All‐Solution‐Processed Indium‐Free Transparent Composite Electrodes based on Ag Nanowire and Metal Oxide for Thin‐Film Solar Cells

Fully solution‐processed Al‐doped ZnO/silver nanowire (AgNW)/Al‐doped ZnO/ZnO multi‐stacked composite electrodes are introduced as a transparent, conductive window layer for thin‐film solar cells. Unlike conventional sol–gel synthetic pathways, a newly developed combustion reaction‐based sol–gel chemical approach allows dense and uniform composite electrodes at temperatures as low as 200 °C. The resulting composite layer exhibits high transmittance (93.4% at 550 nm) and low sheet resistance (11.3 Ω sq^‐1), which are far superior to those of other solution‐processed transparent electrodes and are comparable to their sputtered counterparts. Conductive atomic force microscopy reveals that the multi‐stacked metal‐oxide layers embedded with the AgNWs enhance the photocarrier collection efficiency by broadening the lateral conduction range. This as‐developed composite electrode is successfully applied in Cu(In_1‐x,Ga_x)S₂ (CIGS) thin‐film solar cells and exhibits a power conversion efficiency of 11.03%. The fully solution‐processed indium‐free composite films demonstrate not only good performance as transparent electrodes but also the potential for applications in various optoelectronic and photovoltaic devices as a cost‐effective and sustainable alternative electrode.     

Versatile Nanocomposite Coatings with Tunable Cell Adhesion and Bactericidity

TiO₂‐Ag nanocomposites are known for their bactericidal effect during exposure to appropriate UV radiation. While involving hazardous radiation, and limited to accessible areas, the bactericidity of these coatings is not persistent in the absence of UV light, which impedes their commercial application. Herein it is shown that TiO₂‐Ag nanocomposites can be made highly bactericidal without the need of irradiation. Beyond this, bactericidity can even be mitigated in the presence of pre‐irradiated coatings. Biocompatibility and cell adhesion are also negligibly small for the as‐processed, non‐irradiated coatings, and become fairly high when the coatings are irradiated prior to testing. This opens the possibility to pattern the coatings into areas with high and low cell adhesion properties. Indeed by irradiating the coating through a mechanical mask it is shown that fibroblast cell adherence is sharply confined to the irradiated area. These properties are achieved using TiO₂‐Ag thin films with high silver loadings of 50 wt%. The films are processed on stainless steel substrates using solution deposition. Microstructural characterization by means of X‐ray diffraction, Raman, and X‐ray photoelectron spectroscopy, high‐resolution scanning electron microscopy, and atomic force microscopy show a highly amorphous TiO₂‐Ag_xO nanocomposite matrix with scattered silver nanoparticles. UV irradiation of the films results in the precipitation of a high density of silver nanoparticles at the film surface. Bactericidal properties of the films are tested on α‐haemolyzing streptococci and in‐vitro biocompatibility is assessed on primary human fibroblast cultures. The results mentioned above as to the tunable bactericidity and biocompatibility of the TiO₂‐Ag coatings developed herein, are amenable to silver ion release, to catalytic effects of silver nanoparticles, and to specific wettabilities of the surfaces.     

Low‐Field Magnetoresistance in La0.67Sr0.33MnO3:ZnO Composite Film

A nanocomposite film of La_0.67Sr_0.33MnO₃ (LSMO):ZnO is synthesized by depositing LSMO solution on a vertical array of ZnO nanorods grown on (0001) Al₂O₃ substrate. The magnetic behavior of the composite film differs from that of a pure LSMO film, possibly due to smaller grain size in the composite, small amount of Zn doping, or the presence of nonmagnetic ZnO phase near the LSMO grain boundaries. Magnetotransport measurements show that the low‐field magnetoresistance (LFMR) of the nanocomposite film is significantly enhanced as compared to that observed for pure LSMO film. The highest value of the LFMR of the nanocomposite film at 10 K is –23.9% with a magnetic field of 0.5 T applied parallel to the current.     

Growth of Heteroepitaxial ZnO Thin Films on GaN‐Buffered Al2O3 (0001) Substrates by Low‐Temperature Hydrothermal Synthesis at 90 °C

Heteroepitaxial ZnO films are successfully grown on nondoped GaN‐buffered Al₂O₃ (0001) substrates in water at 90 °C using a two‐step process. In the first step, a discontinuous ZnO thin film (ca. 200 nm in thickness) consisting of hexagonal ZnO crystallites is grown in a solution containing Zn(NO₃)·6 H₂O and NH₄NO₃ at ca. pH 7.5 for 24 h. In the second step, a dense and continuous ZnO film (ca. 2.5 μm) is grown on the first ZnO thin film in a solution containing Zn(NO₃)·6 H₂O and sodium citrate at ca. pH 10.9 for 8 h. Scanning electron microscopy, X‐ray diffraction, UV‐vis absorption spectroscopy, photoluminescence spectroscopy, and Hall‐effect measurement are used to investigate the structural, optical, and electrical properties of the ZnO films. X‐ray diffraction analysis shows that ZnO is a monocrystalline wurtzite structure with an epitaxial orientation relationship of (0001)[11 0]_ZnO∥(0001)[11 0]_GaN. Optical transmission spectroscopy of the two‐step grown ZnO film shows a bandgap energy of 3.26 eV at room temperature. A room‐temperature photoluminescence spectrum of the ZnO film reveals only a main peak at ca. 380 nm without any significant defect‐related deep‐level emissions. The electrical property of ZnO film showed n‐type behavior with a carrier concentration of 3.5 × 10¹⁸ cm^–3 and a mobility of 10.3 cm² V^–1 s^–1.     

Oriented Growth of Al2O3:ZnO Nanolaminates for Use as Electron‐Selective Electrodes in Inverted Polymer Solar Cells

Atomic layer deposition is used to synthesize Al₂O₃:ZnO(1:x) nanolaminates with the number of deposition cycles, x, ranging from 5 to 30 for evaluation as optically transparent, electron‐selective electrodes in polymer‐based inverted solar cells. Al₂O₃:ZnO(1:20) nanolaminates are found to exhibit the highest values of electrical conductivity (1.2 × 10³ S cm^?1; more than six times higher than for neat ZnO films), while retaining a high optical transmittance (≥80% in the visible region) and a low work function (4.0 eV). Such attractive performance is attributed to the structure (ZnO crystal size and crystal alignment) and doping level of this intermediate Al₂O₃:ZnO film composition. Polymer‐based inverted solar cells using poly(3‐hexylthiophene) (P3HT):phenyl‐C₆₁‐butyric acid methyl ester (PCBM) mixtures in the active layer and Al₂O₃:ZnO(1:20) nanolaminates as transparent electron‐selective electrodes exhibit a power conversion efficiency of 3% under simulated AM 1.5 G, 100 mW cm^?2 illumination.     

Electrochemical synthesis of a double‐layer film of ZnO nanosheets/nanoparticles and its application for dye‐sensitized solar cells

A double‐layer film, consisting of an upper layer of ZnO nanosheets and a lower layer of ZnO nanoparticles (designated as ZnONS/NP), was synthesized for the photoanode of a dye‐sensitized solar cell (DSSC) by a one‐step potentiostatic electrodeposition on a conducting fluorine‐doped tin oxide substrate at 70 °C in a solution containing zinc nitrate and sodium acetate, followed by the pyrolysis of the film at 300 °C. The growth mechanism of the double‐layer nanostructure was studied by monitoring the morphological changes at various periods of electrodeposition. The effects of the concentration of acetate anion on the morphology of the double‐layer structure were also studied. The double‐layer film of ZnONS/NP showed a better self‐established light scattering property, compared with that of a thin film of ZnO nanoparticles, prepared without acetate anion. The concentration of an acetate anion in the electrolyte for the electrodeposition of the double‐layer film, the electrodeposition period, and the period for dye adsorption were optimized for obtaining the best performance for a DSSC with a photoanode consisting of the double layer. A metal‐free dye, coded as D149, was used in this research. A conversion efficiency of 4.65% was achieved for a DSSC (0.2376 cm²) with the photoanode, consisting of the double‐layer film, under 100 mW/cm² illumination in the wavelength range of 400–800 nm. X‐ray diffraction patterns, thermo gravimetric curves, elemental analysis, scanning electron microscopic images, transmission electron microscopic image, transmission spectra, and electrochemical impedance spectra were used to explain observations. Copyright © 2012 John Wiley & Sons, Ltd.     

Solution‐Deposited Zinc Oxide and Zinc Oxide/Pentacene Bilayer Transistors: High Mobility n‐Channel,Ambipolar, and Nonvolatile Devices

A solution processed n‐channel zinc oxide (ZnO) field effect transistor (FET) was fabricated by simple dip coating and subsequent heat treatment of a zinc acetate film. The field effect mobility of electrons depends on ZnO grain size, controlled by changing the number of coatings and zinc acetate solution concentration. The highest electron mobility achieved by this method is 7.2 cm² V^?1 s^?1 with On/Off ratio of 70. This electron mobility is higher than for the most recently reported solution processed ZnO transistor. We also fabricated bilayer transistors where the first layer is ZnO, and the second layer is pentacene, a p‐channel organic which is deposited by thermal evaporation. By changing the ZnO grain size (or thickness) this type of bilayer transistor shows p‐channel, ambipolar and n‐channel behavior. For the ambipolar transistor, well balanced electron and hole mobilities are 7.6 × 10^?3 and 6.3 × 10^?3 cm² V^?1 s^?1 respectively. When the ZnO layer is very thin, the transistor shows p‐channel behavior with very high reversible hysteresis. The nonvolatile tuning function of this transistor was investigated.     

Realization of intrinsic p-type ZnO thin films by metal organic chemical vapor deposition

P-type ZnO thin films were grown on sapphire substrates with and without nitrous oxide (N₂O) by metal organic chemical vapor deposition (MOCVD). The intrinsic p-type ZnO films were achieved by controlling the Zn:O ratio in the range of 0.05–0.2 without N₂O flow. Secondary ion mass spectroscopy (SIMS) showed that the films contained little or no nitrogen (N) impurities for all samples. The p-type behavior of the samples should be due to the intrinsic acceptor-like defects V_Zn, for ZnO film grown without nitrous oxide, and N, occupying O sites as acceptors for ZnO film grown with nitrous oxide. The best p-type ZnO film has low resistivity of 0.369 Ω-cm, high carrier density of 1.62×10¹⁹ cm⁻³, and mobility of 3.14 cm²/V-s. The obtained p-type ZnO films possess a transmittance of nearly 100% in the visible region and strong near-band-edge emission.     

Properties of In-Doped ZnO Films Grown by Metalorganic Chemical Vapor Deposition on GaN(0001) Templates

We investigated the properties of indium-doped zinc oxide layers grown by metalorganic chemical vapor deposition on semi-insulating GaN(0001) templates. Specular and transparent films were grown with n-type carrier concentrations up to 1.82 × 10¹⁹ cm⁻³ as determined by Hall measurements, and all In-doped films had carrier concentrations significantly higher than that of a comparable undoped film. For low In flows, the carrier concentration increased accordingly with trimethyl-indium (TMIn) flow until a maximum carrier concentration of 1.82 × 10¹⁹ cm⁻³ was realized. For higher In flows, the carrier concentration decreased with increasing TMIn flow rate. Sheet resistance as low as 185 Ω/sq was achieved for the In-doped films, which is a significant decrease from that of a comparable undoped ZnO film. Our n-type doping studies show that In is an effective dopant for controlling the n-type conductivity of ZnO.     

Atomic Layer Deposition of Conductive Coatings on Cotton,Paper, and Synthetic Fibers: Conductivity Analysis and Functional Chemical Sensing Using “All‐Fiber” Capacitors

Conductive coatings on complex fibrous systems are attracting interest for new electronic and other functional systems. Obtaining a quantitative conductivity value for complex surface coatings is often difficult. This work describes a procedure to quantify the effective electrical conductivity of conductive coatings on non‐conductive fibrous networks. By applying a normal force orthogonal to the current and field direction, fiber/fiber contact is improved and consistent conductance values can be measured. Nylon fibers coated with an electroless silver plating shows effective conductivity up to 1950 S cm^?1, and quartz fibers coated with tungsten by atomic layer deposition (ALD) show values up to ～1150 S cm^?1. Cotton fibers and paper coated with a range of ZnO film thicknesses by ALD show effective conductivity of up to 24 S cm^?1 under applied normal force, and conductivity scaled as expected with film coating thickness. Furthermore, we use the conductive coatings to produce an “all‐fiber” metal–insulator–metal capacitor that functions as a liquid chemical sensor. The ability to reliably analyze the effective conductivity of coatings on complex fiber systems will be important to design and improve performance of similar devices and other electronic textiles structures.     

High‐Performance Amorphous Multilayered ZnO‐SnO2 Heterostructure Thin‐Film Transistors: Fabrication and Characteristics

Multilayered ZnO‐SnO₂ heterostructure thin films consisting of ZnO and SnO₂ layers are produced by alternating the pulsed laser ablation of ZnO and SnO₂ targets, and their structural and field‐effect electronic transport properties are investigated as a function of the thickness of the ZnO and SnO₂ layers. The performance parameters of amorphous multilayered ZnO‐SnO₂ heterostructure thin‐film transistors (TFTs) are highly dependent on the thickness of the ZnO and SnO₂ layers. A highest electron mobility of 43 cm²/V·s, a low subthreshold swing of a 0.22 V/dec, a threshold voltage of 1 V, and a high drain current on‐to‐off ratio of 10¹⁰ are obtained for the amorphous multilayered ZnO(1.5 nm)‐SnO₂(1.5 nm) heterostructure TFTs, which is adequate for the operation of next‐generation microelectronic devices. These results are presumed to be due to the unique electronic structure of amorphous multilayered ZnO‐SnO₂ heterostructure film consisting of ZnO, SnO₂, and ZnO‐SnO₂ interface layers.     

Relaxing the Conductivity/Transparency Trade‐Off in MOCVD ZnO Thin Films by Hydrogen Plasma

Increasing the conductivity of polycrystalline zinc oxide films without impacting the transparency is a key aspect in the race to find affordable and high quality material as replacement of indium‐containing oxides. Usually, ZnO film conductivity is provided by a high doping and electron concentration, detrimental to transparency, because of free carrier absorption. Here we show that hydrogen post‐deposition plasma treatment applied to ZnO films prepared by metalorganic low‐pressure chemical vapor deposition allows a relaxation of the constraints of the conductivity/transparency trade‐off. Upon treatment, an increase in electron concentration and Hall mobility is observed. The mobility reaches high values of 58 and 46 cm²V^?1s^?1 for 2‐μm‐ and 350‐nm‐thick films, respectively, without altering the visible range transparency. From a combination of opto‐electronic measurements, hydrogen is found, in particular, to reduce electron trap density at grain boundaries. After treatment, the values for intragrain or optical mobility are found similar to Hall mobility, and therefore, electron conduction is found to be no longer limited by the phenomenon of grain boundary scattering. This allows to achieve mobilities close to 60 cm²V^?1s^?1, even in ultra‐transparent films with carrier concentration as low as 10¹⁹ cm^?3.     

Growth of Transparent and Conductive Polycrystalline (0001)‐ZnO Films on Glass Substrates Under Low‐Temperature Hydrothermal Conditions

Flat panel display technology seems to be an ever‐expanding field developing into a multibillion dollar market. A set of technical solutions involve a transparent conducting film (TCF) that is today still dominated by indium‐tin‐oxide (ITO). In a race to find alternatives that would avoid the indium pitfalls, mainly due to its increasing price and limited natural availablity, replacement materials have been extensively investigated. This work demonstrates that by exploiting basic principles of crystal growth in geometrically constrained conditions, zinc oxide (ZnO) could easily be utilized for this purpose. ZnO layers were grown on inexpensive glass substrates via low‐temperature citrate‐assisted hydrothermal (HT) method. It was shown that in the nucleation stage the crystal growth can be efficiently controlled by spatially confined oriented growth (SCOG) mechanism to produce smooth and dense (0001) oriented polycrystalline ZnO films with superb optical properties. Our products show optical transparency of 82% and surprisingly low sheet resistance for undoped ZnO, only in the order of few 100 Ω sq^?1. We believe that a very high degree of self‐organization between the ZnO crystals in our polycrystalline films grown under controlled SCOG conditions is main reason for the highest so far reported transparency to conductivity ratio for undoped ZnO thin film ceramics.     

Spontaneous Phase Separation of Poly(3‐hexylthiophene)s with Different Regioregularity for a Stretchable Semiconducting Film

Highly regioregular (RR) poly(3‐hexylthiophene)s PHTs are known to exhibit excellent electrical properties in comparison to chemically identical but regiorandom (rr) PHTs. In this study, distinct RR (97% and 55%)‐graded PHTs are subjected to solution blending to spontaneously separate the high‐RR PHT chains from the low‐RR PHT media and develop highly conjugated nanodomains in both solution and film. In the spun‐cast blend films, the rr PHT matrix imparts sufficient deformability of the channel layer required for stretchable organic thin‐film transistors (OTFTs), compared to neat RR PHTs and blends with a deformable polymer. OTFTs including RR PHT/rr PHT blend films show excellent hole mobility (µ) values up to 0.13 cm² V^?1 s^?1, surpassing that of the best RR PHT films (0.026 cm² V^?1 s^?1) fabricated by ultrasound solution pretreatment. Furthermore, a 50% stretched RR PHT/rr PHT film maintains ≈55% of its µ value at no strain, while RR PHT films show a sudden decrease in µ even at 10% stretch. The simple blending approach imparts deformability to π‐conjugated polymer films for application in stretchable OTFTs.     

Properties of conducting zinc oxide films prepared by R.F. Sputtering

An effective method of dopant incorporation in rf sputtered ZnO film is reported. The electrical, optical and structural properties of zinc doped ZnO films are investigated. Electron mobility of∼10 cm² /V-sec and electron concentration of∼10¹⁹ cm⁻³ have been measured at room temperature. X-ray diffraction data obtained on films prepared on Corning 7059 glass show (002) peak, dominating. The high electrical conductivity and transmission makes ZnO films very attractive as a component for heterojunction solar cells.     

Spontaneous growth by sol-gel process of low temperature ZnO as cathode buffer layer in flexible inverted organic solar cells

In this study, the sol–gel method was employed to prepare zinc oxide (ZnO) thin films as cathode buffer layers for inverted organic solar cells (IOSCs). We used a low temperature sol-gel process for the synthesis of ZnO thin films, in which the molar ratio of zinc acetate dihydrate (ZAD) to ethanolamine (MEA) was varied; subsequently, using the thin films, we successfully fabricated inverted solar cells on flexible plastic substrates. A ZnO sol–gel was first prepared by dissolving ZAD and MEA in ethylene glycol monomethyl ether (EGME). The molar ratios of ZAD to MEA were set as 1:1.2, 1:1, and 1:0.8, and we investigated the characteristics of the resulting ZnO thin films. We investigated the optical transmittance, surface roughness, and surface morphology of the films. Then, we discussed the reasons about the improvement of the device efficiency. The devices were fabricated using the ZnO thin films as cathode buffer layers. The results indicated that the morphology of the thin films prepared using the ZAD to MEA ratios of 1:1 and 1:0.8 changed to a rippled nanostructure after two-step annealing. The PCE was enhanced because of the higher light absorption in the active layer caused by the nanostructure. The structure of the inverted device was ITO/ZnO/P3HT:PC₆₁BM/MoO₃/Ag. The short-circuit current densities (8.59 mA/cm² and 8.34 mA/cm²) of the devices with films prepared using the ZAD to MEA ratios of 1:1 and 1:0.8 ratios, respectively, and annealed at 125 °C were higher than that of the device containing the ZnO thin film that was annealed at 150 °C. Inverted solar cells with ZnO films that were prepared using the ZAD to MEA ratios of 1:1 and 1:0.8 and annealed at 125 °C exhibited PCEs of 3.38% and 3.30%, respectively. More than that, PCEs of the flexible device can reach up to 1.53%.     

High‐Mobility ZnO Thin Film Transistors Based on Solution‐processed Hafnium Oxide Gate Dielectrics

The properties of metal oxides with high dielectric constant (k) are being extensively studied for use as gate dielectric alternatives to silicon dioxide (SiO₂). Despite their attractive properties, these high‐k dielectrics are usually manufactured using costly vacuum‐based techniques. In that respect, recent research has been focused on the development of alternative deposition methods based on solution‐processable metal oxides. Here, the application of the spray pyrolysis (SP) technique for processing high‐quality hafnium oxide (HfO₂) gate dielectrics and their implementation in thin film transistors employing spray‐coated zinc oxide (ZnO) semiconducting channels are reported. The films are studied by means of admittance spectroscopy, atomic force microscopy, X‐ray diffraction, UV–Visible absorption spectroscopy, FTIR, spectroscopic ellipsometry, and field‐effect measurements. Analyses reveal polycrystalline HfO₂ layers of monoclinic structure that exhibit wide band gap (≈5.7 eV), low roughness (≈0.8 nm), high dielectric constant (k ≈ 18.8), and high breakdown voltage (≈2.7 MV/cm). Thin film transistors based on HfO₂/ZnO stacks exhibit excellent electron transport characteristics with low operating voltages (≈6 V), high on/off current modulation ratio (～10⁷) and electron mobility in excess of 40 cm² V^?1 s^?1.     

Enhanced infrared transmission of GZO film by rapid thermal annealing for Si thin film solar cells

Ga doped ZnO (GZO) films prepared by sputtering at room temperature were rapid thermal annealed (RTA) at elevated temperatures. With increasing annealing temperature up to 570°C, film transmission enhanced significantly over wide spectral range especially in infrared region. Hall effect measurements revealed that carrier density decreased from ∼8 × 10²⁰ to ∼ 3 × 10²⁰ cm⁻³ while carrier mobility increased from ∼15 to ∼28 cm²/Vs after the annealing, and consequently low film resistivity was preserved. Hydrogenated microcrystalline Si (µc‐Si:H) and microcrystalline Si_1‐xGe_x (µc‐Si_1‐xGe_x:H, x = 0.1) thin film solar cells fabricated on textured RTA‐treated GZO substrates demonstrated strong enhancement in short‐circuit current density due to improved spectral response, exhibiting quite high conversion efficiencies of 9.5% and 8.2% for µc‐Si:H and µc‐Si_0.9Ge_0.1:H solar cells, respectively. Copyright © 2011 John Wiley & Sons, Ltd.     

Single‐Crystalline Films of the Homologous Series InGaO3(ZnO)m Grown by Reactive Solid‐Phase Epitaxy

Single‐crystalline thin films of the homologous series InGaO₃(ZnO)_m (where m is an integer) are fabricated by the reactive solid‐phase epitaxy (R‐SPE) method. Specifically, the role of ZnO as epitaxial initiator layer for the growth mechanism is clarified. High‐temperature annealing of bilayer films consisting of an amorphous InGaO₃(ZnO)₅ layer deposited at room temperature and an epitaxial ZnO layer on yttria‐stabilized zirconia (YSZ) substrate allows for the growth of single‐crystalline film with controlled chemical composition. The epitaxial ZnO thin layer plays an essential role in determining the crystallographic orientation, while the ratio of the thickness of both layers controls the film composition.     

Characterization and Field‐Emission Properties of Needle‐like Zinc Oxide Nanowires Grown Vertically on Conductive Zinc Oxide Films

Needle‐like ZnO nanowires with high density are grown uniformly and vertically over an entire Ga‐doped conductive ZnO film at 550 °C. The nanowires are grown preferentially in the c‐axis direction. The X‐ray diffraction (XRD) θ‐scan curve shows a full width at half maximum (FWHM) value of 2°. This indicates that the c‐axes of the nanorods are along the normal direction of the substrate surface. The investigation using high‐resolution transmission electron microscopy (HRTEM) confirmed that each nanowire is a single crystal. A room‐temperature photoluminescence (PL) spectrum of the wires consists of a strong and sharp UV emission band at 380 nm and a weak and broad green–yellow band. It reveals a low concentration of oxygen vacancies in the ZnO nanowires and their high optical quality. Field electron emission from the wires was also investigated. The turn‐on field for the ZnO nanowires was found to be about 18 V μm^–1 at a current density of 0.01 μA cm^–2. The emission current density from the ZnO nanowires reached 0.1 mA cm^–2 at a bias field of 24 V μm^–1.