Highly Efficient Fluorescent/Phosphorescent ... - Wiley Online Library

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Oct 6, 2010 - Thomas D. Pawlik, Denis Y. Kondakov, Michael E. Miller, Tommie L. Royster, and Dustin L. Comfort. Eastman Kodak Company, Rochester, ...
17.3 / M. E. Kondakova

17.3: Highly Efficient Fluorescent/Phosphorescent OLED Devices Using Triplet Harvesting Marina E. Kondakova, David J. Giesen, Joseph C. Deaton, Liang-Sheng Liao, Thomas D. Pawlik, Denis Y. Kondakov, Michael E. Miller, Tommie L. Royster, and Dustin L. Comfort Eastman Kodak Company, Rochester, New York 14650, USA

Abstract We demonstrate efficient white and non-white hybrid OLED devices operating by a triplet harvesting mechanism to create light. Triplet excited states are generated in a blue fluorescent light-emitting layer (LEL) and utilized upon their diffusion to the phosphorescent LEL(s). At 1000 cd/m2 a blue/yellow hybrid OLED device shows external quantum efficiency (EQE) of 13.6%, 3.8 V, 30.1 lm/W, and excellent color characteristics suitable for display application. Performance of non-white-emitting hybrids, RGB white, and a tandem hybrid device is discussed. The triplet harvesting mechanism in all hybrid devices was verified by several experimental methods (spectral analysis, time-resolved electroluminescence (EL), magnetic field effect on EL).

1.

Introduction

OLED displays have a number of advantages over LCD displays, including higher power efficiency, better color saturation, and smaller luminance and color variation as a function of viewing angle. As a result, these displays are finding applications in consumer electronics. OLEDs also can be used as an efficient solid-state lighting (SSL) source, and this area of research has attracted worldwide attention. Current benchmarks for efficacy and lifetime targets for SSL are 50 lm/W and 10,000 h at 1000 cd/m2, respectively. With technology development, luminous efficacies in excess of the 85−90 lm/W efficiencies of fluorescent lamps are expected. High efficiency in OLEDs cannot be achieved without use of phosphorescent OLED (PHOLED) technology. Very efficient white devices comprising triplet emitters in light-emitting layers were demonstrated recently [1,2]. One should note that high efficiencies with good stability have been demonstrated for green and red PHOLEDs [3], but stable, deep blue phosphorescent OLEDs still are not available. Therefore, OLEDs combining blue fluorescent emitters with longer wavelength-emitting phosphorescent dopants have been sought as an alternative to achieving high efficiencies in white devices. For instance, individual EL units containing fluorescent and phosphorescent LELs may be used in stacked OLED devices, as was reported by Murano et al. [4]. Very efficient and stable white OLEDs with similar stacked architecture for SSL were announced recently by Novaled [5]. Typically, the use of a fluorescent emitter significantly reduces the efficiency of the device compared to a similar phosphorescent-based device because of the waste of the triplet excitons formed from the majority of recombination events. One way to include blue fluorescent materials and maintain high efficiency is through a triplet harvesting mechanism. Devices employing the triplet harvesting can achieve maximum EQE by confining recombination in a fluorescent LEL and converting the

resulting singlet excitons to blue light, while forcing the triplet excitons to diffuse into a nearby phosphorescent LEL where one or more phosphorescent dopants harvest and convert these triplet excitons to light [6,7]. The goal of this project was to develop efficient OLED devices based on fluorescent/phosphorescent mixed emission and operated by the triplet harvesting mechanism described above. Here we report on several architectures of hybrid white and non-whiteemitting devices.

2.

Experimental

All OLEDs were fabricated on glass substrates precoated with an approximately 25 nm layer of indium-tin oxide (ITO) as the anode. The substrates were sequentially ultrasonicated in a commercial detergent, rinsed in deionized water, and exposed to an oxygen plasma for approximately 1 minute. Then, a 1 nm fluorocarbon (CFx) hole-injecting layer (HIL) was deposited by plasma-assisted deposition of CHF3 over the ITO. The substrates were transferred into a vacuum chamber for sequential deposition of all organic layers by thermal evaporation under a vacuum of 10-6-10-7 Torr. Next, 0.5 nm of LiF was vacuum deposited, followed by a 100 nm layer of aluminum, to form a bilayer cathode. During deposition, the layer thicknesses and doping concentrations were controlled and measured in situ using calibrated thickness sensors. The device emission area is 0.1 cm2. The EL properties of the OLEDs were evaluated using a PR 650 SpectraScan spectrometer and programmable constant-current source-meter at room temperature. The color was reported using CIE (Commission Internationale de l'Eclairage) coordinates. Reported drive voltages are corrected for bus resistance. External quantum efficiency (EQE) was calculated assuming that device emission is Lambertian and angle independent. N,N’-di-1-naphthalenyl-N,N’-diphenyl-4,4’-diaminobiphenyl (NPB) was used in the hole-transporting layer (HTL) and 4,7diphenyl-1,10-phenanthroline (Bphen) in the electron-transporting layer (ETL). Kodak’s proprietary blue dopant (BDM1) was used as a fluorescent emitter in the blue LEL (BLEL) of hybrid devices. The average power consumption values for each display were determined from a system model, which estimates the average power consumption of an active-matrix display panel necessary to display a set of images at a desired luminance and color position. This model has been previously described elsewhere [8]. The color gamut is specified as a percent calculated from the ratio of the display gamut to a gamut determined for the NTSC primaries within the CIE 1931(x,y) chromaticity diagram.

ISSN/008-0966X/08/3901-0219-$1.00 © 2008 SID

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17.3 / M. E. Kondakova Results and Discussion

White light in OLEDs can be generated by simultaneous emission of light from several emitters in the correct proportions from a number of device structures [9]. One simple method of obtaining white-light-emitting OLEDs is to combine blue and yellow LELs in a single device. This combination of fluorescent and phosphorescent emitters has the potential for achieving the highest efficiency and the lowest drive voltage of any architecture employing the triplet harvesting mechanism. Figure 1 shows the structure and energy levels of the fluorescent blue/phosphorescent yellow (BY) hybrid OLED (device A). Because of a sufficiently high energy barrier, electrons are blocked from reaching the BLEL, and recombination is most likely to occur at the BLEL | spacer interface. The spacer layer prevents Förster energy transfer from the BLEL to the yellow emitter while allowing the diffusion of triplets into the yellow LEL (YLEL). Because both energy transfer processes potentially occur over different length scales, the spacer layer thickness can be optimized so that there is little quenching of the blue emission, yet appreciable emission from the YLEL to reach a suitable white emission. For efficient transfer of triplet excited states from the BLEL toward the YLEL, host materials were selected so that the triplet energy of the blue host exceeds the triplet energies of a phosphorescent host and the spacer material. In addition, an exciton-blocking layer (EBL) was used to confine triplet excitons to the BLEL and prevent loss of triplets to the HTL.

To demonstrate triplet harvesting in device A, a device B was constructed similarly to device A except that the phosphorescent emitter was omitted. The blue portion of the spectrum (380−512 nm) of device B exhibits 4.9 % EQE, indicating very efficient fluorescence. Figure 2 shows that devices A and B have nearly identical spectral radiance for the blue component of the EL. The Spectral radiance, W-light/sr/m 2

3.

0.014 blue/yellow (A)

0.012

blue (B)

0.01 0.008 0.006 0.004 0.002 0 380

480

580

680

780

Wavelength, nm

Figure 2. EL spectra of OLEDs: device A contains blue and yellow emitters as shown in Figure 1. The yellow phosphorescent emitter is omitted in device B. Data are taken at 1 mA/cm2. data indicates that the addition of the yellow triplet dopant to device A did not diminish the blue emission relative to that of device B. This demonstrates the efficient use of triplet excitons for yellow emission without affecting utilization of the singlet excitons for blue fluorescence in device A. In addition, the hypothesis of triplet migration in the present device was evidenced by several physical methods, such as time-resolved EL and magnetic field effect on EL [7]. 18 total

16

blue

yellow

14

Figure 1. Energy level diagram of a hybrid white OLED containing blue fluorescent and yellow phosphorescent LELs. HOMO and LUMO values of materials were estimated from solution-determined redox data. Calculated triplet energies (T, eV) are shown. The EL spectrum of device A is shown in Figure 2, where both blue and yellow emission is observed. The BY hybrid device exhibits EQE of 15.8% at a current density of 0.01 mA/cm2. EQE decreases gradually with increasing current, as shown in Figure 3. At 1000 cd/m2 (3.2 mA/cm2), the performance metrics are 13.6% EQE, 3.8 V, 30.1 lm/W, 34.8 cd/A, and CIE x,y of (0.317, 0.364). All results are shown without optical outcoupling enhancement. Color change with current is not significant; ∆CIE u´v´ is 0.0088 in the range of current densities from 0.05 to 80 mA/cm2.

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EQE,%

12 10 8 6 4 2 0 0

10

20

30

Current density, mA/cm

40

2

Figure 3. EQE as a function of current density shown for the total EQE and for blue and yellow components of the EL spectrum of device A. The performance of the BY hybrid OLED as the light source in an RGB color display was evaluated by mathematically applying the

17.3 / M. E. Kondakova

Spectral radiance, W-light/Sr/m 2

transmission curves of color filter arrays (CFA) developed by Eastman Kodak Company [10] to device A and calculating the resulting RGB EQE. The outcome is an RGB system shown in Figure 4. Corresponding color coordinates are also plotted in Figure 5. We should note that CIE x,y of red and blue peaks lie practically on the spectral locus indicating nearly monochoromatic light. Modeling shows that the color gamut of the BY device using CFA is 97% NTSC, which is sufficient for display application.

CIEx,y =0.141, 0.051

0.01 0.008 CIEx,y =0.248, 0.696

0.006

CIEx,y =0.663, 0.332

0.004 0.002 0 380

480

580

680

An RGB white hybrid device can be realized by combining the blue fluorescent emitter and proper proportions of green and red phosphorescent emitters. Hybrid devices having non-white emission may be useful combined with additional fluorescent or phosphorescent elements in series in a stacked OLED. We have demonstrated non-white-emitting efficient devices containing a blue fluorescent LEL and a red or a green phosphorescent LEL, i.e., BR or BG hybrids, respectively. The general device architecture is ITO | HTL | EBL | BLEL | spacer | phosphorescent LEL | ETL | LiF | Al. At 1000 cd/m2 (12.5 mA/cm2) one of the best BR device exhibits 9.1% EQE, 4.6 V, 6.1 lm/W, and CIE x,y of (0.252, 0.175). The highest EQE of 12.4 % is observed at 0.2 mA/cm2 in the device. The best BG device shows 14.0 % EQE, 4 V, 30 lm/W, and CIE x,y of (0.224, 0.408) at 1000 cd/m2. The triplet harvesting mechanism in these devices was verified by the above-mentioned methods.

0.014 0.012

model is the average power usage of the organic stack over a collection of digital still camera images. According to the modeling results, the organic BY stack would require an average power of only 49 mW for a display with the above characteristics.

780

Wavelength,nm

Figure 4. Mathematically modeled EL spectrum of BY device with Kodak’s CFA applied.

To achieve a white color, a 2-stack device containing BR and BG hybrid EL units was constructed (BR||BG). The tandem device has the following structure of layers: ITO | CFx | HIL | HTL | EBL | BLEL | spacer | red LEL | ETL || organic intermediate connector | HTL | EBL | BLEL | spacer | green LEL | ETL | n-doped ETL | Al. At 1000 cd/m2 the performance metrics of the tandem device are 16.9 % EQE, 9.4 V, 9.9 lm/W, and CIE x,y of (0.330, 0.329). The power model estimates an average power usage of 115 mW and a color gamut equal to 112% NTSC. The device spectral data are shown in Figure 5. Spectral radiance, W-Light/Sr/m 2

0.012 BR || BG tandem 0.01

BR BG

0.008 0.006 0.004 0.002 0 380

480

580

680

780

Wavelength, nm

Figure 5. Color coordinates of blue, green and red emitters in BY (device A) with Kodak’s color filters applied. Color gamut of this device is 97% NTSC. Power consumption of a display having this emitter was modeled to understand the impact of this performance on the power usage of an OLED display. A 2.5" display was modeled having a D65 white point with a peak luminance of 180 cd/m2 after a 44% transmittance polarizer. The OLED stack was an unpatterned white emitter using Kodak’s color filters to form an RGBW pixel pattern. In this pattern, the W pixel is unfiltered. The result of the

Figure 6. EL spectra of BR||BG tandem device using combination of 2 non-white hybrid stacks and spectra of the single BR and BG units. Data are taken at 1 mA/cm2. We should note that a tandem device shows higher EQE and better color gamut, however, the BY single-stack hybrid device is still advantageous over the tandem OLED in power efficiency. A third way to form a white device using the harvesting method is by combining green and red triplet emitters in the phosphorescent

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17.3 / M. E. Kondakova emission zone. A device with the layer structure ITO | HTL | EBL | BLEL | spacer | red LEL | green LEL | ETL | LiF | Al showed the following performance at 1000 cd/m2 (5 mA/cm2): 11.1% EQE, 4.5 V, 13.8 lm/W, and CIEx,y of (0.272, 0.307). The color gamut when combined with the color filters is 113% NTSC. The deviation of the color of this device from the D65 white point of the modeled display, however, increases the estimated power consumption due to the additional use of the filtered red and green pixels to form neutral colors near the D65 white point. Because of this, the average power consumption of this OLED stack is 124 mW for the 2.5" panel. The model can rebalance the quantum efficiency of the three individual emitters to a D65 white point without changing the overall external quantum efficiency. This theoretical device would then not require the use of filtered pixels to achieve the desired white point. In this case, the average power usage drops to 83 mW, showing the importance of properly balanced white emission in devices with the RGBW pixel pattern. The performance of BY, RGB, and BR||BG devices is summarized in Table 1. Table 1. Performance of harvesting hybrid devices at 1000 cd/m2. Device

%EQE

V

lm/W

CIEx,y

Power (mW)1

BY

13.6

3.8

30.1

(0.317, 0.364)

49

RGB

11.1

4.5

13.8

(0.272, 0.307)

124 (83*)

BR||BG

16.9

9.4

9.9

(0.330, 0.329)

115

magnetic field effect on device EL spectra. Non-white-emitting EL units were stacked in series producing a white-emitting device with color coordinates very close to D65 white point. These approaches are expected to yield OLED devices having higher EQE than those employing a combination of individual fluorescent and phosphorescent devices and with better lifetimes than can be achieved using complete triplet systems.

5.

Acknowledgements

It is a pleasure to acknowledge many helpful discussions and material research contributions of Dr. R. H. Young, Dr. X. Ren, Dr. W. Begley, Dr. R. Vargas, Dr. V. Jarikov, Dr. T. K. Hatwar, Dr. S. Van Slyke, Mr. K. Klubek, and Mrs. B. Owczarczyk. The authors thank C. Pellow, R. Winter, D. Neill, and D. Arnold for device fabrication and testing. Support and encouragement from Dr. S. Krishnamurthy is gratefully acknowledged.

6.

References:

[1] T. Nakayama, K. Hiyama, K. Furukawa, H. Ohtani, SID Symposium Digest, p.1018, 2007. [2] B.W. D’Andrade, J.-Y. Tsai, C. Lin, M.S. Weaver, P.B. Mackenzie, J.J. Brown, SID Symposium Digest, p. 1026, 2007. [3] Data are taken from http://www.universaldisplay.com/pholed.htm. [4] S. Murano, M. Burghart, J. Birnstock, P. Wellman, M. Vehse, A. Werner, T. Canzler, T. Stubinger, G. He, M. Pfeiffer, H. Boerner, Proc. SPIE, Vol. 5937, p. 59370H, 2005. [5] Data are taken from http://www.novaled.com/news/2007_11_05_pr.html. [6] Y. Sun, N.C. Giebink, H. Kanno, B. Ma, M.E. Thompson, S.R. Forrest, Nature, Vol. 440, p. 908, 2006.

1

Modeled averaged power consumption of the organic stack in a 2.5" OLED panel using an unpatterned white emitter RGBW pixel pattern,

[7] J.C. Deaton, M.E. Kondakova, D.Y. Kondakov, T.D. Pawlik, R.H. Young, D.J. Giesen, SID Symposium Digest, p. 849, 2007.

* power with balanced D65 point (see paragraph above).

[8] J.P. Spindler, T.K. Hatwar, M.E. Miller, M.E. Arnold, M.J. Murdoch, P.K. Kane, J.E. Ludwicki, P.J. Alessi, S.A. Van Slyke, J. Soc. Inf. Display, Vol. 14, p. 37, 2006.

4.

Summary

We have demonstrated various architectures of efficient whiteemitting OLED devices comprising fluorescent and phosphorescent LELs (hybrid approach). Single-stack BY hybrid OLEDs show high EQE, low operational voltage, and excellent color characteristics. The triplet harvesting mechanism in BY and non-white-emitting hybrid OLEDs was investigated and confirmed by EQE analysis of EL spectra, time-resolved EL, and

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[9] A. Misra, P. Kumar, M.N. Kamalasanan, S. Chandra, Semicond. Sci. Technol., Vol. 21, p. R35, 2006. [10] M.J. Helber, P.J. Alessi, M. Burberry, S. Evans, M.C. Brick, D.R. Diehl, R. Cok, SID Symposium Digest, p. 1022, 2007.