Journal of Luminescence 132 (2012) 870–874

Contents lists available at SciVerse ScienceDirect

Journal of Luminescence

0022-23

doi:10.1

n Corr

E-m

lutts@sk

journal homepage: www.elsevier.com/locate/jlumin

Efficient and low potential operative host/guest concentration graded bilayerpolymer electrophosphorescence devices

Jung Kyu Kim a, Dong-Hyun Lee a, Dong Hwan Wang b, Sung M. Cho a, Jun Young Lee a, Jong Hyeok Park a,n

a School of Chemical Engineering and SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University, Suwon 440-746, Republic of Koreab Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology, 373-1 Guseong-dong, Yuseong-gu, Daejeon 305-701, Republic of Korea

a r t i c l e i n f o

Article history:

Received 22 October 2010

Received in revised form

18 September 2011

Accepted 28 September 2011Available online 6 October 2011

Keywords:

Electroluminescence

Electrophosphorescence

Bilayer

Light-emitting diodes

13/$ - see front matter & 2011 Elsevier B.V. A

016/j.jlumin.2011.09.048

esponding author. Tel.: þ82 42 869 3963.

ail addresses: king8104@kaist.ac.kr (D.H. Wan

ku.du (J.H. Park).

a b s t r a c t

This study investigates enhanced electrophosphorescence and its mechanism in poly(N-vinyl carbaz-

ole) (PVK): N,N0-diphenyl-N,N0-bis(3-methylphenyl)-[1,1-biphenyl]-4,40-diamine (TPD)/2-(4-biphenylyl)-5-

(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD): fac-tris(2-phenylpyridine)iridium [Ir(ppy)3] concentration

graded bilayer electroluminescence devices. The two layers are partially intermixed at the bilayer interface

because the upper layer (composed of Ir(ppy)3 and PBD) was spun cast from a solvent that slightly swells

the bottom layer (composed of PVK and TPD). Moreover, PBD in the upper layer can act as an efficient

electron transport layer as well as a hole blocking layer, resulting in greatly enhanced electron–hole

recombination. An indium tin oxide (ITO)/3,4-polyethylenedioxythiophene–polystyrenesulfonate (PEDOT)/

[PVK:TPD/Ir(ppy)3:PBD] bilayer/LiF/Al device showed dramatically decreased turn-on and driving voltages,

enhanced luminescence efficiency, and narrower emission spectra compared to those of conventional ITO/

PEDOT:PSS/[PVK:TPD:Ir(ppy)3:PBD] blend/LiF/Al devices.

& 2011 Elsevier B.V. All rights reserved.

1. Introduction

Semiconductors composed of organics are appealing candi-dates for emitting materials used in light-emitting diodes (LEDs)[1,2]. Organic molecular semiconductors employing triplet-basedemitting centers in organic LEDs (OLEDs) can capture both singletand triplet excited states; hence, the internal quantum efficiencycan approach to 100% in principle [3,4]. Recently, much researchon OLEDs has focused on improving emission efficiency usingphosphorescent materials as emitters and solution processableoriginal device architectures. For example, highly efficient OLEDshave been reported that employ electrophosphorescent materialsvia doping guests (i.e., blue–red emissive phosphorescent dyes) ina low molecular weight organic host [5,6]. Using a heavy atomcapable of a spin–orbit coupling effect, heavy metal complexes suchas fac-tris(2-phenylpyridine)iridium(III)[Ir(ppy)3] enable the other-wise spin-forbidden triplet-to-ground state transition referred to asphosphorescence [7–10]. For example, very high efficiencies havebeen observed from high vacuum evaporated multi-layer structuredOLEDs consisting of small molecules [11–14].

Solution processed electrophosphorescent LEDs based on poly-mers as the host materials are an important development as theycan be manufactured at room temperature by processing thematerials from solution [15,16]. Most phosphorescent polymer

ll rights reserved.

g), anotherpark@gmail.com,

light emitting devices have employed a hole transporting polymerpoly(N-vinyl carbazole) (PVK) blended with charge transportingmaterials such as TPD and PBD, which are widely used as the hostmaterials. The reason for this is that a small molecular weightphosphorescent material such as Ir(ppy)3 should be compatiblewith the host material and possesses an absorption band coin-cident with the photoluminescence spectrum of the host materi-als [17]. However, polymer electrophosphorescent LEDs (PLEDs)composed of PVK hosts have been operated with lower quantumefficiencies than those of LEDs composed of vacuum processedsmall organics as a host, even though they have several advan-tages in terms of processing conditions [18–22]. Furthermore,devices fabricated using PVK as the host exhibit high drivingvoltages, since PVK is a good hole conductor. Thus, both electronand hole transporting materials are co-doped in the PVK materialto balance the carrier injection or transportation, as well as todecrease operating potential. In the case of PLEDs employing aniridium complex, such as Ir(ppy)3 or Ir(piq)2 doped in a blendedhost material, PVK with an electron transporting material such as2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazol (PBD) or a holetransporting material such as N,N0-diphenyl-N,N0-bis(3-methyl-phenyl)-[1,1–biphenyl]-4,40-diamine (TPD) can enhance and/orbalance the carrier injection. However, PBD and TPD are randomlydistributed inside PVK; therefore, the driving voltage of theutilized devices is still too high for commercialization.

In our previous work we found that PVK can provide the emittingmaterial with sufficient excitation energy in PVK(host)/poly(9,90-dihexylfluorene-2,7-divinylene-m-phenylenevinylene-stat-p-pheny-lenvinylene)(guest) bilayer structured polymer LEDs [23,24] and

J.K. Kim et al. / Journal of Luminescence 132 (2012) 870–874 871

in PVK(host)/fac-tris(2-phenylpyridine)iridium [Ir(ppy)3] bilayerstructured polymer LEDs with an intermixing zone [25]. In thepresent paper we design solution processable bilayer OLEDs using ahole transporting bottom layer composed of TPD and PVK, and a toplayer composed of an electron transporting material (PBD) withIr(ppy)3. By optimizing the composition of the top and bottomlayers, we realize novel electrophosphorescent devices that havegreatly decreased turn-on voltages, as well as low driving voltagesand luminance efficiencies comparable to those of conventionalPVK-based blended devices via controlled charge injection.

Fig. 1. Schematic diagram (top) and band gap diagram (bottom) of the bilayer

structure for ITO/PEDOT:PSS/ [PVKþTPD/Ir(ppy)3þPBD] /LiF/Al.

400 450 500 550 600 650 700

Inte

nsity

(a.u

.)

Wavelength (nm)

Bilayer ; PVK+TPD / PBD+Ir(ppy)3Blend ; PVK+TPD+PBD+Ir(ppy)3

Fig. 2. Normalized EL spectra of Ir(ppy)3 doped with PVK in blended (red, circle)

or bilayer (black, rectangle) structures. (For interpretation of the references to

color in this figure legend, the reader is referred to the web version of this article.)

2. Experimental

Light emitting devices composed of bottom/top bi-layers witha concentration gradient were manufactured with the followingstructure: indium tin oxide (ITO)/3,4-polyethylenedioxythio-phene–polystyrenesulfonate(PEDOT)/[PVK:TPD/Ir(ppy):PBD] bil-ayer/LiF/Al. To successfully prepare this bilayer structure, thesolvent for the upper layer must not destroy the bottom layerduring the spin coating process. In the present work, the bottomlayer was prepared by dissolving two materials, a host polymerpoly(N-vinyl carbazole) (PVK, Tokyo chemical industry) and ahole transporter N,N0-diphenyl-N,N0-bis(3-methylphenyl)-[1,1-biphenyl]-4,40-diamine (TPD, Aldrich Chem. Co.) in monochloro-benzene and subsequently spin coating the resultant solution on aPEDOT:PSS layer. The top layer was then prepared by dissolvingboth an electron transporter 2-(4-biphenylyl)-5-(4-tert-butylphe-nyl)-1,3,4-oxadiazole (PBD, Tokyo Chemical Industry) and aphosphorescent green emitter fac-tris(2-phenylpyridine)iridium(Ir(ppy)3, Gracel) in trichloroethylene (TCE) and subsequentlyspin coating onto the bottom layer.

The weight ratio of PVK to TPD in the bottom layer solution(solution A) was 6.78:1. To find the optimal composition of thetop layer, solutions for the bottom layer (solution B) wereprepared by controlling the weight ratio of PBD to Ir(ppy)3 at0:1, 2:1, 2.7:1, 4:1, and 8:1. To prepare a hole injection layer, thePEDOT:PSS (AI 4083) solution was diluted with 2-propanol(Aldrich Chem. Co.) and spin coated to a thickness of 40–50 nmon a pre-cleaned ITO substrate for 30 s at 4000 rpm. After bakingthe substrate at 120 1C for 5 min, solution A for the bottom layer(PVK and TPD dissolved in monochlorobenzene) was spin coatedfor 30 s at 1500 rpm. Then, after baking at 80 1C for 30 min,solution B for the upper layer (Ir(ppy)3 and PBD dissolved in TCE)was spin coated on the bottom layer for 30 s at 1500 rpm andbaked at 80 1C for 30 min. Finally, LiF (1 nm) and Al cathode layers(120 nm) were deposited on the upper layer via thermal evapora-tion under a pressure of about 5�10�6 Torr. The electrical andluminescence (current–voltage–luminance) characteristics of thedevices were analyzed using a source measurement unit (Keithley2400) and a luminance meter (Minolta CS100).

3. Results and discussion

Electro-phosphorescence devices with a [PVK:TPD/Ir(ppy)3:PBD] bilayer structure (Fig. 1) and a concentration gradient werefabricated by a spin coating process. The upper layer solvent (TCE),which can cause swelling of the bottom layer, can generate anintermixing zone at the interface between the two layers, resultingin a bilayer device with a concentration gradient [25]. From theelectroluminescence (EL) spectra of the bilayer device, enhancedcolor purity was observed compared to a device with a blendedemitter (Fig. 2). In a previous work, efficient energy transfer from thePVK host material to the Ir(ppy)3 emitting material was expected ina simple bilayer device with a [PVK/Ir(ppy)3] bilayer structure

because the large emission PVK peak can overlap the metal–ligandcharge transfer absorption peak of Ir(ppy)3. Therefore, Foster-type and Dexter-type energy transfers from PVK to Ir(ppy)3

were predicted to occur in the intermixing zone in the bilayerstructure [25].

Furthermore, the bilayer structure causes the devices toaccumulate injected holes and electrons in the thin intermixedregion. This is due to both the high lowest unoccupied molecularorbital (LUMO) level of PVK, which serves as an effective electronblocking material, and the highest occupied molecular orbital(HOMO) energy level of PBD, which serves as an effective holeblocking material. Thus, as shown in Fig. 1, the bilayer structurecan offer more opportunities for the electrons and holes to berecombined within a very narrow range [25,26]. This may be onereason for the improved color purity in the bilayer devices.

Bilayer structured devices enhanced the balance of electronsand holes in the emissive layer. TPD incorporated into PVK in thebottom layer had the ability to transport the holes, while PBD in

10

12

14

10

12

14

Blend; PBD : Ir(ppy)3 = 4 : 1 (wt%)

Bilayer ; PBD : Ir(ppy)3 = 4 : 1 (wt%)

ncy

(Cd/

A)

J.K. Kim et al. / Journal of Luminescence 132 (2012) 870–874872

the upper layer carried out the important roles of not onlytransporting the electrons, but also blocking the holes due to itslow HOMO level [26]. Because the concentration of Ir(ppy)3 andPBD increases from the intermixed zone to the cathode, themobility of injected holes may be slower as they approachthe cathode due to the increased PBD concentration. Similarly,the injection of holes may slow down as they approach the anode.These gradient device structures provide more opportunities togenerate excitons.

In order to confirm whether the intermixed zone was preparedsuccessfully in the bilayer, the depth profiles of the films werestudied by recording the Ir concentration from the bottom layerto the top layer using Auger spectroscopy in combination withion-beam milling [25]. Fig. 3 shows the peak-to-peak Auger signalof the bilayer films (PVKþTPD (�80 nm)/Ir(ppy)3þPBD) as afunction of depth in the films. From this data we can confirm thatthe Ir(ppy)3 can penetrate into the PVK/TPD mixture layer, thusforming a concentration gradient.

Fig. 4 shows the current density–voltage–luminance (I–V–L)characteristics for two different electroluminescence devices:ITO/PEDOT:PSS/PVKþTPDþ Ir(ppy)3þPBD (blend)/LiF/Al andITO/PEDOT:PSS/[PVKþTPD/Ir(ppy)3þPBD] (bilayer)/LiF/Al. Theweight ratio of PVK, TPD, PBD, and Ir(ppy)3 in the blend structuredevice was 10.2:1.5:4:1, and in the bilayer structure device,PVK:TPD was 10.2:1.5 and TPD:Ir(ppy)3 was 4:1. The bilayerdevice shows much lower turn-on and operating voltages com-pared with the blended EL device. The turn-on and operatingvoltages at 1000 cd/m2 for the bilayer device were 5 V and 9.1 V,respectively, and 10.5 V and 20.5 V for the blended device,respectively. The voltages of the bilayer device were therefore

100500

P-P

Sig

nal (

a.u.

)

Thickness (nm)

Fig. 3. Depth profiles of the PVKþTPD/Ir(ppy)3þPBD bilayer (PBD:Ir(ppy)3¼4:1).

4 8 12 16 20 24 28 320

4000

8000

12000

16000

Bilayer; PBD : Ir(ppy)3 = 4 : 1 (wt%)

Blend; PBD : Ir(ppy)3 = 4 : 1 (wt%)

Lum

ines

cenc

e (C

d/m

2 )

Voltage (V)

0

100

200

300

400

Cur

rent

den

sity

(mA

/cm

2 )

Fig. 4. I–V–L characteristics for two different electroluminescence devices:

Ir(ppy)3 doped with PVK in blended (cycle) or bilayer (square) structures.

half of those of the blended device, but there was no significantdifference in the luminance efficiency (cd/A) between the devices(Fig. 5). That is, the maximum luminance efficiency of the bilayerdevice, which was about 10.16 cd/A, while that of the blendeddevice was about 11.6 cd/A.

Fig. 6 shows the I–V–L characteristics for the electroluminesce-nce PBD-free bilayer devices ITO/PEDOT:PSS/ [PVKþTPD/Ir(ppy)3]bilayer/LiF/Al. The turn-on voltage of the PBD-free bilayer devicewas significantly higher than that of the novel bilayer device. Theturn-on voltage of the PBD free device was about 9 V, which was farhigher than the voltage of the bilayer device (5 V), but was moresimilar to that of the blended device voltage (10.5 V). The maximumluminance of the bilayer device without PBD was around 80 cd/m2,compared with 7400 cd/m2 for the bilayer device with PBD. Fig. 7shows the luminance efficiency (cd/A) of the PBD-free bilayerdevice, which exhibited remarkably low efficiency. The maximumefficiency was 0.1 cd/A, which was about ten times lower than thatof the bilayer device with PBD. Based on these results, we suspectthat PBD, which has both electron transporting and hole blockingroles, in the upper layer mixed with Ir(ppy)3 provides a well-balanced electron–hole transport into the emissive layer. Hence,

0 20 40 60 80 100 120 1400

2

4

6

8

0

2

4

6

8

Lum

inan

ce E

ffici

e

Current density (mA/cm2)

Fig. 5. Luminance efficiency vs. current density characteristics for two different

electroluminescence devices: blended (red, top) or bilayer (black, bottom) struc-

tures. (For interpretation of the references to color in this figure legend, the reader

is referred to the web version of this article.)

0

20

40

60

80

100

120

140

160

180

200

PVK+TPD / Ir(ppy)3 - bilayer

Voltage (V)

Cur

rent

den

sity

(mA

/cm

2 )

6 7 8 9 10 11 12 13 14

6 7 8 9 10 11 12 13 14

0

10

20

30

40

50

60

70

80

Lum

ines

cenc

e (C

d/m

2 )

Fig. 6. I–V–L graph for the ITO/PEDOT:PSS/[PVK, TPD/ Ir(ppy)3 EL device without

PBD]/LiF/Al bilayer structures.

0 20 40 60 80 100 120 140 160 1800.00

0.02

0.04

0.06

0.08

0.10

0.120 20 40 60 80 100 120 140 160 180

0.00

0.02

0.04

0.06

0.08

0.10

0.12PVK+TPD / Ir(ppy)3 - bilayer

Lum

inan

ce E

ffici

ency

(Cd/

A)

Current density (mA/cm2)

Fig. 7. Luminance efficiency vs. current density characteristics for the PVK, TPD/

Ir(ppy)3 EL device without PBD bilayer structures.

6 8 10 12 14-2000

0

2000

4000

6000

8000Bi-layer; PBD : Ir(ppy)3 = 4 : 1 (wt%)Bi-layer; PBD : Ir(ppy)3 = 2.7 : 1 (wt%)Bi-layer; PBD : Ir(ppy)3 = 2 : 1 (wt%)

Voltage (V)

0

50

100

150

200

250

300

Lum

ines

cenc

e (C

d/m

2 )

Cur

rent

den

sity

(mA

/cm

2 )

Fig. 8. I–V–L characteristics for the EL devices of the ITO/PEDOT:PSS/[PVK, TPD/

Ir(ppy)3 1, PBD 4]/LiF/Al, ITO/PEDOT:PSS/[PVK, TPD/Ir(ppy)3 1, PBD 2.7]/LiF/Al, and

ITO/PEDOT:PSS/[PVK, TPD/Ir(ppy)3 1, PBD 2]/LiF/Al bilayer structures.

0 20 40 60 80 100 120 1400

2

4

6

8

10

12

14

0

2

4

6

8

10

12

14Bilayer; PBD : Ir(ppy)3 = 4 : 1 (wt%)Bilayer; PBD : Ir(ppy)3 = 2.7 : 1 (wt%)Bilayer; PBD : Ir(ppy)3 = 2 : 1 (wt%)

Lum

inan

ce E

ffici

ency

(Cd/

A)

Current density (mA/cm2)

Fig. 9. Luminance efficiency vs. current density characteristics for the EL devices

of the ITO/PEDOT:PSS/[PVK, TPD/Ir(ppy)3 4, PBD 1]/LiF/Al, ITO/PEDOT:PSS/[PVK,

TPD/Ir(ppy)3 2.7, PBD 1]/LiF/Al, and ITO/PEDOT:PSS/[PVK, TPD/Ir(ppy)3 2, PBD 1]/

LiF/Al bilayer structures.

Table 1Bilayer device performances at the maximum luminance efficiency (cd/A).

PBD:Ir(ppy)3 (wt. ratio) cd/A mA/cm2 cd/m2 Voltage (V)

0.0:1.0 0.10 3.0 30.5 9.0

2.0:1.0 3.79 9.6 364 9.7

2.7:1.0 6.97 2.8 198 8.2

4.0:1.0 10.16 14.8 1500 9.6

8.0:1.0 0.16 115.6 180 12.0

J.K. Kim et al. / Journal of Luminescence 132 (2012) 870–874 873

more carriers can be recombined, and excitons can be generated atthe intermixed zone.

To investigate how PBD affects the bilayer device, severalbilayer devices with different weight ratios of PBD and Ir(ppy)3

in the upper layer were fabricated. Figs. 8 and 9 show the I–V–L

characteristics and luminance efficiencies (cd/A) for ITO/PED-OT:PSS/[PVKþTPD/Ir(ppy)3þPBD]/LiF/Al bilayer devices withthe following weight ratios: Ir(ppy)3:PBD¼1:2 (device A) 1:2.7(device B) and 1:4 (device C). The turn-on voltages of devices A, B,and C were 6.8 V, 6.5 V, and 5 V, respectively, and the maximumefficiencies of each device were 3.8 cd/A at 364 cd/m2, 7 cd/A at198 cd/m2, and 10.16 cd/A at 1500 cd/m2, respectively. In addi-tion, the operating voltages of devices B and C at 1000 cd/m2 were10 V and 9.1 V, while the voltages of devices A, B, and C at 600 cd/m2

were 11.1 V, 9.3 V, and 8.7 V, respectively. As the ratio of PBDincreased in each bilayer device, the turn-on and operating voltagesdecreased, and the maximum efficiency (cd/A) and luminance(cd/m2) increased greatly. However, the bilayer device containingPBD in the upper layer at a more than 1:4 weight ratio of Ir(ppy)3 toPBD exhibited negative effects, as shown in Table 1. Specifically, theluminance efficiency and maximum luminance decreased. Table 1summarizes the performances of the five bilayer devices withdifferent PBD content in the upper layer. The EL bilayer deviceconsisting of a PVKþTPD/PBDþ Ir(ppy)3 structure had better

performance, turn-on and operating voltages, and luminance effi-ciency than the conventional blended device. Furthermore, thedevice with a 1:4 weight ratio of PBD to Ir(ppy)3 bilayer structureexhibited the optimum EL performance. With this enhanced novelbilayer structure electrophospholuminescence device, some nega-tive aspects of PVK as a host material for electrophosphorescent ELdevices were mitigated without sacrificing other properties.

4. Conclusions

Compared to conventional blended devices, polymer phos-phorescent devices prepared by adapting a bilayer system ofPVK:TPD as the bottom layer and Ir(ppy)3:PBD as the upper layershowed a dramatically decreased turn-on voltage of 5 V and anoperating voltage of 9.1 V, in addition to an efficiency of 10.16 cd/A.The use of a TPD hole transporting additive in the PVK-based bottomlayer and PBD, which had both an electron transporting role and ahole blocking role in the upper layer, resulted in well-balancedelectron–hole carrier transport. Thus, the EL characteristics wereimproved. We believe that this unique approach can solve theproblems associated with PVK-based electrophosphorescentdevices. Hence, we believe that our results support the use of PVKas an effective host material in light emitting diodes.

Acknowledgments

This research was supported by Research Programsthrough the NRF grant funded by MEST (nos. 20100026281,20110023215). This work (Prof. J.H. Park) was also partiallysupported by a grant from the Fundamental R&D Program for

J.K. Kim et al. / Journal of Luminescence 132 (2012) 870–874874

Core Technology of Materials funded by the Ministry of Knowl-edge Economy, Republic of Korea.

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