Physica B 407 (2012) 2753–2757

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Physica B

0921-45

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n Corr

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

Non-doped phosphorescent white organic light-emitting devices with aquadruple-quantum-well structure

Juan Zhao, Junsheng Yu n, Lei Zhang, Jun Wang

State Key Laboratory of Electronic Thin Film and Integrated Devices, School of Optoelectronic Information, University of Electronic Science and Technology of China,

Chengdu 610054, PR China

a r t i c l e i n f o

Article history:

Received 13 February 2012

Received in revised form

7 April 2012

Accepted 9 April 2012Available online 16 April 2012

Keywords:

White organic light-emitting device

(WOLED)

Phosphorescence

Non-doped

Quadruple-quantum-well structure

Potential barrier layer

26/$ - see front matter & 2012 Elsevier B.V. A

x.doi.org/10.1016/j.physb.2012.04.021

esponding author. Tel.: þ86 28 83207157.

ail address: jsyu@uestc.edu.cn (J. Yu).

a b s t r a c t

Non-doped white organic light-emitting devices (WOLEDs) with a quadruple-quantum-well structure

were fabricated. An alternate layer of ultrathin blue and yellow iridium complexes was employed as the

potential well layer, while potential barrier layers (PBLs) were chosen to be 2,2’,2’’-(1,3,5-benzenetriyl)-

tris(1-phenyl-1-H-benzimidazole) (TPBi) or N,N’-dicarbazolyl-3,5-benzene (mCP) combined TPBi. On

adjusting the PBLs for device performance comparison, the results showed that the device with all-TPBi

PBLs exhibited a yellow emission with the color coordinates of (0.50,0.47) at a luminance of 1000 cd/

m2, while stable white emission with the color coordinates of (0.36,0.44) was observed in the device

using mCP combined TPBi as the PBLs. Meanwhile, for the WOLED, with a reduced efficiency roll-off, a

maximum luminance, luminous efficiency, and external quantum efficiency of 12,610 cd/m2, 10.2 cd/A,

and 4.4%, respectively, were achieved. The performance improvement by the introduction of mCP PBL

was ascribed to the well confined exciton and the reduced exciton quenching effect in the multiple

emission regions.

& 2012 Elsevier B.V. All rights reserved.

1. Introduction

During the past few decades, organic light-emitting devices(OLEDs) have drawn extensive academic and industrial interest,especially white OLEDs (WOLEDs) that have potential applicationin displays and lighting sources. Among various configuration forOLED, the multiple-quantum-well (MQW) structure has beenproved to be an effective approach for high device performance[1–3], by confining charge carriers and exciton within the multipleemitting layer (EML); thus the charge carrier recombinationefficiency and exciton formation probability can be beneficiallyenhanced. However, the realization of constructing efficient MQWstructures highly relies on the thickness of potential well layer(PWL), material choice of potential barrier layer (PBL), and soon [4]. Up to the present, several series of the MQW-devices havebeen fabricated by employment of fluorescent or phosphorescentemitters, using the host–guest doping or non-doping method [5,6].It is well known that the fluorescence that can use only 25% singletexcitons for emission would largely limit device efficiency, whilethe phosphorescence can achieve a 100% internal quantum effi-ciency due to the ability of harvesting both singlet and tripletexcitons [7]. Moreover, even though the host–guest doping system

ll rights reserved.

has been widely utilized nowadays, this technique suffers fromcomplex processability and poor reproducibility. In addition, thephase separation phenomenon in guest–host system is anotherpotential problem [8], which can all be avoided in the technologi-cally easy non-doping approach [9]. In view of the shortcomingsaforementioned, considerable efforts for the MQW-devices havebeen devoted to explore the non-doping system by adopting thehigh-efficiency phosphors. Though monochrome phosphorescentdevices with the MQW structure have been developed, the studyof phosphorescent WOLED (PWOLED) based on the non-dopedMQW is rarely reported. Recently, we reported a kind of devicearchitecture for the non-doped PWOLED consisting of a double-quantum-well structure [10], using dual phosphors as the PWLs andN,N’-dicarbazolyl-3,5-benzene (mCP) as the PBLs. Particularly, atten-tion was paid to optimize the thickness of ultrathin PWLs, whilehigh efficiency and stable white emission was obtained. In addition,the fabrication process was greatly simplified and accurately con-trolled, providing a promising solution to lower the production cost.

In this study, we introduced a simple process for the non-doped PWOLEDs with a quadruple-quantum-well (QQW) struc-ture, while ultrathin blue and yellow iridium complexes werealternately formed as the PWLs. Meanwhile, the effect of materialchoice and thickness adjustment of the PBLs on device perfor-mance, which were composed of 2,2’,2’’-(1,3,5-benzenetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) or mCP combined TPBi,was investigated.

J. Zhao et al. / Physica B 407 (2012) 2753–27572754

2. Experimental

The devices were fabricated on indium tin oxide (ITO)-coatedglass substrates, which were consecutively cleaned in ultrasonicbath with detergent, acetone, de-ionized water, and ethanol. Prior tothe deposition of organic layers, the ITO-coated substrates weretreated by oxygen plasma for 5 min to increase the work function ofthe anode. Then, organic and metallic films were subsequentlydeposited by thermal evaporation under a pressure of 3�10�4

and 2�10�3 Pa, at a rate of 0.05–0.1 nm/s and 0.5–1 nm/s, respec-tively. The film thickness and deposition rates were monitoredin situ by oscillating quartz thickness monitors. To study the effectof quadruple-quantum-well structure in PWOLED, the devices werefabricated with a structure of ITO/1,1-bis[(di-4-tolylamino)phenyl]-cyclohexane (TAPC) (30 nm)/iridium(III) bis[(4,6-difluorophenyl)-pyridinato-N,C2’](picolinate) iridium (III) (FIrpic) (1 nm)/PBL1(X1)/bis[2-(4-tertbutylphenyl)benzothiazolato-N,C2’] iridium (acetylace-tonate) [(tbt)2Ir(acac)] (0.5 nm)/PBL2(X2)/FIrpic(1 nm)/PBL3(X3)/(tbt)2Ir(acac)(0.5 nm)/4,7-diphenyl-1,10-phenanthroline (Bphen)(40 nm)/Mg:Ag(200 nm). Therein, phosphorescent blue FIrpic andyellow (tbt)2Ir(acac) were alternately employed as the PWLs/EMLs,while the PBLs were chosen to be either TPBi or mCP combinedTPBi. Thereby, the quadruple-quantum-well structure was formedby spatially separating the PWLs from the PBLs. TAPC and Bphenfunctioned as the hole transporting layer and electron transport-ing layer, respectively, while Mg:Ag alloy was used as a cathode.Meanwhile, in the device architecture depicted in Fig. 1, PBL1, PBL2,and PBL3 represented the PBL that was close to the anode side, inthe middle, and close to the cathode, respectively, while PBL1 andPBL3 were kept to be TPBi. For device characterization, the threePBLs were consecutively adjusted, and five devices were given asfollows.

Device A: PBL2 stands for TPBi, X1¼4 nm, X2¼4 nm andX3¼4 nm.Device B: PBL2 stands for mCP, X1¼4 nm, X2¼4 nm andX3¼4 nm.Device C: PBL2 stands for mCP, X1¼6 nm, X2¼4 nm andX3¼4 nm.Device D: PBL2 stands for mCP, X1¼6 nm, X2¼6 nm andX3¼4 nm.Device E: PBL2 stands for mCP, X1¼6 nm, X2¼6 nm andX3¼6 nm.

Luminance–current density–voltage characteristics wererecorded with a Keithley 4200 source, while electrolumines-cent (EL) spectra and Commission Internationale de l’Eclairage(CIE) coordinates were measured with an OPT-2000 spectro-photometer. The external quantum efficiency (EQE) of thedevice was calculated with a computer program on the basisof previously reported theories in the literature [11]. All the

Fig. 1. Energy level diagram of the QQW-OLEDs.

measurements were performed at room temperature in ambi-ent environment.

3. Results and discussion

Fig. 2 shows the normalized EL spectra of devices A–E at apractical luminance of 1000 cd/m2 for lighting. For all the devices,a peak at 470 nm with a shoulder at 495 nm arises from FIrpicemission, while a peak at 560 nm with a shoulder at 602 nm isfrom (tbt)2Ir(acac). Since there is a large difference of the highestoccupied molecular orbital (HOMO) between PWLs and PBLs,together with a moderate difference of the lowest unoccupiedmolecular orbital (LUMO) levels, as shown in Fig. 1, it renders theblue/yellow PWLs shallow electron traps and deep hole traps.Hence, the charge carriers can be favorably confined in the tripletPWLs/EMLs, followed by direct charge recombination and excitonformation on the emitters, which is the so-called direct carriertrapping mechanism [12], contributing to excite both blue andyellow emissions. Apparently, in devices A–C, the yellow intensityoverwhelms its blue counterpart, giving yellow-dominate emis-sion, whereas white emission can be clearly observed in devices Dand E. Then, the influence of PBLs on color emission of the devicesis demonstrated.

TPBi has been reported to serve as a suitable PBL material inthe MQW-devices because of its high triplet energy (ET) (2.74 eV)and wide energy gap (Eg) [13], which can effectively confine thecharge carrier and exciton in the PWLs. For device A with 4 nmTPBi PBLs, Forster energy transfer from FIrpic to (tbt)2Ir(acac)occurs through exciton diffusion across the PBL, owning to longerForster energy transfer radius (�4 nm) [5] than that of Dexterenergy transfer (�1 nm) [14]. Consequently, the yellow emissionis strengthened. On the other hand, given the electron transport-ing property of TPBi PBLs, it is reasonably speculated that moreexcions are formed in the PWLs near the anode side. For thisreason, the hole transporting material mCP with high ET (2.9 eV)and wide energy gap [15] is introduced as PBL2 in devices B–E, tofacilitate hole transport into the PWLs situated at the cathodeside, followed by recombination with local electrons. Therefore,the excitons could be distributed over the multiple PWLs/EMLs.

Compared to device A, the blue intensity in device B is slightlyboosted while the yellow intensity remains strong, due to thesame reason as discussed above for device A. In the case of deviceC, the thickness of the PBL1 is enlarged to be 6 nm, so the energy

Fig. 2. Normalized EL spectra of the QQW-OLEDs at a luminance of 1000 cd/m2.

Inset: the CIE coordinates of the devices in the luminance range 1000–10,000 cd/m2.

J. Zhao et al. / Physica B 407 (2012) 2753–2757 2755

transfer between blue and yellow PWLs close to the anode sidecan be partly prevented. Nevertheless, the weak blue intensity indevice C suggests that the exciton formation zones have beenpositioned in the PWLs close to the cathode side, where theenergy transfer still exists due to 4 nm-PBL, leading to the highyellow intensity. Increasing the thickness of PBL2 and PBL3 indevices D and E should further restrained the energy transferbetween blue and yellow. As a consequence, white emission canbe realized in devices D and E, which originate from comparableblue and yellow intensities in the spectra, as shown in Fig. 2.Meanwhile, it indicates that the excitons created in the PWLs areinhibited from escaping outside; thus, the recombination zoneshave been well confined in the multiple PWLs/EMLs. Additionallynotice that the blue intensity in device E is moderately higherthan that in device D, implying that the energy transfer is largelysuppressed in the devices with thicker PBLs. Remarkably, as seenfrom the inset of Fig. 2, all the devices show excellent colorstability in a broad luminance range of 1000–10,000 cd/m2.Reasons for the stable color emission are explained by: (1) effec-tive confinement of charge carriers in the multiple EMLs becauseboth HOMO and LUMO levels of the emitters are located betweenthat of the PBLs, as seen from the device architecture of Fig. 1 and(2) well confined excitons in the multiple EMLs as the tripletenergy of the PBLs is higher than that of the emitters. Besides, forthe host-free EMLs, the un-expected exchange energy lossinduced from host to dopant could be favorably eliminated. Theresults reveal that proper material choice and thickness adjust-ment of the PBLs are highly essential for the color emission ofMQW-OLEDs.

The luminance–current density–voltage (L–J–V) characteristicsof devices A–E are displayed in Fig. 3. From the J–V characteristic,it is obvious that devices B and C incorporated with mCP as themiddle PBL (PBL2) show slightly higher current density than thatof device A with all-TPBi PBLs, which is ascribed to the highercarrier mobility of mCP [16], in contrast to that of TPBi [17].Thereby, the charge carrier transport in devices B and C can beaccelerated. With increasing the thickness of the PBLs in devices Dand E, the current density was decreased due to the enlarged totalthickness of the devices. As for the L–V characteristic, it showsthat all the devices exhibit fairly low turn-on and operationvoltages, while the luminance increases rapidly on raising thedriving voltage. Besides, the maximum luminances of devices A, B,C, D and E achieved are 16,220 cd/m2, 27,010 cd/m2, 9,410 cd/m2,12,610 cd/m2 and 8,120 cd/m2, respectively. More details arelisted in Table 1.

Fig. 4 shows the luminous efficiency (LE) versus the currentdensity characteristics of the QQW-devices. At a current density

Fig. 3. Luminance–current density–voltage characteristics of the QQW-devices.

of 20 mA/cm2, the LEs are 17.5 cd/A, 16.3 cd/A, 7.5 cd/A, 6.8 cd/A,and 5.7 cd/A for devices A, B, C, D, and E, respectively. It is worthpointing out that all the device efficiencies remain high over awide range of current density, implying that the exciton quench-ing processes at high current density are effectively preventedand then the efficiency roll-off is reduced. As is well known, thereare mainly two exciton quenching mechanisms in the phosphor-escent EML, namely triplet–triplet annihilation (TTA) and triplet–polaron annihilation (TPA) [18], with the corresponding reactionequations [Eqs. (1) and (2) for TTA and Eqs. (3) and (4) for TPA]shown as follows:

T1þT1-T1þS0 ð1Þ

T1þT1-S1þS0 ð2Þ

T1þe-S0þe ð3Þ

T1þh-S0þh ð4Þ

where T1, S0, S1, e, and h denote the first excited triplet state, theground state, the first excited singlet state, the electron, and thehole, respectively. So, it can be learnt that the TTA as well as TPAprocesses are closely sensitive to the density of the tripletexcitons or charge carriers in the EML. In the region of highcurrent density, the numbers of the excitons and charge carriersare greatly enhanced, while effective suppression of the excitonquenching processes enables low efficiency roll-off. Consideredless than one monolayer thick, the ultrathin non-doped layer ofthe emitter (r1 nm) is discontinuous in atomic force micro-scopy images [19]. When sandwiched between two host-likeneighboring layers, the ultrathin layer of the emitter forms adoping profile similar to the situation where the emitter is co-doped with the two hosts simultaneously. Moreover, in theMQW-devices, the charge carriers and excitons are effectivelydispersed in the multiple EMLs, while these emissive regions areperiodically interrupted by multiple PBLs. Two things are respon-sible for these, i.e. small degree of aggregation and concentrationquenching [20]. Therefore, the TTA and TPA processes as afore-mentioned can be restricted due to the low density of chargecarriers and excitons in the multiple emissive regions, allowingfor the reduced efficiency roll-off.

On the other hand, the formation of excimer [21] could also beimpeded. In addition, at a high current density, taking 80 mA/cm2

for example, the luminous efficiency of device B is 1.8 timeshigher than that of device A, while device D shows an enhance-ment of 1.3 times compared to that of device C. Thereby, it can bededuced that the incorporation and adjustment of the mCP PBL2plays an important role in reducing the efficiency roll-off at thehigh current density, resulting from improved electron–holebalance in the PWLs caused by the enhanced electron mobilityof the mCP PBL at a high electric field [16].

Fig. 5 depicts the external quantum efficiency–voltage (EQE–V)characteristics of the devices based on the QQW structure. It canbe seen that the EQEs of devices A, D, and E are proportional toapplied bias, while those of devices B and C slightly decrease atthe high driving voltage. Consequently, the maximum EQE of thedevices can be realized at high luminance while displaying lowefficiency roll-off, which is an essential characteristic for bothdisplay and lighting applications. The results exhibit that themaximum EQEs of devices A, B, C, D, and E are 6.4%, 6.3%, 4.0%,4.4%, and 2.1%, respectively. Compared to that of typicalphosphorescent OLEDs based on the host–guest doping system,the LEs and EQEs of the QQW-devices are still relatively low.As is well known, most of the phosphorescent emitters sufferfrom a common problem, namely the concentration quenchingeffect in solid state caused by the molecule interactions [22].

Table 1Performance of the quadruple-quantum-well devices.

Device PBL1 (nm) PBL2 (nm) PBL3 (nm) Voltagea (V) Luminanceb (cd/m2) LEb (cd/A) EQEb (%) CIEc (x,y)

A TPBi(4) TPBi(4) TPBi(4) 3.4 16,220 18.8 6.4 (0.50,0.47)

B TPBi(4) mCP(4) TPBi(4) 3.3 27,010 18.7 6.3 (0.49,0.48)

C TPBi(6) mCP(4) TPBi(4) 3.4 9410 7.6 4.0 (0.48,0.47)

D TPBi(6) mCP(6) TPBi(4) 3.7 12,610 10.2 4.4 (0.36,0.44)

E TPBi(6) mCP(6) TPBi(6) 3.5 8120 5.8 2.1 (0.34,0.43)

a At 1 cd/m2.b Maximum value.c At 1000 cd/m2.

Fig. 4. Luminous efficiency–current density characteristics of the QQW-devices.

Fig. 5. External quantum efficiency–current density characteristics of the

QQW-devices.

J. Zhao et al. / Physica B 407 (2012) 2753–27572756

Consequently, the host–guest doping system has become auniversal method for solving this problem. Here, the presentedQQW structure has advantages of simplifying the fabricationprocess and guaranteeing device reproducibility, together withrealizing low efficiency roll-off and excellent color stability. Onthe other hand, the results suggest there is still a large room forperformance enhancement through further device optimiza-tion, and more studies are required on these MQW-devices. Toour best knowledge, this study is the first by applying quad-ruple-quantum-well structure to the non-doped PWOLEDs,

utilizing ultrathin layers of blue and yellow iridium complexesas the PWLs.

4. Conclusions

In summary, we demonstrated the non-doped WOLEDs with aquadruple-quantum-well structure, in which ultrathin layers ofthe blue and yellow iridium complexes were alternately formedas the PWLs. The results showed that the device performance wasgreatly dependent on the material choice and thickness adjust-ment of the PBLs. For the device with all-TPBi PBLs, highperformance was achieved with the sacrifice of white emission.In the case of the device employing mCP combined TPBi as thePBLs, stable white emission along with low efficiency roll-off wasrealized, attributed to well confined excitons in the multiplePWLs caused by high energy levels of the mCP PBL, accompaniedby reduced exciton quenching and improved electron–hole bal-ance. The described non-doped QQW concept indicated thefeasibility of this strategy to obtain high-efficiency WOLEDs withexcellent stability as well as simplified manufacturing.

Acknowledgments

This work was supported by the National Science Foundationof China (NSFC; Grant nos. 61177032 and 61006036), the Foun-dation for Innovative Research Groups of the NSFC (Grant no.61021061), the Fundamental Research Funds for the CentralUniversities (Grant nos. ZYGX2010Z004 and ZYGX2010J004),SRF for ROCS, SEM (Grant no. GGRYJJ08-05), and Doctoral Fundof Ministry of Education of China (Grant no. 20090185110020).

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