Efficient simple structure red phosphorescent organic light emitting devices withnarrow band-gap fluorescent hostTae Jin Park, Woo Sik Jeon, Jung Joo Park, Sun Young Kim, Yong Kyun Lee, Jin Jang, Jang Hyuk Kwon, andRamchandra Pode Citation: Applied Physics Letters 92, 113308 (2008); doi: 10.1063/1.2896641 View online: http://dx.doi.org/10.1063/1.2896641 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/92/11?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Improving efficiency roll-off in organic light emitting devices with a fluorescence-interlayer-phosphorescenceemission architecture Appl. Phys. Lett. 95, 133304 (2009); 10.1063/1.3241079 High-efficiency, low-voltage phosphorescent organic light-emitting diode devices with mixed host J. Appl. Phys. 104, 094501 (2008); 10.1063/1.3000046 A highly efficient wide-band-gap host material for blue electrophosphorescent light-emitting devices Appl. Phys. Lett. 91, 233501 (2007); 10.1063/1.2821116 Highly efficient and stable red phosphorescent organic light-emitting device using bis[2-(2-benzothiazoyl)phenolato]zinc(II) as host material Appl. Phys. Lett. 90, 123509 (2007); 10.1063/1.2643908 Efficient organic light-emitting devices using an iridium complex as a phosphorescent host and a platinumcomplex as a red phosphorescent guest Appl. Phys. Lett. 88, 243511 (2006); 10.1063/1.2213017

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Efficient simple structure red phosphorescent organic light emittingdevices with narrow band-gap fluorescent host

Tae Jin Park,1 Woo Sik Jeon,1 Jung Joo Park,1 Sun Young Kim,1 Yong Kyun Lee,1

Jin Jang,1 Jang Hyuk Kwon,1,a� and Ramchandra Pode2

1Advanced Display Research Center and Department of Information Display, Kyung Hee University,Dongdaemoon-gu, Seoul 130-701, Republic of Korea2Department of Physics, Kyung Hee University, Dongdaemoon-gu, Seoul 130-701, Republic of Korea

�Received 27 December 2007; accepted 11 February 2008; published online 20 March 2008�

Using a narrow band-gap fluorescent host material, bis�10-hydroxybenzo�h� quinolinato�berylliumcomplex, efficient red phosphorescent organic light emitting diodes �PHOLEDs� comprising twolayers have been reported. Significantly low driving voltage of 4.5 V to reach a luminance of1000 cd /m2 is reported in this simple structure PHOLEDs. The current and power efficiencies ofbilayered red PHOLED are 9.66 cd /A and 6.90 lm /W, respectively, promising for low powerdisplay and lighting applications. © 2008 American Institute of Physics. �DOI: 10.1063/1.2896641�

Organic light-emitting diodes �OLEDs� have beenwidely recognized as the technology for presently flat paneldisplay products and potential future use in the lighting in-dustry. Performance and efficiencies of these devices havebeen improved in recent days, particularly in p-i-n typeOLEDs.1 Good charge balance in emitting layers and lowbarrier to charge carriers injection in p-i-n devices demon-strate a low operating voltage and high efficiency. Besides,an upper limit on the external quantum efficiency of 5% influorescent small molecule organic devices has been over-come in phosphorescent OLEDs �PHOLEDs� by harvestingthe singlet and triplet excitons to emission of photons.2,3

Usually, multilayer structures comprising electron and holetransport and blocking layers are desired to fabricatehighly efficient PHOLEDs. However, the turn-on voltageof PHOLEDs is relatively high about 1–2 V comparedto that of fluorescent OLEDs as the device designed hasmultilayer structures for good charge balance and to confineexcitons within an emitting material layer �EML�,4,5 limitingtheir use in display industries. Recently, nearly 100% internalquantum efficiency in devices consisting of host andphosphorescent dopant is demonstrated.6–9 In order tohave highly efficient PHOLEDs, the triplet energy of the hosthigher than that of the phosphorescent guest is desirableto facilitate the exothermic energy transfer from thehost to the guest and to prohibit reverse energy transferfrom the guest back to the host, thereby, effectively confiningtriplet excitons on guest molecules. However, the hostmaterial with a wide band gap often causes an increasein driving voltages of the PHOLEDs. Therefore, the selectionof suitable host materials for PHOLEDs is very imperative toachieve the high efficiency. 4 ,4�-N ,N�-dicarbazolebi-phenyl �CBP� is the most widely used host material inred and green emitting PHOLEDs.10,11 Other host mate-rials such as 4 ,4� ,4�-tris�N-carbazolyl�-triphenylamine�TCTA�, 3-phenyl-4-�1�-naphthyl�-5-phenyl-1,2,4-triazole�TAZ�, 1,3,5-tris�N-phenylbenzimidizol-2yl�benzene �TPBI�,and aluminum�III�bis�2-methyl-8-quinolinato�-4-phenylphe-nolate �Balq� for PHOLEDs are also commercially availableand used as a matter of convenience for many guest-host applications.12–14 Highest occupied molecular orbital

�HOMO�, Lowest unoccupied molecular orbital �LUMO�,and triplet emission energy of these host materials are listedin Table I. The HOMO and LUMO energy levels of bis�10-hydroxybenzo�h�quinolinato�beryllium complex �Bebq2� arereported at 5.5 and 2.8 eV, respectively.15 High luminance inOLEDs with Bebq2 as an emitter was reported by Hamada etal.16 Since Bebq2 and beryllium complexes have very goodelectron transporting characteristics with high electron mo-bility of �10−4 cm2 /V s �Refs. 17–19� and narrow band gap,we believe that Bebq2 can make a suitable candidate for thehost of red light-emitting PHOLEDs.

In this paper, we report a narrow band-gap electrontransporting host material, Bebq2, for red light-emittingPHOLEDs. �Note: Bebq2 host material is highly toxic andhazardous in nature. Take utmost care while workingwith Bebq2.� The triplet energy of Bebq2 host was estimatedusing density functional theory �DFT�. Simple bilayeredPHOLEDs, tris�1-phenylisoquinoline�iridium �Ir�piq�3�doped in Bebq2 host, were fabricated and studied.

Beryllium compound has been reported to have a strongfluorescence characteristic. To estimate the triplet state en-ergy, the phosphorescent spectrum of Bebq2 was investigatedat low temperature. However, no phosphorescent peak inBebq2 complex is observed at 77 K. It only exhibits a strongfluorescence emission at 466 nm. Therefore, the triplet en-ergy state, estimated by molecular modeling and DFT usingDMOL3 program �version 4.2�,20–23 was found to be about3.0 eV. Usually, in phosphorescent host materials, singletand triplet exchange energy value is about 0.5 eV. However,Bebq2 host shows a very small exchange energy value of

a�Electronic mail: jhkwon@khu.ac.kr. FAX: �82-2-961-9154.

TABLE I. HOMO, LUMO, and energy levels of some fluorescent hostmaterials for PHOLEDs.

CompoundsHOMO

�eV�LUMO

�eV�

Reportedtriplet energy

�eV�

Calculatedtriplet energy

�eV�

CBP 5.8 2.5 2.6 2.8TCTA 5.9 2.7 2.8 2.7TPBI 6.3 2.8 ¯ 2.8BAlq 5.9 3.0 2.2 2.6TAZ 6.6 2.6 ¯ 3.3

Bebq2 5.5 2.8 ¯ 2.5

APPLIED PHYSICS LETTERS 92, 113308 �2008�

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0.2 eV and is a signature of strong electron-electron correla-tion. Triplet phosphorescent dopants such as Ir�piq�3 andbis�2-phenylquinoline��acetylacetonate�iridium, used in redlight-emitting PHOLEDs, have triplet energy states �actualLUMO level� at 2.8 and 3.1 eV, respectively:10,24 This tripletenergy of dopant is very close to the triplet energy of theBebq2 host material, thus, facilitating the electron movementin emitting layer.

Furthermore, corroboration of triplet energy state ofBebq2 host and possible energy transfer from the host to thedopant were confirmed by fluorescent and phosphorescentquenching experiments using iridium dopants and Bebq2host in tetrahydrofuran solution. The Bebq2 fluorescencepeak is efficiently quenched by Ir�piq�3 dopant, transferringall its singlet energy directly to the dopant triplet state. As aconsequence, we conclude that the triplet energy level ofBebq2 is higher than that of the Ir�piq�3 dopant and exchangeenergy of host material between singlet and triplet must bevery small. However, the reported triplet energy value ofIr�piq�3 in Ref. 10 is 2.8 eV, which is lower than that of theBebq2 host �3.0 eV�. So, the LUMO energy level �i.e., tripletstate� of Ir�piq�3 dopant was confirmed by the optical band-

gap and cyclic voltammetry measurements and was found tobe 3.1 eV. Both host and dopant molecules seem to havealmost same value of triplet energy.

Figure 1 shows the structures of three red PHOLED de-vices fabricated for the present study. Devices A and Bhave a conventional multilayer structure containing holeand electron transport and injection, and hole blockinglayers with CBP and Bebq2 host materials, respectively. De-vice A with a CBP host material is used as a control device,while the fabricated device C with Bebq2 host has asimple bilayered structure. Red phosphorescent OLEDswere fabricated as follows: devices A and B, indium tinoxide N ,N�-bis�naphthalen-1-yl�-N ,N�-bis�phenyl�ben-zidine ��-NPB� �40 nm�/host:dopant �10 wt% ,30 nm� /Balq�5 nm� / tris-�8-hydroxyquinoline�aluminum �Alq3� �20 nm�/LiF �0.5 nm� /Al �100 nm� and device C, ITO /�-NPB�40 nm�/host:dopant �10 wt% ,50 nm� /LiF �0.5 nm� /Al�100 nm�.

Figure 2 shows the I-V-L characteristics of fabricatedred phosphorescent devices. At a given constant voltage of5 V, current density values of 0.82, 2.83, and 18.99 mA /cm2

in the fabricated devices A, B, and C are noticed, as dis-played in Fig. 2, respectively. The driving voltage for thedevice A to reach 1000 cd /m2 is 8.8 V, 6.8 V for device B,and 4.5 V for device C. A low turn-on voltage of 4.5 V indevice C with a simple bilayered structure compared to con-trol device A with CBP host �8.8 V� is observed. The resis-tance to current conduction in bilayered device C is signifi-cantly reduced. As the HOMO energy of Bebq2 host is at5.5 eV, holes injected from the hole transport layer �HTL�trap directly at the HOMO level �5.1 eV� of dopant. Barrierto hole injection in device C is almost negligible. Also, elec-trons injected from the cathode move freely in the emittinglayer as the LUMO �triplet� of dopant and triplet of host areat the same energy and finally captured at the trapped holesites, giving rise to phosphorescent emission. Multilayerstructures, as displayed in devices A and B introduce heter-obarriers to electron and hole injection into emitting layers,thus, enhancing the turn-on voltages, although some reduc-

FIG. 1. Structures of fabricated three PHOLEDs: device A, CBP:Ir�piq�3;device B, Bebq2: Ir�piq�3; device C, Bebq2: Ir�piq�3 without HBL and ETL.

FIG. 2. I-V-L characteristics of fabri-cated three PHOLEDs: �a� current-voltage, �b� luminance-voltage, �c�current efficiency–luminance, and �d�power efficiency–luminance character-istics. Inset �a� shows the energy dia-gram of red organic bilayeredPHOLEDs.

113308-2 Park et al. Appl. Phys. Lett. 92, 113308 �2008�

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tion of driving voltage in device B due to narrow band-gapBebq2 host materials is noticed. Moreover, in CBP basedPHOLEDs, severe charge trapping at NPB interface has beenreported by several researchers.10,11 The inset of Fig. 2�a�shows the energy band diagram of device C. The current andpower efficiency characteristics of fabricated devices areshown in Figs. 2�c� and 2�d�. At a given constant luminanceof 1000 cd /m2, the current and power efficiencies are9.66 cd /A and 6.90 lm /W for device C, 8.67 cd /A and4.00 lm /W for device B, and 5.05 cd /A and 1.80 lm /W fordevice A, respectively. These values of device C are im-proved by a factor of 1.9 and 3.8 times compared with thoseof device A, respectively. In device reliability tests, devicesA and C show very different behaviors. Device A showsabout 120 h lifetime at 1000 nit, while lifetime of150–160 h is noticed in Bebq2 device. Relatively small ini-tial decrease of brightness value and gradual decay curve areobserved in device C, which indicate that Bebq2 device reli-ability is relatively very good. However, material stability ofBebq2 seems not to be good. Figure 3 shows the electrolu-minescence spectra at a brightness of 1000 cd /m2 of differ-ent fabricated phosphorescent red light-emitting devices.Clean red light at 632 nm observed in device C indicates thecomplete energy transfer from a narrow band-gap Bebq2 hostmaterial to Ir�piq�3 dopant. The CIE coordinate of three reddevices show the same coordinate as CIE �0.67,033�.

Anyway, interesting and intriguing results on the perfor-mance of bilayered device C have been obtained. TheLUMO level of Bebq2 material �2.8 eV�, very close toLUMO values of Balq, Alq3, and LiF cathode, offers almostno barrier to electron injection between the emitting layerand LiF cathode. Furthermore, excellent electron transport-ing property of Bebq2 material favors the mobility of elec-trons which provides a good charge balance in the emittinglayer. HOMO levels of Bebq2 host and NPB HTL in thefabricated device C are very close, while LUMO energy lev-els of host and dopant are almost the same. Therefore, theemission process in PHOLED device C via electron trappingat LUMO and hole trapping at HOMO seems to be mini-mized, giving low driving voltage value. In device C, theemission of red light may be originated from the direct elec-tron capturing from the host and recombining at holestrapped at the HOMO of the dopant in the emitting layer.Indeed, the hole trapping in bilayered device C is not a seri-

ous issue. To investigate this, three PHOLEDs with emittingzones at X=0, 100, and 200 nm from the HTL/EML inter-face were fabricated and studied, as displayed in the inset ofFig. 3. Results reveal excellent emission of red light in alldevices, except some contribution to the emission from theBebq2 host material in devices with X=100 and 200 nm.These results demonstrate that the emission zone in simplebilayered PHOLEDs is very broad and hole trapping is notso severe.

In summary, we report a narrow band-gap host material,Bebq2, for red PHOLEDs with a very small exchange energyvalue of 0.2 eV between singlet and triplet states. It showsalmost no barrier to injection of charge carriers and chargetrapping issue in PHOLEDs is minimized. High current andpower efficiency values of 9.66 cd /A and 6.90 lm /W in bi-layered simple structure PHOLEDs are obtained, respec-tively. The operating voltage of bilayered PHOLEDs at aluminance of 1000 cd /m2 was 4.5 V. In conclusion, a simplebilayered device structure with the narrow band-gap hostcould be a promising way to achieve efficient, economical,and ease in manufacturing process, important for display andlighting production.

This work has been supported by KESRI�R-2007-2-050�which is funded by Ministry of Commerce, Industry, andEnergy �MOCIE�.

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FIG. 3. Normalized electroluminescent spectra of devices A, B, and C at theluminance of 1000 cd /m2. Inset of depicts the recombination zone positionfrom the HTL/EML interface in device C.

113308-3 Park et al. Appl. Phys. Lett. 92, 113308 �2008�

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