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.     

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.     

Highly Transparent and Flexible Organic Light‐Emitting Diodes with Structure Optimized for Anode/Cathode Multilayer Electrodes

In this study, a dielectric layer/metal/dielectric layer (multilayer) electrode is proposed as both anode and cathode for use in the fabrication of transparent and flexible organic light‐emitting diodes (TFOLEDs). The structure of multilayer electrodes is optimized by systematic experiments and optical calculations considering the transmittance and efficiency of the device. The details of the multilayer electrode structure are [ZnS (24 nm)/Ag (7 nm)/MoO₃ (5 nm)] and [ZnS (3 nm)/Cs₂CO₃ (1 nm)/Ag (8 nm)/ZnS (22 nm)], as anode and cathode, respectively. The optimized TFOLED design is fabricated on a polyethylene terephthalate (PET) substrate, and the device shows high transmittance (74.22% around 550 nm) although the PET substrate has lower transmittance than glass. The TFOLEDs operate normally under compressive stress; degradation of electrical characteristics is not observed, comparable to conventional OLEDs with ITO and Al as electrodes. In addition, because the fabricated TFOLEDs show a nearly Lambertian emission pattern and a negligible shift of Commission International de l'Eclairage (CIE) coordination, it is concluded that the fabricated TFOLEDs are suitable for use in displays.     

Transparent Al/WO3/Au film as anode for high efficiency organic light-emitting diodes

A transparent Al/WO₃/Au anode is introduced to fabricate high efficiency organic light-emitting devices (OLEDs). By optimizing the thicknesses of each layers of the Al/WO₃/Au structure, the transmittance of Al(7 nm)/WO₃(3 nm)/Au(13 nm) has reached over 55%. Concerning the performance of OLEDs using the optimized anode, the electroluminescence (EL) current efficiency and brightness are enhanced and the EL spectrum is greatly narrowed as compared to the OLEDs using indium-tin-oxide (ITO) as the anode. The results indicate that the metal/metal oxide/metal transparent electrode is a good structure for the anode of high performance OLEDs. In addition, Al/WO₃/Au can function as a composite transparent electrode for top-emitting OLEDs.     

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.     

Large area roll-to-roll sputtering of transparent ITO/Ag/ITO cathodes for flexible inverted organic solar cell modules

We fabricated solution-processed flexible inverted organic solar cell (IOSC) modules (10 cm × 10 cm) on roll-to-roll (RTR) sputtered ITO/Ag/ITO multilayer cathodes. By using a pilot-scale RTR sputtering system equipped with mid-range frequency power for dual ITO targets and direct current power for the Ag target, we were able to continuously deposit a high-quality ITO/Ag/ITO multilayer on PET substrate with a width of 700 mm and length of 20,000 mm as a function of Ag thickness. At the Ag thickness of 12 nm, the ITO/Ag/ITO multilayer had a very low sheet resistance of 3.03 Ohm/square and high transmittance of 88.17%, which are better values than those of amorphous ITO film. A strip-type ITO/Ag/ITO cathode was successfully patterned using a RTR wet etching process. Successful operation of flexible IOSC modules on RTR sputtered ITO/Ag/ITO cathodes indicate that the RTR sputtering technique is a promising coating process for fabrication of high-quality transparent and flexible cathodes and can advance the commercialization of cost-efficient flexible IOSCs.     

Highly efficient ITO-free polymer solar cells based on metal resonant microcavity using WO3/Au/WO3 as transparent electrodes

Indium tin oxide (ITO)-free polymer solar cells (PSCs) with the structure of Glass/tungsten trioxide (WO₃)/Au/WO₃/PCDTBT: PC₇₀BM/LiF/Al was fabricated and studied. The multilayer structure of WO₃/Au/WO₃ is used as the potential transparent electrode to replace ITO. Metal resonant microcavity, which can enhance light harvesting of active layers, was constructed between Au and Al electrodes. According to the J–V and IPCE characterization with 70 nm active layer, power conversion efficiency (PCE) of the ITO-free microcavity device is approaching 4.55%, which is higher than that of the ITO-based device. However, PCE of the ITO-free device is much lower than that of the ITO-based device when the thickness of active layer increases to 130 nm. The opposite experimental tendency leads to theoretical research toward the simulation of light absorption and optical electric field and the calculation of maximum short circuit current density (J_{sc max}) as a function of active layer thickness based on ITO-free and ITO-based devices. The research results show that microcavity effect is closely linked to intrinsic absorption of active layers.     

三氧化钼修饰银透明电极的光电性能及其应用研究

银(silver,Ag)纳米薄膜具有优异的导电性、延 展性、易制备等优点,是极具潜力的柔性透明电极材 料。通过真空热蒸镀制备不同厚度的银薄膜(6 nm、10 nm、14 nm、18 nm、20 nm、24 nm),由于光散射和 光吸收的共同作用, 其透过率随厚度的增加呈先减小、再增加、再减小的趋势,厚度为18 nm时最优,最高透过率约60%; 而面电阻则随厚度的增加逐渐减小。为提升银膜的透过率,引入高折射率(2.1)电介质三氧化钼(m olybdenum trioxide,MoO₃)对银膜进行修饰,制备了MoO₃/Ag/MoO₃(MAM)多层膜。结 果表明:引入MoO₃可以平滑 银膜表面,降低面电阻,并改善电导率;更重要的是“MoO₃/Ag”界面处会发生折射率耦合 ,大大提升多 层膜的整体透过率,透过率普遍增加至少10%。当银层的厚度为14 nm时,MAM多层膜的透过率最优, 可接近70%。最后,以银作为透明阴极,成功制备了双侧发光的绿光 有机发光二极管(organic light-emitting diode,OLED)。     

透明叠层结构Ag/MoO_3/Ag阳极对绿光OLED器件性能的影响

本文采用一种结构为Ag/MoO_3/Ag的金属/氧化物/金属(M_1/O/M_2)叠层替代ITO作为OLED器件的阳极,研究Ag/MoO_3/Ag叠层结构变化对于OLED器件电极透过率、亮度、光谱等性能的影响。实验采用真空蒸镀方法制备了一系列器件,器件结构为Ag/MoO_3/Ag/MoO_3(10nm)/NPB(40nm)/Alq_3(60nm)/LiF(1nm)/Al(150nm)。对比器件的电压-电流密度、电压-亮度、光谱特性等数据,表明Ag/MoO_3/Ag的结构为20/20/10(nm)时,器件性能较好。在驱动电压为11V时,其亮度达到18 421cd/m~2,电流效率为2.45cd/A;且因器件中存在微腔效应,其EL光谱蓝移,半高宽变窄。但考虑到530nm处其电极透过率仅为17%,所以经换算该器件实际发光亮度比ITO电极器件更高。该Ag/MoO_3/Ag叠层阳极制作相对简单,经优化后在顶发射和柔性OLED器件方面将具有一定的应用前景。     

室温下制备的ZnO/Ag/ZnO多层膜的性能

采用射频磁控溅射ZnO陶瓷靶、直流磁控溅射Ag靶的方法在室温下制备了不同厚度的ZnO/Ag/ZnO多层膜。对样品进行了研究。结果表明:随着Ag层厚度的增加,ZnO(002)衍射峰的强度先增加后减小,Ag(111)衍射峰的强度增强,ZnO/Ag/ZnO多层膜的面电阻先减小后趋于稳定。ZnO膜厚度增加,Ag膜易形成晶状结构,ZnO/Ag/ZnO多层膜的透射峰向长波方向移动。ZnO(60nm)/Ag(11nm)/ZnO(60nm)膜在554nm处的透过率高达92.3%,面电阻为4.2?/□,品质常数?TC最佳,约40×10–3/?。     

Bottom-emission organic light-emitting diodes using semitransparent anode electrode by O2 plasma

To improve the performance of bottom-emission organic light-emitting diodes (BEOLEDs), the effect of oxygen plasma treatment duration on the electrical properties of multi-metal Ni/Ag/Ni thin film anode was investigated. The results revealed that a Ni/Ag/Ni thin-film layer formed upon oxygen plasma treatment for 60 s. Our indium-free bottom-emission OLEDs effectively increased the electrical and optical properties by improving their electron–hole recombination and doing a strong micro-cavity effect with the semitransparent multi-metal anode. The green bottom-emission OLEDs show a luminance of 14,280 cd/m², a luminous efficiency of 8.5 cd/A, external quantum efficiency 2.6% EQE, a Commission Internationale de L’Eclairage coordinates of (0.32, 0.58) on flexible substrate.     

The structural composite effect of Au–WO3–Al interconnecting electrode on performance of each unit in stacked OLEDs

In this article, we report the effects of the thickness of metal and oxide layers of the Al/WO₃/Au interconnecting structure on the electrical and optical characteristics of the upper and bottom units of the two-unit stacked organic-light-emitting-devices (OLEDs). It is found that light emission performance of the upper unit is sensitive to the transmittance of semitransparent Al/WO₃/Au structure, which can be improved by changing the thickness of each layer of the Al/WO₃/Au structure. It is important to note that the introduction of WO₃ between Al and Au significantly enhances the current efficiency of both the upper and bottom units with respect to that of the corresponding Al/Au structure without WO₃. In addition, the emission spectra of both the upper and bottom units are narrower than that of the control device due to microcavity effect. Our results indicate that the Al/WO₃/Au interconnecting structure is a good candidate for fabricating independently controllable high efficiency stacked OLEDs.     

Ag层的厚度对ZnO/Ag/ZnO多层膜性能的影响

采用射频磁控溅射ZnO陶瓷靶、直流磁控溅射Ag靶的方法制备了不同厚度Ag夹层的ZnO(60nm)/Ag/ZnO(60nm)多层膜.分别用X射线衍射仪、紫外可见分光光度计、四探针测试仪对样品的结构、光学性质、电学性质进行了研究.结果表明:随着Ag层厚度的增加,ZnO/Ag/ZnO多层膜呈现多晶结构,Ag(111)衍射峰的强度增强.Ag夹层厚度为11nm时,ZnO(60nm)/Ag/ZnO(60nm)膜在554nm处的透过率高达92.3%.随着Ag层厚度的增加,Ag膜的特征吸收峰呈现红移和宽化,ZnO/Ag/ZnO多层膜的面电阻先减小后趋于稳定.     

Design of a MoOx/Au/MoOx transparent electrode for high-performance OLEDs

A MoO_x(top)/Au/MoO_x(bottom) multilayer was systematically designed for transparent electrodes in green OLEDs in terms of optical transmission and series resistance of the device. The enhancement in optical transmission of MoO_x/Au/MoO_x (MAM) structures is a result of a series of events, including the optical interference within the multilayers and the interaction of light with surface plasmon polaritons in the metal layer. For the maximum transmission, the optical interference occurring within the multilayers was simulated using a transfer matrix model to determine the optimum thickness of MoO_x layers, and then the thickness of the Au interlayer was experimentally optimized for extraordinary optical transmission. In addition, the series resistance added by the top MoO_x was characterized to confirm its negligible impact on the performance of the device. The optimum MoO_x (40 nm)/Au (10 nm)/MoO_x (40 nm) structure showed much higher transmission in the green-red region and lower sheet resistance than indium tin oxide (ITO). We have fabricated MAM-based OLEDs the driving voltage of which was significantly reduced to ∼5.5 V at a current density of 20 mA/cm², and the current efficiency (11.46 Cd/A) was higher than that (10.91 Cd/A) of ITO-based OLEDs, demonstrating that the MAM electrode is a potential replacement for ITO in optical devices.     

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.     

Buffer and anode-integrated WO3-doped In2O3 electrodes for PEDOT:PSS-free organic photovoltaics

We developed PEDOT:PSS-free organic solar cells (OSCs) using WO₃ and In₂O₃ (IWO) mixed electrodes acting as a buffer hole injection layer (HIL) and anode simultaneously. Through the co-sputtering and rapid thermal annealing (RTA) of WO₃ and In₂O₃, we achieved buffer and anode-integrated transparent electrodes with a sheet resistance of 17 Ohm/square, a transmittance of 90.32%, and a work function of 4.83 eV, all of which are comparable to values obtained with a conventional ITO anode. Due to the existence of WO₃ in the In₂O₃ matrix, OSCs fabricated on an IWO electrode with no acidic PEDOT:PSS buffer layer showed a PCE of 2.87%. Therefore, a transparent IWO electrode simultaneously acting as an HIL and anode layer can be considered a promising transparent electrode for cost-efficient and reliable OSCs because it could eliminate the use of acidic PEDOT:PSS buffer layer.     

Al/MoO3 阳极薄膜：一种有效高性能的柔性有机光电子的阳极结构

在本文中,我们报导了一种Al/MoO3复合阳极结构的高性能OLEDs。发现该结构器件的独特效率是由于在阳极/有机界面上的界面偶极子有很大的下降,从而导致空穴注入到常用的空穴传输分子上能力得到增强。强微腔作用进一步增强了电致发光强度,同时由于Al/MoO3易于加工,我们成功的在朔料基板上研制出发光均匀的大面积柔性OLEDs。     

Bulk-like Al/Ag bilayer film due to suppression of surface plasmon resonance for high transparent organic light emitting diodes

We demonstrate the enhanced optical and electrical properties of an ultrathin silver (Ag) film by applying an aluminum (Al) seed layer between LiF and Ag as a transparent cathode for higher-transparency organic light-emitting diodes (OLEDs). Although the thickness ranges from 4 to 8 nm, the ultrathin Ag film is a continuous and uniform bulk-like film with an Al seed layer, which suppresses the surface plasmon absorption. Compared to an Ag-only cathode, the measured transmittance spectra were considerably increased, comparable with the theoretical calculations of a bulk Al/Ag bilayer film. The Al/Ag bilayer cathode has a transmittance of 87% at a 550 nm wavelength and a sheet resistance of 19.5 Ω/sq with a 4-nm-thick Ag layer. The transparent OLED devices that employed the Al/Ag cathode showed a transmittance of 72% at a 550 nm wavelength for an Ag thickness of 6 nm.