Solution-processed Zinc Tin Oxide Research Articles

Improvement in Switching Characteristics and Bias Stability of Solution-Processed Zinc-Tin Oxide Thin Film Transistors via Simple Low-Pressure Thermal Annealing Treatment.

In this study, we used a low-pressure thermal annealing (LPTA) treatment to improve the switching characteristics and bias stability of zinc-tin oxide (ZTO) thin film transistors (TFTs). For this, we first fabricated the TFT and then applied the LPTA treatment at temperatures of 80 °C and 140 °C. The LPTA treatment reduced the number of defects in the bulk and interface of the ZTO TFTs. In addition, the changes in the water contact angle on the ZTO TFT surface indicated that the LPTA treatment reduced the surface defects. Hydrophobicity suppressed the off-current and instability under negative bias stress because of the limited absorption of moisture on the oxide surface. Moreover, the ratio of metal-oxygen bonds increased, while the ratio of oxygen-hydrogen bonds decreased. The reduced action of hydrogen as a shallow donor induced improvements in the on/off ratio (from 5.5 × 103 to 1.1 × 107) and subthreshold swing (8.63 to V·dec-1 and 0.73 V·dec-1), producing ZTO TFTs with excellent switching characteristics. In addition, device-to-device uniformity was significantly improved because of the reduced defects in the LPTA-treated ZTO TFTs.

Dual Light Temporal Coding Modes Enabled by Nanoparticle-Mediated Phototransistors via Gate Bias Modulation for Brain-Inspired Visual Perception.

The core integration and cooperation of the retina, neurons, and synapses in the visual systems enable humans to effectively sense and process visual information with low power consumption. To mimic the human visual system, an artificial sensory nerve, along with optical sensing─a paired-pulse ratio (PPR) of the light pulse stimulated currents─and neural coding has been developed. For performing the artificial visual perception functions, we consistently reveal the positive and negative correlations between the PPR index and light pulse time interval by applying two consecutive light stimuli with gate voltages of -10 and 5 V, respectively, to a phototransistor. This phototransistor contains a heterostructured channel layer composed of zinc-oxide nanoparticles (ZnO NPs) interconnected with a solution-processed zinc-tin oxide (ZTO) film. The oxygen adsorption and desorption on the ZnO NP surface under light illumination are responsible for the positive-sloped PPR; the electron trapping effect at the ZnO NP/SiO2 interface is attributed to the negative-sloped PPR. The various accountable light power densities and number of surface trap states are considered to be directly realizing these spike-timing interval-dependent characteristics. The actual benefit of these characteristics is the dual temporal coding modes based on multiplicative operation using a ZTO/ZnO NP phototransistor realized via the active gate voltage modulation. The contrary tendency of the PPR index and temporal coding─a major biological neural coding─is well demonstrated by the potential of ZTO/ZnO NP phototransistors to be implemented in sensor networks for an artificial visual perception.

Importance of Solvent Evaporation Temperature in Pre-Annealing Stage for Solution-Processed Zinc Tin Oxide Thin-Film Transistors

We focused on the importance of solvent evaporation governed by the temperature of the pre-annealing stage (TS) in solution-processed zinc tin oxide (ZTO) thin-film transistors (TFTs). We controlled TS based on the boiling point (BP) of the solvent used. When TS reaches the BP, the field effect mobility is found to be about 1.03 cm2/V s, which is 10 times larger than the TS < Bp case (0.13 cm2/V s). The reason is presumed to be that residual organic defects are effectively removed as TS increases. In addition, when Ts is beyond Bp, the mobility is rather decreased due to structural defects such as pores and pinholes. Based on our results, it is noted that TS plays a significant role in the enhancement of electrical performance and stability of solution-processed ZTO TFTs.

Importance of Structural Relaxation on the Electrical Characteristics and Bias Stability of Solution-Processed ZnSnO Thin-Film Transistors.

Effect of structural relaxation (SR) on the electrical characteristics and bias stability of solution-processed zinc–tin oxide (ZTO) thin-film transistors (TFTs) were systematically investigated by controlling the annealing time of the ZTO semiconductor films. Note that SR was found to increase with increased annealing time. Due to the increased SR, the ratio of oxygen vacancies (VO) increased from 21.5% to 38.2%. According to increased VO, the mobility in the saturation region was exhibited by a sixfold increase from 0.38 to 2.41 cm2 V−1 s−1. In addition, we found that the threshold voltage negatively shifted from 3.08 to −0.95 V. Regarding the issue of bias stability, according to increased SR, positive-bias stress of the ZTO TFTs was enhanced, compared with reverse features of negative-bias stress. Our understanding is expected to provide a basic way to improve the electrical characteristics and bias stability of rare-metal-free oxide semiconductor TFTs, which have not been sufficiently studied.

Study of Self-Patterned Zinc Tin Oxide Thin-Film Transistors Prepared from Photocurable Precursor Solution with Photoacid Generator

Solution-processed zinc tin oxide (ZTO) thin-film transistors (TFTs) have great potential uses in next-generation wearable and flexible electronic products. Zinc and tin precursor materials are naturally abundant and have low fabrication costs. To integrate a single ZTO TFT into logic circuits including inverters, NAND, and NOR gates will require the development of a facile patterning process to replace conventional and complicated photolithography techniques which are usually time-consuming and toxic. In this study, self-patterned ZTO thin films were prepared using a photo-patternable precursor solution including a photoacid generator, (4-methylthiophenyl)methyl phenyl sulfonium triflate. Solution-processed ZTO precursor films fabricated with the photoacid generator were successfully micropatterned by UV exposure, and transitioned to a semiconducting ZTO thin film by heat treatment. The UV-irradiated precursor films became insoluble in developing solvent as the generated proton from the photoacid generator affected the metal-containing ligand and changed the solubility of the metal oxide precursors. The resulting ZTO thin films were utilized as the active layers of n-type TFTs, which exhibited a typical n-type transfer, and output characteristics with appropriate threshold voltage, on/off current ratio, and field-effect mobility. We believe that our work provides a convenient solution-based route to the fabrication of metal-oxide semiconductor patterns.

Effects of thermal annealing time and molar ratio of channel layers on solution-processed ZnO/SnO2 thin-film transistors

In solution-processed zinc tin oxide (ZTO) dual-active-layer (DAL) thin-film transistors (TFTs), a thermal limitation exists whereby the ZTO channel layer requires a high annealing temperature—above 470 °C—to achieve stable performance. This disadvantage has been overcome by applying ZnO/SnO2 channel structures and additional annealing methods. However, these methods are expensive due to equipment requirements. Therefore, we aimed at lowering the annealing temperature of solution-processed ZnO/SnO2 TFTs by varying the molar ratio and annealing conditions of the channel layers. The optimized TFTs were fabricated at an annealing temperature of 350 °C without employing additional annealing methods. The fabricated TFTs showed superior and more stable performance as compared to ZTO TFTs annealed at 350 °C. The electrical characteristics of the fabricated ZnO/SnO2 TFTs included a saturation mobility (µsat) of 3.04 cm2/V·s, an on-off-current ratio (ION/OFF) of 1.41 × 106, a threshold voltage (Vth) of 3.04 V, and a subthreshold swing (SS) of 1.49 V/dec. Based on X-ray photoelectron spectroscopy results, compared with the ZTO DAL channel layer, the ZnO/SnO2 channel layer showed an increased ratio of metal-oxygen bonds and a decreased ratio of metal-hydroxyl bonds in O 1s deconvolution peaks.

Solution-Processed Yttrium-Doped IZTO Semiconductors for High-Stability Thin Film Transistor Applications

In this article, solution-processed zinc-tin–oxide (ZTO), indium–zinc–tin–oxide (IZTO), and yttrium-doped indium–zinc–tin–oxide (IZTO:Y) thin film transistors (TFTs) were investigated. The results indicate that the field effect mobility of the ZTO TFTs is enhanced by indium doping, whereas leading to the degradation of stability. Y-doping IZTO TFTs were also studied aiming to improve the stability of IZTO TFTs. The IZTO:Y shows a relatively higher transmittance in the visible light region compared to the nondoped IZTO, which could be due to the decrease of shallow defect levels caused by oxygen vacancy in the bandgap. Based on the X-ray photoelectron spectroscopy analysis, the oxygen vacancy concentration of IZTO:Y thin films decreases dramatically from 36.5% to 18.5% with Y incorporation. Moreover, the stability of IZTO:Y TFTs under positive bias stress (PBS) is also significantly improved. The device with 5-mol% Y element shows a low threshold voltage shift of 0.7 V compared to 2.6 V of IZTO TFT. The improvement in stability and electrical properties of IZTO TFTs are attributed to the Y doping, which can suppress the oxygen vacancy defects. Therefore, the high stability and excellent electrical performance of Y-doped IZTO TFTs have shown great application potential in transparent devices.

Highly stable and efficient planar perovskite solar cells using ternary metal oxide electron transport layers

In planar perovskite solar cells, the electron transport layer (ETL) plays a vital role in effective extraction and transportation of photogenerated electrons from the perovskite layer to the cathode. Ternary metal oxides exhibit excellent potentials as ETLs since their electrical and optical properties are attunable through simple compositional adjustments. In this paper, we demonstrate the use of solution-processed zinc oxide (ZnO) and zinc tin oxide (ZTO) films as highly efficient ETLs for perovskite solar cells. We observe poor compatibility between ZnO and perovskite which impedes device reproducibility, stability, and performance unlike ZTO ETL devices. Furthermore, we modify the ZTO/perovskite interface by introducing a thin passivating SnO2 interlayer. The Zn1Sn1Ox/SnO2 ETL device demonstrates paramount power conversion efficiency (PCE) of 19.01% with corresponding short circuit current density (Jsc), open circuit voltage (Voc), and fill factor (FF) values of 21.93 mA cm−2, 1.10 V, and 78.82%. Moreover, the Zn1Sn1Ox/SnO2 ETL device displays superior stability, maintaining 90% of its initial PCE after 90 days in the absence of encapsulation at relative humidity of 30–40%. Enhancement in charge extraction, favourable energy alignment, and reduction in recombination sites greatly contribute to the optimal performance, stability, and reproducibility of the Zn1Sn1Ox/SnO2 ETL device.

Passivation of Thin Channel Zinc Tin Oxide TFTs Using Al2O3 Deposited by O3-Based Atomic Layer Deposition

Here, we fabricate solution processed zinc tin oxide bottom gate top contact thin film transistors (TFTs) with active layer thickness down to 6 nm by varying the solution molarity and number of layers. As-fabricated devices show hysteresis as large as 12.65 V. The hysteresis is then studied under vacuum to understand the important role of the back channel in electrical performance. After vacuum treatment, the hysteresis measured under vacuum was reduced to −0.05 V. Alumina deposited on the back channel using atomic layer deposition (ALD) is used for passivation. The TFTs passivated with an H2O-based ALD process show conductive behavior after passivation while an O3-based ALD process results in a negative threshold voltage shift dependent on the thickness of the active layer. O3-based passivated devices show good stability with a field effect mobility of 8.3 cm2V−1s−1, ${V}_{t}$ of 5.5 V, and hysteresis of 0.85 V.

A Study of Solution-Processed Zinc–Tin-Oxide Semiconductors for Thin-Film Transistors

Thin-film transistors (TFTs) fabricated using solution-processed zinc–tin-oxide (ZTO) semiconductor thin films as the channel layers are proposed in this paper. Effect of the ZTO annealing temperature on the TFT performance is studied. Significant reduction of oxygen-vacancy and chlorine residue contents in the ZTO semiconductor can be obtained when the annealing temperatures are 500 °C–700 °C. The ZTO semiconductor with the annealing process at 600 °C in air can give an optimal ZTO TFT having a low leakage current of 10−12 A and a high ${I}_{ \mathrm{\scriptscriptstyle ON}}/{I}_{ \mathrm{\scriptscriptstyle OFF}}$ ratio of 3 $\times \,\,10^{7}$ . The subthreshold swing is 0.39 V/decade corresponding to an interface trap density of $1.2\times \,\,10^{12}\,\,$ cm−2eV−1.

P‐23: Enhancement in the Mobility and the Stability of Solution‐Processed Zinc‐Tin Oxide Thin‐Film Transistors Using Alkali Metal Superoxide

We have studied how to improve the mobility and stability of solution processed zinc‐tin oxide thin‐film transistors (ZTO TFTs) simultaneously using multifunctional potassium superoxide precursor. Potassium cations in the potassium superoxide precursor acts as a shallow donor in the ZTO thin film to improve the carrier concentration (electron), which allows the potassium‐doped ZTO TFT to exhibit high mobility. Then, the anion of the precursor exists as a superoxide radical, and it showed the effect of reducing the oxygen vacancy in the process of forming the oxide thin film Consequently, potassium‐doped ZTO TFT using potassium superoxide precursor exhibited improved mobility and stability, showing an increase in the mobility from 5.11 to 8.36 cm2/Vs and a decrease in the threshold voltage shift from 4.65 to 3.36 V under a negative bias temperature illumination stress test conducted over 5,000 sec..

Patterning and passivation effects of zinc-tin-oxide thin-film transistors using an electrohydrodynamic jet printer

Solution-processed zinc-tin oxide (ZTO) thin-film transistors (TFTs) were prepared using an electrohydrodynamic (EHD) jet printer with different patterning methods, line patterns and direct patterns with mask. The printing parameters were optimized to obtain suitable patterns and mask effects in the printing process with a low viscosity precursor solution. The electrical properties of the EHD jet-printed ZTO show a mobility of 7.0 cm2/Vs, a threshold voltage of 7 V, and a subthreshold slope of 0.4 V/dec. The gate leakage current can be reduced by minimizing the overlap of active layer with source and drain electrodes without patterning the gate electrode because the patterning of the active layer more effectively controlled the gate leakage current. Blocking oxygen by passivation is the key to protect the threshold voltage shift under positive bias stress.

Dual-Functional Superoxide Precursor To Improve the Electrical Characteristics of Oxide Thin Film Transistors.

We investigated a method to simultaneously improve the mobility and reliability of solution-processed zinc tin oxide thin film transistors (ZTO TFTs) using a dual-functional potassium superoxide precursor. Potassium cations in the potassium superoxide (KO2) precursor act as carrier suppliers in the ZTO thin film to improve the carrier (electron) concentration, which allows the potassium-doped ZTO TFT to exhibit high mobility. The anions in the precursor exist as superoxide radicals that reduce oxygen vacancies during the formation of thin oxide film. Consequently, the KO2-treated ZTO TFTs exhibited improved mobility and reliability compared with pristine ZTO TFTs, with an increase in field effect mobility from 5.57 to 8.74 cm2/V s and a decrease in the threshold voltage shift from 7.18 to 3.85 V, after a positive bias temperature stress test conducted over 5000 s.

The effects of localized tail states on charge transport mechanisms in amorphous zinc tin oxide Schottky diodes

Temperature-dependent current–voltage measurements were performed on vertical Schottky diodes made with solution-processed amorphous zinc tin oxide (a-ZTO) semiconductor and palladium rectifying contacts. Above 260 K, forward bias electron transport occurs via thermionic emission over an inhomogeneous, voltage-dependent Schottky barrier with = 0.72 eV, σ0 = 0.12 eV, and A* = 44 A cm−2 K−2, where and are the mean potential barrier and its standard deviation at zero bias, respectively, and A* is Richardson’s constant. For large currents, the series ohmic resistance of the bulk semiconductor dominates. At temperatures below 260 K, less carriers are excited from localized states below the conduction band edge, and space-charge-limited current (SCLC) dominates. The exponential tail density of states parameters extracted for a-ZTO are gtc = 1.34 × 1019 cm−3 eV−1 and kTt = 26 meV. The intermediate tail state density in a-ZTO, less than that of amorphous silicon and greater than that of amorphous indium gallium zinc oxide, allows for experimental observation of a temperature-dependent transition of bulk charge transport mechanisms in strong forward bias from semiconductor-like ohmic conduction near room temperature to insulator-like SCLC at lower temperatures. In reverse bias, the same tail states lead to modified Poole–Frenkel emission, reducing the leakage current. The frequency response of a half-wave rectifier and diode impedance spectroscopy confirm that the Schottky diode cut-off frequency is above 1 MHz.

Fabrication of solution-processed nitrogen-doped niobium zinc tin oxide thin film transistors using ethanolamine additives

In this study, we fabricated the nitrogen-doped NbZnSnO channel layers by using the sol-gel method. Monoethanolamine (MEA) was used as a nitrogen additive. From the XPS results, the concentration of oxygen vacancies changes as a function of MEA/Nb ratio. A NbZnSnO film with MEA/Nb of 0.2 shows the lowest amount of oxygen vacancies. TFT electrical performance also shows a device with an MEA/Nb ratio of 0.2 possesses a high carrier mobility (7.4 cm2 V−1s−1) and good bias stress stability. In addition, we also investigated the effect of the aging time of precursor solution on the electrical characteristics of the TFT. After adding MEA, the annealing temperature of the NbZnSnO channels can be reduced, pertaining to the acceleration of the hydrolysis and condensation reaction.

Stable and fast switching of liquid crystals on solution-derived compound oxide films irradiated by ion beam

We fabricated stable, homogeneous, fast-switching liquid crystal cells on solution-processed zinc tin oxide films treated by ion-beam irradiation >1200 eV. The ion-beam-irradiated zinc tin oxide films were amorphous and demonstrated good transparency. X-ray photoelectron spectroscopy analysis was used to determine the liquid crystal alignment mechanism with respect to the ion-beam-irradiated zinc tin oxide film. The total response time of electrically controlled birefringence-liquid crystal cells on 2200-eV ion-beam-irradiated zinc tin oxide films was 5.04 ms; the threshold voltage was reduced by 15% compared with rubbed polyimide. Thus, ion-beam-irradiated zinc tin oxide films showed great potential as an alignment layer for high-performance electrically controlled birefringence-liquid crystal cells in industrial applications.

Toward an Understanding of Thin-Film Transistor Performance in Solution-Processed Amorphous Zinc Tin Oxide (ZTO) Thin Films.

Amorphous zinc tin oxide (ZTO) thin films are accessible by a molecular precursor approach using mononuclear zinc(II) and tin(II) compounds with methoxyiminopropionic acid ligands. Solution processing of two precursor solutions containing a mixture of zinc and tin(II)-methoxyiminopropinato complexes results in the formation of smooth homogeneous thin films, which upon calcination are converted into the desired semiconducting amorphous ZTO thin films. ZTO films integrated within a field-effect transistor (FET) device exhibit an active semiconducting behavior in the temperature range between 250 and 400 °C, giving an increased performance, with mobility values between μ = 0.03 and 5.5 cm2/V s, with on/off ratios increasing from 105 to 108 when going from 250 to 400 °C. Herein, our main emphasis, however, was on an improved understanding of the material transformation pathway from weak to high performance of the semiconductor in a solution-processed FET as a function of the processing temperature. We have correlated this with the chemical composition and defects states within the microstructure of the obtained ZTO thin film via photoelectron spectroscopy (X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy), Auger electron spectroscopy, electron paramagnetic resonance spectroscopy, atomic force microscopy, and photoluminescence investigations. The critical factor observed for the improved performance within this ZTO material could be attributed to a higher tin concentration, wherein the contributions of point defects arising from the tin oxide within the final amorphous ZTO material play the dominant role in governing the transistor performance.

Comparative Study of Antimony Doping Effects on the Performance of Solution-Processed ZIO and ZTO Field-Effect Transistors.

ZnO-based oxide films are emerging as high-performance semiconductors for field-effect transistors (FETs) in optoelectronics. Carrier mobility and stability in these FETs are improved by introducing indium (In) and gallium (Ga) cations, respectively. However, the strong trade-off between the mobility and stability, which come from In or Ga incorporation, still limits the widespread use of metal oxide FETs in ultrahigh pixel density and device area-independent flat panel applications. We demonstrated that the incorporation of antimony (Sb) cations in amorphous zinc indium oxide (ZIO) simultaneously enhanced the field-effect mobility (μFET) and electrical stability of the resulting Sb-doped ZIO FETs. The rationale for the unexpected synergic effect was related to the unique electron configuration of Sb5+ ([Kr]4d105s05p0). However, the benefit of Sb doping was not observed in the zinc tin oxide (ZTO) system. All the Sb-doped ZTO FETs suffered from a reduction in μFET and a deterioration of gate bias stress stability with an increase in Sb loading. This can be attributed to the formation of heterogeneous defects due to Sb-induced phase separation and the creation of Sb3+ induced acceptor-like trap states.

Low-Temperature Solution-Processed Zinc Tin Oxide Film as a Cathode Interlayer for Organic Solar Cells.

In this study, Sn-doped ZnO (ZTO) is prepared by a sol-gel method and is employed as an electron transport material for organic solar cells (OSCs). After Sn modification, the fabricated ZTO films exhibited better charge transport properties and smoother surface morphology, especially for those processed at a low temperature of 120 °C. By incorporation of the high-temperature (200 °C) processed ZTO films, inverted OSCs showed the highest power conversion efficiency (PCE) of 9.32%, which is higher than those based on the same temperature processed ZnO films. For the devices based on the low-temperature processed ZTO, a high PCE over 9.0% with long-term stability was achieved, which is much better than those based on the same temperature processed ZnO (8.46% PCE). Here, the ZTO films can be fabricated without high-temperature annealing, demonstrating their great potential as electron transport layers for efficient flexible OSCs.

Effect of relative humidity and pre-annealing temperature on spin-coated zinc tin oxide films made via the metal–organic decomposition route

Solution-processed zinc tin oxide thin-film transistor using metal–organic decomposition achieves wide process window in terms of ambient humidity during deposition.