Solution-processed Zinc Oxide Thin Film Research Articles

Effects of Hydrogen Plasma Treatment on the Electrical Behavior of Solution-Processed ZnO Thin Films.

In this study, the effect of atmospheric hydrogen plasma treatment on the in-plane conductivity of solution-processed zinc oxide (ZnO) in various environments is reported. The hydrogen-plasma-treated and untreated ZnO films exhibited ohmic behavior with room-temperature in-plane conductivity in a vacuum. When the untreated ZnO film was exposed to a dry oxygen environment, the conductivity rapidly decreased, and an oscillating current was observed. In certain cases, the thin film reversibly 'switched' between the high- and low-conductivity states. In contrast, the conductivity of the hydrogen-plasma-treated ZnO film remained nearly constant under different ambient conditions. We infer that hydrogen acts as a shallow donor, increasing the carrier concentration and generating oxygen vacancies by eliminating the surface contamination layer. Hence, atmospheric hydrogen plasma treatment could play a crucial role in stabilizing the conductivity of ZnO films.

Tailored β-diketones as effective surface passivation for solution processed zinc oxide thin film transistors

Diketone passivation of defect states at the surface of solution-processed zinc oxide thin film transistors is investigated. Trifluoroacetylacetophenone is used as the basis for passivating molecules due to its easy chemical variability on the phenyl ring. Electron withdrawing or donating substituents can be introduced at the phenyl ring to vary the electron density at the ketone anchoring positions of the passivating molecule. The relative change of electron density is quantified by DFT calculations for different substituents. Experimentally thin zinc oxide films are prepared by aqueous spray pyrolysis, while passivating diketones are deposited in vacuum on the ZnO surface. XPS and AFM measurements confirm the successful deposition of passivating molecules at the zinc oxide surface. Electrical characterization of passivated zinc oxide transistors is carried out for different exposure times. All diketones improve transistor stability for positive and negative bias stress. The passivated transistors show improved electrical properties with increased electron mobility and decreased disorder parameter. Finally, threshold voltages shift systematically to negative values and hysteresis clearly reduces. The experimentally determined electrical properties of passivated zinc oxide transistors show a systematic correlation to the effective electron density at the oxygen anchor atoms of diketones and higher charge results in a better performance. • 1,3-diketone molecules as passivation with adjustable effective charge at oxygen. • Chemical passivation of zinc oxide surface defects. • High electron density at oxygen anchor atoms improves transistor performance best.

Opto-electronic properties of solution-processed zinc oxide thin films: role of solvents and doping

Undoped zinc oxide (ZnO) and nitrogen-doped zinc oxide (NZO) thin films were prepared on transparent conducting oxide-coated glass substrates by employing sol–gel technique. The effect of different solvents and nitrogen doping on the optical, structural, and electrical properties was investigated by UV–visible absorption spectroscopy, atomic force microscopy (AFM), X-ray diffraction (XRD), profilometry, and Hall effect studies. ZnO films yielded transmittance above 85% and the bandgap of ZnO thin films decreased with doping. XRD pattern confirmed hexagonal wurtzite structure of ZnO. NZO thin films were found to be in the nano-thin film phase with thickness of 40 nm. Hall effect studies yielded carrier concentration of 1.2 × 1015 cm−3 and 2.03 × 1014 cm−3, respectively, for undoped and doped ZnO thin films. The changes in vibrational modes of ZnO due to nitrogen doping were detected using Fourier transform infrared (FTIR) analysis. It was found that p-type doping, leading to an improved surface morphology, led to a reduction in optical bandgap and an increased charge carrier mobility. The choice of the solvent was found to have a profound influence on the surface morphology, optical bandgap, tail states distribution, and charge carrier mobility.

Bipolar resistive switching, synaptic plasticity and non-volatile memory effects in the solution-processed zinc oxide thin film

The memristive devices are getting a lot of interest in recent years due to its applications in the field of resistive memory and brain-inspired computing. In the present work, we have developed a ZnO memristive device using the reflux method. We have demonstrated non-volatile memory properties and mimicked the basic synaptic properties of using Al/ZnO/FTO thin film device. The structural, morphological and compositional studies of ZnO thin film were carried out by X-ray diffraction, scanning electron microscopy and X-ray photoelectron spectroscopy, respectively. The co-existence of analog and digital resistive switching in a ZnO memristive device was achieved by properly tuning the external electrical stimulus. The developed ZnO memristive device mimics the basic synaptic properties such as potentiation-depression, symmetric Hebbian and antisymmetric Hebbian learning rules at a lower voltage bias (±1 V to ± 3 V). Whereas, non-volatile memory properties (endurance and retention) were achieved at a higher voltage bias (±4 V to ± 6 V). The Al/ZnO/FTO thin film memory device shows good resistive switching memory properties such as 104 endurance cycles and 104 s retention period with good uniformity during the cycle to cycle operation. The detailed analysis of I–V results suggested that the Schottky conduction model is responsible for the analog mode of operation, whereas, space charge limited current governs the device dynamics during the digital mode of operation. A possible resistive switching model is also presented.

On the UV-light-induced Desorption/Adsorption Mechanism of Atmospheric Species in Solution-processed Zinc Oxide Thin Films

AbstractZinc oxide (ZnO) is a n-type transparent semiconductor which can be processed by low cost techniques (such as spray-pyrolysis and spin-coating) and can be applied as the active layer of thin-films transistors (TFTs). The electrical properties of ZnO films are strongly affected when the device is exposed to room conditions and/or UV-light, suggesting possible applications as UV or/and gas sensors. Atmospheric oxygen molecules adsorbed on ZnO surface act as charge traps, decreasing the material conductivity. The incidence of UV-light causes an increase of the material conductivity due to the photogeneration of electron-hole pairs via direct band-to-band transitions (classic photoconductivity process) and due to the desorption of oxygen molecules, which presents a relatively slower response and is a less understood mechanism. In the current paper, we study the influence of environmental parameters, such as temperature, humidity and UV-light intensity, on the electrical properties of spin-coated ZnO thin films to understand the role of the desorption mechanism on the photoconductivity process. The analysis of the device current vs. time curves shows the existence of two light-induced desorption mechanisms: i) one which increases the electrical conductivity of the ZnO film (desorption-like process) and ii) a second one which decreases the conductivity (adsorption-like process). A Plackett-Burman design of experiment (DOE) was used to study the influence of characterization factors like UV intensity, temperature and humidity on electrical parameters obtained from the experimental curves. We observed that the desorption-like process is a first order mechanism, exhibiting desorption rate proportional to n(t), where n(t) represents the adsorbate concentration as a function of the time, whereas the adsorption-like mechanism exhibits a desorption rate proportional to the forth power of n(t).

Low-Concentration Indium Doping in Solution-Processed Zinc Oxide Films for Thin-Film Transistors.

We investigated the influence of low-concentration indium (In) doping on the chemical and structural properties of solution-processed zinc oxide (ZnO) films and the electrical characteristics of bottom-gate/top-contact In-doped ZnO thin-film transistors (TFTs). The thermogravimetry and differential scanning calorimetry analysis results showed that thermal annealing at 400 °C for 40 min produces In-doped ZnO films. As the In content of ZnO films was increased from 1% to 9%, the metal-oxygen bonding increased from 5.56% to 71.33%, while the metal-hydroxyl bonding decreased from 72.03% to 9.63%. The X-ray diffraction peaks and field-emission scanning microscope images of the ZnO films with different In concentrations revealed a better crystalline quality and reduced grain size of the solution-processed ZnO thin films. The thickness of the In-doped ZnO films also increased when the In content was increased up to 5%; however, the thickness decreased on further increasing the In content. The field-effect mobility and on/off current ratio of In-doped ZnO TFTs were notably affected by any change in the In concentration. Considering the overall TFT performance, the optimal In doping concentration in the solution-processed ZnO semiconductor was determined to be 5% in this study. These results suggest that low-concentration In incorporation is crucial for modulating the morphological characteristics of solution-processed ZnO thin films and the TFT performance.

Optical Reflectivity of Spin-Coated Multilayered ZnO and Al:ZnO Thin Films

Zinc oxide (ZnO) and its doped counterparts such as Al:ZnO are extensively investigated as indium-free alternatives for oxide electronics including solar cells, light emitting diodes and also display technologies. For the latter application, thin film transistor (TFT) devices are the building-block structures. High fidelity TFTs require conductive channel materials, with good field effect mobility for majority carriers and tunable optical transparency. Controlling growth, doping, crystallization, and thickness of thin films of these materials is required in order to improve and control physical properties, primarily electronic conductivity and optical transparency. With the advent of flexible electronics and curved TFT-based display panels, low cost, solution-processed methods are important and provide scalable coating methods on a range of substrates. This work demonstrates the changes to the morphology, crystalline structure, optical reflectivity and electrical conductance of solution-processed ZnO thin films by the inclusion of an aluminium dopant during spin-coating. The is work also determines the compositional chemical state of the Al:ZnO structures compared to ZnO using X-ray photoelectron spectroscopy in conjunction with detailed X-ray diffraction and transmission electron microscopy examination of the film morphology. We also demonstrate a method of determining the optical thickness of multilayer thin films using simple, non-destructive angle-resolved reflectance measurements. Using optical interference, the optical thickness of the multi-layered deposited ZnO and Al:ZnO can be determined and show good agreement with the thicknesses measured by transmission electron microscopy of electron transparent lamellar cross-sections. We also show the visible and near-infra red (VIS-NIR) light spectroscopy of ZnO and Al:ZnO multi-layered thin film structures grown on oxidized silicon substrates also define the growth conditions and processing to provide tunable antireflection coatings of ZnO and AZO.

Solution-processed ZnO thin films for low voltage and low temperature application in flexible resistive random access memory

The demand for flexible resistive random access memories (ReRAM) has increased because wearable devices, such as smart watches, e-paper, and flexible displays, have become a trend. These devices have advantages such as portability, light weight, and user-friendly interfaces, which are better than the currently available rigid electronic systems. A flexible nonvolatile memory is an essential part of electronic systems. In this study, zinc oxide (ZnO) thin films were used for fabricating ReRAM devices. The fabrication was achieved by a simple solution processing method and low temperature annealing (approximately 100°C) on a hot plate. In addition, the devices fabricated on polyimide substrates showed excellent operational characteristics with a high ratio (≥104) of the high resistance state to the low resistance state. The operational voltage of the ZnO ReRAM devices was <2V, and the reset operational voltage was less than −1V.

Synthesis of Highly Concentrated ZnO Nanorod Sol by Sol-gel Method and their Applications for Inverted Organic Solar Cells

The effects of the zinc oxide (ZnO) preparing process on the performance of inverted organic photovoltaic cells (OPVs) were explored. The morphology and size of ZnO nanoparticles were controlled, leading to more efficient charge collection from device and higher electron mobility compared with nanospheres. Nanosized ZnO particles were synthesized by using zinc acetate dihydrate and potassium hydroxide in methanol. Also, water was added into the reac- tion medium to control the morphology of ZnO nanocrystals from spherical particles to rods, and NH 4 OH was used to prevent the gelation of dispersion. Solution-processed ZnO thin films were deposited onto the ITO/glass substrate by using spin coating process and then ZnO films were used as an electron transport layer in inverted organic photovoltaic cells. The analyses were carried out by using TEM, FE-SEM, AFM, DLS, UV-Vis spectroscopy, current density-voltage characteristics and solar simulator.

High-mobility solution-processed zinc oxide thin films on silicon nitride

A high effective electron mobility of 33 cm2 V–1 s–1 was achieved in solution-processed undoped zinc oxide (ZnO) thin films. The introduction of silicon nitride (Si3N4) as growth substrate resulted in a mobility improvement by a factor of 2.5 with respect to the commonly used silicon oxide (SiO2). The solution-processed ZnO thin films grown on Si3N4, prepared by low-pressure chemical vapor deposition, revealed bigger grain sizes, lower strain and better crystalline quality in comparison to the films grown on thermal SiO2. These results show that the nucleation and growth mechanisms of solution-processed films are substrate dependent and affect the final film structure accordingly. The substantial difference in electron mobilities suggests that, in addition to the grain morphology and crystalline structure effects, defect chemistry is a contributing factor that also depends on the particular substrate. In this respect, interface trap densities measured in high-κ HfO2/ZnO MOSCAPs were about ten times lower in those fabricated on Si3N4 substrates. (© 2014 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)

Improved Electrical and Thermal Stability of Solution-Processed Li-Doped ZnO Thin-Film Transistors

The effects of lithium (Li) doping on the performance of solution-processed zinc oxide (ZnO) thin-film transistors (TFTs) grown using the sol-gel method are investigated. Li-doped ZnO films as channel layers are chemically prepared by spin coating the gel with an aqueous solution of zinc acetate dihydrate, Li nitrate, and ethanolamine. TFT devices fabricated with 2 at.% Li-doped films show a good field-effect mobility of 3.31 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /V·s, a subthreshold slope of 0.82 V/dec, and an on-off current ratio of over 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> . These TFT devices also show good thermal and electrical stability, which is mainly attributed to the compact surface morphology of the channel layer, small carrier concentration, and less oxygen deficiency, which reduces the interface electrical trapping at the gate insulator.

Inverted polymer solar cells with a solution-processed zinc oxide thin film as an electron collection layer

A solution-processed zinc oxide (ZnO) thin film as an electron collection layer for polymer solar cells (PSCs) with an inverted device structure was investigated. Power conversion efficiencies (PCEs) of PSCs made with a blend of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) are 3.50% and 1.21% for PSCs with and without the ZnO thin film, respectively. Light intensity dependence of the photocurrent and the capacitance-voltage measurement demonstrate that the increased PCEs are due to the restriction of the strong bimolecular recombination in the interface when a thin ZnO layer is inserted between the polymer active layer and the ITO electrode. These results demonstrate that the ZnO thin film plays an important role in the performance of PSCs with an inverted device structure.

High performance low temperature solution-processed zinc oxide thin film transistor

Amorphous zinc oxide thin films have been processed out of an aqueous solution applying a one step synthesis procedure. For this, zinc oxide containing crystalline water (ZnO⋅×H2O) is dissolved in aqueous ammonia (NH3), making use of the higher solubility of ZnO⋅×H2O compared with the commonly used zinc oxide. Characteristically, as-produced layers have a thickness of below 10nm. The films have been probed in standard thin film transistor devices, using silicon dioxide as dielectric layer. Keeping the maximum process temperature at 125°C, a device mobility of 0.25cm2V−1s−1 at an on/off ratio of 106 was demonstrated. At an annealing temperature of 300°C, the performance could be optimized up to a mobility of 0.8cm2V−1s−1.

Solution-Processed Zinc Oxide Thin Film as a Buffer Layer for Polymer Solar Cells with an Inverted Device Structure

A solution-processed zinc oxide (ZnO) thin film as a buffer layer for polymer solar cells (PSCs) with an inverted device structure has been demonstrated. A power conversion efficiency (PCE) of 3.8% was observed from an inverted device structure with the ZnO buffer layer. Without the ZnO layer, PSCs only show a PCE of 1.67%, which is less than half the value observed from PSCs with the ZnO buffer layer. When operated at room temperature, no obvious degradation was observed from the PSCs with the ZnO layer after continuously illuminating the devices for 4 h. However, a significant degradation was observed from the PSCs without the ZnO buffer layer after illuminating the devices only for 1 h. Furthermore, PSCs with the ZnO buffer layer also show very good shelf stability; only 5% degradation was observed in PCEs after 47 days. All these results demonstrate that the ZnO buffer layer plays an important role in the enhancement of PSCs’ performance with an inverted device structure.

Effects of Li doping on the performance and environmental stability of solution processed ZnO thin film transistors

We report the effects of lithium (Li) doping on the performance and environmental stability of solution processed zinc oxide (ZnO) thin film transistors (TFTs). It was found that appropriate amount of Li doping significantly reduced the background conductivity of ZnO films and also improved the orientation of ZnO crystallites along the c-axis. A highest field-effect mobility of 3.07 cm2/V s was found for the 5 at. % Li-doped ZnO TFTs. However, 15 and 25 at. % Li-doped ZnO TFTs showed good environmental stability of Ion/Ioff ratio with reasonable field-effect mobility.