Low-Temperature Postfunctionalization of Highly Conductive Oxide

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Low Temperature Post-Functionalization of Highly Conductive Oxide Thin-Films Toward Solution-Based Large-Scale Electronics Seok-Gyu Ban, Kyung-Tae Kim, Byung Doo Choi, Jeong-Wan Jo, YongHoon Kim, Antonio Facchetti, Myung-Gil Kim, and Sung Kyu Park ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07528 • Publication Date (Web): 20 Jul 2017 Downloaded from http://pubs.acs.org on July 23, 2017

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ACS Applied Materials & Interfaces

Low Temperature Post-Functionalization of Highly Conductive Oxide ThinFilms Toward Solution-Based Large-Scale Electronics

Seok-Gyu Ban1, Kyung-Tae Kim1, Byung Doo Choi2, Jeong-Wan Jo1, Yong-Hoon Kim3, Antonio Facchetti4, Myung-Gil Kim2*, and Sung Kyu Park1*

1

School of Electrical and Electronic Engineering, Chung-Ang University, Seoul, Korea

2

Department of Chemistry, Chung-Ang University, Seoul, Korea

3

SKKU Advanced Institute of Nanotechnology (SAINT) and School of Advanced Materials Science and

Engineering, Sungkyunkwan University, Suwon, South Korea 4

Department of Chemistry and the Materials Research Center, Northwestern University, Evanston, IL

60208, and Flexterra. Inc., Skokie, IL 60077, USA

*Corresponding Author: Prof. Sung Kyu Park and Prof. Myung-Gil Kim Phone: 82-2-820-5347, Fax: 82-2-820-5347, Email: [email protected] and [email protected]

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ABSTRACT Although transparent conducting oxides (TCOs) have played a key role in a wide range of solid-state electronics from conventional optoelectronics to emerging electronic systems, the processing temperature and conductivity of solution-processed materials seem to be far exceeding the thermal limitations of soft materials and insufficient for high-perfomance large-area systems, respectively. Here, we report a strategy to form highly conductive and scalable solution-processed oxide materials and their successful translation into large-area electronic applications, which is enabled by photo-assisted post-functionalization at lowtemperature. The low-temperature fabrication of indium-tin-oxide (ITO) thin films was achieved by using photo-ignited combustion synthesis combined with photo-assisted reduction process under hydrogen atmosphere. It was noteworthy that the photochemically activated hydrogens on ITO surface could be triggered to facilitate highly crystalline oxygen deficient structure allowing significant increase of carrier concentration and mobility through film microstructure modifications. The low-temperature postfunctionalized ITO films demonstrated conductivity of > 1607 S/cm and sheet resistance of < 104 Ω/ under the process temperature of less than 300 ˚C, which are comparable to those of vacuum-deposited and hightemperature annealed ITO films. Based on the photo-assisted post-functionalization route, all solutionprocessed transparent metal-oxide thin-film-transistors and large-area integrated circuits with the ITO bus lines were demonstrated, showing field-effect mobilities of > 6.5 cm2 V-1 s-1 with relatively good operational stability and oscillation frequency of more than 1 MHz in 7-stage ring oscillators, respectively.

Keywords : transparent conducting electrodes, all-solution processed transparent large-scale electronics, thin film transistor, photo-chemically driven combustion process, low temperature functionalization


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INTRODUCTION Metal-oxide semiconductors have emerged as a potential replacement for amorphous silicon (a-Si) and low-temperature polycrystalline silicon (LTPS) devices in active-matrix electronics including displays, sensor arrays, and X-ray detectors due to their relatively high carrier mobility and good uniformity over a large-area.1–6 Additionally, their compatibility with solution-process and high optical transparency in the visible range has opened a new promising application for cheap and transparent electronics. Nevertheless, for the full realization of large volume production in the transparent metal-oxide electronics, it would be still needed to achieve all solution-processed metal-oxide functional films and their successful integrity. To date, transparent functional oxides have been extensively investigated for the applications of large-area and flexible electronics by continuous solution process, resulting in high-performance semiconducting and insulating thin films at low-temperature.2,7,8 Although these previous advances are noteworthy, solutionprocessed transparent conducting oxides (TCOs) are still not fully developed possibly due to their highprocessing temperature (>500 – 600 ˚C) and difficulties for obtaining high carrier concentration and sufficient carrier mobility in the TCO films. In fact, solution-based conductive thin films utilizing carbon materials (CNT, graphene, graphene oxide)9–11, organic or polymer materials12,13, metal-nanowire meshes14–17, and an oxide nanocomposite18–20 have been studied and the advances are noteworthy. However, they are still suffered from insufficient optoelectronic performances, operational stability, and process compatibility with conventional device integration process, limiting their general utilization into current industry. Furthermore, it is also noted that solution-processed electrodes based on organic, Au or Ag typically show poor contact properties for oxide semiconductors comparing with conducting oxides. Although recent demonstration of solution-processed TCOs2,20–23 have shown promising results which suffice for device contact electrodes, the insufficient conductivity for bus-line have been problematic for the scalable realization of the transparent electronics at circuit or system levels. Throughout a variety of researches of novel precursors and processing methods, relatively conductive TCO films could be achieved. However, the control of carrier concentration and film crystallinity have not been readily obtained, resulting in thier limited utilization in device levels. In order to enable the films to include substantial number of carriers and crystallinity, high temperature reducing 3 ACS Paragon Plus Environment


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treatment (Treducing > 500 ˚C) is generally required to activate dopants and generate oxygen vacancies in the films. Therefore, new material systems and efficient post-processing methods of low-temperature solutionprocessed TCOs are strongly demanded to fulfil the industrial standards of optoelectronic performances such as optical transmittance (T550 nm) > 80% and electrical conductivity (σ) > 1000 S/cm.2,24–28 Here, we explore a facile route to achieve effective post-functionalization for high-performance and scalable solution-processed TCOs, and their translation into all solution-processed large-area metal-oxide systems. The efficacy of the present low-temperature combustion synthesis combined with deep ultraviolet (DUV) assisted photochemical activation to oxide semiconducting materials motivated our research. Comparing to previous reports on the photo-post-functionalization of ITO sol-gel films where UV and ozone treatment are usually used to remove organic impurity compounds19, the main purpose of our photo-postfunctionalization process is inducing hydrogen activation more strongly at lower temperature. In addition to the synergetic combination, the effective post-functionalization of the oxide semiconductors to include substantial carrier concentration and high crystallinity have been developed by employing photo-assisted hydrogen reduction treatment at low temperature (Treduction < 300 ˚C). The resultant indium-tin-oxide (ITO) films demonstrated excellent optoelectronic performance of optical transmittance at 550 nm (T550nm) ~ 90% and electrical conductance (σ) > 1607 S/cm. Comprehensive spectroscopic investigations, surface chemical analyses, and optoelectronic material/device characterisations were performed to clarify the underlying mechanism for the successful formation of highly conductive and scalable solution-processed ITO thin films at low-temperature. Finally, we successfully demonstrate all solution-processed oxide based highperformance thin-film-transistors (TFTs) and their scalable translation into integrated circuits via engineering compatible manners.

EXPERIMENTAL SECTION Solution preparation and characterization A solution for ITO was prepared by the following procedure. Indium nitrate hydrate (In(NO3)3·xH2O), tin chloride (SnCl2) and ammonium nitrate (NH4NO3) powders (all from Sigma-Aldrich) were dissolved in 2-methoxyethanol (anhydrous, Sigma-Aldrich). The total molar concentration of ITO 4 ACS Paragon Plus Environment

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precursor was 0.4 M. After dissolving the precursors in the solvent, acetylacetone and ammonium hydroxide were added. The solution was aged for 12 hours at room temperature. After aging the each solution, the indium and tin precursor solutions were mixed with a volume ratio of 9:1 (In:Sn). The precursor solutions for IGZO and In2O3 were prepared as follows. Indium nitrate hydrate, gallium nitrate hydrate (Ga(NO3)3·xH2O) and zinc nitrate hydrate (Zn(NO3)3·xH2O) powders (all from Sigma-Aldrich) were dissolved in 2-methoxyethanol. The total molar concentration of IGZO precursor was 0.1 M. The molar concentration of In2O3 precursor was 0.05 M. After dissolving the precursors in the solvent, acetylacetone and ammonium hydroxide were added. The solutions were aged for 12 hours at room temperature. In the case of IGZO, after aging the each solution, the indium, gallium and zinc precursor solutions were mixed with a volume ratio of 8.5:1:1.7. The precursor solution for Al2O3 was prepared by following procedure. Aluminum nitrate nonahydrate (Al(NO3)3·9H2O) (Sigma-Aldrich) were dissolved in 2-methoxyethanol (anhydrous, Sigma-Aldrich). The molar concentration of aluminum nitrate nonahydrate was 0.8 M. After dissolving the precursor in the solvent, the solution was stirred for more than 12 hours at 75 ˚C.

Light-assisted photochemistry process and film characterization The light-assisted photochemistry processes were conducted by using high-density UV treatment systems equipped with a low-pressure mercury lamp (wavelengths of 253.7 nm (90%) and 184.9 nm (10%), energy density of 28 mW cm-2) (UV253H, Filgen) or an excimer lamp (wavelength of 172nm (>95%), energy density of 50 mW cm-2) (EX-mini, Hamamatsu Photonics). The as-spun samples and ITO thin film samples were placed under the DUV lamp at approximately 5 cm apart. In the case of low-pressure mercury lamp, N2 gas was continuously supplied to prevent formation of ozone. In the case of excimer lamp, H2:N2 (1:9) mixture gas was continuously inserted to provide a reduction environment condition inside the chamber. The DUV irradiation time was 2 hours for photo-annealing process and 10 min for photo-activated reduction process. The temperature of the substrate surface was measured by an infrared camera (InfraCAM, FLIR System). XPS spectra were recorded on Thermo VG Scientific Escalab 220i-XL spectrometer, using a monochromated Al Kα source at 1486.6 eV with a base pressure of 7.8 x 10-10 mbar. For each sample, Ar ion etching was carried out before the analysis. 5 ACS Paragon Plus Environment


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Transistor and circuit fabrication and electrical measurements For the fabrication of an ITO thin film on glass substrate, 0.7 mm-thick glass substrates (Eagle XG, Corning) have been used. The ITO solution was spin-coated and photo-annealed by low-pressure mercury lamp in N2 atmosphere for 2 hours at the temperature of 200 ˚C. This process was repeated four times to obtain the 60 nm-thick ITO films. Afterwards, photo-assisted hydrogen reduction process was conducted on ITO films by using an excimer lamp under H2:N2 mixture ambient at desired temperature (200 – 300 ˚C) for 10 min. For reference ITO films, ITO solution was spin-coated and annealed on a hot plate under air ambient at desired temperature (200 – 500 ˚C) for 30 min. This process was also repeated four times. After thermal annealing, the ITO films were post-treated on a hot plate under H2:N2 mixture ambient desired temperature (200 – 500 ˚C) for 10 min. For the electrical characterization of ITO thin films on glass substrate, 4-probe measurement and Hall effect measurement were performed using Van der Pauw method. For the fabrication of oxide TFTs on glass substrates, 0.7 mm-thick glass substrates (Eagle XG, Corning) have been used. As a gate electrode, ITO solution was spin-coated and photo-annealed by lowpressure mercury lamp in N2 atmosphere for 2 hours. This process was repeated four times to obtain the 60 nm-thick ITO films. After patterning the gate layer by photolithography and wet etching, photo-assisted hydrogen reduction process was conducted. On the gate electrode, Al2O3 solution was spin-coated and photo-annealed by low-pressure mercury lamp in N2 atmosphere for 2 hours. This process was repeated twice to obtain a 100 nm-thick Al2O3 film. Via holes were wet etched by using photolithography. For the source/drain electrodes, ITO solution was spin-coated and photo-annealed by low-pressure mercury lamp in N2 atmosphere for 2 hours. This process was repeated four times to obtain the 60 nm-thick ITO films. After patterning the source/drain electrodes by photolithography and wet etching, photo-assisted hydrogen reduction process was carried out. For the channel layer, IGZO or In2O3 solution was spin-coated and photoannealed in N2 atmosphere for 2 hours. This process was repeated twice. IGZO and In2O3 layer was wet etched by photolithography. The wet etching of the ITO, IGZO and In2O3 layer was carried out by using LCE-12K (an ITO etchant) from Cyantek corporation. The wet etching of the Al2O3 layer was carried out by using a mixture of DPD-200 (a photoresist developer) and deionized water 1:1 volume ratio. 6 ACS Paragon Plus Environment

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The electrical characterization of oxide TFTs were performed using a semiconductor parameter analyzer (Agilent 4156C, Agilent Technologies). The capacitance of the gate dielectric was measured with an Agilent 4284A precision LCR meter. For ring oscillators, a digital storage oscilloscope (TDS2012B, Tektronix) was used to measure the oscillation frequency. All of the measurements were carried out under dark in air ambient. The saturation field-effect mobility (µsat) and subthreshold swing (SS) of TFTs were calculated by using following equations (1) and (2), respectively.

2L µ sat = WCi

SS=

 ∂ I DS   VG 

   

2

dVG d (log I DS )

(1)

(2)

For operational stability test of the metal-oxide TFTs, constant positive gate bias (VGS = + 10 V) and sourcedrain bias (VDS = + 0.1 V) were applied to the device for a preset time.

RESULTS AND DISCUSSIONS Unlike the conventional metal-oxide semiconductors and insulators, it is considered that the successful formation of solution-based conducting oxide films would require two conversion steps. Initially, the high quality metal-oxide semiconducting materials could be synthesized with appropriate precursors and processing methods. Next, in order to afford a conductive function to the pristine semiconducting materials, subsequent post-functionalization (reduction) step is needed. Detailed analysis of reaction coordinate vs. energy states in accordance with precursor systems and processing methods was schematically depicted in Figure 1a. Typically, in the synthesis of thermally driven solution-processed ITO films with conventional precursor systems, high temperature (Ta > 500 ˚C) synthesis and post-functionalization process have been carried out to remove organic residues and induce a large number of free carriers, respectively. In contrast, the thermally driven combustion oxide semiconductor can be synthesized at low temperature (Ta ~ 150 – 200 ˚C). The balanced redox chemistry and the local oxidizer existence can give high exothermicity and 7 ACS Paragon Plus Environment


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efficient organic impurity removal, which allow the low temperature formation of oxide films. Nevertheless, the high temperature post-functionalization is still critical since dopant activation and oxygen vacancy generation are involved with different mechanism from the reduced driving force for oxide lattice formation in the combustion synthesis. It also should be noted that the instantaneous heat generation and significant gas formation along with the combustion reaction typically produced porous structured thick-film (thickness > 50 nm) with insufficient film densification period, indicating the limited electrical conductance of ITO films as high as 680 S/cm even with the high processing temperature.2 (Tprocess > 500 ˚C) To overcome these high temperature process and film densification issues, low temperature functionalization and efficient reduction process have been developed by employing photo-assisted combustion synthesis and a unique reducing treatment. As shown in Figure 1a, for low-temperature solution-based ITO film formation, a redox-based combustion synthetic approach which was ignited and processed by both low-temperature thermal energy (200 ˚C) and photo-energy from the DUV irradiation (from LPM lamp with main peaks at 184.9 (10%) and 253.7 nm (90%)) under N2-purging condition was provided, eliminating the need for externally applied high temperature and efficiently overcoming the energy crest for stable formation of metal-oxide-metal (M-O-M) lattices. Although combustion only process typically provides instantaneous energy for M-O-M lattice formation, the lack of continuous energy supply resulted indeed the poor dopant activation and insufficient structural relaxation within ITO films. Whereas in the photo-combined combustion reaction with the identical minimal temperature, additional continuous activation energy can be offered and thus more densified M-O-M structures with activated dopant could be obtained. As expected, comparing to combustion only processed sample, much improved conductivity (inactive vs. 110 S/cm) and densified structures were observed in the photo-assisted combustion synthesized ITO films. Besides the substitutional doping with n-type dopant (Sn in In2O3), ionized oxygen vacancies (single(VO+) or doubly charged (VO2+)) are generally believed to be another main origin of the stable excess carrier generation in TCO materials.29,30 Therefore, in order to enable large excess electrons to be released into conduction band and increase the conductivity of the pristine ITO films, additional processing step for obtaing oxygen deficient structures is strongly required. For the effective post-functionalization in vapor8 ACS Paragon Plus Environment

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deposited or solution-processed ITO films, several methods have been investigated, such as vacuum annealing, hydrogen plasma treatment, fluorine doping, and thermal annealing under hydrogen reductant ambient.27,30–33 The post-functionalization under reducing ambient has been commonly utilized to improve electrical performance of the ITO films, supplying significantly increased free-carriers. More importantly, it can be seen that the photo-assisted reducing treatment of the pristine ITO film under H2 environment can afford more functionalization on the pristine films. In most of reducing treatment, hydrogen molecules could be dissociated into activated species once they were adsorbed onto the ITO surface and supplied with enough energy. Subsequently, the activated hydrogen onto the ITO surface eventually cause the reduction of the oxide lattice while generating substantial amount of oxygen vacancies, anion site substitution, and interstitial insertion.27,34–36 As reported, highly energetic photons typically from DUV light, can be a more favourable option not only to facilitate the dissociation of molecular bonding but to enhance the transition of material phases by supplying non-thermal energetic sources.7 Therefore, releasing highly energetic photons (excimer lamp with main peak of 172 nm wavelength, > 95%) under H2 ambient appears to give significant influence on effective activation and dissociation of the hydrogen molecules. The home-made experimental setup, which is utilized in the photo-assisted hydrogen reduction process, is schematically depicted in Figure 1c. As shown in this figure, pre-deposited ITO samples from the photo-ignited combustion synthesis were directly exposed to DUV irradiation from the excimer lamp under flowing hydrogen and nitrogen mixed atmosphere (1:9 volume ratio). In parallel, mild heating (200 – 300 ˚C) was supplied to the ITO samples which is placed in the reliably sealed quartz glass (front side) chamber. Figure 1d describes the course of reduction mechanism which exhibits the elimination of oxygen species in the ITO structure and hydrogen incorporation into the generated oxygen vacancies. The energetic photon irradiation under the mixed hydrogen ambient may lead to dissociation and adsorption of hydrogen onto the ITO surface. During the DUV irradiation, moderate heating may induce both the accelerated diffusion of the hydrogen and the thermal agitation (i.e., vibration) for lowering the activation barrier, facilitating strong reaction with oxygen in the ITO lattice. Although it is still a topic of debate, ionized oxygen vacancies are generally believed to be the origin of free carrier (electron) generation as well as defect sites for deteriorating charge carrying ability. Therefore, while the excess electrons released into the 9 ACS Paragon Plus Environment


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ITO film increase the carrier concentration of the film immediately upon the ionization, reduction of the carrier mobility has been also observed.24 In order to understand the underlying course of material physics in the hydrogen-enhanced reductive ITO films, we exhaustively investigated chemical bonding composition of the ITO films upon various post-functionalization (reducing treatment) conditions. It was evident from the X-ray photoelectron spectroscopy (XPS) spectra of O1s peak that relative portion of oxygen vacancies was increased in accordance with supplied temperatures (Figures 2a-d and Supplementary Figure S2). The high quality pristine ITO film formation with photo-assisted combustion synthesis was verified by the observation of lower M-OH areal ratios than those of themally annealed film. Compared to the thermally induced reduction treatment, the photo-assisted reducing treatment appears to be much more effective to post-functionalize the ITO films. Indeed, as shown in Figures 2c-d, the ITO films offered by photo-assisted reducing treament at 250 ˚C exhibits area ratio of M-Ovac of 20.9% which is even higher than those (20.0%) of thermally reduced films at 500 ˚C. The significant oxygen vacancy generation with photo-assisted reduction are also monitored with UV-Vis absorption and tranamission analysis. The Figures 3a-b show optical band gap and transmittance of the ITO films from different post-treatments. From the spectroscopic analysis, the optical bandgap values were estimated as 3.45, 3.40, 3.35 and 3.2 eV for the photo-assisted reduced ITO films with supplied temperatures of 300, 250, 200 ˚C, and pristine, respectively. Intuitively, ITO films including a large amount of oxygen vacancies and corresponding substantial carrier concentration can be expected to show larger optical bandgap. The apparent band gap of degenerated semiconductors like ITO could be increased as the fundamental absorption edge, which lies in the near-ultraviolet of the visible spectrum, shifts to shorter wavelength (higher energy) as a result of all states close to the conduction band being populated by the released excess electrons from the ionized oxygen vacancies, pushing the Fermi level higher into conduction band. Therefore, such a low level of M-OH bondings and a large amount of M-Ovac in the photo-assisted reduced ITO films are one of the convincing explanation for the enhanced electrical properties of the ITO films. These results indicates that the hydrogen reduction process is promoted upon highly energetic photon irradiation, and the resultant films exhibited remarkable increase of carrier concentration in solution-processed ITO films with lower supplied temperature. 10 ACS Paragon Plus Environment

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Apart from the highly increased electron concentration in the conduction band, another possible mechanism which may influence on the conductivity of the ITO films is charge carrying ability. As described in Table 1, the photo-assisted reduced ITO films reveal an abrupt drop of Hall mobility once the pristine films is exposed to the reducing treatment. The decreased Hall mobility can be attributted to the dramitically increased carrier concentration and oxygen vancies, which have been often considered as carrier scattering centers and trapping sites, respectively. Subsequently, as increasing the process temperature, the Hall mobility is increased again, which is likely due to the improved structural integrity of the ITO films. The structural effect on the conductivity of the post-functionalized ITO films was further interrogated to understand how the charge carrying ability interplays with film microstructure modification upon the aforementioned photo-assisted reduction treatment. For comprehensive investigations, X-ray diffraction (XRD), transmission electron microscopy (TEM), and atomic force microscopy (AFM) analysis were performed and presented in Figures 3c-f and Supplementary Figures S3 and 4 as evidences to support the efficancy of the proposed approach and to explain the underlying mechanism. In order to examine the structural modification with reducing process conditions, the film crystallinities were monitored throughout XRD analysis over the entire ITO films. As shown in Figure 2c-d and Supplementary Figure S3 the peaks of (222), (400), (440), (622) were observed and became prominent with increasing temperature. Using Scherrer equation from the XRD spectra, the significant increase of crystallite size (amorphous, 8.52, and 11.54 nm) is observed in the photo-assisted reduced ITO samples with supplied temperature of 200, 250, and 300 ˚C, respectively. In contrast, the crystallite sizes of thermally reduced ITO films show rather slower increase from 2.7 to 3.12 nm with increasing temperature. The TEM analysis in Figures 3e-f shows clear difference of crystallinity and grain size between the photo-assisted and thermally induced post-functionalized ITO films, demonstrating grain size of ~10 and ~3 nm, respectively. Generally, the crystallite growth of ITO with thermal annealing was observed at high temperature over 650 ˚C.24–26 Whereas, under the reducing ambient at intermediate temperature (~ 350 ˚C), the reduction of ITO into metallic indium resulted low temperature melting metallic indium precipitation which facilitate lowtemperature crystallite growth.37 Furthermore, considering the facile activation of hydrogen with the photoassisted reducing treatment, the larger grain size and crystallinity of the photo-assisted reduced ITO films 11 ACS Paragon Plus Environment


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can be attributed to the significant indium precipitation formation and subsequent crystallite growth throughout the entire film depth of the ITO films. The result indicates that, as we expected, the ITO films fabricated by the highly energetic photon irradiation constitute well-ordered lattice structure with larger size grains, implying that the supplied energy may play a key role in determining the crystallinity and grain size, and consequently the charge carrying ability. The electical characteristics of the ITO films are summarized in Table 1 and Figure 4. To measure and obtain the electrical performance, we carried out 4-point measurement with van der pauw method. The electrical conductivity of the ITO films in Figure 4a show continous increase with increasing the postfunctionalization temperature. The photo-assisted reduced ITO films processed at 300 ˚C exhibit the electrical conductivity of 1607 S/cm, which is, to the best of our knowledge, the highest electrical conductivity of solution-processed ITO films at the processing temperature. Also, it can be seen from Figure 4c-d that the photo-assisted reduced ITO films demonstrate higher mobility and larger carrier concentration than those of thermally processed films, which support the superior film quality and more effective post-functionalization of the photo-assisted reducing methods. From these results we can observe that the direct evidences of low-temperature post-functionalization of solution-processed TCOs and the effect of the hydrogen-mediated photochemical reduction on high-performance optoelectronic thin films. To demonstrate the versatile translation of the synthesized conducting oxides into high-performance and scalable transparent electronics, all solution-processed metal-oxide TFTs and large-area integrated circuits were fabricated on commercially available borosilicate glass substrates. In these implementations, the solution derived ITO has been utilized as gate and source/drain electrodes for the TFT devices as well as cross-wiring and data lines for the large-area circuits. As we know, this is the first demonstration for allsolution processed metal-oxide TFT circuits, utilizing solution oxide conductors as the data bus line in largearea electronic systems. To provide an efficient fabrication process for all-solution based metal-oxide TFT devices, bottom gate and bottom source/drain contact configuration was offered as depicted in Figure 5a, which is fully compatible with current CMOS technology. Using this configuration, almost no chemical damages on each functional layer were observed during the device building process, minimizing the influence of different chemical selectivity between the highly crystalline ITO electrodes and amorphous 12 ACS Paragon Plus Environment

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metal-oxide semiconductors such as indium-gallium-zinc-oxide (IGZO) and indium-oxide (In2O3) layers. TEM images in Figure 5a demonstrates the cross-sectional images of the TFT devices, showing± the stacked sol-gel processed metal-oxide structures along with the dashed line. It clearly describes that the proposed device configuration with all-solution metal-oxide integrity could be sustainable during the conventional CMOS process. In the whole device fabrication, photochemically driven sol-gel metal-oxide functional layers were integrated as gate/source/drain electrodes, dielectrics/insulators, semiconductors, and cross-wiring/data electrodes via all wetting process in air ambient. Besides the photo-assisted post-functionalized ITO electrodes, highly stable and functional metal-oxide gate dielectrics and semiconductors7, 8 were obtained using the similar synthesizing process (Supplementary Figure S5). Figure 5b and 5d show the transfer and output characteristics of all-solution processed IGZO and In2O3 TFTs with channel lengths and widths of 10 µm and 100 µm, respectively. It can be seen that the all-solution processed TFTs exhibit well-defined current on/off function and on-state current levels, showing minimal gate leakage current. Figure 5c and 5e illustrate filed-effect mobility distribution of the TFT devices showing averaged saturation field-effect mobility of 3.2 cm2 V-1 s-1 (maximum value of around 4.33 cm2 V-1 s-1) with a standard deviation of 0.49 cm2 V-1 s-1 (24 devices) for IGZO and 5.76 cm2 V-1 s-1 (maximum value of around 6.72 cm2 V-1 s-1) with a standard deviation of 0.36 cm2 V-1 s-1 (30 devices) for In2O3 TFTs. Also, the devices had current on/off modulation, subthreshold swing (SS), and threshold voltage (VT) values of 109, 108, 0.223 ± 0.012, 0.296 ± 0.007 V decade-1, and 1.03 ± 0.08, -2.39 ± 0.1 V, for IGZO and In2O3, respectively, which are acceptable for applications of active-matrix displays such as organic light-emitting diode and liquid-crystal displays. In parallel, to take the generality of the post-functionalized highly conductive ITO films, large-area integrity of all metal-oxide TFTs were implementated as depicted in Figure 6a. The circuits were implementated by cascading 7 inveters in which each inverter has a β-ratio of 20 with (W/L)drive of 100 µm/5 µm and (W/L)load of 20 µm/20 µm, via cross-wiring of the solution-processed high quality ITO electrodes. The overlap distance between the gate and source/drain electrodes was 4 µm. Figure 6b plotted oscillation frequency and propagation delay as a function of supply bais, and the output waveforms of the 7-stage ring 13 ACS Paragon Plus Environment


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oscillator were provided in Supplementay Figure S6. With a supply voltage of VDD = 50 V, an oscillation frequency of > 1.018 MHz and corresponding propagation delay of < ~ 100 ns stage-1 were measured, and even at a low supply voltage of VDD = 15 V, an oscillation frequency over 250 kHz were recorded. To examine the operational stability of the all-solutiom processed metal-oxide TFTs, positive-gate bias-stress (PBS) tests were performed under air ambient conditions without packaging process (Figure 6c). Interestingly, even without passivation(packaging) layers, the IGZO TFTs exhibited an relatively acceptable operational stability with a VT shift (∆VT) of less than 3.5 V during the 10,000 sec of PBS with electric field of 1 MV cm-1 (Vgate of 10V). It is noted that although the devices were fabricated with all-solution process at relatively low-temeprature, featuring bottom gate and bottom contact structures which are vulnerable to environmental ambient, the resulting stability is nearly comparable to that of solution-processed top contact configured devices with vacuum-deposited electrodes.

CONCLUSIONS In summary, using the unique photo-assisted reduction routes with photo-annealed combustion precursor, we have successfully implemented highly conductive and scalable metal-oxide thin films and correspondingly all solution-processed metal-oxide TFTs, logic circuits, and 7-stage ring-oscillators. This unprecedentedly outstanding optoelectronic property of low-temperature solution-processed ITO films may be a breakthrough in the transition to the large-scale fabrication of all solution-processed electronics on ultra-flexible plastic substrates, providing a solid path to low-cost high performance and large-scale electronics.

ASSOCIATED CONTENT Supporting Information AFM data, gate dielectric properties, details of XPS data, XRD data, TEM images and 7-stage ring oscillation circuit measurement.


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ACKOWLEDGEMENT This work was supported by Institute for Information & communications Technology Promotion(IITP) grant funded by the Korea government(MSIP) (No.2017-0-00048, Development of Core Technologies for Tactile Input/Output Panels in Skintronics(Skin Electronics) and also supported by the Chung-Ang University Excellent Student Scholarship.



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Figure Captions Figure 1. Low-temperature photochemical combustion synthesis and post-functionalization of oxide conductors. (a) The reaction coordinates vs. energy states of thermally driven conventional process and photochemically driven combustion synthesis. (b) The two-step photochemical combustion synthesis and post-functionalization process for highly conductive ITO films. (c) An illustration of post-functionalization for ITO films (photo-induced reduction process) using a homemade hotplate chamber with quartz glasses. (d) Schematics showing the photo-induced reduction process of ITO films using an excimer lamp.

Figure 2. X-ray photoelectron spectroscopy (XPS) analysis of post-functionalized ITO films. (a-b) The O1s XPS spectra of photo-assisted (a) and thermally induced (b) post-functionalized ITO thin films as a function of process temperature. (c-d) The areal ratio of M-OH, M-Ovac and M-O-M bonding states in ITO thin films with photo-assisted (c) and thermally induced (d) postfunctionalization.

Figure 3. Optical Transmittance, X-ray diffraction (XRD) and transmission electron microscope (TEM) data of ITO films. (a-b) Optical transmittance (a) and Tauc plot (b) of ITO films as a function of process temperature with photo-assisted post-functionalization process. (c-d) The XRD spectra of ITO thin films as a function of process temperature with photo-assisted (c) and thermally induced (d) post-functionalization process. (e-f) TEM images and electron diffractions (inset) of ITO thin films with photo-assisted post-functionalization at 300 ˚C (e) and thermally induced post-functionalization at 500 ˚C (f).

Figure 4. Electrical characterization of ITO thin films. (a) Electrical conductivity, (b) sheet resistance, (c) Hall mobility, and (d) carrier concentration of post-functionalized ITO thin films processed at different temperatures with photo-assisted and thermally induced reducing treatments.


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Figure 5. The transfer characteristics and electrical performance of all solution-processed transparent IGZO and In2O3 TFTs on a glass substrate. (a) Schematic cross section of all solutionprocessed IGZO TFTs and a relevant cross sectional TEM image. (b) Transfer and output characteristics of all solution-processed IGZO TFTs. (c) Distribution of saturation mobility, threshold voltage (VT) and subthreshold slope (SS) of all solution-processed IGZO TFTs. (d) Transfer and output characteristics of an all solution-processed In2O3 TFTs. (e) Distribution of saturation mobility, VT and SS of all solution-processed In2O3 TFTs.

Figure 6. Electrical characteristics of 7-stage ring oscillators fabricated with photo-assisted postfunctionalized TCOs. (a) Optical images of all solution-processed IGZO TFTs and 7-stage ring oscillators with a β-ratio of 20. (b) Oscillation frequency (red) and per-stage propagation delay (blue) of a 7-stage ring oscillator as a function of supply voltage, VDD. (c) VT of IGZO TFTs under a positive gate-bias stress. (VGS = + 10 V, VDS = + 0.1 V)

Table 1. Electrical properties of photo-assisted and thermally induced post-functionalized ITO thin films fabricated on glass substrates.



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Figure 1. Low-temperature photochemical combustion synthesis and post-functionalization of oxide conductors. (a) The reaction coordinates vs. energy states of thermally driven conventional process and photochemically driven combustion synthesis. (b) The two-step photochemical combustion synthesis and post-functionalization process for highly conductive ITO films. (c) An illustration of post-functionalization for ITO films (photo-induced reduction process) using a homemade hotplate chamber with quartz glasses. (d) Schematics showing the photo-induced reduction process of ITO films using an excimer lamp. 424x202mm (96 x 96 DPI)

ACS Paragon Plus Environment


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Figure 2. X-ray photoelectron spectroscopy (XPS) analysis of post-functionalized ITO films. (a-b) The O1s XPS spectra of photo-assisted (a) and thermally induced (b) post-functionalized ITO thin films as a function of process temperature. (c-d) The areal ratio of M-OH, M-Ovac and M-O-M bonding states in ITO thin films with photo-assisted (c) and thermally induced (d) post-functionalization. 517x209mm (96 x 96 DPI)


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Figure 3. Optical Transmittance, X-ray diffraction (XRD) and transmission electron microscope (TEM) data of ITO films. (a-b) Optical transmittance (a) and Tauc plot (b) of ITO films as a function of process temperature with photo-assisted post-functionalization process. (c-d) The XRD spectra of ITO thin films as a function of process temperature with photo-assisted (c) and thermally induced (d) post-functionalization process. (e-f) TEM images and electron diffractions (inset) of ITO thin films with photo-assisted postfunctionalization at 300 ˚C (e) and thermally induced post-functionalization at 500 ˚C (f). 523x255mm (96 x 96 DPI)



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Figure 4. Electrical characterization of ITO thin films. (a) Electrical conductivity, (b) sheet resistance, (c) Hall mobility, and (d) carrier concentration of post-functionalized ITO thin films processed at different temperatures with photo-assisted and thermally induced reducing treatments. 375x253mm (96 x 96 DPI)


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Figure 5. The transfer characteristics and electrical performance of all solution-processed transparent IGZO and In2O3 TFTs on a glass substrate. (a) Schematic cross section of all solution-processed IGZO TFTs and a relevant cross sectional TEM image. (b) Transfer and output characteristics of all solution-processed IGZO TFTs. (c) Distribution of saturation mobility, threshold voltage (VT) and subthreshold slope (SS) of all solution-processed IGZO TFTs. (d) Transfer and output characteristics of an all solution-processed In2O3 TFTs. (e) Distribution of saturation mobility, VT and SS of all solution-processed In2O3 TFTs. 527x197mm (96 x 96 DPI)



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Figure 6. Electrical characteristics of 7-stage ring oscillators fabricated with photo-assisted postfunctionalized TCOs. (a) Optical images of all solution-processed IGZO TFTs and 7-stage ring oscillators with a β-ratio of 20. (b) Oscillation frequency (red) and per-stage propagation delay (blue) of a 7-stage ring oscillator as a function of supply voltage, VDD. (c) VT of IGZO TFTs under a positive gate-bias stress. (VGS = + 10 V, VDS = + 0.1 V) 353x255mm (96 x 96 DPI)


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Table 1. Electrical properties of photo-assisted and thermally induced post-functionalized ITO thin films fabricated on glass substrates. 481x315mm (96 x 96 DPI)


Low-Temperature Postfunctionalization of Highly Conductive Oxide

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