Improving the performance of blue phosphorescent organic
light-emitting devices using a composite emitter
Miao-Tsai Chu
a,b, Meng-Ting Lee
a,*, Chin H. Chen
c, Mei-Rurng Tseng
a aMaterial and Chemical Research Laboratories, Industrial Technology Research Institute (ITRI), Hsinchu 310, Taiwan
b
Master Degree Program of Flat Panel Display Technology, National Chiao Tung University, Hsinchu 300, Taiwan
cDisplay Institute, Microelectronics and Information Systems Research Center, National Chiao Tung University, Hsinchu 300, Taiwan
a r t i c l e
i n f o
Article history: Received 26 April 2009
Received in revised form 6 June 2009 Accepted 6 June 2009
Available online 10 June 2009
PACS: 73.50.Gr 73.61.Ph 78.60.Fi 85.60.Jb Keywords: OLED Phosphorescent Blue emitter Carrier transport
a b s t r a c t
A composite emitter is constructed by doping a carrier-transporting material into a con-ventional emitter composing of only host and dopant. The transport of carriers from either hole- or electron-transporting layer into the emitter can be promoted through the carrier-transporting material, in particular, when a wide-band-gap host material is used. A blue phosphorescent OLED based on iridium(III)bis((4,6-difluorophenyl)-pyridinate-N,C20
)-pico-linate (FIrpic) as dopant in the composite emitter achieved a power efficiency of 20 lm/W and a low driving voltage of 4.2 V at 1000 cd/m2, whose current efficiency at 20 mA/cm2 was 2.5 times better than that of device using the conventional emitter.
Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
Recently, many reports have been disclosed that the efficiency of white organic light-emitting devices (OLEDs) is already superior to that of traditional lighting sources of incandescent bulb and fluorescent lamp[1–9]. These re-sults can mainly be attributed to the progressive develop-ment of phosphorescent materials, which utilize both singlet and triplet excitons for generating light emission. Therefore, OLEDs based on phosphorescent emitters can, theoretically, achieve internal quantum efficiency (IQE) up to 100%[10,11].
A blue emitter is indispensable component for generat-ing white light and its device performance is critical to that of white OLEDs. To achieve efficient blue phosphorescent
emission, a high triplet excited state (>2.8 eV) is essential in order to prevent the quenching of the dopant emission [12]. However, a high triplet excited state implies a large electrical band gap, i.e., a low-lying highest occupied molecular orbital (HOMO) and/or high-lying lowest occu-pied molecular orbital (LUMO) level, that will act as energy barriers for the transport of carriers from nearby hole (HTL) or electron-transporting layer (ETL) to emitter, which consequently decrease the probability of carrier recombination and increase the driving voltage. Several or-ganic carbazole-based materials have been proposed to be as blue phosphorescent host [13–16]. Those devices re-sulted in good quantum efficiency but a relatively high driving voltage, close to 7 V at practical brightness of 1000 cd/m2, which is due to the energy barriers created be-tween carrier-transporting layers and the host.
Recently, it was reported that blue PHOLEDs operated at practical brightness of 1000 cd/m2 with a rather low
1566-1199/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2009.06.005
* Corresponding author.
E-mail address:[email protected](M.-T. Lee).
Contents lists available atScienceDirect
Organic Electronics
driving voltage of 5 V can be achieved by using diphos-phine oxide derivatives as host[17–19]. The low driving voltage can be rationalized by the following two reasons. One is the intrinsic good electron-transporting ability of diphosphine oxide derivatives, which can be helpful to the transport of electrons in the bulky organic layer. The other contribution comes from the reduction of the energy barrier between diphoshpine oxide derivatives and LiF/Al cathode since these materials have a low LUMO level of 2.9 eV. However, the diphosphine oxide derivatives have a low-lying HOMO level of 6.6 eV, which causes a large en-ergy barrier for the transport of holes from nearby HTL into the host. Consequently, the quantum efficiency of blue PHOLEDs based on diphosphine dioxide derivatives as host was significantly below 100%.
To summarize the development of host material in the previous studies, an appropriate blue phosphorescent host material should not only have a high triplet excited state as well as bipolar transport property, but also a minimum en-ergy barrier between the host and adjacent carrier-trans-porting material. Unfortunately, up to now, it is not an easy task to find a host material that will satisfy all the above-mentioned requirements. In this paper, we will introduce a composite emitter incorporating a carrier-transporting material into the emitter composed of diphos-phine oxide derivatives host and organometallic iridium dopant. The problematic transport of holes from the HTL into diphosphine oxide derivatives host can be alleviated through the adoption of carrier-transporting material, thus the probability of carrier recombination can be increased and the driving voltage can be decreased. Consequently, a blue PHOLED with a power efficiency of 20 lm/W and low driving voltage of 4.2 V at practical brightness of 1000 cd/m2is achieved.
2. Experimental
The composite emitter comprises a wide-band-gap host, 2,8-bis(diphenylphosphoryl)dibenzothiophen (PPT) [18], which has a triplet excited state of 3.0 eV, a blue phosphorescent dopant, iridium(III)bis((4,6-difluoro-phenyl)-pyridinate-N,C20
)picolinate (FIrpic), which pos-sessed a triplet excited state of 2.62 eV[2], and a carrier-transporting material, 4,40,400 -tri(N-carbazolyl)triphenyl-amine (TCTA), which possessed a triplet excited state of 2.76 eV [2] as well as a good hole-transporting ability (1.6 104cm2/Vs at 105V/cm) [The data was measured by time-of-flight method, the detailed results will be re-ported in elsewhere]. In addition, a electron-blocking layer (EBL), bis[4-(p,p0-ditolylamino)phenyl]diphenylsilane (DTASi) [20], with high triplet excited state of 2.95 eV and a high-lying LUMO level of 2.2 eV was used to confine all generated exciton in emitter. A high electron mobility material (104cm2/Vs at 105V/cm), 4,7-diphenyl-1,10-phenanthroline (Bphen)[21], and 20% (vol%) cesium car-bonation (Cs2CO3) doped with Bphen [22] were used as ETL and n-ETL, respectively, to achieve low driving voltage. The device architecture used is ITO (150 nm)/NPB (45 nm)/ DTASi (15 nm)/composite emitter (15 nm)/Bphen (25 nm)/ n-ETL (20 nm)/Al (100 nm), where 4,40 -bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl (NPB) is HTL. In order to investi-gate the effect of doping carrier-transporting material in FIrpic/PPT emitter, the doping concentration of blue dop-ant was fixed at 15% and the doping concentration of 0%, 10%, and 20% for the carrier-transporting material were tested.
Fig. 1shows the detailed molecular structures of
mate-rials and architecture of device used in this study. All the materials were deposited by thermal evaporation in an
VAC Solciet OLED coater at a base vacuum of 107Torr. The device performance of luminance yield and EL spectra, CIEx,ycolor coordinates were measured by a Minolta lumi-nance meter and a Photo Research PR-650 spectrophotom-eter driven by a programmable dc source, respectively. 3. Results and discussion
Fig. 2shows the dependence of current density–voltage
(J–V) characteristics of the blue PHOLEDs on the doping concentration of carrier-transporting material in emitter. Lower driving voltage was observed in the device with doping carrier-transporting material in emitter as com-pared with that of undoped device. For instance, the driv-ing voltage at current density of 20 mA/cm2 is 5.0 V for doping 20% carrier-transporting material in the emitter and 5.6 V for the undoped device. The decreased driving voltage can be attributed to the transport of holes from EBL (DTASi) into host (PPT) which was promoted through doping carrier-transporting material (TCTA) in emitter, since TCTA possesses a good hole-transporting ability and
a HOMO level (5.7 eV) lying between that of DTASi (5.6 eV) and PPT (6.6 eV). The device energy diagram is shown in the inset ofFig. 2.
Fig. 3shows the dependence of current
efficiency–cur-rent density (cd/A–J) characteristics of the blue PHOLEDs on the doping concentration of carrier-transporting mate-rial in the emitter. When doping 10% carrier-transporting material in the emitter, the device exhibited a current effi-ciency of 26.8 cd/A with low driving voltage of 4.2 V (J 4 mA/cm2), which is 2 times higher than that of the un-doped device with 12.3 cd/A and 5.0 V (J 8.2 mA/cm2) at a practical brightness of 1000 cd/m2. The current efficiency sustains as high as 25 cd/A when the doping concentration of carrier-transporting material is increased to 20% in the emitter. We believe the dramatic enhancement in current efficiency when doping carrier-transporting material in the emitter is resulted from the ‘‘hole-facilitating” charac-ter of carrier-transporting macharac-terial, which can increase the concentration of holes in the emitter as opposed to that of undoped device. Therefore, the probability of carrier recombination can be increased.
In addition, at current density of 20 mA/cm2, the device with 10% carrier-transporting material doped in emitter exhibited a current efficiency of 22.6 cd/A, which is 2.5 times higher than that of undoped device with only 9.2 cd/A. This result demonstrates that the problematic efficiency roll-off in blue PHOLEDs can also be suppressed by doping carrier-transporting material in the emitter. For instance, the efficiency roll-off from low (1 mA/cm2) to high current density (40 mA/cm2) is 29% and 23% for 10% and 20% carrier-transporting material in the emitter, respectively, while it is nearly 60% for the undoped device. The suppression of efficiency roll-off in blue PHOLEDs when doping carrier-transporting material in the emitter can be attributed to the ‘‘hole-facilitating” character of car-rier-transporting material, which can reduce the accumu-lated holes at the EBL (DTASi) and FIrpic/PPT emitter interface to inhibit triplet-polaron quenching. Triplet-po-laron quenching has been reported to be one of the major reasons for efficiency roll-off in PHOLEDs[23]. The detailed device performances of these blue PHOLEDs are summa-rized inTables 1 and 2.
In order to prove that the transport of holes from EBL (DTASi) into FIrpic/PPT emitter was promoted by doping carrier-transporting material, two additional device archi-tectures were fabricated; one has a pure PPT layer inserted in-between the EBL and emitter while the other has a TCTA:PPT (10:90) composite layer. The detailed device architectures and energy diagram are depicted in Fig. 4. When inserted a pure PPT layer in-between the EBL and FIrpic/PPT emitter, the device exhibited a very low current efficiency (<1 cd/A) and its EL spectrum displayed three dominant peak emissions at 420, 472, and 580 nm as shown inFig. 5, which were originated from NPB, FIrpic, and the exciplex emission of DTASi/PPT, respectively. This phenomenon can be rationalized by the following mecha-nism. Firstly, according to the energy diagram, the holes and electrons were accumulated at the EBL/pure PPT inter-face. Secondly, a portion of the carriers recombined and the others formed the DTASi/PPT exciplex. Thirdly, the exci-tons diffused to either anode or cathode side across the
Fig. 2. Current density–voltage (J–V) characteristics of blue PHOLEDs as well as the device energy diagram (inset).
Fig. 3. Current efficiency–current density (cd/A–J) characteristics of blue PHOLEDs.
interfacial barriers of DTASi and PPT both of which have high excited state. Finally, the low excited energy of NPB and FIrpic captured the excitons to give the light emission. On the contrary, when inserted a TCTA:PPT (10:90) composite layer in-between the EBL and FIrpic/PPT emit-ter, the device with a current efficiency of 11.5 cd/A at a brightness of 1000 cd/m2was closed to that of device with whole FIrpic/PPT emitter shown inFig. 3. And also, the EL spectrum displayed only one dominant peak emission at 472 nm without detecting the emission from the exciplex of DTASi/PPT. This phenomenon further supports the
spec-ulation that the transport of holes from the EBL into PPT is promoted by doping carrier-transporting material. There-fore, the holes and electrons can no longer accumulate at EBL/PPT interface and the recombination zone is shifted to-ward the FIrpic/PPT emitter.
4. Summary
For solid-state lighting application, the low efficiency and the problematic efficiency roll-off are critical issues to be overcome in blue PHOLEDs. Here, we have proposed
Table 1
Device performances of blue PHOLEDs.
Emitter Voltage (V) Current efficiency (cd/A) Power efficiency (lm/W) EQE (%) CIEx,y Efficiency roll-off (%)
Host v (%) TCTA v (%) FIrpic At 1000 cd/m2
, (at 20 mA/cm2 ) From 1 to 40 mA/cm2 PPT 0 15 5.0 (5.6) 12.3 (9.2) 7.7 5.5 0.18, 0.38 60 10 4.2 (5.2) 26.8 (22.6) 20 12.1 0.17, 0.36 29 20 4.2 (5.0) 25 (22.2) 18.7 11.4 0.17, 0.36 23 Table 2
Summary of the characteristics of devices using FIrpic as the blue phosphorescent dopant.
Emitter (Dopant: FIrpic) Voltage (V) Current efficiency (cd/A) Power efficiency (lm/W) EQE (%) Reference Host Carrier-transporting material At 1000 cd/m2
PPT TCTA 4.2 26.8 20 12.1 This work
TCZ1 – 6.8 23 10.5 10.2 [13] CzSi – 5.0a 24a 16a 12a [14] SimCP – – – 11.9b 14.4b [15] mCP – – – 8.9b 7.5b [16] MPO12 – 4.8c – – 9.1c [17] PO15 – 5.2c – – 9.8c [18] DBF – 5.4c 21.2c 12.5c 8.0c [19] aReported at 100 cd/m2. bReported at maximal value. c
Reported at 800 cd/m2
.
a composite emitter concept and shown that doping a car-rier-transporting material in the conventional blue phos-phorescent emitter composed of wide-band-gap host and dopant can efficiently facilitate the transport of carriers into the emitter, as well as increase the carrier recombina-tion and reduce carrier accumularecombina-tion at interface. We showed that this composite blue phosphorescent emitter not only can decrease the driving voltage of device, but also increase the current efficiency as well as suppress the efficiency roll-off problem often encountered at high current density.
Acknowledgment
This work was supported by National Science and Tech-nology Development Fund (Grant No. 8354DL1000) of the National Science Council of Taiwan.
References
[1] B.W. D’Andrade, J. Esler, C. Lin, V. Adamovich, S. Xia, M.S. Weaver, R. Kwong, J.J. Brown, in: Proceedings of the International Display Workshops, Nigata, 2008, p. 143.
[2] S.J. Su, E. Gonmori, H. Sasabe, J. Kido, Adv. Mater. 20 (2008) 4194. [3] M.T. Lee, J.S. Lin, M.T. Chu, M.R. Tseng, Appl. Phys. Lett. 93 (2008)
13306.
[4] S.H. Eom, Y. Zheng, E. Wrzesniewski, J. Lee, N. Chopra, F. So, J. Xue, Appl. Phys. Lett. 94 (2009) 153303.
[5] T. Nakayama, K. Hiyama, K. Furukawa, H. Ohtani, in: Proceedings of Society for Information Display, Long Beach, CA, 2007, p. 1018. [6] Y. Sun, S.R. Forrest, Appl. Phys. Lett. 91 (2007) 263503.
[7] G. Schwartz, M. Pfeiffer, S. Reineke, K. Walzer, K. Leo, Adv. Mater. 19 (2007) 3672.
[8] Y. Sun, N.C. Giebink, H. Kanno, B. Ma, M.E. Thompson, S.R. Forrest, Nature 440 (2006) 908.
[9] B.W. D’Andrade, R.J. Holmes, S.R. Forrest, Adv. Mater. 16 (2004) 624. [10] M.A. Baldo, D.F. O’Brien, Y. You, A. Shoustikov, M.E. Thompson, S.R.
Forrest, Nature 395 (1998) 151.
[11] C. Adachi, M.A. Baldo, M.E. Thompson, S.R. Forrest, J. Appl. Phys. 90 (2001) 5048.
[12] S. Tokito, T. Iijima, Y. Suzuri, H. Kita, T. Tsuzuki, F. Sato, Appl. Phys. Lett. 83 (2003) 569.
[13] M.H. Tsai, Y.H. Hong, C.H. Chang, H.C. Su, C.C. Wu, A. Matoliukstyte, J. Simokaitiene, S. Grigalevicius, J.V. Grazulevicius, C.P. Hsu, Adv. Mater. 19 (2007) 862.
[14] M.H. Tsai, H.W. Lin, H.C. Su, T.H. Ke, C.C. Wu, F.C. Fang, Y.L. Liao, K.T. Wong, C.I. Wu, Adv. Mater. 18 (2006) 1216.
[15] S.J. Yeh, W.C. Wu, C.T. Chen, Y.H. Hong, Y. Chi, M.H. Ho, S.F. Hsu, C.H. Chen, Adv. Mater. 17 (2005) 285.
[16] R.J. Homes, S.R. Forrest, Y.J. Tung, R.C. Kwong, J.J. Brown, S. Garon, M.E. Thompson, Appl. Phys. Lett. 82 (2003) 2422.
[17] X. Cai, A.B. Padmaperuma, L.S. Sapochak, P.A. Vecchi, P.E. Burrows, Appl. Phys. Lett. 92 (2008) 083308.
[18] L.S. Sapochak, A.B. Padmaperuma, P.A. Vecchi, X. Cai, P.E. Burrows, Proc. SPIE 6655 (2007) 665506-1.
[19] P.A. Vecchi, A.B. Padmaperuma, H. Qiao, L.S. Sapochak, P.E. Burrows, Org. Lett. 8 (2006) 4211.
[20] N. Ide, T. Komoda, J. Kido, Proc. SPIE 6333 (2006) 63330M-1. [21] S. Naka, H. Okada, H. Onnagawa, T. Tsutsui, Appl. Phys. Lett. 76
(2000) 197.
[22] T. Hasegawa, S. Miura, T. Moriyama, T. Kimura, I. Takaya, Y. Osata, H. Mizutai, in: Proceedings of the Society for Information Display, Seattle, WA, 2004, p.154.
[23] M.A. Baldo, C. Adachi, S.R. Forrest, Phys. Rev. B 62 (2000) 10967. Fig. 5. EL spectra of device with a pure PPT or TCTA:PPT layer inserted
in-between EBL and FIrpic/PPT emitter as well as current efficiency–current density (cd/A–J) characteristics (inset).
