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White organic light-emitting diodes showing nearly 100% internal quantum efficiency
Organic Electronics 11 (2010) 1759–1766
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White organic light-emitting diodes showing nearly 100% internal quantum efficiency Ji Hoon Seo a, Seok Jae Lee a, Bo Min Seo a, Se Jin Moon a, Kum Hee Lee b, Jung Keun Park b, Seung Soo Yoon b,*, Young Kwan Kim a,** a b
Department of Information Display, Hongik University, Seoul 121-791, South Korea Department of Chemistry, Sungkyunkwan University, Suwon, Kyeonggi-do 440-746, South Korea
a r t i c l e
i n f o
Article history: Received 2 February 2010 Received in revised form 13 July 2010 Accepted 18 July 2010 Available online 7 August 2010 Keywords: White organic light-emitting diodes Various electron-transporting materials Various 100% internal quantum efficiency
a b s t r a c t The authors have demonstrated white organic light-emitting diodes (WOLED) using all phosphorescent emitters for blue, green, and red, such as FCNIrpic, Ir(ppy)3 and Ir(pq)2(acac). Various electron-transporting materials such as Bebq2, BPhen, TAZ, BAlq, and TPBI were also investigated for improving quantum efficiency of phosphorescence of WOLED by fabricating and characterizing white devices using these materials as an electron-transporting layer (ETL). It was found that WOLED with an ETL of TPBI exhibits a peak external quantum efficiency (EQE) of 19.5% and a peak power efficiency of 39.2 lm/W at very low light intensity. The optimized device also shows an EQE of 16.4% at 1000 cd/m2. It was also found that the optimized white device with TPBI shows minimal change with D Commission Internationale de I’Eclairage coordinates of ±(0.02, 0.00) from 100 to 10,000 cd/m2. In order to understand the mechanism for achieving nearly 100% IQE with this device structure, various analyses were also executed, such as UV/visible absorbance measurements of FCNIrpic and photoluminance (PL) measurements of mCP, fabrications of hole- and electron-only devices, UV/visible absorbance measurements of Ir(ppy)3 and Ir(pq)2(acac) and PL of FCNIrpic, and surface roughness measurements of various ETL films. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction White organic light-emitting diodes (WOLEDs) have drawn increasing attention as a solid-state light (SSL) source and backlights in liquid crystal displays and full-color OLEDs due to their light weight, low operating voltage, thinness, diffusiveness, and harmlessness for the restriction of certain hazardous substances directive/waste electrical and electronic equipment [1–6]. There are two ways of realizing WOLEDs as SSL in the aspects of materials used. One is phosphorescent WOLEDs (PHWOLEDs), which use all phosphorescent emitters, and the other is hybrid WOLEDs
* Corresponding author. Tel.: +82 31 290 7071; fax: +82 31 290 7075. ** Corresponding author. Tel.: +82 2 320 1646; fax: +82 2 3141 8928. E-mail addresses: [email protected] (S.S. Yoon), [email protected] (Y.K. Kim). 1566-1199/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2010.07.015
(HWOLEDs), which use fluorescent blue and phosphorescent red–green emitters. Sun et al. have demonstrated highly efficient HWOLEDs using fluorescent blue and phosphorescent red–green emitters, where a spacer of 4,40 -N,N0 dicarbazole-biphenyl was used in order to prevent singlet energy transfer from fluorescent blue emitter to phosphorescent red–green emitters; the exchange energy losses were minimized to increase the efficiency and lifetime [7]. Nevertheless, they showed low external quantum efficiency (EQE) and power efficiency (PE) to be about 10% and 14 lm/W at 500 cd/m2, respectively [8]. PHWOLEDs have also been investigated intensively because we can harvest both singlet and triplet excitons with PHWOLEDs [9–11]. Unfortunately, PHWOLEDs, despite the possibility of achieving 100% internal quantum efficiency (IQE), do not yet have a proper phosphorescent blue emitter with high-energy gap, long life, or host materials for this emitter.
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Nevertheless, lately, a few researchers have reported highly efficient phosphorescent deep-blue OLEDs and have also investigated the synthesis of novel blue Ir and Pt complexes [12,13]. White light emission can be obtained from monomeric or polymeric emitters [14,15], depending on molecular length, from all phosphorescent, fluorescent or hybrid phosphorescent–fluorescent materials [16–18], depending on the emission mechanism, and from single-emitting layer (EML) or multi-EML [19,20], excimer or exciplex [21,22], microcavity, down conversion, p–i–n, and tandem junction [23–26], depending on device structures. Among these options, we have investigated multi-EML junctions using all phosphorescent emitters in this paper. Materials for hole-transporting layers (HTL) have been investigated widely in various aspects such as hole mobility, band gap, triplet exciton energy, and glass transition temperature. As a results, N0 -bis-(1-naphyl)-N,N0 -diphenyl-1,10 -biphenyl-4,40 -diamine (NPB) is now used as a common HTL material because it shows the most outstanding characteristics such as thermal stability, high hole mobility, and appropriate band gap. Even though there are many kinds of electron-transporting layers (ETL), such as bis(10-hydroxybenzo[h]quinolinato)beryllium complex (Bebq2), 4,7-diphenyl-1,10-phenanthroline (BPhen), 3-(4biphenylyl)-4-phenyl-5-tert-butylphenyl-1,2,4-triazole (TAZ), bis-(2-methyl-8-quinolinolate)-4-(phenylphenolato)aluminum (BAlq), and 2,20 ,200 -(1,3,5-benzinetriyl)-tris(1-phenyl1-H-benzimidazole) (TPBI), there were no systematic investigations to improve the efficiency of WOLEDs having multi-EML junctions using all phosphorescent emitters in the aspects of various physical properties. In this paper, we investigated the effects of photophysical analysis for host-dopant trade-off of 1,3-bis(9-carbazolyl)benzene (mCP) and iridium(III) bis[(5-cyano-4-fluorophenyl)pyridinato-N,C20 ]picolinate (FCNIrpic) and various ETL materials on the efficiency of these WOLEDs by analyzing hole block ability, lowest unoccupied molecular orbital (LUMO) level, electron mobility, triplet energy level, and surface morphology of various ETL materials. Hence, 100% IQE and 19.5% EQE were achieved by using the optimized WOLEDs with TPBI as an ETL material. The structure of all devices and energy level diagrams of WOLEDs fabricated in this study are shown in Fig. 1(a) and b), respectively, where the device structure was indiumtin oxide (ITO) (150 nm)/NPB (50 nm)/mCP (10 nm)/ FCNIrpic:mCP (7 nm)/fac-tris(2-phenylpyridine)iridium (Ir(ppy)3):iridium(III) bis(2-phenylquinoline) acetylacetonate(Ir(pq)2(acac)):TPBI (3 nm)/various ETLs (30 nm)/lithium quinolate (Liq) (2 nm)/Al (100 nm), respectively. Bebq2, BPhen, TAZ, BAlq, and TPBI were used as ETL material in devices A, B, C, D, and E, respectively. The doping concentrations of FCNIrpic in mCP, Ir(ppy)3, and Ir(pq)2(acac) in TPBI were optimized to 8%, 8%, and 3%, respectively. The thickness of EML was optimized to 10 nm since this thickness has many advantages such as low operating voltage, higher efficiency, and lower color shift [9]. NPB as an HTL, mCP as an exciton-blocking layer (XBL) and a phosphorescent blue host, FCNIrpic, as a phosphorescent blue emitter, TPBI as a phosphorescent red host and an ETL, Ir(ppy)3, as a phosphorescent green emitter, Ir(pq)2(acac) as a phospho-
Fig. 1. (a) The structures of all devices fabricated in this study. (b) Energy level diagrams for all white devices. Numbers denote the energy level for the HOMO and LUMO of various materials used in this study, where the HOMO and LUMO of FCNIrpic, Ir(ppy)3, and Ir(pq)2(acac) are presented by blue, green, and red dotted line, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
rescent red emitter, and Liq as an electron injection layer were also used in these device structures. In device E, TPBI was used as a red host and ETL simultaneously, for this setup makes the fabrication process simple. The highest occupied molecular orbital (HOMO), LUMO, electron mobility, and triplet emission energy for various ETLs are listed in Table 1 [27–33]. It is clear in Table 1 that Bebq2 and BPhen have a considerably higher electron mobility of 104 cm2/V s compared to TAZ, BAlq, and TPBI, which has an electron mobility of 105 cm2/V s. TAZ and TPBI have higher triplet emission energies than FCNIrpic (2.65 eV). 2. Experimental An indium-tin oxide (ITO)-coated glass substrate was cleaned in an ultrasonic bath using the following sequence of solvents: acetone, methanol, distilled water, and isopropyl alcohol. Thereafter, a precleaned ITO layer was treated with O2 plasma under the following conditions: 2 102 Torr and 125 W for 2 min. HWOLEDs were fabricated by the high-vacuum (5 107 Torr) thermal evaporation of organic materials onto the surface of the ITO-coated glass substrate (10 X/sq, emitting area was 3 3 mm2). The deposition rates were 0.1 nm/s for all organic materials and 0.01 nm/s for Liq. Without breaking the vacuum, after the deposition of the organic layers, the Al cathode was deposited at a rate of 1 nm/s. The ultraviolet (UV)/visible absorption and PL
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J.H. Seo et al. / Organic Electronics 11 (2010) 1759–1766 Table 1 HOMO, LUMO, electron mobility, and triplet energy level of various electron transporting layers. ETL
HOMO (eV)
LUMO (eV)
Electron mobility (cm2/V s)
Triplet energy (eV)
Bebq2 BPhen TAZ BAlq TPBI
5.5 6.4 6.3 5.9 6.1
2.8 3.0 2.7 2.9 2.8
104 104 105 105 105
2.5 2.5 3.3 2.6 2.8
[27] [29] [31] [32] [31]
[28] [30] [28] [33] [27]
spectra were measured with HP-8453 (Hewlett Packard) and LS-55 (Perkin Elmer). In a presence of a DC voltage bias, the optical and electrical properties of WOLEDs such as the current density, luminance, PE, EQE, Commission Internationale de I’Eclairage (CIEx,y) coordinates, and EL spectra of the emission characteristics were measured with Keithley 2400 (Keithley) and Chroma meter CS1000A (Konica Minolta) instruments simultaneously. The films of various ETLs on silicon wafer substrates were used for atomic force microscopy (AFM) imaging. The various ETLs were deposited at a thickness of 20 nm at a rate of 0.1 nm/s. The AFM images were obtained with an XE-150 (Park Systems) instrument. The film conditions for the AFM measurements were kept the same as for OLEDs. All measurements were carried out under ambient conditions at room temperature.
Since FCNIrpic is an iridium derivative having a strong electron-withdrawing substituent of CN at the position of fluorine in iridium(III) bis(4,6-(difluorophenyl)pyridinato0 N,C2 )picolinate (FIrpic), FCNIrpic has a wider bandgap (2.5/5.8 eV) than that of FIrpic (2.5/5.7 eV). It can be expected that the efficient Förster energy transfer would be possible from mCP to FCNIrpic since 1MLCT and 3MLCT absorption spectra of FCNIrpic has broader spectral overlap with fluorescence spectra of mCP from 330 to 450 nm. Efficient Dexter energy transfer is also possible from phosphorescence of mCP (430 nm) to 3MLCT absorption of FCNIrpic (413 nm) [34]. Therefore, both Förster and Dexter energy transfers occur through the paths as follows; singlet excited state (S1) of mCP ? S1 of FCNIrpic ? intersystem crossing ? triplet exited state (T1) of FCNIrpic and T1 of mCP ? T1 of FCNIrpic, respectively [19].
3. Results and discussion
3.2. Mechanism analysis for achieving nearly 100% IQE of WOLEDs
3.1. Photophysical analysis for host-dopant trade-off Fig. 2 shows the UV/visible absorption spectrum of FCNIrpic and PL spectra of FCNIrpic, and of mCP in chlorobenzene, respectively. It was found in Fig. 2 that FCNIrpic shows the maximum UV/visible absorption peak at 250 nm with a sub-peak at 293 nm and peaks also at 375 and 413 nm, which are due to spin-allowed 1metal–ligand charge transfer band (MLCT) and spin-forbidden 3MLCT, respectively. It was also found in Fig. 2 that the PL emission peaks of FCNIrpic and mCP were located at 465.5 nm, with vibrational peaks at 492 nm and at 361.5 nm, respectively.
Fig. 2. UV/visible absorption spectrum of FCNIrpic and normalized PL spectra of mCP and FCNIrpic.
Fig. 3(a) and (b) show EQE and PE-luminance characteristics of all devices, respectively. It was found in Fig. 3(a) and (b) that devices A, B, C, D, and E have peak EQEs of 8.9%, 11.8%, 12.0%, 15.1%, and 19.3% and peak PEs of 19.5, 28.9, 26.2, 33.5, and 39.2 lm/W at very low light intensities, respectively. The EQE of 19.3% and the PE 39.2 lm/W at very low light intensity are equivalent to nearly 100% IQE [35,13]. All devices also show EQEs of 7.2%, 9.1%, 9.1%, 12.0%, and 16.4% at 1000 cd/m2. Device E, having TBPI as an ETL which is one of the highest values for the previously reported WOLEDs to the best of our knowledge. These excellent experimental results seem to be due to the following reasons: (1) blue device (10.70%) using FCNIrpic had an efficiency 1.5 times that of the control blue device (7.43%) using FIrpic [36] (see Ref. [36]); (2) TPBI as an ETL has an ideal hole-blocking property (see Section 3.2.2); (3) all the electrons injected into the EML pass without electron leakage since the same material was used as the red host and ETL (see Section 3.2.2); (4) since TPBI (2.8 eV) has a higher triplet energy than FCNIrpic (2.65 eV), device E, having TPBI as an ETL, can confine triplet energy of FCNIrpic to EML effectively (see Section 3.2.4); (5) TPBI film exhibits a very smooth surface with r 0.14 nm, which is the lowest value among those of Bebq2, BPhen, TAZ, BAlq, and TPBI film (see Section 3.2.5). Thus, these five factors may contribute partially to the achievement of excellent efficiency in WOLEDs shown in this study. We will discuss some experimental results obtained by various analyses in detail in order to explain excellent efficiency in the next chapter.
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Fig. 3. (a) EQE versus luminance characteristics of all devices with various ETLs. (b) PE versus luminance characteristics of all devices with various ETLs.
3.2.1. Electrical and optical characteristics of WOLEDs using various ETLs Fig. 4 and the inset in Fig. 4 show current density–voltage and luminance–voltage characteristics of all five devices, respectively. It was found in Fig. 4 that they have maximum current densities of 600, 143, 19, 64, and 113 mA/cm2 at 10 V and luminances of 13,200, 7910, 2780, 9620, and 11,500 cd/m2 at 10 V, respectively. Device A shows the highest current density at operating voltages, which seems to be due to the inadequate role of Bebq2 having the low HOMO (5.5 eV) as a hole-blocking layer. On the other hand, device C shows the lowest current density among other devices, which also seems to be due to the largest electron injection barrier between the Liq-Al and LUMO of TAZ and its lower electron mobility of 105 cm2/V s compared to Bebq2 and BPhen as shown in Table 1. It was also found that although BAlq in device D and TPBI in device E have the same electron mobility
Fig. 4. Current density versus voltage characteristics of all devices. Inset: luminance versus voltage characteristics of all devices.
as shown in Table 1, device D shows lower current density than device E. This can be explained as follows: device E did not have an electron injection barrier between the red EML and ETL because the same material, TPBI, was used for both the EML and ETL in device E. It is shown in the inset of Fig. 4 that devices A and B have similar saturation characteristics at driving voltages higher than 9.5 V, which seems to be due to the inadequate hole-blocking ability of Bebq2 in device A and also bad surface morphology of the Bphen layer in device B (see Sections 3.2.2 and 3.2.5). 3.2.2. Fabrication and characterization of unipolar devices Hole-only devices (HODs) and electron-only devices (EODs) were fabricated to prove the previous assumption. Fig. 5 and the inset of Fig. 5 show current density–voltage
Fig. 5. Current density versus voltage characteristics of HODs. Inset: HODs structure which have five structures of ITO (150 nm)/NPB-1 (15 nm)/mCP (5 nm)/TPBI (3 nm)/various ETLs (7 nm)/NPB-2 (20 nm)/Al (100 nm).
J.H. Seo et al. / Organic Electronics 11 (2010) 1759–1766
Fig. 6. Current density versus voltage characteristics of EODs. Inset: EODs structure which have five structures of ITO (150 nm)/TPBI (10 nm)/ETLs (40 nm)/Liq-Al (100 nm).
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characteristics of HODs, and the device structure for five HODs is as follows: ITO (150 nm)/NPB-1 (15 nm)/mCP (5 nm)/TPBI (3 nm)/various ETLs (HOD-1–5) (7 nm)/NPB2 (20 nm)/Al (100 nm), where various ETLs are Bebq2 (HOD-1), BPhen (HOD-2), TAZ (HOD-3), BAlq (HOD-4), and TPBI (HOD-5). To maintain the same HOMO value in HODs as that of white devices, NPB-1, mCP, and TPBI were used as the HTL and NPB-2 was used as the electron-blocking layer. It was found in Fig. 5 that the HODs show current densities of 7.0 104, 6.5 107, 1.4 106, 5.0 106, and 9.0 106 mA/cm2 at 1 V and also 0.538, 0.003, 0.002, 0.020, and 0.009 mA/cm2 at 5 V. HOD-1 shows the highest current density at the whole voltage, and this trend becomes more conspicuous at a lower voltage. Therefore, this indicates that Bebq2 could not play an adequate role of hole-blocking in device A. The lower the HOMO level of ETLs, the more holes that pass through toward the cathode; all ETLs except Bebq2 can effectively prevent a hole from passing through an EML and, therefore, confine hole carriers within the EML.
Fig. 7. (a) EL spectra of all white devices at 1000 cd/m2. (b) CIEx,y coordinates versus luminance characteristics from 100 to 10,000 cd/m2 of all white devices.
Fig. 8. (a) Blue device A and B where the device structures were ITO (150 nm)/NPB (50 nm)/mCP (10 nm)/FCNIrpic:mCP (8%, 10 nm)/BAlq (blue device A) or TPBI (blue device B) (30 nm)/LiF (2 nm)/Al (100 nm), respectively. (b) EQE and PE versus luminance characteristics of blue device A and B.
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Fig. 6 and the inset of Fig. 6 show current density–voltage characteristics of EODs, and the device structure for five EODs is as follows: ITO (150 nm)/TPBI (10 nm)/various ETLs (40 nm)/Liq (2 nm)/Al (100 nm), where various ETLs are Bebq2 (EOD-1), BPhen (EOD-2), TAZ (EOD-3), BAlq (EOD4), and TPBI (EOD-5). To maintain the same LUMO value in EODs as that of white devices, TPBI was used as an HTL. It was found in Fig. 6 that the EODs show current densities of 1.54, 0.66, 0.02, 0.02, and 0.06 mA/cm2 at 3 V and 47.8, 30.9, 1.2, 0.9, and 1.3 mA/cm2 at 5 V, where the current densities of EODs having Bebq2 and BPhen were much higher than those of other devices. This indicates that the electron mobility of ETLs plays a more important role in the current density of EODs than the LUMO value of ETLs. 3.2.3. EL characteristics of WOLEDs using various ETLs Fig. 7(a) shows the EL spectra of five white OLEDs at 1000 cd/m2. It was found in Fig. 7(a) that all devices have
Fig. 9. Possible triplet energy transfer paths for all emitters in the two hosts and ETLs.
blue peaks at 465–469 nm, green peaks at 501–503 nm, and red peaks at 592–594 nm. Fig. 7(b) also shows CIEx,y coordinates-luminance characteristics from 100 to 10,000 cd/m2 for all devices, where they have an emission of CIEx,y coordinates from (0.42, 0.41), (0.42, 0.45), (0.45, 0.42), (0.38, 0.40), and (0.47, 0.40) at 100 cd/m2 to (0.40, 0.42), (0.41, 0.46), (0.38, 0.42), (0.38, 0.41), and (0.45, 0.40) at 5000 cd/m2. It was found in Fig. 7(b) that device E has minimal change with DCIEx,y of ±(0.02, 0.00) from 100 to 10,000 cd/m2. It was also found in Fig. 7(a) that red peak intensity in devices C and E is enhanced compared to that in other devices, which means that devices C and E have more warmish white-emission characteristics rather than coldish white-emission characteristics compared to other devices. 3.2.4. Efficiency comparison of white devices with BAlq and TPBI having different triplet energy Device E with an ETL of TPBI showed higher efficiency than device D with an ETL of BAlq because TPBI (2.8 eV) has higher triplet energy than FCNIrpic (2.65 eV). On the other hand, BAlq (2.6 eV) has lower triplet energy than FCNIrpic as shown in Table 1. Therefore, device E could confine triplet energy in EML well. To confirm this result, as shown in Fig. 8(a), we fabricated blue devices A and B that had the following structure: ITO (150 nm)/NPB (50 nm)/mCP (10 nm)/FCNIrpic:mCP (8%, 10 nm)/BAlq (blue device A) or TPBI (blue device B) (30 nm)/LiF (2 nm)/Al (100 nm), where the doping concentrations of FCNIrpic in mCP were the same as those used for white devices. Fig. 8(b) also showed EQE and PE-luminance charac-
Fig. 10. AFM images of various ETLs. AFM images of various ETLs showing a 5 5 lm surface area: (a) Bebq2, (b) BPhen, (c) TAZ, (d) BAlq, and (e)TPBI.
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teristics of blue devices A and B, where an EQE of blue devices A and B was 7.55% and 10.01% at 100 cd/m2 and 4.62% and 8.42% at 1000 cd/m2, respectively. It was also found in Fig. 8(b) that a PE of blue devices A and B was 8.63 and 13.18 lm/W at 100 cd/m2 and 4.16 and 8.94 lm/ W at 1000 cd/m2, respectively. Blue device B with an ETL of TPBI showed higher efficiency by a factor of 1.3 and two at 100 and 1000 cd/m2 than blue device A with an ETL of BAlq, respectively. The efficiency difference between blue devices A and B explains the reason why device E has higher efficiency than device D. In other words, the ETL of TPBI, which has a higher triplet energy level than that of FCNIrpic, could confine triplet energy in EML well. Fig. 9 shows possible triplet energy transfer paths for all emitters in the two hosts and ETLs, where mCP used as a host for FCNIrpic and XBL, and TPBI, used as a host for Ir(ppy)3 and Ir(pq)2(acac), had higher triplet energy leading to effective confinement of triplet excitons in EML. 3.2.5. Surface morphology of various ETLs Fig. 10 shows the AFM images of Bebq2, BPhen, TAZ, BAlq, and TPBI films. It was found in Fig. 10 that for the films of BPhen and TAZ, the surfaces were very rough with r.m.s. roughness values (r) of 14.75 and 54.60 nm, respectively. On the other hand, TPBI film showed the smoothest r, at 0.14 nm, among them. Bebq2 and BAlq films also showed r of 0.15 and 0.56 nm, respectively. We can expect that the smooth surface of the film may be one of the important factors for achieving high efficiency in OLEDs due to less defective sites at the smooth interface of films. Therefore, the highest efficiency of device E with TPBI as the ETL may be partially due to the good film-forming property of TPBI. 4. Conclusions WOLEDs were fabricated using blue, green, and red phosphorescent emitters such as FCNIrpic, Ir(ppy)3, and Ir(pq)2(acac) and various ETLs such as Bebq2, BPhen, TAZ, BAlq, and TPBI. White devices with an ETL of TPBI exhibited nearly IQE of 100% and PE of 39.2 lm/W, which had a doubly higher efficiency than the control white device. Remarkably, it also showed high EQE of 16.4% at 1000 cd/ m2. In order to understand the reason why white devices with an ETL of TPBI exhibit nearly 100% IQE, many devices with various structures were fabricated and characterized in this study. In summary, the reasons for nearly 100% IQE are as follows: (1) high-efficiency phosphorescent blue emitter of FCNIrpic: the efficiency of phosphorescent blue emitters is generally lower than that of phosphorescent green and red emitters. But, the blue device with FCNIrpic in this study was found to have 1.5 times higher efficiency than the blue device with FIrpic; (2) good hole-blocking ability: TPBI has a deep HOMO value of 6.2 eV, which is similar to BPhen (6.4 eV) and TAZ (6.3 eV); (3) no electron leakage: no electron leakage can occur at the interface of red EML and ETL since the optimized white device E used TPBI as a host for the red emitter and also as ETL material; (4) higher triplet energy than blue emitter: TPBI can play the role of hole- and exciton-blocking layer simulta-
Table 2 Various characteristics of all devices, including, external quantum efficiency, current efficiency, power efficiency, and Commission Internationale de L’Eclairage (CIEx,y) coordinates at 500 cd/m2. Device
External quantum efficiency (%)
Current efficiency (cd/A)
Power efficiency (lm/W)
CIEx,y
0.42, 0.41 0.42, 0.45 0.44, 0.42 0.39, 0.40 0.46, 0.39
A
7.70
17.14
10.87
B
9.93
22.15
13.20
C
10.04
24.46
9.07
D
12.92
28.51
13.54
E
17.47
35.74
19.85
neously. Therefore, white devices with TPBI showed good confinement of triplet excitons within the EML; (5) a very smooth surface: TPBI has the smoothest morphology of r 0.14 nm among other ETLs. Thus, white devices with TPBI do not seem to show a saturated luminance–voltage curve at high voltage. We have exhibited high-efficiency PHWOLEDs along with a suggestion for the mechanism for achieving nearly 100% IQE. PHWOLEDs fabricated using device structures and materials described here may be expected to be applicable for SSL. Various characteristics of all devices are summarized in Table 2, such as EQE, current efficiency, PE, and CIEx,y coordinates at 500 cd/m2. Acknowledgements This work was supported by the ERC program of the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea Ministry of Education, Science and Technology (MEST) (No. R11-2007-045-03001-0). References [1] B.W. D’Andrade, S.R. Forrest, Adv. Mater. 16 (2006) 1585. [2] J.H. Seo, J.H. Seo, J.H. Park, Y.K. Kim, J.H. Kim, G.W. Hyung, K.H. Lee, S.S. Yoon, Appl. Phys. Lett. 90 (2007) 203507. [3] K.S. Yook, J.Y. Lee, Appl. Phys. Lett. 92 (2008) 193308. [4] Q.X. Tong, S.L. Lai, M.Y. Chan, J.X. Tang, H.L. Kwong, C.S. Lee, S.Y. Lee, Appl. Phys. Lett. 91 (2007) 023503. [5] M.H. Ho, S.F. Hsu, J.W. Ma, S.W. Hwang, P.C. Yeh, C.H. Chen, Appl. Phys. Lett. 91 (2007) 113518. [6] Y. Sun, S.R. Forrest, Appl. Phys. Lett. 91 (2007) 263503. [7] Y. Sun, N.C. Giebink, H. Kanno, B. Ma, M.E. Thompson, S.R. Forrest, Nature 440 (2006) 908. [8] G. Schwartz, S. Reineke, T.C. Rosenow, K. Walzer, K. Leo, Adv. Funct. Mater. 19 (2009) 1319. [9] S.J. Su, E. Gonmori, H. Sasabe, J. Kido, Adv. Mater. 20 (2008) 4189. [10] S. Reineke, F. Lindner, G. Schwartz, N. Seidler, K. Wazer, B. Küssem, K. Leo, Nature 459 (2009) 234. [11] Q. Wang, J. Ding, D. Ma, Y. Cheng, L. Wang, X. Jing, F. Wang, Adv. Funct. Mater. 19 (2008) 84. [12] Y. Zheng, S.H. Eom, N. Chopra, J.W. Lee, F. So, J. Xue, Appl. Phys. Lett. 92 (2008) 223301. [13] E.L. Williams, K. Haavisto, J. Li, G.E. Jabbour, Adv. Mater. 19 (2007) 197. [14] H.I. Baek, C.H. Lee, J. Appl. Phys. 108 (2008) 124504. [15] F. Hide, P. Kozodoy, S.P. DenBaars, A.J. Heeger, Appl. Phys. Lett. 70 (1997) 2664. [16] S. Tokito, T. Lijima, T. Tsuzuki, F. Sato, Appl. Phys. Lett. 83 (2003) 2459.
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Organic Electronics journal homepage: www.elsevier.com/locate/orgel
White organic light-emitting diodes showing nearly 100% internal quantum efficiency Ji Hoon Seo a, Seok Jae Lee a, Bo Min Seo a, Se Jin Moon a, Kum Hee Lee b, Jung Keun Park b, Seung Soo Yoon b,*, Young Kwan Kim a,** a b
Department of Information Display, Hongik University, Seoul 121-791, South Korea Department of Chemistry, Sungkyunkwan University, Suwon, Kyeonggi-do 440-746, South Korea
a r t i c l e
i n f o
Article history: Received 2 February 2010 Received in revised form 13 July 2010 Accepted 18 July 2010 Available online 7 August 2010 Keywords: White organic light-emitting diodes Various electron-transporting materials Various 100% internal quantum efficiency
a b s t r a c t The authors have demonstrated white organic light-emitting diodes (WOLED) using all phosphorescent emitters for blue, green, and red, such as FCNIrpic, Ir(ppy)3 and Ir(pq)2(acac). Various electron-transporting materials such as Bebq2, BPhen, TAZ, BAlq, and TPBI were also investigated for improving quantum efficiency of phosphorescence of WOLED by fabricating and characterizing white devices using these materials as an electron-transporting layer (ETL). It was found that WOLED with an ETL of TPBI exhibits a peak external quantum efficiency (EQE) of 19.5% and a peak power efficiency of 39.2 lm/W at very low light intensity. The optimized device also shows an EQE of 16.4% at 1000 cd/m2. It was also found that the optimized white device with TPBI shows minimal change with D Commission Internationale de I’Eclairage coordinates of ±(0.02, 0.00) from 100 to 10,000 cd/m2. In order to understand the mechanism for achieving nearly 100% IQE with this device structure, various analyses were also executed, such as UV/visible absorbance measurements of FCNIrpic and photoluminance (PL) measurements of mCP, fabrications of hole- and electron-only devices, UV/visible absorbance measurements of Ir(ppy)3 and Ir(pq)2(acac) and PL of FCNIrpic, and surface roughness measurements of various ETL films. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction White organic light-emitting diodes (WOLEDs) have drawn increasing attention as a solid-state light (SSL) source and backlights in liquid crystal displays and full-color OLEDs due to their light weight, low operating voltage, thinness, diffusiveness, and harmlessness for the restriction of certain hazardous substances directive/waste electrical and electronic equipment [1–6]. There are two ways of realizing WOLEDs as SSL in the aspects of materials used. One is phosphorescent WOLEDs (PHWOLEDs), which use all phosphorescent emitters, and the other is hybrid WOLEDs
* Corresponding author. Tel.: +82 31 290 7071; fax: +82 31 290 7075. ** Corresponding author. Tel.: +82 2 320 1646; fax: +82 2 3141 8928. E-mail addresses: [email protected] (S.S. Yoon), [email protected] (Y.K. Kim). 1566-1199/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2010.07.015
(HWOLEDs), which use fluorescent blue and phosphorescent red–green emitters. Sun et al. have demonstrated highly efficient HWOLEDs using fluorescent blue and phosphorescent red–green emitters, where a spacer of 4,40 -N,N0 dicarbazole-biphenyl was used in order to prevent singlet energy transfer from fluorescent blue emitter to phosphorescent red–green emitters; the exchange energy losses were minimized to increase the efficiency and lifetime [7]. Nevertheless, they showed low external quantum efficiency (EQE) and power efficiency (PE) to be about 10% and 14 lm/W at 500 cd/m2, respectively [8]. PHWOLEDs have also been investigated intensively because we can harvest both singlet and triplet excitons with PHWOLEDs [9–11]. Unfortunately, PHWOLEDs, despite the possibility of achieving 100% internal quantum efficiency (IQE), do not yet have a proper phosphorescent blue emitter with high-energy gap, long life, or host materials for this emitter.
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Nevertheless, lately, a few researchers have reported highly efficient phosphorescent deep-blue OLEDs and have also investigated the synthesis of novel blue Ir and Pt complexes [12,13]. White light emission can be obtained from monomeric or polymeric emitters [14,15], depending on molecular length, from all phosphorescent, fluorescent or hybrid phosphorescent–fluorescent materials [16–18], depending on the emission mechanism, and from single-emitting layer (EML) or multi-EML [19,20], excimer or exciplex [21,22], microcavity, down conversion, p–i–n, and tandem junction [23–26], depending on device structures. Among these options, we have investigated multi-EML junctions using all phosphorescent emitters in this paper. Materials for hole-transporting layers (HTL) have been investigated widely in various aspects such as hole mobility, band gap, triplet exciton energy, and glass transition temperature. As a results, N0 -bis-(1-naphyl)-N,N0 -diphenyl-1,10 -biphenyl-4,40 -diamine (NPB) is now used as a common HTL material because it shows the most outstanding characteristics such as thermal stability, high hole mobility, and appropriate band gap. Even though there are many kinds of electron-transporting layers (ETL), such as bis(10-hydroxybenzo[h]quinolinato)beryllium complex (Bebq2), 4,7-diphenyl-1,10-phenanthroline (BPhen), 3-(4biphenylyl)-4-phenyl-5-tert-butylphenyl-1,2,4-triazole (TAZ), bis-(2-methyl-8-quinolinolate)-4-(phenylphenolato)aluminum (BAlq), and 2,20 ,200 -(1,3,5-benzinetriyl)-tris(1-phenyl1-H-benzimidazole) (TPBI), there were no systematic investigations to improve the efficiency of WOLEDs having multi-EML junctions using all phosphorescent emitters in the aspects of various physical properties. In this paper, we investigated the effects of photophysical analysis for host-dopant trade-off of 1,3-bis(9-carbazolyl)benzene (mCP) and iridium(III) bis[(5-cyano-4-fluorophenyl)pyridinato-N,C20 ]picolinate (FCNIrpic) and various ETL materials on the efficiency of these WOLEDs by analyzing hole block ability, lowest unoccupied molecular orbital (LUMO) level, electron mobility, triplet energy level, and surface morphology of various ETL materials. Hence, 100% IQE and 19.5% EQE were achieved by using the optimized WOLEDs with TPBI as an ETL material. The structure of all devices and energy level diagrams of WOLEDs fabricated in this study are shown in Fig. 1(a) and b), respectively, where the device structure was indiumtin oxide (ITO) (150 nm)/NPB (50 nm)/mCP (10 nm)/ FCNIrpic:mCP (7 nm)/fac-tris(2-phenylpyridine)iridium (Ir(ppy)3):iridium(III) bis(2-phenylquinoline) acetylacetonate(Ir(pq)2(acac)):TPBI (3 nm)/various ETLs (30 nm)/lithium quinolate (Liq) (2 nm)/Al (100 nm), respectively. Bebq2, BPhen, TAZ, BAlq, and TPBI were used as ETL material in devices A, B, C, D, and E, respectively. The doping concentrations of FCNIrpic in mCP, Ir(ppy)3, and Ir(pq)2(acac) in TPBI were optimized to 8%, 8%, and 3%, respectively. The thickness of EML was optimized to 10 nm since this thickness has many advantages such as low operating voltage, higher efficiency, and lower color shift [9]. NPB as an HTL, mCP as an exciton-blocking layer (XBL) and a phosphorescent blue host, FCNIrpic, as a phosphorescent blue emitter, TPBI as a phosphorescent red host and an ETL, Ir(ppy)3, as a phosphorescent green emitter, Ir(pq)2(acac) as a phospho-
Fig. 1. (a) The structures of all devices fabricated in this study. (b) Energy level diagrams for all white devices. Numbers denote the energy level for the HOMO and LUMO of various materials used in this study, where the HOMO and LUMO of FCNIrpic, Ir(ppy)3, and Ir(pq)2(acac) are presented by blue, green, and red dotted line, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
rescent red emitter, and Liq as an electron injection layer were also used in these device structures. In device E, TPBI was used as a red host and ETL simultaneously, for this setup makes the fabrication process simple. The highest occupied molecular orbital (HOMO), LUMO, electron mobility, and triplet emission energy for various ETLs are listed in Table 1 [27–33]. It is clear in Table 1 that Bebq2 and BPhen have a considerably higher electron mobility of 104 cm2/V s compared to TAZ, BAlq, and TPBI, which has an electron mobility of 105 cm2/V s. TAZ and TPBI have higher triplet emission energies than FCNIrpic (2.65 eV). 2. Experimental An indium-tin oxide (ITO)-coated glass substrate was cleaned in an ultrasonic bath using the following sequence of solvents: acetone, methanol, distilled water, and isopropyl alcohol. Thereafter, a precleaned ITO layer was treated with O2 plasma under the following conditions: 2 102 Torr and 125 W for 2 min. HWOLEDs were fabricated by the high-vacuum (5 107 Torr) thermal evaporation of organic materials onto the surface of the ITO-coated glass substrate (10 X/sq, emitting area was 3 3 mm2). The deposition rates were 0.1 nm/s for all organic materials and 0.01 nm/s for Liq. Without breaking the vacuum, after the deposition of the organic layers, the Al cathode was deposited at a rate of 1 nm/s. The ultraviolet (UV)/visible absorption and PL
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HOMO (eV)
LUMO (eV)
Electron mobility (cm2/V s)
Triplet energy (eV)
Bebq2 BPhen TAZ BAlq TPBI
5.5 6.4 6.3 5.9 6.1
2.8 3.0 2.7 2.9 2.8
104 104 105 105 105
2.5 2.5 3.3 2.6 2.8
[27] [29] [31] [32] [31]
[28] [30] [28] [33] [27]
spectra were measured with HP-8453 (Hewlett Packard) and LS-55 (Perkin Elmer). In a presence of a DC voltage bias, the optical and electrical properties of WOLEDs such as the current density, luminance, PE, EQE, Commission Internationale de I’Eclairage (CIEx,y) coordinates, and EL spectra of the emission characteristics were measured with Keithley 2400 (Keithley) and Chroma meter CS1000A (Konica Minolta) instruments simultaneously. The films of various ETLs on silicon wafer substrates were used for atomic force microscopy (AFM) imaging. The various ETLs were deposited at a thickness of 20 nm at a rate of 0.1 nm/s. The AFM images were obtained with an XE-150 (Park Systems) instrument. The film conditions for the AFM measurements were kept the same as for OLEDs. All measurements were carried out under ambient conditions at room temperature.
Since FCNIrpic is an iridium derivative having a strong electron-withdrawing substituent of CN at the position of fluorine in iridium(III) bis(4,6-(difluorophenyl)pyridinato0 N,C2 )picolinate (FIrpic), FCNIrpic has a wider bandgap (2.5/5.8 eV) than that of FIrpic (2.5/5.7 eV). It can be expected that the efficient Förster energy transfer would be possible from mCP to FCNIrpic since 1MLCT and 3MLCT absorption spectra of FCNIrpic has broader spectral overlap with fluorescence spectra of mCP from 330 to 450 nm. Efficient Dexter energy transfer is also possible from phosphorescence of mCP (430 nm) to 3MLCT absorption of FCNIrpic (413 nm) [34]. Therefore, both Förster and Dexter energy transfers occur through the paths as follows; singlet excited state (S1) of mCP ? S1 of FCNIrpic ? intersystem crossing ? triplet exited state (T1) of FCNIrpic and T1 of mCP ? T1 of FCNIrpic, respectively [19].
3. Results and discussion
3.2. Mechanism analysis for achieving nearly 100% IQE of WOLEDs
3.1. Photophysical analysis for host-dopant trade-off Fig. 2 shows the UV/visible absorption spectrum of FCNIrpic and PL spectra of FCNIrpic, and of mCP in chlorobenzene, respectively. It was found in Fig. 2 that FCNIrpic shows the maximum UV/visible absorption peak at 250 nm with a sub-peak at 293 nm and peaks also at 375 and 413 nm, which are due to spin-allowed 1metal–ligand charge transfer band (MLCT) and spin-forbidden 3MLCT, respectively. It was also found in Fig. 2 that the PL emission peaks of FCNIrpic and mCP were located at 465.5 nm, with vibrational peaks at 492 nm and at 361.5 nm, respectively.
Fig. 2. UV/visible absorption spectrum of FCNIrpic and normalized PL spectra of mCP and FCNIrpic.
Fig. 3(a) and (b) show EQE and PE-luminance characteristics of all devices, respectively. It was found in Fig. 3(a) and (b) that devices A, B, C, D, and E have peak EQEs of 8.9%, 11.8%, 12.0%, 15.1%, and 19.3% and peak PEs of 19.5, 28.9, 26.2, 33.5, and 39.2 lm/W at very low light intensities, respectively. The EQE of 19.3% and the PE 39.2 lm/W at very low light intensity are equivalent to nearly 100% IQE [35,13]. All devices also show EQEs of 7.2%, 9.1%, 9.1%, 12.0%, and 16.4% at 1000 cd/m2. Device E, having TBPI as an ETL which is one of the highest values for the previously reported WOLEDs to the best of our knowledge. These excellent experimental results seem to be due to the following reasons: (1) blue device (10.70%) using FCNIrpic had an efficiency 1.5 times that of the control blue device (7.43%) using FIrpic [36] (see Ref. [36]); (2) TPBI as an ETL has an ideal hole-blocking property (see Section 3.2.2); (3) all the electrons injected into the EML pass without electron leakage since the same material was used as the red host and ETL (see Section 3.2.2); (4) since TPBI (2.8 eV) has a higher triplet energy than FCNIrpic (2.65 eV), device E, having TPBI as an ETL, can confine triplet energy of FCNIrpic to EML effectively (see Section 3.2.4); (5) TPBI film exhibits a very smooth surface with r 0.14 nm, which is the lowest value among those of Bebq2, BPhen, TAZ, BAlq, and TPBI film (see Section 3.2.5). Thus, these five factors may contribute partially to the achievement of excellent efficiency in WOLEDs shown in this study. We will discuss some experimental results obtained by various analyses in detail in order to explain excellent efficiency in the next chapter.
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Fig. 3. (a) EQE versus luminance characteristics of all devices with various ETLs. (b) PE versus luminance characteristics of all devices with various ETLs.
3.2.1. Electrical and optical characteristics of WOLEDs using various ETLs Fig. 4 and the inset in Fig. 4 show current density–voltage and luminance–voltage characteristics of all five devices, respectively. It was found in Fig. 4 that they have maximum current densities of 600, 143, 19, 64, and 113 mA/cm2 at 10 V and luminances of 13,200, 7910, 2780, 9620, and 11,500 cd/m2 at 10 V, respectively. Device A shows the highest current density at operating voltages, which seems to be due to the inadequate role of Bebq2 having the low HOMO (5.5 eV) as a hole-blocking layer. On the other hand, device C shows the lowest current density among other devices, which also seems to be due to the largest electron injection barrier between the Liq-Al and LUMO of TAZ and its lower electron mobility of 105 cm2/V s compared to Bebq2 and BPhen as shown in Table 1. It was also found that although BAlq in device D and TPBI in device E have the same electron mobility
Fig. 4. Current density versus voltage characteristics of all devices. Inset: luminance versus voltage characteristics of all devices.
as shown in Table 1, device D shows lower current density than device E. This can be explained as follows: device E did not have an electron injection barrier between the red EML and ETL because the same material, TPBI, was used for both the EML and ETL in device E. It is shown in the inset of Fig. 4 that devices A and B have similar saturation characteristics at driving voltages higher than 9.5 V, which seems to be due to the inadequate hole-blocking ability of Bebq2 in device A and also bad surface morphology of the Bphen layer in device B (see Sections 3.2.2 and 3.2.5). 3.2.2. Fabrication and characterization of unipolar devices Hole-only devices (HODs) and electron-only devices (EODs) were fabricated to prove the previous assumption. Fig. 5 and the inset of Fig. 5 show current density–voltage
Fig. 5. Current density versus voltage characteristics of HODs. Inset: HODs structure which have five structures of ITO (150 nm)/NPB-1 (15 nm)/mCP (5 nm)/TPBI (3 nm)/various ETLs (7 nm)/NPB-2 (20 nm)/Al (100 nm).
J.H. Seo et al. / Organic Electronics 11 (2010) 1759–1766
Fig. 6. Current density versus voltage characteristics of EODs. Inset: EODs structure which have five structures of ITO (150 nm)/TPBI (10 nm)/ETLs (40 nm)/Liq-Al (100 nm).
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characteristics of HODs, and the device structure for five HODs is as follows: ITO (150 nm)/NPB-1 (15 nm)/mCP (5 nm)/TPBI (3 nm)/various ETLs (HOD-1–5) (7 nm)/NPB2 (20 nm)/Al (100 nm), where various ETLs are Bebq2 (HOD-1), BPhen (HOD-2), TAZ (HOD-3), BAlq (HOD-4), and TPBI (HOD-5). To maintain the same HOMO value in HODs as that of white devices, NPB-1, mCP, and TPBI were used as the HTL and NPB-2 was used as the electron-blocking layer. It was found in Fig. 5 that the HODs show current densities of 7.0 104, 6.5 107, 1.4 106, 5.0 106, and 9.0 106 mA/cm2 at 1 V and also 0.538, 0.003, 0.002, 0.020, and 0.009 mA/cm2 at 5 V. HOD-1 shows the highest current density at the whole voltage, and this trend becomes more conspicuous at a lower voltage. Therefore, this indicates that Bebq2 could not play an adequate role of hole-blocking in device A. The lower the HOMO level of ETLs, the more holes that pass through toward the cathode; all ETLs except Bebq2 can effectively prevent a hole from passing through an EML and, therefore, confine hole carriers within the EML.
Fig. 7. (a) EL spectra of all white devices at 1000 cd/m2. (b) CIEx,y coordinates versus luminance characteristics from 100 to 10,000 cd/m2 of all white devices.
Fig. 8. (a) Blue device A and B where the device structures were ITO (150 nm)/NPB (50 nm)/mCP (10 nm)/FCNIrpic:mCP (8%, 10 nm)/BAlq (blue device A) or TPBI (blue device B) (30 nm)/LiF (2 nm)/Al (100 nm), respectively. (b) EQE and PE versus luminance characteristics of blue device A and B.
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Fig. 6 and the inset of Fig. 6 show current density–voltage characteristics of EODs, and the device structure for five EODs is as follows: ITO (150 nm)/TPBI (10 nm)/various ETLs (40 nm)/Liq (2 nm)/Al (100 nm), where various ETLs are Bebq2 (EOD-1), BPhen (EOD-2), TAZ (EOD-3), BAlq (EOD4), and TPBI (EOD-5). To maintain the same LUMO value in EODs as that of white devices, TPBI was used as an HTL. It was found in Fig. 6 that the EODs show current densities of 1.54, 0.66, 0.02, 0.02, and 0.06 mA/cm2 at 3 V and 47.8, 30.9, 1.2, 0.9, and 1.3 mA/cm2 at 5 V, where the current densities of EODs having Bebq2 and BPhen were much higher than those of other devices. This indicates that the electron mobility of ETLs plays a more important role in the current density of EODs than the LUMO value of ETLs. 3.2.3. EL characteristics of WOLEDs using various ETLs Fig. 7(a) shows the EL spectra of five white OLEDs at 1000 cd/m2. It was found in Fig. 7(a) that all devices have
Fig. 9. Possible triplet energy transfer paths for all emitters in the two hosts and ETLs.
blue peaks at 465–469 nm, green peaks at 501–503 nm, and red peaks at 592–594 nm. Fig. 7(b) also shows CIEx,y coordinates-luminance characteristics from 100 to 10,000 cd/m2 for all devices, where they have an emission of CIEx,y coordinates from (0.42, 0.41), (0.42, 0.45), (0.45, 0.42), (0.38, 0.40), and (0.47, 0.40) at 100 cd/m2 to (0.40, 0.42), (0.41, 0.46), (0.38, 0.42), (0.38, 0.41), and (0.45, 0.40) at 5000 cd/m2. It was found in Fig. 7(b) that device E has minimal change with DCIEx,y of ±(0.02, 0.00) from 100 to 10,000 cd/m2. It was also found in Fig. 7(a) that red peak intensity in devices C and E is enhanced compared to that in other devices, which means that devices C and E have more warmish white-emission characteristics rather than coldish white-emission characteristics compared to other devices. 3.2.4. Efficiency comparison of white devices with BAlq and TPBI having different triplet energy Device E with an ETL of TPBI showed higher efficiency than device D with an ETL of BAlq because TPBI (2.8 eV) has higher triplet energy than FCNIrpic (2.65 eV). On the other hand, BAlq (2.6 eV) has lower triplet energy than FCNIrpic as shown in Table 1. Therefore, device E could confine triplet energy in EML well. To confirm this result, as shown in Fig. 8(a), we fabricated blue devices A and B that had the following structure: ITO (150 nm)/NPB (50 nm)/mCP (10 nm)/FCNIrpic:mCP (8%, 10 nm)/BAlq (blue device A) or TPBI (blue device B) (30 nm)/LiF (2 nm)/Al (100 nm), where the doping concentrations of FCNIrpic in mCP were the same as those used for white devices. Fig. 8(b) also showed EQE and PE-luminance charac-
Fig. 10. AFM images of various ETLs. AFM images of various ETLs showing a 5 5 lm surface area: (a) Bebq2, (b) BPhen, (c) TAZ, (d) BAlq, and (e)TPBI.
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teristics of blue devices A and B, where an EQE of blue devices A and B was 7.55% and 10.01% at 100 cd/m2 and 4.62% and 8.42% at 1000 cd/m2, respectively. It was also found in Fig. 8(b) that a PE of blue devices A and B was 8.63 and 13.18 lm/W at 100 cd/m2 and 4.16 and 8.94 lm/ W at 1000 cd/m2, respectively. Blue device B with an ETL of TPBI showed higher efficiency by a factor of 1.3 and two at 100 and 1000 cd/m2 than blue device A with an ETL of BAlq, respectively. The efficiency difference between blue devices A and B explains the reason why device E has higher efficiency than device D. In other words, the ETL of TPBI, which has a higher triplet energy level than that of FCNIrpic, could confine triplet energy in EML well. Fig. 9 shows possible triplet energy transfer paths for all emitters in the two hosts and ETLs, where mCP used as a host for FCNIrpic and XBL, and TPBI, used as a host for Ir(ppy)3 and Ir(pq)2(acac), had higher triplet energy leading to effective confinement of triplet excitons in EML. 3.2.5. Surface morphology of various ETLs Fig. 10 shows the AFM images of Bebq2, BPhen, TAZ, BAlq, and TPBI films. It was found in Fig. 10 that for the films of BPhen and TAZ, the surfaces were very rough with r.m.s. roughness values (r) of 14.75 and 54.60 nm, respectively. On the other hand, TPBI film showed the smoothest r, at 0.14 nm, among them. Bebq2 and BAlq films also showed r of 0.15 and 0.56 nm, respectively. We can expect that the smooth surface of the film may be one of the important factors for achieving high efficiency in OLEDs due to less defective sites at the smooth interface of films. Therefore, the highest efficiency of device E with TPBI as the ETL may be partially due to the good film-forming property of TPBI. 4. Conclusions WOLEDs were fabricated using blue, green, and red phosphorescent emitters such as FCNIrpic, Ir(ppy)3, and Ir(pq)2(acac) and various ETLs such as Bebq2, BPhen, TAZ, BAlq, and TPBI. White devices with an ETL of TPBI exhibited nearly IQE of 100% and PE of 39.2 lm/W, which had a doubly higher efficiency than the control white device. Remarkably, it also showed high EQE of 16.4% at 1000 cd/ m2. In order to understand the reason why white devices with an ETL of TPBI exhibit nearly 100% IQE, many devices with various structures were fabricated and characterized in this study. In summary, the reasons for nearly 100% IQE are as follows: (1) high-efficiency phosphorescent blue emitter of FCNIrpic: the efficiency of phosphorescent blue emitters is generally lower than that of phosphorescent green and red emitters. But, the blue device with FCNIrpic in this study was found to have 1.5 times higher efficiency than the blue device with FIrpic; (2) good hole-blocking ability: TPBI has a deep HOMO value of 6.2 eV, which is similar to BPhen (6.4 eV) and TAZ (6.3 eV); (3) no electron leakage: no electron leakage can occur at the interface of red EML and ETL since the optimized white device E used TPBI as a host for the red emitter and also as ETL material; (4) higher triplet energy than blue emitter: TPBI can play the role of hole- and exciton-blocking layer simulta-
Table 2 Various characteristics of all devices, including, external quantum efficiency, current efficiency, power efficiency, and Commission Internationale de L’Eclairage (CIEx,y) coordinates at 500 cd/m2. Device
External quantum efficiency (%)
Current efficiency (cd/A)
Power efficiency (lm/W)
CIEx,y
0.42, 0.41 0.42, 0.45 0.44, 0.42 0.39, 0.40 0.46, 0.39
A
7.70
17.14
10.87
B
9.93
22.15
13.20
C
10.04
24.46
9.07
D
12.92
28.51
13.54
E
17.47
35.74
19.85
neously. Therefore, white devices with TPBI showed good confinement of triplet excitons within the EML; (5) a very smooth surface: TPBI has the smoothest morphology of r 0.14 nm among other ETLs. Thus, white devices with TPBI do not seem to show a saturated luminance–voltage curve at high voltage. We have exhibited high-efficiency PHWOLEDs along with a suggestion for the mechanism for achieving nearly 100% IQE. PHWOLEDs fabricated using device structures and materials described here may be expected to be applicable for SSL. Various characteristics of all devices are summarized in Table 2, such as EQE, current efficiency, PE, and CIEx,y coordinates at 500 cd/m2. Acknowledgements This work was supported by the ERC program of the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea Ministry of Education, Science and Technology (MEST) (No. R11-2007-045-03001-0). References [1] B.W. D’Andrade, S.R. Forrest, Adv. Mater. 16 (2006) 1585. [2] J.H. Seo, J.H. Seo, J.H. Park, Y.K. Kim, J.H. Kim, G.W. Hyung, K.H. Lee, S.S. Yoon, Appl. Phys. Lett. 90 (2007) 203507. [3] K.S. Yook, J.Y. Lee, Appl. Phys. Lett. 92 (2008) 193308. [4] Q.X. Tong, S.L. Lai, M.Y. Chan, J.X. Tang, H.L. Kwong, C.S. Lee, S.Y. Lee, Appl. Phys. Lett. 91 (2007) 023503. [5] M.H. Ho, S.F. Hsu, J.W. Ma, S.W. Hwang, P.C. Yeh, C.H. Chen, Appl. Phys. Lett. 91 (2007) 113518. [6] Y. Sun, S.R. Forrest, Appl. Phys. Lett. 91 (2007) 263503. [7] Y. Sun, N.C. Giebink, H. Kanno, B. Ma, M.E. Thompson, S.R. Forrest, Nature 440 (2006) 908. [8] G. Schwartz, S. Reineke, T.C. Rosenow, K. Walzer, K. Leo, Adv. Funct. Mater. 19 (2009) 1319. [9] S.J. Su, E. Gonmori, H. Sasabe, J. Kido, Adv. Mater. 20 (2008) 4189. [10] S. Reineke, F. Lindner, G. Schwartz, N. Seidler, K. Wazer, B. Küssem, K. Leo, Nature 459 (2009) 234. [11] Q. Wang, J. Ding, D. Ma, Y. Cheng, L. Wang, X. Jing, F. Wang, Adv. Funct. Mater. 19 (2008) 84. [12] Y. Zheng, S.H. Eom, N. Chopra, J.W. Lee, F. So, J. Xue, Appl. Phys. Lett. 92 (2008) 223301. [13] E.L. Williams, K. Haavisto, J. Li, G.E. Jabbour, Adv. Mater. 19 (2007) 197. [14] H.I. Baek, C.H. Lee, J. Appl. Phys. 108 (2008) 124504. [15] F. Hide, P. Kozodoy, S.P. DenBaars, A.J. Heeger, Appl. Phys. Lett. 70 (1997) 2664. [16] S. Tokito, T. Lijima, T. Tsuzuki, F. Sato, Appl. Phys. Lett. 83 (2003) 2459.
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