Flexible organic light-emitting diodes with ZnS/Ag/ZnO/Ag/WO3 multilayer electrode as a transparent anode

We investigated the highly flexible, transparent and very low resistance ZnS/1st Ag/ZnO/2nd Ag/WO₃ (ZAZAW) multilayer electrodes on PET substrate as an anode in flexible organic light-emitting diodes (OLEDs). A theoretical calculation was first conducted to obtain the optimal thickness of the ZAZAW multilayer for high transparency. Its measured luminous transmittance was over 80% in the visible range with a very low sheet resistance of 2.17 Ω/sq., and it had good mechanical flexibility due to the ductility of Ag. Ag’s effect on optical and electrical properties was also studied. Flexible OLEDs devices that were fabricated on ZAZAW multilayer anode showed good hole injection properties comparable to those of ITO-based OLEDs due to the use of WO₃ as a hole injection layer. However, the electroluminescent properties of the ZAZAW-based OLEDs varied depending on WO₃ thickness. Although the transmittance of the ZAZAW electrode was reduced by tuning the WO₃ thickness to adjust the microcavity effect, the device efficiency could be enhanced above that of ITO-based OLEDs.     

Highly flexible organic light-emitting diodes based on ZnS/Ag/WO3 multilayer transparent electrodes

We report our study on highly flexible organic light-emitting diodes based on ZnS/Ag/WO₃ (ZAW) multilayer transparent electrodes in which high conductivity and ductility of Ag layers allow for efficient sheet conduction and flexibility while ZnS and WO₃ layers provide a means for enhancement in optical transmission and/or carrier-injection. Devices with ZAW anodes fabricated on planarized plastic substrates not only exhibit a performance and operational stability comparable to or better than those of ITO-based devices but also show a mechanical flexibility that is far superior to that of ITO-based devices. Experimental results show that a consistent performance can be obtained in ZAW-based devices upon repeated bending down to a radius of curvature of 5 mm, below which the flexibility of the devices is limited ultimately by the delamination occurring at cathode/organic interfaces rather than by the ZAW electrodes themselves.     

Fluoropolymer-assisted graphene electrode for organic light-emitting diodes

Organic light-emitting diodes (OLEDs) were fabricated on a graphene electrode, with synthesized graphene being transferred and simultaneously doped with supporting polymers. Poly[methyl methacrylate] (PMMA) and fluoropolymer (CYTOP) layers were used as the supporting polymers. The sheet resistance of CYTOP-assisted graphene (CYTOP-G) with 4 layers of graphene is 200 Ω/sq., which is lower than that of PMMA-assisted graphene (PMMA-G, 330 Ω/sq.) The transmittance value of PMMA-G and CYTOP-G with 4 graphene layers is higher than 85%. CYTOP-G is shown to exhibit a higher tolerance to UV–O₃ treatment and thermal annealing than PMMA-G. Work function of CYTOP-G is 4.7 eV, which is higher than that of PMMA-G (4.3 eV). X-ray photoemission and Raman spectroscopy data indicate that CYTOP-G has numerous C-F bonds on the surface exhibiting p-type semiconductor properties, owing to the high electronegativity of fluorine. The turn-on voltage of an OLED based on CYTOP-G with 4 graphene layers is 4.2 V, which is lower than that of indium tin oxide (ITO)-based one (4.5 eV). Furthermore, the luminance ratio of graphene-based OLEDs to ITO-based OLEDs was calculated to be 104% for CYTOP-G, and 97% for PMMA-G. According to the ultraviolet photoemission spectra, the hole injection barrier in CYTOP-G is lower by about 0.5 eV than the hole injection barrier in PMMA-G. These results are very encouraging to the prospect of replacing ITO electrodes with graphene ones in OLED applications.     

Enhanced light extraction efficiency using self-textured aluminum-doped zinc oxide in organic light-emitting diodes

We report enhanced light extraction efficiency in organic light-emitting diodes (OLEDs) fabricated on a self-textured aluminum-doped zinc oxide (AZO) anode layer. The self-textured AZO (ST-AZO) layer was fabricated by radio-frequency magnetron sputtering with a short period of thermal treatment without employing any additional etching processes. The green-emitting OLEDs exhibited a maximum power efficiency of 56.1 lm/W with 33.7% external quantum efficiency (EQE). We achieved a 3.24-fold enhancement in power efficiency and 2.55-fold increase in EQE for the OLED fabricated on the ST-AZO anode compared to that fabricated on the ITO anode. Furthermore, a low driving voltage and high current efficiency were obtained simultaneously for the OLED fabricated on the ST-AZO layer compared to that fabricated on the flat ITO anode layer. The ST-AZO layer acted as a random scattering layer that enabled the efficient extraction of generated light and served as the anode layer instead of the commonly used ITO. Our study showed that the ST-AZO layer fabricated by a simple sputtering process effectively improved the optical and electrical properties of the OLED.     

Neutral-pH PEDOT:PSS as over-coating layer for stable silver nanowire flexible transparent conductive films

The silver nanowire (AgNW) mesh film with poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as the over-coating layer is a promising flexible transparent conductive film technology. In this work, experimental studies show that the hygroscopic and acid properties of the common PEDOT:PSS lead to poor stabilities of the composite films, due to the conductivity degradation of PEDOT:PSS by the water absorption and the acid corrosion of AgNWs by PEDOT:PSS. By using the modified PEDOT:PSS of neutral pH as the over-coating layer, the long term shelf-life time, thermal and current stressing stabilities are all significantly improved without sacrifice of transparency, electrical conductivity and mechanical flexibility. Under both cases of thermal aging test at 210 °C for 20 min and 12 h continuous current stressing at a current density of 30 mA/cm², no obvious change of the conductivity is observed. The results clearly demonstrate that using the neutral-pH PEDOT:PSS as an over-coating layer can help to achieve flexible AgNW transparent conductive films with superior stability for flexible optoelectronic devices.     

Ink formulation of in-situ crosslinkable hole-transporting composite for multilayer inkjet-printed organic light-emitting diodes

Inkjet printing is considered to be the most promising technique for high-resolution, large-scale and low-cost organic light-emitting diode (OLED) displays. However, the device performance with multilayer printing is still a challenge that needs to be tackled, because of the serious re-dissolving problem of bottom film into the subsequent ink during printing processes. This work demonstrates an in-situ crosslinkable composite of hole-transporting polymer poly(N-vinylcarbazole) (PVK) and oxidative coupling agent phosphomolybdic acid (PMA) without ultra-violet irradiation or high temperature annealing treatment. The hole-transporting ink is formulated by adding high-boiling point solvents including o-dichlorobenzene, butyl benzoate, cyclohexylbenzene and 1-chloronaphthalene to eliminate the coffee ring effect. Their influence on the ink property is systematically investigated, and the corresponding solidification mechanism is also proposed. The modified PVK:PMA ink leads to a crosslinked hole-transporting layer (HTL) with an excellent solvent resistance and temperature-independent property. Finally, the emitting layer is successfully inkjet-printed onto this HTL, and multilayer inkjet-printed OLEDs are achieved with a current efficiency of 21.7 cd/A. In summary, this in-situ crosslinkable hole-transporting composite ink is facile and effective for multilayer inkjet-printed OLEDs.     

ITO-free,efficient, and inverted phosphorescent organic light-emitting diodes using a WO3/Ag/WO3 multilayer electrode

We present an indium tin oxide (ITO)-free, bottom-emission inverted phosphorescent organic light-emitting diode (PHOLED) with a maximum luminance of 280,000 cd/m² at 8 V, total maximum current efficiency of 81.4 cd/A, and external quantum efficiency of 22.4%. The inverted OLED structure is composed of glass/WO₃ (30 nm)/Ag (15 nm)/WO₃ (5 nm)/BPhen:15wt% CS₂CO₃ (5 nm)/BPhen (30 nm)/CBP: 8wt% Ir(ppy)₃ (10 nm)/TAPC (50 nm)/WO₃ (5 nm)/Ag (150 nm) multilayers. In this device structure, the WO₃/Ag/WO₃ (WAW) multilayer serving as a transparent cathode demonstrates a low sheet resistance (3.5 Ω/sq) and high optical transmittance (approximately 80%) in a visible light range of 400–600 nm; this multilayer was prepared by thermal evaporation to form a relatively smooth morphology of the conductive thin film on the glass substrate. In addition, an electron-only WAW device was subjected to electrical characterization, and the results revealed that this device exhibited a more efficient electron injection property at the WAW/BPhen:CS₂CO₃ interface than the contact electrode of a standard ITO-based device.     

The application of single-layer graphene modified with solution-processed TiOx and PEDOT:PSS as a transparent conductive anode in organic light-emitting diodes

There are many challenges for a direct application of graphene as the electrodes in organic electronics due to its hydrophobic surfaces, low work function (WF) and poor conductance. The authors demonstrate a modified single-layer graphene (SLG) as the anode in organic light-emitting diodes (OLEDs). The SLG, doped with the solution-processed titanium suboxide (TiO_x) and poly(3,4-ethylenedio-xythiophene)/poly(styrene sulfonic acid) (PEDOT:PSS), exhibits excellent optoelectronic characteristics with reduced sheet resistance (R_sq), increased work function, as well as over 92% transmittance in the visible region. It is notable that the R_sq of graphene decreased by ∼86% from 628 Ω/sq to 86 Ω/sq and the WF of graphene increased about 0.82 eV from 4.30 eV to 5.12 eV after a modification by using the TiO_x–PEDOT:PSS double interlayers. In addition, the existence of additional TiO_x and PEDOT:PSS layers offers a good coverage to the PMMA residuals on SLG, which are often introduced during graphene transfer processes. As a result, the electrical shorting due to the PMMA residues in the device can be effectively suppressed. By using the modified SLG as a bottom anode in OLEDs, the device exhibited comparable current efficiency and power efficiency to those of the ITO based reference OLEDs. The approach demonstrated in this work could potentially provide a viable way to fabricate highly efficient and flexible OLEDs based on graphene anode.     

Low resistive transparent and flexible ZnO/Ag/ZnO/Ag/WO3 electrode for organic light-emitting diodes

Transparent electrodes cannot easily be created with high transmittance and low sheet resistance simultaneously, although some optoelectronic devices, such as large organic light-emitting diode (OLED) displays and lightings, require very low resistive transparent electrodes. Here, we propose a very low resistive transparent electrode (～1.6 Ω/sq) with a high transmittance (～75%) for OLED devices, the transmittance level of which represents the highest reported value to date given such a low sheet resistance level. It consists of a stacked silver (Ag)/zinc oxide (ZnO)/Ag multilayer covered by high refractive index dielectric layers. The proposed multilayer electrode with optimal layer thicknesses has a high and wide spectral transmittance peak due to interference. The low sheet resistance is a result of two Ag layers connected via the sandwiched ZnO layer. In addition to its low sheet resistance coupled with high transmittance, the proposed multilayer electrode has good flexibility. An OLED with an anode of the stacked Ag/ZnO/Ag multilayer shows performance comparable to that of an anode of indium tin oxide.     

ITO-free flexible organic light-emitting diode using ZnS/Ag/MoO3 anode incorporating a quasi-perfect Ag thin film

The multi-layer electrode (ZnS/Ag/MoO₃) was optimized by investigating the formation of a continuous Ag thin film according to the base layer. The aggregation of the Ag atom was strictly limited on the ZnS layer, which showed the best thermal stability for Ag. The thermally evaporated 7-nm-thick Ag film with surface coverage of 99.6% was achieved on the ZnS layer. We fabricated the ZnS (25 nm)/Ag (7 nm)/MoO₃ (5 nm) (Z25A7M5) multi-layer electrode, optimized through the numerical calculation. The transmittance of 83% at λ = 550 nm and sheet resistance of 9.6 Ohm/sq were recorded from the Z25A7M5 electrode. These results were mainly attributed to the uniform film-like morphology of the Ag thin film. The flexible OLEDs, based on the Z25A7M5 anode also showed feasible I–V–L characteristics compared to those of ITO-based devices.     

Intense-pulsed-light irradiation of Ag nanowire-based transparent electrodes for use in flexible organic light emitting diodes

Silver nanowire (AgNW) based transparent electrodes are inherently coarse and therefore typically are only ever weakly bonded to a substrate. A remarkable improvement in the characteristics of a AgNW network film has, however, been achieved through a simple and short process of irradiating it with intense pulsed light (IPL). This not only avoids any severe deterioration in the optical characteristics of the AgNW film, but also significantly improves its electrical conductivity, adhesion to a polymeric substrate, and ability to endure bending stress. Most important of all, however, is the finding that the surface roughness of AgNW networks can also be improved by radiation. In a series of measurements made of organic light emitting diodes fabricated using these treated electrodes, it was revealed that the leakage current can be notably reduced by IPL treatment.     

Organic/metal hybrid cathode for transparent organic light-emitting diodes

We report a highly transparent organic/metal hybrid cathode of a Cs-doped electron transport layer (Cs-ETL)/Ag for transparent organic light-emitting diode (TOLED) applications. Particular attention is paid to the surface morphology on the Ag film and its influence on the optical transparency and electrical conductivity. With the use of Cs-ETL, a smooth and continuous surface morphology of Ag film was achieved, leading to a high transmittance of ～75% in TOLED with a low sheet resistance of 4.5 Ω/Sq in cathode film. We successfully applied our Cs-ETL/Ag transparent cathode to fabricate highly transparent OLEDs. Our approach suggests a new electrode structure for transparent OLED applications.     

Fabrication of flexible organic light-emitting diodes with Alq3 as emitting layer

Fabrication of flexible organic light-emitting diodes（FOLEDs） with ITO/PVK ：TPD/Alq3/Al configuration prepared on PET substrates is reported. Alq3 is used as the light-emitting material. The curves of the current density vs. voltage,optical current vs. voltage and quantum efficiency vs. current density of the devices are investigated. Compared the devices with the ones that have the same configuration and are fabricated under the same conditions but on glass substrates,the characteristics of the two kinds of devices are very similar except that the threshold voltage of the flexible FOLEDs is a little higher. Under the driving voltage of 20V,the corresponding brightness and the external quantum efficiency are 1000 cd/m^2 and 0. 27%, respectively. In addition, the anti-bend ability of the devices is tested and the reasons of failure of the devices are analyzed.     

Transparent conductive PVP/AgNWs films for flexible organic light emitting diodes by spraying method

In this study, a simple spraying method is used to prepare the transparent conductive films (TCFs) based on Ag nanowires (AgNWs). Polyvinylpyrrolidone (PVP) is introduced to modify the interface of substrate. The transmittance and bending performance are improved by optimizing the number of spraying times and the solution concentration and controlling the annealing time. The spraying times of 20, the concentration of 2 mg/mL and the annealing time of 10 min are chosen to fabricate the PVP/AgNWs films. The transmittance of PVP/AgNWs films is 53.4%—67.9% at 380—780 nm, and the sheet resistance is 30 Ω/□ which is equivalent to that of commercial indium tin oxide (ITO). During cyclic bending tests to 500 cycles with bending radius of 5 mm, the changes of resistivity are negligible. The performance of PVP/AgNW transparent electrodes has little change after being exposed to the normal environment for 1 000 h. The adhesion to polymeric substrate and the ability to endure bending stress in AgNWs network films are both significantly improved by introducing PVP. Spraying method makes AgNWs form a stratified structure on large-area polymer substrates, and the vacuum annealing method is used to weld the AgNWs together at junctions and substrates, which can improve the electrical conductivity. The experimental results indicate that PVP/AgNW transparent electrodes can be used as transparent conductive electrodes in flexible organic light emitting diodes (OLEDs).     

衬底温度对Al2O3掺杂ZnO透明导电薄膜性能的影响

以纯度为99.9％的陶瓷靶(w(ZnO)=98％,w(Al2O3)=2％)为溅射靶材,采用射频磁控溅射法在玻璃衬底上沉积制备了Al2O3掺杂的ZnO(AZO)薄膜.采用X射线衍射(XRD)仪、扫描电子显微镜(SEM)、紫外可见光谱(UV-Vis)仪等仪器,对AZO薄膜的形貌结构、光电学性能进行了测试,从薄膜生长方式和缺...     

绒面AZO最新研究进展及应用

掺铝氧化锌(AZO)透明导电膜作为一种光电性能优异的透明导电膜(TCO)受到研究人员的广泛关注,并被认为是当前大规模使用的传统铟锡氧化物(ITO)的替换材料。绒面AZO薄膜因其电阻率低、高透过率且具有良好的陷光效果,可以提高太阳能电池的光电转换效率,而被认为是太阳能电池前电极的理想材料。综述了绒面AZO透明导电膜的制备方法和性能研究现状,并针对AZO的国内外研究状况提出了今后的发展趋势和研究方向。     

Synthesis of blue light-emitting graphene quantum dots and their application in flexible nonvolatile memory

We presented a facile method to prepare graphene quantum dots (GQDs) from double-walled carbon nanotube with blue light emission in a chlorobenzene solution, which enabled the preparation of GQD–polymer hybrid nanocomposite. The wavelength-dependent fluorescent lifetime of the GQDs was investigated by using time-resolved photoluminescence technique. Significantly, nonvolatile rewritable memory effect was observed for the GQD-based nanocomposite, suggesting the promising applications of GQDs in data storage. Moreover, due to the easy solution process, we demonstrated the design and realization of flexible GQD-based memory device. This work may expand the application of GQDs to the portable electronic devices.     

Reduced graphene oxide prepared at low temperature thermal treatment as transparent conductors for organic electronic applications

In this work, we obtain transparent conducting thin films of both chemically and thermally reduced graphene oxide. High-quality films are normally obtained with thermal treatments at temperatures about 1000 °C, while the highest temperatures employed during the thermal treatment in this work were as low as 400 °C, which is a mandatory condition when dealing with organic electronic devices on glass substrates. To reach such a low thermal treatment, a two-step oxidation process was employed in order to allow the formation of carbonyl chemical groups rather than epoxy functionalization. Each GO sample was structurally and chemically analysed by Infrared spectroscopy (ATR-FTIR), X-ray diffraction (XRD), Ultraviolet–visible absorption spectroscopy (UV-VIS), Thermogravimetric analysis (TGA), Raman spectroscopy (RS), X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The GO conducting thin films exhibited a sheet resistance of 3.2 × 10³ Ω/sq as well as a high transmittance: up to 80% at 550 nm. Furthermore, Raman spectroscopy, X-ray diffraction and AFM show that the thermally reduced thin films are mainly composed of single and bilayer GO sheets with a very low average roughness. Also, these GO thin films, with such surprising quality, have been employed as non-doped and metal free electrodes in organic light emitting diodes.     

Improved efficiency of indium-tin-oxide-free organic light-emitting devices using PEDOT:PSS/graphene oxide composite anode

We have demonstrated an indium-tin-oxide free organic light-emitting device (OLED) with improved efficiency by doping poly (3,4-ethylene dioxythiophene):poly (styrene sulfonate) (PEDOT:PSS) with graphene oxide (GO) as a composite anode. In comparison with a pure PEDOT:PSS anode, 55% enhancement in efficiency has been obtained for the OLEDs based on the PEDOT:PSS/GO composite anode at an optimal condition. The PEDOT:PSS/GO composite anode shows a lower hole-injection barrier, which contributes to the improved device efficiency. Moreover, both high transmittance and good surface morphology similar to that of the pure PEDOT:PSS film also contribute to the enhanced efficiency. It is obvious that composite anode will generally be applicable in organic optoelectronic devices which require smooth and transparent anode.