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High color rending index and high-efficiency white organic light-emitting diodes based on the control of red phosphorescent dye-doped hole transport layer M. Y. Zhang,1 F. F. Wang,1 N. Wei, 1 P. C. Zhou,1 K. J. Peng,2 J. N. Yu,1 Z. X. Wang,1 and B. Wei,1,* 1

Key Laboratory of Advanced Display and System Applications, Ministry of Education, Shanghai University, 149 Yanchang Road, Shanghai, 200072, China 2 The Department of Chemistry, Shanghai University, 99 Shangda Road, Shanghai, 200444, China * [email protected]

Abstract: We have investigated the transport characteristics of red phosphorescent dye bis(1-(phenyl)isoquinoline) iridium (III) doped 4,4’,4”-tri(Nacetylanetonate (Ir(piq)2acac) carbazolyl)triphenylamine (TCTA), and found that the increasing doping ratio was facilitated to improve the ability of hole transporting. A high color rendering index (CRI) and high-efficiency WOLED was achieved by employing Ir(piq)2acac doped TCTA film as an effective red emissive layer due to the generation of charge transfer complex (CTC) at the interface. The relative proportion in red: green: blue emission intensity can be controlled by the CTC concentration to obtain high CRI WOLEDs. The WOLED with an optimal red dye doping concentration of 5 wt% exhibits a high CRI of 89 and a power efficiency of 31.2 lm/W and 27.5 lm/W at the initial luminance and 100 cd/m2, respectively. The devices show little variation of the Commission Internationale de I’Eclairage coordinates in a wide range of luminance. ©2012 Optical Society of America OCIS codes: (310.6860) Thin films, optical properties; (230.3670) Light-emitting diodes; (330.1715) Color, rendering and metamerism; (160.4670) Organic materials.

References and links 1.

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Received 26 Jul 2012; revised 17 Oct 2012; accepted 17 Oct 2012; published 21 Dec 2012 14 January 2013 / Vol. 21, No. S1 / OPTICS EXPRESS A173

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1. Introduction White organic light-emitting diodes (OLEDs) are under intense investigation for the application of solid-state lighting and full color display nowadays and more countries specify minimum requirements for both the luminous efficiency and color rendering index (CRI) for solid-state lighting products [1, 2]. As a unique property of a light source, CRI is defined as “effect of an illuminant on the color appearance of objects by conscious or subconscious comparison with their color appearance under a reference illuminant” and plays a critical role in high-quality lighting system and full color display [3]. To date, some approaches to fabricating a high efficient and high-CRI white OLED have been developed, including a single emissive layer structure doped with different materials [4,5], and stacked multi-emissive layers structure in which each layer emits different light colors to generate combined white light [6–11] or electro-phosphorescent white OLEDs [12], as well as a tandem cell structure. Among these various devices, a multi-emissive layers structure white OLEDs have been fabricated [13, 14]. For example, Leo et al. reported a white OLED that used deep-blue fluorescent as well as green and red phosphorescent emitters that exhibited a CRI of 86 and 22 lm/W efficiency at 1000 cd/m2 [15]. Yang et al. reported a triple emissive-layer device with 91 CRI and 3.1 lm/W efficiency [16]. Jou et al. reported a double white emissive-layers device with a CRI of 96 and 5.2 lm/W efficiency [17]. Many reports on high CRI white OLEDs have been published shows that there is still considerable room for improvement in the efficiency and high CRI devices. 2. Experimental We have first fabricated hole-only devices: ITO/MoO3(2 nm)/NPB(20 nm)/TCTA: X wt% Ir(piq)2acac(30 nm)/Ag(10 nm)/Al(100 nm). In order to investigate the effect of dye doping ration on the hole-transporting property, we varied X to be 0, 2, 5 and 10 respectively. We further employed the investigated hole transport layer in white OLED: ITO/2T-NATA(45 nm)/TCTA(3 nm)/TCTA: X wt% Ir(piq)2acac(2 nm) /CBP:10 wt% Ir(ppy)3(5 nm)/CBP(5 nm)/ TBADN:2 wt% DPVPA(5 nm)/ Bphen(30 nm) / LiF(0.3 nm)/ Al (100 nm). We name white OLEDs Devices A, B, and C corresponding to the various X value of 2, 5 and 10. Here, 4,4,4-{N-(2-naphthyl)-N-phenylamino}-triphenylamine (2T-NATA) was used as the hole-injection layer (HIL), 4,4’,4”-tri(N-carbazolyl)triphenylamine (TCTA) as the holetransport layer (HTL) and 4,7-diphenyl-1,10-phenanthroline (Bphen) as the electrontransport layer(ETL). 4,4'-Bis(9H-carbazol-9-yl)biphenyl (CBP) layer was employed as the space layer. X wt% bis(1-(phenyl)isoquinoline) iridium (III) acetylanetonate (Ir(piq)2acac) doped TCTA, 10 wt% tris(2-phenylpyridine)iridium(III) (Ir(ppy)3) doped CBP and 2 wt% 9,10-bis[4-(2,2-diphenylvinyl)phenyl]anthracene (DPVPA) doped 2-tert-butyl-9,10-bis-(βnaphthyl)-anthracene (TBADN) were used as red, green and blue emitting layers, respectively To verify whether the white OLED with host material of TCTA and the electron transport layer TBADN exhibit the best performances, we replaced the host of red emission layer TCTA with CBP to study its EL properties in Device D while TBADN was taken over by

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CBP in Device E. Also, Device F was fabricated in which the host of red, green and blue emission layers was the same material CBP (both TCTA and TBADN were replaced by CBP). Besides, all the devices had a red dye doping ration of 5 wt%. The devices were fabricated by means of vacuum thermal evaporation method. All organic layers and Al cathode were deposited by high vacuum (10−5 mbar) thermal evaporation onto a clean glass substrate precoated with a 220 nm thick indium tin oxide (ITO) layer which was UV-ozone treated before evaporation process. After the evaporation process, the current-voltage and luminance characteristics of devices were measured using a Keithley 2400 source meter and a PR-650 spectrometer, respectively. All measurements were made in the dark at room temperature. 3. Results and discussion 3.1 Effective hole transport characteristics of red phosphorescent doped host-guest system An effective hole injection is required in order to increase the efficiency of OLEDs. To improve the injection of hole, we fabricated a series of hole-only devices with the structure of red phosphorescent dye (Ir(piq)2acac) doped TCTA matrix. Ag was chosen for the bottom electrode due to its high work function which not only benefited the injection of holes but also reduced the electrons injected. Both the highest occupied molecular orbital (HOMO) level of MoO3 and N, N′-bis-(naphthyl)-N, N′-diphenyl-1, 1′-biphenyl-4, 4′-diamine (NPB) are around 5.3 eV, indicating that a small energy barrier (~0.4 eV) for hole injection exists when compared with the HOMO level for TCTA, which is used as hole transporting material. The small energy barrier facilitates the hole injection and a small improvement of charge injection is of great significance in this case. The proposed energy diagram and general structure of hole-only devices are shown in Fig. 1(a) and 1(b).

Fig. 1. (a) The proposed energy diagram, (b) general structure of hole-only devices and (c) the J-V characteristics of hole-only devices.

The J-V characteristics of hole-only devices are shown in Fig. 1(c). It is observed that the current density of hole-only devices increases markedly as the doping ratio increases. This dramatic increasing may be attributed to the effective hole trapping of Ir(piq)2acac in which part of holes is trapped by Ir(piq)2acac while transporting in TCTA and results in additional injection of charges. However, it should be noted that the current density increases slowly when the doping ratio is larger than 5 wt%. The results indicate that the effect of hole trapping is close to saturation and Ir(piq)2acac plays a minor role in the hole injection while the property of hole injection is determined by the TCTA largely. Our research demonstrates that appropriate doping concentration of Ir(piq)2acac could lead to good performance of holeonly devices. 3.2 Effect of red dye doping ratio on the performance of white OLEDs White OLEDs using a multi-emission layers structures in which the three primary colors were emitted from different organic have been fabricated. In these devices, Ir(piq)2acac is applicable not only as a red emission dye but also as an assistant for the hole transport layer.

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To obtain spectra similar to that of incandescent light as well as high efficiency, varied doping ratio is required. We have designed the following structure for Devices A, B and C while the doping ratio(X) is set to be 2, 5 and 10 separately. The band structure of Devices is illustrated in Fig. 2. The HOMO and lowest unoccupied molecular orbital (LUMO) levels of the DPVPA and TBADN reveal that holes can be directly captured by the DPVPA guest while electrons injected from Al are captured by TBADN largely. Moreover, holes will be confined in the TBADN layer for the large holeinjection barrier of 0.7 eV between TBADN and Bphen.

Fig. 2. The proposed energy diagram of Devices A, B and C.

It can be seen from Fig. 3(a) that the white spectra for red phosphorescent dye-doped devices cover the whole visible spectra with three peaks at 468 nm, 512 nm and 608 nm respectively. The intensities of red emission increased and blue emission decreased with respect to green emission is observed when the doping ratio increases from 2 wt% to 5 wt%. We deduce that a charge transfer complex (CTC) be generated at the interface between red and green emission layers. The CTC zone comprises a strong acceptor, Ir(piq)2acac doped TCTA layer and electron donor CBP layer. Electric-field or light may induce the formation of CTC which causes more electrons transport forward hole injection side. The lower triplet energy level of Ir(piq)2acac than that of Ir(ppy)3 makes electrons easier to be trapped by red dye. Therefore, with the increase in the doping ratio, more excitons are produced in the red emissive layer which results in a shift of recombination zone towards the red EML from blue EML. However, both red and blue emitting intensity drop off at the concentration of 10 wt% which may be caused by the quenching of singlet and triplet excitons (see Fig. 3(b)). We demonstrated that the highest CRI of 89 was achieved for the device with a doping ratio of 5.0 wt%, while CRI values decreased to 86 and 84 for the devices with the doping ratios of 2.0 wt% and 10.0 wt%, respectively.

Fig. 3. (a) EL spectra of reference Device (x = 0) and Devices A (x = 2), B (x = 5) and C (x = 10) at the current density of 0.08 mA/cm2. (b) The current density-voltage-luminance (J-V-L) characteristics of Devices A, B and C.

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The current density (J)-voltage (V)-luminance (L) and the current efficiency-luminancepower efficiency curves are plotted in Fig. 4(a) and 4(b). The Device B shows a better EL performance e.g. a high CRI of 89 with 31.2 lm/W at the current density of 0.08 mA/cm2 (30.7 cd/m2), or 27.5 lm/W at 100 cd/m2 and a maximum current efficiency of 33.7 cd/A, attributing to a balanced charge injection and an effective energy transfer.

Fig. 4. (a) The J-V-L characteristics of Device B. (b) The current efficiency-luminance-power efficiency characters of Device B. (c)The electroluminescence (EL) spectra of Device B as the voltage applied from 3.4 V to 7.4 V. The inset shows the blue emitting part of the spectra. (d) The CRI histogram at various voltages.

Figure 4(c) and 4(d) shows the EL spectra of Device B as the driving voltage ranges from 3.4 V to 7.4 V and CRI histogram at various bias voltages. It is intriguing to note that the red emission intensity of Device B decreases sharply from the 3.4 V to 4.4 V while it only changes a little as the voltage larger than 4.4 V. This change is considered to arise from different emission mechanism as the voltages varies, in which the charge trapping dominates at low bias voltage while triplet energy transfer takes it over at high voltages. On the contrary, there is a greatly reduce in blue emission at the beginning (inset in Fig. 4(c)), while the intensity of blue light rises remarkably as the bias voltage of above 4.4 V. This could be explained that excitons diffusion from the undoped CBP dominates when the bias voltage is larger than 4.4 V. Furthermore, slight emission peaked at 420 nm is observed which can be attributed to the incomplete energy transfer from undoped CBP in which more excitons are formed while only part of triplet excitons are efficiently transferred to green and red emission layers. Figure 4(d) exhibits the increasing trend for CRI value as the increasing voltage and we can see that the CRI reaches as high as 92 at the voltage of 8.0 V. The Commission Internationale de l’Eclairage (CIE) coordinates of the emissions shifts from (0.48, 0.44) at 3.4 V to (0.45, 0.45) at 7.4 V. 3.3 Effect of host material of blue and red emitters on the performance of white OLEDs To verify whether the host of red emission layer TCTA or blue emission layer TBADN fit the structure mostly, we have designed Devices D, E and F using different hosts of three emitting layers, as listed in Table 1.

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Table 1. Electroluminescent Properties of the Devices B, D, E and F Hosts of emitters Device

Red

Green

Doping ratio % Blue

Efficiency Von (V)

ŋp, max

lm/W

ŋp,100 cd/m2

lm/W

Device TCTA CBP TBADN 5.0 3.4 31.2 27.5 B Device CBP CBP TBADN 5.0 3.6 9.9 7.2 D Device TCTA CBP CBP 5.0 3.8 12.5 11.6 E Device CBP CBP CBP 5.0 4.2 8.4 8.3 F *Note: the CRI column is corresponding to the CRI value at the maximum power efficiency.

CRI *

ŋp,1000 cd/m2

lm/W 17.4

89

3.6

85

8.9

72

6.2

87

The EL properties of the devices are showed in Table 1. It is clearly that Device B shows a high efficiency of 31.2 lm/W and high CRI of 89 at the initial luminance, which is proved to be a better performance compared to other devices. Devices D, E and F exhibit a maximum efficiency of only 9.9 lm/W, 12.5 lm/W and 8.4 lm/W, respectively, which can be attributed to the following reasons: 1) electrons are less injected for lower charge mobility of CBP compared to TBADN; 2) there exists a big energy barrier at the interfaces between electron injection layer Bphen and blue emissive layer CBP or between hole injection and red emissive layer. 4. Conclusion We have investigated the hole transport characteristics of red phosphorescent dye (Ir(piq)2acac)-doped TCTA, and found that the increasing of the doping ratio could greatly improve the ability of hole transport. The high-efficiency and high-CRI WOLEDs have been achieved by using Ir(piq)2acac doped TCTA functioning both hole transport layer and a red emissive layer. The component ratio of blue emission to red emission can be controlled by the generation of charge transfer complex at the interface. In addition, different host materials which have both high charge transport mobility and appropriate energy levels have been investigated. An optimized WOLED has been demonstrated to exhibit a high CRI of 89 with a power efficiency of 31.2 lm/W and 27.5 lm/W at the initial luminance and 100 cd/m2, respectively. Acknowledgments This work was supported by the Key Innovation Project of Education Commission of Shanghai Municipality (12ZZ091, 12YZ021), the National Natural Science Foundation of China (61136003, 61275041) and Mechatronics Engineering Innovation Group Project, Innovation Program (11ZZ81).

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