Current Applied Physics 10 (2010) 1108–1111

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Current Applied Physics

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Low voltage red phosphorescent organic light-emitting deviceswith triphenylphosphine oxide and 4,40-bis(2,20-diphenylvinyl)-1,10-biphenylelectron transport layers

Tae-Yong Kim, Dae-Gyu Moon *

Department of Materials Engineering, Soonchunhyang University, 646, Eupnae-ri, Shinchang-myeon, Asan-si, Chungcheongnam-do 336-745, Republic of Korea

a r t i c l e i n f o a b s t r a c t

Article history:Received 16 March 2009Received in revised form 24 June 2009Accepted 21 January 2010Available online 29 January 2010

Keywords:Organic light-emitting devicePhosphorescenceDriving-voltageElectron transport layer

1567-1739/$ - see front matter � 2010 Elsevier B.V. Adoi:10.1016/j.cap.2010.01.009

* Corresponding author. Tel.: +82 41 530 1312; faxE-mail address: dgmoon@sch.ac.kr (D.-G. Moon).

We have developed red phosphorescent organic light-emitting devices operating at low voltages by usingtriphenylphosphine oxide (Ph3PO) and 4,40-bis(2,20-diphenylvinyl)-1,10-biphenyl (DPVBi) electron trans-port layers. 4,40-bis(N-carbazolyl)-1,10-biphenyl (CBP) and tris-(1-phenylisoquinolinolato-C2,N) irid-ium(III) [Ir(piq)3] were used as host and guest materials, respectively. Small voltage drops across theelectron transport layers and direct injection of holes from 4,40 ,400-tris[N-(2-naphthyl)-N-phenyl-amino]-triphenylamine (2-TNATA) hole transport layer into the Ir(piq)3 guests are responsible for thehigh current density at low voltage, resulting in a high luminance of 1000 cd/m2 at low voltages of2.8–3.0 V in devices with a structure of ITO/2-TNATA/CBP:Ir(piq)3/DPVBi/Ph3PO/LiF/Al.

� 2010 Elsevier B.V. All rights reserved.

1. Introduction

Phosphorescent organic light-emitting devices (PHOLEDs) havebeen interested in display and lighting applications because theycan provide a high internal quantum efficiency by harvesting bothelectro-generated singlet and triplet excitons [1,2]. Although thegreen-emitting organic electrophosphorescent devices with nearly100% internal quantum efficiency have been demonstrated [3],high driving voltage is still one of drawbacks in PHOLED applica-tions. The PHOLEDs typically have multilayer structures composedof large band gap materials, thereby limiting the injection of carri-ers at organic/organic and organic/electrode interfaces. In addition,the transport of injected carriers is limited by the low mobility ofcarrier transport layer. Furthermore, the electron mobility of elec-tron transport layer is generally much lower than the hole mobilityof hole transport one. Kido et al. [4] proposed the electrical dopingof low work function metals such as Li and Sr into the electrontransport layer. These dopants create free electrons in the electrontransport layer, resulting in low injection barrier and high electronconductivity. Pfeiffer et al. [5] demonstrated the low voltage greenPHOLEDs using a pin structure [6] where both electron and holetransport layers are doped with organic or inorganic dopants. Onthe other hands, Park et al. [7] reported the red PHOLEDs with anoperating voltage of 4.5 V at 1000 cd/m2, without using an electri-cally doped transport layer. They developed simple bilayer struc-

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ture consisting of hole transporting N,N0-bis(naphthalene-l-yl)-N,N0-bis(phenyl) benzidine and electron transporting bis(10-hydroxybenzo[h]quinolinato) beryllium layers.

Recently, organic phosphorous compounds have been proposedas electron transport and host layers for low voltage PHOLEDs[8,9]. 4,40-bis(diphenylphosphine oxide) biphenyl [8] and 2,8-bis(diphenylphosphoryl) dibenzothiophene [9] have been used aselectron transporting materials for blue-emitting PHOLEDs. In thispaper, we report the low voltage red PHOLEDs with double elec-tron transport layer structure consisting of triphenylphosphineoxide (Ph3PO) and 4,40-bis(2,20-diphenylvinyl)-1,10-biphenyl(DPVBi) layers. The Ph3PO, which has been used to form complexeswith lanthanide metal ions for using an emitter in OLEDs [10], hasa high electron transporting ability [11]. The DPVBi has beenwidely used as a charge transporting host material in fluorescentOLEDs [12]. In our devices, we used the DPVBi layer as an electrontransporting material. By using the Ph3PO and DPVBi electrontransport layers, we demonstrate the red PHOLEDs with operatingvoltages of 2.8–3.0 V at a luminance of 1000 cd/m2.

2. Experimental

Red PHOLEDs were fabricated on indium tin oxide (ITO) coatedglass substrates. The sheet resistance of ITO film was about 10 X/sq. After defining the ITO anode patterns using standard photoli-thography process, the substrates were cleaned with isopropylalcohol and deionized water followed by exposing to oxygen plas-ma. All organic and metal layers were deposited by using a thermal

Fig. 1. Schematic diagram of the red PHOLED structure.

T.-Y. Kim, D.-G. Moon / Current Applied Physics 10 (2010) 1108–1111 1109

evaporation method in a base pressure of about 1 � 10�6 Torr. A15 nm thick 4,40,400-tris[N-(2-naphthyl)-N-phenyl-amino]-triphen-ylamine (2-TNATA) layer was deposited on the patterned ITO sub-strate, and then 4,40-bis(N-carbazolyl)-1,10-biphenyl (CBP) layerwas co-deposited with a red phosphorescent dopant, tris-(1-phen-ylisoquinolinolato-C2,N) iridium(III) [Ir(piq)3], whose concentra-tion was varied from 2 to 20 wt.%. After then, 30–60 nm thickDPVBi layers were deposited, followed by the deposition of a60 nm thick Ph3PO layer. After depositing organic layers, a0.5 nm thick LiF and a 100 nm thick Al layers were sequentiallyevaporated through a shadow mask. Fig. 1 shows a schematic dia-gram of the completed device structure. All the completed deviceswere encapsulated without exposing to air in a nitrogen atmo-sphere glove box. Current density–voltage–luminance (J–V–L)characteristics of the devices were measured using computer con-trolled Keithley 2400 source-measure units and a luminance meter(Minolta LS 100). Electroluminescence (EL) spectra and color coor-dinates were measured with a spectroradiometer (Minolta CS1000).

Fig. 2. Current density (solid)–voltage–luminance (open) curves of the red PHOL-EDs with different thicknesses of DPVBi layers. Device structure: ITO/2-TNATA(15 nm)/CBP:Ir(piq)3 (10 nm, 6 wt.%)/DPVBi (x nm)/Ph3PO (60 nm)/LiF (0.5 nm)/Al,where x = 30, 40, and 60.

Fig. 3. Current density (solid)–voltage–luminance (open) curves of the red PHOL-EDs with different concentrations of Ir(piq)3. Device structure: ITO/2-TNATA(15 nm)/CBP:Ir(piq)3 (10 nm, x wt.%)/DPVBi (30 nm)/Ph3PO (60 nm)/LiF (0.5 nm)/Al, where x = 2, 6, 10, and 20.

3. Result and discussion

Fig. 2 shows the J–V–L curves of the red-emitting phosphores-cent devices. The device structure is ITO/2-TNATA (15 nm)/CBP:Ir(-piq)3 (10 nm, 6 wt.%)/DPVBi (30–60 nm)/Ph3PO (60 nm)/LiF/Al. Thedevices operate at significantly low voltages. For example, theturn-on voltage for a luminance of 1 cd/m2 is 2.2 V in the devicewith a 30 nm thick DPVBi layer. The luminance in this device in-creases very rapidly with increasing the voltage, being a high lumi-nance of 1000 cd/m2 at a low voltage of 3.0 V. The driving voltagesof our devices are lower than those of the reported red phosphores-cent devices [7,13]. For example, Tsuzuki and Tokito [13] demon-strated a red device with an operating voltage of 5 V at aluminance of 1000 cd/m2, using a bis(2-phenylpyridinato-N,C20)iridium(acetylacetonate) as a narrow band gap host for thered phosphorescent Ir(piq)3 guest. The driving voltage is also lowerthan that of the pin structured red PHOLED (�3.5 V at 1000 cd/m2)reported by Reineke et al. [14]. The low driving voltages in our de-vices are clearly attributed to the high current densities as shownin Fig. 2. For example, a voltage of 3.2 V is required to achieve acurrent density of 20 mA/cm2 in the device with a 30 nm thickDPVBi layer. On the other hand, a 60 nm thick DPVBi device needsabout 0.2 V higher voltage (3.4 V) for the same current density.Thus, the results indicate that the voltage drop across the DPVBilayer is small. It also suggests that the DPVBi has a good capability

of transporting electrons. In addition, as described in a previous re-port [11], the required voltages for the same current density werealmost independent of Ph3PO thickness (2.9–3.1 V for 20 mA/cm2)in the devices with a structure of ITO/2-TNATA/DPVBi/Ph3PO/LiF/Al. The electrons can be easily transported thorough the Ph3POlayer and can be easily injected from Ph3PO layer into the DPVBilayer. Therefore, most of applied voltage is dropped across the 2-TNATA and doped CBP layers rather than the DPVBi and Ph3PO lay-ers. Since the thicknesses of 2-TNATA and Ir(piq)3 doped CBP layersare 15 and 10 nm, respectively, high electric fields are built in thesethin layers, resulting in high current density at low applied voltage.

Fig. 3 shows the J–V–L curves of the red PHOLEDs doped withvarious concentrations of Ir(piq)3. Although the Ir(piq)3 dopedCBP layer is only 10 nm, the current density at the same voltageis higher at higher concentration of Ir(piq)3. For example, the cur-rent density at 4 V increases from 76 to 135 mA/cm2 when the Ir(-piq)3 concentration increases from 2 to 20 wt.%, so that the devicedoped with 20 wt.% Ir(piq)3 exhibits a significantly low drivingvoltage. The 20 wt.% Ir(piq)3 doped device shows the operatingvoltages of 2.4 and 2.8 V at 100 and 1000 cd/m2, respectively. Thisdependency of the Ir(piq)3 concentration on the current densityindicates the direct injection of charge carriers into the guest mol-ecules from adjacent carrier transport layers. The highest occupiedmolecular orbital (HOMO) levels of 2-TNATA and Ir(piq)3 are 5.1

Fig. 4. EL spectra of the red PHOLEDs doped with Ir(piq)3; (a) 2 wt.% Ir(piq)3, (b)6 wt.% Ir(piq)3.

Fig. 5. Current efficiency (solid) and power efficiency (open) curves as a function ofluminance for the red PHOLEDs with different concentrations of Ir(piq)3.

1110 T.-Y. Kim, D.-G. Moon / Current Applied Physics 10 (2010) 1108–1111

and 5.0 eV, respectively [15,16]. HOMO level of CBP is known to be6.0–6.3 eV [17,18]. Therefore, the direct injection of holes from 2-TNATA into the Ir(piq)3 guest molecules is more energeticallyfavorable than the injection of holes from 2-TNATA into the CBPhost. On the other hand, electrons can be easily injected from theDPVBi into the CBP or Ir(piq)3 because the LUMO level of DPVBi(2.8 eV) is shallower than those of CBP (2.9–3.1 eV) and Ir(piq)3

(3.0 eV) [12,16–18].Fig. 4a shows the EL spectra for the device with 2 wt.% Ir(piq)3.

The EL spectrum at 3 V shows a strong emission peak at a wave-length of 622 nm, which is attributed to the phosphorescencefrom Ir(piq)3. At 5 V, another small peak at 457 nm is observedin the EL spectrum. This peak may not be due to the emissionfrom CBP host because it has a fluorescence peak at 390–400 nm [19]. Since the DPVBi has a fluorescence peak at 450–460 nm [20], the small peak at 457 nm results from the DPVBilayer. It means that some of holes pass through the doped CBPlayer, recombining with electrons in the DPVBi layer. Due to thissmall blue peak, the Commission Internationale de l’Eclairage(CIE) coordinates change from (0.67, 0.33) at 3 V to (0.62, 0.32)at 5 V. However, as shown in Fig. 4b, as the Ir(piq)3 concentrationincreases to 6 wt.%, the blue peak at 457 nm becomes negligiblysmall. In addition, the device with 6 wt.% Ir(piq)3 shows a phos-phorescence peak at 625 nm, exhibiting a slight red shift com-pared to the device with 2 wt.% Ir(piq)3. This red shiftcontributes to the purity of red color. In the device with 6 wt.%Ir(piq)3, the CIE coordinates are (0.68, 0.32) and (0.67, 0.32) at3 and 5 V, respectively.

Fig. 5 shows the current efficiency and power efficiency curvesas a function of luminance for the devices. The current efficiencyincreases with luminance, reaches a maximum, and then decreasesfast with increasing the luminance. The initial rise in current effi-ciency may be ascribed to the increase in the probability of radia-

tive triplet exciton formation at the Ir(piq)3 guest sites. The fastdrop of current efficiency above 200–600 cd/m2 results from trip-let–triplet annihilation as reported by many authors [21]. It shouldbe noted that the maximum current efficiency depends on the Ir(-piq)3 concentration. The device with 6 wt.% Ir(piq)3 shows a highercurrent efficiency than the other devices. It may be due to the com-promise between the direct recombination probability and concen-tration quenching. As the holes are directly injected from 2-TNATAinto the Ir(piq)3 guest molecules in our devices, the light is gener-ated by the direct recombination of holes and electrons on the Ir(-piq)3 molecules so that the direct recombination probabilityincreases with increasing the Ir(piq)3 concentration. However, athigh Ir(piq)3 concentration, the excitons can be easily quenchedby the self-quenching interactions between Ir(piq)3 molecules[22]. The 6 wt.% Ir(piq)3 device shows a maximum current effi-ciency of 7.3 cd/A and a power efficiency of 8.7 lm/W at a lumi-nance of 250 cd/m2. While the current efficiency is similar withthe reported values of 4–8 cd/A [23–26], the power efficiency ismuch better in our device, because the operating voltage is aslow as 2.6 V at 250 cd/m2 in this 6 wt.% Ir(piq)3 device (see Fig. 3).

4. Conclusions

We demonstrated low voltage red PHOLEDs using DPVBi andPh3PO electron transport layers. The devices exhibit a high lumi-nance of 1000 cd/m2 at low voltage of 2.8–3.0 V by the small volt-age drops across these electron transport layers and directinjection of holes into the red phosphorescent Ir(piq)3 guest mole-cules. The low driving voltage leads to a high power efficiency of8.7 lm/W at 250 cd/m2 in the red PHOLEDs doped with 6 wt.%Ir(piq)3.

Acknowledgements

This work was supported by Ministry of Education, ScienceTechnology (MEST)/Korea Institute for Advancement of Technol-ogy (KIAT) through the Human Resource Training Project for Regio-nal Innovation and by the IT R&D program of the Ministry ofKnowledge Economy (MKE)/Korea Evaluation Institute of Technol-ogy (KEIT) (Development of Fundamental Technologies for FlexibleCombined-Function Organic Electronic Device).

References

[1] M.A. Baldo, D.F. O’Brien, Y. You, A. Shoustikov, S. Sibley, M.E. Thompson, S.R.Forrest, Nature (London) 395 (1998) 151.

T.-Y. Kim, D.-G. Moon / Current Applied Physics 10 (2010) 1108–1111 1111

[2] V. Cleave, G. Yahioglu, P.L. Barny, R.H. Friend, S.R. Tessler, Adv. Mater. 11(1999) 285.

[3] C. Adachi, M.A. Baldo, M.E. Thompson, S.R. Forrest, J. Appl. Phys. 90 (2001)5048.

[4] J. Kido, T. Matsumoto, Appl. Phys. Lett. 73 (1998) 2866.[5] M. Pfeiffer, S.R. Forrest, K. Leo, M.E. Thompson, Adv. Mater. 14 (2002) 1633.[6] J. Huang, M. Pfeiffer, A. Werner, J. Blochwitz, K. Leo, S. Liu, Appl. Phys. Lett. 80

(2002) 139.[7] T.J. Park, W.S. Jeon, J.J. Park, S.Y. Kim, Y.K. Lee, J. Jang, J.H. Kwon, R. Pode, Appl.

Phys. Lett. 92 (2008) 113308.[8] P.E. Burrows, A.B. Padmaperuma, L.S. Sapochak, P. Djurovich, M.E. Thompson,

Appl. Phys. Lett. 88 (2006) 183503.[9] X. Cai, A.B. Padmaperuma, L.S. Sapochak, P.A. Vecchi, P.E. Burrows, Appl. Phys.

Lett. 92 (2008) 083308.[10] S. Capecchi, O. Renault, D.G. Moon, M. Halim, M. Etchells, P.J. Dobson, O.V.

Salata, V. Christou, Adv. Mater. 12 (2000) 1591.[11] M.Y. Ha, D.G. Moon, Appl. Phys. Lett. 93 (2008) 043306.[12] C. Hosokawa, H. Higashi, H. Nakamura, T. Kusumoto, Appl. Phys. Lett. 67

(1995) 3853.

[13] T. Tsuzuki, S. Tokito, Adv. Mater. 19 (2007) 276.[14] S. Reineke, K. Walzer, K. Leo, Phys. Rev. B 75 (2007) 125328.[15] K. Okumoto, Y. Shirota, J. Lumin. 87 (2000) 1171.[16] B.D. Chin, C.H. Lee, Adv. Mater. 19 (2007) 2061.[17] I.G. Hill, A. Kahn, J. Appl. Phys. 86 (1998) 5583.[18] M.A. Baldo, S. Lamansky, P.E. Burrows, M.E. Thompson, S.R. Forrest, Appl. Phys.

Lett. 75 (1999) 4.[19] L. Zou, V. Savvate’ev, J. Booher, C.H. Kim, J. Shinar, Appl. Phys. Lett. 79 (2001)

2282.[20] G. Li, J. Shinar, Appl. Phys. Lett. 83 (2003) 5359.[21] M.A. Baldo, C. Adachi, S.R. Forrest, Phys. Rev. B 62 (2000) 10967.[22] Y. Kawamura, K. Goushi, J. Brooks, J.J. Brown, H. Sasabe, C. Adachi, Appl. Phys.

Lett. 86 (2005) 071104.[23] T. Tsuzuki, S. Tokito, Appl. Phys. Lett. 94 (2009) 033302.[24] S.H. Kim, J. Jang, J.Y. Lee, Synth. Met. 157 (2007) 228.[25] D. Qin, Y. Tao, J. Appl. Phys. 97 (2005) 044505.[26] A. Tsuboyama, H. Iwawaki, M. Furugori, T. Mukaide, J. Kamatani, S. Igawa, T.

Moriyama, S. Miura, T. Takiguchi, S. Okada, M. Hoshino, K. Ueno, J. Am. Chem.Soc. 125 (2003) 12971.