Solution-processed Metal Oxide Research Articles

Effects of Environmental Water Absorption by Solution-Deposited Al2O3 Gate Dielectrics on Thin Film Transistor Performance and Mobility

In recent years, many solution-processed oxide transistors have been reported with mobility rivaling or exceeding their vacuum-deposited counterparts. Here, we show that water absorption from the environment by solution-processed dielectric materialsexplains this enhanced mobility. By monitoring the water content of Al2O3, ZrO2, and bilayer dielectric materials, we demonstrate how water absorption by the dielectric affects electrical characteristics in solution-processed metal oxide transistors. These effects, including capacitance-frequency dispersion, counterclockwise hysteresis in transfer curves, and high channel mobility, are elucidated by electron transfer between the gate/channel and trap states within the band gap of the dielectric created by the water.

Solution-processed oxide thin film transistors on shape memory polymer enabled by photochemical self-patterning

Abstract

Simple and reliable direct patterning method for carbon-free solution-processed metal oxide TFTs

Metal oxide TFT fabrication based on a solution-processing method is considered a promising alternative to conventional vacuum processing and has a number of advantages such as low cost, large-area fabrication, and process simplicity. A simple and reliable, direct patterning method for obtaining a carbon-free aqueous metal oxide film is presented herein. Patterning, which is achieved by selective photoreaction of water molecules under ultraviolet irradiation and by a safe, environment-friendly chemical etching process using a non-toxic organic acid, is followed by an annealing process at a temperature of 350 °C to obtain carbon-free metal oxide TFTs. In–Ga–Zn oxide (IGZO), TFTs on SiO2 dielectrics that were fabricated with a direct patterning method exhibited an average mobility of 4.3 cm2/V·s with good uniformity, which is comparable to TFTs formed by conventional photolithography. The TFTs exhibited stable performance with small (within 0.5 V) shifts in switch-on voltage under positive and negative bias stress. Fabrication of flexible IGZO TFTs by direct patterning was also achieved.

Effects of process variables on aqueous-based AlOx insulators for high-performance solution-processed oxide thin-film transistors

Recently, aqueous method has attracted lots of attention because it enables the solution-processed metal oxide thin film with high electrical properties in low temperature fabrication condition to various flexible devices. Focusing the development of aqueous route, many researchers are only focused on metal oxide materials. However, for expansive application of the aqueous-based metal oxide films, the systematic study of performance change with process variables for the development of aqueous-based metal oxide insulator film is urgently required. Here, we propose importance of process variables to achieve high electrical-performance metal oxide insulator based on the aqueous method. We found that the significant process variables including precursor solution temperature and humidity during the spin-coating process strongly affect chemical, physical, and electrical properties of AlOx insulators. Through the optimization of significant variables in process, an AlOx insulator with a leakage current value approximately 105 times smaller and a breakdown voltage value approximately 2–3 times greater than un-optimized AlOx was realized. Finally, by introducing the optimized AlOx insulators to solution-processed InOx TFTs, we successfully achieved InOx/AlOx TFTs with remarkably high average field-effect mobility of ∼52cm2V−1s−1 and on/off current ratio of 106 at fabrication temperature of 250°C.

(Invited) Fabrication Technique for Low-Temperature Aqueous Solution-Processed Oxide Thin-Film Transistors

Solution-processing of oxide semiconductor is a promising technique for the simple and low-cost fabrication of oxide thin-film transistors (TFTs). We demonstrate improved TFT characteristics by application of a hydrogen injection and oxidation (HIO) process to the fabrication of high-performance low-temperature oxide TFTs. The compatibility of this technique for flexible substrates was also confirmed when the HIO method was applied to solution-processed metal oxide TFTs.

Regulation of precursor solution concentration for In-Zn oxide thin film transistors

The tunable electronic performance of the solution-processed semiconductor metal oxide is of great significance for the printing electronics. In current work, transparent thin-film transistors (TFTs) with indium-zinc oxide (IZO) were fabricated as active layer by a simple eco-friendly aqueous route. The aqueous precursor solution is composed of water without any other organic additives and the IZO films are amorphous revealed by the X-ray diffraction (XRD). With systematic studies of atomic force microscopy (AFM), X-ray photoemission spectroscopy (XPS) and the semiconductor property characterizations, it was revealed that the electrical performance of the IZO TFTs is dependent on the concentration of precursor solution. As well, the optimum preparation process was obtained. The concentrations induced the regulation of the electronic performance was clearly demonstrated with a proposed mechanism. The results are expected to be beneficial for development of solution-processed metal oxide TFTs.

Solution-processed phosphorus-tungsten oxide film as hole injection layer for application in efficient organic light-emitting diode

The research on new functional layer materials and optimization of fabrication in organic light-emitting diode (OLED) has been continuously concerned. In this work, phosphotungstic acid solution used as a precursor is spin-coated onto ITO anode, and then a phosphorus-tungsten oxide (WO3-P2O5) film is obtained after annealing treatment. Various parameters which include film composition, surface morphology, work function and optical property are measured. It suggests that the film has the composition of 24 WO3·P2O5 annealed at 200 °C, which exhibits work function of 5.55 eV, surface root mean square (RMS) roughness value of 1.525 nm and optical transmittance of over 92%. The WO3-P2O5 film is used as hole injection layer (HIL), and the OLED with the structure of [ITO/ WO3-P2O5/ TPD/ Alq3/ LiF/ Al] is fabricated. The performance of device shows turn-on voltage of 2.6 V, maximum luminance of 13553 cd/m2, maximum current efficiency of 5.87 cd/A and maximum power efficiency of 3.44 lm/W, respectively. The results provide a new way to deposit HIL based on solution-processed metal oxide in efficient OLED.

Solution Processed Metal Oxide High-κ Dielectrics for Emerging Transistors and Circuits.

The electronic functionalities of metal oxides comprise conductors, semiconductors, and insulators. Metal oxides have attracted great interest for construction of large-area electronics, particularly thin-film transistors (TFTs), for their high optical transparency, excellent chemical and thermal stability, and mechanical tolerance. High-permittivity (κ) oxide dielectrics are a key component for achieving low-voltage and high-performance TFTs. With the expanding integration of complementary metal oxide semiconductor transistors, the replacement of SiO2 with high-κ oxide dielectrics has become urgently required, because their provided thicker layers suppress quantum mechanical tunneling. Toward low-cost devices, tremendous efforts have been devoted to vacuum-free, solution processable fabrication, such as spin coating, spray pyrolysis, and printing techniques. This review focuses on recent progress in solution processed high-κ oxide dielectrics and their applications to emerging TFTs. First, the history, basics, theories, and leakage current mechanisms of high-κ oxide dielectrics are presented, and the underlying mechanism for mobility enhancement over conventional SiO2 is outlined. Recent achievements of solution-processed high-κ oxide materials and their applications in TFTs are summarized and traditional coating methods and emerging printing techniques are introduced. Finally, low temperature approaches, e.g., ecofriendly water-induced, self-combustion reaction, and energy-assisted post treatments, for the realization of flexible electronics and circuits are discussed.

Electron mobility enhancement in solution-processed low-voltage In2O3 transistors via channel interface planarization

The quality of the gate dielectric/semiconductor interface in thin-film transistors (TFTs) is known to determine the optimum operating characteristics attainable. As a result in recent years the development of methodologies that aim to improve the channel interface quality has become a priority. Herein, we study the impact of the surface morphology of three solution-processed high-k metal oxide dielectrics, namely AlOx, HfOx, and ZrOx, on the operating characteristics of In2O3 TFTs. Six different dielectric configurations were produced via single or double-step spin-casting of the various precursor formulations. All layers exhibited high areal capacitance in the range of 200 to 575 nF/cm2, hence proving suitable, for application in low-voltage n-channel In2O3 TFTs. Study of the surface topography of the various layers indicates that double spin-cast dielectrics exhibit consistently smoother layer surfaces and yield TFTs with improved operating characteristics manifested, primarily, as an increase in the electron mobility (µ). To this end, µ is found to increase from 1 to 2 cm2/Vs for AlOx, 1.8 to 6.4 cm2/Vs for HfOx, and 2.8 to 18.7 cm2/Vs for ZrOx-based In2O3 TFTs utilizing single and double-layer dielectric, respectively. The proposed method is simple and potentially applicable to other metal oxide dielectrics and semiconductors.

Improvement in Electrical Characteristics of Eco-friendly Indium Zinc Oxide Thin-Film Transistors by Photocatalytic Reaction.

Eco-friendly solution-processed oxide thin-film transistors (TFTs) were fabricated through photocatalytic reaction of titanium dioxide (PRT). The titanium dioxide (TiO2) surface reacts with H2O under ultraviolet (UV) light irradiation and generates hydroxyl radicals (OH•). These hydroxyl radicals accelerate the decomposition of large organic compounds such as 2-methoxyethanol (2ME; one of the representative solvents for solution-processed metal oxides), creating smaller organic molecular structures compared with 2ME. The decomposed small organic materials have low molar masses and low boiling points, which help improving electrical properties via diminishing defect sites in oxide channel layers and fabricating low-temperature solution-processed oxide TFTs. As a result, the field-effect mobility improved from 4.29 to 10.24 cm2/V·s for IGZO TFTs and from 2.78 to 7.82 cm2/V·s for IZO TFTs, and the Vth shift caused by positive bias stress and negative bias illumination stress over 1000 s under 5700 lux decreased from 6.2 to 2.9 V and from 15.3 to 2.8 V, respectively. In theory, TiO2 has a permanent photocatalytic reaction; as such, hydroxyl radicals are generated continuously under UV irradiation, improving the electrical characteristics of solution-processed IZO TFTs even after four iterations of TiO2 recycling in this study. Thus, the PRT method provides an eco-friendly approach for high-performance solution-processed oxide TFTs.

P-1.10: Solution-processed metal oxide semiconductors fabricated with oxygen radical assisting perchlorate aqueous precursors through a new low-temperature reaction route

P-1.10: Solution-processed metal oxide semiconductors fabricated with oxygen radical assisting perchlorate aqueous precursors through a new low-temperature reaction route

Recent Advances of Solution-Processed Metal Oxide Thin-Film Transistors.

Solution-processed metal oxide thin-film transistors (TFTs) are considered as one of the most promising transistor technologies for future large-area flexible electronics. This work surveys the recent advances in solution-processed metal oxide TFTs, including n-type oxide semiconductors, oxide dielectrics, and p-type oxide semiconductors. We first deliver a review on the history and present status of metal oxide TFTs. Then, we present the recent progress in solution-processed n-type oxide semiconductors, with a special focus on low-temperature and large-area solution-based approaches as well as emerging nondisplay applications. Next, we give a detailed analysis of the state-of-the-art solution-processed oxide dielectrics for low-power electronics. We further discuss the recent advances in solution-based p-type oxide semiconductors, which will enable the highly desirable future low-cost large-area complementary circuits. Finally, we draw conclusions and outline the perspectives over the research field.

Solution processed transition metal oxide anode buffer layers for efficiency and stability enhancement of polymer solar cells

Polymer solar cells were fabricated with solution-processed transition metal oxides, MoO3 and V2O5 as anode buffer layers (ABLs). The optimized device with V2O5 ABL exhibited considerably higher power conversion efficiency (PCE) compared to the devices based on MoO3 and poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) ABLs. The space charge limited current measurements and impedance spectroscopy results of hole-only devices revealed that V2O5 provided a very low charge transfer resistance and high hole mobility, facilitating efficient hole transfer from the active layer to the ITO anode. More importantly, incorporation of V2O5 as ABL resulted in substantial improvement in device stability compared to MoO3 and PEDOT:PSS based devices. Unencapsulated PEDOT:PSS-based devices stored at a relative humidity of 45% have shown complete failure within 96 h. Whereas, MoO3 and V2O5 based devices stored in similar conditions retained 22% and 80% of their initial PCEs after 96 h. Significantly higher stability of the V2O5-based device is ascribed to the reduction in degradation of the anode/active layer interface, as evident from the electrical measurements.

Efficient inverted flexible polymer solar cells with transparent top MoO_3/Au/Ag/NPB electrodes

We demonstrated ITO-free flexible inverted polymer solar cells based on multilayer transparent top electrodes with the structures of MoO3/Au/Ag/NPB (N,N'-diphenyl-N,N'-bis(1-naphthyl)(1,1'-biphenyl)-4,4'diamine). An ultrathin Ag film was prepared in the MoO3/Au/Ag/NPB electrode with MoO3 as the wetting layer and Au as the seed layer. By regulating the thickness of NPB anti-reflection layers, the transmittance of the MAN structure could be improved in the wavelength range of 430–800 nm. Considering the thermal compatibility of flexible substrate, the thermal evaporation-based 2,2’,2”-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H- benzemidazole) (TPBi) layer was used to replace high temperature solution-processed metal oxides as the interlayer to modify the effective work function of the Ag cathode. The optimized flexible inverted polymer solar cell with a MoO3/Au/Ag/NPB top electrode showed high power conversion efficiency (PCE) of 6.02%, which was comparable with the ITO-based plate equivalent. Moreover, the inverted flexible polymer solar cells exhibited good flexibility and mechanical stability.

Effects of interface energy modification in solution-processed In2O3 thin film transistors for sensing applications

Thin film transistors (TFTs) based on metal oxides have high mobility, transparency, and excellent stability, and are considered to be a key part of the next generation transparent electronic devices. Solution-processed metal oxide thin films using the sol-gel method has enabled the deposition of metal oxide thin films without a vacuum system, and it allows metal oxide TFTs to be fabricated more efficiently at low cost and over a large area through a simple process. In this study, we fabricated In2O3 using the sol-gel method and adopted various self-assembled monolayers (SAMs) to modify the interface between the semiconductor and dielectric layer. We used octadecyltrichlorosilane as a hydrophobic surface and partially removed it through exposure to UV radiation to form a patterned semiconductor using a simple spin-coating procedure that remarkably reduced the leakage current. 3-aminopropylsilane, (3-mercaptopropyl)trimethoxysilane, and cyclopentadienyltrimethylsilane as SAM materials and toluene and water as solvents to dilute them were used to modify the surface of the SiO2. We measured the contact angle under various conditions and the electrical characteristics of the TFTs annealed in various temperatures of 230, 250, and 300°C to confirm the effect of interface energy modification, and concluded that surface energy affected the mobility of the In2O3 transistors and the lower surface energy enhanced the mobility of the TFTs.

Reduced water vapor transmission rates of low-temperature solution-processed metal oxide barrier films via ultraviolet annealing

Here, we report the fabrication of low-temperature sol-gel-derived aluminum oxide (AlOx) films via ultraviolet (UV) annealing and the investigation of their water vapor blocking properties by measuring the water vapor transmission rates (WVTRs). The UV annealing process induced the formation of a dense metal-oxygen-metal bond (Al-O-Al structure) at low temperatures (<200°C) that are compatible with commercial plastic substrates. The density of the UV-annealed AlOx thin film at 180°C was comparable to that of AlOx thin films that have been thermally annealed at 350°C. Furthermore, the UV-annealed AlOx thin films exhibited a high optical transparency in the visible region (>99%) and good electrical insulating properties (∼10−7A/cm2 at 2MV/cm). Finally, we confirmed that a dense AlOx thin film was successfully deposited onto the plastic substrate via UV annealing at low temperatures, leading to a substantial reduction in the WVTRs. The Ca corrosion test was used to measure the WVTRs of AlOx thin films deposited onto polyethylene naphthalate or polyimide substrates, determined to be 0.0095gm−2day−1 (25°C, 50% relative humidity) and 0.26gm−2day−1, respectively.

Accurate Defect Density-of-State Extraction Based on Back-Channel Surface Potential Measurement for Solution-Processed Metal-Oxide Thin-Film Transistors

We report more accurate extraction method of the defect density of states for solution-processed indium-gallium-zinc-oxide (IGZO) thin-film transistors (TFTs). Since the solution-processed IGZO TFTs have a very thin (~8 nm) active semiconductor layer, their back-channel surface potential should be considered in the field-effect method. If the back-channel surface potential is ignored, deviation between theoretically derived and experimentally measured activation energy data becomes more significant as the thickness of the semiconductor layer decreases in comparison with its Debye length. Dependence of the back-channel surface potential on the applied gate voltages was verified by scanning Kelvin probe microscopy and found to be proportional to the gate voltages. The modified field-effect method provided a more accurate model of the activation energy over the subthreshold region and correspondingly more accurate defect density of states of the IGZO TFTs.

Low-temperature synthesis TiOx passivation layer for organic-silicon heterojunction solar cell with a high open-circuit voltage

Organic/silicon (Si) heterojunction solar cells may promise for high performance and low cost solar cell manufacture because they can combine advantages of Si and organic semiconductors. However, the performances of organic-Si solar cells were still mainly jeopardized by Si rear contact recombination losses. Here, titanium oxide (TiOx) layer via low-temperature solution synthesis way was deposited on Si rear side to reduce electrical losses. Minority carrier lifetime, transient photovoltage decay and external quantum efficiency measurements indicated that TiOx could effectively suppress the recombination losses of Si rear side, which reflected in the improvement of open circuit voltage (Voc) and long-wavelength photoresponse. Contact resistance and capacitance-voltage measurements further showed that resistance and rear adverse barrier height (Φ-) between Si and aluminum could be dramatically reduced. A power conversion efficiency (PCE) of 14.3% was achieved for textured Si/poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT: PSS) solar cell by incorporating TiOx layer between Si and aluminum. A Voc of 643mV was achieved based on this low-temperature processed organic-Si heterojunction solar cell. The outstanding performance was ascribed to Si-O-Ti bonding at TiOx/Si interface, which was identified by X-ray photoelectron spectroscopy. This simple low-temperature solution processed metal oxide layer provided a facile way for Si surface passivation to achieve high performance solar cells.

Fully solution-processed metal oxide thin-film transistors via a low-temperature aqueous route

We report a facile and low-temperature aqueous route for the fabrication of various oxide thin films (Al2O3, In2O3 and InZnO). A detail study is carried out to reveal the formation and properties of these sol-gel-derived thin films. The results show that the water-based oxide thin films undergo the decomposition of nitrate group as well as conversion of metal hydroxides to form metal oxide framework. High quality oxide thin film could be achieved at low temperature by this aqueous route. Furthermore, these oxide thin films are integrated to form thin-film transistors (TFTs) and the electrical performance is systematically studied. In particular, we successfully demonstrate In2O3/Al2O3 TFTs with high mobility of 30.88cm2V−1 s−1 and low operation voltage of 4V at a maximum processing temperature of 250°C.

Low-temperature, high-mobility, solution-processed metal oxide semiconductors fabricated with oxygen radical assisted perchlorate aqueous precursors.

In this report, a simple and general chemical route for fabricating MO semiconducting films at a relatively low temperature without any fuel additives or special annealing steps was demonstrated. The precursor, which consisted of perchlorate, nitrate, and water, is easily converted into In2O3 at an annealing temperature of 250 °C due to oxygen radical assisted decomposition and generation of a large amount of heat. It is found that perchlorate salt can decompose and form an oxide film with high quality at a lower temperature when assisted by nitrate salt. The optimized In2O3-TFT fabricated via this precursor exhibits a saturation mobility of 14.5 cm2 V-1 s-1. Furthermore, this approach has been expanded to the fabrication of ZnO films and attained improved performance, indicating its universality.