Weavable and Highly Efficient Organic Light-Emitting Fibers for

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Weavable and Highly Efficient Organic Light-Emitting Fibers for Wearable Electronics: Achieved by a Scalable, Low-Temperature Process Seonil Kwon, Hyuncheol Kim, Seungyeop Choi, Eun Gyo Jeong, Dohong Kim, Somin Lee, Hoseung Lee, Young Cheol Seo, and Kyung Cheol Choi Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b04204 • Publication Date (Web): 06 Dec 2017 Downloaded from http://pubs.acs.org on December 7, 2017

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Nano Letters

Weavable and Highly Efficient Organic LightEmitting Fibers for Wearable Electronics: Achieved by a Scalable, Low-Temperature Process Seonil Kwon1, Hyuncheol Kim1, Seungyeop Choi1, Eun Gyo Jeong1, Dohong Kim1, Somin Lee1, Ho Seung Lee1, Young Cheol Seo1, and Kyung Cheol Choi1* 1

School of Electrical Engineering, Korea Advanced Institute of Science and Technology

(KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 34141, South Korea

ABSTRACT Fiber-based wearable displays, one of the most desirable requisites of electronic textiles (etextiles), 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 fiber-based organic light-emitting diodes (fiber OLEDs) based on a simple, costeffective 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

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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

TEXT Advances 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 hands-free 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 light-emitting 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


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electronics field. Various strategies have been developed to realize wearable displays, including the use of ultra-thin 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 (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 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 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. Considering that most clothes consist of thermally delicate


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fibers such as cotton, polyester and nylon, manufacturing without a high temperature process is as important as eliminating the high vacuum process in the commercial development of etextiles. 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 solutionprocessed stably. In particular, the ultrathin electron injection layer (such as LiF, Cs2CO3) cannot be uniformly deposited on cylindrical 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 low-temperature solution-processable cathode. The proposed fiber-based 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


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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):

polystyrene

sulfonate

(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 polyethyleneimine (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 glassbased 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


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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 150200°C is required for the deposition of high-quality ZnO thin films,41–43 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 ~ 5nm) were used for HIL (see Experimental Section 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 eV 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


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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 (HRSTEM) and energy dispersive X-ray spectroscopy (EDS) elemental mapping analysis. Crosssectional 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 multi-layer architecture is of great importance, because state-of-the-art, highly efficient solution processed planar OLEDs are based on a multi-layer stack architecture. This result confirms that a nanometer scale multi-layer stack was successfully and uniformly dip-coated on fibers. Figure 2b presents the fiber OLEDs operated at 7 V bias. Figure 2c and 2d 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 < 10V. The current efficiency for the control devices and the fiber OLEDs were 11.6 cd A-1 and 11.1 cd A-1, respectively, with a highest luminance of 48,010 cd m-2 and 11,780 cd m-2, respectively. The fiber OLEDs showed an unprecedentedly high current efficiency over 11 cd A-1, which is the highest value ever and comparable to their conventional


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counterpart.36–38 However, a significant difference was observed between the fiber OLEDs and glass-based control devices in the highest luminance value, due to the increase in series resistance, which is attributed to the difference in conductivity of the PEDOT:PSS in the fiber OLEDs, and the ITO in the glass-based devices (Figure S4). Furthermore, to demonstrate the mechanical flexural properties of the fiber OLEDs, two identical samples (#1-solid symbol, #2-open symbol) were used for bending tests, which were conducted under several radii of curvature, 7 and 3.5 mm, or 5.5 and 2.8 mm, respectively, as shown in Figure 2e. The fiber OLEDs were shown to operate well even after a 1,000 cycle bending test, at a radius of curvature of 3.5 mm. Figure 2f summarize the relative current efficiency changes measured at 4V before and after the cyclic bending tests at various bending radii. The corresponding tensile strain (S) is calculated as S = ℎ⁄2 ,27 where h is the thickness of the substrate and R is the bending radius of curvature. Consequently, the fiber OLEDs were demonstrated to withstand tensile strains of up to 4.3% at a radius of curvature of 3.5 mm, while retaining more than 90% of their current efficiency. A photograph of a bent fiber OLED in operation is shown in Figure 2g. Depending on the textile weaving pattern, the induced strain is known to be adjustable from 10% to 2.2%,17 and thus the above flexibility test implies that the fiber OLEDs will be robust enough for the fabric weaving process. In order to understand the flexural behavior of fiber OLEDs, we performed numerical simulations of the surface strain against external forces depending on fibers and films using the finite-element method. The details of the simulation model are described in Supplementary Note 1. The simulation results show a surface strain applied to the devices on the fiber and the film, respectively, under a bending radius of 3.5 mm (Figure 2h). While the medial surface of the fiber undergoes a tensile strain similar to the one applied to the film, the tensile strain applied to the


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lateral surface of the fibers gradually decreases due to a fall-off in the effective thickness of the substrate. Therefore, it is expected that the flexural behavior of the fiber OLEDs will be superior to that of planar devices. In order to quantitatively analyze the enhancement in outcoupling efficiency due to the effect of the cylindrical shaped geometry quantitatively, optical simulations were performed as a function of the refractive index of the organic layer, using the ray-tracing LightTools simulation. The simulated outcoupling efficiency values of the planar and cylindrically shaped OLEDs are displayed in Figure 3a. The refractive indices of the Super Yellow, PEDOT:PSS, and substrates were 1.89, 1.49 and 1.575, respectively, at a wavelength of 545 nm, which is the main peak of the fabricated OLEDs (Figure S6, Table S2). Assuming that any absorption losses are ignored, the outcoupling efficiency of the conventional planar shaped OLEDs is 13.4%. This result is in quantitative agreement with the calculated value based on the simple ray-optics model using the MATLAB simulation, in the range of the refractive index of the organic layer (Figure S6b). For the cylindrical shaped fibers, outcoupling efficiency of the fiber OLEDs is 15.2%, which means that the cylindrical shaped geometry helps the light extraction slightly more than the planar geometry. But in reality, all of the dip-coated layers in the structure of the fiber OLEDs were coated coaxially on the surface of the entire fiber, and the redundant layers on the opposite side caused extra absorbing losses (Figure S6c). Considering this extra absorbing loss caused by the redundant dip-coated parts in the fiber OLEDs, the simulated outcoupling efficiency of the fiber OLEDs was reduced from 15.2% to 13.3% in Figure 3a. As a result, this extra absorbing loss cancels out the small enhancement in outcoupling efficiency due to the cylindrically shaped geometry. In other words, these simulation results verify that the resulting optoelectronic


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performance of the fiber OLEDs is similar to that of a conventional planar OLED, and the equivalent performance of the fiber OLEDs was mainly due to an enhancement in internal quantum efficiency stemming from the improvement in charge balance, rather than by an enhancement in outcoupling efficiency due to the optical properties of the fiber geometry, as expected. The emission profiles of the fiber OLEDs were also measured depending on viewing angles, to identify the influence of the cylindrical shaped geometry on emission profiles over viewing angles. As depicted in Figure 3b, θ and ϕ directions are parallel and perpendicular to the fiber OLEDs,

respectively.

Figure

3c

and

3d

shows

the

angular

dependence

of

the

electroluminescence (EL) emission characteristics as a function of emission angle in the two directions (θ, ϕ). Angular EL profiles of the fiber OLEDs were also analyzed via LightTools simulation. An isotropic point source was located in the middle of the organic layer by varying the angular range (ψ), from 0 degree (normal direction) to 30-89 degrees (Figure 3b), assuming that the IQE in the fiber OLEDs is invariant with the location of point source. In the θ direction, the normalized EL intensity of the fiber OLED is similar to the simulation result for a lambertian emission, meaning that the light emitted parallel to the fibers is scarcely affected by the geometry of the fibers, and thus the angular EL intensity profiles are analogous to those of the conventional planar devices. In addition, although the angular range of ψ was varied from 30 degrees to 89 degrees, there was virtually no difference in the trend of angular EL intensity (Figure 3c). Meanwhile, the light emitted perpendicular to the fibers (in the ϕ direction) exhibited a unique angular EL characteristic over the viewing angle, a varying angular range of ψ (Figure 3d), due to the effect of the cylindrical shaped geometry. As a result, the actual charge balance of the fiber OLEDs could be estimated using the measured angular EL profiles in the ϕ


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direction. These measured values indicate that the fiber OLEDs have an e-h balance similar to the medial side at angles below 35 degrees, and then the e-h balance for the fiber OLEDs begins to rapidly deteriorate above 35 degrees. This is because the thermally deposited MoO3/Al anode is thinner on the lateral surface of the cylindrical fiber than on the medial surface, and thus, the thinner MoO3 layer fails to act as the HIL, which exacerbates the electron-hole charge carrier balance on the lateral side. Although the angular EL intensity profiles differ from each other depending on the direction, fortunately, in both directions they show negligible change in Commission International d’Echairage (CIE) 1931 chromaticity coordinates at various angles, as shown in Figure S7. After thin film encapsulation, no degradation in performance of the fiber OLEDs was observed (Figure 4a and 4b). The current efficiency and highest luminance value for the encapsulated fiber OLEDs were 11.6 cd A-1 and 13,937 cd m-2, respectively. During the encapsulation process, the fiber OLEDs were exposed to temperatures below 70 °C for about 3h. This incidental postdeposition thermal treatment might lead to an improvement in efficiency and a reduction in the injection barrier of the devices.45 To examine the reliability of encapsulated fiber OLEDs, the operating lifetimes were tested under ambient air with two different conditions; one while being driven at a constant (duty 100%) current density of 1.4 mA cm-2, and the other while being pulse driven (duty 50%, 60Hz) at the same current, which corresponds to an initial luminance (L0) of 150 cd m-2. The half-lifetime (t50) was ca. 80 h for the duty 100% and 230 h for the 50%, respectively (Figure 4c). In addition to high efficiency, the fiber OLEDs demonstrated continuous operating lifetimes at levels comparable to the control devices (~80 h) presented in Figure S5, meaning that the state-of-the-art, highly efficient solution processed planar OLEDs can be applied to cylindrical shaped fibers without a reduction in performance. To the best of our


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knowledge, the luminance, current efficiency as well as lifetime obtained for the fiber OLEDs are among the highest for fiber-based light emitting devices reported to date (Table S3). To investigate the versatile applicability of the proposed fabrication method, fiber OLEDs with different diameters were demonstrated. According to the Landau, Levich and Derjaguin (LLD) theory,46 the thickness of dip-coated layers is known to be determined by several factors, such as the radius of the fiber (r), the viscosity (η) and surface tension (γ) of the solution and the withdrawal velocity (V) of the dip coating. To modulate the thickness of the dip-coated layers on fibers with different diameters, we used the following equation, h ∝ r Ca2/3, where the capillary number (Ca) is defined as Ca = ηV⁄ . As the diameter of the fibers decreases, the thickness of the dip-coated film decreases, and thus the withdrawal velocity should increase to form a uniform thickness of film on fibers with different diameters. Therefore, we developed fiber OLEDs with diameters ranging from 300 µm down to 90 µm just by modulating the withdrawal velocity of the fibers in the dip-coating process. Fiber OLEDs with diameters of 220, 120 and 90 µm were demonstrated at a 7 V bias and a 115-µm-thick human hair was placed next to them for comparison (Figure 5c). As shown in Figure 5a, the thinner fiber OLEDs with a diameter of 90 µm, are highly flexible and the emitting cell is also clear, as before. Due to the limitations of the measured spot (see more details in the Experimental section), the optical characteristics of fiber OLEDs with diameters over 100 µm could be measured. The J-VL characteristics of fiber OLEDs with a diameter of 120 um were obtained to be similar to the characteristics of 300-µm-thick fiber OLEDs (Figure 5b). As the fiber substrates become thinner, the circumference grows smaller, and thus, series resistance increases inevitably, which leads to a small slope of the J-V curve at higher biases (> 8 V) compared with that of 300-µm-thick fibers. Consequently, we developed fiber OLEDs with different diameters of 300, 220, 120 and


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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 demonstrated to successfully form letters of the ‘KAIST’ pattern in operation as shown in Figure 5e-5g. The ultimate goal in fiber-based wearable display research is to produce high performance fiber OLEDs by reel-to-reel production at little cost. Despite the high performance of the fiber OLEDs achieved in this work, the proposed fabrication process still has some room for improvement, because eliminating the thermal deposition process under vacuum is key to the scale-up of fiber OLEDs manufacturing. Several approaches could be taken to address this. One alternative is to use a buffer layer to prevent solvent permeation when depositing the anode by the solution process.47 A lamination technique to fabricate the anode without a solution process could also be a suitable alternative to a thermally evaporated anode.48,49 These and other potential methods would permit the manufacture of high performance fiber OLEDs by costeffective reel-to-reel production in the future. In summary, we demonstrated the fabrication of weavable and highly efficient fiber OLEDs using a simple, cost-effective, and low-temperature dip-coating process. The fabricated fiber OLEDs exhibited high luminance and efficiency and reasonable operating lifetimes, of over 10,000 cd m-2, 11 cd A-1, and 80 h, respectively, which are unprecedented values for fiber based light-emitting devices, and comparable to conventional glass-based OLEDs. The fiber OLEDs withstood tensile strains up to 4.3% at a radius of curvature of 3.5 mm and were successfully woven into textiles by hand. Furthermore, the proposed fabrication methods verified scalability by successfully demonstrating fiber OLEDs with diameters ranging from 300 µm down to 90


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µm. The low-temperature process offers versatility, and is compatible with thermally vulnerable polymer fibers as well. Given these results in this work, we believe that our cost-effective, low-temperature, and scalable approaches pave the way for commercially feasible fiber-based wearable displays and will serve as promising building blocks for future research activities in related fields of wearable electronics.


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METHODS ZnO NPs synthesis. ZnO NPs were synthesized by the following method, previously reported.50,51 First, 1.23 g of zinc acetate dihyrate (Zn(Ac)2·2H2O) was dissolved in 55 ml of methanol (MeOH) at room temperature and placed in a 3-neck round bottom flask. 0.48 g of KOH was dissolved in 25 ml of MeOH and added dropwise at 60⁰C. The reaction mixture was vigorously stirred at that temperature for 125 min and then the synthesized ZnO NPs were washed with MeOH twice by centrifugation at 4000 rpm for 10 min. The precipitate was redispersed in 1-butanol at 2 wt% for further use.

OLED fabrication and evaluation. ITO coated glass substrates (R□ ≈ 15 Ω/□) and polymer fibers were successively cleaned with acetone, isopropyl alcohol and deionized water in an ultrasonic bath. PEDOT:PSS (Clevios PH1000) was filtered with a 0.45 µm PVDF membrane filter and then mixed with 5 wt% dimethyl sulfoxide (DMSO, Sigma Aldrich) and 0.5 wt% Zonyl FS-300 to improve conductivity and wettability.52 For the fiber OLEDs, pre-cleaned fibers were dip-coated in a PEDOT:PSS solution at a withdrawal speed of 40 mm min-1 6 times, for use as a cathode, and then were annealed for 30 min. The ZnO NPs were dip-coated onto the PEDOT:PSS coated fibers at 120 mm min-1 and then annealed for 30 min. PEI diluted with 0.4 wt% 2-methoxyethanol was dip-coated at 1200 mm min-1. After annealing for 10 min, the fibers were dip-rinsed three times at 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 glove box and all annealing processes were implemented at 105 ⁰C as depicted in Fig. 1A. For the ITO coated glass-based counterparts, the synthesized ZnO NPs were spin-coated (3000 rpm, 60 s) on


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ITO coated glass and annealed at 90 ⁰C for 30 min. The PEI solution was spin-coated (5000 rpm, 60 s), annealed at 110 ⁰C 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 ⁰C for 30 min. MoO3 (10nm) and Al (80nm) were deposited on the active layer as the anode by thermal evaporation under vacuum conditions (2 × 10-6 Torr). The fabricated devices were encapsulated by a 50-nm-thick Al2O3 layer via thermal atomic layer deposition (THALD) system at a temperature of 70 ⁰C. Trimethylaluminium (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 5⁰ 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, USA) was used to prepare the cross-section samples of the fiber OLEDs. HR-STEM analysis were performed using a HRTEM (Talos F200X, FEI company, USA) operated at an accelerating voltage of 200 kV and equipped with a Super X EDS system and high angle annular dark-field detector (HAADF).


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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) (0⁰ to 30-89⁰ with increments of 1⁰). The light source was set at a wavelength of 545 nm, which is the main peak of the fabricated OLEDs (Fig. 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. ASSOCIATED CONTENT Supporting Information. Supporting Information is available free of charge. Sheet resistance of the PEDOT:PSS dip coated fibers based on the number of dip-coated layers. Powder X-Ray Diffraction (XRD) 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


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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 present results with those of previously reported works. AUTHOR INFORMATION Corresponding Author *Email: [email protected] Author Contributions S. Kwon conceived the idea and strategy, designed and fabricated the devices and performed the measurements and analysis with assistance from H. Kim, S. Choi, Y.C. Seo., and S. Lee. H. Kim, and H. S. Lee synthesized the ZnO nanoparticles. E. G. Jeong fabricated the passivation layer. S. Kwon, H. Kim, E. G. Jeong and D. Kim prepared the manuscript with assistance from all other co-authors. 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. ACKNOWLEDGMENTS This work was supported by the Engineering Research Center of Excellence (ERC) Program supported by National Research Foundation (NRF), Korean Ministry of Science & ICT (MSIT) (Grant No. NRF-2017R1A5A1014708). We would like to acknowledge the technical support from ANSYS Korea.


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Figure captions 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. 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): (c) Luminance and current density vs voltage; (d) Current efficiency vs luminance. (e) The luminance vs current density of the fiber OLEDs before and after 1,000 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 4V 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 OLEDs 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.


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Figure 3. (a) Outcoupling efficiency of the planar (black) and cylindrical OLEDs with (orange) /without (red) extra absorbing losses produced the redundant dip-coated parts, based on the raytracing simulation (LightTools 8.4). (b) Schematics illustrating that the point sources are located in the middle of the organic layer, by varying the angular range (ψ), and that θ and ϕ directions are parallel and perpendicular to the fiber OLEDs, respectively. Measured and simulated results of normalized electroluminescence emission intensities for viewing angles in increments of 5⁰: (c) θ direction; (d) ϕ direction. The simulation results for viewing angles were integrated by changing the angular range of the light source from 0-30⁰ to 0-89⁰.

Figure 4. OLED device performance before (circle) and after (triangle) encapsulation process: (a) Luminance and current density vs voltage; (b) Current efficiency vs luminance. Inset: a microscopic image of the encapsulated fiber OLED in operation. (c) Operating lifetime of encapsulated fiber OLEDs under continuous (duty 100%, black) and 60Hz pulse (duty 50%, red) driving at a current density of 1.4 mA cm-2. Initial luminance (L0) is 150 cd m-2. 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, 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 handwoven into textiles. Photographs of fiber OLEDs integrated into (f) textiles to form the letters of ‘KAIST’ and (g) a knitwear in operation.


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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) Crosssectional 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. 180x246mm (300 x 300 DPI)


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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): (c) Luminance and current density vs voltage; (d) Current efficiency vs luminance. (e) The luminance vs current density of the fiber OLEDs before and after 1,000 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 4V 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 OLEDs 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.


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Figure 3. (a) Outcoupling efficiency of the planar (black) and cylindrical OLEDs with (orange) /without (red) extra absorbing losses produced the redundant dip-coated parts, based on the ray-tracing simulation (LightTools 8.4). (b) Schematics illustrating that the point sources are located in the middle of the organic layer, by varying the angular range (ψ), and that θ and ϕ directions are parallel and perpendicular to the fiber OLEDs, respectively. Measured and simulated results of normalized electroluminescence emission intensities for viewing angles in increments of 5⁰: (c) θ direction; (d) ϕ direction. The simulation results for viewing angles were integrated by changing the angular range of the light source from 0-30⁰ to 0-89⁰. 111x88mm (300 x 300 DPI)


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Figure 4. OLED device performance before (circle) and after (triangle) encapsulation process: (a) Luminance and current density vs voltage; (b) Current efficiency vs luminance. Inset: a microscopic image of the encapsulated fiber OLED in operation. (c) Operating lifetime of encapsulated fiber OLEDs under continuous (duty 100%, black) and 60Hz pulse (duty 50%, red) driving at a current density of 1.4 mA cm-2. Initial luminance (L0) is 150 cd m-2. 166x362mm (300 x 300 DPI)


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Weavable and Highly Efficient Organic Light-Emitting Fibers for

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