Highly Flexible Indium Tin Oxide Nanofiber Transparent Electrodes by

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Highly flexible indium tin oxides nanofiber transparent electrodes by blow spinning Haolun Wang, Suiyang Liao, Xiaopeng Bai, Zhenglian Liu, Minghao Fang, Tao Liu, Ning Wang, and Hui Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13255 • Publication Date (Web): 23 Nov 2016 Downloaded from http://pubs.acs.org on November 24, 2016

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ACS Applied Materials & Interfaces

Highly flexible indium tin oxides nanofiber transparent electrodes by blow spinning Haolun Wang1,2, Suiyang Liao2, Xiaopeng Bai2, Zhenglian Liu3, Minghao Fang3, Tao Liu1, Ning Wang1,*, and Hui Wu2,* 1

State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic

Science and Technology of China, Chengdu, 610054, China. 2

State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and

Engineering, Tsinghua University, Beijing, 100084, China. 3

School of Materials Science and Technology, China University of Geosciences, Beijing, 100083

China. KEYWORDS: Transparent electrodes, flexibility, indium tin oxides, blow spinning, nanofiber

ABSTRACT: Transparent conductive film (TCF) has found wide applications. Indium tin oxide (ITO) is currently the most widely used transparent electrode. However, major problem of ITO is the lacking of flexibility, which totally limits its applications. Here, we report a highly flexible transparent electrode consisting of freestanding ITO nanofiber network fabricated by blow spinning, the advantage of which is its high-efficiency, low cost and safety. When the bending radius decreased to 0.5 mm, the resistance of the transparent electrodes only increased by 18.4%. Furthermore, the resistance was almost unchanged after thousands of bending cycles at 3.5 mm bending radius.

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The next generation electronic devices will be towards transparent and wearable direction. Thus, the fabrication of flexible transparent electrode with low resistance is much important. Recently, continuous efforts have been devoted to the preparation methods of flexible TCF using different materials, such as CNT and graphenes,1 metallic nanoparticles and nanowires,2 conductive polymers.3 While, the synthesis of these materials exist some problem such as complicated process, high cost and difficult to produce in mass. It is known that transparent conducting oxides (TCOs) such as fluorine-doped tin oxide (FTO) and ITO are indispensable transparent electrodes in solar cells, thin-film transistors and organic light-emitting diodes.4 Nowadays, ITO is the most commonly used transparent electrode with more than 97% market share of transparent conductive coatings, because of its excellent properties of low sheet resistance and high transparency.5 At present, the main synthesis methods of multifunctional and high-quality ITO films include pulsed laser deposition,6 magnetron sputtering,7 ion-beam assisted deposition,8 the sol-gel dip and spin-coating progress.9 The sheet resistance of ITO can reach around 10 Ω/sq at 90% transparency,10 while the realization of flexibility is always an enormous challenge. The brittleness property of ITO film severely limits its applications to the flexible devices.11 Hence, it’s necessary to develop a low cost and large-scale method to solve this problem. In recent years, some research presented flexible property of ceramic nanofiber benefited from their large aspect ratios.12 Given this, the continue ITO nanofiber network can be expected to be an efficient flexible ITO film. Currently, electrospinning has been successfully used to produce nanofiber, while some problems still remain, such as low production efficiency, the safety issue caused by high voltage and the requirement of collectors’ conductivity which limit its being applied into industrialization.13 In our study, we designed and fabricated ITO nanofiber transparent electrodes by blow spinning, which is a low cost, industrial-scale and safe method to produces continue


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nanofiber with large aspect ratio.14 A variety of substrates and sol-gel precursor systems can be applied to blow spinning process, which does not need to have a conductive collector with high voltage. For transparent electrodes based on nanofibers, the diameter of film’s nanofibers is closely related to the conductivity and transparency.15 And, the diameter of fibers can easily be controlled by polymer concentration, gas pressure, working distance, injection rate and so on.14 So, blow spinning is a potential and convenient method to fabricate flexible transparent electrodes with nanofibers. The precursor solution was prepared through a typical procedure. In(NO3)3·4.5H2O and SnCl4·5H2O were dissolved in ethanol. Polyvinyl butyral (PVB) was then added into the solution at a concentration of 5 wt%. In blow spinning process, the In and Sn ratio in nanofiber can be controlled by adjusting the precursor ratio easily. The electrical properties of ITO was greatly affected by tin doping concentration. According to the literatures,16,17 the ratio of Sn/In is 1:9 in the ITO precursor solution. The mixture was magnetically stirred for ~4 h in a capped bottle at room temperature. Then, the solution was loaded into a 1 mL syringe with a special coaxial needle. ITO precursor solution was injected from the inner nozzle with a speed of 3 mL/h, and the gas pressure was 8 psi from the outer nozzle (airflow velocity is ~16 m/s at the exit).The blow spun nanofibers were collected onto a glass fiber mat, which was placed 30 cm away from the nozzle. After different spinning time at room temperature, the nanofiber film of PVB/ In(NO3)3/ SnCl4 was heated in air at 450 °C for 2 h at a heating rate of 2 °C/min, and cooled in the furnace. Then, with a heating rate of 2 °C/min, the film was annealed at 300 °C in a hydrogen atmosphere for 90 min in a tube furnace. Through the above steps, we can get the flexible and transparent ITO nanofiber electrodes. The crystal structure was recorded using X-ray diffraction (XRD, D/max-2500, Rigaku) with Cu Kα radiation at a scanning rate of 7°/min with 2θ ranging


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from 20° to 80°. The microstructure of samples was observed using a field-emission scanning electron microscope (LEO-1530, Zeiss). The TEM images were obtained using JEOL-2010 TEM operating at 120 kV. Transparence spectra were measured by an UV–Vis–NIR spectroscope (Lambda 950, Perkin-Elmer, America) in the range of 400 nm to 700 nm. The sheet resistance was recorded by a ST-2258C multifunction digital four-point probe tester. Binding energy of element was measured using X-ray photoelectron spectrometer (Escalab 250Xi, Thermo Fisher) equipped with an Al Kα radiation source (1487.6 eV) and hemispherical analyzer with a pass energy of 30.0 eV and an energy step size of 0.05 eV. Figure 1a shows the schematic diagram of blow spinning. A coaxial nozzle used in blow spinning, consisted of an inner nozzle for precursor solution and an outer nozzle through airflow. The precursor solution was pumped through the inner nozzle, and was stretched by the gas from outer nozzle during the fixed working distance between collector and nozzle (Figure S1and Movie S1, supporting information). Most solvent of the precursor solution volatilized at this process. The compressed gas can be consisted of argon, nitrogen, air, etc. Glass fiber mat is one of the perfect choices to collect the as-prepared precursor nanofiber, due to its resistance to high temperature so that can be transfered to the muffle furnace for further heat treatment. We can see a layer of the large-area ITO precursor nanofiber film on the glass fiber mat (Figure S2, supportting information). After being annealed in a muffle furnace, the ITO nanofiber film can be peeled off from the glass fiber mat. Subsequently, the collected ITO nanofiber film was further annealed in hydrogen atmosphere. Finally, the obtained product is extremely transparent that we can clearly see the words on the paper though it (Figure 1b). The crystal structure of the nanofiber is cubic bixbyite In2O3 structure without any other phases about Sn compounds from X-ray diffraction (XRD) analysis (Figure 1c). Optical microscope photograph and scanning


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electron microscope (SEM) images (Figure 1d and Figure S3, supporting information) display that the average diameter of blow spun ITO nanofibers is around 330 nm with large aspect ratio ITO nanofiber is continuous and distributed uniformly. There are some nodes between ITO nanofibers, which is great help for enhancing the electrical conductivity of the ITO nanofiber film (inset in Figure 1d). Figure 1e shows transmission electron microscope (TEM) image of blow spun ITO nanofibers. The surface of the nanofiber is smooth with roughness less than 8 nm. And the ITO nanofiber is dense, because no pores can be found in nanofiber from SEM and TEM images. The grain size is less than 10 nm according to the high-resolution TEM (HRTEM) image (lower inset in Figure 1e). And the selected-area electron diffraction (SAED) pattern shows that the ITO nanofiber is polycrystalline (upper inset in Figure 1e). Energy-dispersive Xray spectroscopy (EDS) elemental mapping image can reveal the chemical composition of nanofiber, which In and Sn are distributed in individual nanofiber uniformly (Figure 1f). Combining with the XRD result of ITO, these phenomena indicate that the Sn is in a solid solution with In2O3. ITO is a wide-bandgap, n-type semiconductor with high transmittance in the visible optical region. Moreover, the good conductivity of ITO is attributed to the presence of oxygen vacancies in In2O3 crystal structure and substitutional tin.16 In our work, the electrical conductivity of the blow spun ITO nanofiber films after annealing in a hydrogen atmosphere is much better than untreated films. To explain the reasons, the X-ray photoelectron spectroscopy (XPS) spectra of unannealed and annealed blow spun ITO nanofiber are shown in Figure 2a-d. Figure 2a shows the XPS wide scan of the blow spun ITO nanofibers after annealing. Multi-peaks corresponding to O1s, In3p, In3d, In4s, In4p, and In4d were detected. The spectra of the O1s core level were evaluated by Lorentzian-Gaussian fitting, displaying three peaks with the binding energy of


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530.2 eV, 531.6 eV and 533.0 eV, respectively. From the literature, it is found that the first peak at ~530 eV corresponds to the bulk lattice oxygen.18-20 The second peak at ~532 eV is assigned to oxygen deficient region of matrix.18,21 And the last peak centered about 533 eV is assigned to surface contamination such as water related species and chemisorbed oxygen.19,21 Comparing Figure 2b with Figure 2c, the relative area of the second peak at ~532 eV obviously increased after hydrogen annealing, which means more oxygen vacancies in ITO nanofibers. Moreover, as shown in Figure 2d, the spectra of In3d core level reveals that the binding energy after annealing increases by 0.1 eV. In some highly oxygen deficient In2O3 films, such as MOCVD,22 the higher shift of binding energy can also be found. Therefore, we can infer that the main reason of the increased electric conductivity after annealing could be attributed to the increase of oxygen vacancies based on the analysis of XPS. Optical transmittance and sheet resistance of the ITO nanofiber film are two important factors for transparent conductive film (TCF). These properties are closely related to the thickness and density of the ITO nanofiber film, which can be easily controlled by varying the fabrication time of blow spinning. The blow spinning times of ITO nanofiber in Figure 3a are 30 s, 1 min, 2 min, 3 min and 4 min, respectively. Figure 3a shows the relationship between transmittance at 550 nm and sheet resistance of blow spun ITO nanofiber film, electrospun ITO nanofiber film17 and sputtered ITO on poly(ethylene terephthlate) (PET),23 respectively. For example, the sheet resistance of blow spun ITO nanofiber film is 304 Ω/sq and the transmittance is 81%. It is obvious that the transmittance increase with sheet resistance when the blow spinning time decreased. At the same transmittance, the sheet resistance of blow spun ITO nanofiber film is lower than electrospun ITO nanofiber and some researches such as Ni film,24 single wall carbon nanotube25 and PEDOT: PSS.26 The reason of lower resistance than the film by electrospinning17


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is that the larger diameter of our nanofibers (~330 nm) than the electrospun counterpart (80 ± 20 nm).15 In the 400 nm to 700 nm range of visible light spectrum, the transmittance values of blow spun ITO nanofiber films are still at a decent level for the corresponding sheet resistance (Figure 3b). And the transmittance increases as the sheet resistance increases, which also agrees well with results in Figure 3a. Besides transmittance and sheet resistance, flexibility is another requirement if these electrodes are going to play a role in building electronic devices of the future. It’s a challenge to achieve flexible ITO film because of its brittle nature. Electrospun ITO nanofiber film is a decent choice for flexible TCF. However, the production efficiency of electrospinning is too low, which limits the applications for industry. Thus, we researched a high flexibility of ITO nanofiber transparent electrodes by blow spinning, a low cost, industrial-scale and safe method (Movie S2, supporting information). As shown in Figure 3c, after the blow spun ITO film (the diameter of ~330 nm) was bend to radius as low as 0.5 mm, the resistance of the film only increased by 18.4%. By comparison, the quality of this film is a bit superior to electrospun ITO film. And the resistance of the sputtered ITO film is thousands of times larger after bending than pristine one. While there is an impact of thickness on the their limited flexibility, the main reason for the sharp increase of the resistance is the break of conducting path for carriers caused by the parallel cracks in the sputtered layer during the bending test.17,27 After bending to 3.5 mm radius for 1000 times, the resistance of blow spun ITO nanofiber film just has a slight change (Figure 3d), which is superior to the electrospun ITO film and the sputtered ITO film.17 And we think the modest improvement of flexibility is due to a higher mass ratio of the inorganic salt (In(NO3)3·4.5H2O/SnCl4·5H2O) to the polymer (PVB) in the precursor, leading to less porous nanofibers in our experiment than the cited work.28 This is one advantage of our method, since


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the precursor with a higher concentration of inorganic salts can still be blow spun into nanofibers with ease. While for electrospinning, a “jam” phenomenon maybe happen at the tip of the syringe needle, since the addition of inorganic salt will increase the conductivity of the precursor solution.29 This result further indicates that blow spun ITO nanofiber film has high flexibility, more durable, and is more suitable to apply to industry. We can easily transfer the film onto different substrates without any surface treatment, because it’s freestanding and macroscopic level web. Figure 4a shows the “conducting leaf”, prepared by transferring the ITO nanofiber film onto leaves. When repeatedly bending and recovering the leaves, it’s still conductive and intact (Figure 4b and Movie 3, supporting information). This transparent electrode has significant advantages for using in biomimetic materials such as electronic skin. As shown in Figure 4c, details of photograph of the freestanding film exhibit no fractures when it bended to ~0.5 mm radius. The SEM image in Figure 4d shows the outstanding flexibility of the ITO nanofiber film. The film was easily folded to a very tiny radius (

Highly Flexible Indium Tin Oxide Nanofiber Transparent Electrodes by

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