Efficiency is crucial for organic light emitting diodes (OLEDs) to be energy-saving and to have a long lifetime for display and solid state lighting applications. Numerous approaches have been proposed to attain high efficiency OLEDs through the synthesis of novel organic materials, the design of light extraction structures and the design of efficiency-effective device architectures. In this report, we first summarise the efficiency records of OLED devices using fluorescent, phosphorescent, and thermally activated delay fluorescent materials. Importantly, we review all the available efficiency-effective device architectural approaches, which include using thin layer structures, low carrier injection barriers, high carrier mobility, balanced carrier injection, effective carrier confinement, effective host-to-guest energy transfer, effective recombination zone, effective exciton generation on the host, effective exciton confinement, p–i–n structures, and tandem structures. It is hoped that better device structures can therefore be devised upon suitable device engineering to achieve higher efficiency for OLED devices.

Approaches for fabricating high efficiency organic light emitting diodes

Degradation Mechanisms and Reactions in Organic Light-Emitting Devices.

The light-emitting element of the present invention includes a light-emitting layer and a layer for controlling movement of carriers between a pair of electrodes. The layer for controlling movement of carriers includes a first organic compound having a carrier transporting property and a second organic compound for reducing the carrier transporting property of the first organic compound, and the second organic compound is dispersed in the first organic compound. The layer for controlling movement of carriers is provided in such a manner, whereby change in carrier balance with time can be suppressed. Therefore, a light-emitting element having a long lifetime can be obtained.

Light-emitting element, light-emitting device, and electronic device

Adv Mater 2009, 21, 1845–1849 2009 WILEY-VCH Verlag G Gas-diffusion barriers play an important role in many applications today, such as flat-panel displays or solar cells grown on plastic substrates In particular, organic light-emitting diodes (OLEDs) need efficient encapsulation because of the high sensitivity of many organic or electrode materials to moisture and oxygen, which has been demonstrated to cause device degradation and limited lifetime Maximum allowable permeation rates for a viable encapsulant to be used in organic optoelectronics are still under debate, but it is common sense that the requirements are far more demanding than those for typical barrier films used in food or pharmacy packaging A very commonly quoted figure for the upper limit of the water-vaportransmission rate (WVTR) in order to reach a minimum OLED lifetime of 10000 h is 10 6 gm 2 day  This value originated from an estimate of the amount of water needed to degrade the reactive cathode material For the oxygen-transmission rate (OTR), maximum values for similar OLED lifetimes have been reported in the region of 10 –10 3 cmm 2 day  Nowadays, a very common technique is the encapsulation of organic devices with a glass or metal lid, which is applied under nitrogen atmosphere using, for example, an epoxy resin as glue In addition, the residual moisture in the cavity between the organic device and lid is further minimized by an opaque getter material For large-area, flexible, or transparent applications, glass-lid encapsulation is not suitable A promising alternative is thin-film barriers of metal oxides or nitrides, such as Al2O3, SiO2, TiO2, or SiN These materials can be deposited by several techniques Plasma-assisted processes, such as sputtering or plasma-enhanced chemical vapor deposition (PECVD), provide high quality films, but their typically reported OTR and WVTR values of about 05 cmm 2 day 1 and 03 gm 2 day , respectively, limited by imperfections in the films, are not sufficient for OLED applications Moreover, efficient step coverage and conformal coating are not straightforward with these techniques As a workaround, hybrid organic/inorganic multilayer structures have been introduced In these, the polymeric intermediate layer levels the inorganic layer so that diffusion pathways through the entire stack of the barrier can be reduced However, the combination of alternating nonvacuum-based polymer deposition processes and vacuum-based deposition processes is timeconsuming and costly, and therefore not really practical for production An alternative technique that promises highly uniform thin-film coatings is atomic-layer deposition (ALD) ALD relies on the sequential exposure of the surface to be coated to a metal–organic precursor and a reactant (H2O, O3, NH3, etc) The sequential dosing of precursors leads to a self-limiting process with concomitant precise control over thickness and homogeneity The technique is well-established in the processing of insulating films for gate dielectrics and capacitors ALD has the advantage of allowing the deposition of very dense films at low temperatures (<100 8C), and thus appears to be a promising technique suitable for preparing encapsulation layers on top of organic electronic devices There are a few reports dealing with ALD-grown thin films for OLED encapsulation Very-low permeation rates of 65 10 5 gm 2 day 1 at 60 8C have been reported for Al2O3 films that were grown at 120 8C [13] However, deposition temperatures above 100 8C might be critical, because glass-transition temperatures for many functional OLED materials have been reported in this very temperature range In this Communication, we report on thin encapsulation layers prepared by ALD at a temperature of 80 8C Specifically, we study neat Al2O3 films as a reference, and introduce novel highly efficient permeation barriers based on so-called nanolaminate structures A nanolaminate, in this case, is a composite film consisting of repetitively deposited ultrathin (a few nanometers thick) alternating layers of Al2O3 and ZrO2 The alternating nanolayer structure suppresses the formation of both microscopic voids and nanocrystals As a result, the probability of the occurrence of statistical defects in the barrier structure related to permeation paths along connected voids or grain boundaries is significantly reducedWe demonstrate that nanolaminates render pinhole-free thin-film encapsulation layers suitable for large-area organic devices possible First, the quantitative assessment of permeation rates on the order of 10 6 gm 2 day 1 for water and 10 3 cmm 2 day 1 for oxygen is very challenging and, as yet, no commercially available method can be applied in this range Therefore, we used a very sensitive permeation-measurement technique that was introduced by Paetzold et al The method allows the measurement

Al2O3/ZrO2 Nanolaminates as Ultrahigh Gas-Diffusion Barriers—A Strategy for Reliable Encapsulation of Organic Electronics

Organic light-emitting diodes have become a mainstream display technology because of their desirable features. Third-generation electroluminescent devices that emit light through a mechanism called thermally activated delayed fluorescence are currently garnering much attention. However, unsatisfactory device stability is still an unresolved issue in this field. Here we demonstrate that electron-transporting n-type hosts, which typically include an acceptor moiety in their chemical structure, have the intrinsic ability to balance the charge fluxes and broaden the recombination zone in delayed fluorescence organic electroluminescent devices, while at the same time preventing the formation of high-energy excitons. The n-type hosts lengthen the lifetimes of green and blue delayed fluorescence devices by > 30 and 1000 times, respectively. Our results indicate that n-type hosts are suitable to realize stable delayed fluorescence organic electroluminescent devices. OLEDs based on thermally activated delayed fluorescence have shown high fluorescence efficiency but poor lifetime. Herein, Cui et al. demonstrate that the use of n-type host molecules can increase the device lifetime by 30 times and 1000 times for green and blue OLEDs, respectively.

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Long-lived efficient delayed fluorescence organic light-emitting diodes using n-type hosts

In this letter, we present and analyze the device performance of the mixed host (MH) organic light-emitting devices (OLEDs). The host of the emitting layer (EML) material in this device consists of a hole transport layer (HTL) and an electron transport layer (ETL) fabricated by coevaporation. The bipolar transport characteristic of the MH layer helps to reduce the driving voltage. Device lifetime is increased due to the elimination of the sharp boundary of the HTL/EML interface. Combining the MH structure with a high mobility electron ETL material, bis(10-hydroxybenzo[h]qinolinato)beryllium, the OLED has shown a brightness of 27600cd∕m2 at a driving voltage of 5 V, and a lifetime four times longer than that of a conventional OLED.

Mixed host organic light-emitting devices with low driving voltage and long lifetime

High contrast ratio organic light-emitting devices based on CuPC as electron transport material

Operation lifetimes of organic light-emitting devices with different layer structures

Silicon gratings with periods from 180 to 550 nm were fabricated with a laser ablation technique in which the interference fringe from an intense and coherent ultraviolet laser at 266 nm directly melted silicon surface. The scanning electron microscopy and atomic force microscopy pictures showed that the corrugations were in quite good quality with the depth as large as 70 nm. The measurement of grating period dependence on temperature showed that rapid thermal annealing could release the thermal strains, which were built during the melting and cooling process in laser ablation, and make the grating period variation more regular. Also, with an air gap between the sample and prism surfaces, the fabricated gratings had weaker thermal strains and more regular temperature dependencies. All the measurement results of temperature dependence were consistent with theoretical predictions.

Direct writing of silicon gratings with highly coherent ultraviolet laser

GaAs-AlGaAs multiple quantum well semiconductor circular ring lasers with a /spl sigma/ shape were fabricated by using the deep UV laser-assisted cryo-etching technique. Most of the fabricated lasers had external quantum efficiencies of more than 18% which mere higher than similar devices previously reported. The modal spacing observed from the resonance spectrum near threshold was always several times that corresponding to the circular oscillation of the ring cavity. The observed modal spacing was quite consistent with the theoretical result based on a coupled-cavity model.

Yih Chang

Papers

Mixed host organic light-emitting devices with low driving voltage and long lifetime

High contrast ratio organic light-emitting devices based on CuPC as electron transport material

Operation lifetimes of organic light-emitting devices with different layer structures

Direct writing of silicon gratings with highly coherent ultraviolet laser

Semiconductor circular ring lasers fabricated with the cryo-etching technique