development of high performance oleds for general lighting

Journal ofMaterials Chemistry C

FEATURE ARTICLE

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Development of h

HPDiUenata2am

neering, Yamagata University inprofessor (2009–now).

aDepartment of Organic Device Engineer

Engineering, Yamagata University, Yoneza

[email protected]; [email protected] Center for Organic Electronics (

Yamagata, 992-8510 Japan

Cite this: J. Mater. Chem. C, 2013, 1,1699

Received 8th November 2012Accepted 12th December 2012

DOI: 10.1039/c2tc00584k

www.rsc.org/MaterialsC

This journal is ª The Royal Society of

igh performance OLEDs for generallighting

Hisahiro Sasabe*ab and Junji Kido*ab

Since the development of the first white organic light-emitting device (OLED) in 1993, twenty years have

passed. The power efficiency and lifetime of this white OLED were reportedly only <1 lm W�1 and <1 day,

respectively. However, recent rapid advances in material chemistry have enabled the use of white OLEDs for

general lighting. In 2012, white OLED panel efficiency has reached 90 lm W�1 at 1000 cd m�2, and a

tandem white OLED panel has realized a lifetime of over 100 000 hours. What is more important in

OLEDs is to shed clear light on the new design products, such as transparent lighting panels and

luminescent wallpapers. These fascinating features enable OLEDs as a whole new invention of artificial

lighting. In this review, we would like to overview the recent developments of white OLED, especially

three key elemental technologies related to material chemistry: (1) low operating voltage technology,

(2) phosphorescent OLED technology and (3) multi-photon emission (MPE) device technology.

1 Introduction: OLED lighting technology

In 1993, Kido and co-workers developed the rst white organiclight-emitting device (OLED). Although, the performance of thiswhite OLED was reportedly only �1 lm W�1 with an externalquantum efficiency of 1% and a short lifetime of less than 1day.1 However, now researchers believe that white OLEDs will bea promising technology for the next-generation light sourcewithin a few years (Fig. 1).

isahiro Sasabe received hish.D. degree in 2005 from theepartment of Applied Chem-stry of Osaka Prefectureniversity. He joined the Opto-lectronic Industry and Tech-ology Development Associationt Yamagata in 2005–2007, andhen went for a postdoctoral stayt Yamagata University in007–2009. He started hiscademic career at the Depart-ent of Organic Device Engi-Yonezawa as an assistant

ing, Graduate School of Science and

wa, Yamagata, 992-8510 Japan. E-mail:

agata-u.ac.jp

ROEL), Yamagata University, Yonezawa,

Chemistry 2013

For general lighting application, the light source requires abrightness of 3000–5000 cd m�2, and a standard uorescenttube achieves 70 lm W�1 and 10 000 hours of lifetime simul-taneously. Thus, the next generation light source should ach-ieve a higher power efficiency and a longer lifetimesimultaneously at high brightness. For a source to be humaneye-friendly, it should possess a high color rendering index(CRI, Ra > 80) to reproduce the colors of an object. For OLEDs,one can use the various emissive materials with different colorsto produce a white emission with a high Ra and a desiredcorrelated color temperature (CCT).2–8 In addition, white OLEDsare mercury-free illumination light-sources and meet therequirements of the EU WEEE & RoHS directives. From thepower consumption point of view, a large part of energy supplyin conventional light sources is converted into heat instead of

Junji Kido is a full professor ofthe Department of OrganicDevice Engineering at YamagataUniversity. He received his Ph.D.in polymer chemistry from Poly-technic University, New York, in1989. From 2003 to 2010, Kidoserved as the general director ofthe Research Institute forOrganic Electronics founded bythe government of YamagataPrefecture. He invented a whiteOLED in 1993 and is working onthe development of high perfor-mance OLEDs.

J. Mater. Chem. C, 2013, 1, 1699–1707 | 1699

http://dx.doi.org/10.1039/c2tc00584k

http://pubs.rsc.org/en/journals/journal/TC

http://pubs.rsc.org/en/journals/journal/TC?issueid=TC001009

Fig. 1 Recent demonstration of OLED panels from Lumiotec in Japan (Copyright2012, Lumiotec, http://www.lumiotec.com).

Journal of Materials Chemistry C Feature Article

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light. For example, the surface of an incandescent bulb reachesca. 90 �C and that of a uorescent tube reaches ca. 60 �C. Inorder to prevent a re, it is crucial for the next-generation lightsource to remain cool at the surface by realizing high powerconversion efficiency and reducing the operating voltage. In thisregard, OLEDs are an ideal light source and able to keep thesurface much cooler at around 30 �C than those of theconventional light sources. What is more important in OLEDs isto shed clear light on the new design products, such as trans-parent lighting panels and luminescent wallpapers. Thesefascinating features enable OLEDs as a whole new inventionof articial lighting. In 2012, researchers at Panasonic andToshiba independently introduced white OLED panels with 90lmW�1 efficiency at 1000 cd m�2.9,10 Panasonic researchers alsorealized tandem white OLEDs with an extremely long lifetime ofover 100 000 hours at 1000 cd m�2. On the other hand, LGchemical announced to develop a 300 � 300 mm2 size whiteOLED panel with 135 lm W�1 and 40 000 hours lifetime at ahigher brightness of 3000 cd m�2 by 2015.11

In this review, we would like to overview the recent devel-opments of white OLEDs, especially three key elemental tech-nologies related in material chemistry: (1) low operating voltagetechnology, (2) phosphorescent OLED technology and (3) multi-photon emission (MPE) device technology. Since the powerconsumption is directly proportional to the operating voltage inan OLED, the operating voltage must be minimized for eco-friendly lighting. At the same time, it is essential to realize highinternal quantum efficiency, which means high electron-to-photon conversion efficiency, by using phosphorescent mate-rials. Generally, standard OLEDs show a signicant decrease inefficiency and lifetime under use at high brightness, whichaccompanies high-current density. A kind of tandem OLEDcalled the MPE device gives a key solution to realize high effi-ciency and long lifetime simultaneously even at highbrightness.

Fig. 2 Li complexes used as an EIL.

2 Low operating voltage technology

The reduction of the electrical power consumption is absolutelyessential from a practical point of view. From a theoreticalperspective, Meerheim and co-workers have predicted a ther-modynamic limit of operating voltage by using an equation

1700 | J. Mater. Chem. C, 2013, 1, 1699–1707

based on black body radiation.12 For example, the thermody-namic limit in a green OLED is evaluated to be 1.95 V at 100 cdm�2. Compared to the lowest voltage of 2.40 V in fac-tris-(2-phenylpyridine)iridium(III) [Ir(ppy)3]-based OLEDs at thesame brightness reported by Su and co-workers, there is still alarge difference of 0.45 V to be reduced.13

OLED is a multilayered device, which typically consists of atransparent metal oxide anode, hole-transport layer (HTL),emissive layer (EML), electron-transporting layer (ETL) andmetal cathode. Thus, there are a number of interfaces at metal/organic and organic/organic layers. One approach to reduce theoperating voltage is to insert an inorganic electron injectionlayer at the ETL/cathode metal interface. One typical example isthe use of an ultra thin lithium uoride (LiF) layer as an elec-tron-injection layer (EIL). In 1997, Mason and co-workersreported an N,N0-di(naphalene-1-yl)-N,N0-diphenylbenzidine(NPD)/8-hydroxyquinoline aluminum (Alq) device with LiF.14

Compared with the corresponding Mg:Ag based device, theLiF/Al based device showed a 7 V reduction in the operatingvoltage at 100 mA cm�2. Similarly, Wakimoto and co-workershave reported the use of inorganic salts, such as lithium oxide(Li2O) and sodium chloride (NaCl).15

Another approach is the use of an ultra thin metal layer or ametal-doped organic layer at the organic/cathode interface.16

For the use of an ultra thin metal layer, Kido and co-workersreported a device with Li/Ag in 1993.16a This device showedsuperior performance to those with a conventional cathodeMg:Ag.17 A metal-doping technology so-called chemical dopinggenerates the radical anions as intrinsic electron carriers, whichresult in a low barrier height for electron injection and highconductivity of the doped layer. In 1998, Kido and co-workersreported that a device with a Li-doped Alq/Al layer showed highluminance of over 30 000 cd m�2 at 10.5 V, while a devicewithout the metal-doped Alq layer exhibited only 3400 cd m�2 at14 V.18 Later in 2002, Huang and Leo reported an Alq device witha Li-doped 4,7-diphenyl-1,10-phenanthroline (Bphen)/Al layer,which showed 1000 cd m�2 at 2.9 V.19 Compared to Tang'sinitial Alq device,17 this Alq device showed a 7 V reduction indriving voltage at 1000 cd m�2. Furthermore, in 2002, Leo andco-workers reported an Ir(ppy)3-based device with a Cs-dopedBphen/Al layer, which showed 1000 cd m�2 at 3.0 V.20 However,these highly active alkaline metals such as Li and Cs are noteasy-to-handle and can easily oxidize in the presence of ambientoxygen and water.

As an alternative strategy, a new type of cathode interfacelayer composed of metal complexes such as 8-quinolinolatolithium (Liq), 8-quinolinolato sodium (Naq), lithium acetyla-cetonate (Liacac) and lithium dipivaloylmethane (Lidpm) was

This journal is ª The Royal Society of Chemistry 2013


Fig. 3 Chemical structure of materials.

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reported by Kido and co-workers in 1998 (Fig. 2).21–23 Thesemetal complexes can be evaporated at a relatively low temper-ature of 200–300 �C compared with LiF (>700 �C), and are easy-to-handle under ambient conditions. By using a co-depositedlayer of Alq and Liq as an EIL, the electron injection from the Alcathode to Alq layer can be facilitated effectively.

Kido and co-workers have deduced that thermal reduction ofLi cations by Al occurs at the Liq/Alq interface leading to aneffective Li-doping of Alq. Other than Liq and Naq, the use of8-quinolinolato cesium (Csq) as a cathode interface layer hasbeen reported by Qiao and Qiu in 2008.24 Aer the report of Liq,Fujihira and C. Ganzorig reported lithium carboxylates thatshowed superior performance to LiF in 1999.25 Kim and co-workers reported the use of 2-(2-hydroxyphenyl)benzoxazolatelithium (LiPBO).26 In 2003, Wang and Ma reported a hydroxy-loxadiazole lithium complex.27 Recently, Pu and co-workershave reported lithium phenolate derivatives.28 These materialsshow much lower sublimation temperature of around 300 �Cwith comparable performance. Surprisingly, a 40 nm thick lmof lithium 2-(20,20 0-bipyridine-60-yl)phenolate (LiBPP) or lithium2-(2-pyridyl)phenolate (LiPP) is effective as an EIL, providinglow driving voltages, while a thick lm of LiF serves as acomplete insulator, resulting in high driving voltages. From a


practical point of view, this is a huge advantage of the use ofmetal complexes. Very recently, metallocene compounds29 andimidazolium salt30 have been reported for use as air stabledopants for organic n-type semiconductors.

3 Phosphorescent OLED technology

Recent advances in material chemistry have enabled whiteOLED efficiency beyond that of uorescent tubes by usingphosphorescent technology and light-outcoupling techniques.31

The use of phosphorescent OLED technology is critical forrealizing energy-saving general lighting, because phosphores-cent emitters such as Ir(ppy)3 and iridium(III)bis(4,6-(diuor-ophenyl)pyridinato-N,C20)picolinate (FIrpic) enable an internalefficiency as high as 100% converting both singlet and tripletexcitons into photons and make OLED efficiency 4 times higherthan those with uorescent emitters.32,33 Generally, a phos-phorescent emitter is dispersed in a suitable host material toobtain a high photoluminescent quantum efficiency (hPL) sup-pressing concentration quenching. In order to realize a high hPL

for a phosphorescent emitter, the host material needs to have ahigher triplet energy (ET) than that of the emitter.34–37 Especiallyin blue phosphorescent OLEDs, host materials with high ET

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over 2.75 eV are necessary. A representative example usingFIrpic as a blue phosphorescent emitter has been demonstratedby Tokito and co-workers.38 In this device, carbazole-based hostmaterials 4,40-N,N0-dicarbazolylbiphenyl (CBP) and 4,4-bis-(9-carbazolyl)-2,2-dimethyl-biphenyl (CDBP) were used as hostmaterials (Fig. 3).

The power efficiency was reported to be as low as 6.3 lm W�1

for CBP, but the use of high ET host materials like CDBP gaveimproved efficiency of up to 10.5 lm W�1. In general, recom-bination of holes and electrons takes place very close to theHTL/EML and/or EML/ETL interface, because there are largeenergy barriers from HTL/ETL to EML. Thus, to obtain a higherdevice efficiency, it is highly important to use high ET HTL andETL. In 2005, Kido and co-workers developed an FIrpic-basedOLED with an hp,max of 37 lm W�1 by using novel highET materials, such as 2,20-bis[30 0-(N,N0-ditolylamino)phenyl]-biphenyl (3DTAPBP), 2,20-bis(4-carbazolylphenyl)-1,10-biphenyl(4CzPBP) and 1,3,5-tris[3,5-bis(pyrid-3-yl)phenyl]benzene(mTPPP) (Fig. 3).39 This device used high ET materials not onlyas a host but also as an ETL and HTL.

Later in 2008, Sasabe and co-workers realized an FIrpic-based OLED with an hp,max over 60 lm W�1 using a wide-energygap ETL, 3,5,300,50 0-tetra-3-pyridyl-[1,10,30,10 0]terphenyl (B3PyPB)(Fig. 4).40 These devices use high ET HTL and ETL, which act ascarrier-transporters as well as triplet exciton blockers to mini-mize the efficiency loss at the HTL/EML and/or EML/ETL

Fig. 4 (a) Device architecture and used materials in high-performance FIrpic-based OLEDs. (b) Power efficiency–luminance–current efficiency characteristics ofOLED with B3PyPB. (Reprinted with permission from ref. 40. Copyright 2008American Chemical Society.)

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interface. Recently, some researchers have suggested that adirect recombination on the emitter would be the key processof exciton generation in phosphorescent OLEDs41–44 becauseelectrons are easily trapped by the highly doped FIrpic andelectrophosphorescence appears to take place mainly near theinterface between EML and ETL.45 On the other hand, carrierbalance between electrons and holes in EML is anotherconsideration to obtain a high efficiency OLED.46,47 For a bettercarrier balance, it is thought to be important to adjust frontiermolecular orbitals (FMO) of the host material with FMOs ofHTL and ETL. Considering each emitter has its own electronicand optical properties depending on the chemical structure,48–50

each emitter should be installed in a suitable material system tofulll the highest potential.

For fabricating white OLEDs using phosphorescent emitters,there are two common approaches. One approach is to combinea blue uorescent emitter with phosphorescent emitters of theother colors, creating a hybrid white OLED. Because all phos-phorescent emitter devices could introduce an intrinsicexchange energy loss derived from energy transfer, which takesplace from the singlets of the host to the triplets of the guest,hybrid white OLEDs are considered to take advantage of theoperating voltage.51–54 Themain requirement of this approach isthe use of a blue uorescent emitter with higher ET than that ofthe other phosphorescent emitters. The blue uorescentemitter also needs to have a high hPL. Several such hybrid whiteOLEDs have been developed. For the most recent example, Leeand co-workers reported hybrid a white OLED with a powerefficiency at 100 cdm�2 (hp,100) of 50 lmW�1 and hp,1000 of 34 lmW�1 by using 2,8-di[4-(diphenylamino)phenyl] dibenzothio-phene-S,S-dioxide (DADBT) as a blue uorescent emitter andfac-tris(2-phenylquinoline)iridium(III) [Ir(2-phq)3] as an orangephosphorescent emitter (Fig. 3).54

Importantly, this work has shed light on the possibility ofhybrid white OLEDs presenting high performances, which arecomparable with that using all phosphorescent emitters. At thisstage, there are large spectrum changes in electroluminescencecaused by triplet–triplet annihilation depending on the currentdensity in this white OLED. This could be improved by using anovel material system and/or sophisticated device engineering.

Another approach is to use all phosphorescent emitters.Universal Display corporation reported a white OLED with 102lm W�1 efficiency at 1000 cd m�2 using light outcouplingenhancement techniques.55 However, the detailed material anddevice structures are unrevealed. While in the scientic litera-ture, Reineke and co-workers reported white OLEDs with uo-rescent tube efficiency of 81 lm W�1 at 1000 cd m�2 achievingRa of 80 by combining RGB phosphorescent emitters and lens-based outcoupling enhancement (�2.7) techniques.56 However,the efficiency was decreased to hp,1000 of 33 lm W�1 withoutlight-outcoupling enhancement. On the other hand, Su and co-workers have reported a high-efficiency two-color white OLEDwith an hp,1000 of 44 lm W�1 without using outcouplingenhancement.57 Although the efficiency is much higher than theother white OLEDs, the CRI is 68, which is not acceptable for thegeneral lighting use. Because the CRI of white OLEDs can begreatly improved by using pure blue emitters having peak



Fig. 5 Current density–external quantum efficiency characteristics of blue andwhite OLEDs.

Fig. 7 Chemical structures of phenylpyridines.


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wavelength at 450 nm, a three-color white OLED by using a pureblue phosphorescent emitter, mer-tris(N-dibenzofuranyl-N0-methylimidazole)iridium(III) [Ir(dbfmi)], has been investi-gated.58 Ir(dbfmi) has a long phosphorescent lifetime (sp) of19.6 ms, which is ca. 12 times longer than that of FIrpic (1.48 ms)and the corresponding pure blue OLED showed strong effi-ciency roll-off at high current density. However, by using acombination of green and red emitters with short sp (�1.5 ms),this white OLED dramatically reduced the efficiency roll-offmost likely by harvesting the triplet energy of the blue emitter(Fig. 5). This white OLED showed an hp,max of 59.9 lm W�1 andan hp,1000 of 43.3 lmW�1 with a high CRI over 80. Therefore, themethodology is quite useful because the choice of blue phos-phorescent emitter can be greatly expanded. The efficiency of awhite OLED strongly depends on the performance of the blueemissive unit contained therein, development of high-perfor-mance blue phosphorescent OLEDs and relatedmaterials is stilla challenging issue.59–61

For the fabrication of vacuum-deposited OLEDs, amorphousmolecular glasses are commonly used, and small moleculestherein had been regarded to be randomly oriented so far.However, recently, researchers have realized that the perfor-mance of amorphous molecular glasses can be improved via asophisticatedmolecular design taking advantage of intra and/orintermolecular interactions.62 A typical example is to use theweak CH–N hydrogen bond (Fig. 6).

The binding energy of this weak CH–N hydrogen bond isestimated to be 10–20 kJ mol�1, which is about half the energyof a typical hydrogen bond (20–30 kJ mol�1).63 This can be seenin the difference in the boiling points of benzene (80.1 �C) andpyridine (115.2 �C). Although the molecular weights of thesemolecules are almost the same, the weak CH–N hydrogen bond

Fig. 6 A weak CH–N hydrogen bond interaction between pyridines.


interaction between pyridines leads to an increase in the energyneeded to evaporate the liquid. In fact, Hohenstein andSherill have reported the binding energy of a pyridine dimer tobe 13 kJ mol�1 using quantum chemical calculations.64

For a material in OLED, Ichikawa and co-workers reported a2,20-bipyridine derivative, 1,3-bis[2-(2,20-bipyridin-6-yl)-1,3,4-oxadiazo-5-yl]benzene (BpyOXD) which uses a weak CH–Nhydrogen bond interaction between pyridines. Compared to 1,3-bis[2-(4-tert-butylphenyl)-1,3,4-oxadiazo-5-yl]benzene (OXD7),BpyOXD showed superior performance in an NPD/Alq devicewith a much lower driving voltage (Fig. 3).65

Yokoyama and co-workers have investigated the molecularaggregation state of oxadiazole derivatives by using a variableangle spectroscopic ellipsometry.66 They reported that intra-molecular CH–N hydrogen bond interactions of pyridine ringscause the molecules to become planar and enhance the hori-zontal molecular orientation, leading to high mobility.

On the other hand, by using 3- and/or 4-pyridine deriva-tive(s), an “inter”-molecular CH–N hydrogen bond network canbe created in the solid lm (Fig. 7).

Recently, several bis-4,6-(3,5-dipyridylphenyl)-2-methylpyr-imidine (BPyMPM) derivatives have been developed and used toinvestigate the effect of nitrogen orientation on physical prop-erties (Fig. 3).67 Each of these BPyMPM derivatives have adifferent nitrogen orientation on the peripheral pyridine ringsand show a large difference in electron mobility (me). At 298 K,the me of B4 is measured to be 10 times higher than that of B3and 100 times higher than that of B2 (Fig. 8, Table 1).68

Recently, Yokoyama and co-workers have reported molecularorientation and intermolecular CH–N hydrogen bonding inBPyMPM derivatives by using a variable angle spectroscopicellipsometry and an infrared spectrometry analyses.69 They have

Fig. 8 Electronmobility of BPyMPM derivatives. (Reprinted with permission fromref. 67).

J. Mater. Chem. C, 2013, 1, 1699–1707 | 1703


Fig. 9 Examples of proposed network formation of B3PyMPM and B4PyMPMbased on weak CH–N hydrogen-bonding interaction.

Table 1 Charge transport parameters of BPyMPM derivatives

Compound m3 (cm2 V�1 s�1)a m0 (cm

2 V�1 s�1)b s (meV)c ed

B2PyMPM 1.6 � 10�6 5.1 � 10�5 91 2.7B3PyMPM 1.5 � 10�5 6.9 � 10�5 88 1.2B4PyMPM 1.0 � 10�4 4.5 � 10�4 76 0.6

a Electron mobility calculated by the TOF method at 298 K.b Hypothetical mobility in the disorder-free system. c Energeticdisorder. d Positional disorder.

Fig. 11 A structure of 2 unit MEP device.


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pointed out that B3PyMPM and B4PyMPM gave highly orientedlms likely driven by CH–N hydrogen bonding (Fig. 9).

4 Multi-photon emission device technology

As mentioned above, standard OLEDs show a signicantdecrease in efficiency and lifetime under use in general lighting(Fig. 10).

Fig. 10 Required OLED performance for lighting and display application.

1704 | J. Mater. Chem. C, 2013, 1, 1699–1707

A tandem OLED, so-called multi-photon emission (MPE)OLED, is recognized as a key technology for use in generallighting, because one can obtain high-brightness at low currentdensity leading to an OLED with high efficiency and long life-time (Fig. 11).70 For recent example, Lumiotec and Panasonicuse MPE device technology. An MPE device consists of multipleemissive units connected with a charge generation layer (CGL).In this device, each CGL generates electrons and holes uponvoltage application. Injected holes and electrons recombine ineach emissive layers (EML). In N-unit MPE devices, the oper-ating voltage increasesN times, however, theN times luminancecan be obtained compared to that in a 1 unit device at the samecurrent density. In general, the lifetime of an OLED is inverselyproportional to the amount of the current, which is equivalentto how many times oxidation and reduction take place on themolecule therein. Therefore, the OLED lifetime can bedramatically improved by using an MPE device structure. At thesame time, especially in phosphorescent OLEDs, a problem ofefficiency decrease at high current density, which is calledtriplet–triplet annihilation, can be avoided. Further, thickerMPE devices result in fewer short-circuit defects. Use of atransparent indium tin oxide (ITO) electrode as a part of CGL isan effective method, however, use of ITO results in sputteringdamage to an organic layer, and the high conductivity causescross-talk problem (Fig. 12).

On the other hand, a CGL consisting of insulating organicmaterials, such 2,3,5,6-tetrauoro-7,7,8,8-tetracyanoquinodi-methane (F4TCNQ) and 1,4,5,8,9,11-hexaazatriphenylene hexa-carbonitrile (HATCN) has been focused as an alternative

Fig. 12 A photograph of the cross-talk problem without patterning connector.



Fig. 13 Chemical structures of F4TCNQ and HATCN.


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approach (Fig. 13).71 An organic acceptor compound, HATCNhas no absorption in the visible range and has a deep lowestunoccupied molecular orbital (LUMO) level to accept an elec-tron from the neighboring donor material, such as NPD uponvoltage application. Liao and co-workers reported a uorescentMPE device incorporating a combination of HATCN/NPD layersas a CGL.72 They demonstrated that use of a HACN layer is aneffective way to replace the p-type doped HTL in CGL, which canreduce the operating voltage, and improve the stability andpower efficiency.

Recently, Chiba and co-workers have reported ultra high-efficiency Ir(ppy)3-based MPE devices using HATCN.73 A threestacked MPE device realized an extremely high current effi-ciency of 244 cd A�1 (an external efficiency of 66%) at 1000 cdm�2, which is three times higher than that in a 1 unit device(Fig. 14). Assuming an outcoupling efficiency of 30% from thenormal glass/ITO substrate, this efficiency means an internalquantum efficiency of 220%.

Very recently, Small and co-workers have demonstratedexperimentally that the hole injection efficiency of the HATCN/NPD interface reaches close to 100%.74 Although HATCN hasseveral attractive features as an acceptor in CGL, there is aproblem related with the deep LUMO energy. Recently, Kim andco-workers have pointed out that the electron injection fromHATCN to the neighboring ETL with a deep LUMO level isefficient, while that to the ETL with a shallow LUMO level is

Fig. 14 Current efficiency–luminance characteristics of green phosphorescentMPE devices (1-unt: circles, 2-unit: triangles, and 3-unit: squares) (Reprinted withpermission from ref. 73).


inefficient due to the large energy offset between the LUMOenergies.75 Because the use of ETL with high ET is generallyeffective for a blue phosphorescent OLED,40,76 effective electron-injection from a CGL to ETL with a high-lying LUMO level canbe a hurdle in a blue phosphorescent MPE device. Indeed, anFIrpic-based MPE device using a combination of TAPC/MoO3/Al/LiF layers as a CGL unit did not work well as a MPE device,and showed a much higher driving voltage of 16.2 V at 100 cdm�2, which is 5.6 times higher than a 1 unit device.76 Therefore,a novel combination of TAPC/MoO3/Al/Liq layers has beenproposed as a CGL unit. In this case, the use of Liq instead ofLiF is critical to inject electrons from the CGL unit. This result iscompletely different in the green phosphorescent MPE device.73

5 Conclusion and outlook

At this stage, the efficiency of white OLED panels has justreached 90 lm W�1 at 1000 cd m�2, slightly above the perfor-mance of uorescent tubes, and there is still much room forimprovement of the power efficiency compared to the theoret-ical limit of 248 lmW�1 (standard light source (D65) from 400 to700 nm wavelength).77 As mentioned, the power consumption isdirectly proportional to the operating voltage, therefore, theoperating voltage must be minimized by using a smart materialand device engineering. Simultaneously, it is absolutely essen-tial to realize high internal quantum efficiency. Thus, thedevelopment of novel smart materials with multiple function-alities is still a challenging task. In this regard, what isrequested to material chemists is to make clear the relation-ships among chemical structure, quantum chemical simula-tion, physical properties and device performances.43,78 Thiswould contribute greatly to extract a guideline to maximize thepotential of a material and device performance. In a futureOLEDs, molecular orientation will be actively controllable by asophisticated molecular design including intra- and intermo-lecular interactions and an installed organic semiconductorlayer will perform beyond the molecular formula.79 Especially inthe EML, a horizontally oriented emitter is expected to boost theefficiency of OLED up to 150%.80 A new type of uorescent OLEDincorporating thermally activated delayed uorescence (TADF)materials has been recently developed by Adachi and co-workers.81 For this, uorescent emitters with extremely smallDEST, which means a small energy difference between singletand triplet excited states, are necessary to be explored. Ulti-mately, TADF OLED is expected to achieve high internalquantum efficiency close to that of phosphorescent OLEDswithout using platinum group metals, such as iridium, plat-inum and osmium. Standard current-driven OLEDs show asignicant decrease in efficiency and lifetime at high brightnessof 3000–5000 cd m�2. A key solution is to use a MPE devicestructure that achieves high efficiency and long lifetimesimultaneously. However, even in the state-of-the-art MPEdevices, the operating voltage is still much higher than that oftheoretical limit. Thus, development of novel materials for CGLand detailed investigation of the operating mechanism aroundCGL would become highly desirable. Material chemists can playa critical role in the creation of smart materials for the future

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OLED lighting. We believe that OLED lighting can replacetraditional light sources in the near future.

Acknowledgements

We would like to thank all the researchers who participated inour works discussed in the paper and whose names appear inreferences. We also greatly acknowledge the nancial support inpart by New Energy and Industrial Technology DevelopmentOrganization (NEDO) through the “Advanced Organic DeviceProject”, by Japan Regional Innovation Strategy Program by theExcellence (J-RISE) (creating international research hub foradvanced organic electronics) of Japan Science and TechnologyAgency (JST), and by KAKENHI (23750204).

Notes and references

1 J. Kido, M. Kimura and K. Nagai, Science, 1995, 267, 1332–1334.

2 Organic Electronics-Materials, Processing, Devices andApplications, ed. F. So, CRC Press, 2010.

3 B. W. D'andrade and S. R. Forrest, Adv. Mater., 2004, 16,1585–1595.

4 F. So, J. Kido and P. Burrows, MRS Bull., 2008, 33, 663–669.5 G. Zhou, W.-Y. Wong and S. Suo, J. Photochem. Photobiol., C,2010, 11, 133–156.

6 K. T. Kamtekar, A. P. Monkman and M. R. Bryce, Adv Mater.,2010, 22, 572–582.

7 Q. Wang and D. G. Ma, Chem. Soc. Rev., 2010, 39, 2387–2398.8 M. C. Gather, A. Kohnen and K. Meerholz, Adv Mater., 2011,23, 233–248.

9 T. Komoda, K. Yamae, V. Kittichungchit, H. Tsuji and N. Ide,SID 12 Digest, 2012, 610.

10 K. Sugi, T. Ono, D. Kato, T. Yonehara, T. Sawabe, S. Enomotoand I. Amemiya, SID 12 Digest, 2012, 1548.

11 http://www.lgchem.com.12 R. Meerheim, K. Walzer, G. He, M. Pfeiffer and K. Leo, Proc.

SPIE, 2006, 6192, 61920P.13 S. J. Su, H. Sasabe, Y. J. Pu, K. Nakayama and J. Kido, Adv

Mater., 2010, 22, 3311–3316.14 L. S. Hung, C. W. Tang and M. G. Mason, Appl. Phys. Lett.,

1997, 70, 152–154.15 T. Wakimoto, Y. Fukuda, K. Nagayama, A. Yokoi, H. Nakada

and M. Tsuchida, IEEE Trans. Electron Device, 1997, 44, 1245.16 (a) J. Kido, K. Nagai and Y. Okamoto, IEEE Trans. Electron

Device, 1993, 40, 1342; (b) K. Walzer, B. Maennig,M. Pfeiffer and K. Leo, Chem Rev., 2007, 107, 1233–1271.

17 C. W. Tang and S. A. VanSlyke, Appl. Phys. Lett., 1987, 51, 913.18 J. Kido and T. Matsumoto, Appl. Phys. Lett., 1998, 73, 2866.19 J. Huang, M. Pfeiffer, A. Werner, J. Blochwitz, K. Leo and

S. Liu, Appl. Phys. Lett., 2002, 80, 139.20 M. Pfeiffer, S. R. Forrest, K. Leo and M. E. Thompson, Adv.

Mater., 2002, 14, 1633.21 J. Endo, J. Kido and T. Matsumoto, Ext. Abst. (59th Autumn

Meet. 1998), Jpn. Soc. Appl. Phys., 16a-YH-10, p. 1086.22 J. Endo, T. Matsumoto and J. Kido, Jpn. J. Appl. Phys., Part 2,

2002, 41, L800.

1706 | J. Mater. Chem. C, 2013, 1, 1699–1707

23 C. Schmitz, H.-W. Schmidt and M. Thelakkat, Chem. Mater.,2000, 12, 3012.

24 K. Xie, J. Qiao, L. Duan, Y. Li, D. Zhang, G. Dong, L. Wangand Y. Qiu, Appl. Phys. Lett., 2008, 93, 183302.

25 C. Ganzorig and M. Fujihira, Jpn. J. Appl. Phys., Part 2, 1989,38, L1348.

26 Y. Kim, J.-G. Lee and S. Kim, Adv. Mater., 1999, 11, 1463.27 F. Liang, J. Chen, L. Wang, D. Ma, X. Jing and F. Wang,

J. Mater. Chem., 2003, 13, 2922.28 Y.-J. Pu, M. Miyamoto, K. Nakayama, T. Oyama,

M. Yokoyama and J. Kido, Org. Electron., 2009, 10, 228.29 (a) C. K. Chan, W. Zhao, S. Barlow, S. Marder and A. Kahn,

Org. Electron., 2008, 9, 575; (b) Y. Qi, S. K. Mohapatra,S. B. Kim, S. Barlow, S. R. Marder and A. Kahn, Appl. Phys.Lett., 2012, 100, 083305.

30 P. Wei, T. Menke, B. D. Naab, K. Leo, M. Riede and Z. Bao,J. Am. Chem. Soc., 2012, 134, 3999.

31 Highly Efficient OLEDs with Phosphorescent Materials, ed. H.Yersin, Wiley-VCH, Weinheim, 2008.

32 M. A. Baldo, S. L. Lamansky, P. E. Burrows, M. E. Thompsonand S. R. Forrest, Appl. Phys. Lett., 1999, 75, 4.

33 Y. Kawamura, K. Goushi, J. Brooks, J. J. Brown, H. Sasabeand C. Adachi, Appl. Phys. Lett., 2005, 86, 071104.

34 B. Mi, Z. Gao, Z. Liao, W. Huang and C. H. Chen, Sci. China:Chem., 2010, 53, 1679–1694.

35 Y. Tao, C. Yang and J. Qin, Chem. Soc. Rev., 2011, 40, 2943–2970.

36 A. Chaskar, H. F. Chen and K. T. Wong, Adv Mater., 2011, 23,3876–3895.

37 S. O. Jeon and J. Y. Lee, J. Mater. Chem., 2012, 22, 4233.38 S. Tokito, T. Iijima, Y. Suzuri, H. Kita, T. Tsuzuki and F. Sato,

Appl. Phys. Lett., 2003, 83, 569.39 J. Kido, N. Ide, Y.-J. Li, Y. Agata and H. Shimizu, IQEC/CLEO-

PR, 2005, CWN1–2.40 H. Sasabe, E. Gonmori, T. Chiba, Y.-J. Li, D. Tanaka, S.-J. Su,

T. Takeda, Y.-J. Pu, K. Nakayama and J. Kido, Chem. Mater.,2008, 20, 5951.

41 H. Sasabe, N. Toyota, H. Nakanishi, T. Ishizaka, Y. J. Pu andJ. Kido, Adv. Mater., 2012, 24, 3212–3217.

42 H. Sasabe, Y. Seino, M. Kimura and J. Kido, Chem. Mater.,2012, 24, 1404–1406.

43 D. Kim, L. Zhu and J.-L. Bredas, Chem. Mater., 2012, 24,2604–2610.

44 C. Weichsel, L. Burtone, S. Reineke, S. Hintschich,M. Gather, K. Leo and B. Lussem, Phys. Rev. B: Condens.Matter Mater. Phys., 2012, 86.

45 M. H. Tsai, H. W. Lin, H. C. Su, T. H. Ke, C. c. Wu, F. C. Fang,Y. L. Liao, K. T. Wong and C. I. Wu, Adv. Mater., 2006, 18,1216–1220.

46 T. Tsutsui, MRS Bull., 1997, 39.47 N. C. Giebink and S. R. Forrest, Phys. Rev. B: Condens. Matter

Mater. Phys., 2008, 77, 235215.48 Y. You and S. Y. Park, Dalton Trans. 2009, 1267.49 P. T. Chou and Y. Chi, Chem.–Eur. J., 2007, 13, 380–

395.50 H. Yersin, A. F. Rausch, R. Czerwieniec, T. Hoeck and

T. Fischer, Coord. Chem. Rev., 2011, 255, 2622–2652.




Dow

nloa

ded

by C

ape

Bre

ton

Uni

vers

ity o

n 06

Mar

ch 2

013

Publ

ishe

d on

13

Dec

embe

r 20

12 o

n ht

tp://

pubs

.rsc

.org

| do

i:10.

1039

/C2T

C00

584K

View Article Online

51 G. Schwartz, S. Reineke, T. C. Rosenow, K. Walzer andK. Leo, Adv. Funct. Mater., 2009, 19, 1319–1333.

52 Y. Sun, N. C. Giebink, H. Kanno, B. Ma, M. E. Thompson andS. R. Forrest, Nature, 2006, 440, 908.

53 M. E. Kondakova, J. C. Deaton, T. D. Pawlik, D. J. Giesen,D. Y. Kondakov, R. H. Young, T. L. Royster, D. L. Comfortand J. D. Shore, J. Appl. Phys., 2010, 107, 014515.

54 J. Ye, C. J. Zheng, X. M. Ou, X. H. Zhang, M. K. Fung andC. S. Lee, Adv Mater., 2012, 24, 3410–3414.

55 B. W. D'Andrade, J. Esler, C. Lin, V. Adamovich, S. Xia,M. S. Weaver, R. Kwong and J. J. Brown, Proc. SPIE, 2008,7051, 70510Q.

56 S. Reineke, F. Lindner, G. Schwartz, N. Seidler, K. Walzer,B. Lussem and K. Leo, Nature, 2009, 459, 234.

57 S. J. Su, E. Gonmori, H. Sasabe and J. Kido, Adv. Mater., 2008,20, 4189.

58 H. Sasabe, J. Takamatsu, T. Motoyama, S. Watanabe,G. Wagenblast, N. Langer, O. Molt, E. Fuchs, C. Lennartzand J. Kido, Adv. Mater., 2010, 22, 5003.

59 L. Xiao, Z. Chen, B. Qu, J. Luo, S. Kong, Q. Gong and J. Kido,Adv Mater., 2011, 23, 926.

60 H. Sasabe and J. Kido, Chem. Mater., 2011, 23, 621–630.61 K. S. Yook and J. Y. Lee, Adv Mater., 2012, 24, 3169–3190.62 D. Yokoyama, J. Mater. Chem., 2011, 21, 19187.63 G. R. Desiraju and T. Steiner, The Weak Hydrogen Bond–in

Structural Chemistry and Biology, IUCr, Oxford universitypress, 1999.

64 E. G. Hohenstein and C. D. Sherrill, J. Phys. Chem. A, 2009,113, 878–886.

65 M. Ichikawa, T. Kawaguchi, K. Kobayashi, T. Miki,K. Furukawa, T. Koyama and Y. Taniguchi, J. Mater. Chem.,2006, 16, 221.

66 D. Yokoyama, A. Sakaguchi, M. Suzuki and C. Adachi, Appl.Phys. Lett., 2009, 95, 243303.


67 H. Sasabe, D. Tanaka, D. Yokoyama, T. Chiba, Y.-J. Pu,K. Nakayama, M. Yokoyama and J. Kido, Adv. Funct. Mater.,2011, 21, 336–342.

68 (a) H. Bassler, Phys. Status Solidi B, 1993, 175, 15; (b) OrganicPhotoreceptors for Imaging Systems, ed. P. M. Borsenbergerand D. S. Weiss, Marcel Dekker Inc., New York, 1993.

69 D. Yokoyama, H. Sasabe, H. Furukawa, C. Adachi andJ. Kido, Adv. Funct. Mater., 2011, 21, 1375.

70 J. Kido, T. Matsumoto, T. Nakada, J. Endo, K. Mori,N. Kawamura and A. Yokoi, SID 03 Digest, 2003, 964.

71 J.-X. Tang, M.-K. Fung, C.-S. Lee and S.-T. Lee, J. Mater.Chem., 2010, 20, 2539.

72 L. S. Liao, W. K. Slusarek, T. K. Hatwar, M. L. Ricks andD. L. Comfort, Adv. Mater., 2008, 20, 324.

73 T. Chiba, Y.-J. Pu, R. Miyazaki, K. Nakayama, H. Sasabe andJ. Kido, Org. Electron., 2011, 12, 710.

74 C. E. Small, S.-W. Tsang, J. Kido, S. K. So and F. So, Adv.Funct. Mater., 2012, 22, 3261.

75 S. Lee, J.-H. Lee, J.-H. Lee and J. J. Kim, Adv. Funct. Mater.,2012, 22, 855.

76 H. Sasabe, K. Minamoto, Y.-J. Pu, M. Hirasawa and J. Kido,Org. Electron., 2012, 13, 2615–2619.

77 Y. Ohno, Proc. SPIE, 2004, 5530, 88.78 D. Kim, V. Coropceanu and J. L. Bredas, J. Am. Chem. Soc.,

2011, 133, 17895.79 (a) Z. B. Henson, K. Mullen and G. C. Bazan, Nat. Chem.,

2012, 4, 699–704; (b) L. Bartels, Nat. Chem., 2010, 2, 87–95.80 M. Flammich, J. Frischeisen, D. S. Setz, D. Michaelis,

B. C. Krummacher, T. D. Schmidt, W. Brutting andN. Danz, Org. Electron., 2011, 12, 1663.

81 (a) A. Endo, M. Ogasawara, A. Takahashi, D. Yokoyama,Y. Kato and C. Adachi, Adv. Mater., 2009, 21, 4802; (b)Q. Zhang, J. Li, K. Shizu, S. Huang, S. Hirata, H. Miyazakiand C. Adachi, J. Am. Chem. Soc., 2012, 134, 14706.

J. Mater. Chem. C, 2013, 1, 1699–1707 | 1707


development of high performance oleds for general lighting

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