/Eosin Y electrode in DSSC, the degradation of the cell with time was reduced and I_SC, V_OC and fill factor values were increased. The use of a thin ZnO layer, permitted the formation of an energy barrier at the electrode/electrolyte interface, thus reducing recombination rate and improving cell performance. In addition, the adsorbed dye molecules prepared by one-step cathodic electrodeposition with ZnO were very stable compared with that prepared by conventional immersing method, as evidenced by UV/vis absorption spectroscopy measurements.     

Effect of heat treatment on microstructure and transparent conducting properties of sol-gel ZnO/Al thin films

Abstract

Transparent and conducting Al doped ZnO (AZO) thin films with c axis preferred orientation were prepared on glass substrates via the sol-gel route. The physical and chemical changes of the precursors during thermal treatment were systematically investigated by differential scanning calorimeter and thermogravimetric analysis and the crystallinity of AZO films was characterised by X-ray diffraction. The surface morphology evolution of the films post-heated at 420, 450, 530 and 550°C respectively was observed by SEM. Results reveal that the crystallising process of ZnO/Al comprises two stages: the primary nucleation and growth occurs at 318°C and the continuous crystallisation at 488°C. The optimised preheating and post-heating temperatures are determined at 420 and 530°C respectively. The annealing treatment in vacuum can contribute considerably to the electrical conductivity. The film post-heated at 530°C shows a homogenous dense microstructure and exhibits the minimum sheet resistance of 140 Ω/sq. The visible optical transmittance of all films is more than 90%.     

Synthesis of nanostructured Al-doped zinc oxide films on Si for solar cells applications

Al-doped ZnO thin films have been prepared by a novel successive chemical solution deposition technique. The variation in morphological, structural, electrical, and optical properties of nanostructured films with doping concentration is investigated in details. It was demonstrated that rapid photothermal processing (RPP) improves the quality of nanostructured ZnO films according to the enhancement of resonant Raman scattering efficiency, and the suppression of the visible luminescence with the increase of RPP temperature. It was found from the I-V characteristics of ZnO/Si heterojunction that the average short-circuit current density is about 8 mA/cm². For 1%Al-doped ZnO/SiO₂/Si structure, the short-circuit current density is about 28 mA/cm². The improvement shown in the characteristics may be assigned partially to the reduction of the defect density in the nanostructured Al-doped ZnO films after RPP. The correlations between the composition, microstructure of the films and the properties of the solar cell structures are discussed. The successive chemically deposited Al-doped ZnO thin film offers wider applications of low-cost solar cells in heterojunction structures.     

Improvement of the performance of dye-sensitized solar cells using Sn-doped ZnO nanoparticles

Sn-doped and undoped ZnO nanoparticles were synthesized by hydrothermal method and their performance as the photoanode of dye-sensitized solar cells (DSSCs) was investigated. Energy dispersive X-ray spectroscopy and X-ray diffraction showed that the Sn had been doped into the ZnO lattice. A red shift of photoluminescence spectra which was induced by Sn doping was observed. The photocurrent density-voltage curves of DSSCs indicated that the efficiency was increased by as high as 140% on bare-FTO substrate and 105% on ZnO compact layer/FTO substrate via Sn doping. Also the effect of the ZnO compact layer was discussed by both of Sn-doped or undoped DSSCs.     

柔性不锈钢衬底CIGS薄膜太阳电池

以轻质柔性不锈钢材料为衬底,利用三步共蒸发法制备较高质量的四元化合物Cu(In,Ga)Se_2薄膜,CIGS层在Mo导电层上具有很强的附着力。利用XRD和XRF分别分析了所制备薄膜的晶相和组分。以ZnO:Al/i-ZnO/ CdS/CIGS/Mo/Stainless steel结构为基础得到最高转换效率为9.39%的柔性太阳电池。最后讨论了衬底粗糙度、有害杂质的扩散和不含有Na元素等不利因素对于电池性能的影响。     

Si doped ZnO thin films for transparent conducting oxides

Abstract

Transparent conductive silicon doped zinc oxide (SZO, 3%Si) thin films are grown by direct current magnetron sputtering on glass substrates at room temperature. Experimental results show that the sputtering time has a significance impact on the growth rate, crystal quality and electrical properties of the films, and have little impact on the optical properties of the films. The growth rate decreases with the sputtering time. The resistivity of ZnO/Si films decreases as the sputtering time increases from 8 to 20 min. However, as the sputtering time increases further, the electrical resistivity increases instead. When other sputtering conditions are kept unchanged, it is found that the optimum sputtering time is 20 min and the achieved lowest resistivity is 4·92×10^?4 Ω cm (sheet resistance?=?11·5 Ω/sq for thickness 427·5 nm). The UV-vis transmission spectrum shows that all film samples present a transmission of above 90·0% in the visible range.     

Doping, vacuum annealing, and thickness effect on the physical properties of zinc oxide films deposited by spray pyrolysis

Gallium, aluminum, and indium-doped ZnO (ZnO:Ga, ZnO:Al, and ZnO:In) films have been deposited by the chemical spray method on sodocalcic substrates. The effect of different dopant elements, a post-annealing treatment in vacuum, and the film thickness on the electrical, optical, structural, and morphological properties of the films has been investigated. The best electrical properties were observed in the thickest indium-doped ZnO films; the lowest electrical resistivity was of the order of 10⁻³ Ω cm. In general, the optical transmittance value in the visible spectrum oscillated around of 87% in the thinnest films. The structural and morphological properties of ZnO:Ga and ZnO:Al films are similar, as in both cases the (0 0 2) orientation is dominant on the rest of the peaks, and both surfaces have a rough appearance. In the case of ZnO:In films, the (1 0 1) was the preferential growth orientation, and the surfaces seem to be smoother than the corresponding ZnO:Ga and ZnO:Al films.     

Facile fabrication and optimization of bowl-like ZnO/CdS nano-composite thin films with hierarchical nanopores and nano-cracks for high-performance photoelectrochemistry

A special nano-structured composite ZnO/CdS thin film with hierarchical nanopores and nano-cracks has been synthesized by a facile two-step method for the first time, in which both loadings of ZnO and CdS are optimized. We first fabricated the hierarchical nanoporous ZnO thin film through rapid gas/liquid interface assembly and layer-by-layer transfers of bowl-like ZnO nanoparticles for thirteen times. The ZnO nanobowls are prepared by a simple solution chemical reaction without using any templates. After annealing, the assembled ZnO film is sensitized with CdS nanoparticles by successive ionic layer adsorption and reactions for six cycles. Nano-cracks form for the ZnO/CdS nano-composite film by calcination, which is due to the different thermal expansion behavior between the ZnO film and the CdS layer. The facilely optimized ZnO/CdS films can serve as a promising photoanode in a photoelectrochemical cell, and it can generate a saturated photocurrent density as high as 7.8 mA cm^?2 at ?0.9 V (vs. Hg|Hg₂SO₄|saturated K₂SO₄) under visible light illumination of 100 mW cm^?2 in an aqueous solution of 0.5 M Na₂S, corresponding to a solar-to-electricity conversion efficiency of 6.6%.     

Optical transmittance and photoconductivity studies on ZnO : Al thin films prepared by the sol–gel technique

Polycrystalline ZnO : Al thin films have been prepared by the (Sol–gel) chemical deposition method. The ZnO : Al thin films are very transparent (90%) in the near UV, VIS and IR regions. The films are oriented along the c-axis ([0 0 2] direction) in the hexagonal structure. It is known that pure ZnO thin films are not chemically stable in corrosive media, but aluminium stabilizes the ZnO system and increases its electrical conductivity. Finally, the ZnO : Al thin films are reasonably stable under storage in air and in reactive atmospheres like O₂, H₂O, H₂ or in weak acids. Dark- and photo-conductivity of the ZnO : Al films are very high (1–100 Ω⁻¹ cm⁻¹), so that they can be used as transparent conductors in solar cells or in electrochromic devices.     

Effect of composition gradient in Cu(In,Al)Se2 solar cells

Cu(In,Al)Se₂ (CIAS) thin films were prepared by a three-stage evaporation process. In this experiment, the composition ratio of Cu/(In+Al) at the end of the second stage (Cu/III_2nd) was changed from 1.1 to 1.7. The CIAS films showed an Al distribution with a V-shape profile. The valley depth of the V-shape from the surface increased with increasing the Cu/III_2nd ratio. The valleys of the V-shape for the films with the Cu/III_2nd ratio of 1.1–1.7 were located at approximately 0.3–1.0 μm from the film surface, respectively. The rms surface roughness increased from 40 nm for Cu/III_2nd=1.1 to 90 nm at Cu/III_2nd=1.3 and then saturated for greater Cu/III_2nd ratios. Solar cells with the Al/ITO/ZnO/CdS/CIAS/Mo/soda-lime glass structure were fabricated. The fill factor was seen to decrease while the product of short-circuit current and open-circuit voltage remained constant. The reverse saturation current increased when the Cu/III_2nd ratio is greater than 1.3 which is a behavior of the surface roughness. Cu/III_2nd ratios greater than 1.3 lead to the distant position of V-shape from the surface and the increase in surface roughness.     

RF sputter deposition of the high-quality intrinsic and n-type ZnO window layers for Cu(In,Ga)Se2-based solar cell applications

Transparent ZnO films were prepared by rf magnetron sputtering, and their electrical, optical, and structural properties were investigated under various sputtering conditions. Aluminum-doped n-type(n-ZnO) and undoped intrinsic-ZnO (i-ZnO) layers were deposited on a glass substrate by incorporating different targets in the same reaction chamber. The n-ZnO films were strongly affected by argon ambient pressure and substrate temperature, and films deposited at 2 mTorr and 100°C showed superior properties in resistivity, transmission, and figure of merit (FOM). The sheet resistance of ZnO film was less dependent on film thickness when the substrate was heated during deposition. These positive effects of elevated substrate temperature are presumably attributed to the rearrangement of the sputtered atoms by the heat energy. Also, the films are electrically uniform through the 5 cm×5 cm substrate. The maximum deviation in sheet resistance is less than 10%. All of the films showed strong (0 0 2) diffraction peak near 2θ =34°. The undoped i-ZnO films deposited in the mixture of argon and oxygen gases showed high transmission properties in the visible range, independent of the Ar/O₂ ratio, while resistivity rose with increased oxygen partial pressure. The Cu(In,Ga)Se₂ solar cells, incorporating bi-layer ZnO films (n-ZnO/i-ZnO) as window layer, were finally fabricated. The fabricated solar cells showed 14.48% solar efficiency under AM 1.5 conditions (100 mW/cm²).