New hybrid encapsulation for flexible organic light-emitting devices on plastic substrates

The hybrid encapsulation for flexible organic light-emitting devices on plastic substrate was investi- gated. The hybrid encapsulation consisted of four periods of Alq3/LiF layers as the pre-encapsulation layer and a flexible aluminum foil coated with getter as the encapsulation cap. We measured the device lifetime at a continuous constant current of 20 mA/cm2, which corresponded to an initial luminance of 2000 cd/m2. The half-luminance decay time of the encapsulated device was about 458 h. More over, the hybrid encapsulation is ultrathin and flexible, ensuring device bendability.

Device design principles and bioelectronic applications for flexible organic electrochemical transistors

Organic electrochemical transistors(OECTs) exhibit significant potential for applications in healthcare and human-machine interfaces, due to their tunable synthesis, facile deposition, and excellent biocompatibility. Expanding OECTs to the fexible devices will significantly facilitate stable contact with the skin and enable more possible bioelectronic applications. In this work,we summarize the device physics of fexible OECTs, aiming to offer a foundational understanding and guidelines for material selection and device architecture. Particular attention is paid to the advanced manufacturing approaches, including photolithography and printing techniques, which establish a robust foundation for the commercialization and large-scale fabrication. And abundantly demonstrated examples ranging from biosensors, artificial synapses/neurons, to bioinspired nervous systems are summarized to highlight the considerable prospects of smart healthcare. In the end, the challenges and opportunities are proposed for fexible OECTs. The purpose of this review is not only to elaborate on the basic design principles of fexible OECTs, but also to act as a roadmap for further exploration of wearable OECTs in advanced bio-applications.

Air-Stable Ultrabright Inverted Organic Light-Emitting Devices with Metal Ion-Chelated Polymer Injection Layer

Here,this work presents an air-stable ultrabright inverted organic lightemitting device(OLED)by using zinc ionchelated polyethylenimine(PEI)as electron injection layer.The zinc chelation is demonstrated to increase the conductivity of the PEI by three orders of magnitude and passivate the polar amine groups.With these physicochemical properties,the inverted OLED shows a record-high external quantum efficiency of 10.0% at a high brightness of 45,610 cd m^(-2) and can deliver a maximum brightness of 121,865 cd m^(-2).Besides,the inverted OLED is also demonstrated to possess an excellent air stability(humidity,35%)with a half-brightness operating time of 541 h@1000 cd m^(-2) without any protection nor encapsulation.

Improving efficiency of organic light-emitting devices by optimizing the LiF interlayer in the hole transport layer

The efficiency of organic light-emitting devices （OLEDs） based on N,N＇-bis（1-naphthyl）-N,N＇-diphenyl-N,1＇- biphenyl-4,4＇-diamine （NPB） （the hole transport layer） and tris（8-hydroxyquinoline） aluminum （Alq3） （both emission and electron transport layers） is improved remarkably by inserting a LiF interlayer into the hole transport layer. This thin LiF interlayer can effectively influence electrical performance and significantly improve the current efficiency of the device. A device with an optimum LiF layer thickness at the optimum position in NPB exhibits a maximum current efficiency of 5.96 cd/A at 215.79 mA/cm2, which is about 86% higher than that of an ordinary device （without a LiF interlayer, 3.2 cd/A）. An explanation can be put forward that LiF in the NPB layer can block holes and balance the recombination of holes and electrons. The results may provide some valuable references for improving OLED current efficiency.

Dependence of Performance of Organic Light-emitting Devices on Sheet Resistance of Indium-tin-oxide Anodes

The dependence of the performance of organic light-emitting devices（OLEDs） on the sheet resistance of indiumtin-oxide（ITO） anodes was investigated by measuring the steady state current density brightness voltage characteristics and the electroluminescent spectra. The device with a higher sheet resistance anode shows a lower current density, a lower brightness level, and a higher operation voltage. The electroluminescence（EL） efficiencies of the devices with the same structure but different ITO anodes show more complicated differences. Furthermore, the shift of the light-emitting zone toward the anode was found when an anode with a higher sheet resistance was used. These performance differences are discussed and attributed to the reduction of hole injection and the increase in voltage drop over ITO anode with the increase in sheet resistance.

Numerical model of multilayer organic light-emitting devices

A numerical model of multilayer organic light-emitting devices is presented in this article. This model is based on the drift-diffusion equations which include charge injection, transport, space charge effects, trapping, heterojunction interface and recombination process. The device structure in the simulation is ITO/CuPc （20 nm）/NPD （40 nm）/Alq3 （60 nm）/LiF/Al. There are two heterojunctions which should be dealt with in the simulation. The I-V characteristics, carrier distribution and recombination rate of a device are calculated. The simulation results and measured data are in good agreement.

Negative capacitance in doped bi-layer organic light-emitting devices

This paper reports that the doped bi-layer organic light-emitting devices are fabricated by doping in different regions of the light-emitting layer, the admittance and luminance spectra to characterize the capacitance and luminance of the device are measured. Negative capacitance （NC） appeared at low frequencies when the doped devices are biased with high voltages. The measured phase difference between AC voltage applied across the device and AC current flowing through the device show that the device is inductive when NC appears.

Enhancement of Frster energy transfer from thermally activated delayed fluorophores layer to ultrathin phosphor layer for high color stability in non-doped hybrid white organic light-emitting devices

Fluorescence/phosphorescence hybrid white organic light-emitting devices（WOLEDs） based on double emitting layers（EMLs） with high color stability are fabricated.The simplified EMLs consist of a non-doped blue thermally activated delayed fluorescence（TADF） layer using 9,9-dimethyl-9,10-dihydroacridine-diphenylsulfone（DMAC-DPS） and an ultrathin non-doped yellow phosphorescence layer employing bis[2-（4-tertbutylphenyl）benzothiazolato-N,C2＇]iridium（acetylacetonate）（（tbt）_2Ir（acac））.Two kinds of materials of 4,7-diphenyl-1,10-phenanthroline（Bphen） and 1,3,5-tris（2-Nphenylbenzimidazolyl） benzene（TPBi） are selected as the electron transporting layer（ETL）,and the thickness of yellow EML is adjusted to optimize device performance.The device based on a 0.3-nm-thick yellow EML and Bphen exhibits high color stability with a slight Commission International de l＇Eclairage（CIE） coordinates variation of（0.017,0.009） at a luminance ranging from 52 cd/m^2 to 6998 cd/m^2.The TPBi-based device yields a high efficiency with a maximum external quantum efficiency（EQE）,current efficiency,and power efficiency of 10%,21.1 cd/A,and 21.3 lm/W,respectively.The ultrathin yellow EML suppresses hole trapping and short-radius Dexter energy transfer,so that Forster energy transfer（FRET）from DMAC-DPS to（tbt）_2Ir（acac） is dominant,which is beneficial to keep the color stable.The employment of TPBi with higher triplet excited state effectively alleviates the triplet exciton quenching by ETL to improve device efficiency.

Flexible organic photovoltaic devices based on oligothiophene derivatives

For the purpose of developing flexible organic photovoltaic devices, we have fabricated two flexible devices using 5-formyl- 2,2′：5′,2″：5″,2′″-quaterthiophene （4T-CHO）, 5-formyl-2,2′：5′, 2″：5″,2′″：5′″,2″″-quinquethiophene （5T-CHO） and 3,4,9,10-perylenetertracarboxylic dianhydride （PTCDA）. The PET-ITO/4T-CHO/PTCDA/A1 device has an open circuit voltage （Voc） of 1.56 V, photoelectric conversion efficiency of 0.77%. The PET-ITO/5T-CHO/PTCDA/A1 device has a Voc of 1.70 V, photoelectric conversion efficiency of 0.84%. The two flexible devices have high Voc （1.56 and 1.70 V）. It is possible that intermolecular hydrogen bonding between -CHO group of nT-CHO and carboxylic dianhydride of PTCDA contributes to enhancing the efficiency by promoting interfacial electron transfer and eliminating the subconducting band trap sites.

Luminescent Enhancement of Heterostructure Organic Light-Emitting Devices Based on Aluminum Quinolines

High performance organic light-emitting devices （OLEDs） have been investigated by using fluorescent bis （2-methyl-8-quinolinolato）（para-phenylphenolato）aluminum（BAlq） as an emissive layer on the performance of multicolor devices consisting of N, N＇-bis-（1-naphthyl）-N,N＇diphenyl- 1,1＇-biphenyl-4,4＇- diamine （NPB） as hole transport layer. The results show that the performance of heterostructure blue light-emitting device composed of 8-hydroxyquinoline aluminum （Alq3） as an electron transport layer has been dramatically enhanced. In the case of high performance heterostructure devices, the electroluminescent spectra has been perceived to vary strongly with the thickness of the organic layers due to the different recombination region, which indicates that various color devices composed of identical components could be implemented by changing the film thickness of different functional layers.

High-contrast top-emitting organic light-emitting devices

In this paper we report on a high-contrast top-emitting organic light-emitting device utilizing a moderate-reflection contrast-enhancement stack and a high refractive index anti-reflection layer.The contrast-enhancement stack consists of a thin metal anode layer,a dielectric bilayer,and a thick metal underlayer.The resulting device,with the optimized contrast-enhancement stack thicknesses of Ni（30 nm）/MgF 2（62 nm）/ZnS（16 nm）/Ni（20 nm） and the 25-nm-thick ZnS anti-reflection layer,achieves a luminous reflectance of 4.01% in the visible region and a maximum current efficiency of 0.99 cd/A（at 62.3 mA/cm 2） together with a very stable chromaticity.The contrast ratio reaches 561：1 at an on-state brightness of 1000 cd/m^2 under an ambient illumination of 140 lx.In addition,the anti-reflection layer can also enhance the transmissivity of the cathode and improve light out-coupling by the effective restraint of microcavity effects.

Improvement of electron injection of organic light-emitting devices by inserting a thin aluminum layer into cesium carbonate injection layer

We investigate the electron injection effect of inserting a thin aluminum（Al） layer into cesium carbonate（Cs2CO3）injection layer. Two groups of organic light-emitting devices（OLEDs） are fabricated. For the first group of devices based on Alq3, we insert a thin Al layer of different thickness into Cs2CO3 injection layer, and the device＇s maximum current efficiency of 6.5 cd/A is obtained when the thickness of the thin Al layer is 0.4 nm. However, when the thickness of Al layer is 0.8 nm, the capacity of electron injection is the strongest. To validate the universality of this approach, then we fabricate another group of devices based on another blue emitting material. The maximum current efficiency of the device without and with a thin Al layer is 4.51 cd/A and 4.84 cd/A, respectively. Inserting a thin Al layer of an appropriate thickness into Cs2CO3 layer can result in the reduction of electron injection barrier, enhancement of the electron injection, and improvement of the performance of OLEDs. This can be attributed to the mechanism that thermally evaporated Cs2CO3 decomposes into cesium oxides, the thin Al layer reacts with cesium oxides to form Al–O–Cs complex, and the amount of the Al–O–Cs complex can be controlled by adjusting the thickness of the thin Al layer.

Recombination Efficiency and Width in Bilayer Organic Light-emitting Devices

A bilayer model with ohmic anode contact and injection limited cathode contact has been proposed to calculate the recombination efficiency and recombination zone width of the device. The effects of the thickness of hole transport layer and the barriers of organic/organic interface on the combination efficiency and recombination width have been discussed. It is found that： （1） When the electrons are blocked fully and the holes are not blocked significantly at the organic/organic interface, for a given Lh/L, the recombination efficiency increases with increasing the applied voltage, but at a higher applied voltage, the recombination efficiency decreases with increasing Lh/L; （2） The recombination efficiency increases with increasing applied voltage and Hh＇, and when applied voltage and Hh＇ exceed some value, the recombination efficiency appears as a plateau; （3） The recombination width decreases with increasing the applied voltage and Lh/L. This model might explain the relative experiment phenomena.

Study on Microcavity Organic Light-emitting Devices Containing Negative Refractive Index Dielectric Layer

A new structure containing negative refractive index dielectric layer（NRlDL） is introduced into microcavity. The properties of the new mierocavity organic light-emitting devices（MOLEDs） are investigated. In the experiment, the transfer matrix method is adopted. The dependence of reflectance and transmittance on the refractive index and thickness of NRIDL are analyzed in detail. Compared with the electroluminescence spectra of non-NRIDL diodes, the line widths of the spectra of the MOLEDs are narrower and all the peaks enhance. The results show that the new structure is beneficial to improve the performance and reduce the thickness of microcavity devices.

Lowering the driving voltage and improving the luminance of blue fluorescent organic light-emitting devices by thermal annealing a hole injection layer of pentacene

We chose pentacene as a hole injection layer（HIL） to fabricate the high performance blue fluorescent organic lightemitting devices（OLEDs）. We found that the carrier mobility of the pentacene thin films could be efficiently improved after a critical annealing at temperature 120℃. Then we performed the tests of scanning electron microscopy, atomic force microscopy, and Kelvin probe to explore the effect of annealing on the pentacene films. The pentacene film exhibited a more crystalline form with better continuities and smoothness after annealing. The optimal device with 120℃ annealed pentacene film and n-doped electron transport layer（ETL） presents a low turn-on voltage of 2.6 V and a highest luminance of 134800 cd/m^2 at 12 V, which are reduced by 26% and improved by 50% compared with those of the control device.

High-Efficiency Green Phosphorescent Organic Light-Emitting Diode Based on Simplified Device Structures

A high-efficiency green phosphorescent organic light emitting diode with a simplified structure is achieved that is free of a hole transport layer. The design of this kind of device structure not only saves the consumption of organic materials but also greatly reduces the structural heterogeneities and effectively facilitates the charge injection into the emissive layer. The resulting green phosphorescent organic light-emitting diodes （PHOLEDs） exhibit higher electroluminescent efficiency. The maximum external quantum efficiency and current efficiency reach 23.7% and 88 cd/A, respectively. Moreover the device demonstrates satisfactory stability, keeping 23.7% and 88cd/A, 22% and 82cd/A, respectively, at a luminance of 100 and 1000cd/m2. The working mechanism for achieving high efficiency based on such a simple device structure is discussed correspondingly. The improved charge carrier injection and transport balance are proved to prominently contribute to achieve the high efficiency and great stability at high luminance in the green PHOLEDs.

Green Light-Emitting Organic Electroluminescent Device with a New Fluorescent Dye Dispersed in Poly(N-vinylcarbazole) Emitter Layer

Double-layer organic electroluminescent devices have been constructed. A new fluorescent dye, 9,10-bis(phenylethynyl)anthracence, was chosen as the dopant which was molecularly dispersed in the polymer film, and green light was observed from the device with luminance of 130cd/m(2) at 17V.

Improving Efficiency by Doping PtOEP into Spiro Light-Emitting Devices

To investigate effective means of improving the efficiency of organic light-emitting devices （OLEDs） by making full use of ,triplet emission, a phosphorescent material Pt （II） Octaethylporphine （PtOEP） is doped into polymer host polyspirobifluorene （Spiro） to allow radiative recombination of triplet excitons. The current and brightness characteristics of the devices are tested and the electroluminescent spectra are described. Both fluorescence and phosphorescence are ob- served,and an obvious increase in external quantum efficiency is realized compared to undoped devices when different phosphorescent dopant concentrations are tried. Thus,the phosphorescent emission from triplet excited states might be an effective way to increase the efficiency of OLEDs when the concentration of the phosphorescent dopant is properiy controlled.

Ultra‑Transparent and Multifunctional IZVO Mesh Electrodes for Next‑Generation Flexible Optoelectronics

Mechanically durable transparent electrodes are essential for achieving long-term stability in flexible optoelectronic devices.Furthermore,they are crucial for applications in the fields of energy,display,healthcare,and soft robotics.Conducting meshes represent a promising alternative to traditional,brittle,metal oxide conductors due to their high electrical conductivity,optical transparency,and enhanced mechanical flexibility.In this paper,we present a simple method for fabricating an ultra-transparent conducting metal oxide mesh electrode using selfcracking-assisted templates.Using this method,we produced an electrode with ultra-transparency(97.39%),high conductance(Rs=21.24Ωsq^(−1)),elevated work function(5.16 eV),and good mechanical stability.We also evaluated the effectiveness of the fabricated electrodes by integrating them into organic photovoltaics,organic light-emitting diodes,and flexible transparent memristor devices for neuromorphic computing,resulting in exceptional device performance.In addition,the unique porous structure of the vanadium-doped indium zinc oxide mesh electrodes provided excellent flexibility,rendering them a promising option for application in flexible optoelectronics.

Inorganic and Organic Solution-Processed Thin Film Devices

Thin films and thin film devices have a ubiquitous presence in numerous conventional and emerging technologies. This is because of the recent advances in nanotechnology, the development of functional and smart materials,conducting polymers, molecular semiconductors, carbon nanotubes, and graphene, and the employment of unique properties of thin films and ultrathin films, such as high surface area, controlled nanostructure for effective charge transfer, and special physical and chemical properties, to develop new thin film devices. This paper is therefore intended to provide a concise critical review and research directions on most thin film devices, including thin film transistors, data storage memory, solar cells, organic light-emitting diodes, thermoelectric devices, smart materials, sensors, and actuators. The thin film devices may consist of organic, inorganic, and composite thin layers, and share similar functionality, properties, and fabrication routes. Therefore, due to the multidisciplinary nature of thin film devices, knowledge and advances already made in one area may be applicable to other similar areas. Owing to the importance of developing low-cost, scalable, and vacuum-free fabrication routes, this paper focuses on thin film devices that may be processed and deposited from solution.