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Synthetic Metals 161 (2011) 2417– 2421

Contents lists available at SciVerse ScienceDirect

Synthetic Metals

journa l h o me page: www.elsev ier .com/ locate /synmet

ptimization of yellow phosphorescent organic light-emitting devices based onriplet exciton diffusion length

uan Zhao, Junsheng Yu ∗, Zhu Ma, Lu Li, Yadong Jiangtate Key Laboratory of Electronic Thin Film and Integrated Devices, School of Optoelectronic Information, University of Electronic Science and Technology of China,hengdu 610054, China

r t i c l e i n f o

rticle history:eceived 26 June 2011eceived in revised form 1 September 2011ccepted 9 September 2011vailable online 1 October 2011

a b s t r a c t

Study on the diffusion of triplet excitons from light emitting layer (EML) in organic light-emitting devicesdoped with yellow phosphorescent material to the adjacent hole transporting layer (HTL) is carried out, inwhich a non-doped red phosphorescent ultra-thin layer as an exciton-sensing layer is set in various posi-tion of HTL. A diffusion length of triplet exciton between 3 nm and 5 nm is inferred from the observed and

eywords:rganic light-emitting devicesellow phosphorescentriplet exciton diffusion lengthxciton-sensing layer

disappeared red light emission in electroluminescent spectra. For further device optimization, either 5 nm4,4′,4′′-tri(N-carbazolyl)triphenylamine (TCTA) or 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC)as a exciton blocking layer is introduced to block the exciton diffusion. The maximum current efficiencyand power efficiency of the optimized device are reached to 24.6 cd/A and 16.3 lm/W, respectively. Thehigh performance is ascribed to confined diffusion of triplet excitons from light-emitting zone and higherradiation efficiency of the triplet exciton.

. Introduction

Organic light-emitting devices (OLEDs) have been widely usedn flat panel display, and especially for lighting in the currentrend of advocating eco-friendly lifestyle [1–3]. High efficiency haseen achieved in contrast to the initial with a power efficiency of.5 lm/W by Tang and VanSlyke [4]. It is well known that there

s two types of excitons, i.e., singlet and triplet, can be createdpon charge carrier recombination during light-emitting process

n OLEDs. As for phosphorescent OLEDs, phosphorescent dopantentered by a heavy metal atom, of which iridium is the most com-only employed, is introduced to make use of both singlet and

riplet excitons for emission. Dexter energy transfer between hostriplets and dopant triplets predominates through hopping amongeighboring molecules by electron-exchange [5–7], and the former

s intensely required to have higher triplet energy than that of theater. The triplet energy of both hole transporting layer (HTL) andlectron transporting layer (ETL) has a great effect on device per-ormance and should be high enough to confine excitons in themitting layer (EML) [8]. Otherwise, the triplet excitons in the EMLre expected to be lost by diffusion out into HTL or ETL due to their

ong diffusion length caused by long lifetime, from microsecond to

illisecond range [9,10].

∗ Corresponding author.E-mail address: jsyu@uestc.edu.cn (J. Yu).

379-6779/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.synthmet.2011.09.018

© 2011 Elsevier B.V. All rights reserved.

Recently, Kim et al. [11] reported that not all excitons can beconfined in EML even the generation zone is located in the EML,and excitons should be distributed with certain dispersion aroundan average position resulted from its certain diffusion length. Thediffusion length of triplet exciton has been widely investigated[12,13]. For phosphorescent OLEDs with the doping system havethe highest device performance [14,15], when the diffusion lengthof triplet exciton is to be measured, the sensitive layer and dop-ing system should also be phosphorescent materials. In this work,we investigated the diffusion length of triplet exciton from yellowphosphorescent EML to adjacent HTL by using a 0.3 nm red phos-phorescent ultra-thin layer as exciton-sensing layer. The evidencefor the diffusion of triplet excitons and energy loss was obtained.Based on the results we had got, exciton blocking layer (EBL) suchas either TCTA or TAPC with higher triplet energy was incorporatedto confine the excitons diffusion, facilitate the triplet exciton radia-tion and reduce the energy loss. High performance of the optimizeddevices had been achieved.

2. Experimental

All the devices are fabricated on pre-cleaned indium–tin–oxide(ITO) substrate, and then treated by oxygen plasma for 5 min toincrease the work function prior to the deposition of organic layers.

Organic and metallic layers are subsequently deposited at a pres-sure on the order of magnitude 10−4 and 10−3 Pa, respectively. N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)-benzidine (NPB) is usedas both HTL and host in the EML, where yellow phosphorescent

2418 J. Zhao et al. / Synthetic Metals 161 (2011) 2417– 2421

cture

blut[taE((3dot((cclTdtr

3

baa3Efdiedrg

ieto[saomdomc

diffusion length, D is the diffusion coefficient, and � is the lifetimefor excitons.

Even though the excitons formed at HTL/EML interface should bemuch less than that at EML/ETL interface deduced from Eq. (1), this

Fig. 1. The chemical stru

is[2-(4-tertbutylphenyl)benzothiazolato-N,C2′] iridium (acety-

acetonate) [(tbt)2Ir(acac)] synthesized by our laboratory [16] issed as the dopant. To study the diffusion length of triplet exci-on, the red phosphorescent material tris(1-phenylisoquinoline)Ir(piq)3] as exciton-sensing layer is inserted in the HTL, and thehickness is 0.3 nm to reduce its influence on the device as muchs possible. 4,7-Diphenyl-1-10-phenanthroline (Bphen) is used asTL, and Mg:Ag alloy as cathode. The device structure is ITO/NPB40 − d nm)/Ir(piq)3 (0.3 nm)/NPB (d nm)/NPB:(tbt)2Ir(acac)5%, 10 nm)/Bphen (30 nm)/Mg:Ag (200 nm), where d = 0, 1,, 5, 8, 10 corresponding to devices A1–A6. In optimizedevices, either 4,4′,4′′-tri(N-carbazolyl)triphenylamine (TCTA)r 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) func-ioned as EBL is introduced, with a structure of ITO/NPB40 − y nm)/EBL(y nm)/NPB:(tbt)2Ir(acac) (5%, 10 nm)/Bphen30 nm)/Mg:Ag (200 nm), where y = 0, 5-nm TAPC, 5-nm TCTA,orresponding to devices A–C. The chemical structure of typi-al materials used in this work is shown in Fig. 1. The energyevel structure for the two kinds of devices is shown in Fig. 2.he electroluminescent (EL) spectra of the un-capsulatedevices are measured with an OPT-2000 spectrometer andhe luminance–current density–voltage (L–J–V) characteristics areecorded with Keithley 4200 source.

. Results and discussion

Fig. 3a shows the normalized EL spectra of devices A1–A6 at aias voltage of 4 V. Obvious red emission from Ir(piq)3 with a peakt 615 nm [17], and yellow emission from (tbt)2Ir(acac) with a peakt 560 nm can be observed in devices A1–A3, where d is less than

nm. As shown from Fig. 2a, the NPB layer on the right side is theML. The red intensity decreases as Ir(piq)3 is placed further awayrom HTL/EML interface, which contributes to 95%, 84% and 54% forevices A1–A3, respectively, by integrating the EL spectra, as shown

n the inset of Fig. 3a. On the other hand, only pure (tbt)2Ir(acac)mission with a peak at 560 nm and a shoulder at 603 nm is seen inevices A4–A6, where d is larger than 5 nm. The specific reason fored light emission is unclear currently, and herein we would like toive a tentative explanation.

The recombination of electrons and holes will create excitonsn neutral bound state [18,19], consisted of both singlet and tripletxcitons. As the hole mobility of NPB is at least two times higherhan the electron mobility of Bphen [20], all exciton formationccurs at EML/ETL interface inferred by the model Forrest et al.9] had proposed. The singlet excitons formed would decay tran-iently because of short lifetime in nanosecond range [10], whichre several orders shorter than that of the triplet excitons. On thether hand, the triplet excitons with longer diffusion length areore prone to diffuse if there is no energy barrier. Moreover, Kon-

akova et al. [21] had evidenced from the time-resolved EL basedn their triplet-harvesting hybrid OLEDs, it was the triplet excitonigration through the sensor layer that contributed to phosphores-

ent emission. The distribution density of excitons n(x) is described

of some materials used.

to be exponentially decreased in the 10 nm EML by the followingequations [22]:

n(x) = n0 exp(

− x

LD

)(1)

LD =√

D� (2)

where n0 is the exciton density in the exciton generation zone, xis the distance from the exciton generation zone, LD is the exciton

Fig. 2. Energy level structure of (a) devices with exciton-sensing layer and (b)devices with EBL.

J. Zhao et al. / Synthetic Metals 161 (2011) 2417– 2421 2419

F ssion

h

pEdEd[eae

taeitcff(metOAf

ig. 3. (a) EL spectra of devices A1–A6 (inset: intensity ratio of red and yellow emiole trapping effect by Ir(piq)3 when d ≥ 5 nm (right).

art of triplet excitons have a higher possibility to diffuse outsideML and extend to a certain distance in HTL. In addition, the excitoniffusion will be facilitated by energy barrier-free between HTL andML. It should be noted that the diffusion length of triplet excitonsepends on many factors, such as property and purity of materials10], �–� stacking of molecules [23], aggregation and crystal ori-ntation [24]. It’s different from each other, which can be as larges a few micrometers [25] or only a few nanometers [26,27], it isven different from the same material reported elsewhere [28–30].

It is confirmed from EL spectra in Fig. 3a that, for devices A1–A3,he triplet excitons at HTL/EML interface can diffuse into the HTLnd then captured by Ir(piq)3 molecules, leading to red emission bynergy transfer. Leo et al. had suggested that longer triplet lifetimen NPB than in Ir(piq)3 leaved plenty of time for NPB excitons toravel before they decay [31]. Thus, when the sensor layer is placedlosed to the EML, the dominated red emission is barely resultedrom singlet excitons trapped by the sensor layer but from the dif-usion of triplet excitons. In addition, the triplet excitons of yellowtbt)2Ir(acac) would also naturally diffuse toward Ir(piq)3 layer and

ake contribution to the red emission, resulting from lower tripletnergy at longer wavelength for Ir(piq)3. The schematic diagram of

he exciton diffusion when d ≤ 3 nm is shown on the left in Fig. 3b.n the other hand, no red emission can be observed in devices4–A6, indicating that the triplet excitons at HTL/EML interface

ail to migrate further than 5 nm. By above knowable, the diffusion

vs. d) and (b) Schematic diagram for the exciton diffusion when d ≤ 3 nm (left) and

length of triplet exciton from EML to HTL is inferred to be larger than3 nm and shorter than 5 nm in our devices. As aforementioned, thediffusion length depends on many factors, and it’s even greatly dif-ferent for the same material reported by different researchers. LikeNPB, the diffusion length of another hole transporting material a-NPD had reported to be 2.6 nm [32]. Even the diffusion length weobtained was short with respect to some others, it could provideimportant evidence that the diffusion of triplet excitons should becarefully considered to optimize device structures.

Fig. 4 shows the luminance–current density–voltage (L–J–V)characteristics of devices A1–A6. The current density of devicesA1–A3 is higher than that of devices A4–A6, whereas the J–V profileof the former three devices is less different from each other thanthat of the latter three ones, implying that the Ir(piq)3 layer has anobvious effect on carrier transport when d ≥ 5 nm. As known fromthe energy level in Fig. 2a, Ir(piq)3 molecules act as hole traps in NPBbecause of its higher-lying HOMO (the highest occupied molecularorbital) level of 5.1 eV compared to 5.5 eV for NPB, leading to thereduced hole dominated transport. From the L–J characteristics wecan see that devices A4–A6 show higher luminance performance,in which the carrier balance is likely to be improved as a result

of the hole trapping effect by Ir(piq)3 [33–35]. The schematic dia-gram of the hole trapping effect by Ir(piq)3 when d ≥ 5 nm is shownon the right in Fig. 3b. Secondly, we can infer from above thathole trapping effect by Ir(piq)3 is more effective to improve charge

2420 J. Zhao et al. / Synthetic Metals 161 (2011) 2417– 2421

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Fe

Table 1Characteristics of devices A, B, C. Von, Lmax, �L, �P present turn-on voltage, the max-imum luminance, luminance efficiency, power efficiency, respectively.

Device Von (V) Lmax (cd/m2) @ voltage (V) �L (cd/A) �P (lm/W)

A 2.8 26,240 @ 11.6 11a 7.7a

9.5b 7.5b

9.2c 5.3c

B 3.0 31,400 @ 12.8 17.3a 11.4a

15.3b 10.2b

15.1c 8.0c

C 2.8 33,300 @ 13.6 24.6a 16.3a

18.8b 12.6b

23.3c 12.9c

a The maximum value obtained.b The value obtained at 1000 cd/m2.c The value obtained at 10,000 cd/m2.

ig. 4. Luminance–current density–voltage characteristics of the devices A1–A6.

arrier balance when it is located next to the anode where holesre injected originally, and this could possibly provide an approacho improve charge balance of devices where holes are the majorityharge carriers.

Triplet exciton can diffuse a certain distance away from EMLased on our results above, and then a loss in terms of device perfor-ance occurs. Therefore, exciton blocking layer is usually needed

o confine excitons in the EML. Here, both TCTA (ET = 2.86 eV) and

APC (ET = 2.96 eV) [21] are introduced as EBL between HTL andML to optimize our devices. Fig. 5a and b shows the L–J–V anduminance efficiency–power efficiency–current density (�L–�P–J)

ig. 5. (a) Luminance–current density–voltage and (b) Luminance efficiency–powerfficiency–current density characteristics of devices A, B, C.

characteristics of device A (without EBL, as standard device),respectively, compared with the optimized devices B (TCTA as EBL)and C (TAPC as EBL). For details, the main characteristics of thesedevices are listed in Table 1.

Fig. 5a indicates that the current density of optimized devicesB and C are reduced in contrast to conventional device A, which isprobably due to the introduced hetero-junction [36] after employ-ing EBL. Meanwhile, the maximum luminance is increased from26,240 cd/m2 for device A to 31,400 cd/m2 and 33,300 cd/m2 fordevices B and C, resulting in an enhancement of 20% and 27%,respectively. Fig. 5b reveals that when the current density is35 mA/cm2, the �L is increased from 10.3 cd/A for device A to17.2 cd/A and 24.6 cd/A for B and device C, showing the improve-ment of 57% and 124%, respectively. Meanwhile, the �P is increasedfrom 7.6 lm/W to 11.1 lm/W and 14.7 lm/W for devices A to B andC. In addition, high efficiency can still be maintained in devices Band C, even though the luminance is enlarged from 1000 cd/m2 to10,000 cd/m2 provided in Table 1. Therefore, greater improvementis realized when EBL is incorporated, especially for TAPC-baseddevice. The reasons are given as follows: firstly, higher tripletenergy of EBL to confine excitons in the EML, 2.96 eV for TAPCcompared to 2.86 eV for TCTA. Secondly, higher hole mobility tobenefit hole transport, 1.0 × 10−2 cm2 V−1 s−1 [8] for TAPC com-pared to 1.9 × 10−4 cm2 V−1 s−1 [37] for TCTA. Thirdly, higher LUMO(the lowest unoccupied molecular orbital) level can block elec-trons from entering the HTL, 2.0 eV for TAPC compared to 2.3 eV forTCTA. On the whole, the recombination efficiency of holes and elec-trons is consequently increased so that high device performance isobtained.

4. Conclusion

In conclusion, we have investigated the diffusion of triplet exci-tons from EML to HTL based on yellow phosphorescent OLEDs, inwhich a non-doped red phosphorescent Ir(piq)3 as the exciton-sensing layer is inserted in the HTL with a variable distance (d)away from HTL/EML interface. The diffusion of triplet excitons isconfirmed by analysis of EL spectra: red emission is observed whend ≤ 3 nm, while it disappears when d ≥ 5 nm. The results provide anexplanation for why EBL like TAPC or TCTA is needed to make device

optimization, and great improvement of device performance havebeen achieved. The highest performance is obtained with TAPC asEBL, showing the lowest Von of 2.8 V, the highest luminance of33,300 cd/m2 at 13.6 V and the highest efficiency of 23.3 cd/A at10,000 cd/m2. The method we take in this work will pave a way tooptimize the performance of other OLEDs and get a bright future.

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cknowledgments

This work was partially supported by the National Science Foun-ation of China (NSFC) (Grant No. 60736005 and 60425101-1), theoundation for Innovative Research Groups of the NSFC (Grant No.1021061), the Fundamental Research Funds for the Central Uni-ersities (Grant No. ZYGX2010Z004), SRF for ROCS, SEM (Granto. GGRYJJ08-05), Doctoral Fund of Ministry of Education of China

Grant No. 20090185110020).

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