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  • Editor: Christian Seassal
  • Vol. 21, Iss. S1 — Jan. 14, 2013
  • pp: A173–A178
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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, F. F. Wang, N. Wei, P. C. Zhou, K. J. Peng, J. N. Yu, Z. X. Wang, and B. Wei  »View Author Affiliations


Optics Express, Vol. 21, Issue S1, pp. A173-A178 (2013)
http://dx.doi.org/10.1364/OE.21.00A173


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Abstract

We have investigated the transport characteristics of red phosphorescent dye bis(1-(phenyl)isoquinoline) iridium (III) acetylanetonate (Ir(piq)2acac) doped 4,4’,4”-tri(N-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 OSA

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

1. B. W. D’Andrade and S. R. Forrest, “White organic light-emitting devices for solid-state lighting,” Adv. Mater. (Deerfield Beach Fla.) 16(18), 1585–1595 (2004). [CrossRef]

, 2

2. Y. Ohno, “Color rendering and luminous efficacy of white LED spectra,” Proc. SPIE 5530, 88–98 (2004). [CrossRef]

]. 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

3. CIE 17.4–1987, “International lighting vocabulary,” No. 845–02–59 (1987).

].

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

4. Y. W. Ko, C. H. Chung, J. H. Lee, Y. H. Kim, C. Y. Sohn, B. C. Kim, C. S. Hwang, Y. H. Song, J. Lim, Y. J. Ahn, G. W. Kang, N. Lee, and C. Lee, “Efficient white organic light emission by single emitting layer,” Thin Solid Films 426(1-2), 246–249 (2003). [CrossRef]

,5

5. R. S. Deshpande, V. Bulovic´, and S. R. Forrest, “White-light-emitting organic electroluminescent devices based on interlayer sequential energy transfer,” Appl. Phys. Lett. 75(7), 888–890 (1999). [CrossRef]

], and stacked multi-emissive layers structure in which each layer emits different light colors to generate combined white light [6

6. J. Kido, M. Kimura, and K. Nagai, “Multilayer white light-emitting organic electroluminescent device,” Science 267(5202), 1332–1334 (1995). [CrossRef] [PubMed]

11

11. T. H. Liu, Y. S. Wu, M. T. Lee, H. H. Chen, C. H. Liao, and C. H. Chen, “Highly efficient yellow and with organic electroluminescent devices base on sterically hindered tetra (t-butyl) rubrene dopant,” Appl. Phys. Lett. 85, 4304–4306 (2004). [CrossRef]

] or electro-phosphorescent white OLEDs [12

12. Y. Sun, N. C. Giebink, H. Kanno, B. Ma, M. E. Thompson, and S. R. Forrest, “Management of singlet and triplet excitons for efficient white organic light-emitting devices,” Nature 440(7086), 908–912 (2006). [CrossRef] [PubMed]

], as well as a tandem cell structure. Among these various devices, a multi-emissive layers structure white OLEDs have been fabricated [13

13. G. Li and J. Shinar, “Combinatorial fabrication and studies of bright white organic light-emitting devices based on emission from rubrene-doped 4,4′-bis(2,2′-diphenylvinyl)-1,1′-biphenyl,” Appl. Phys. Lett. 83(26), 5359–5361 (2003). [CrossRef]

, 14

14. K. O. Cheon and J. Shinar, “Bright white small molecular organic light-emitting devices based on a red-emitting guest-host layer and blue-emitting 4,4′-bis(2,2′-diphenylvinyl)-1,1′-biphenyl,” Appl. Phys. Lett. 81(9), 1738–1740 (2002). [CrossRef]

]. 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

15. S. Reineke, G. Schwartz, K. Walzer, and K. Leo, “Reduced efficiency roll-off in phosphorescent organic light emitting diodes by suppression of triplet-triplet annihilation,” Appl. Phys. Lett. 91, 123508 (2007).

]. Yang et al. reported a triple emissive-layer device with 91 CRI and 3.1 lm/W efficiency [16

16. H. S. Yang, Y. W. Shi, Y. Zhao, Y. L. Meng, W. Hu, J. Y. Hou, and S. Y. Liu, “High colour rendering index white organic light-emitting devices with three emitting layers,” Displays 29(4), 327–332 (2008). [CrossRef]

]. Jou et al. reported a double white emissive-layers device with a CRI of 96 and 5.2 lm/W efficiency [17

17. J. H. Jou, S. M. Shen, C. R. Lin, Y. S. Wang, Y. C. Chou, S. Z. Chen, and Y. C. Jou, “Efficient very-high rendering index organic light-emitting diode,” Org. Electron. 12(5), 865–868 (2011). [CrossRef]

]. 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 hole-transport layer (HTL) and 4,7-diphenyl-1,10-phenanthroline (Bphen) as the electron-transport 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-(β-naphthyl)-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 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)
Fig. 1 (a) The proposed energy diagram, (b) general structure of hole-only devices and (c) the J-V characteristics of hole-only devices.
and 1(b).

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 hole-only 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. 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
Fig. 2 The proposed energy diagram of Devices A, B and C.
. 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 hole-injection barrier of 0.7 eV between TBADN and Bphen.

It can be seen from Fig. 3(a)
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.
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.

The current density (J)-voltage (V)-luminance (L) and the current efficiency-luminance-power efficiency curves are plotted in Fig. 4(a)
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.
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.

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

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

table-icon
View This Table
.

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).

References and links

1.

B. W. D’Andrade and S. R. Forrest, “White organic light-emitting devices for solid-state lighting,” Adv. Mater. (Deerfield Beach Fla.) 16(18), 1585–1595 (2004). [CrossRef]

2.

Y. Ohno, “Color rendering and luminous efficacy of white LED spectra,” Proc. SPIE 5530, 88–98 (2004). [CrossRef]

3.

CIE 17.4–1987, “International lighting vocabulary,” No. 845–02–59 (1987).

4.

Y. W. Ko, C. H. Chung, J. H. Lee, Y. H. Kim, C. Y. Sohn, B. C. Kim, C. S. Hwang, Y. H. Song, J. Lim, Y. J. Ahn, G. W. Kang, N. Lee, and C. Lee, “Efficient white organic light emission by single emitting layer,” Thin Solid Films 426(1-2), 246–249 (2003). [CrossRef]

5.

R. S. Deshpande, V. Bulovic´, and S. R. Forrest, “White-light-emitting organic electroluminescent devices based on interlayer sequential energy transfer,” Appl. Phys. Lett. 75(7), 888–890 (1999). [CrossRef]

6.

J. Kido, M. Kimura, and K. Nagai, “Multilayer white light-emitting organic electroluminescent device,” Science 267(5202), 1332–1334 (1995). [CrossRef] [PubMed]

7.

G. T. Lei, L. D. Wang, and Y. Qiu, “Multilayer organic electrophosphorescent white light-emitting diodes without exciton-blocking layer,” Appl. Phys. Lett. 88(10), 103508 (2006). [CrossRef]

8.

C. W. Ko and Y. T. Tao, “Bright white organic light-emitting diode,” Appl. Phys. Lett. 79(25), 4234–4236 (2001). [CrossRef]

9.

Y. Sato, T. Ogata, S. Ichinosawa, and Y. Murata, “Characteristics of organic electroluminescent devices with new dopants,” Synth. Met. 91(1-3), 103–107 (1997). [CrossRef]

10.

T. K. Hatwar, J. P. Spindler, M. L. Ricks, R. H. Young, Y. Hamada, N. Saito, K. Mameno, R. Nishikawa, H. Takahashi, and G. Rajeswaran, “High-efficiency white OLEDs based on small molecules,” Proc. SPIE 5214, 233–240 (2004). [CrossRef]

11.

T. H. Liu, Y. S. Wu, M. T. Lee, H. H. Chen, C. H. Liao, and C. H. Chen, “Highly efficient yellow and with organic electroluminescent devices base on sterically hindered tetra (t-butyl) rubrene dopant,” Appl. Phys. Lett. 85, 4304–4306 (2004). [CrossRef]

12.

Y. Sun, N. C. Giebink, H. Kanno, B. Ma, M. E. Thompson, and S. R. Forrest, “Management of singlet and triplet excitons for efficient white organic light-emitting devices,” Nature 440(7086), 908–912 (2006). [CrossRef] [PubMed]

13.

G. Li and J. Shinar, “Combinatorial fabrication and studies of bright white organic light-emitting devices based on emission from rubrene-doped 4,4′-bis(2,2′-diphenylvinyl)-1,1′-biphenyl,” Appl. Phys. Lett. 83(26), 5359–5361 (2003). [CrossRef]

14.

K. O. Cheon and J. Shinar, “Bright white small molecular organic light-emitting devices based on a red-emitting guest-host layer and blue-emitting 4,4′-bis(2,2′-diphenylvinyl)-1,1′-biphenyl,” Appl. Phys. Lett. 81(9), 1738–1740 (2002). [CrossRef]

15.

S. Reineke, G. Schwartz, K. Walzer, and K. Leo, “Reduced efficiency roll-off in phosphorescent organic light emitting diodes by suppression of triplet-triplet annihilation,” Appl. Phys. Lett. 91, 123508 (2007).

16.

H. S. Yang, Y. W. Shi, Y. Zhao, Y. L. Meng, W. Hu, J. Y. Hou, and S. Y. Liu, “High colour rendering index white organic light-emitting devices with three emitting layers,” Displays 29(4), 327–332 (2008). [CrossRef]

17.

J. H. Jou, S. M. Shen, C. R. Lin, Y. S. Wang, Y. C. Chou, S. Z. Chen, and Y. C. Jou, “Efficient very-high rendering index organic light-emitting diode,” Org. Electron. 12(5), 865–868 (2011). [CrossRef]

OCIS Codes
(160.4670) Materials : Optical materials
(230.3670) Optical devices : Light-emitting diodes
(310.6860) Thin films : Thin films, optical properties
(330.1715) Vision, color, and visual optics : Color, rendering and metamerism

ToC Category:
Light-Emitting Diodes

History
Original Manuscript: July 26, 2012
Revised Manuscript: October 17, 2012
Manuscript Accepted: October 17, 2012
Published: December 21, 2012

Citation
M. Y. Zhang, F. F. Wang, N. Wei, P. C. Zhou, K. J. Peng, J. N. Yu, Z. X. Wang, and B. Wei, "High color rending index and high-efficiency white organic light-emitting diodes based on the control of red phosphorescent dye-doped hole transport layer," Opt. Express 21, A173-A178 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-S1-A173


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References

  1. B. W. D’Andrade and S. R. Forrest, “White organic light-emitting devices for solid-state lighting,” Adv. Mater. (Deerfield Beach Fla.)16(18), 1585–1595 (2004). [CrossRef]
  2. Y. Ohno, “Color rendering and luminous efficacy of white LED spectra,” Proc. SPIE5530, 88–98 (2004). [CrossRef]
  3. CIE 17.4–1987, “International lighting vocabulary,” No. 845–02–59 (1987).
  4. Y. W. Ko, C. H. Chung, J. H. Lee, Y. H. Kim, C. Y. Sohn, B. C. Kim, C. S. Hwang, Y. H. Song, J. Lim, Y. J. Ahn, G. W. Kang, N. Lee, and C. Lee, “Efficient white organic light emission by single emitting layer,” Thin Solid Films426(1-2), 246–249 (2003). [CrossRef]
  5. R. S. Deshpande, V. Bulovic´, and S. R. Forrest, “White-light-emitting organic electroluminescent devices based on interlayer sequential energy transfer,” Appl. Phys. Lett.75(7), 888–890 (1999). [CrossRef]
  6. J. Kido, M. Kimura, and K. Nagai, “Multilayer white light-emitting organic electroluminescent device,” Science267(5202), 1332–1334 (1995). [CrossRef] [PubMed]
  7. G. T. Lei, L. D. Wang, and Y. Qiu, “Multilayer organic electrophosphorescent white light-emitting diodes without exciton-blocking layer,” Appl. Phys. Lett.88(10), 103508 (2006). [CrossRef]
  8. C. W. Ko and Y. T. Tao, “Bright white organic light-emitting diode,” Appl. Phys. Lett.79(25), 4234–4236 (2001). [CrossRef]
  9. Y. Sato, T. Ogata, S. Ichinosawa, and Y. Murata, “Characteristics of organic electroluminescent devices with new dopants,” Synth. Met.91(1-3), 103–107 (1997). [CrossRef]
  10. T. K. Hatwar, J. P. Spindler, M. L. Ricks, R. H. Young, Y. Hamada, N. Saito, K. Mameno, R. Nishikawa, H. Takahashi, and G. Rajeswaran, “High-efficiency white OLEDs based on small molecules,” Proc. SPIE5214, 233–240 (2004). [CrossRef]
  11. T. H. Liu, Y. S. Wu, M. T. Lee, H. H. Chen, C. H. Liao, and C. H. Chen, “Highly efficient yellow and with organic electroluminescent devices base on sterically hindered tetra (t-butyl) rubrene dopant,” Appl. Phys. Lett.85, 4304–4306 (2004). [CrossRef]
  12. Y. Sun, N. C. Giebink, H. Kanno, B. Ma, M. E. Thompson, and S. R. Forrest, “Management of singlet and triplet excitons for efficient white organic light-emitting devices,” Nature440(7086), 908–912 (2006). [CrossRef] [PubMed]
  13. G. Li and J. Shinar, “Combinatorial fabrication and studies of bright white organic light-emitting devices based on emission from rubrene-doped 4,4′-bis(2,2′-diphenylvinyl)-1,1′-biphenyl,” Appl. Phys. Lett.83(26), 5359–5361 (2003). [CrossRef]
  14. K. O. Cheon and J. Shinar, “Bright white small molecular organic light-emitting devices based on a red-emitting guest-host layer and blue-emitting 4,4′-bis(2,2′-diphenylvinyl)-1,1′-biphenyl,” Appl. Phys. Lett.81(9), 1738–1740 (2002). [CrossRef]
  15. S. Reineke, G. Schwartz, K. Walzer, and K. Leo, “Reduced efficiency roll-off in phosphorescent organic light emitting diodes by suppression of triplet-triplet annihilation,” Appl. Phys. Lett.91, 123508 (2007).
  16. H. S. Yang, Y. W. Shi, Y. Zhao, Y. L. Meng, W. Hu, J. Y. Hou, and S. Y. Liu, “High colour rendering index white organic light-emitting devices with three emitting layers,” Displays29(4), 327–332 (2008). [CrossRef]
  17. J. H. Jou, S. M. Shen, C. R. Lin, Y. S. Wang, Y. C. Chou, S. Z. Chen, and Y. C. Jou, “Efficient very-high rendering index organic light-emitting diode,” Org. Electron.12(5), 865–868 (2011). [CrossRef]

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