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High-color-rendering pure-white phosphorescent organic light-emitting devices employing only two complementary colors
Organic Electronics 11 (2010) 266–272
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Organic Electronics journal homepage: www.elsevier.com/locate/orgel
High-color-rendering pure-white phosphorescent organic light-emitting devices employing only two complementary colors Chih-Hao Chang a, Chung-Chia Chen a, Chung-Chih Wu a,*, Sheng-Yuan Chang b, Jui-Yi Hung b, Yun Chi b,* a
Department of Electrical Engineering, Graduate Institute of Photonics and Optoelectronics, Graduate Institute of Electronics Engineering, National Taiwan University, Taipei, Taiwan 10617, ROC b Department of Chemistry, National Tsing Hua University, Hsinchu, Taiwan 300, ROC
a r t i c l e
i n f o
Article history: Received 8 September 2009 Received in revised form 26 October 2009 Accepted 1 November 2009 Available online 6 November 2009 Keywords: Osmium complex Iridium complex White OLEDs Color rendering index
a b s t r a c t We report successful fabrication of high-color-rendering pure-white phosphorescent organic light-emitting devices (OLEDs) by employing a true-blue iridium complex Ir(dfbppy)(fbppz)2 and a wide-bandwidth yellow emitting osmium complex Os(bptz)2(dppee). The two-component phosphorescent white OLED exhibited a high color rendering index (CRI) of 81 and CIEx,y coordinates close to the ideal white emission (0.33, 0.33). By doping the yellow phosphors into the hole-transport layer and the electron-transport layer adjacent to the blue-emitting layer and thereby forming a triple-emitting-layer device structure, we obtained WOLED which exhibited rather stable colors at different biases/ brightnesses. Such high-CRI pure-white two-component WOLEDs yielded EL efficiencies of up to 9.5%, 22.9 cd/A, and 20 lm/W for the forward directions. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction With intrinsically high quantum efficiency, white-emitting organic light-emitting devices (WOLEDs) incorporating phosphorescent emitters have become the most promising candidates for meeting the stringent efficiency requirements in lighting applications [1,2]. For high-quality white-light illumination, WOLEDs with CIEx,y coordinates similar to those of the blackbody radiation (with a correlated color temperature (CCT) between 2500 and 6500 K), and a color rendering index (CRI) above 80 are required [3]. Intuitively, the CRI can be enhanced by increasing the number of dopants to extend the spectral range of the electroluminescence (EL). For instance, in 2002, D’Andrade et al. demonstrated that the CRI of phosphorescent WOLEDs could be greatly improved from 50 to 83 by using
* Corresponding authors. Tel.: +886 2 33663636; fax: +886 2 23689178 (C.-C. Wu). E-mail addresses: [email protected] (C.-C. Wu), [email protected] (Y. Chi). 1566-1199/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2009.11.004
three (red, green and bluish-green) phosphorescent dopants instead of two complementary (red and bluishgreen) dopants [4]. As another example, in 2006 the same group combined blue fluorophors and red/green phosphors to generate white EL, in which deeper blue emission was obtained from the singlet emission of the fluorophors (to relax the requirement for the more challenging true-blue phosphors with high emission efficiencies) while red/green emission was obtained from the triplet emission of the phosphors [5]. In such hybrid WOLEDs, the CRI was improved to 85. In 2007, Schwartz et al. also successfully demonstrated high-CRI WOLEDs (CRI = 86) using a similar hybrid fluorophor/phosphor approach [6]. However, the adoption of a larger number of emissive dopants not only complicates device fabrication, but also renders the color control in WOLEDs fraught with difficulties. In addition, most reported phosphorescent WOLEDs yielded warm white, which would limit their applications in lighting. As such, generating white electrophosphorescence using less dopants (e.g. two complementary colors) while retaining a useful CRI is highly desired, since it would
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vastly simplify the device structures of WOLEDs. To achieve such a goal, it is necessary to develop complementary-color phosphors with extended bandwidths as well as good color rendering capabilities. Until recently, one of the major bottlenecks in implementing high-CRI two-component phosphorescent WOLEDs was the lack of efficient blue phosphors generating enough light output in the shorter wavelength region. For instance, the adoption of the common bluish-green phosphor FIrpic in the two-component WOLEDs generally yielded a poor CRI of <70 due to deficient color rendering in the blue region [7–9]. However, this can be compensated by adopting novel true-blue organic phosphors developed recently [10–13]. On the other hand, from the perspective of color mixing, widebandwidth complementary yellow phosphors are also needed to generate white emission with high CRI. In this paper, through the employment of a recently developed true-blue iridium complex Ir(dfbppy)(fbppz)2 and a newly developed wide-bandwidth yellow osmium complex Os(bptz)2(dppee) in appropriate device architectures, we report two-component pure-white phosphorescent WOLEDs with CRI of >80 and CIE coordinates close to the ideal equal-energy white emission (0.33, 0.33).
2. Experimental 2.1. Materials The blue phosphorescent iridium complex Ir(dfbppy)(fbppz)2, where dfbppy and fbppz stand for 4-tert-butyl-2-(2,4-difluorophenyl)pyridinato and 3-(trifluoromethyl)-5-(4-tert-butylpyridyl) pyrazolate, and the yellow phosphorescent osmium complex Os(bptz)2(dppee), where bptz and dppee stands for 3-tert-butyl-5-(2-pyridyl)-1,2,4triazolate and cis-1,2-bis(diphenylphosphino) ethene, whose chemical structures are shown in Fig. 1a, were employed in the preparation of two-component WOLEDs. As shown in literature, the heteroleptic IrIII complex Ir(dfbppy)(fbppz)2 revealed true-blue phosphorescence with CIEy of 0.2 [10,13]. In particular, placing bulky tertiary butyl substituents around the chelates had improved dispersion of dopant in the host matrix and reduced back energy transfer from dopant to host due to their increased spatial separation [13,14]. On the other hand, the yellow emitting Os(II) complex Os(bptz)2(dppee), showing a featureless emission peak centered at k = 611 nm, phosphorescence lifetime sobs = 1 ls and emission quantum yield / = 0.3 in degassed CH2Cl2 solution, was synthesized from treatment of Os3(CO)12 with bptzH and dppee in sequence, employing the synthetic methodology that described in literature [15]. Selected spectral data of Os(bptz)2(dppee): MS (FAB, 192 Os): m/z 990 (M+). 1H NMR (500 MHz, d6-acetone, 298 K): d8.04 (t, JHH = 9.0 Hz, 4H), 7.83–7.71 (m, 2H), 7.59–7.55 (m, 4H), 7.36 (dd, JHH = 7.5, 7.0 Hz, 2H), 7.29 (dd, JHH = 8.0, 7.0 Hz, 4H), 7.05–7.03 (m, 4H), 6.85 (dd, JHH = 7.5, 7.3 Hz, 4H), 6.79–6.75 (m, 6H), 1.46 (s, 18H). 31P NMR (202 MHz, d6-acetone, 298 K): d31.04 (s). Anal. Calcd. for C48H48N8OsP2: C, 58.29; N, 11.33; H, 4.89. Found: C, 58.26; N, 11.24; H, 4.85.
Fig. 1. (a) Chemical structures, (b) PL spectra (measured in CH2Cl2), and (c) CIE coordinates (PL) of Ir(dfbppy)(fbppz)2 and Os(bptz)2(dppee).
2.2. OLED fabrication All synthesized compounds were subject to temperature-gradient sublimation under high vacuum before use. OLEDs were fabricated on the ITO-coated glass substrates with multiple organic layers sandwiched between the transparent bottom indium–tin–oxide (ITO) anode and the top metal cathode. The organic and metal layers were deposited by thermal evaporation in a vacuum chamber with a base pressure of <106 torr. The deposition rate of organic layers was kept at 0.2 nm/s. The deposition system permitted the fabrication of the complete device structure in a single pump-down without breaking vacuum. The active area of the device was 2 2 mm2, as defined by the shadow mask for cathode deposition.
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2.3. Device characterizations Current–voltage–luminance (I–V–L) characterization of the devices was measured using an Agilent 4155B semiconductor parameter analyzer and a Si photodiode calibrated with Photo Research PR650. EL spectra of devices were collected by a calibrated CCD spectrograph. Total photon output from the device (either from the viewing direction or from all directions of the device) was measured in an integrating sphere containing a calibrated photodetector. 3. Results and discussions 3.1. Emission properties of the Ir and Os complexes Photoluminescence (PL) of Ir and Os complexes were measured using a Charge-Coupled Device (CCD) spectrograph and the 325 nm line of the He-Cd laser as the excitation source. For determining PL quantum yields, the samples were mounted in a calibrated integrating sphere
coupled with the CCD spectrograph. By comparing the spectral intensities of the excitation laser and the PL emission, quantum yields (PLQY) were determined [16]. The structural drawings, PL spectrum of Ir(dfbppy)(fbppz)2 and Os(bptz)2(dppee) in CH2Cl2 and their corresponding CIE coordinates are shown in Fig. 1a–c, respectively. The PL quantum yield of Ir(dfbppy)(fbppz)2 in degassed CH2Cl2 is calculated to be 0.68, for which true-blue emission showed a CIEy close to 0.2, while the PL spectrum of Os(bptz)2(dppee) in CH2Cl2 revealed a yellow–orange emission with a quantum yield of 0.3. With the CIE coordinates shown in Fig. 1c, these two phosphors are indeed quite complementary in colors and appropriate combinations of these two colors could possibly generate nearly ideal white emission with CIE coordinates close to (0.33, 0.33) (Fig. 1c). The main PL peak of Ir(dfbppy)(fbppz)2 locates at 449 nm and is accompanied by the vibronic bands peaking at 479 and 507 nm and extending into the blue–green region, which together would provide color rendering in the shorter wavelength region. The yellow emitter Os(bptz)2(dppee) exhibits a
Fig. 2. (a) Structural drawings of materials used in the fabrication of OLEDs; (b) Schematic structures of the various OLED devices.
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Fig. 4. The energy levels of different materials used in devices.
Fig. 3. (a) Current–voltage (I–V) characteristics, (b) external quantum efficiency versus luminance, and (c) power efficiency versus luminance for devices A, B and C.
broad PL peak at 611 nm and full-width at half-maximum (FWHM) of 120 nm, covering the spectral range all the way to saturated red. The exceeding large FWHMs of both the blue and yellow emission render possible fabrication of two-component phosphorescent WOLEDs with high CRI, if appropriate color mixing could be implemented by certain device architectures. 3.2. WOLEDs with double emitting layers In general, the WOLED device structures can be roughly classified into single- and multiple-emitting-layer (EML)
architectures. The fabrication of WOLEDs with the single EML is usually difficult because the multiple dopants and the host have to be evaporated simultaneously with precise ratio control. Furthermore, the relative intensities of the different emissions might vary upon variation of the operational biases. On the other hand, the device structure with multiple EMLs sometimes suffers from shifting of the recombination zone (RZ) upon increasing the biases. As such, the buffer layer is often inserted between the EMLs to prevent the RZ shift and to stabilize the emission color [17,18]. However, the hindered carrier transport at the buffer layer might degrade the power efficiency. In this work, without adopting the buffer layer, we studied various architectures for multiple-EML WOLEDs, with the aim of achieving stabilized colors. Three multiple-EML WOLED structures without the buffer layer were fabricated in this study (Fig. 2). To prevent the back transfer of the triplet excitons from the blue phosphors to the host material, a wide-gap host 9(4-tert-butylphenyl)-3,6-bis(triphenylsilyl)-9H-carbazole (CzSi), with a large triplet energy of 3.02 eV, was adopted for the blue-emitting layer [19]. In considering the carrier-transport properties and the energy-gap matching, both the hole-transport material 4,40 ,400 -tris(carbazole-9yl)-triphenylamine (TCTA) and the electron-transport material 2,9-dimethyl-4,7-diphenyl-1,10-phenanhroline (BCP) were tested as the host for the yellow emitting layer. Both are commonly used in green to red phosphorescent OLEDs [20]. From the phosphorescence measurements at 77 K, triplet energies of TCTA and BCP were determined to be 2.79 and 2.54 eV, respectively, both larger than that of Os(bptz)2(dppee). Hence, the yellow emitter was doped in the carrier transport layer (BCP or TCTA or both) next to the blue EML. We first tested two architectures with double EMLs. The first device configuration (device A, in which the blue phosphor was doped into the CzSi, while the yellow phosphor was doped into TCTA) was: ITO/a-NPD (30 nm)/TCTA (10 nm)/TCTA doped with 1 wt.% of Os(bptz)2(dppee) (5 nm)/CzSi doped with 6 wt.% of Ir(dfbppy)(fbppz)2
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(15 nm)/BCP (50 nm)/Cs2CO3 (2 nm)/Al (150 nm). 4,40 bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl (a-NPD) and TCTA were used as the hole-transport layer (HTLs), BCP as the electron-transport layer (ETL), and Cs2CO3 as the electron-injection layer (EIL). ITO and Al were used as the anode and cathode respectively. In device B, the yellow emitter was doped into the electron-transport layer BCP. The configuration of device B was: ITO/a-NPD (30 nm)/ TCTA (15 nm)/CzSi doped with 6 wt.% of Ir(dfbppy)(fbppz)2 (15 nm)/BCP doped with 2 wt.% of Os(bptz)2(dppee) (5 nm)/BCP (45 nm)/Cs2CO3 (2 nm)/Al (150 nm). The chemical structures of the materials used and the device structures are shown in Fig. 2. The current–voltage (I–V) characteristics of devices A and B are shown in Fig. 3a. From the I–V curves, the lower voltage of device B compared to that of device A suggested hole trapping associated with the yellow emitter in TCTA. Fig. 4 depicts the energy levels of materials used in the devices. The energy levels of hosts and carrier-transport materials were collected from the literatures while energy levels of the phosphorescent dopants [Os(bptz)2(dppee), Ir(dfbppy)(fbppz)2] were determined by cyclic voltammetry (CV) [10,19,21,22]. The ionization potential of Os(bptz)2(dppee) (4.8 eV) is substantially lower than those of TCTA, CzSi and Ir(dfbppy)(fbppz)2 (all of which range between 5.8 and 6.0 eV). In view of such an energy-level relationship, it is expected that the Os complexes will function as effective hole traps in TCTA in device A [23–28], but less likely as electron traps in BCP in device B. The EL spectra of devices A and B at different brightness levels are shown in Fig. 5a and b, respectively. With more contribution from the blue Ir complex and significantly less emission from the yellow Os complex, device A exhibited a bluish-white EL. Such EL characteristics indicated that the exciton formation occurred predominantly inside the CzSi layer, perhaps closer to the CzSi/BCP interface (even though the Os complex in TCTA demonstrated certain hole-trapping characteristics), since the CzSi was dominated by hole-transporting [19,29]. The significant efficiency roll-off in device A might also be associated with such a scenario, since the electron-transporting BCP (with lower triplet energy of 2.54 eV) can quench the high-energy triplet excitons of Ir(dfbppy)(fbppz)2) (2.8 eV) near the interface. On the other hand, doping the yellow Os complex into the electron-transporting BCP near the CzSi/ BCP interface (device B) greatly enhanced the yellow emission from the Os complex, resulting in a white EL with CIE coordinates close to (0.33, 0.33) (Table 1). This was due to the fact that the exciton formation zone was almost entirely located at the CzSi/BCP interface. Thus, the increased yellow emission was likely the result of both the direct exciton formation on the Os complex and the energy transfer from the Ir complex (or host) to the Os complex. Furthermore, although both devices A and B exhibited similar EL efficiencies at low current densities (peak external EL quantum efficiencies of 9% and 9.6% for devices A and B, respectively; Fig. 3b and Table 1), the efficiency roll-off in device B was nevertheless much less pronounced.
Fig. 5. Normalized EL spectra of OLED devices A, B and C at different brightness levels. EL spectra of all devices were collected at some fixed driving voltages (e.g. 4, 5, 6, 7, 8, 9 V, etc.), and the EL spectra shown in a– c represent those observed at the voltages giving brightness levels closest to 100 and 1000 cd/m2 for each device.
3.3. WOLEDs with triple emitting layers Although device B yielded nearly pure white EL, its CRI value was still below 80 and the color variation with brightness levels was noticeable. For device B, the intensity
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C.-H. Chang et al. / Organic Electronics 11 (2010) 266–272 Table 1 EL characteristics of devices A, B and C.
External quantum efficiency (%) Luminance efficiency (cd/A) Power efficiency (lm/W) Max. brightness (cd/m2) (V) CIE coordinates CRI/CCT (K)
Peak 102 cd/m2 Peak 102 cd/m2 Peak 102 cd/m2 102 cd/m2 103 cd/m2 102 cd/m2 103 cd/m2
Device A (2 EML)
Device B (2 EML)
Device C (3 EML)
9.6 4.2 21.5 7.5 18.8 4.1 7731 (11.6 V) (0.211, 0.249) (0.208, 0.245) – –
9.6 6.8 22.5 15.3 19.6 10.0 13368 (10.8 V) (0.324, 0.343) (0.339, 0.354) 79 / 5861 78 / 5229
9.5 7.0 22.9 15.0 20.0 8.7 10271 (11.0 V) (0.311, 0.327) (0.314, 0.332) 81 / 6592 81 / 6400
of yellow emission grew relative to the intensity of the blue emission and the CRI dropped with the bias/brightness. It might imply that the exciton recombination zone migrated towards the BCP EML when the bias was increased, leading to larger contribution from the yellow emission of the Os complex. Such effects may be suppressed or reduced by utilizing the hole trapping phenomenon in the device structure A; the hole-trapping mechanism might be used to retard the shift of the recombination zone toward the BCP interface and thus to stabilize EL colors with biases. Based on these considerations, we further tested a device C with the yellow Os complex doped into portions of both the hole-transport layer TCTA and electron-transport layer BCP adjacent to the blue-emitting layer. The configuration of device C with triple EMLs was: ITO/a-NPD (30 nm)/TCTA (10 nm)/TCTA doped with 1 wt.% of Os(bptz)2(dppee) (5 nm)/CzSi doped with 6 wt.% of Ir(dfbppy)(fbppz)2 (15 nm)/BCP doped with 2 wt.% of Os(bptz)2(dppee) (5 nm)/BCP (45 nm)/Cs2CO3 (2 nm)/Al (150 nm) (shown in Fig. 2). In Fig. 3a, one observed that device C exhibited I–V characteristics very similar to those of device A (and higher voltage than device B), indicating similar hole-trapping effects from the Os(bptz)2(dppee) doped TCTA. Fig. 5c shows the EL spectra of device C at different brightness levels. The corresponding CIE coordinates of the EL spectra are tabulated in Table 1. For the brightness from <102–103 cd/m2, the device exhibited nearly pure white EL with CIE coordinates of (0.311, 0.327)–(0.314, 0.332), which are close to the ideal equal-energy white (0.33, 0.33). With a color variation of only about (0.003, 0.005), the color was rather stable, indicating the effectiveness of the aforementioned device strategy and architecture. Furthermore, the CRI of device C at different brightness levels (Table 1) were higher than 80 (81 indeed). To the best of our knowledge, reports of two-component pure-white phosphorescent OLEDs with a CRI of >80 are very rare. The EL efficiency characteristics of device C were similar to those of device B (Fig. 3(b), 3c and Table 1). The device C exhibited peak EL efficiencies of 9.5% photon/electron, 22.9 cd/A, and 20 lm/W for the forward directions. At the practical brightness of 100 cd/m2, the forward efficiency remained high at around 7.0%, 15.0 cd/A, and 8.7 lm/W. For lighting applications, the light emitted from all surfaces of the substrate can in principle be redirected to the
forward direction via certain lighting fixtures; hence the total efficiency of the device was also characterized by using an integrating sphere setup [30]. The total quantum and power efficiencies measured in our sphere setup were about 1.7 larger than the forward viewing efficiencies, which were consistent with previous reports [3,31–33]. In this regard, device C indeed had a total peak external quantum efficiency and a total power efficiency of 16.2% and 34 lm/W, respectively. 4. Conclusions In conclusion, by utilizing a true-blue iridium complex Ir(dfbppy)(fbppz)2 and a wide-bandwidth yellow osmium complex Os(bptz)2(dppee), we obtained a highly efficient phosphorescent WOLEDs. The two-component phosphorescent WOLED exhibited a high CRI of 81 and CIE coordinates close to the ideal white emission (0.33, 0.33). By doping the yellow phosphors into the hole-transport layer and the electron-transport layer adjacent to the blue-emitting layer and thereby forming a triple-emitting-layer device structure, we obtained WOLED which exhibited rather stable colors at different biases/brightnesses. Such high-CRI pure-white two-component WOLEDs yielded EL efficiencies of up to 9.5%, 22.9 cd/A, and 20 lm/W for the forward directions. Acknowledgements The authors gratefully acknowledge the financial support from National Science Council and Ministry of Economic Affairs of Taiwan. References [1] Y. Sun, S.R. Forrest, Appl. Phys. Lett. 91 (2007) 263503. [2] S. Reineke, F. Lindner, G. Schwartz, N. Seidler, K. Walzer, B. Lüssem, K. Leo, Nature 459 (2009) 234–238. [3] B.W. D’Andrade, S.R. Forrest, Adv. Mater. 16 (2004) 1585–1595. [4] B.W. D’Andrade, M.E. Thompson, S.R. Forrest, Adv. Mater. 14 (2002) 147–151. [5] Y. Sun, N.C. Giebink, H. Kanno, B. Ma, M.E. Thompson, S.R. Forrest, Nature 440 (2006) 908–912. [6] G. Schwartz, M. Pfeiffer, S. Reineke, K. Walzer, K. Leo, Adv. Mater. 19 (2007) 3672–3676. [7] C.-H. Chang, Y.-J. Lu, C.-C. Liu, Y.-H. Yeh, C.-C. Wu, J. Disp. Technol. 3 (2007) 193–199. [8] S.H. Kim, J. Jang, J.Y. Lee, Appl. Phys. Lett. 91 (2007) 123509.
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Organic Electronics journal homepage: www.elsevier.com/locate/orgel
High-color-rendering pure-white phosphorescent organic light-emitting devices employing only two complementary colors Chih-Hao Chang a, Chung-Chia Chen a, Chung-Chih Wu a,*, Sheng-Yuan Chang b, Jui-Yi Hung b, Yun Chi b,* a
Department of Electrical Engineering, Graduate Institute of Photonics and Optoelectronics, Graduate Institute of Electronics Engineering, National Taiwan University, Taipei, Taiwan 10617, ROC b Department of Chemistry, National Tsing Hua University, Hsinchu, Taiwan 300, ROC
a r t i c l e
i n f o
Article history: Received 8 September 2009 Received in revised form 26 October 2009 Accepted 1 November 2009 Available online 6 November 2009 Keywords: Osmium complex Iridium complex White OLEDs Color rendering index
a b s t r a c t We report successful fabrication of high-color-rendering pure-white phosphorescent organic light-emitting devices (OLEDs) by employing a true-blue iridium complex Ir(dfbppy)(fbppz)2 and a wide-bandwidth yellow emitting osmium complex Os(bptz)2(dppee). The two-component phosphorescent white OLED exhibited a high color rendering index (CRI) of 81 and CIEx,y coordinates close to the ideal white emission (0.33, 0.33). By doping the yellow phosphors into the hole-transport layer and the electron-transport layer adjacent to the blue-emitting layer and thereby forming a triple-emitting-layer device structure, we obtained WOLED which exhibited rather stable colors at different biases/ brightnesses. Such high-CRI pure-white two-component WOLEDs yielded EL efficiencies of up to 9.5%, 22.9 cd/A, and 20 lm/W for the forward directions. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction With intrinsically high quantum efficiency, white-emitting organic light-emitting devices (WOLEDs) incorporating phosphorescent emitters have become the most promising candidates for meeting the stringent efficiency requirements in lighting applications [1,2]. For high-quality white-light illumination, WOLEDs with CIEx,y coordinates similar to those of the blackbody radiation (with a correlated color temperature (CCT) between 2500 and 6500 K), and a color rendering index (CRI) above 80 are required [3]. Intuitively, the CRI can be enhanced by increasing the number of dopants to extend the spectral range of the electroluminescence (EL). For instance, in 2002, D’Andrade et al. demonstrated that the CRI of phosphorescent WOLEDs could be greatly improved from 50 to 83 by using
* Corresponding authors. Tel.: +886 2 33663636; fax: +886 2 23689178 (C.-C. Wu). E-mail addresses: [email protected] (C.-C. Wu), [email protected] (Y. Chi). 1566-1199/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2009.11.004
three (red, green and bluish-green) phosphorescent dopants instead of two complementary (red and bluishgreen) dopants [4]. As another example, in 2006 the same group combined blue fluorophors and red/green phosphors to generate white EL, in which deeper blue emission was obtained from the singlet emission of the fluorophors (to relax the requirement for the more challenging true-blue phosphors with high emission efficiencies) while red/green emission was obtained from the triplet emission of the phosphors [5]. In such hybrid WOLEDs, the CRI was improved to 85. In 2007, Schwartz et al. also successfully demonstrated high-CRI WOLEDs (CRI = 86) using a similar hybrid fluorophor/phosphor approach [6]. However, the adoption of a larger number of emissive dopants not only complicates device fabrication, but also renders the color control in WOLEDs fraught with difficulties. In addition, most reported phosphorescent WOLEDs yielded warm white, which would limit their applications in lighting. As such, generating white electrophosphorescence using less dopants (e.g. two complementary colors) while retaining a useful CRI is highly desired, since it would
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vastly simplify the device structures of WOLEDs. To achieve such a goal, it is necessary to develop complementary-color phosphors with extended bandwidths as well as good color rendering capabilities. Until recently, one of the major bottlenecks in implementing high-CRI two-component phosphorescent WOLEDs was the lack of efficient blue phosphors generating enough light output in the shorter wavelength region. For instance, the adoption of the common bluish-green phosphor FIrpic in the two-component WOLEDs generally yielded a poor CRI of <70 due to deficient color rendering in the blue region [7–9]. However, this can be compensated by adopting novel true-blue organic phosphors developed recently [10–13]. On the other hand, from the perspective of color mixing, widebandwidth complementary yellow phosphors are also needed to generate white emission with high CRI. In this paper, through the employment of a recently developed true-blue iridium complex Ir(dfbppy)(fbppz)2 and a newly developed wide-bandwidth yellow osmium complex Os(bptz)2(dppee) in appropriate device architectures, we report two-component pure-white phosphorescent WOLEDs with CRI of >80 and CIE coordinates close to the ideal equal-energy white emission (0.33, 0.33).
2. Experimental 2.1. Materials The blue phosphorescent iridium complex Ir(dfbppy)(fbppz)2, where dfbppy and fbppz stand for 4-tert-butyl-2-(2,4-difluorophenyl)pyridinato and 3-(trifluoromethyl)-5-(4-tert-butylpyridyl) pyrazolate, and the yellow phosphorescent osmium complex Os(bptz)2(dppee), where bptz and dppee stands for 3-tert-butyl-5-(2-pyridyl)-1,2,4triazolate and cis-1,2-bis(diphenylphosphino) ethene, whose chemical structures are shown in Fig. 1a, were employed in the preparation of two-component WOLEDs. As shown in literature, the heteroleptic IrIII complex Ir(dfbppy)(fbppz)2 revealed true-blue phosphorescence with CIEy of 0.2 [10,13]. In particular, placing bulky tertiary butyl substituents around the chelates had improved dispersion of dopant in the host matrix and reduced back energy transfer from dopant to host due to their increased spatial separation [13,14]. On the other hand, the yellow emitting Os(II) complex Os(bptz)2(dppee), showing a featureless emission peak centered at k = 611 nm, phosphorescence lifetime sobs = 1 ls and emission quantum yield / = 0.3 in degassed CH2Cl2 solution, was synthesized from treatment of Os3(CO)12 with bptzH and dppee in sequence, employing the synthetic methodology that described in literature [15]. Selected spectral data of Os(bptz)2(dppee): MS (FAB, 192 Os): m/z 990 (M+). 1H NMR (500 MHz, d6-acetone, 298 K): d8.04 (t, JHH = 9.0 Hz, 4H), 7.83–7.71 (m, 2H), 7.59–7.55 (m, 4H), 7.36 (dd, JHH = 7.5, 7.0 Hz, 2H), 7.29 (dd, JHH = 8.0, 7.0 Hz, 4H), 7.05–7.03 (m, 4H), 6.85 (dd, JHH = 7.5, 7.3 Hz, 4H), 6.79–6.75 (m, 6H), 1.46 (s, 18H). 31P NMR (202 MHz, d6-acetone, 298 K): d31.04 (s). Anal. Calcd. for C48H48N8OsP2: C, 58.29; N, 11.33; H, 4.89. Found: C, 58.26; N, 11.24; H, 4.85.
Fig. 1. (a) Chemical structures, (b) PL spectra (measured in CH2Cl2), and (c) CIE coordinates (PL) of Ir(dfbppy)(fbppz)2 and Os(bptz)2(dppee).
2.2. OLED fabrication All synthesized compounds were subject to temperature-gradient sublimation under high vacuum before use. OLEDs were fabricated on the ITO-coated glass substrates with multiple organic layers sandwiched between the transparent bottom indium–tin–oxide (ITO) anode and the top metal cathode. The organic and metal layers were deposited by thermal evaporation in a vacuum chamber with a base pressure of <106 torr. The deposition rate of organic layers was kept at 0.2 nm/s. The deposition system permitted the fabrication of the complete device structure in a single pump-down without breaking vacuum. The active area of the device was 2 2 mm2, as defined by the shadow mask for cathode deposition.
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2.3. Device characterizations Current–voltage–luminance (I–V–L) characterization of the devices was measured using an Agilent 4155B semiconductor parameter analyzer and a Si photodiode calibrated with Photo Research PR650. EL spectra of devices were collected by a calibrated CCD spectrograph. Total photon output from the device (either from the viewing direction or from all directions of the device) was measured in an integrating sphere containing a calibrated photodetector. 3. Results and discussions 3.1. Emission properties of the Ir and Os complexes Photoluminescence (PL) of Ir and Os complexes were measured using a Charge-Coupled Device (CCD) spectrograph and the 325 nm line of the He-Cd laser as the excitation source. For determining PL quantum yields, the samples were mounted in a calibrated integrating sphere
coupled with the CCD spectrograph. By comparing the spectral intensities of the excitation laser and the PL emission, quantum yields (PLQY) were determined [16]. The structural drawings, PL spectrum of Ir(dfbppy)(fbppz)2 and Os(bptz)2(dppee) in CH2Cl2 and their corresponding CIE coordinates are shown in Fig. 1a–c, respectively. The PL quantum yield of Ir(dfbppy)(fbppz)2 in degassed CH2Cl2 is calculated to be 0.68, for which true-blue emission showed a CIEy close to 0.2, while the PL spectrum of Os(bptz)2(dppee) in CH2Cl2 revealed a yellow–orange emission with a quantum yield of 0.3. With the CIE coordinates shown in Fig. 1c, these two phosphors are indeed quite complementary in colors and appropriate combinations of these two colors could possibly generate nearly ideal white emission with CIE coordinates close to (0.33, 0.33) (Fig. 1c). The main PL peak of Ir(dfbppy)(fbppz)2 locates at 449 nm and is accompanied by the vibronic bands peaking at 479 and 507 nm and extending into the blue–green region, which together would provide color rendering in the shorter wavelength region. The yellow emitter Os(bptz)2(dppee) exhibits a
Fig. 2. (a) Structural drawings of materials used in the fabrication of OLEDs; (b) Schematic structures of the various OLED devices.
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Fig. 4. The energy levels of different materials used in devices.
Fig. 3. (a) Current–voltage (I–V) characteristics, (b) external quantum efficiency versus luminance, and (c) power efficiency versus luminance for devices A, B and C.
broad PL peak at 611 nm and full-width at half-maximum (FWHM) of 120 nm, covering the spectral range all the way to saturated red. The exceeding large FWHMs of both the blue and yellow emission render possible fabrication of two-component phosphorescent WOLEDs with high CRI, if appropriate color mixing could be implemented by certain device architectures. 3.2. WOLEDs with double emitting layers In general, the WOLED device structures can be roughly classified into single- and multiple-emitting-layer (EML)
architectures. The fabrication of WOLEDs with the single EML is usually difficult because the multiple dopants and the host have to be evaporated simultaneously with precise ratio control. Furthermore, the relative intensities of the different emissions might vary upon variation of the operational biases. On the other hand, the device structure with multiple EMLs sometimes suffers from shifting of the recombination zone (RZ) upon increasing the biases. As such, the buffer layer is often inserted between the EMLs to prevent the RZ shift and to stabilize the emission color [17,18]. However, the hindered carrier transport at the buffer layer might degrade the power efficiency. In this work, without adopting the buffer layer, we studied various architectures for multiple-EML WOLEDs, with the aim of achieving stabilized colors. Three multiple-EML WOLED structures without the buffer layer were fabricated in this study (Fig. 2). To prevent the back transfer of the triplet excitons from the blue phosphors to the host material, a wide-gap host 9(4-tert-butylphenyl)-3,6-bis(triphenylsilyl)-9H-carbazole (CzSi), with a large triplet energy of 3.02 eV, was adopted for the blue-emitting layer [19]. In considering the carrier-transport properties and the energy-gap matching, both the hole-transport material 4,40 ,400 -tris(carbazole-9yl)-triphenylamine (TCTA) and the electron-transport material 2,9-dimethyl-4,7-diphenyl-1,10-phenanhroline (BCP) were tested as the host for the yellow emitting layer. Both are commonly used in green to red phosphorescent OLEDs [20]. From the phosphorescence measurements at 77 K, triplet energies of TCTA and BCP were determined to be 2.79 and 2.54 eV, respectively, both larger than that of Os(bptz)2(dppee). Hence, the yellow emitter was doped in the carrier transport layer (BCP or TCTA or both) next to the blue EML. We first tested two architectures with double EMLs. The first device configuration (device A, in which the blue phosphor was doped into the CzSi, while the yellow phosphor was doped into TCTA) was: ITO/a-NPD (30 nm)/TCTA (10 nm)/TCTA doped with 1 wt.% of Os(bptz)2(dppee) (5 nm)/CzSi doped with 6 wt.% of Ir(dfbppy)(fbppz)2
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(15 nm)/BCP (50 nm)/Cs2CO3 (2 nm)/Al (150 nm). 4,40 bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl (a-NPD) and TCTA were used as the hole-transport layer (HTLs), BCP as the electron-transport layer (ETL), and Cs2CO3 as the electron-injection layer (EIL). ITO and Al were used as the anode and cathode respectively. In device B, the yellow emitter was doped into the electron-transport layer BCP. The configuration of device B was: ITO/a-NPD (30 nm)/ TCTA (15 nm)/CzSi doped with 6 wt.% of Ir(dfbppy)(fbppz)2 (15 nm)/BCP doped with 2 wt.% of Os(bptz)2(dppee) (5 nm)/BCP (45 nm)/Cs2CO3 (2 nm)/Al (150 nm). The chemical structures of the materials used and the device structures are shown in Fig. 2. The current–voltage (I–V) characteristics of devices A and B are shown in Fig. 3a. From the I–V curves, the lower voltage of device B compared to that of device A suggested hole trapping associated with the yellow emitter in TCTA. Fig. 4 depicts the energy levels of materials used in the devices. The energy levels of hosts and carrier-transport materials were collected from the literatures while energy levels of the phosphorescent dopants [Os(bptz)2(dppee), Ir(dfbppy)(fbppz)2] were determined by cyclic voltammetry (CV) [10,19,21,22]. The ionization potential of Os(bptz)2(dppee) (4.8 eV) is substantially lower than those of TCTA, CzSi and Ir(dfbppy)(fbppz)2 (all of which range between 5.8 and 6.0 eV). In view of such an energy-level relationship, it is expected that the Os complexes will function as effective hole traps in TCTA in device A [23–28], but less likely as electron traps in BCP in device B. The EL spectra of devices A and B at different brightness levels are shown in Fig. 5a and b, respectively. With more contribution from the blue Ir complex and significantly less emission from the yellow Os complex, device A exhibited a bluish-white EL. Such EL characteristics indicated that the exciton formation occurred predominantly inside the CzSi layer, perhaps closer to the CzSi/BCP interface (even though the Os complex in TCTA demonstrated certain hole-trapping characteristics), since the CzSi was dominated by hole-transporting [19,29]. The significant efficiency roll-off in device A might also be associated with such a scenario, since the electron-transporting BCP (with lower triplet energy of 2.54 eV) can quench the high-energy triplet excitons of Ir(dfbppy)(fbppz)2) (2.8 eV) near the interface. On the other hand, doping the yellow Os complex into the electron-transporting BCP near the CzSi/ BCP interface (device B) greatly enhanced the yellow emission from the Os complex, resulting in a white EL with CIE coordinates close to (0.33, 0.33) (Table 1). This was due to the fact that the exciton formation zone was almost entirely located at the CzSi/BCP interface. Thus, the increased yellow emission was likely the result of both the direct exciton formation on the Os complex and the energy transfer from the Ir complex (or host) to the Os complex. Furthermore, although both devices A and B exhibited similar EL efficiencies at low current densities (peak external EL quantum efficiencies of 9% and 9.6% for devices A and B, respectively; Fig. 3b and Table 1), the efficiency roll-off in device B was nevertheless much less pronounced.
Fig. 5. Normalized EL spectra of OLED devices A, B and C at different brightness levels. EL spectra of all devices were collected at some fixed driving voltages (e.g. 4, 5, 6, 7, 8, 9 V, etc.), and the EL spectra shown in a– c represent those observed at the voltages giving brightness levels closest to 100 and 1000 cd/m2 for each device.
3.3. WOLEDs with triple emitting layers Although device B yielded nearly pure white EL, its CRI value was still below 80 and the color variation with brightness levels was noticeable. For device B, the intensity
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C.-H. Chang et al. / Organic Electronics 11 (2010) 266–272 Table 1 EL characteristics of devices A, B and C.
External quantum efficiency (%) Luminance efficiency (cd/A) Power efficiency (lm/W) Max. brightness (cd/m2) (V) CIE coordinates CRI/CCT (K)
Peak 102 cd/m2 Peak 102 cd/m2 Peak 102 cd/m2 102 cd/m2 103 cd/m2 102 cd/m2 103 cd/m2
Device A (2 EML)
Device B (2 EML)
Device C (3 EML)
9.6 4.2 21.5 7.5 18.8 4.1 7731 (11.6 V) (0.211, 0.249) (0.208, 0.245) – –
9.6 6.8 22.5 15.3 19.6 10.0 13368 (10.8 V) (0.324, 0.343) (0.339, 0.354) 79 / 5861 78 / 5229
9.5 7.0 22.9 15.0 20.0 8.7 10271 (11.0 V) (0.311, 0.327) (0.314, 0.332) 81 / 6592 81 / 6400
of yellow emission grew relative to the intensity of the blue emission and the CRI dropped with the bias/brightness. It might imply that the exciton recombination zone migrated towards the BCP EML when the bias was increased, leading to larger contribution from the yellow emission of the Os complex. Such effects may be suppressed or reduced by utilizing the hole trapping phenomenon in the device structure A; the hole-trapping mechanism might be used to retard the shift of the recombination zone toward the BCP interface and thus to stabilize EL colors with biases. Based on these considerations, we further tested a device C with the yellow Os complex doped into portions of both the hole-transport layer TCTA and electron-transport layer BCP adjacent to the blue-emitting layer. The configuration of device C with triple EMLs was: ITO/a-NPD (30 nm)/TCTA (10 nm)/TCTA doped with 1 wt.% of Os(bptz)2(dppee) (5 nm)/CzSi doped with 6 wt.% of Ir(dfbppy)(fbppz)2 (15 nm)/BCP doped with 2 wt.% of Os(bptz)2(dppee) (5 nm)/BCP (45 nm)/Cs2CO3 (2 nm)/Al (150 nm) (shown in Fig. 2). In Fig. 3a, one observed that device C exhibited I–V characteristics very similar to those of device A (and higher voltage than device B), indicating similar hole-trapping effects from the Os(bptz)2(dppee) doped TCTA. Fig. 5c shows the EL spectra of device C at different brightness levels. The corresponding CIE coordinates of the EL spectra are tabulated in Table 1. For the brightness from <102–103 cd/m2, the device exhibited nearly pure white EL with CIE coordinates of (0.311, 0.327)–(0.314, 0.332), which are close to the ideal equal-energy white (0.33, 0.33). With a color variation of only about (0.003, 0.005), the color was rather stable, indicating the effectiveness of the aforementioned device strategy and architecture. Furthermore, the CRI of device C at different brightness levels (Table 1) were higher than 80 (81 indeed). To the best of our knowledge, reports of two-component pure-white phosphorescent OLEDs with a CRI of >80 are very rare. The EL efficiency characteristics of device C were similar to those of device B (Fig. 3(b), 3c and Table 1). The device C exhibited peak EL efficiencies of 9.5% photon/electron, 22.9 cd/A, and 20 lm/W for the forward directions. At the practical brightness of 100 cd/m2, the forward efficiency remained high at around 7.0%, 15.0 cd/A, and 8.7 lm/W. For lighting applications, the light emitted from all surfaces of the substrate can in principle be redirected to the
forward direction via certain lighting fixtures; hence the total efficiency of the device was also characterized by using an integrating sphere setup [30]. The total quantum and power efficiencies measured in our sphere setup were about 1.7 larger than the forward viewing efficiencies, which were consistent with previous reports [3,31–33]. In this regard, device C indeed had a total peak external quantum efficiency and a total power efficiency of 16.2% and 34 lm/W, respectively. 4. Conclusions In conclusion, by utilizing a true-blue iridium complex Ir(dfbppy)(fbppz)2 and a wide-bandwidth yellow osmium complex Os(bptz)2(dppee), we obtained a highly efficient phosphorescent WOLEDs. The two-component phosphorescent WOLED exhibited a high CRI of 81 and CIE coordinates close to the ideal white emission (0.33, 0.33). By doping the yellow phosphors into the hole-transport layer and the electron-transport layer adjacent to the blue-emitting layer and thereby forming a triple-emitting-layer device structure, we obtained WOLED which exhibited rather stable colors at different biases/brightnesses. Such high-CRI pure-white two-component WOLEDs yielded EL efficiencies of up to 9.5%, 22.9 cd/A, and 20 lm/W for the forward directions. Acknowledgements The authors gratefully acknowledge the financial support from National Science Council and Ministry of Economic Affairs of Taiwan. References [1] Y. Sun, S.R. Forrest, Appl. Phys. Lett. 91 (2007) 263503. [2] S. Reineke, F. Lindner, G. Schwartz, N. Seidler, K. Walzer, B. Lüssem, K. Leo, Nature 459 (2009) 234–238. [3] B.W. D’Andrade, S.R. Forrest, Adv. Mater. 16 (2004) 1585–1595. [4] B.W. D’Andrade, M.E. Thompson, S.R. Forrest, Adv. Mater. 14 (2002) 147–151. [5] Y. Sun, N.C. Giebink, H. Kanno, B. Ma, M.E. Thompson, S.R. Forrest, Nature 440 (2006) 908–912. [6] G. Schwartz, M. Pfeiffer, S. Reineke, K. Walzer, K. Leo, Adv. Mater. 19 (2007) 3672–3676. [7] C.-H. Chang, Y.-J. Lu, C.-C. Liu, Y.-H. Yeh, C.-C. Wu, J. Disp. Technol. 3 (2007) 193–199. [8] S.H. Kim, J. Jang, J.Y. Lee, Appl. Phys. Lett. 91 (2007) 123509.
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