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Color-stable WRGB emission from blue OLEDs with quantum dots-based patterned down-conversion layer
Accepted Manuscript Color-stable WRGB emission from blue OLEDs with quantum dots-based patterned down-conversion layer Maojun Yin, Teng Pan, Ziwei Yu, Xiaomei Peng, Xiang Zhang, Wenfa Xie, Shihao Liu, Letian Zhang PII:
S1566-1199(18)30442-7
DOI:
10.1016/j.orgel.2018.08.036
Reference:
ORGELE 4850
To appear in:
Organic Electronics
Received Date: 31 July 2018 Revised Date:
15 August 2018
Accepted Date: 20 August 2018
Please cite this article as: M. Yin, T. Pan, Z. Yu, X. Peng, X. Zhang, W. Xie, S. Liu, L. Zhang, Colorstable WRGB emission from blue OLEDs with quantum dots-based patterned down-conversion layer, Organic Electronics (2018), doi: 10.1016/j.orgel.2018.08.036. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Color-Stable WRGB emission from blue OLEDs with quantum dots-based patterned down-conversion layer Maojun Yin, Teng Pan, Ziwei Yu, Xiaomei Peng, Xiang Zhang, Wenfa Xie, Shihao Liu*, Letian Zhang*
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State Key Laboratory of Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun, 130012, People’s Republic of China *
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Corresponding author: [email protected], [email protected]
Abstract
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For full-color displays, white organic light-emitting devices (OLEDs) combining with color filters are a promising technology. It avoids the precise shadow masking that is often used to separate RGB pixels. But the spectra of traditional white OLEDs are changeable, which would make the full-color displays show poor color stability. In this work, in order to realize color-stable white, red, green and blue emission, blue
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bottom-emitting OLEDs are integrated with color filters and two types of patterned down-conversion films containing red and green quantum dots (QDs). Besides, by solution-based transfer printing, micro-pillar arrays are formed on the surface of down-conversion films to improve the light extraction property. Based on this
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principle, color-stable white, red, green and blue emission are achieved from efficient blue OLEDs. Due to the highly saturated color emission of QDs, wider color gamut
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can be anticipated by employing these devices as three primary colors, nearly 15% wider than that based on commercial white LEDs or white OLEDs with the same color filters.
Keywords: Organic light-emitting devices, Quantum dots, Micro-pillar arrays, Down-conversion, Color gamut
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1.
Introduction Today, organic light-emitting devices (OLEDs) have successfully come into
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commercialization [1, 2]. Novel technologies for displays are developed based on OLEDs, such as separated white, red, green and blue (WRGB) or RGB sub-pixels [3, 4] and white OLEDs combining with color filters [5-12]. Although the former is easy
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to realize high brightness and energy-saving displays, its high manufacture cost and high defect ratio limit its application in large scale display. Nevertheless, the latter can
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avoid the traditional fine metal mask and reduce the difficulty and costs in realizing high resolution or large scale displays. Therefore, it can be seen that white light is also very essential for display industry. Compared with white LEDs, white OLEDs are
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better white-light sources for display, because OLEDs have the advantage of self-emitting and low power consumption. Besides, white OLED can be also used as the fourth white sub-pixel to increase display luminance and contrast. However, the
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color stability of traditional white OLEDs is poor. There are spectral changes caused
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by different degradation processes of organic electroluminescence (EL) materials [13] and the shifting of recombination region as voltages increasing [14, 15]. It can lead to color distortion when the traditional white OLEDs are applied to displays. Nevertheless, the down-conversion systems have the potential to realize
color-stable WRGB emission via blue OLEDs [16, 17]. Blue OLEDs are used as pumping sources, leading to photoluminescence (PL) after the excitation of down-conversion materials by blue light. Then WRGB emission can be anticipated,
ACCEPTED MANUSCRIPT through combining blue OLEDs with red-emitting materials, red-emitting materials and red filters, green-emitting materials and green filters, and blue filters, respectively. It can be noted that the spectra of blue OLEDs are stable due to the single emitter.
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Besides, the PL spectra of down-conversion materials are nearly invariable with the stable pumping sources. There would be synchronous changes between blue light and the converted light. As a result, the color stability of the generated WRGB emission
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against voltage and operation time can be easily achieved. But it is difficult to choose
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appropriate down-conversion materials. Because the absorption spectra of organic phosphors widely used in OLEDs are usually in the ultraviolet (UV) region [18]. However, with the fast development, quantum dots (QDs) receive particular attention as potential down-conversion materials. QDs have many favorable optoelectronic
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features, such as high PL quantum yield, size-dependent emission and excellent stability [19-24]. Moreover, the broad absorption spectra and beneficially high absorption capability in the near-UV–blue region provide high conversion efficiency
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[25]. QDs also feature high color saturation due to the narrow emission spectra.
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Therefore, it will be also beneficial to expand color gamut of displays. In addition, the luminous efficiency is equally important for displays. But, a large
number of generated photons are trapped inside the glass substrates of bottom-emitting OLEDs. As a result, the luminous efficiency is considerably limited by the substrate and waveguide mode [26, 27]. To extract the trapped photons and increase efficiency, the films with microscopic structure are widely adopted [28-30]. In this work, red and green CdSe/ZnS QDs are embedded into polymethyl
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(PMMA)
matrices
to
prepare
down-conversion
films.
Blue
bottom-emitting OLEDs integrated with color filters and two types of patterned down-conversion films are proposed to realize color-stable white, red, green and blue
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emission. The influences of micro-pillar arrays of the PMMA matrices on the device performance are also explored.
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2. Experimental Section
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Fig. 1. Device structures of (a) BOLED and (b) WRGB OLEDs achieved by combining BOLED with patterned down-conversion films and color filters.
2.1 Device structures of OLEDs
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Fig.1 (a) shows the device structure of blue OLED (BOLED) that is used as
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pumping source. The device structure of the BOLED is indium tin oxide (ITO)/MoO3 (3 nm)/di-[4-(N,N-di-ptolyl-amino)-phenyl] cyclohexane (TAPC, 35 nm)/4,4',4''-tris (carbazo-9-yl)triphenylamine (TCTA, 5 nm)/4,4',4''-tris(carbazol-9-yl)-triphenylamine: bis[(4,6-difluorophenyl)-pyridinato-N,C2'] (picolinato) Ir(III) (TCTA:12wt% FIrpic, 20 nm)/1,3,5-tri[(3-pyridyl)-phen-3-yl] benzene (TmPyPB, 60 nm)/LiF (0.5 nm)/Mg: 6wt% Ag (100 nm). As shown in Fig. 1 (b), because of the excitation of blue light, the patterned down-conversion films with red and green QDs can produce red and green
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To fabricate BOLED, the ITO-coated glass substrates are cleaned with Decon 90, and ultrasonically cleaned in deionized water baths for 15 min. After dried at 120 oC for 20 min and handled in plasma for about 5 min, the substrates are loaded in a
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vacuum evaporator. Then organic layers and cathode materials are deposited on the
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substrates under high vacuum (~6.0×10–4 Pa). The material deposition rate is monitored with quartz crystal. The current density-voltage-luminance characteristics and spectra are measured by using a Goniophotometric Measurement System (GP500, Otsuka Electronics Co. Osaka, Japan) in air at room temperature simultaneously. The
sphere.
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power efficiency and external quantum efficiency are measured with an integrating
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2.3 Fabrication of down-conversion films
Fig. 2. Schemes of the fabrication process for the down-conversion film with micro-pillar arrays.
The fabrication process of down-conversion films is illustrated in Fig. 2. For the preparation of down-conversion films, 0.285 g of PMMA is completely dissolved in 1
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is drop-casted onto the silicon substrate. The patterned silicon substrate with micro-pits arrays is used to fabricate the down-conversion films with micro-pillar arrays, while the flat silicon substrate is used to fabricate the flat down-conversion
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films without any patterns. After heating at 70 oC for 40 min to promote the solvent
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evaporation, the solidified down-conversion film with about 1 mm thickness can be readily peeled off without damage. In this process, red and green CdSe/ZnS QDs (100 mg/ml and 50 mg/ml in toluene) are used and the QD concentration of the film can be easily controlled by the volume of QD solution added to the QD-PMMA mixture
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solution. 3. Results and discussion 3.1 White OLEDs
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For bottom-emitting OLEDs, many photons are trapped within the glass substrates
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due to the substrate and waveguide mode. Therefore, the patterned down-conversion films with micro-pillar arrays are proposed to improve the light extraction. To study the effects of micro-pillar arrays on the device performance, we prepare flat down-conversion films as references. The red QD solution with the same volume (1.0 µL) is used for preparing the film with or without micro-pillar arrays. White OLED with flat down-conversion film and patterned down-conversion film are fabricated and their EL performances are studied. For simplicity, the former is marked as
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Device-WF, while the latter is marked as Device-W.
Fig. 3 (a) Absorption spectra of the flat film (Abs-FF), the patterned film (Abs-PF), red QDs
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(Abs-R), green QDs (Abs-G), and EL spectra of BOLED in the forward direction. (b) Normalized spectra of BOLED, Device-WF and Device-W. (c) Current efficiency–luminance characteristics and (d) current density-voltage-luminance characteristics of BOLED, Device-WF and Device-W. The inset is the emitting image of BOLED at 6 V. (e) Power efficiency–current density–external quantum efficiency (EQE) characteristics of Device-WF and Device-W. (f) Angular distribution of EL intensity for Device-WF and Device-W (from 0° to 90°), and the Lambertian distribution.
Fig.3 (a) shows the absorption spectra of the flat and patterned down-conversion films in the forward direction, respectively. The two types of down-conversion films
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absorption properties of down-conversion films are dominated by QDs materials. Because the concentration of QDs in PMMA is not very high, the absorption properties of down-conversion films in the forward direction are less susceptible to
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microcylinders.
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The existence of micro-pillar arrays makes no difference to emission spectra. As shown in Fig.3 (b), the pumping source BOLED exhibits a peak at 470 nm with a shoulder peak at 500 nm, and its CIE coordinates are (0.16, 0.34). For both white OLEDs with down-conversion films, red emission whose peak at 635 nm appears and
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their spectra are almost identical due to the same QD concentration. Fig.3 (c), (d) and (e) show the current efficiency-luminance, current density-voltage-luminance and power efficiency-current density-external quantum
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efficiency characteristics of BOLED, Device-WF and Device-W. All devices present
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the same current density-voltage characteristics, as shown in Fig.3 (d). It is because the free-standing flat down-conversion films are fixed outside BOLEDs, and they hardly affect the organic and metal layers of BOLEDs. BOLED exhibits the maximum current efficiency of 30.9 cd/A and the maximum luminance of 25761 cd/m2. Device-WF shows the maximum current efficiency of 18.5 cd/A and the maximum luminance of 13779 cd/m2. For Device-W with the patterned down-conversion film, the maximum current efficiency is 19.1 cd/A and the maximum luminance is 14330
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Furthermore, the comparisons between Device-WF and Device-W in power efficiency and EQE illustrate that the microstructure on the surface of down-conversion film can effectively improve the device performance. The maximum
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power efficiency and EQE of Device-WF are 16.5 lm/W and 7.4%, while Device-W
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exhibits much higher maximum power efficiency and EQE of 19.5 lm/W and 8.8%. Besides, it is observed that there are about 19% improvements in power efficiency and EQE, which are greater than the improvements in current efficiency and luminance. It is because part of photons that trapped in the glass substrates and
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down-conversion films are extracted into the air by using the ordered micro-pillar arrays. More extracted photons lead to higher efficiency. Significantly, the down-conversion film with micro-pillar arrays improves light extraction efficiency.
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But with the scattering effect, the micro-pillar arrays scatter the photons in all
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directions and lead them to deviate from their direction of propagation. Therefore, power efficiency and EQE, which are related with the photons propagated toward different directions, are improved obviously. While the changes in current efficiency and luminance which are measured in the forward direction, are not so evident. The normalized angular emission characteristics of Device-WF and Device-W are shown in Fig. 3(f). It can be seen that compared with Device-WF, Device-W shows higher emission intensities in the range of 10-80°. It should be attributed to the excellent
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extraction and scattering effect caused by the micro-pillar arrays.
Fig.4 (a) Normalized EL spectra of Device-W under different driving voltages. The inset is the
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emitting image of Device-W at 6 V. (b) SEM image of the down-conversion film with micro-pillar arrays.
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The normalized spectra of Device-W under different voltages are shown in Fig.4 (a). The spectra are nearly invariable under different voltages. The reason is the blue-emissive and red-emissive components simultaneously change with the same
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degree of variation. The CIE coordinates only shift from (0.302, 0.410) at 4 V to (0.304, 0.417) at 8 V. It means that Device-W based on down-conversion system has relatively high spectral stability.
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The surface topography of the down-conversion film with micro-pillar arrays is
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analyzed using scanning electron microscopy (SEM), as shown in Fig.4 (b). We can clearly see the regular micro-pillar arrays. The height of each micro pillar is 5 µm, and both the diameter and spacing are 20 µm. These pillars with complete shape and clear contour are the same size as the pits on the surface of patterned Si substrate. 3.2 RGB OLEDs
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Fig. 5 (a) Transmission spectra of RGB color filters (CFs). (b) Normalized spectra of Device-R, Device-G and Device-B under different driving voltages. The inset is the photograph of the
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patterned down-conversion films with red and green QDs under UV light. (c) Current efficiency– luminance characteristics of Device-R, Device-G and Device-B. The inset presents the CIE coordinates of the three devices.
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RGB emission is obtained by passing the emission of BOLED through different
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color filters and down-conversion films with micro-pillar arrays as shown in Fig.1 (b). Fig.5 (a) shows the transmission spectra of ordinary RGB color filters used in this work. Every filter allows specific wavelengths of light to pass through. Fig.5 (b) shows the normalized spectra of Device-R, Device-G and Device-B. Device-R has the emission peak at 635 nm. The emission spectrum is very narrow, which is attributed to the red QDs. In addition, the patterned down-conversion film containing 12 µL of green QD solution is prepared. It is combined with BOLED and green filter to
ACCEPTED MANUSCRIPT fabricate Device-G. We can see that Device-G has two peaks. The peak at 550 nm is originated from green QDs, and the other peak at 510 nm is associated with the shoulder peak of the EL spectra of BOLED. To improve color quality, Device-B is
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obtained by BOLED simply integrated with blue filter, and it exhibits a much narrower blue emission with peak at 470 nm. Moreover, it is obvious that the spectra of the RGB OLEDs are stable under different voltages, and the CIE coordinates of
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and (0.0003, 0.0075) from 4 V to 8 V.
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Device-R, Device-G and Device-B only shift by (0.0009, 0.0016), (0.0009, 0.0015)
Fig.5 (c) indicates the current efficiency–luminance characteristics of Device-R, Device-G and Device-B. The RGB OLEDs show the maximum CE of 3.2, 20.8 and 10.3 cd/A, respectively. The efficiency decline and brightness decay are attributed to
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the low transmittance of color filters, the energy loss in the down-conversion process and the mismatch between emission spectra and the transmission of color filters. The device performance can be improved by using high quantum-yield QDs with emission
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matching well with color filters.
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As shown in the inset of Fig.5 (c), the CIE coordinates of RGB OLEDs are (0.70, 0.30), (0.28, 0.64) and (0.10, 0.16), respectively. The color gamut reaches up to 83.1% NTSC (National Television Standards Committee). We also adopt commercial available white OLED (P03A0404N-A121, Lumiotec Inc.) and white LED (DN01B, LatticePower) with color filters mentioned above to achieve RGB emission for displays. Fig.6 (a) shows the normalized spectra of the white OLED and white LED. Besides, the normalized spectra of their respective filtered RGB emission are shown
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from white LED are (0.66, 0.34), (0.32, 0.63), (0.15, 0.09) and 73.6% NTSC, respectively. Apparently, the color gamut of the devices investigated in this work is
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nearly 15% higher than the devices based on common white LED or white OLED.
Fig. 6 Normalized spectra of (a) white OLED and white LED, and the filtered RGB emission from (b) white OLED and (c) white LED.
To comprehend the improvement in color gamut, the corrected full width at half
maximums (FWHMs) of RGB emission are calculated by superposing products of the proportion and FWHM of each peak in RGB emission. The corrected FWHMs of the RGB emission in our work are about 31, 30 and 36 nm, respectively. The corrected FWHMs of the filtered RGB emission from white OLED are about 25, 48 and 44 nm,
ACCEPTED MANUSCRIPT while those from white LED are 67, 73 and 23 nm. Most of organic material and phosphor used in OLED and LED usually exhibit broad spectra. After adopting QDs, the obtained RGB OLEDs exhibit narrower peaks and higher color purity. The narrow
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peaks are beneficial to save energy and maintain efficiency, because a lot of energy of the light with large FWHM can be absorbed by color filters, leading to lower efficiency. Moreover, QDs with various light–emitting wavelengths can be used to
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meet different requirements, due to the advantage of size-dependent emission.
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Furthermore, with the development of material science, it is possible to achieve 100% NTSC color gamut by adopting deep blue organic EL materials and better QDs. 4. Conclusion
In summary, efficient and color-stable WRGB OLEDs with QDs are achieved.
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Micro-pillar arrays on the surface of the down-conversion film by transfer imprint are also proved to be beneficial to improve the light extraction from substrate. Furthermore, by adopting color filters and the patterned down-conversion films
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containing different QDs, we achieve color-stable full-color WRGB emission, whose
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color gamut reaches up to 83.1% NTSC. We believe that this study provides an effective and convenient method to fabricate white OLEDs and full-color displays.
Acknowledgement
This work was supported by the National Natural Science Foundation of China (Grant Nos. 61474054, 61475060, 61774074).
References: [1] H. Sasabe, J. Kido, Development of high performance OLEDs for general lighting,
ACCEPTED MANUSCRIPT J. Mater. Chem. C 1 (2013) 1699-1707. [2] T. Sekitani, H. Nakajima, H. Maeda, T. Fukushima, T. Aida, K. Hata, T. Someya, Stretchable active-matrix organic light-emitting diode display using printable elastic
RI PT
conductors, Nat. Mater. 8 (2009) 494-499. [3] J.W. Hamer, A.D. Arnold, M.L. Boroson, M. Itoh, T.K. Hatwar, M.J. Helber, K. Miwa, C.I. Levey, M. Long, J.E. Ludwicki, D.C. Scheirer, J.P. Spindler, S.A. Van
SC
Slyke, System design for a wide-color-gamut TV-sized AMOLED display, J. Soc. Inf.
M AN U
Display 16 (2008) 3-14.
[4] F. Chesterman, G. Muliuk, B. Piepers, T. Kimpe, P. De Visschere, K. Neyts, Power consumption and temperature distribution in WRGB active-matrix OLED displays, J. Disp. Technol. 12 (2016) 616-625.
TE D
[5] Y. Xiong, W. Xu, C. Li, B. Liang, L. Zhao, J.B. Peng, Y. Cao, J. Wang, Utilizing white OLED for full color reproduction in flat panel display, Org. Electron. 9 (2008) 533-538.
EP
[6] C.W. Han, H.K. Kim, H.S. Pang, S.H. Pieh, C.J. Sung, H.S. Choi, W.C. Kim, M.S.
AC C
Kim, Y.H. Tak, Dual-plate OLED display (DOD) embedded with white OLED, J. Display Technol. 5 (2009) 541–545. [7] J.W. Park, D.C. Shin, S.H. Park, Large-area OLED lightings and their applications, Semicond. Sci. Tech. 26 (2011) 034002. [8] C.W. Han, K.M. Kim, S.J. Bae, H.S. Choi, J.M. Lee, T.S. Kim, Y.H. Tak, S.Y. Cha, B.C. Ahn, 55-inch FHD OLED TV employing new tandem WOLEDs, SID Symp. Dig. Tech. Pap. 43 (2012) 279−281.
ACCEPTED MANUSCRIPT [9] Y. Fan, H.M. Zhang, J.S. Chen, D.G. Ma, Three-peak top-emitting white organic emitting diodes with wide color gamut for display application, Org. Electron. 14 (2013) 1898-1902.
RI PT
[10] H.J. Kim, M.H. Shin, Y.J. Kim, Optical efficiency enhancement in white organic light-emitting diode display with high color gamut using patterned quantum dot film and long pass filter, Jpn. J. Appl. Phys. 55 (2016) 08RF01.
SC
[11] W.W. Zhang, H.Y. Liu, R.G. Sun, Full color organic light-emitting devices with
M AN U
microcavity structure and color filter, Opt. Express 17 (2009) 8005-8011. [12] C.W. Han, Y.H. Tak, B.C. Ahn, 15-in. RGBW panel using two-stacked white OLED and color filters for large-sized display applications, J. Soc. Inf. Display 19 (2011) 190-195.
TE D
[13] M. Schaer, F. Nuesch, D. Berner, W. Leo, L. Zuppiroli, Water vapor and oxygen degradation mechanisms in organic light emitting diodes, Adv. Funct. Mater. 11 (2001) 116-121.
EP
[14] S.F. Chen, Q. Wu, M. Kong, X.F. Zhao, Z. Yu, P.P. Jia, W. Huang, On the origin
AC C
of the shift in color in white organic light-emitting diodes, J. Mater. Chem. C 1 (2013) 3508-3524.
[15] S.S. Lee, T.J. Song, S.M. Cho, Organic white-light-emitting devices based on balanced exciton-recombination-zone split using a carrier blocking layer, Mat. Sci. Eng. B 95 (2002) 24-28. [16] B.C. Krummacher, V.E. Choong, M.K. Mathai, S.A. Choulis, F. So, F. Jermann, T. Fiedler, M. Zachau, Highly efficient white organic light-emitting diode, Appl. Phys.
ACCEPTED MANUSCRIPT Lett. 88 (2006) 113506. [17] S.H. Liu, X.M. Wen, W.B. Liu, W. Zhang, J. Yu, W.F. Xie, L.T. Zhang, Angle-stable top-emitting white organic light-emitting devices employing a
RI PT
down-conversion layer, Curr. Appl. Phys. 14 (2014) 1451-1454. [18] H.F. Xiang, S.C. Yu, C.M. Che, P.T. Lai, Efficient white and red light emission from
GaN/tris-(8-hydroxyquinolato)
aluminum/platinum(II)
SC
meso-tetrakis(pentafluorophenyl) porphyrin hybrid light-emitting diodes, Appl. Phys.
M AN U
Lett. 83 (2003) 1518-1520.
[19] C. Yoon, T. Kim, M.H. Shin, Y.G. Song, K. Shin, Y.J. Kim, K. Lee, Highly luminescent and stable white light-emitting diodes created by direct incorporation of Cd-free quantum dots in silicone resins using the thiol group, J. Mater. Chem. C 3
TE D
(2015) 6908-6915.
[20] M. Danek, K. F. Jensen, C. B. Murray, M. G. Bawendi, Synthesis of luminescent thin-film CdSe/ZnSe quantum dot composites using CdSe quantum dots passivated
EP
with an overlayer of ZnSe, Chem. Mater. 8 (1996) 173–180.
AC C
[21] J.Y. Woo, K.N. Kim, S. Jeong, C.S. Han, Thermal behavior of a quantum dot nanocomposite as a color converting material and its application to white LED, Nanotechnology 21 (2010) 495704. [22] E. Jang, S. Jun, H. Jang, J. Llim, B. Kim, Y. Kim, White-light-emitting diodes with quantum dot color converters for display backlights, Adv. Mater. 22 (2010) 3076-3080. [23] X.B. Wang, X.S. Yan, W.W. Li, K. Sun, Doped quantum dots for
ACCEPTED MANUSCRIPT white-light-emitting diodes without reabsorption of multiphase phosphors, Adv. Mater. 24 (2012) 2742-2747. [24] C.C. Tu, L. Tang, J.D. Huang, A. Voutsas, L.Y. Lin, Visible electroluminescence
RI PT
from hybrid colloidal silicon quantum dot-organic light-emitting diodes, Appl. Phys. Lett. 98 (2011) 213102.
[25] J.M. Li, Y. Liu, J. Hua, L.H. Tian, J.L. Zhao, Photoluminescence properties of
SC
transition metal-doped Zn-In-S/ZnS core/shell quantum dots in solid films, Rsc Adv.
M AN U
6 (2016) 44859-44864.
[26] S. Lee, E. Wrzesniewski, W.R. Cao, J.G. Xue, E.P. Douglas, Printed microlens arrays for enhancing light extraction from organic light-emitting devices, J. Disp. Technol. 9 (2013) 497-503.
TE D
[27] R. Meerheim, M. Furno, S. Hofmann, B. Lussem, K. Leo, Quantification of energy loss mechanisms in organic light-emitting diodes, Appl. Phys. Lett. 97 (2010) 275.
EP
[28] H.Y. Lin, Y.H. Ho, J.H. Lee, K.Y. Chen, J.H. Fang, S.C. Hsu, M.K. Wei, H.Y.
AC C
Lin, J.H. Tsai, T.C. Wu, Patterned microlens array for efficiency improvement of small-pixelated organic light-emitting devices, Opt. Express 16 (2008) 11044-11051. [29] Y.F. Liu, J. Feng, Y.F. Zhang, H.F. Cui, D. Yin, Y.G. Bi, J.F. Song, Q.D. Chen, H.B. Sun, Polymer encapsulation of flexible top-emitting organic light-emitting devices with improved light extraction by integrating a microstructure, Org. Electron. 15 (2014) 2661-2666. [30] J.Y. Oh, J.H. Kim, Y.K. Seo, C.W. Joo, J. Lee, J.I. Lee, S. Yu, C. Yun, M.H.
ACCEPTED MANUSCRIPT Kang, B.H. Choi, Y.H. Kim, Down-conversion light outcoupling films using imprinted microlens arrays for white organic light-emitting diodes, Dyes Pigments
AC C
EP
TE D
M AN U
SC
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136 (2017) 92-96.
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• Blue bottom-emitting OLEDs integrating with color filters and two types of patterned down-conversion films containing red and green QDs are proposed to realize color-stable WRGB emission.
proposed to improve the light extraction property.
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• Micro-pillar arrays formed on the surface of down-conversion films are
• 83.1% NTSC color gamut is achieved only based on blue
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bottom-emitting OLEDs and QDs down-conversion films.
S1566-1199(18)30442-7
DOI:
10.1016/j.orgel.2018.08.036
Reference:
ORGELE 4850
To appear in:
Organic Electronics
Received Date: 31 July 2018 Revised Date:
15 August 2018
Accepted Date: 20 August 2018
Please cite this article as: M. Yin, T. Pan, Z. Yu, X. Peng, X. Zhang, W. Xie, S. Liu, L. Zhang, Colorstable WRGB emission from blue OLEDs with quantum dots-based patterned down-conversion layer, Organic Electronics (2018), doi: 10.1016/j.orgel.2018.08.036. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Color-Stable WRGB emission from blue OLEDs with quantum dots-based patterned down-conversion layer Maojun Yin, Teng Pan, Ziwei Yu, Xiaomei Peng, Xiang Zhang, Wenfa Xie, Shihao Liu*, Letian Zhang*
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State Key Laboratory of Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun, 130012, People’s Republic of China *
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Corresponding author: [email protected], [email protected]
Abstract
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For full-color displays, white organic light-emitting devices (OLEDs) combining with color filters are a promising technology. It avoids the precise shadow masking that is often used to separate RGB pixels. But the spectra of traditional white OLEDs are changeable, which would make the full-color displays show poor color stability. In this work, in order to realize color-stable white, red, green and blue emission, blue
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bottom-emitting OLEDs are integrated with color filters and two types of patterned down-conversion films containing red and green quantum dots (QDs). Besides, by solution-based transfer printing, micro-pillar arrays are formed on the surface of down-conversion films to improve the light extraction property. Based on this
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principle, color-stable white, red, green and blue emission are achieved from efficient blue OLEDs. Due to the highly saturated color emission of QDs, wider color gamut
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can be anticipated by employing these devices as three primary colors, nearly 15% wider than that based on commercial white LEDs or white OLEDs with the same color filters.
Keywords: Organic light-emitting devices, Quantum dots, Micro-pillar arrays, Down-conversion, Color gamut
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1.
Introduction Today, organic light-emitting devices (OLEDs) have successfully come into
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commercialization [1, 2]. Novel technologies for displays are developed based on OLEDs, such as separated white, red, green and blue (WRGB) or RGB sub-pixels [3, 4] and white OLEDs combining with color filters [5-12]. Although the former is easy
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to realize high brightness and energy-saving displays, its high manufacture cost and high defect ratio limit its application in large scale display. Nevertheless, the latter can
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avoid the traditional fine metal mask and reduce the difficulty and costs in realizing high resolution or large scale displays. Therefore, it can be seen that white light is also very essential for display industry. Compared with white LEDs, white OLEDs are
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better white-light sources for display, because OLEDs have the advantage of self-emitting and low power consumption. Besides, white OLED can be also used as the fourth white sub-pixel to increase display luminance and contrast. However, the
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color stability of traditional white OLEDs is poor. There are spectral changes caused
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by different degradation processes of organic electroluminescence (EL) materials [13] and the shifting of recombination region as voltages increasing [14, 15]. It can lead to color distortion when the traditional white OLEDs are applied to displays. Nevertheless, the down-conversion systems have the potential to realize
color-stable WRGB emission via blue OLEDs [16, 17]. Blue OLEDs are used as pumping sources, leading to photoluminescence (PL) after the excitation of down-conversion materials by blue light. Then WRGB emission can be anticipated,
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Besides, the PL spectra of down-conversion materials are nearly invariable with the stable pumping sources. There would be synchronous changes between blue light and the converted light. As a result, the color stability of the generated WRGB emission
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against voltage and operation time can be easily achieved. But it is difficult to choose
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appropriate down-conversion materials. Because the absorption spectra of organic phosphors widely used in OLEDs are usually in the ultraviolet (UV) region [18]. However, with the fast development, quantum dots (QDs) receive particular attention as potential down-conversion materials. QDs have many favorable optoelectronic
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features, such as high PL quantum yield, size-dependent emission and excellent stability [19-24]. Moreover, the broad absorption spectra and beneficially high absorption capability in the near-UV–blue region provide high conversion efficiency
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[25]. QDs also feature high color saturation due to the narrow emission spectra.
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Therefore, it will be also beneficial to expand color gamut of displays. In addition, the luminous efficiency is equally important for displays. But, a large
number of generated photons are trapped inside the glass substrates of bottom-emitting OLEDs. As a result, the luminous efficiency is considerably limited by the substrate and waveguide mode [26, 27]. To extract the trapped photons and increase efficiency, the films with microscopic structure are widely adopted [28-30]. In this work, red and green CdSe/ZnS QDs are embedded into polymethyl
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(PMMA)
matrices
to
prepare
down-conversion
films.
Blue
bottom-emitting OLEDs integrated with color filters and two types of patterned down-conversion films are proposed to realize color-stable white, red, green and blue
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emission. The influences of micro-pillar arrays of the PMMA matrices on the device performance are also explored.
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2. Experimental Section
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Fig. 1. Device structures of (a) BOLED and (b) WRGB OLEDs achieved by combining BOLED with patterned down-conversion films and color filters.
2.1 Device structures of OLEDs
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Fig.1 (a) shows the device structure of blue OLED (BOLED) that is used as
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pumping source. The device structure of the BOLED is indium tin oxide (ITO)/MoO3 (3 nm)/di-[4-(N,N-di-ptolyl-amino)-phenyl] cyclohexane (TAPC, 35 nm)/4,4',4''-tris (carbazo-9-yl)triphenylamine (TCTA, 5 nm)/4,4',4''-tris(carbazol-9-yl)-triphenylamine: bis[(4,6-difluorophenyl)-pyridinato-N,C2'] (picolinato) Ir(III) (TCTA:12wt% FIrpic, 20 nm)/1,3,5-tri[(3-pyridyl)-phen-3-yl] benzene (TmPyPB, 60 nm)/LiF (0.5 nm)/Mg: 6wt% Ag (100 nm). As shown in Fig. 1 (b), because of the excitation of blue light, the patterned down-conversion films with red and green QDs can produce red and green
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To fabricate BOLED, the ITO-coated glass substrates are cleaned with Decon 90, and ultrasonically cleaned in deionized water baths for 15 min. After dried at 120 oC for 20 min and handled in plasma for about 5 min, the substrates are loaded in a
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vacuum evaporator. Then organic layers and cathode materials are deposited on the
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substrates under high vacuum (~6.0×10–4 Pa). The material deposition rate is monitored with quartz crystal. The current density-voltage-luminance characteristics and spectra are measured by using a Goniophotometric Measurement System (GP500, Otsuka Electronics Co. Osaka, Japan) in air at room temperature simultaneously. The
sphere.
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power efficiency and external quantum efficiency are measured with an integrating
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2.3 Fabrication of down-conversion films
Fig. 2. Schemes of the fabrication process for the down-conversion film with micro-pillar arrays.
The fabrication process of down-conversion films is illustrated in Fig. 2. For the preparation of down-conversion films, 0.285 g of PMMA is completely dissolved in 1
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is drop-casted onto the silicon substrate. The patterned silicon substrate with micro-pits arrays is used to fabricate the down-conversion films with micro-pillar arrays, while the flat silicon substrate is used to fabricate the flat down-conversion
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films without any patterns. After heating at 70 oC for 40 min to promote the solvent
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evaporation, the solidified down-conversion film with about 1 mm thickness can be readily peeled off without damage. In this process, red and green CdSe/ZnS QDs (100 mg/ml and 50 mg/ml in toluene) are used and the QD concentration of the film can be easily controlled by the volume of QD solution added to the QD-PMMA mixture
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solution. 3. Results and discussion 3.1 White OLEDs
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For bottom-emitting OLEDs, many photons are trapped within the glass substrates
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due to the substrate and waveguide mode. Therefore, the patterned down-conversion films with micro-pillar arrays are proposed to improve the light extraction. To study the effects of micro-pillar arrays on the device performance, we prepare flat down-conversion films as references. The red QD solution with the same volume (1.0 µL) is used for preparing the film with or without micro-pillar arrays. White OLED with flat down-conversion film and patterned down-conversion film are fabricated and their EL performances are studied. For simplicity, the former is marked as
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Device-WF, while the latter is marked as Device-W.
Fig. 3 (a) Absorption spectra of the flat film (Abs-FF), the patterned film (Abs-PF), red QDs
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(Abs-R), green QDs (Abs-G), and EL spectra of BOLED in the forward direction. (b) Normalized spectra of BOLED, Device-WF and Device-W. (c) Current efficiency–luminance characteristics and (d) current density-voltage-luminance characteristics of BOLED, Device-WF and Device-W. The inset is the emitting image of BOLED at 6 V. (e) Power efficiency–current density–external quantum efficiency (EQE) characteristics of Device-WF and Device-W. (f) Angular distribution of EL intensity for Device-WF and Device-W (from 0° to 90°), and the Lambertian distribution.
Fig.3 (a) shows the absorption spectra of the flat and patterned down-conversion films in the forward direction, respectively. The two types of down-conversion films
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absorption properties of down-conversion films are dominated by QDs materials. Because the concentration of QDs in PMMA is not very high, the absorption properties of down-conversion films in the forward direction are less susceptible to
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microcylinders.
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The existence of micro-pillar arrays makes no difference to emission spectra. As shown in Fig.3 (b), the pumping source BOLED exhibits a peak at 470 nm with a shoulder peak at 500 nm, and its CIE coordinates are (0.16, 0.34). For both white OLEDs with down-conversion films, red emission whose peak at 635 nm appears and
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their spectra are almost identical due to the same QD concentration. Fig.3 (c), (d) and (e) show the current efficiency-luminance, current density-voltage-luminance and power efficiency-current density-external quantum
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efficiency characteristics of BOLED, Device-WF and Device-W. All devices present
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the same current density-voltage characteristics, as shown in Fig.3 (d). It is because the free-standing flat down-conversion films are fixed outside BOLEDs, and they hardly affect the organic and metal layers of BOLEDs. BOLED exhibits the maximum current efficiency of 30.9 cd/A and the maximum luminance of 25761 cd/m2. Device-WF shows the maximum current efficiency of 18.5 cd/A and the maximum luminance of 13779 cd/m2. For Device-W with the patterned down-conversion film, the maximum current efficiency is 19.1 cd/A and the maximum luminance is 14330
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Furthermore, the comparisons between Device-WF and Device-W in power efficiency and EQE illustrate that the microstructure on the surface of down-conversion film can effectively improve the device performance. The maximum
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power efficiency and EQE of Device-WF are 16.5 lm/W and 7.4%, while Device-W
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exhibits much higher maximum power efficiency and EQE of 19.5 lm/W and 8.8%. Besides, it is observed that there are about 19% improvements in power efficiency and EQE, which are greater than the improvements in current efficiency and luminance. It is because part of photons that trapped in the glass substrates and
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down-conversion films are extracted into the air by using the ordered micro-pillar arrays. More extracted photons lead to higher efficiency. Significantly, the down-conversion film with micro-pillar arrays improves light extraction efficiency.
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But with the scattering effect, the micro-pillar arrays scatter the photons in all
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directions and lead them to deviate from their direction of propagation. Therefore, power efficiency and EQE, which are related with the photons propagated toward different directions, are improved obviously. While the changes in current efficiency and luminance which are measured in the forward direction, are not so evident. The normalized angular emission characteristics of Device-WF and Device-W are shown in Fig. 3(f). It can be seen that compared with Device-WF, Device-W shows higher emission intensities in the range of 10-80°. It should be attributed to the excellent
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extraction and scattering effect caused by the micro-pillar arrays.
Fig.4 (a) Normalized EL spectra of Device-W under different driving voltages. The inset is the
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emitting image of Device-W at 6 V. (b) SEM image of the down-conversion film with micro-pillar arrays.
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The normalized spectra of Device-W under different voltages are shown in Fig.4 (a). The spectra are nearly invariable under different voltages. The reason is the blue-emissive and red-emissive components simultaneously change with the same
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degree of variation. The CIE coordinates only shift from (0.302, 0.410) at 4 V to (0.304, 0.417) at 8 V. It means that Device-W based on down-conversion system has relatively high spectral stability.
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The surface topography of the down-conversion film with micro-pillar arrays is
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analyzed using scanning electron microscopy (SEM), as shown in Fig.4 (b). We can clearly see the regular micro-pillar arrays. The height of each micro pillar is 5 µm, and both the diameter and spacing are 20 µm. These pillars with complete shape and clear contour are the same size as the pits on the surface of patterned Si substrate. 3.2 RGB OLEDs
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Fig. 5 (a) Transmission spectra of RGB color filters (CFs). (b) Normalized spectra of Device-R, Device-G and Device-B under different driving voltages. The inset is the photograph of the
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patterned down-conversion films with red and green QDs under UV light. (c) Current efficiency– luminance characteristics of Device-R, Device-G and Device-B. The inset presents the CIE coordinates of the three devices.
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RGB emission is obtained by passing the emission of BOLED through different
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color filters and down-conversion films with micro-pillar arrays as shown in Fig.1 (b). Fig.5 (a) shows the transmission spectra of ordinary RGB color filters used in this work. Every filter allows specific wavelengths of light to pass through. Fig.5 (b) shows the normalized spectra of Device-R, Device-G and Device-B. Device-R has the emission peak at 635 nm. The emission spectrum is very narrow, which is attributed to the red QDs. In addition, the patterned down-conversion film containing 12 µL of green QD solution is prepared. It is combined with BOLED and green filter to
ACCEPTED MANUSCRIPT fabricate Device-G. We can see that Device-G has two peaks. The peak at 550 nm is originated from green QDs, and the other peak at 510 nm is associated with the shoulder peak of the EL spectra of BOLED. To improve color quality, Device-B is
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obtained by BOLED simply integrated with blue filter, and it exhibits a much narrower blue emission with peak at 470 nm. Moreover, it is obvious that the spectra of the RGB OLEDs are stable under different voltages, and the CIE coordinates of
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and (0.0003, 0.0075) from 4 V to 8 V.
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Device-R, Device-G and Device-B only shift by (0.0009, 0.0016), (0.0009, 0.0015)
Fig.5 (c) indicates the current efficiency–luminance characteristics of Device-R, Device-G and Device-B. The RGB OLEDs show the maximum CE of 3.2, 20.8 and 10.3 cd/A, respectively. The efficiency decline and brightness decay are attributed to
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the low transmittance of color filters, the energy loss in the down-conversion process and the mismatch between emission spectra and the transmission of color filters. The device performance can be improved by using high quantum-yield QDs with emission
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matching well with color filters.
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As shown in the inset of Fig.5 (c), the CIE coordinates of RGB OLEDs are (0.70, 0.30), (0.28, 0.64) and (0.10, 0.16), respectively. The color gamut reaches up to 83.1% NTSC (National Television Standards Committee). We also adopt commercial available white OLED (P03A0404N-A121, Lumiotec Inc.) and white LED (DN01B, LatticePower) with color filters mentioned above to achieve RGB emission for displays. Fig.6 (a) shows the normalized spectra of the white OLED and white LED. Besides, the normalized spectra of their respective filtered RGB emission are shown
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from white LED are (0.66, 0.34), (0.32, 0.63), (0.15, 0.09) and 73.6% NTSC, respectively. Apparently, the color gamut of the devices investigated in this work is
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nearly 15% higher than the devices based on common white LED or white OLED.
Fig. 6 Normalized spectra of (a) white OLED and white LED, and the filtered RGB emission from (b) white OLED and (c) white LED.
To comprehend the improvement in color gamut, the corrected full width at half
maximums (FWHMs) of RGB emission are calculated by superposing products of the proportion and FWHM of each peak in RGB emission. The corrected FWHMs of the RGB emission in our work are about 31, 30 and 36 nm, respectively. The corrected FWHMs of the filtered RGB emission from white OLED are about 25, 48 and 44 nm,
ACCEPTED MANUSCRIPT while those from white LED are 67, 73 and 23 nm. Most of organic material and phosphor used in OLED and LED usually exhibit broad spectra. After adopting QDs, the obtained RGB OLEDs exhibit narrower peaks and higher color purity. The narrow
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peaks are beneficial to save energy and maintain efficiency, because a lot of energy of the light with large FWHM can be absorbed by color filters, leading to lower efficiency. Moreover, QDs with various light–emitting wavelengths can be used to
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meet different requirements, due to the advantage of size-dependent emission.
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Furthermore, with the development of material science, it is possible to achieve 100% NTSC color gamut by adopting deep blue organic EL materials and better QDs. 4. Conclusion
In summary, efficient and color-stable WRGB OLEDs with QDs are achieved.
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Micro-pillar arrays on the surface of the down-conversion film by transfer imprint are also proved to be beneficial to improve the light extraction from substrate. Furthermore, by adopting color filters and the patterned down-conversion films
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containing different QDs, we achieve color-stable full-color WRGB emission, whose
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color gamut reaches up to 83.1% NTSC. We believe that this study provides an effective and convenient method to fabricate white OLEDs and full-color displays.
Acknowledgement
This work was supported by the National Natural Science Foundation of China (Grant Nos. 61474054, 61475060, 61774074).
References: [1] H. Sasabe, J. Kido, Development of high performance OLEDs for general lighting,
ACCEPTED MANUSCRIPT J. Mater. Chem. C 1 (2013) 1699-1707. [2] T. Sekitani, H. Nakajima, H. Maeda, T. Fukushima, T. Aida, K. Hata, T. Someya, Stretchable active-matrix organic light-emitting diode display using printable elastic
RI PT
conductors, Nat. Mater. 8 (2009) 494-499. [3] J.W. Hamer, A.D. Arnold, M.L. Boroson, M. Itoh, T.K. Hatwar, M.J. Helber, K. Miwa, C.I. Levey, M. Long, J.E. Ludwicki, D.C. Scheirer, J.P. Spindler, S.A. Van
SC
Slyke, System design for a wide-color-gamut TV-sized AMOLED display, J. Soc. Inf.
M AN U
Display 16 (2008) 3-14.
[4] F. Chesterman, G. Muliuk, B. Piepers, T. Kimpe, P. De Visschere, K. Neyts, Power consumption and temperature distribution in WRGB active-matrix OLED displays, J. Disp. Technol. 12 (2016) 616-625.
TE D
[5] Y. Xiong, W. Xu, C. Li, B. Liang, L. Zhao, J.B. Peng, Y. Cao, J. Wang, Utilizing white OLED for full color reproduction in flat panel display, Org. Electron. 9 (2008) 533-538.
EP
[6] C.W. Han, H.K. Kim, H.S. Pang, S.H. Pieh, C.J. Sung, H.S. Choi, W.C. Kim, M.S.
AC C
Kim, Y.H. Tak, Dual-plate OLED display (DOD) embedded with white OLED, J. Display Technol. 5 (2009) 541–545. [7] J.W. Park, D.C. Shin, S.H. Park, Large-area OLED lightings and their applications, Semicond. Sci. Tech. 26 (2011) 034002. [8] C.W. Han, K.M. Kim, S.J. Bae, H.S. Choi, J.M. Lee, T.S. Kim, Y.H. Tak, S.Y. Cha, B.C. Ahn, 55-inch FHD OLED TV employing new tandem WOLEDs, SID Symp. Dig. Tech. Pap. 43 (2012) 279−281.
ACCEPTED MANUSCRIPT [9] Y. Fan, H.M. Zhang, J.S. Chen, D.G. Ma, Three-peak top-emitting white organic emitting diodes with wide color gamut for display application, Org. Electron. 14 (2013) 1898-1902.
RI PT
[10] H.J. Kim, M.H. Shin, Y.J. Kim, Optical efficiency enhancement in white organic light-emitting diode display with high color gamut using patterned quantum dot film and long pass filter, Jpn. J. Appl. Phys. 55 (2016) 08RF01.
SC
[11] W.W. Zhang, H.Y. Liu, R.G. Sun, Full color organic light-emitting devices with
M AN U
microcavity structure and color filter, Opt. Express 17 (2009) 8005-8011. [12] C.W. Han, Y.H. Tak, B.C. Ahn, 15-in. RGBW panel using two-stacked white OLED and color filters for large-sized display applications, J. Soc. Inf. Display 19 (2011) 190-195.
TE D
[13] M. Schaer, F. Nuesch, D. Berner, W. Leo, L. Zuppiroli, Water vapor and oxygen degradation mechanisms in organic light emitting diodes, Adv. Funct. Mater. 11 (2001) 116-121.
EP
[14] S.F. Chen, Q. Wu, M. Kong, X.F. Zhao, Z. Yu, P.P. Jia, W. Huang, On the origin
AC C
of the shift in color in white organic light-emitting diodes, J. Mater. Chem. C 1 (2013) 3508-3524.
[15] S.S. Lee, T.J. Song, S.M. Cho, Organic white-light-emitting devices based on balanced exciton-recombination-zone split using a carrier blocking layer, Mat. Sci. Eng. B 95 (2002) 24-28. [16] B.C. Krummacher, V.E. Choong, M.K. Mathai, S.A. Choulis, F. So, F. Jermann, T. Fiedler, M. Zachau, Highly efficient white organic light-emitting diode, Appl. Phys.
ACCEPTED MANUSCRIPT Lett. 88 (2006) 113506. [17] S.H. Liu, X.M. Wen, W.B. Liu, W. Zhang, J. Yu, W.F. Xie, L.T. Zhang, Angle-stable top-emitting white organic light-emitting devices employing a
RI PT
down-conversion layer, Curr. Appl. Phys. 14 (2014) 1451-1454. [18] H.F. Xiang, S.C. Yu, C.M. Che, P.T. Lai, Efficient white and red light emission from
GaN/tris-(8-hydroxyquinolato)
aluminum/platinum(II)
SC
meso-tetrakis(pentafluorophenyl) porphyrin hybrid light-emitting diodes, Appl. Phys.
M AN U
Lett. 83 (2003) 1518-1520.
[19] C. Yoon, T. Kim, M.H. Shin, Y.G. Song, K. Shin, Y.J. Kim, K. Lee, Highly luminescent and stable white light-emitting diodes created by direct incorporation of Cd-free quantum dots in silicone resins using the thiol group, J. Mater. Chem. C 3
TE D
(2015) 6908-6915.
[20] M. Danek, K. F. Jensen, C. B. Murray, M. G. Bawendi, Synthesis of luminescent thin-film CdSe/ZnSe quantum dot composites using CdSe quantum dots passivated
EP
with an overlayer of ZnSe, Chem. Mater. 8 (1996) 173–180.
AC C
[21] J.Y. Woo, K.N. Kim, S. Jeong, C.S. Han, Thermal behavior of a quantum dot nanocomposite as a color converting material and its application to white LED, Nanotechnology 21 (2010) 495704. [22] E. Jang, S. Jun, H. Jang, J. Llim, B. Kim, Y. Kim, White-light-emitting diodes with quantum dot color converters for display backlights, Adv. Mater. 22 (2010) 3076-3080. [23] X.B. Wang, X.S. Yan, W.W. Li, K. Sun, Doped quantum dots for
ACCEPTED MANUSCRIPT white-light-emitting diodes without reabsorption of multiphase phosphors, Adv. Mater. 24 (2012) 2742-2747. [24] C.C. Tu, L. Tang, J.D. Huang, A. Voutsas, L.Y. Lin, Visible electroluminescence
RI PT
from hybrid colloidal silicon quantum dot-organic light-emitting diodes, Appl. Phys. Lett. 98 (2011) 213102.
[25] J.M. Li, Y. Liu, J. Hua, L.H. Tian, J.L. Zhao, Photoluminescence properties of
SC
transition metal-doped Zn-In-S/ZnS core/shell quantum dots in solid films, Rsc Adv.
M AN U
6 (2016) 44859-44864.
[26] S. Lee, E. Wrzesniewski, W.R. Cao, J.G. Xue, E.P. Douglas, Printed microlens arrays for enhancing light extraction from organic light-emitting devices, J. Disp. Technol. 9 (2013) 497-503.
TE D
[27] R. Meerheim, M. Furno, S. Hofmann, B. Lussem, K. Leo, Quantification of energy loss mechanisms in organic light-emitting diodes, Appl. Phys. Lett. 97 (2010) 275.
EP
[28] H.Y. Lin, Y.H. Ho, J.H. Lee, K.Y. Chen, J.H. Fang, S.C. Hsu, M.K. Wei, H.Y.
AC C
Lin, J.H. Tsai, T.C. Wu, Patterned microlens array for efficiency improvement of small-pixelated organic light-emitting devices, Opt. Express 16 (2008) 11044-11051. [29] Y.F. Liu, J. Feng, Y.F. Zhang, H.F. Cui, D. Yin, Y.G. Bi, J.F. Song, Q.D. Chen, H.B. Sun, Polymer encapsulation of flexible top-emitting organic light-emitting devices with improved light extraction by integrating a microstructure, Org. Electron. 15 (2014) 2661-2666. [30] J.Y. Oh, J.H. Kim, Y.K. Seo, C.W. Joo, J. Lee, J.I. Lee, S. Yu, C. Yun, M.H.
ACCEPTED MANUSCRIPT Kang, B.H. Choi, Y.H. Kim, Down-conversion light outcoupling films using imprinted microlens arrays for white organic light-emitting diodes, Dyes Pigments
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EP
TE D
M AN U
SC
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136 (2017) 92-96.
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• Blue bottom-emitting OLEDs integrating with color filters and two types of patterned down-conversion films containing red and green QDs are proposed to realize color-stable WRGB emission.
proposed to improve the light extraction property.
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• Micro-pillar arrays formed on the surface of down-conversion films are
• 83.1% NTSC color gamut is achieved only based on blue
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bottom-emitting OLEDs and QDs down-conversion films.