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.     

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.     

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.     

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.     

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.     

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.     

All-solution-processed foldable transparent electrodes of Ag nanowire mesh and metal matrix films for flexible electronics

In this study, we developed foldable transparent electrodes composed of Ag nanowire (AgNW) networks welded by Ag nanoparticles (AgNPs) reduced from commercial Ag ink. All the processes used were solution-based. Using the Meyer rod method, uniform AgNW networks were roll-to-roll coated on large-area polymer substrates, and the spin-coated AgNPs firmly welded the AgNWs together at junctions and to substrates. The hybrid films consisting of AgNWs and the Ag film matrix exhibited higher electrical conductivity (5.0–7.3 × 10⁵ S/m) than and equivalent transparency (90–95%) to the AgNW networks. Furthermore, the hybrid films showed significantly better bending stability than AgNW networks. During cyclic bending tests to 10,000 cycles at 5 mm bending radius and even when almost folded with r_b of 1 mm, the resistivity changes were negligible because AgNWs were tightly held and adhered to the substrate by Ag films covering wires, thereby hindering fracturing of AgNWs under tension. Because the films were fabricated at a low temperature, there was no oxidation on the surfaces of the films. Hence, flexible organic light-emitting diodes (f-OLEDs) were successfully fabricated on polyethylene terephthalates (PET) coated with the hybrid films. The f-OLED in the bent state was comparable to that in the flat state, validating the potential applications of these transparent hybrid films as electrodes in various flexible electronics.     

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.     

Charge conduction study of phosphorescent iridium compounds for organic light-emitting diodes application

The charge conduction properties of a series of iridium-based compounds for phosphorescent organic light-emitting diodes (OLEDs) have been investigated by thin-film transistor (TFT) technique. These compounds include four homoleptic compounds: Ir(ppy)₃, Ir(piq)₃, Ir(Tpa-py)₃, Ir(Cz-py)₃, and two heteroleptic compounds Ir(Cz-py)₂(acac) and FIrpic. Ir(ppy)₃, Ir(piq)₃ and FIrpic are commercially available compounds, while Ir(Tpa-py)₃, Ir(Cz-py)₃ and Ir(Cz-py)₂(acac) are specially designed to test their conductivities with respect to the commercial compounds. In neat films, with the exception of FIrpic, all Ir-compounds possess significant hole transporting capabilities, with hole mobilities in the range of about 5 × 10⁻⁶–2 × 10⁻⁵ cm² V⁻¹ s⁻¹. FIrpic, however, is non-conducting as revealed by TFT measurements. We further investigate how Ir-compounds modify carrier transport as dopants when they are doped into a phosphorescent host material CBP. The commercial compounds are chosen for the investigation. Small amounts of Ir(ppy)₃ and Ir(piq)₃ (<10%) behave as hole traps when they are doped into CBP. The hole conduction of the doped CBP films can be reduced by as much as 4 orders of magnitude. Percolating conduction of Ir-compounds occurs when the doping concentrations of the Ir-compounds exceed 10%, and the hole mobilities gradually increase as their values reach those of the neat Ir films. In contrast to Ir(ppy)₃ and Ir(piq)₃, FIrpic does not participate in hole conduction when it is doped into CBP. The hole mobility decreases monotonically as the concentration of FIrpic increases due to the increase of the average charge hopping distance in CBP.     

Inorganic/organic multilayer passivation incorporating alternating stacks of organic/inorganic multilayers for long-term air-stable organic light-emitting diodes

Inorganic/organic multilayer passivation films were fabricated in an effort to enhance the long-term air stability of an organic light-emitting diode (OLED). AlO_x was used as an inorganic layer and 6.5 pairs of alternating silicon monoxide (SiO) and poly zinc acrylate (pZA) layers were used as the organic layers. The organic layers (Alternate stack of SiO and pZA layers) made it possible to decouple the defects present in the inorganic layer (AlO_x) and to increase the penetration length of water and oxygen into the devices. The resulting film was characterized using transmission electron microscopy. The film displayed a high transmittance (>95%) and a low water vapor transmission rate (WVTR, 7.98 × 10⁻⁵ g/m² day) under accelerated environmental aging conditions (a temperature of 60 °C and a relative humidity (R.H.) of 90%). In addition, The OLEDs did not incur significant damage caused by temperature (100 °C) and plasma (100 W) during fabrication process by depositing of SiO onto the OLED device prior to applying the passivation layers. The passivated OLED displayed a prolonged shelf-lifetimes over a period of 3000 h at 60 °C and 90% R.H.     

粉末靶磁控溅射制备AZO/Ag/AZO透明导电薄膜的特性

室温下采用射频磁控溅射粉末靶,在玻璃基底上制备了掺铝氧化锌/银/掺铝氧化锌(AZO/Ag/AZO)三层透明导电薄膜.通过优化中间银层厚度,优化了三层透明导电薄膜的光电性能.采用原子力显微镜和X射线衍射仪分别对薄膜的形貌和结构进行检测分析;采用紫外可见分光光度计和霍尔效应仪分别对薄膜的光电性能进行检测分析.结果表明,所制备的三层膜表面平整,颗粒大小错落均称;三层膜呈现多晶结构,AZO层薄膜具有(002)择优取向的六方纤锌矿结构,Ag层薄膜具有(111)择优取向的立方结构;当三层薄膜为AZO (20 nm) /Ag(12 nm) /AZO (20 nm)时,在550 nm处的透光率为88％,方块电阻为4.3Ω/□,电阻率为2.2×10-5 Ω·cm,载流子浓度为2.8×1022/cm3,迁移率为10 cm2/ (V·s),品质因子为3.5×10-2 Ω-1.     

Electrical characteristics of In/ITO p-type ohmic contacts for high-performance GaN-based light-emitting diodes

We have investigated the annealing-induced improved electrical properties of In(10 nm)/ITO(200 nm) contacts with p-type GaN. The contacts become ohmic with a specific contact resistance of 2.75×10^–3 Ω cm² upon annealing at 650 °C in air. X-ray photoemission spectroscopy (XPS) Ga 2p core levels obtained from the interface regions before and after annealing indicate a large band-bending of p-GaN, resulting in an increase in the Schottky barrier height. STEM/energy dispersive X-ray (EDX) profiling results exhibit the formation of interfacial In-Ga-Sn-oxide. Based on the STEM and XPS results, the ohmic formation mechanisms are described and discussed. It is also shown that patterning by nano-imprint lithography improves the light output power of blue LEDs by 18–28% as compared to that of LEDs fabricated with unpatterned In/ITO contacts.     

High performance top-emitting and transparent white organic light-emitting diodes based on Al/Cu/TcTa transparent electrodes for active matrix displays and lighting applications

It is challenging to obtain broadband emission covering as much of the visible light spectrum as possible in top-emitting white organic light-emitting diodes (TEWOLEDs) due to the well known microcavity effects. In this work, we achieved TEWOLED with three separate peak and negligible angular dependence by employing a high transmittance stack cathode Al (2 nm)/Cu (18)/TcTa (60 nm). The TEWOLED shows an efficiency of 25.6 cd/A, 20.1 Lm/W at 1000 cd/m², and low voltage of 4.2 V for 1222 cd/m². Synchronously, we achieved transparent white organic light-emitting diode (TWOLED) using this high transmittance stack cathode, the TWOLED exhibits similar spectrum and comparable luminance from both sides, and the maximum total efficiencies of the TWOLED are 28.6 cd/A, 24.9 Lm/W.     

Fabrication of CNTs/Cu composite thin films for interconnects application

Carbon nanotubes/copper (CNTs/Cu) composite thin films were fabricated by combined electrophoresis and electroplating techniques. Electrical properties and structure of both CNTs/Cu thin films and the reference pure Cu thin films were investigated after annealing at different temperatures. The sheet electrical resistance of CNTs/Cu films decreases faster than that of pure Cu films with increase of annealing temperature. The grain size of CNTs/Cu film becomes much larger than that of pure Cu film at the same annealing temperature. The peak relative intensity of Cu (1 1 1) plane in CNTs/Cu film was stronger than that of pure Cu film. CNTs/Cu composite thin films, with better electrical properties than that of conventional pure Cu thin films, have been fabricated by electrophoresis and electroplating deposition techniques.