Journal of Luminescence 132 (2012) 1994–1998

Contents lists available at SciVerse ScienceDirect

Journal of Luminescence

0022-23

http://d

n Corr

E-m

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

Combined host–guest doping and host-free systems for high-efficiency whiteorganic light-emitting devices

Juan Zhao, Junsheng Yu n, Shengqiang Liu, Yadong Jiang

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 November 2011

Received in revised form

11 February 2012

Accepted 7 March 2012Available online 17 March 2012

Keywords:

White organic light-emitting device

(WOLED)

Phosphorescence

Host–guest doping

Host-free

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

x.doi.org/10.1016/j.jlumin.2012.03.025

esponding author. Tel.: þ86 28 83207157.

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

a b s t r a c t

Highly efficient white organic light-emitting devices (WOLEDs) with a four-layer structure were

realized by utilizing phosphorescent blue and yellow emitters. The key concept of device construction

is to combine host–guest doping system of the blue emitting layer (EML) and the host-free system of

yellow EML. Two kinds of WOLEDs incorporated with distinct host materials, namely N,N’-dicarbazolyl-

3,5-benzene (mCP) and p-bis(triphenylsilyly)benzene (UGH2), were fabricated. Without using light

out-coupling technology, a maximum current efficiency (ZC) of 58.8 cd/A and a maximum external

quantum efficiency (ZEQE) of 18.77% were obtained for the mCP-based WOLED; while a maximum ZC of

65.3 cd/A and a maximum ZEQE of 19.04% were achieved for the UGH2-based WOLED. Meanwhile, both

WOLEDs presented higher performance than that of conventionally full-doping WOLEDs. Furthermore,

systematic studies of the high-efficiency WOLEDs were progressed.

& 2012 Elsevier B.V. All rights reserved.

1. Introduction

White organic light-emitting devices (WOLEDs) have attracteda large volume of attention because of their wide applications indisplays and lighting sources. To generate white emission, themost efficient approach is to utilize host–guest doping system bymixing three primary colors or two complementary colors into asingle- or multiple- stacked emitting layer (EML). As for electro-luminescent materials, phosphorescent emitters which canachieve the much desired 100% internal quantum efficiency arewidely used, due to the capability of harvesting both singlet andtriplet excitons [1,2]. Nowadays, WOLEDs with high performancehave already been achieved, such as high efficiency exceeding100 lm/W [3], wide color temperature ranging from 2300 K to8200 K as sunlight-style illumination [4], high color renderingindex of 90 together with a long lifetime of 100,000 h [5], andhigh contrast ratio for full color active matrix displays [6]. Evenso, for the development of WOLEDs in the near future, manychallenges still lay ahead while no fundamental obstacles are inthe way. Among various techniques for fabricating WOLEDs, themost efficient and used way is to adopt the host–guest dopingsystem with a multilayer structure, as the WOLEDs aforemen-tioned. However, it suffers from problems of manufacturingcomplexity and difficulty. For example, the doping concentration

ll rights reserved.

of each emitter in EMLs should be precisely controlled so as toobtain white emission, since natural energy transfer from higher-energy emitters to lower ones occurs. Therefore, the specifieddoping concentration of red, yellow or even green emitters isusually required to be very low, which makes it tough toguarantee the accuracy of the doping concentration, the usageefficiency of the materials and the reproducibility of the devices.To simplify the fabrication process, non-doped WOLEDs made ofhost-free EMLs have been investigated [7,8], and co-evaporationof multiple emitters and hosts can be avoided. Whereas, deviceperformance is strongly susceptible to the thickness of host-freeEMLs due to the aggregation-induced quenching effect, resultingfrom strong intermolecular interaction that is caused by the shortdistance between the emitter molecules [9].

Taking their own advantages into account, a concept of combin-ing the host–guest doping system with the host-free system intoone device has been introduced to construct WOLEDs, which issimilar to the delta-doping method reported by Zhao et al. [10]. As aresult, this method can yield a modified device that renderssimplified structure and high efficiency. So far, the strategy hasbeen mainly employed in fluorescent or fluorescent/phosphorescenthybrid devices [11,12], where the fluorescent emitters greatlyinfluence the device efficiency. Recently, Lee et al. applied a similarmethod and reported an all-phosphorescent WOLED with highefficiency [13], by dispersing host-free yellow emitter betweentwo triple-doped blue EMLs. Nevertheless, the fabrication processis still very complex. Hence, it is necessary and promising to performmore studies on simplified WOLEDs with high efficiency.

J. Zhao et al. / Journal of Luminescence 132 (2012) 1994–1998 1995

In this work, we propose an easily realizable strategy tofabricate highly efficient WOLEDs consisting of phosphorescentblue–yellow emitters, by combining the host–guest doping sys-tem of blue EML with the host-free system of yellow EML into onedevice. Two host materials, N,N’-dicarbazolyl-3,5-benzene (mCP)and p-bis(triphenylsilyly)benzene (UGH2), were selected asp-host and n-host for the blue EML, respectively. Additionally,conventional WOLEDs using the full-doping system were alsoconstructed for comparison. Furthermore, the reasons for high-efficiency of WOLEDs which employed the host-free yellow EMLwere also analyzed.

Fig. 1. Luminance–current density–voltage characteristics of devices A–D.

2. Experimental

All the devices were grown on clean indium tin oxide (ITO)-coated glass substrates, which were treated with oxygen plasma for5 min prior to the deposition of organic layers to increase the workfunction of anode [14]. Organic and metallic layers were subse-quently deposited without breaking the vacuum, while keeping thepressure in the order of the magnitude 10�4 and 10�3 Pa, respec-tively. Film thicknesses and deposition rates were monitored in situby oscillating quartz thickness monitors. By combining the host–guest doping system of blue EML with the host-free system ofyellow EML, simply structured WOLEDs were fabricated. To inves-tigate the effectivity of host-free yellow EML in WOLEDs, additionalWOLEDs with full-doping EMLs were fabricated. Device structureswere given as device A, ITO/TAPC (40 nm)/mCP:8%FIrpic (20 nm)/(tbt)2Ir(acac) (1 nm)/Bphen (30 nm)/Mg:Ag(200 nm); device B, ITO/TAPC (40 nm)/mCP:8%FIrpic (16 nm)/mCP:3%(tbt)2Ir(acac) (5 nm)/Bphen (30 nm)/Mg:Ag(200 nm); device C, ITO/TAPC (40 nm)/(tbt)2

Ir(acac) (1 nm)/UGH2:8%FIrpic (20 nm)/Bphen (30 nm)/Mg:Ag(200 nm); and device D, ITO/TAPC (40 nm)/UGH2:3%(tbt)2Ir(acac)(5 nm)/UGH2:8%FIrpic (16 nm)/Bphen (30 nm)/Mg:Ag(200 nm). Thethickness of host-free yellow EML had been optimized to be 1 nm inour preceding works. Phosphorescent bis[(4,6-difluorophenyl)-pyr-idinato-N,C2’](picolinate) iridium (III) (FIrpic) was doped into themCP or wide-energy-gap UGH2 host as the blue EML, while bis[2-(4-tertbutylphenyl)benzothiazolato-N,C2’] iridium (acetylacetonate)[(tbt)2Ir(acac)] was employed as the host-free yellow EML. 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) with a high holemobility of 1.0�10�2 cm2 V�1 s�1 [15] was used as both holetransporting layer (HTL) and exciton blocking layer, while 4,7-diphenyl-1,10-phenanthroline (Bphen) with a high electron mobilityof 3�10�4 cm2 V�1 s�1 [16] functioned as both electron transport-ing layer (ETL) and hole blocking layer, and Mg:Ag alloy was used asa cathode.

Luminance–current density–voltage characteristics wererecorded with a Keithley 4200 source, while electroluminescentspectra (EL) and Commission Internationale de l’Eclairage coordi-nates of the uncapsulated devices were measured with an OPT-2000 spectrometer. External quantum efficiencies of the deviceswere calculated with a computer program on the basis of thetheory as reported in the literature [17]. Absorption (Abs) andphotoluminescence (PL) spectra were recorded with the UV–visible spectrophotometer SHIMAZU UV1700 and OPT-2000,respectively.

3. Results and discussion

3.1. Electrical characteristics of WOLEDs

Fig. 1 shows the luminance–current density–voltage (L–J–V)characteristics of devices A–D. As seen, the current density ofdevice A is almost identical to that of device B, but slightly lower.

On the other hand, the current density of device C is obviouslyhigher than that of device D at an identical driving voltage. Thus,the result displays that the host-free-based devices have compar-able or relatively lower driving voltage than that of the full-doping-based devices. Moreover, the mCP-based devices A and Bhave lower driving voltage than that of the UGH2-based devices Cand D, attributing to not only wider energy gap of UGH2 (4.4 eV)than that of mCP (3.5 eV) as seen from the energy level diagram inFig. 3, but also lower conductivity and higher resistance of theUGH2 host [18]. For the L–V characteristics, the luminance ofdevice A is higher than that of device B at low driving voltage,while the luminance of device C is much higher than that ofdevice D at high voltage. The turn-on voltage (defined as thevoltage to attain 1 cd/m2) of 3.1, 3.4, 4.2, 4.4 V, along withmaximum luminance of 41,790, 32,740, 24,700, and 8700 cd/m2

are obtained for devices A, B, C, and D, respectively. The resultsdemonstrate that the WOLEDs based on the concept of combininghost–guest doping system and host-free system show moreexcellent performance than that of the conventionally full-dopingWOLEDs counterparts.

Fig. 2(a) and (b) shows the current efficiency–power efficiency–current density (ZC–ZP–J) characteristics of WOLEDs. Compared todevice B (33.6 cd/A), maximum ZC has been improved by 76% indevice A (58.8 cd/A), while the enhancement is 94% by comparingthe maximum ZC of device C (65.3 cd/A) and device D (33.6 cd/A). Tothe best of our knowledge, both higher current efficiencies arecomparable to the state-of-art efficiencies of WOLEDs without usinglight out-coupling technology in the literature [19–21]. Remarkably,at a practical luminance of 1000 cd/m2, ZC and ZP are maintained ashigh as 49.3 cd/A and 19.6 lm/W for device A, and 40.5 cd/A and13.5 lm/W for device C, while 30.8 cd/A and 11.8 lm/W for device B,and 7.6 cd/A and 2.1 lm/W for device D. It has been known thatdevice efficiencies usually decrease at high current density due totriplet–triplet annihilation in phosphorescent OLEDs [22]. Whereas,efficiency roll-off is greatly reduced in the mCP-based devices (A andB) on comparison with the UGH2-based devices (C and D). Detailedexplanations for the device efficiency and the efficiency roll-off willbe given as follows.

The energy level diagram of the devices is shown in Fig. 3. AsFig. 3(a) shows, in device A, electrons passed from Bphen can bereadily transferred into the blue EML through the neat (tbt)2Ir(acac) layer because of LUMO (the lowest unoccupied molecularorbital) match between (tbt)2Ir(acac) and Bphen, and then com-bined with holes in the blue EML. Whereas, in device B, there is ahigh energy barrier of 0.6 eV for electrons to be transported intoEMLs, caused by the LUMO difference between mCP and Bphen,resulting in electrons being accumulated at the mCP/Bphen

J. Zhao et al. / Journal of Luminescence 132 (2012) 1994–19981996

interface. Similarly, in device C, holes passed from TAPC can befacilitated to transfer into the blue EML across the neat (tbt)2Ir(acac) layer as shown in Fig. 3(b), and then combined withelectrons in the blue EML. Whereas, in device D, a high energybarrier of 1.7 eV for holes is generated at TAPC/UGH2 interface,which hinders holes from transporting into the EMLs, althoughcharge transport via the dopant sites may exist. Therefore,recombination zone (RZ) can be effectively broadened in devicesA and C, while RZ is located at a narrow region near the yellowEML/ETL and HTL/yellow EML interfaces in devices B and D,respectively. In consequence, the charge carrier accumulationthat has a negative effect on device performance at these inter-faces can be greatly avoided in devices A and C [23]. Moreover, as

Fig. 2. Current efficiency–power efficiency–current density characteristics of

devices (a) A and B; (b) C and D.

Fig. 3. Energy level diagrams of de

for the host–guest doping system of the yellow EML in devices Band D, unexpected exchange energy loss induced from host todopant would be serious because of the large energy gap betweenthe host (mCP or UGH2) and (tbt)2Ir(acac), while such energy losscan be effectively eliminated in the host-free system of the yellowEML. On the other hand, the intermolecular distance in theultrathin layer is larger than a typical quenching radius, leadingto a small degree of molecular aggregation and concentrationquenching [7]. Thereby, higher efficiencies can be realized indevices A and C. Additionally, the cascade-type energy bandstructure for both holes from the anode and electrons from thecathode to EMLs has a significant influence on the enhancementof device performance [24]. Thus, the cascade-type structure indevices A and B enables more balanced charge carrier and bettersuppressed concentration quenching, resulting in lower efficiencyroll-off accordingly [25].

The external quantum efficiencies–current density (ZEQE–J)characteristics of the devices are shown in Fig. 4. It can be seenthat, high ZEQE close to the maximum out-coupling efficiency of20% [26] has been realized in devices A and C. Device A exhibits amaximum ZEQE of 18.77%, which is 1.48 times higher than that of12.66% for device B. Meanwhile, device C has a maximum ZEQE upto 19.04%, 1.25 times more than that of 15.21% for device D. Asthe spectral power distribution has a remarkable effect on deviceperformance [27], ZEQE is also dependent on the standard photo-pic efficiency function K(l), described by [24]

ZEQE ¼CZC

RlIðlÞdl

RIðlÞKðlÞdl

vices (a) A and B; (b) C and D.

Fig. 4. External quantum efficiencies–current density characteristics of devices A–D.

Fig. 6. UV–visible absorption and PL spectra of organic materials. The Abs and PL

spectra were measured using chloroform solution at room temperature, while PL

spectra of mCP and UGH2 were from Refs. [30,31].

J. Zhao et al. / Journal of Luminescence 132 (2012) 1994–1998 1997

where C is a constant relying on Plank’s constant, the velocity oflight and electron charge, ZC is the current efficiency, l is thewavelength, I(l) is the EL intensity, and K(l) has a maximum at555 nm and decreases rapidly toward either blue or red region. Itis inferred that higher ZEQE reached in devices A and C is partlydue to the higher ZC with respect to those of devices B and D.

3.2. EL characteristics of WOLEDs

Fig. 5 shows the normalized EL spectra of devices A–D at a biasof 6 V. For all the devices, a peak at 465 nm with a shoulder at494 nm arises from FIrpic emission, while a peak at 560 nm witha shoulder at 602 nm is from (tbt)2Ir(acac). In devices A and C, it isobserved that the intensity of blue and yellow light is comparable,although device A has moderately stronger yellow emission thanthat of device C. However, in the case of devices B and D, theyellow intensity overwhelms its blue counterpart, giving yellow-dominant emission. Commission Internationale de l’Eclairage(CIE) coordinates of the devices over a voltage range of 5–9 Vare shown in the inset of Fig. 5, while the CIE coordinates arevaried from (0.32, 0.41) to (0.34, 0.43) in device A, from (0.43,0.44) to (0.44, 0.45) in device B, from (0.30, 0.38) to (0.33, 0.40) indevice C, and from (0.48, 0.47) to (0.49, 0.48) in device D. The ELspectra variation upon the bias voltage can be explained firstly,because of the RZ shift [28]; secondly, due to field-dependentcompetition between charge carrier trapping on the emitters andundisturbed charge carrier transport across EMLs [29].

3.3. Emission mechanisms for WOLEDs

Fig. 6 shows the Abs and PL spectra of materials used fordevice fabrication. The Abs and PL spectra of the emitters weremeasured using chloroform solution at room temperature, whilethe PL spectra of the two hosts were from Refs. [30,31]. As for theAbs spectra of FIrpic, strong absorption bands below 350 nm areassigned to intraligand 1p–pn transitions, while two weak peaksof 380 nm and 410 nm are associated with 1MLCT and 3MLCT(where MLCT is metal-to-ligand charge transfer) [32], respec-tively, contributing to phosphorescence. Similarly, as the Absspectra of (tbt)2Ir(acac), two peaks resulting from 1MLCT and3MLCT appear at 409 nm and 450 nm, respectively. As is wellknown there are two emission mechanisms in the operatingOLEDs, namely, energy transfer and direct charge trapping [19].

Fig. 5. Normalized EL spectra of the four devices at a bias of 7 V. Inset: CIE

coordinates of the devices over a bias ranging from 5 to 9 V.

Fig. 6 shows that spectral overlap between the PL spectra of mCPand the Abs spectra of FIrpic is much larger than that betweenUGH2 and FIrpic, predicting more efficient energy transfer frommCP to FIrpic, while the latter is considered as the major source ofexciton formation in UGH2:FIrpic system [33]. On the other hand,it is obvious that the PL spectra of FIrpic partly overlaps the Absspectrum of (tbt)2Ir(acac), implying that natural energy transferfrom FIrpic to (tbt)2Ir(acac) occurs. As for devices A and C, RZcovers the host-free yellow EML, although it is mainly located atthe blue EML, and direct charge trapping on (tbt)2Ir(acac) sites issupposed to be the main mechanism for yellow emission. In thecase of devices B and D, RZ is mainly located at the yellow EML,contributing to the intensified yellow emission.

3.4. Influence of the host-free EML’s position on the emission color

In order to investigate the effect of host-free EML’s position onthe emission color of the devices, two additional devices werefabricated: device E, ITO/TAPC (40 nm)/(tbt)2Ir(acac) (1 nm)/mCP:8%FIrpic (20 nm)/Bphen (30 nm)/Mg:Ag(200 nm); and device F,ITO/TAPC (40 nm)/UGH2:8%FIrpic (20 nm)/(tbt)2Ir(acac) (1 nm)/Bphen (30 nm)/Mg:Ag(200 nm). The structure of device E differsfrom that of device A in the change of EMLs’ sequence, which isthe same situation for comparing devices F and C.

Fig. 7(a) and (b) shows the EL spectra of devices E and F at avoltage range of 5–9 V. With increasing voltage, the CIE coordi-nates moderately vary from (0.30, 0.38) to (0.28, 0.37) in deviceE, while the CIE coordinates vary from (0.20, 0.37) to (0.24, 0.38)in device F, as a consequence of the reasons aforementioned. Asseen, the blue intensity is more intensified than its yellowcounterpart in both devices E and F, and white emission cannotbe obtained. Given the hole transporting property of p-host mCPand the electron transporting property of n-host UGH2, RZ isexpected to be mainly located at the blue EML in the two devices,while the pure (tbt)2Ir(acac) layer is far away from RZ. Thus, it isnot favorable for the yellow emission through both energytransfer and direct charge trapping mechanisms, as a conse-quence of weak yellow intensity in the devices. Therefore, wecan deduce that the position of the host-free yellow EML has greatinfluence on the emissive color of the devices, by considering thecarrier transporting property of the host material.

Fig. 7. Normalized EL spectra of devices (a) E and (b) F over a bias ranging from 5

to 9 V.

J. Zhao et al. / Journal of Luminescence 132 (2012) 1994–19981998

4. Conclusions

In summary, we have introduced the concept of combininghost–guest doping system of the blue EML and host-free systemof the yellow EML to fabricate WOLEDs. This simple method issignificantly less demanding in terms of multilayer structure andaccurate manufacturing. Based on the device concept, the mCP-and UGH2-based WOLEDs successfully achieve maximum ZEQE of18.77% and 19.04%, respectively, which are among the highestones of WOLEDs without using out-coupling enhancement. Thedevice efficiencies are also superior to those of conventionalWOLEDs by using the doping system to form the yellow EML.The encouraging results represent a significant improvement ofWOLEDs, and we believe that the appropriate choice of otheremitters by using this strategy can also realize high-performancedevices.

Acknowledgments

This work was supported by the National Science Foundationof China (NSFC) (Grant no. 61177032), the Foundation for Inno-vative Research Groups of the NSFC (Grant no. 61021061), theFundamental Research Funds for the Central Universities (Grantno. ZYGX2010Z004), SRF for ROCS, SEM (Grant no. GGRYJJ08-05),and Doctoral Fund of Ministry of Education of China (Grant no.20090185110020).

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