Flexible Organic Light-emitting Devices Research Articles

Stable Ultrathin Ag Electrodes by Tailoring the Surface of Plastic Substrates for Flexible Organic Light-Emitting Devices.

Flexible transparent metal electrodes (FTMEs) have significant application potentials in the fields of flexible optoelectronic devices due to their outstanding optical transmittance and electrical conductivity. However, obtaining excellent optoelectrical properties and mechanical flexibility of FTMEs is challenging because ultrathin metal layers usually follow an island growth mode. In this paper, flexible transparent ultrathin Ag electrodes with high mechanical stability and good optoelectrical properties were exploited by tailoring the surface properties of plastic substrates with ultraviolet-ozone (UVO) treatment for regulating the nucleation and growth kinetics of Ag films. The composite transparent electrodes of Ag (9 nm)/MoO3 (20 nm) fabricated on the UVO-treated polyethylene terephthalate (PET) substrates possess a low sheet resistance of ∼7.9 Ω/sq, a high optical transmittance of ∼87.2% at 550 nm, a long-period environmental stability of 30 days (∼65 °C, ∼80% humidity), and excellent mechanical flexibility of 100,000 bending cycles at a bending radius of 1.5 mm. These properties are derived from the surface treatment of PET substrates by UVO, which increases substrate surface energy and produces chemical nucleation sites of the phenolic hydroxyl groups. The phenolic hydroxyl groups generated on the PET surface not only provided efficient nucleation sites for subsequent Ag film growth but also formed C-O-Ag bonds between the substrate surface and the Ag layer, which act as "anchor chains" to fix firmly the Ag atoms on the substrate surface. As a universal applicability strategy, the composite electrodes on the UVO-treated polyethylene naphthalate (PEN) and norland optical adhesive 63 (NOA63) substrates also possess excellent optoelectrical properties and mechanical flexibility. Based on the ultrathin Ag composite electrodes, the flexible white organic light-emitting devices with PET, PEN, and NOA63 as substrates present the maximum current efficiencies of 53.0, 77.0, and 65.2 cd/A, respectively.

Ultra-Smooth and Efficient NiO/Ag/NiO Transparent Electrodes for Flexible Organic Light-Emitting Devices.

We demonstrate highly flexible and efficient organic light-emitting devices (OLEDs) using an ultra-smooth NAN anode. A template-stripping process was employed to create an ultra-smooth NAN anode on a photopolymer substrate. The flexible OLEDs obtained by this method maintained good electroluminescent properties and mechanical stability after bending. The efficiency of the flexible OLEDs was improved by 30.6% compared with conventional OLEDs deposited on PET/ITO substrate due to the enhanced hole injection from the ultra-smooth anode.

Flexible alumina films prepared using high-bias pulse power for OLED thin film encapsulation

To overcome the moisture sensitivity of flexible organic light-emitting diode devices (OLEDs), physical vapor deposition (PVD) technology can be used to prepare films with excellent density for thin film encapsulation (TFE). The internal stress in the film should be modulated to enhance the flexibility, which can prevent structural degradation. To address these issues, a filtered cathode vacuum arc (FCVA) equipped with a superlow duty ratio high-bias pulsed was used to prepare Al2O3 films with low internal stress (−57.6 MPa), excellent visible light transmittance, high packaging density, and good bending resistance. After 1000 bending cycles, the water vapor transmission rate (WVTR) for a film is increased to 5.57 × 10−3 g/m2/day at 85 °C and 85% relative humidity (R.H.). In addition, the molecular dynamics (MD) is employed to investigate the mechanism of film internal stress release, as well as the shallow implantation of ions brought about by high-bias pulse.

Analysis on the Mechanical Behavior of Flexible Screens.

Recently, flexible organic light-emitting devices (OLEDs) have become more and more popular. However, the force distribution and deformation are very complex during the bending process, and it is difficult to analyze the stress and strain by theoretical analysis and direct experiment. In this paper, finite element analysis of the bending model for the flexible screen was performed. For common U-shaped bending, it was shown that the maximum Mises stress increases rapidly as the bending radius decreases, and the redistribution of the tensile zone and the compression zone should be the key to the layer material selection. The results were verified by an imaging experiment. Further, a water-drop-shaped bending mode was analyzed to reduce the risk of structure failure.

Organic light-emitting devices based on conducting polymer treated with benzoic acid

We report on the enhanced conductivity of the benzoic-acid-treated poly(3,4-ethlenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) electrode for use in highly flexible, organic light-emitting devices (OLEDs). The conductivity of the benzoic-acid-treated PEDOT:PSS electrode increased from 1 to 1583.2 S/cm, in comparison with that of the pristine PEDOT:PSS electrode, due to a complex factor of the H+ mole % and the dielectric constant of the benzoic solution. Among the post-treatment methods of the PEDOT:PSS electrodes, the operating voltage at 1000 cd/m2 of OLEDs fabricated utilizing the PEDOT:PSS electrode with the benzoic acid treatment has the lowest value, and its maximum luminance is 24,400 cd/m2, which are 1.54 and 2.15 times higher than those of OLEDs using PEDOT:PSS electrodes treated with dimethyl sulfoxide and methanol, respectively. The luminance of a flexible OLED with a benzoic-acid-treated PEDOT:PSS electrode after 1400 bending cycles decreased to 83% of the initial luminance, resulting in excellent mechanical stability.

A Systematic Study of the Interactions in the Top Electrode/Capping Layer/Thin Film Encapsulation of Transparent OLEDs

Development of flexible transparent organic light-emitting devices (TOLEDs) still requires a number of advancements in transparent conducting electrodes with low reflection and absorption, a capping layer (CL) acting as refractive index-matching, and thin film encapsulation (TFE) with high water vapor barrier properties, among others. While substantial research has been reported on isolated examples in each area, there has been no detailed and systemic research related to the overall interactions of top electrodes, CLs, TFE, and their interfaces. In this work, TOLEDs have been fabricated with a thin Ag top electrode and CLs of different surface energy, which was encapsulated with high water vapor barrier property (1.35Ⅹ10−4 gm−2day−1 at 37.8 °C and 100 % RH). The encapsulation barrier was comprised of 50-nm-thick Al2O3 thin films deposited using a low-temperature (95 °C) ALD process. Once prepared, the TOLEDs were studied using a variety of techniques to determine the enhancements to electrical, optical, and water vapor barrier properties. Although the nature of the CL materials affects the film formation on the top electrode layer, there is no impact on the properties of the Al2O3 thin films. In the formation process of the Al2O3 TFE, the device incorporating a tris(8-hydroxyquinoline)aluminum (Alq3) CL showed superior performance, whereas device performance degredation was noted with the 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile (HATCN) CL. The lack of degradation in the Alq3 CL device during ALD processing is attributed to the high thermal stability of Alq3, which exhibits a high glass transition temperature of 175 °C. However, in the HATCN CL device, it is expected that formation of aromatic radical anions [HAT(CN)6]•− and dianions [HAT(CN)6]2−, and/or band bending of fermi energy of the HATCN at the interface of the thin Ag layer, results in the observed degraded performance.

Plasmonic ultrathin metal grid electrode induced optical outcoupling enhancement in flexible organic light-emitting device

We propose a plasmonic ultrathin metal grid electrode as a promising light outcoupling strategy in a flexible organic light-emitting device (OLED). The plasmonic ultrathin metal grid electrode consists of an ultrathin Au grid film incorporating with plasmonic subwavelength corrugations, which was fabricated by lithography and nanoimprint technologies, demonstrates excellent optical and electrical properties as the substitute of ITO transparent electrode in flexible OLED. The transmittance of the plasmonic ultrathin Au grid electrode is 88.5% at 550 nm and the sheet resistance is about 36 Ω/sq. The periodic subwavelength corrugations of the plasmonic ultrathin Au grid electrode excite the surface plasmon-polariton (SPP) resonance which is an adverse optical mode with power loss in OLED, and the photons trapped in SPP mode are effectively outcoupled and extracted by the plasmonic corrugations. Compared to the ITO-based device, OLED based on plasmonic ultrathin Au grid electrode realizes a desirable enhancement of 60.3% in current efficiency. In addition, the flexible OLED based on the proposed electrode also exhibits decent mechanical robustness and flexibility with stable electroluminescence performances.

Highly transparent and conductive metal oxide/metal/polymer composite electrodes for high-efficiency flexible organic light-emitting devices

Abstract Ultrathin metal films emerge as an innovative category of transparent electrodes in recent decades, holding great promises enabling the next-generation flexible organic light-emitting devices (OLEDs). Although metal thin films with polymer nucleation inducers have been extensively studied in OLEDs, satisfying the requirements of both superior optoelectrical and high optical outcoupling characteristics is still challenging. Here, we demonstrate a metal oxide/ultrathin Ag/polymer (MAP) composite electrode with low sheet resistance of 15.1 Ω/sq, high transmittance of 87.4% at 550 nm, and smooth morphology with surface roughness of 0.768 nm. Besides, the composite electrodes significantly enhance the outcoupling of the light trapped in OLEDs due to the relatively high-refractive index polymer. Flexible OLEDs with the MAP anodes exhibit over 2.3 times enhancement in efficiency to that of indium tin oxide (ITO)-based OLEDs. The flexible OLEDs can survive 1000 bending cycles at a bending radius of 8 mm with negligible decrease in electroluminescent performance.

Highly linearly polarized light emission from flexible organic light-emitting devices capitalized on integrated ultrathin metal-dielectric nanograting.

Although there have been tremendous achievements ever since the first work on an organic electroluminescent (EL) device that emitted polarized light, the development of flexible polarized emission organic light-emitting devices (OLEDs) is not without hurdles, and the challenge towards real-world applications still requires tremendous effort. In this paper, we proposed highly linearly polarized light-emission from flexible green OLEDs capitalized on integrated ultrathin metal-dielectric nanograting. The acquired polarized device with meticulously optimized geometric parameters yields an angle-invariant average extinction ratio beyond 20.0 dB within a viewing angle range of ± 60°. The detailed analysis illustrates that surface plasmons and cavity modes are simultaneously contributed to the TM-polarized light selection. We hope that the presented approach will open new opportunities for designing flexible polarized light sources.

Transparent ultra-thin silver electrodes formed via a maskless evaporation process for applications in flexible organic light-emitting devices

Abstract We have fabricated flexible, transparent, ultra-thin silver (Ag) electrodes by using a maskless deposition process and applied them to organic light-emitting devices (OLEDs). A selective ultraviolet/ozone (UV/O3) treatment was performed on the polyethylenimine (PEI) polymer layer, which contained amine groups and was coated onto a silicon substrate, in order to form an electrode pattern. An ultra-thin Ag film was deposited in vacuum on the UV/O3-treated substrate without a mask, resulting in the formation of a flexible, transparent, ultra-thin Ag electrode pattern. The optimum PEI concentration and the optimum UV/O3 treatment time for the formation of flexible, transparent, Ag electrode patterns were determined from their sheet resistances, optical transmittances, Fourier-transform infrared spectra and X-ray photoelectron spectra. The sheet resistance of the ultra-thin Ag electrode transferred to an epoxy substrate changed within 5% only when the bending diameter of curvature and the number of bending cycles were 5 mm and 3000, respectively. The current density and the luminance of an OLED with a flexible, transparent, ultra-thin Ag electrode deposited on an epoxy substrate were larger than those of an OLED with an indium-tin-oxide (ITO) electrode formed on a polyethylene naphthalate substrate. We have confirmed that the flexible, transparent, ultra-thin Ag electrodes hold promise for use in the fabrication of flexible OLEDs.

A NEW STRUCTURE OF FLEXIBLE OLED WITH COPPER NANOWIRE ANODE AND GRAPHERE OXIDE/PEDOT: PSS ANODE BUFFER LAYER

A flexible organic light-emitting device (OLED) was produced using copper nanowire (CuNW) film as anode and Graphene oxide (GO)/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) film as anode buffer layer. Compared with other transparent conductive films (TCFs), CuNWs are low cost, easy to fabricate, and compatible with flexible substrates over a large area. Due to these advantages, CuNWs are showing greater and greater promise for the next generation of TCF. Modified by PEDOT:PSS, the conductivity and work function of the CuNW film can be dramatically enhanced. However, PEDOT:PSS is highly acidic and easy to corrode the CuNW film, which will reduce maximum luminous brightness and current efficiency of the OLED. In this paper, GO/PEDOT:PSS was used as anode buffer layer to modify the CuNW anode and the composite transparent electrode exhibited excellent optoelectrical properties. The driving voltage of the OLED with CuNW/PEDOT:PSS is 6.2[Formula: see text]V, and the maximum luminous brightness is 2737.2[Formula: see text]cd/m2. The driving voltage of the OLED with CuNW/GO/PEDOT:PSS anode was reduced to 5.1[Formula: see text]V, and the maximum luminous brightness was improved to 3007.4[Formula: see text]cd/m2.

Highly transparent and flexible fabric-based organic light emitting devices for unnoticeable wearable displays

Abstract Fabric displays integrated into clothing are important members of wearable electronics. Highly transparent and flexible organic light emitting devices (OLEDs) fabricated on fabrics can make clothing keep the original appearance and wearing comfort and so are under focused research. Indium tin oxide is the main transparent electrodes due to its high transparency. However, its brittleness makes OLEDs lack of flexibility. Here ultrathin metal films are used as anodes and cathodes in OLEDs for both high transparency and flexibility. The anode and cathode are 7 nm Au film and 9 nm Ag film respectively. As a result, the total transmittance of the OLEDs is 74% at 550 nm wavelength when combining with a capping layer. Fabric displays based on the transparent and flexible OLEDs are realized. The patterns of the fabric with different colors can be seen clearly through the OLEDs. At the same time, the fabric display can withstand a 1 mm bending radius. This is the first reported transparent and flexible fabric displays, which will be useful for smart IT devices that are comfortable and fashionable to wear.

Highly efficient flexible organic light-emitting devices based on PEDOT:PSS electrodes doped with highly conductive Pyronin B

Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is a promising material for utilization in flexible transparent electrodes. However, because the conductivity of PEDOT:PSS is as low as 1 S/cm or even lower, PEDOT:PSS has the inherent problem of a conductivity that is too low for it to be used in the fabrication of flexible, transparent electrodes for use in flexible organic light-emitting devices (OLEDs). Here, we report on hole injection enhancement in a highly conductive Pyronin-B-doped PEDOT:PSS electrode/hexaazatriphenylene hexacarbonitrile (HAT-CN) layer, a strong electron-accepting charge-generation layer, for use in highly efficient flexible OLEDs. While the conductivity of the pristine PEDOT:PSS is 1 S/cm, that of Pyronin-B-doped PEDOT:PSS is significantly increased to 1467 S/cm due to the dramatically enhanced screening effect for PEDOT:PSS films. The energy-band bending barrier at the interface between the Pyronin-B-doped PEDOT:PSS electrode and the HAT-CN layer is significantly decreased compared to that between the pristine PEDOT:PSS electrode and the HAT-CN layer, resulting in improved hole injection. The operating voltage and the current efficiency of the flexible OLEDs with Pyronin-B-doped PEDOT:PSS/HAT-CN electrodes (6.8 V, 26.03 cd/A) are similar to those of OLEDs with ITO/HAT-CN electrodes (7.7 V, 27.95 cd/A). The brightness of the OLEDs after 900 cycles of bending with a radius of curvature of 5 mm is 80% the initial brightness, indicative of the stability of these flexible devices under repeated bendings.

Effects of hole injection layer on performance of green OLEDs based on flexible ITO

Hybrid 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile(HAT-CN)/molydium oxide (MoO3) is employed as hole injection layer (HIL) to modify the flexible ITO anode in bis(2-phenylpyridine)(acetylacetonate)iridium (Ir(ppy)2acac)-based green phosphorescence flexible organic light-emitting devices (FOLEDs). The hybrid HAT-CN/MoO3 leads to high hole injection efficiency and improvement in device performance. The device with HAT-CN(5 nm)/MoO3(1 nm) as HIL shows a turn-on voltage of 3.0 V, a maximum current efficiency (CEmax) of 41.29 cd/A and maximum power current efficiency (PEmax) of 15.92 lm/W, indicating the maximum efficiency increase by 41.6% and 53.1% compared with that of the single MoO3 as HIL, respectively. The smooth current injection can be achieved by the HAT-CN's ability of cleaning ITO surface. The hybrid HAT-CN/MoO3 reduces the hole injection barrier and suppresses the space change accumulation in electrode/organic layer interface to enhance hole injection efficiently. Our work suggests that the effective modification for flexible ITO anode is necessary to obtain high-efficiency FOLEDs.

Flexible, transparent patterned electrodes based on graphene oxide/silver nanowire nanocomposites fabricated utilizing an accelerated ultraviolet/ozone process to control silver nanowire degradation

We developed flexible, transparent patterned electrodes, which were fabricated utilizing accelerated ultraviolet/ozone (UV/O3)-treated graphene oxide (GO)/silver nanowire (Ag-NW) nanocomposites via a simple, low-cost pattern process to investigate the feasibility of promising applications in flexible/wearable electronic and optoelectronic devices. The UV/O3 process of the GO/Ag-NW electrode was accelerated by the pre-heat treatment, and the degradation interruption of Ag NWs was removed by the GO treatment. After the deposition of the GO-treated Ag NW electrodes, the sheet resistance of the thermally annealed GO-treated Ag-NW electrodes was significantly increased by using the UV/O3 treatment, resulting in a deterioration of the GO-treated Ag NWs in areas exposed to the UV/O3 treatment. The degradation of the Ag NWs caused by the UV/O3 treatment was confirmed by using the sheet resistances, scanning electron microscopy images, X-ray photoelectron microscopy spectra, and transmittance spectra. While the sheet resistance of the low-density Ag-NW electrode was considerably increased due to the pre-thermal treatment at 90 °C for 10 min, that of the high-density Ag-NW electrode did not vary significantly even after a UV/O3 treatment for a long time. The degradation interference phenomenon caused by the UV/O3 treatment in the high-density Ag NWs could be removed by using a GO treatment, which resulted in the formation of a Ag-NW electrode pattern suitable for promising applications in flexible organic light-emitting devices. The GO treatment decreased the sheet resistance of the Ag-NW electrode and enabled the pattern to be formed by using the UV/O3 treatment. The selective degradation of Ag NWs due to UV/O3 treatment decreased the transparency of the Ag-NW electrode by about 8% and significantly increased its sheet resistance more than 100 times.

Ultrathin, lightweight and flexible organic light-emitting devices with a high light outcoupling efficiency

Abstract High-efficiency, flexible organic light-emitting diodes (OLEDs) are of interest for display and lighting applications. However, they often suffer from inefficient light extraction, and many outcoupling schemes are incompatible with flexible OLEDs. Here, we demonstrate a corrugated, ultrathin (10 μm), light-weight (20 g/m2), and flexible OLED on a polychloro-p-xylylene (parylene) substrate. A visible-wavelength-scale random corrugation pattern is imprinted on both surfaces of the parylene substrate that efficiently outcouples trapped substrate, waveguide and surface plasmon modes. A green phosphorescent OLED fabricated on a corrugated parylene substrate (CP-OLED) has an external quantum efficiency of 28 ± 2% compared with 21 ± 1% for devices on a conventional flat glass substrate. The CP-OLED shows a Lambertian intensity profile whose spectra are unchanged at different viewing angles. The very thin and flexible substrates offer a solution for foldable displays over very small radii for use in mobile devices and medical applications.

Efficient and chromaticity-stable flexible white organic light-emitting devices based on organic-inorganic hybrid color-conversion electrodes.

We have developed a novel organic–inorganic hybrid color conversion electrode composed of Ag NWs/poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) via a solution process, which is the first report on a color conversion electrode for applications in flexible optoelectronics. Using the Ag NWs/MEH-PPV composite film as the anode on polyethylene terephthalate substrate and combined with a blue organic light emitting devices (OLEDs) unit employing bis(3,5-difluoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl)iridium(iii)) (Flrpic) in 1,3-bis(carbazol-9-yl)benzene (mCP) as the emitting layer, a highly efficient and chromaticity-stable color-conversion flexible white OLEDs (WOLEDs) is achieved with a maximum current efficiency of 20.5 cd A−1. To the best of our knowledge, this is the highest efficiency reported for color-conversion based flexible WOLEDs. Our work provides an approach to achieving high-performance flexible WOLEDs devices and demonstrates great potential for lighting and display applications.

Highly Flexible and Mechanically Robust Ultrathin Au Grid as Electrodes for Flexible Organic Light-Emitting Devices

We report a highly flexible ultrathin Au grid with high mechanical robustness as transparent electrode for flexible organic light-emitting devices (OLEDs). The grids are introduced to an ultrathin Au film to improve its optical transmittance and mechanical performance. With the line width of 15 μm and the gap width of 30 μm, the optimal ultrathin Au grid presents a desirable transmittance of 88.7% at 550 nm and a sheet resistance of 36 Ω/sq. Owing to the reduced tensile stress by the ultrathin grid patterns, the flexible ultrathin Au grid exhibits extraordinary mechanical stability with maintained conductivity after 48 000 bending cycles under a bending radius of 1 mm. Flexible OLEDs with high flexibility and mechanical robustness have been obtained using the ultrathin Au grid as the transparent anode by maintaining 92% initial efficiency after 4200 bending cycles. Moreover, flexible OLEDs exhibit a significantly improved current efficiency in comparison to the devices based on the continuous Au or traditional ITO anode.

Spontaneously formed random corrugations for efficient light extraction enhancement in flexible organic light-emitting devices

Abstract We report spontaneously formed random corrugations as an efficient light extraction strategy in flexible organic light-emitting devices (OLEDs). The corrugations with directional randomness and broad distribution are spontaneously formed on the modified surface of SU-8 elastic film due to the mismatch of thermal expansion coefficients between the surface and bulk of SU-8. The topographic parameters of the random corrugations are desirably tunable by simply adjusting the annealing temperatures. The flexible OLEDs based on the random-corrugated SU-8 substrate demonstrate the improved light extraction characteristics in broadband, corresponding to the enhancements of 34% in luminance and 15.09% in current efficiency without directionality and spectral changes. The corrugated OLEDs also show high flexibility and mechanical stability without dramatical performance falloff under bending, folding as well as twisting.

Secondary-Transferring Graphene Electrode for Stable FOLED

In this work, sharp wrinkles on graphene films, caused by graphene duplicating the grain boundary cracks of copper foil during the preparation process, were carefully explored. A secondary-transferring graphene film process was proposed to re-transform the “Peak” morphology of graphene surface into “Valley” form. The process we have developed is highly effective and almost nondestructive to the graphene through testing the surface morphology and photo-electric properties before and after the secondary-transferring process. Flexible organic light-emitting device (FOLED) with PEDOT:PSS/SLG/NOA63 framework as a targeted application was fabricated to illustrate the value of our proposed method in fabricating stable devices, the maximum luminance can reach about 35000 cd/m2, and the maximum current efficiency was 16.19 cd/A. This method can also be applied to the roll-to-roll preparation of large area high-quality graphene.