Single-emission-layer white organic light-emitting devices:
Chromaticity and colour-rendering consideration
Chia-Chan Fan
a, Ming-Hong Huang
a, Wei-Chieh Lin
a, Hao-Wu Lin
a,⇑, Yun Chi
b,
Hsin-Fei Meng
c, Teng-Chih Chao
d, Mei-Rurng Tseng
da
Department of Materials Science and Engineering, National Tsing Hua University, Hsin Chu, Taiwan
bDepartment of Chemistry, National Tsing Hua University, Hsin Chu, Taiwan cInstitute of Physics, National Chiao Tung University, Hsin Chu, Taiwan d
Material and Chemical Research Laboratories, Industrial Technology Research Institute (ITRI), Hsin Chu, Taiwan
a r t i c l e
i n f o
Article history: Received 6 October 2013
Received in revised form 13 November 2013 Accepted 30 November 2013
Available online 12 December 2013 Keywords:
Solution-processed Organic light-emitting diodes Colour-rendering index Daylight locus
a b s t r a c t
The chromaticity and colour-rendering capability of solution-processed single emission layer (EML) white organic light-emitting diodes (W-OLEDs) can be precisely tuned by manipulating the dopant compositions in the EMLs. In this work, we numerically modelled binary, ternary, and quaternary doping single EML W-OLEDs. The correlated colour temperature (CCT), colour-rendering index (CRI), and spectral designs were correlated. The simulation predicted that the quaternary doping system possesses the best chromatic-ity performance. The corresponding binary, ternary and quaternary doping single EML W-OLEDs were fabricated and characterised to verify the calculation. The solution-processed quaternary doping W-OLEDs were designed with CRI values up to 85, deviations from the Planckian locus (Du0v0) as low as +0.0009, an EQE of 13.7%, a power efficacy of 14.7 lm/W
and current efficiency of up to 24.9 cd/A at 1000 cd/m2.
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
White organic light-emitting devices (W-OLEDs) based on solution processes exhibit both high reproducibility and material utilisation, which make them promising can-didates for large-area, low-cost solid-state lighting[1–8]. To prevent interfacial mixing, the single emission layer (EML) that consists of a host and multi-colour dopants is usually adopted in solution processed W-OLEDs [1–3]. Due to this unique characteristic, solution-processed W-OLEDs constitute a high-quality lighting source (e.g., high colour rendering index (CRI), target correlated colour temperature (CCT) and small deviation (Du0v0) from
Planckian locus in CIE 1976 coordinates) with different approaches than their thermal-evaporation counterparts. In thermal-evaporation W-OLEDs, the stacking of
multi-EMLs in which a single EML consists of one host and one emissive dopant is commonly utilised to achieve the required illumination quality [9–17]. This approach is adopted because controlling the ratio of compositions (1 host and P2 dopants) for multiple sources of co-deposi-tion is difficult. Conversely, the ratios of various compo-nents can be precisely controlled in solution processes by varying the weight percentage of dissolved materials
[18–20]. Furthermore, the colour-shift under different operational current densities can be prohibited in solution processed single EML W-OLEDs because of their spatial homogeneity in the emission zone[21,22]. However, the mechanism by which the manipulation of the EML of solu-tion processed W-OLEDs increases the illuminasolu-tion quality has not been revealed. In this study, we investigated the chromaticity and the colour-rendering performance of single EML solution-processed W-OLEDs based on commercial available emission dopants (e.g., red emitter Osmium(II)bis(5-(benzothiazol-2-yl)-3-trifluoromethylpy-razole)1,2-bis(phospholano)benzene (Os(btfp)2(pp2b), 1566-1199/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.orgel.2013.11.040
⇑Corresponding author. Tel.: +886 3 5715131x33879. E-mail address:[email protected](H.-W. Lin).
Contents lists available atScienceDirect
Organic Electronics
yellow emitter Iridium(III)bis(4-phenylthieno [3,2-c]pyrid-inato-N,C2) (PO-01-TB), green emitter iridium(III)tris
(2-(4-tolyl)phenylpyridine) (Ir(mppy)3), and blue emitter
Bis[2-(4,6-difluorophenyl)pyridinato-C2 ,N](picolinato)irid-ium(III) (FIrpic))[23–26]. The illumination characteristics were first simulated by superimposing the electrolumines-cence (EL) spectra of individual monochromatic devices. For binary-emissive dopant (red R and blue B) devices, only one R:B combination can fall on the Planckian locus. Both the simulation and experimental results indicate that this type of device shows a high CCT value but a poor colour-rendering capability. To increase the CRI, green (G) or yel-low (Y) emissive-dopants must be introduced inside the EMLs. Multiple possibilities of R:G:B, R:Y:B or R:G:Y:B combinations fit well with the requirement of a small Du0v0 value [27,28]. However, these combinations show
various CCT and CRI characteristics. A correlation between the initial R:B ratio (before adding G or Y parts), the CRI and CCT was determined. R:B, R:G:B, R:Y:B and R:G:Y:B vices whose emission spectra approach the numerical de-sign were fabricated. The experimental results closely approximate the simulation values. The R:B devices pro-duced white emission with a Commission internationale de l’éclairage (CIE) 1931 coordinates of (0.33, 0.34), exter-nal quantum efficiency (EQE) of 13.2%, current efficiency of 22.6 cd/A, power efficacy of 13.1 lm/W, and CRI of 59 at
1000 cd/m2. The R:G:B and R:Y:B W-OLEDs (CCT 3000 K)
exhibited an improved CRI of 79. Finally, the R:G:Y:B de-vices exhibited an excellent CRI value up to 85 and very low Du0v0 values as low as + 0.0009 and a CCT = 2699 K,
accompanied with an acceptable EQE of 13.7%, current effi-ciency of 24.9 cd/A and power efficacy of 14.7 lm/W at 1000 cd/m2.
2. Results and discussion 2.1. Photophysical properties
The molecular structures of the host and emissive dopants used in the EMLs are shown inFig. 1. The monochromatic OLEDs were fabricated first. The device structure was opti-mised as follows (optimisation procedures see Supporting Information S1): indium tin oxide (ITO)/Poly(3,4-ethylenedi-oxythiophene:poly(styrenesulfonate) (PEDOT:PSS) (40 nm)/ 4,40,400-tris(N-carbazolyl)-triphenylamine (TCTA) (40 nm)/
2,6-bis[(3-carbazol-9-yl)phenyl)]pyridine (26DCzPPy): dopants (30 nm)/1,3,5-tri(m-pyrid-3-yl-phenyl)-benzene (TmPyPB) (50 nm)/CsF (1 nm)/Al (140 nm). The EL spectra and the corresponding CIE 1931 coordinates of red, yellow, green and blue devices are shown inFig. 2a. The EL spectra
Fig. 1. The molecular structures of dopants (Firpic, Ir(mppy)3, PO-01-TB,
and Os(btfp)2(pp2b)) and host (26DCzPPy) used in the EMLs.
400 500 600 700 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
Normalized Intensity (a.u.)
Wavelength (nm) FIrpic Ir(mppy)3 PO-01-TB Os(btfp)2(pp2b)
(a)
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 FIrpic Ir(mppy)3 PO-01-TB Os(btfp)2(pp2b) y x(b)
Fig. 2. (a) The electroluminescence spectra and (b) their corresponding chromaticity coordinates in the CIE 1931 chromaticity space of FIrpic, Ir(mppy)3, PO-01-TB, and Os(btfp)2(pp2b) devices.
were later superimposed to simulate and predict the illumi-nation properties of W-OLEDs.
2.2. Binary doping system
The dashed line inFig. 2b shows the colour that are pos-sible by combining the FIrpic ((0.17, 0.35)CIE 1931) and
Os(btfp)2(pp2b) ((0.64, 0.36)CIE 1931) emission. Only one
intersections between this line and the Planckian locus at (0.33, 0.34)CIE 1931 was found to correspond to a CCT of
5700 K.Fig. 3a shows the simulated and experimental EL spectra of the R/B binary system W-OLEDs. The J–V–L curves and efficiency characteristics of the device are shown inFig. 4, and the performances are summarised in
Table 1. Although the device showed a promising perfor-mance with an EQE of 13.2%, power efficacy of 13.1 lm/W, and current efficiency of 22.6 cd/A with a Du0v0as low as
+0.0034 at 1000 cd/m2, the CRI value of this device was
low (59) due to a concave feature at 560 nm in the EL spectrum.
2.3. Ternary doping system
To increase the CRI value, introducing a third emissive dopant into the EMLs that covers the green to yellow part of the lumination spectrum is reasonable[16,29–31]. The green-emitting dopant Ir(mppy)3 and yellow-emitting
dopant PO-01-TB are both suitable for this purpose. As shown inFig. 5, white illumination with a CCT range from 5700 K to 2000 K can be obtained by introducing Ir(mppy)3
(G) and PO-01-TB (Y) into the former R:B system. Interest-ingly, the R:G:B W-OLEDs showed the highest achievable CRI value of 83 at a low CCT near 2130 K ((0.52, 0.41)CIE 1931),
and the CRI value gradually decreased as the CCT increased, as shown in Fig. 5a. Conversely, the R:Y:B W-OLEDs showed a peak CRI value up to 81 at a CT of 3100 K ((0.42, 0.40)CIE 1931), while the CRI values decreased
both at CT > 3100 K and CT < 3100 K, as shown inFig. 5b. The CRI values of the devices with a pre-determined CCT
could be predicted with this simulation prior to the device fabrication and measurement. For instance, if an incandes-cent lamp-like W-OLED (CCT 3000 K) is required, The R:G:B system yields a a CRI of 76, while R:Y:B W-OLEDs can potentially generate a CRI of 78. Accordingly, R:G:B and R:Y:B W-OLEDs with a CCT of approximately 3000 K were fabricated. As shown inFig. 3b and c, the simulated and measured EL spectra showed little deviation, which indicated that the modelling can properly predict the illu-mination properties of the ternary devices.Fig. 4shows the J–V–L curves and the efficiency characteristics of the ter-nary doping devices. Compared to the biter-nary system, the R:Y:B ternary doping devices showed comparably high EQE values of 12.5%, a power efficacy of 14.3 lm/W and current efficiency of 22.5 cd/A at 1000 cd/m2, while the
R:G:B devices showed a slightly lower performance of 11.4% EQE, 10.6 lm/W and 20.8 cd/A at 1000 cd/m2, which was presumably due to a lower quantum yield of Ir(mppy)3 (70% measured in integrate sphere quantum
yield measurement system). 2.4. Quaternary doping system
Modern lighting applications require high CRI values >80[15,32]. In principle, these values can be reached by introducing a fourth or more emissive dopants into the EMLs of W-OLEDs. Compared to the co-evaporation tech-nique in vacuum deposition, complicated doping (P4 do-pants in a host) in a single EML is simplified with a solution process. To demonstrate this advantage, R:Y:G:B quaternary doping W-OLEDs were modelled and fabricated as shown inFig. 6. In the quaternary doping system, the ra-tios of four components can be manipulated to ensure that the CIE coordinates of device emission are close to the Planckian locus. To simplify the modelling process, the ra-tio of red and blue components was first fixed as a starting point. As shown inFig. 7, the starting points A to F repre-sent the bluish to reddish spectra, which can be generated by FIrpic ((0.17, 0.35)CIE 1931) and Os(btfp)2(pp2b) ((0.64,
400 500 600 700 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Model CRI = 78, 2941K R:Y:B Device CRI = 78, 2870K
Wavelength (nm)
(c)
400 500 600 700
Wavelength (nm)
Model CRI = 85, 2696K R:Y:G:B Device CRI = 85, 2699K
(d)
Model CRI = 76, 2929K R:G:B Device CRI = 79, 2967K(b)
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 Model CRI = 57, 5677 K R:B Device CRI = 59, 5554 K(a)
Normalized Intensity (a.u.) Normalized Intensity (a.u.)Fig. 3. Comparison between modelled and measured electroluminescence spectra of W-OLED devices: (a) R:B binary W-OELD, (b) R:G:B ternary W-OLED, (c) R:Y:B ternary W-OLED, and (d) R:Y:G:B quaternary W-OLED.
0.36)CIE 1931) emission. The Du0v0values can be minimised
and thus approaching the Planckian locus by adding a cer-tain amount of Ir(mppy)3and PO-01-TB emission. The CIE
coordinates (colour) and starting point-dependent CRI val-ues are shown in the figures, and the highest achievable CRI values are indicated with red arrows. The simulation results show that the CRI values of quaternary doping W-OLEDs are generally higher than those of ternary W-OLEDs. A comparison of the highest CRI values in
Fig. 7shows that higher CRI value can be obtained at lower CCT values (87 (2291 K) > 85 (2696 K) > 84 (2854 K) > 83 (3114 K) > 81 (3499 K) = 81 (3464 K)). This indicates that the achievable highest CRI in quaternary W-OLED is still highly related to its CCT. The preferable high CRI results at lower CCT values could be due to the lack of deep blue emission for commercial available phosphorescence blue emitter (i.e. FIrPic). However, a CRI > 80 can be anticipated in R:Y:G:B W-OLEDs with a CCT range from 3500 K to 2000 K. Moreover, as in the binary and ternary systems, the highest achievable CCT value in the quaternary system is 5700 K, limited by high CIE y value (0.35) of FIrpic. A wider CCT tunable range is anticipated if a blue dopant with emission of lower CIE y value (60.28) is available. Figs.3d and4show the EL spectrum and J–V–L character-istics of the CT = 2699 K quaternary device. As predicted by the simulation, the device not only showed the highest CRI value of 85 and a very small Du0v0value of +0.0009 but also
possessed the best performance of 13.7% EQE, 14.7 lm/W and 13.6 cd/A at 1000 cd/m2. A 30–40 lm/W of power
effi-cacy is believed possible with the proper light extraction structures [9,33–36]. Device performance of the quater-nary doping W-OLEDs with different CCT ranges from 4363 K to 2390 K is listed inTable 2. They all showed supe-rior illumination properties than their binary and ternary counterparts of the same colour. The results indicate that multi-doping single EML W-OLEDs are promising candi-dates for efficient and high-quality lighting applications. Furthermore, as shown inS3, the colours of all WOLED de-vices were stable from 100 cd/m2to 3000 cd/m2. A slight blue-shift occurred on each device operated above 10,000 cd/m2.Fig. 8shows a photo of the working
quater-nary doping and biquater-nary doping devices. Note the improved colour rendering capability of quaternary doping devices.
3. Conclusions
In summary, binary, ternary, and quaternary doping single EML W-OLEDs were comprehensively modelled. The relationship between CCT, CRI, and spectral designs was revealed. The simulation predicted that quaternary doping system show the best chromaticity performance. Solution-processed binary, ternary, and quaternary doping single EML W-OLEDs were fabricated and characterised to verify the numerical calculation. The experimental and simulated results closely approximate one another. Finally, a quaternary doping W-OLED with a CRI value up to 85, a Du0v0as low as +0.0009, an EQE of 13.7%, a power efficacy
of 14.7 lm/W, and current efficiency as high as 24.6 cd/A at 1000 cd/m2 was demonstrated. The methodology and
implementation presented in this work should guide the future development of high CRI, low-cost, solution processed single EML W-OLEDs.
0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 RB = 1:11 RGB = 3:1:16 RYB = 5:2:42 RYGB = 8:3:1:44 6000K 4000K 5000K 3500K 3000K 2500K v' u' 2000K
(a)
0 2 4 6 8 10 0.1 1 10 100 1000 RB Binary RGB Terany RYB Terany RYGB QuaternaryCurrent density (mA/cm
2) Voltage (V)
(b)
100 101 102 103 104 Luminance (cd/m 2) 10 100 1000 10000 5 10 15 20 25 30 Power Efficacy (lm/W) RB Binary RGB Terany RYB Terany RYGB QuaternaryCurrent Efficiency (cd/A)
Luminance (cd/m2)
(c)
0 5 10 15 20 25 30 1 10 100 1000 5 10 15 RB Binary RGB Terany RYB Terany RYGB Quaternary EQE (%) Luminance (cd/m2)(d)
Fig. 4. (a) The chromaticity coordinates of the W-OLEDs in the CIE 1976 (L0, u0, v0) colour space at different molecular weight ratios of dopants.
(b) J–V–L characteristics, (c) current efficiencies and power efficacies, and (d) external quantum efficiencies of the W-OLEDs. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
4. Experimental 4.1. Device fabrication
Indium tin oxide (ITO)-coated glass substrates were sequentially cleaned in an ultrasonic bath with de-ionised water, acetone, and methanol. The substrates were then treated with UV-ozone before thin film deposition. PED-OT:PSS was spin-coated onto the substrates, which were then dried in air at 135 °C for 30 min. A solution of TCTA (10 mg/mL) was prepared using toluene as a solvent and spin-coated onto the PEDOT:PSS layer. The cast films were
baked at 100 °C for 10 min in a nitrogen atmosphere to remove the residual solvents. The emissive dopants and 26DCzPPy host were dissolved in toluene (6.67 mg/mL) and deposited by bar coating on a hot plate at 80 °C. The gap between the bar and the substrates was 60
l
m, and the speed of the bar was controlled at 350 mm/s by an auto-bar coating machine. TmPyPB, CsF, and Al were ther-mally vaporised in a vacuum chamber with a base pressure <106Torr.4.2. Measurements
The device characteristics were obtained by a Keithley 2636A source meter and silicon photodetector that was calibrated using a Photo Research, Inc. PR-650 SpectraScan
Table 1
Device performance of R:B, R:G:B, and R:Y:B W-OLEDs. The weight ratio of total dopants and host are fixed to 1:9. Weight ratio of dopants
in device fabrication
Ratio of EL spectrum in optical simulation
gp, max/gc, max/EQE
(lm W1/cd A1/%) gp, 1000/gc, 1000/EQE (lm W1/cd A1/%) CIEx, y CCT (K) CRI Du0v0 R:B R:B 1:11* 1:2.2 15.6/23.1/13.5 13.1/22.6/13.2 (0.33, 0.34) 5681 59 +0.0034 R:G:B R:G:B 3:1:8 12:3:1 10.6/17.0/10.9 8.8/16.9/10.8 (0.53, 0.41) 2011 83 -0.0015 3:1:16* 2:7:1 11.6/20.9/11.4 10.6/20.8/11.4 (0.44, 0.41) 2967 79 +0.0016 3:1:24 1:0.4:1.9 11.5/22.9/11.9 10.2/22.6/11.8 (0.33, 0.38) 5683 71 +0.0177 R:Y:B R:Y:B 1:1:6 1.8:2:1 17.3/25.9/12.5 15.9/25.0/12.2 (0.48, 0.43) 2627 75 +0.0058 3:1:16 3.1:1.1:1 14.0/18.7/10.4 12.9/18.6/10.4 (0.48, 0.40) 2428 80 -0.0046 4:1:20 2.2:0.8:1 16.4/26.6/12.9 14.2/26.2/12.7 (0.48, 0.39) 2366 78 -0.0079 5:2:42* 2:0.7:1 15.8/22.8/12.7 14.3/22.5/12.5 (0.45, 0.40) 2950 78 -0.0041
The corresponding devices shown in Figure 4.
0.3 0.4 0.5 0.6 0.30 0.35 0.40 0.45 0.50 Os(btfp)2(pp2b) Rich y x FIrpic Rich
(a)
57.00 63.00 69.00 75.00 81.00 87.00 CRI 83 57 0.3 0.4 0.5 0.6 0.3 0.4 0.5 0.6 y x Os(btfp)2(pp2b) Rich FIrpic Rich 57.00 63.00 69.00 75.00 81.00 87.00 81 57 CRI(b)
Fig. 5. Modelled CRI values of the ternary W-OLED. The Firpic and Os(btfp)2(pp2b) RB binary systems were used as the starting points and
the desired chromaticity approaching the Planknian locus could be obtained by adding the third (a) Ir(mppy)3or (b) PO-01-TB dopants.
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Os(btfp) 2(pp2b) y x FIrpic Ir(mppy)3 PO-01-TB
Fig. 6. Schematic of model simulating in quaternary case: the dashed line represents the section from Os(btfp)2(pp2b) (0.64, 0.36) to the
intersec-tion at (0.33, 0.34)*. The dashed line could be separated by several starting
points from bluish (FIrpic-rich) to reddish (Os(btfp)2(pp2b)-rich). Arrows
pointing to Ir(mppy)3of (0.25, 0.63) and PO-01-TB of (0.49, 0.5) are
constructed on one of these starting points. Because the arrows attach to the Planckian locus, a region of the CIE value that approaches the Planckian locus was obtained from the starting point as well as CRI value and correlated spectrum.*(The connection between FIrpic (0.17, 0.35) and
Os(btfp)2(pp2b) (0.64, 0.36) intersects with Planckian locus at (0.33,
0.34)). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
colorimeter. The electroluminescence (EL) spectra were measured by a calibrated Ocean Optics spectrometer. The thicknesses of each layer was measured with a V-VASE var-iable-angle spectroscopic ellipsometer (J. A. Woollam Inc.).
Acknowledgements
The work was financially supported by the National Science Council of Taiwan (NSC 101-2112-M-007-017-MY3, NSC 102-2221-E-007-125-MY3, NSC 102-2119-M-007-006) and the Low Carbon Energy Research Centre at the National Tsing-Hua University.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in the online version, athttp://dx.doi.org/10.1016/ j.orgel.2013.11.040.
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CRI
Fig. 7. Modelled CRI values of the quaternary W-OLED. The Firpic and Os(btfp)2(pp2b) RB binary systems were used as the starting points (points A–F) and
the desired chromaticity approaching Planknian locus could be obtained by adding Ir(mppy)3and PO-01-TB dopants.
Table 2
Device performance of R:Y:G:B W-OLEDs. The weight ratio of total dopants and host are fixed to 1:9). Weight ratio of dopants
in device fabrication
Ratio of EL spectrum in optical simulation
gp, max/gc, max/EQE
(lm W1 /cd A1 /%) gp, 1000/gc, 1000/EQE (lm W1 /cd A1 /%) CIEx, y CCT (K) CRI Du0v0 R:Y:G:B R:Y:G:B 5:1:1:42 15.4/21.7/10.5 13.5/21.0/10.2 (0.46, 0.44) 4363 79 +0.0179 10:3:3:84 15.8/21.7/10.5 14.6/22.2/10.3 (0.39, 0.43) 4095 80 +0.0199 5:2:2:50 17.2/23.3/12.1 16.1/24.9/11.9 (0.40, 0.43) 3970 82 +0.0191 10:4:3:84 14.4/19.5/8.6 13.5/19.3/8.5 (0.40, 0.44) 4034 82 +0.0232 5:2:4:42 17.9/24.6/10.2 15.5/24.1/10.2 (0.40, 0.46) 4002 74 +0.0294 6:2:1:42 13.7/19.5/10.7 11.8/19.0/10.4 (0.42, 0.42) 3368 83 +0.0076 8:3:1:44* 2:0.6:0.48:1 15.5/24.8/13.7 14.7/24.6/13.7 (0.47, 0.41) 2699 85 +0.0009 16:4:2:88 14.1/19.8/10.1 12.3/19.3/9.9 (0.48, 0.42) 2535 84 -0.0023 7:2:1:44 14.5/20.2/10.9 11.8/19.3/10.6 (0.48, 0.41) 2498 84 0.0000 8:3:1:48 14.5/21.3/11.4 12.2/20.9/11.2 (0.49, 0.42) 2390 83 -0.0001
The corresponding devices shown in Figure 4.
Fig. 8. Picture of the R:B binary and R:Y:G:B quaternary W-OLEDs. The R:B binary W-OLED exhibits cooler colour temperature at 5700 K and a low CRI value of 59 and the R:Y:G:B W-OLED showed a warm colour temperature at 2700 K with a high CRI value up to 85. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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