Thin Solid Films 520 (2012) 4003–4007

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High-efficiency white organic light-emitting devices with a non-doped yellowphosphorescent emissive layer

Juan Zhao, Junsheng Yu ⁎, Xiao Hu, Menghan Hou, Yadong JiangState 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

⁎ Corresponding author. Tel.: +86 28 83207157.E-mail address: jsyu@uestc.edu.cn (J. Yu).

0040-6090/$ – see front matter © 2012 Elsevier B.V. Alldoi:10.1016/j.tsf.2012.01.006

a b s t r a c t

a r t i c l e i n f o

Article history:Received 21 July 2011Received in revised form 31 December 2011Accepted 9 January 2012Available online 13 January 2012

Keywords:White organic light-emitting devicesPhosphorescent emittersDoping blue systemNon-doping yellow system

Highly efficient phosphorescent white organic light-emitting devices (PHWOLEDs) with a simple structure ofITO/TAPC (40 nm)/mCP:FIrpic (20 nm, x wt.%)/bis[2-(4-tertbutylphenyl)benzothiazolato-N,C2′] iridium(acetylacetonate) (tbt)2Ir(acac) (y nm)/Bphen (30 nm)/Mg:Ag (200 nm) have been developed, by insertinga thin layer of non-doped yellow phosphorescent (tbt)2Ir(acac) between doped blue emitting layer (EML)and electron transporting layer. By changing the doping concentration of the blue EML and the thicknessof the non-doped yellow EML, a PHWOLED comprised of higher blue doping concentration and thinner yel-low EML achieves a high current efficiency of 31.7 cd/A and Commission Internationale de l'Eclairage coordi-nates of (0.33, 0.41) at a luminance of 3000 cd/m2 could be observed.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

White organic light-emitting devices (WOLEDs) have attractedconsiderable attention because of versatile applications in displays,solid state lighting, backlight sources for flat panel displays and soon. It is well known that white light can be realized by mixing threeprimary colors red–green–blue, or two complementary colors suchas blue-yellow, using phosphorescent or fluorescent materials,among which phosphorescent WOLEDs (PHWOLEDs) have attractedmore attention due to a maximum internal quantum efficiency of100% [1–5]. Although the highest reported efficiencies have beenachieved in PHWOLEDs by employing multilayer system as well asdoping system, the device structures are complicated. For example,several widely used device configurations are proposed as: (i) singledoped emitting layer (EML) with multiple emitters [3,4], (ii) multiplesingle-doped EMLs [5,6], (iii) p-i-n structure [7] and so on. Noticeably,a very low doping concentration (~0.4%–2%) of red, yellow or greendopant is usually required, considering natural energy transfer be-tween dopants from that with higher energy to lower. However, it re-mains technologically difficult to manipulate the low dopingconcentration accurately, resulting in low reproducibility and highproduction cost. Thus, it is of great necessary to develop efficientWOLEDs with more simplified structure.

Recently, to solve the problems above, Lee et al. have reported aPHWOLED with a host-free, yellow phosphorescent material dispersedbetween two blue EMLs [8], eliminating time consumption to control

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the low doping concentration of the yellow emitter. However, theirtwo triple-doped blue EMLs, seven organic layers in all, together withadditional introduction of light-extraction film and modification ofhost material, make the manufacture process still complex.

In this study, we will introduce a simple method for high-efficiency PHWOLEDs, most importantly, without sacrificing the de-vice performance. The key to the device concept is the combinationof doping and non-doping systems into one PHWOLED. The devicesare optimized in terms of doping concentration and thickness of theEMLs, and a highly efficient PHWOLED with relatively stable white-emission is achieved. Examples that have reported about this ap-proach are very rare in the literature.

2. Experimental details

All the devices were deposited on pre-cleaned indium tin oxide (ITO)glass substrates, followed by oxygen plasma treatment, then the organicand metallic layers were deposited at a pressure on the order of magni-tude 10−4 and 10−3 Pa, respectively. A 40-nm 1,1-bis[(di-4-tolyla-mino)phenyl]cyclohexane (TAPC) was used as both hole transportinglayer and exciton blocking layer. Then, a 20 nm thick blue EML was con-structed by doping blue phosphorescent bis[(4,6-difluorophenyl)-pyridi-nato-N,C2′](picolinate) iridium (III) (FIrpic) into a carbazole-based hostN,N’-dicarbazolyl-3,5-benzene (mCP). A thin layer of non-doped yellowphosphorescent bis[2-(4-tertbutylphenyl)benzothiazolato-N,C2′] iridium(acetylacetonate) [(tbt)2Ir(acac)] synthesized by our laboratory [9,10]was dispersed between the blue EML and 4,7-diphenyl-1,10-phenan-throline (Bphen), which functioned as electron transporting layer (ETL)and hole blocking layer. And finally, Mg:Ag alloy was used as cathode.Electroluminescent (EL) spectra and Commission Internationale de

Table 1Structures of the single carrier devices.

Device type Sample Bottom layer Middle layer Top layer

Hole-only S1 TAPC (40 nm) mCP (20 nm) TAPC (10 nm)Hole-only S2 TAPC (40 nm) mCP:5%FIrpic

(20 nm)TAPC (10 nm)

Electron-only S3 Bphen (10 nm) mCP (20 nm) Bphen (30 nm)Electron-only S4 Bphen (10 nm) mCP:5%FIrpic

(20 nm)Bphen (30 nm)

Table 2Characteristics of the PHWOLEDs. Von, L, ηC, ηP present turn-on voltage, luminance, cur-rent efficiency, power efficiency, respectively.

Device mCP: x%FIrpic

(tbt)2Ir(acac)(y nm)

Von (V) Maximum L(cd/m2)@voltage (V)

MaximumηC (cd/A)

MaximumηP (lm/W)

A 3 0.8 3.3 34613@12.8 21.9 10.6B 5 0.8 3.2 31760@11 28.1 13.8C 8 0.8 3 36841@10.8 31.7 16.3D 8 1.4 3.2 30386@12.4 12.6 6.1E 8 2 3.3 34838@12.4 8.0 4.3

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l'Eclairage (CIE) coordinates of un-capsulated devices were measuredwith an OPT-2000 spectrometer and luminance-current density–voltagecharacteristics were recorded with Keithley 4200 semiconductor source.

The PHWOLEDs had a simple structure of ITO/TAPC (40 nm)/mCP:FIrpic (20 nm, x%)/(tbt)2Ir(acac) (y nm)/Bphen (30 nm)/Mg:Ag(200 nm). While the thickness of the yellow EML y was kept at0.8 nm, the doping concentration of the blue EML x varied from 3%(device A), 5% (device B) to 8% (device C), respectively. On theother hand, while the doping concentration of the blue EML x wasfixed at 8%, the thickness of the yellow EML y was further increasedto 1.4 nm (device D) and 2 nm (device E), and the device parameterswere shown in Table 2. Moreover, the molecular structures of the or-ganic materials and energy diagram of the devices are shown in Fig. 1.

3. Results and discussion

Fig. 2 shows the luminance–current density–voltage characteristics(L–J–V) of devices A-E, and the detailed characteristics are summarized

Fig. 1. Molecular structures of the organic mat

in Table 2. It can be seen that, the current density is increased as FIrpicdoping concentration increasing from 3%, 5% to 8% in devices A, B, C.Meanwhile, the luminance is gradually enhanced, such as, the high lu-minance of 18,550, 28,100, 36,350 cd/m2 are obtained at a bias of10 V, respectively. As well known, for charge transport in the doped de-vices, hopping mechanism plays a main role, and the hopping is facili-tated at high doping concentration [11,12]. To investigate the role ofFIrpic molecules on the current density, single carrier devices are fabri-cated as shown in Table 1. Fig. 3 shows the current density-voltagecharacteristics of devices S1–S4. We can see that, devices S2 and S4with FIrpic doped mCP have higher current density than that of S1and S3 with puremCP layer. The results confirm that FIrpic can providean additional channel to facilitate charge transport [13] in the EML,which can also be inferred from its highest occupied molecular orbital(HOMO)/lowest unoccupied molecular orbital (LUMO) levels shownin Fig. 1. Therefore, higher current density is available formore radiativerecombination, contributing to higher luminance [14].

Simultaneously, for devices C, D and E as shown in Fig. 2, as thethickness of (tbt)2Ir(acac) layer increases from 0.8 to 1.4 and 2 nm,respectively, the current density shows steady increment. As theneat layer grows thicker, it functions more like ETL deduced fromwell matched LUMO level between (tbt)2Ir(acac) and Bphen asproved in Fig. 1, showing an electron-transporting characteristic.Consequently, higher current density is achieved with thicker(tbt)2Ir(acac) layer. Whereas, the luminance is decreased with in-creasing the thickness of (tbt)2Ir(acac), the luminance of 18,250 and25,090 cd/m2 are obtained at 10 V for devices D and E, respectively.The lower luminance is ascribed to the notorious concentrationquenching effect, resulting from intermolecular dipole–dipole inter-action caused by short distance between the emitter molecules [15].

Fig. 4(a) shows the current efficiency–power efficiency–currentdensity (ηC–ηP–J) characteristics of devices A, B, and C. The efficiencyis increased with increasing FIrpic doping concentration, while theconcentration quenching effect [16] is expected to be serious if theconcentration continues to increase. Compared to devices A, B,and C, the efficiencies of devices D and E shown in Fig. 4(b) are obvi-ously reduced. The lower efficiency is attributed to the concentrationquenching effect as mentioned above. It can be evidenced from nor-malized EL spectra of the two representative devices C and D overrange from 5 V to 10 V, as shown inset in Fig. 4(a) and (b),respectively.

erials and energy diagram of the devices.

Fig. 2. Luminance–current density–voltage characteristics of devices A–E.

Fig. 4. Current efficiency–power efficiency–current density characteristics of (a) devicesA, B and C. Inset: EL spectra of device C; (b) devices D and E. Inset: EL spectra of device D.

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It can be clearly seen from the EL spectra, the yellow intensity in de-vice D is indeed sufficiently weakened with respect to that in device C.In addition,we can learn from the spectra versus applied voltage, for de-vice C, the yellow intensity is intensified with increasing the voltage,while it is decreased in device D. In aspect of the electric field, weknow that most voltage drop is in the ETL with increasing the field (asdiscussed later), resulting in more charge carriers in the (tbt)2Ir(acac)layer. Therefore, the concentration quenching effect will be more seri-ous in devices D and E with thicker (tbt)2Ir(acac) layer, while it can berelatively limited in devices with a partial layer formed by 0.8 nm(tbt)2Ir(acac). This is another proof for higher efficiency in device C in-stead of device D. Of particular note is that the efficiency also dependson the eye sensitivity of spectra, which is higher in the yellow regionthan that in the blue [17]. Thereby, the efficiencies are reasonably re-duced in devices D and E. Additionally, both EL spectra vary slightly atlower voltages of 5, 7, and 8 V, while moderate spectra variations ap-pear at higher voltage than 10 V. At a practical luminance of 3000 cd/m2, the CIE coordinates of devices C and D are (0.33, 0.41) and (0.27,0.38), respectively, corresponding to warmish- and coldish-whiteemission.

Furthermore, it can be inferred that all the devices exhibit a rela-tively low efficiency roll-off over range from a low Jwhere the highestefficiency is obtained, to a high J of 200 mA/cm2 that is equally corre-sponding to high luminance of 27,300, 30,800, 36,400 18,250,22,240 cd/m2 for devices A to E, respectively. Whereas, the similar de-vice in Ref. [8] revealed low efficiency roll-off over the current densityrange from low to high value of only 40 mA/cm2 (10000 cd/m2).

Fig. 3. Current density–voltage characteristics of the hole-only and electron-only devices.

Thus, it implies that the main degradation mechanisms [18,19], in-cluding triplet–triplet annihilation, triplet–polaron quenching andelectric field induced dissociation of excitons are suppressed to takea minor effect on device performance.

For comparison with device C, PHWOLED based on doped yellowEML instead of non-doped is also fabricated (not shown here), and ayellow-dominated WOLED with lower efficiency is gained (the resultswill be shown elsewhere). Apparently, our WOLEDs based on the com-bination of doping and non-doping systems outperform its convention-al full-doping counterparts. Reasonable explanations for high efficiencyof device C will be addressed from two aspects as follows.

It is generally known that there are two mechanisms for excitonformation and emission, namely, energy transfer and direct chargetrapping [20]. As shown in the Fig. 1, the former dominates in mCP:FIrpic layer (pathway I), in which doping system a maximum photo-luminescence quantum efficiency of 100% had been reported [21].Taking energy level differences between mCP and FIrpic into consid-eration, direct charge trapping on FIrpic (pathway II) occurs, whichcan compensate for the consumed blue emission resulted from ener-gy transfer to yellow emitter via Förster or Dexter process [22]. On theother hand, the highly efficient later one [23,24] for exchange energyloss induced from host to dopant is eliminated [25], makes significantcontribution in neat (tbt)2Ir(acac) layer (pathway III). Moreover, it isof great importance that the intermolecular distance in very thin neatemitter layer (b1 nm) is larger than typical quenching radius (~1–1.3 nm), leading to a small degree of molecular aggregation and con-centration quenching [26]. In addition, high triplet energy of TAPC(2.96 eV) and Bphen (2.59 eV) [27] can effectively prevent excitonsgenerated in the blue and yellow EMLs, respectively, from driftingand diffusing outside where quenching centers might exist [28].

Fig. 5. Logarithmic carrier mobility of materials versus square root of the electric field.Inset: Fitting results by Poole–Frenkel expression.

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On the other hand, from the perspective of electric field in a thindevice, sharply increasing electric field [29] is highly sensitive to theapplied voltage. Then the field dependent carrier mobility of organicmaterial μ(E) will change differently from each other, redistributinginternal non-uniform electric field [30]. Prior to the following analy-sis, we propose that the effect of partial layer formed by 0.8-nmthick (tbt)2Ir(acac) is ignored [31] while the influence of FIrpic inthe EML is assumed to be minimized for convenience. Carrier mobilityof TAPC, mCP and Bphen is taken from literatures [32,33] as shown inFig. 5, and the data are fitted using Poole-Frenkel expression given as[34]:

μ Eð Þ ¼ μ0 exp γffiffiffiE

p� �

where E is electric field, μ0 is zero mobility, γ is exponential factor thatcan be understood as slope of lnμ∝E½ linear curves. It demonstratesthat the hole mobility of TAPC (μTAPC-h) is the highest, directly indicat-ing that the EML and ETL, either individually or in conjunction,accounted for most of the voltage drop [29].

Fig. 6 shows a schematic view of the internal electric potential.Given that each organic layer can be considered as a resistor con-nected in series [35], an equivalent circuit model is adopted. Assoon as the device is forward biased, TAPC layer can reach a flat-band condition, together with a zero internal electric field related todrastically reduced resistance, just like N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine (NPB) layer in forward biasedNPB/ tris(8-hydroxyquinolato)aluminium (Alq3) hetero-device [35].

At low electric field, the main voltage drop is in the EML as seen inFig. 6(a), due to higher electron mobility of Bphen (μBphen-e) de-scribed in Fig. 5, making the field discontinuous at EML/Bphen inter-face, and carrier recombination zone is expected to extend the wholeEML. When the field increases, the carrier mobility of mCP that have

Fig. 6. Electric potential inside the device at (a) low electricfield and (b) high electricfield.

stronger dependence on E increase more rapidly deduced from highervalue of γ as showed inset in Fig. 5. Then, the flat-band condition willbe attained in the EML with increasing the field, and then the voltagedrop is completely in the ETL shown as Fig. 6(b), which can effectivelyenhance electron injection and transport [36]. Interestingly noticethat, the electron mobility of mCP (μmCP-e) has become higher thanits hole mobility (μmCP-h), implying mCP somewhat a bipolar host.Therefore, all contribute to an improved electron–hole balance sinceholes are in excess for the device.

4. Conclusions

In summary, the utilization of combined doping and non-dopingsystems allows us to obtain simple PHWOLEDs with high efficiencyand low efficiency roll-off. The manufacturing process is simplified interms of not only removing the controlling of a low doping concentra-tion required for conventional full-doping device, but also withoutemploying any charge carrier injection lays or metal-organic dopinglayers. A PHOLED consisting of higher blue doping concentration andthinner yellow EML possesses a maximum luminance of 36,840 cd/m2

and current efficiency of 31.7 cd/A. Based on our analysis, it revealsthat high device performance can be procured by using efficient emit-ters and improving carrier balance in the devices. We believe that thistechnology is a remarkable advance to provide an avenue for realizingefficient WOLED.

Acknowledgments

This work was supported by the National Science Foundation ofChina (NSFC) (Grant No. 60736005 and 60425101–1), the Foundationfor Innovative Research Groups of the NSFC (Grant No. 61021061), theFundamental Research Funds for the Central Universities (Grant No.ZYGX2010Z004), SRF for ROCS, SEM (Grant No. GGRYJJ08-05), DoctoralFund of Ministry of Education of China (Grant No. 20090185110020).

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