Letter Cite This: Nano Lett. 2018, 18, 347−356
pubs.acs.org/NanoLett
Weavable and Highly Efficient Organic Light-Emitting Fibers for Wearable Electronics: A Scalable, Low-Temperature Process Seonil Kwon, Hyuncheol Kim, Seungyeop Choi, Eun Gyo Jeong, Dohong Kim, Somin Lee, Ho Seung Lee, Young Cheol Seo, and Kyung Cheol Choi* School of Electrical Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 34141, South Korea
Nano Lett. 2018.18:347-356. Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 09/10/18. For personal use only.
S Supporting Information *
ABSTRACT: Fiber-based wearable displays, one of the most desirable requisites of electronic textiles (e-textiles), have emerged as a technology for their capability to revolutionize textile and fashion industries in collaboration with the state-of-the-art electronics. Nonetheless, challenges remain for the fibertronic approaches, because fiber-based light-emitting devices suffer from much lower performance than those fabricated on planar substrates. Here, we report weavable and highly efficient fiberbased organic light-emitting diodes (fiber OLEDs) based on a simple, cost-effective and low-temperature solution process. The values obtained for the fiber OLEDs, including efficiency and lifetime, are similar to that of conventional glass-based counterparts, which means that these state-of-the-art, highly efficient solution processed planar OLEDs can be applied to cylindrical shaped fibers without a reduction in performance. The fiber OLEDs withstand tensile strain up to 4.3% at a radius of 3.5 mm and are verified to be weavable into textiles and knitted clothes by hand-weaving demonstrations. Furthermore, to ensure the scalability of the proposed scheme fiber OLEDs with several diameters of 300, 220, 120, and 90 μm, thinner than a human hair, are demonstrated successfully. We believe that this approach, suitable for cost-effective reel-to-reel production, can realize low-cost commercially feasible fiber-based wearable displays in the future. KEYWORDS: Wearable electronics, wearable displays, fiber electronics, thread displays, dip coating
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(OLEDs),31−37 and polymer light-emitting electrochemical cells (PLECs).38 Among these strategies, fiber-based wearable display devices are considered to be highly desirable because they allow display functions to be incorporated without losing the inherent properties of hierarchically woven clothes, which include important characteristics such as flexibility, comfort, and being durable/washable all at the same time. For such applications, the fiber-based wearable display has a competitive advantage over other strategies for the following several reasons: (i) The method which employs ultrathin substrates has shown very high flexibility due to their thinness but tends to not only easily tear, making the devices vulnerable to wear, but also allowing devices adhered to textiles to easily delaminate. (ii) The fabric/textile-based strategy has the merit of using existing fabrication processes on planar glass and plastic substrates, but the device planarization process can cause their partner fabrics to lose some important properties such as breathability (porosity), which is important for maintaining a
dvances in material science and electronics technologies have enabled the form-factor of electronic devices to be miniaturized into a nearly imperceptible configurations, including wearable/stretchable platforms that permit both deformability and portability.1,2 Electronic clothing systems are widely believed to have great potential as ideal wearable platforms because they can be worn at all times, provide handsfree operation, flexibility, portability, and comfort.3,4 Accordingly, numerous studies have recently been dedicated to realizing that potential. Wearable devices including passive and active components such as conductors,5−8 sensors,9 supercapacitors,10−12 transistors and logic circuits,13−16 lightemitting devices,17 and power sources including photovoltaic cells and nanogenerators,18−26 have been demonstrated for promising applications in smart clothing (e-textiles). Among these applications, wearable displays have attracted attention as an emerging technology in the textile and fashion industries, in collaboration with the state-of-the-art electronics field. Various strategies have been developed to realize wearable displays, including the use of ultrathin and/or stretchable substrates.27−29 In addition, methods of directly fabricating displays on textiles and/or fibers have been pursued by utilizing inorganic light-emitting diodes (LEDs),17,30 organic LEDs © 2017 American Chemical Society
Received: September 29, 2017 Revised: November 23, 2017 Published: December 6, 2017 347
DOI: 10.1021/acs.nanolett.7b04204 Nano Lett. 2018, 18, 347−356
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Figure 1. (a) Schematic illustration of the proposed fabrication scheme on cylindrical fibers. Except for the anode deposition, the entire deposition process is implemented by dip coating and thermal annealing under 105 °C. (b) Cross-sectional SEM (left) and HR-STEM (right) image of the fabricated fiber OLEDs. (c) Cross-sectional HAADF-STEM image and the corresponding STEM-EDS elemental mapping images of sulfur (S) (red), zinc (green), carbon (gray), molybdenum (Mo) (blue), Al (yellow) and oxygen (cyan) showing their different colors. Spatially resolved STEM-EDS elemental mapping showed good separation of the layers. Mo peaks are observed in the S mapping image due to the overlap of the Mo and S peaks.
Considering that most clothes consist of thermally delicate fibers such as cotton, polyeste, and nylon, manufacturing without a high temperature process is as important as eliminating the high vacuum process in the commercial development of e-textiles. In our previous studies, fiber-based PLEDs were realized using a dip-coating method, although it was necessary to deposit the low work-function LiF/Al bilayer cathode by thermal evaporation process because of its reactivity. Chemically reactive cathodes generally cannot be solution-processed stably. In particular, the ultrathin electron injection layer (such as LiF, Cs2CO3) cannot be uniformly deposited on cylindrical
comfortable and consistent body temperature, by allowing the evaporation of sweat from skin. Fiber-shaped PLECs have recently been developed using an all-solution process, which promises a simple and low-cost route for roll-to-roll manufacturing without a high vacuum process.38 However, unlike OLEDs PLECs have a unique operating mechanism which involves the diffusion of mobile ions, and this leads to an intrinsically slow response time and rather poor lifetime. Those characteristics have limited the practical use of PLECs in display applications. In addition, all-solution processed fiber-shaped PLECs have been fabricated on steel wires using a high temperature process. 348
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Figure 2. (a) Schematic showing the device structure of ITO glass-based OLEDs (left) and the proposed fiber OLEDs (right). (b) Photograph of the proposed fiber OLED, which has a diameter of 300 μm, in operation. Inset: a microscopic image of the emitting cell of the fiber OLED in operation. OLED device performance on glass (black) and fiber (red) for (c) luminance and current density versus voltage and (d) current efficiency versus luminance. (e) The luminance versus current density of the fiber OLEDs before and after 1000 cyclic bending tests at various bending radii. Two identical samples (#1-closed symbols, #2-open symbols) were used for the cyclic bending tests under several radii of curvature, 7 and 3.5 mm or 5.5 and 2.8 mm, respectively. (f) The relative current efficiency changes measured at 4 V before and after the cyclic bending tests and the corresponding tensile strain. Inset: a side view of the 3.5 mm bending (Scale bar: 2 mm). (g) Photograph of a bent fiber OLED in operation. A numerical simulation of the elastic strain distribution under a bending radius of 3.5 mm was conducted using the finite-element method: (h) The surface strain on the cylindrical fibers and the planar films along the path shown in the inset. Inset: the cross-sectional images of a fiber and a film, respectively. Logo reproduced with permission from KAIST, 2017. 349
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Nano Letters shaped fibers via thermal evaporation without using a costly rotational thermal evaporation technique. The resulting unevenness causes a disproportionate distribution of electron−holes on the fibers, resulting in much lower device performance compared with planar OLEDs. To address this issue, further research is needed, to improve fiber optoelectronic performance and reliability including luminance, efficiency, and lifetime up to levels comparable to flat OLEDs, and more importantly, to realize all-solution processed fiber-based OLEDs with high performance. In this paper, we propose weavable and highly efficient organic light-emitting fibers based on a simple, cost-effective solution fabrication technique with low-temperature process. We revised the device structure to provide uniformly efficient electron injection on the fibers and adopt an air-stable and lowtemperature solution-processable cathode. The proposed fiberbased OLEDs (fiber OLEDs) exhibited luminance and current efficiency values of over 10,000 cd m−2 and 11 cd A−1, respectively, values which are on par with ITO glass-based counterparts. After thin film encapsulation, the encapsulated fiber OLEDs also showed operating lifetimes similar to control devices. The fiber substrates used in this work were polyethylene terephthalate (PET) fibers, which are widely used and can be obtained at little cost. Also, the proposed fabrication scheme mainly uses a dip coating method, a simple yet cost-effective solution process, which can significantly reduce production costs for fiber-based wearable displays. Furthermore, the proposed fabrication scheme employs an annealing process with as low a temperature (∼105 °C) as possible and OLEDs were also demonstrated on 90 μm thick fibers thinner than human hair, which attests to their versatility for application on delicate fibers, and the scalability of the proposed fabrication scheme. The developed fiber OLEDs were verified to be flexible and weavable by flexural strain tests and hand-weaving demonstration. Figure 1a shows a schematic diagram of the proposed fabrication process on fibers. All of the fabrication steps before anode deposition use a dip-coating method, which is a simple yet highly effective solution fabrication technique to concentrically coat the cylindrical-shaped fibers. The OLED structure we adopted here is a bottom-emission inverted fluorescent structure, in which light emitted in the emitting layer (EML) passes through the substrates. The conducting polymer poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) (Heraeus PH1000) and indium tin oxide (ITO) electrodes were used as the cathodes of the fiber OLEDs and the glass-based counterparts, respectively. Zinc oxide nanoparticles (ZnO NPs) and polyethylenimine (PEI), Super Yellow (Merck, ELUMO ≈ −3.0 eV, EHOMO ≈ −5.4 eV),39,40 molybdenum oxide (MoO3) and Al acted as the electron injection layer (EIL), EML, hole injection layer (HIL) and anode, respectively. The inverted OLEDs were fabricated by dip coating the fibers or by spin coating for the glass-based counterparts, depending on the PEDOT:PSS or ITO/ZnO/PEI/Super Yellow/MoO3/Al device architecture depicted in Figure 2a. One may note that inexpensive, industrially used polymer fibers usually have a rough surface morphology, unlike rather expensive optical fibers or metal wires, and conventionally a planarization layer is considered to be essential when using plastic substrates. However, this planarization step can introduce unwanted effects such as stiffening. In our previous study, dip coating PEDOT:PSS on a cylindrical fiber was shown
to improve surface roughness and was sufficient to ensure device stability without any planarization layer.36 In the present design, the conducting polymer acts simultaneously as a cathode and a planarization layer. This was accomplished by dip coating the fiber six times to achieve a conductive electrode with sufficiently low sheet resistance (Figure S1). ZnO is well-known as an electron injection layer in organic semiconductors, as an alternative to the use of chemically reactive low work function (WF) materials such as Ca, Mg/Ag and Ba/Al, or ultrathin EIL (e.g., LiF, Cs2CO3). Generally, a high-temperature process exceeding 150−200 °C is required for the deposition of high-quality ZnO thin films41−43 but this would not be compatible with temperature-sensitive polymer substrates. Unlike other deposition methods, such as sol−gel, colloidal NPs can be deposited using a low-temperature annealing process and still produce a high-quality layer because this is accomplished by decoupling the nanoparticle chemical synthesis and the layer deposition process. In this work, synthesized ZnO NPs (average size ∼5 nm) were used for HIL (see Methods and Figure S2 for details). However, a large energy barrier exists between the ZnO and Super Yellow (≈ 1.2−1.6 eV) which can limit effective electron injection into the EML. Recently, PEI and polyethylenimine ethoxylated (PEIE) were introduced as an interfacial layer to reduce the WF of metal oxides by inducing a strong dipole moment on the metal oxide layers.44 In particular, following the application of an ultrathin PEI interfacial layer, the WF of ZnO was reported to decrease from 4.3 to 3.1 eV. Accordingly, a combination of ZnO and PEI was adopted for efficient electron injection in the present inverted OLED structure, which subsequently exhibited performance similar to a conventional OLED structure with an LiF/Al configuration as the cathode (Figure S3). After MoO3/Al was thermally evaporated on the EML, if necessary, the fiber OLED devices were encapsulated by an Al2O3 single layer. Before measuring the optoelectronic performance of the devices, the structure of the proposed fiber OLEDs was investigated to verify successful deposition on the fibers, using scanning electron microscopy (SEM), high-resolution scanning transmission electron microscopy (HR-STEM), and energy dispersive X-ray spectroscopy (EDS) elemental mapping analysis. Cross-sectional SEM and HR-STEM images of the fiber OLEDs are shown in Figure 1b. Although the PEDOT:PSS electrode and the fiber substrate are difficult to distinguish in the HR-STEM image, the individual layers are unambiguously divided in the results of the EDS mapping analysis (Figure 1c). The device consists of a 175 nm PEDOT:PSS electrode, a 20 nm ZnO and PEI layer as the EIL, a 45 nm Super Yellow as the EML, 10 nm MoO3 layer as the HIL, ∼80 nm Al anode, and 50 nm Al2O3 encapsulation layer, which shows good agreement with the proposed device structure presented in Figure 2a. The solution deposition of a multilayer architecture is of great importance, because state-ofthe-art, highly efficient solution processed planar OLEDs are based on a multilayer stack architecture. This result confirms that a nanometer scale multilayer stack was successfully and uniformly dip-coated on fibers. Figure 2b presents the fiber OLEDs operated at 7 V bias. Figure 2c,d shows the current density−voltage−luminance (J− V−L) characteristics of the ITO glass-based OLED and the fiber OLED. The resulting fiber OLED devices exhibited a 2.25−2.5 V turn-on voltage and reasonable operating voltage 8 V) compared with that of 300 μm thick fibers. Consequently, we developed fiber OLEDs with different diameters of 300, 220, 120, and 90 μm, thinner than a human hair, demonstrating that the proposed fabrication method is scalable. Finally, inspired by the aforementioned flexibility test, the fabricated fiber OLEDs were weaved into commercially available textiles and knitted clothes by hand and 353
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Figure 5. (a) Photograph of an operating fiber OLED with a diameter of 90 μm. (b) J−V−L characteristics of the fiber OLEDs with different diameters of 120 and 300 μm. (c) Demonstrations of 220, 120, and 90 μm thick fiber OLEDs in operation. A human hair is placed next to the fiber OLEDs for comparison. (d) SEM images of fibers with different diameters of 220, 120, and 90 μm, and a human hair (Scale bar: 200 μm). (e) Demonstration of fiber OLEDs hand-woven into textiles. Photographs of fiber OLEDs integrated into (f) textiles to form the letters of “KAIST” and (g) a knitwear in operation.
600 mm min−1 in ethanol to eliminate any PEI surplus. After an annealing process for 5 min, a chlorobenzene solution consisting of Super Yellow (0.8 wt %) was dip-coated at 40 mm min−1. All solution processes were performed inside a nitrogen-filled glovebox and all annealing processes were implemented at 105 °C as depicted in Figure 1A. For the ITO coated glass-based counterparts, the synthesized ZnO NPs were spin-coated (3000 rpm, 60 s) on ITO coated glass and
annealed at 90 °C for 30 min. The PEI solution was spin-coated (5000 rpm, 60 s), annealed at 110 °C for 30 min and rinsed with ethanol. For the emitting layer, Super Yellow dissolved in a chlorobenzene at 0.5 wt % was spin-coated (3500 rpm, 40 s) and annealed 110 °C for 30 min. MoO3 (10 nm) and Al (80 nm) were deposited on the active layer as the anode by thermal evaporation under vacuum conditions (2 × 10−6 Torr). 354
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Nano Letters The fabricated devices were encapsulated by a 50 nm thick Al2O3 layer via thermal atomic layer deposition (THALD) system at a temperature of 70 °C. Trimethylaluminum (TMA) and H2O were used as the precursors of the Al2O3 and the reactant, respectively. The current density−voltage−luminance characteristics were measured using a Keithley 2400 source measurement unit and a calibrated BM-7A luminance colorimeter with a closeup lens (AL-12) in a nitrogen-filled glovebox. The cyclic bending test was done with a high-precision customized bending machine in a nitrogen-filled glovebox. The angle-resolved measurement was conducted at increments of 5° using a CS-2000 spectroradiometer with a closeup lens (CS-A35) under ambient air. For the precise calculation of the emitting area of the fiber OLEDs, the area of the emitting cells was calculated using the following equation, considering the cylindrical shape of fiber OLEDs: Area = π × r × L, where r is the radius of the fibers and L is the length of the emitting cells. A cross-section polisher (IB-09010CP, JEOL) and a focused ion beam (FIB) system (Quanta 3D FEG, FEI company, U.S.A.) was used to prepare the cross-section samples of the fiber OLEDs. HR-STEM analysis were performed using a HRTEM (Talos F200X, FEI company, U.S.A.) operated at an accelerating voltage of 200 kV and equipped with a Super X EDS system and high-angle annular dark-field detector (HAADF). Geometrical Optics Simulation. We performed a geometrical simulation to verify the outcoupling efficiency and radiant angular profiles based on the geometrical shape of the fibers, using a ray-tracing simulation (LightTools 8.4). For the sake of convenience, the OLED structure was simplified into three layers consisting of anode, organic layer, and cathode. The thickness of the cylindrical fiber was set at the actual value. An isotropic point source located in the middle of the organic layer was used for the conventional planar OLEDs. For the cylindrical fiber OLEDs, an isotropic point source was located in the middle of the organic layer by varying the angular range (ψ) (0° to 30−89° with increments of 1°). The light source was set at a wavelength of 545 nm, which is the main peak of the fabricated OLEDs (Figure S6a). The refractive indices of the organic layer and PEDOT:PSS were either measured by ellipsometry measurements or were taken from the literature (Table S2). Radiant flux was integrated using a spherical detector surrounding the entire structure.
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present results with those of previously reported works (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Kyung Cheol Choi: 0000-0001-6483-9516 Author Contributions
S.K. conceived the idea and strategy, designed and fabricated the devices and performed the measurements and analysis with assistance from H.K., S.C., Y.C.S., and S.L. H.K., and H.S.L. synthesized the ZnO nanoparticles. E.G.J. fabricated the passivation layer. S.K., H.K., E. G.J., and D.K. prepared the manuscript with assistance from all other coauthors. K.C.C supervised this project. The manuscript was written with contributions from all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Engineering Research Center of Excellence (ERC) Program (Grant No. NRF2017R1A5A1014708) and Nano·Material Technology Development Program (Grant No. NRF-2016M3A7B4910635) supported by the National Research Foundation of Korea (NRF), the Ministry of Science & ICT (MSIT). We thank Prof. Jeong Ku Kang and Sang Rim Shin from KAIST for help in the use of a cross-section polisher. We would like to acknowledge the technical support from ANSYS Korea.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.7b04204. Sheet resistance of the PEDOT:PSS dip-coated fibers based on the number of dip-coated layers. Powder X-ray diffraction of the synthesized ZnO NPs and calculated nanoparticle size. J−V−L characteristics of the normal and inverted OLEDs. J−V−L characteristics of the normal OLEDs using PEDOT:PSS or ITO electrodes. Operating lifetime and applied bias of the inverted planar OLEDs based on ITO-coated glass substrates. Comparison of the LightTools raytracing optical simulation with a simple ray-optics model simulation result. Change in CIE 1931 coordinates at different emission angles. FEM simulation of the flexural behavior. Table comparing the 355
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DOI: 10.1021/acs.nanolett.7b04204 Nano Lett. 2018, 18, 347−356
