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Hybrid color-conversion layers for white emission from fluorescent blue organic light-emitting diodes
Current Applied Physics 17 (2017) 1108e1113
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Hybrid color-conversion layers for white emission from fluorescent blue organic light-emitting diodes Seung-Hwan Lee a, 1, Deuk Su Jo b, 1, Bong Sung Kim c, Dae-Ho Yoon b, Heeyeop Chae a, Ho-Kyoon Chung c, Sung Min Cho a, * a b c
School of Chemical Engineering, Sunkyunkwan University, Suwon 440-746, Republic of Korea School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon 440-746, Republic of Korea SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University, Suwon 440-746, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 December 2016 Received in revised form 17 April 2017 Accepted 8 May 2017 Available online 9 May 2017
Hybrid color-conversion layers (CCLs) were developed to convert a blue emission from fluorescent organic light-emitting diodes (OLEDs) to obtain a white emission with a high color rendering index (CRI). The hybrid CCLs were composed of an inorganic phosphor, organic dye, and silicon dioxide (SiO2) scattering nanoparticles. The inorganic phosphors convert a part of blue emission from OLEDs to a greenyellow emission effectively. A part of the green-yellow emission was consecutively converted to a red emission with the organic dye. Using the hybrid CCLs, we obtained a balanced white emission with the highest CRI of 93 and the color temperature of 3500 K. The high CRI white OLED showed the power efficiency of 11 lm/W which was enhanced by 1.9 times from that of the blue fluorescent OLED. We showed that the utilization of the SiO2 nanoparticles did not only enhance the power efficiency but also significantly reduce the white color variation to the viewing angle. © 2017 Elsevier B.V. All rights reserved.
Keywords: Organic light-emitting diode Color-conversion layer Color rendering index Phosphor
1. Introduction White organic light-emitting diodes (OLEDs) have already been commercialized as a general solid-state lighting. Unlike inorganic light-emitting diodes, OLEDs possess high spectrum-tailoring flexibility because there is a wide variety of organic emitters ranging from red to violet. White OLEDs can be easily achieved by stacking two or more different color emitters in a large area. However, the white OLEDs require complicated device engineering in order to accurately tune their chromaticity. Due to their complicated structures, they also require careful fabrication processes in order to maintain a high production yield. Another way to achieve white OLEDs is to utilize down-conversion of simple blueemitting OLEDs. The down-conversion from blue to white can be accomplished by inorganic phosphors [1,2], organic emitters [3e7], quantum dots [8], or combined emitters [9]. Even though the down-conversion method to achieve white OLEDs are simple in view of device and process, it requires high-efficiency and stable
* Corresponding author. E-mail address: [email protected] (S.M. Cho). 1 Co-first authors with equal contributions. http://dx.doi.org/10.1016/j.cap.2017.05.004 1567-1739/© 2017 Elsevier B.V. All rights reserved.
blue OLEDs with a proper emission spectrum for the color conversion. To date, stable fluorescent blue materials have a limited power efficiency (PE), while highly efficient blue phosphorescent materials have a short lifetime and high longer-wavelength portion in the emission. In this study, we fabricated a fluorescent blue OLED and converted the blue emission to white by utilizing a combined hybrid structure of an inorganic phosphor, organic emitter, and scattering nanoparticle. Due to the low PE of fluorescent blue OLEDs, the down-converted white OLEDs show lower PE than that of the state-of-the-art stacked phosphorescent white OLEDs. However, many research groups are actively developing highly efficient phosphorescent and thermally-activated-delayedfluorescent blue materials. If the efficient blue-emission materials are available, the down-converted white OLEDs could be an importance method especially for large-area OLED applications because the mass production of OLEDs with simpler structures could potentially have a higher production yield. In order to achieve an efficient color conversion, the blueemission spectrum should be well matched with the absorption spectrum of the color converters. At the same time, the amount of inorganic phosphor and organic emitter should be well controlled to convert a proper portion of the blue emission. In this regards, we carefully selected an inorganic phosphor of which the absorption
S.-H. Lee et al. / Current Applied Physics 17 (2017) 1108e1113
spectrum overlapped the blue emission spectrum. To control the amount of the color-conversion materials, we piled up a monolayer of inorganic phosphor particles with an adhesive resin containing an organic emitter. In this study, we aim to develop an optimized structure of color-conversion layer (CCL) and achieve the downconverted white emission with a high color rendering index (CRI) from a blue OLED. We also report the utilization of the scattering silicon dioxide (SiO2) particles to further enhance the white OLED efficiency and reduce the angular dependence of white emission. 2. Experimental Indium-tin-oxide (ITO)-coated glass substrates of 50 mm 50 mm size were utilized as the transparent-electrode substrate for the fabrication of OLEDs. The ITO was pre-patterned to form the emission area of 20 mm 20 mm. The patterned ITO substrates were ultrasonically cleaned in acetone, methanol, and isopropyl alcohol baths for 10 min each. After the substrates were treated in an ultraviolet (UV)-ozone environment for 20 min, they were loaded in a vacuum evaporator for the OLED deposition. We fabricated blue-emitting fluorescent OLEDs, of which the emission spectrum has the peak wavelength at 460 nm. Organic materials and cathode metal were evaporated under 8 107 torr. 1,4,5,8,9,11Hexaazatriphenylene-hexacarbonitrile (HAT-CN) and N,N0 -bis-(1naphyl)-N,N0 -diphenyl-1,10 -biphenyl-4,40 -diamine (NPB) were utilized as the hole injection layer (HIL) and hole transport layer (HTL), respectively. 4,40 ,400 -Tri(N-carbazolyl)triphenylamine (TCTA) was utilized as the electron blocking layer (EBL). For blue fluorescent emission, 2-methyl-9,10-di(2-naphthyl) anthracene (MADN) and 1,6-bis(N-phenyl-p-CN-phenylamino)-pyrenes (Pyrene-CN) were utilized as the host and dopant materials, respectively. To complete blue OLED devices, electron transporting LG201 (LG Chem. Ltd) doped with lithium quinolate (Liq), electron injecting Liq, and aluminum (Al) cathode were successively deposited. The blueemitting OLED structure was HAT-CN (35 nm)/NPB (80 nm)/TCTA (20 nm)/MADN:Pyrene-CN (5%, 20 nm)/LG201:Liq (50%, 40 nm)/Liq (1.5 nm)/Al (100 nm). The fabricated OLEDs were encapsulated with a glass for the device-performance measurement. The OLED performance was measured using a spectro-radiometer (CS-2000, Konika Minolta) and source meter (Keithley 2400). Fluorescent blue OLEDs were first fabricated on ITO/glass substrates. On the face of the glass substrate, a poly-acrylic acid (PAA) layer was formed using a bar coater and the layer was cured at 60 C for 3 min. The PAA layer was utilized as an optically clear adhesive (OCA). A green-yellow Sr2SiO4:Eu2þ phosphor was evenly spread on the adherent PAA surface. Non-bonded phosphor particles to the PAA surface were removed by nitrogen blowing. In this way, a monolayer of the phosphors was formed on the PAA surface. In order to put another phosphor layer, the same processes were repeated to coat another PAA layer and Sr2SiO4:Eu2þ phosphor layer. The SiO2 nanoparticle layer was coated using the same process for the phosphor layer. For the inclusion of an organic dye into our CCL structures, a red organic dye (ATTO 590, Sigma Aldrich) was dispersed with the poly-acrylic acid (PAA) at the concentration of 0.1 wt.%. The hybrid CCL structures were fabricated with the same processes using ATTO 590-dispersed PAA instead of the pure PAA. 3. Results and discussion We prepared 6 different CCL structures using Sr2SiO4:Eu2þ phosphor, ATTO 590 organic dye, and SiO2 scattering nanoparticles as shown in Fig. 1. The CCL 1 and 2 structures have one and two monolayers of the phosphor particles, respectively. The phosphor particles were directly bonded with an OCA as a monolayer. The OCA was a PAA layer. As shown in Fig. 2, the size of the phosphor
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particles ranges from 5 to 10 mm and the OCA thickness is around 20 mm. We controlled the amount of the phosphors in CCLs by increasing the number of monolayer. The CCL 3 structure has an additional monolayer of SiO2 nanoparticles on the CCL 2 surface in order to find out the scattering effect of the SiO2 nanoparticles. The structures CCL 4e6 are the same as CCL 1e3 except the inclusion of ATTO 590 organic dye in each OCA layer. Therefore, we could control the amount of the organic dye by increasing the number of OCA layer. Fig. 2 shows the actual cross-sectional scanning-electron-microscopy (SEM) images of all the CCL structures fabricated in this study. The CCL thickness was as high as 60 mm for the three story CCL 3 and 6 structures. In the insets of CCL 3 and 6 structures, we showed the enlarged SEM images of the SiO2 scattering nanoparticles of which the diameter was 300 nm. Fig. 3 shows the emission or absorption spectra of the blueemission OLED, Sr2SiO4:Eu2þ phosphor, and ATTO 590 organic dye. The blue electroluminescence (EL) spectrum has a peak wavelength at 460 nm wavelength with a small shoulder peak at 480 nm wavelength. The full-width-half-maximum of the spectrum was 60 nm. As shown in the figure, the EL spectrum overlaps well the absorption spectrum of the Sr2SiO4:Eu2þ phosphor in the range from 425 to 530 nm. We expect the efficient absorption of blue emission from the OLED by the Sr2SiO4:Eu2þ phosphors. The green-yellow emission centered at 570 nm from the excited Sr2SiO4:Eu2þ phosphors can be effectively absorbed by the ATTO 590 organic dye in the wavelength region of overlap between the emission and absorption spectra of the Sr2SiO4:Eu2þ phosphor and ATTO 590 dye, respectively. The excited ATTO 590 dye can then generate a red emission centered at 624 nm wavelength. The ATTO 590 belongs to the class of rhodamine dye. It shows strong absorption and high fluorescence at 593 nm and 622 nm wavelengths with a quantum yield of 80%. The organic dye is effective to achieve white emission from blue OLEDs since it does not absorb the light in blue-wavelength region but selectively absorb the light in yelloworange wavelength region unlike common inorganic red phosphors. Because the 6 different CCL structures are attached to the face of the blue OLEDs with a same structure, the current-voltage curves are identical for all the devices as shown in Fig. 4(a). The luminance of the CCL-attached white OLEDs was always higher than that of the bare blue OLED. It is due to the difference in the emission spectra caused by the different color conversion. The highest luminance was obtained with the CCL 2 and 3. In Table 1, we listed the performance of the bare blue OLED and down-converted white OLEDs. Luminous efficacy of radiation (LER) measures the fraction of electromagnetic power which is useful for lighting. It is obtained by dividing the luminous flux by the radiant flux. Light with wavelengths outside the visible spectrum reduces LER because it contributes to the radiant flux while the luminous flux of such light is zero. The response of a typical human eye to light is the strongest at a wavelength of 555 nm and diminishes gradually to the longer and shorter wavelengths. As shown in Fig. 4(c)-(d), and Table 1, the emission spectra obtained with the CCL 2 and 3 resemble the most to the photopic response function among all the measured spectra and thus show the highest LER values. The luminance and power efficiency (PE) of down-converted white OLEDs with CCLs showed the same order as the LER values listed in Table 1. The external quantum efficiency (EQE) shows a little different behavior since it is related with the color conversion efficiency of Sr2SiO4:Eu2þ phosphor and ATTO 590 organic dye. The highest EQE was obtained with the CCL 1 and 4 which were the simplest CCL structure in all the CCLs. More complex CCLs attached to the bare blue OLED result in more loss in EQE. The performance of the bare blue OLED and down-converted white OLEDs was summarized in Table 1. The bare fluorescent blue OLED has the LER of 200 and PE of 5.85 lm/W at
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Fig. 1. Schematics of six different CCL structures fabricated in this study. The OCA and phosphor particle were a poly-acrylic acid and Sr2SiO4:Eu2þ phosphor layer, respectively. The ATTO denotes the ATTO 590 organic dye dispersed in the OCA.
Fig. 2. Cross-sectional SEM images of six different CCL structures. The CCL 1e3 and CCL 4e6 structures are structurally the same except the ATTO 590 dispersed in the OCA layers in CCL 4e6 structures.
1000 cd/m2 luminance. The CCL 2 attachment enhanced the PE to 16.97 lm/W at the luminance. The utilization of SiO2 nanoparticle layer further enhanced the PE to 17.47 lm/W, which was almost
three times higher than that of the bare blue OLED. Without ATTO 590 organic dye, the CRI was always less than 70 since there was no red emission. With two layers of ATTO 590 dye, the CRI increased
S.-H. Lee et al. / Current Applied Physics 17 (2017) 1108e1113
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Table 1 CRI, LER, PE, EQE, and the enhancement ratios of the blue OLED and down-converted white OLEDs in the normal direction at 1000 cd/m2 luminance. Sample
Blue OLED CCL 1 CCL 2 CCL 3 CCL 4 CCL 5 CCL 6
Fig. 3. Normalized EL spectrum of a blue OLED and absorption/photoluminescence spectra of the Sr2SiO4:Eu2þ phosphor and ATTO 590 dye. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
up to 93 even though the PE and PE enhancement ratio dropped to 11.06 lm/W and 1.82 times, respectively. Fig. 4(c) and (d) shows the emission spectra from the bare blue OLED and down-converted white OLEDs, which are measured in
CRI
68 60 61 76 93 93
LER
200 341 399 390 311 298 297
1000 cd/m2
Enhancement ratio
lm/W
EQE
lm/W
EQE
LER
5.85 16.05 16.97 17.47 14.39 10.66 11.06
5.73 8.45 7.79 8.23 8.42 7.29 7.50
1.00 2.74 2.90 2.99 2.46 1.82 1.89
1.00 1.47 1.36 1.44 1.47 1.27 1.31
1.00 1.71 2.00 1.95 1.56 1.49 1.49
the direction normal to the OLED surface and averaged over all viewing directions at the luminance of 1000 cd/m2, respectively. The emission spectrum varies as the viewing angle changes. In Fig. 4(c), the blue emission intensity from the device with CCL 4 was stronger than that with CCL 1, even though the blue emission intensity with CCL 4 should be similar to or lower than that with CCL 1. We think that this experimental inconsistency comes from the random nature of phosphor particles on the CCL surface as can be seen in Fig. 2(a) and (d). The experimental inconsistency seems reduced as the CCL thickness increases. In Fig. 5, we showed the angular dependence of the emission spectra of OLEDs with different CCLs at the luminance of 1000 cd/ m2. The measurement was carried out by changing the viewing angle from 0 to 80 in 10 intervals. As the viewing angle increases,
Fig. 4. (a) Current density-luminance-applied voltage curves; (b) EQE and PE with respect to the luminance for the bare blue OLED and down-converted white OLEDs; (c) EL spectra of the OLEDs measured in the direction normal to the surface; (d) averaged EL spectra in all direction angles at 1000 cd/m2 luminance. (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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Fig. 5. Emission spectra of the down-converted white OLEDs at different viewing angles from 0 to 80 in 10 intervals. Insets show the emission positions in the International Commission on Illumination (CIE) coordinate and the relative positions to the Planckian locus.
the light travels longer path through the CCLs by experiencing stronger scattering with the Sr2SiO4:Eu2þ phosphor particles. As the result, the stronger light conversion occurs at the higher viewing angle. Fig. 5 shows the angularly dependent emission spectra of all the down-converted white OLEDs along with the positions in the color coordinate of the emissions. A single layer of Sr2SiO4:Eu2þ phosphor and ATTO 590 organic dye was not enough to convert blue and green-yellow emission as shown in Fig. 5(a) and (d), respectively. The CCL 2 and CCL 5 showed a balanced color conversion from the blue OLED to Sr2SiO4:Eu2þ phosphor or from blue OLED to ATTO 590 organic dye via Sr2SiO4:Eu2þ phosphor. The CCL 3 and 6 showed similar emission spectra to the CCL 2 and 5 but reduced variation of the spectra with the viewing angle. The positions in the color coordinates of the emissions look more stationary than those of other emissions. It seems to be due to the effective scattering of light by the SiO2 nanoparticles. Therefore, we can conclude that the inclusion of the SiO2 nanoparticle layer did not only increase PE but also reduce the emission spectrum variation to the viewing angle. The solid curves in the inset figures are the Planckian locus representing the color temperatures of a black body. The inset figure of Fig. 5(f) shows that the OLED with CCL 6 emits white emission with
an approximate color temperature of 3500 K at the color coordinate of (0.4, 0.4). In order to see the effect of SiO2 nanoparticle layer on the angular dependence more quantitatively, the changes in x and y components of CIE coordinate are shown in Fig. 6 when the viewing angle varies from 0 to 80 . The angular dependence of emission from device with CCL 2 and CCL 5 is similar to or a little weaker than that with CCL 1 and CCL 4. However, the changes in CIE coordinate of emission from device with CCL 3 and CCL 6 have been significantly lowered as shown in the figure. This observation clearly shows that the inclusion of SiO2 scattering nanoparticles reduces the emission spectrum variation to the viewing angle. In summary, we developed hybrid color-conversion layers to convert a blue OLED emission to a high CRI white emission. With a fluorescent blue emission centered at 460 nm wavelength, a white emission with CRI of 93 CRI and color temperature of 3500 K was obtained. The PE of the down-converted white OLED was 11 lm/W, which was enhanced by 1.9 times from that of the bare blue OLED. The relatively low PE is because we utilized a fluorescent blue OLED. We expect that our optimized CCL can produce a high PE and good quality white emission if highly efficient phosphorescent or thermallyactivated-delayed-fluorescent blue materials are available.
Fig. 6. Changes in the CIE coordinate when the viewing angle varies from 0 to 80 .
S.-H. Lee et al. / Current Applied Physics 17 (2017) 1108e1113
4. Conclusions In this study, we demonstrated a white OLED by the down conversion of a fluorescent blue emission. The down conversion was achieved by the Sr2SiO4:Eu2þ yellow-green phosphor and ATTO 590 red organic dye. In order to obtain a balanced white emission with a high CRI, the amount of the color converters was controlled by increasing the number of the phosphor monolayer using an adhesive layer containing the organic dye. A high CRI of 93 was obtained with the CCL 5 and 6 structures which had two layers of the organic dye. We showed that the SiO2 scattering nanoparticle was effective not only to enhance the PE of the white OLEDs but also to reduce the angular dependence of the emission. Acknowledgements This work was supported by the MSIP (Ministry of Science, ICT and Future Planning)under the ITRC (Information Technology Research Center)support program (IITP-2016-H8501-16-1009)
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supervised by the IITP (Institute for Information & communications Technology Promotion), Korea.
References [1] J. Lee, N. Chopra, D. Bera, S. Maslow, S.-H. Eom, Y. Zheng, P. Holloway, J. Xue, F. So, Adv. Energy Mater. 1 (2011) 174e178. [2] S.-H. Cho, J.R. Oh, H.K. Park, H.K. Kim, Y.-H. Lee, J.-G. Lee, Y.R. Do, Opt. Express 18 (2) (2010) 1099e1104. [3] N.J. Findlay, J. Bruckbauer, A.R. Inigo, B. Breig, S. Arumugam, D.J. Wallis, R.W. Martin, P.J. Skabara, Adv. Mater. 26 (2014) 7290e7294. [4] Y.-H. Ho, D.-W. Huang, Y.-T. Chang, Y.-H. Ye, C.-W. Chu, W.-C. Tian, C.-T. Chen, P.K. Wei, Opt. Express 20 (3) (2012) 3005e3014. [5] Y.B. Yuan, S. Li, Z. Wang, H.T. Xu, X. Zhou, Opt. Express 17 (3) (2009) 1577e1582. [6] S. Chen, H.-S. Kwok, Org. Electron 12 (2011) 677e681. [7] S. Liu, X. Wen, W. Liu, W. Zhang, J. Yu, W. Xie, L. Zhang, Curr. Appl. Phys. 14 (2014) 1451e1454. [8] H.W. Lee, K.-H. Lee, J.W. Lee, J.-H. Kim, H. Yang, Y.K. Kim, Jpn. J. Appl. Phys. 54 (2015) 02BC02. [9] A.R. Duggal, J.J. Shiang, C.M. Heller, D.F. Foust, Appl. Phys. Lett. 80 (19) (2002) 3470e3472.
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Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Hybrid color-conversion layers for white emission from fluorescent blue organic light-emitting diodes Seung-Hwan Lee a, 1, Deuk Su Jo b, 1, Bong Sung Kim c, Dae-Ho Yoon b, Heeyeop Chae a, Ho-Kyoon Chung c, Sung Min Cho a, * a b c
School of Chemical Engineering, Sunkyunkwan University, Suwon 440-746, Republic of Korea School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon 440-746, Republic of Korea SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University, Suwon 440-746, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 December 2016 Received in revised form 17 April 2017 Accepted 8 May 2017 Available online 9 May 2017
Hybrid color-conversion layers (CCLs) were developed to convert a blue emission from fluorescent organic light-emitting diodes (OLEDs) to obtain a white emission with a high color rendering index (CRI). The hybrid CCLs were composed of an inorganic phosphor, organic dye, and silicon dioxide (SiO2) scattering nanoparticles. The inorganic phosphors convert a part of blue emission from OLEDs to a greenyellow emission effectively. A part of the green-yellow emission was consecutively converted to a red emission with the organic dye. Using the hybrid CCLs, we obtained a balanced white emission with the highest CRI of 93 and the color temperature of 3500 K. The high CRI white OLED showed the power efficiency of 11 lm/W which was enhanced by 1.9 times from that of the blue fluorescent OLED. We showed that the utilization of the SiO2 nanoparticles did not only enhance the power efficiency but also significantly reduce the white color variation to the viewing angle. © 2017 Elsevier B.V. All rights reserved.
Keywords: Organic light-emitting diode Color-conversion layer Color rendering index Phosphor
1. Introduction White organic light-emitting diodes (OLEDs) have already been commercialized as a general solid-state lighting. Unlike inorganic light-emitting diodes, OLEDs possess high spectrum-tailoring flexibility because there is a wide variety of organic emitters ranging from red to violet. White OLEDs can be easily achieved by stacking two or more different color emitters in a large area. However, the white OLEDs require complicated device engineering in order to accurately tune their chromaticity. Due to their complicated structures, they also require careful fabrication processes in order to maintain a high production yield. Another way to achieve white OLEDs is to utilize down-conversion of simple blueemitting OLEDs. The down-conversion from blue to white can be accomplished by inorganic phosphors [1,2], organic emitters [3e7], quantum dots [8], or combined emitters [9]. Even though the down-conversion method to achieve white OLEDs are simple in view of device and process, it requires high-efficiency and stable
* Corresponding author. E-mail address: [email protected] (S.M. Cho). 1 Co-first authors with equal contributions. http://dx.doi.org/10.1016/j.cap.2017.05.004 1567-1739/© 2017 Elsevier B.V. All rights reserved.
blue OLEDs with a proper emission spectrum for the color conversion. To date, stable fluorescent blue materials have a limited power efficiency (PE), while highly efficient blue phosphorescent materials have a short lifetime and high longer-wavelength portion in the emission. In this study, we fabricated a fluorescent blue OLED and converted the blue emission to white by utilizing a combined hybrid structure of an inorganic phosphor, organic emitter, and scattering nanoparticle. Due to the low PE of fluorescent blue OLEDs, the down-converted white OLEDs show lower PE than that of the state-of-the-art stacked phosphorescent white OLEDs. However, many research groups are actively developing highly efficient phosphorescent and thermally-activated-delayedfluorescent blue materials. If the efficient blue-emission materials are available, the down-converted white OLEDs could be an importance method especially for large-area OLED applications because the mass production of OLEDs with simpler structures could potentially have a higher production yield. In order to achieve an efficient color conversion, the blueemission spectrum should be well matched with the absorption spectrum of the color converters. At the same time, the amount of inorganic phosphor and organic emitter should be well controlled to convert a proper portion of the blue emission. In this regards, we carefully selected an inorganic phosphor of which the absorption
S.-H. Lee et al. / Current Applied Physics 17 (2017) 1108e1113
spectrum overlapped the blue emission spectrum. To control the amount of the color-conversion materials, we piled up a monolayer of inorganic phosphor particles with an adhesive resin containing an organic emitter. In this study, we aim to develop an optimized structure of color-conversion layer (CCL) and achieve the downconverted white emission with a high color rendering index (CRI) from a blue OLED. We also report the utilization of the scattering silicon dioxide (SiO2) particles to further enhance the white OLED efficiency and reduce the angular dependence of white emission. 2. Experimental Indium-tin-oxide (ITO)-coated glass substrates of 50 mm 50 mm size were utilized as the transparent-electrode substrate for the fabrication of OLEDs. The ITO was pre-patterned to form the emission area of 20 mm 20 mm. The patterned ITO substrates were ultrasonically cleaned in acetone, methanol, and isopropyl alcohol baths for 10 min each. After the substrates were treated in an ultraviolet (UV)-ozone environment for 20 min, they were loaded in a vacuum evaporator for the OLED deposition. We fabricated blue-emitting fluorescent OLEDs, of which the emission spectrum has the peak wavelength at 460 nm. Organic materials and cathode metal were evaporated under 8 107 torr. 1,4,5,8,9,11Hexaazatriphenylene-hexacarbonitrile (HAT-CN) and N,N0 -bis-(1naphyl)-N,N0 -diphenyl-1,10 -biphenyl-4,40 -diamine (NPB) were utilized as the hole injection layer (HIL) and hole transport layer (HTL), respectively. 4,40 ,400 -Tri(N-carbazolyl)triphenylamine (TCTA) was utilized as the electron blocking layer (EBL). For blue fluorescent emission, 2-methyl-9,10-di(2-naphthyl) anthracene (MADN) and 1,6-bis(N-phenyl-p-CN-phenylamino)-pyrenes (Pyrene-CN) were utilized as the host and dopant materials, respectively. To complete blue OLED devices, electron transporting LG201 (LG Chem. Ltd) doped with lithium quinolate (Liq), electron injecting Liq, and aluminum (Al) cathode were successively deposited. The blueemitting OLED structure was HAT-CN (35 nm)/NPB (80 nm)/TCTA (20 nm)/MADN:Pyrene-CN (5%, 20 nm)/LG201:Liq (50%, 40 nm)/Liq (1.5 nm)/Al (100 nm). The fabricated OLEDs were encapsulated with a glass for the device-performance measurement. The OLED performance was measured using a spectro-radiometer (CS-2000, Konika Minolta) and source meter (Keithley 2400). Fluorescent blue OLEDs were first fabricated on ITO/glass substrates. On the face of the glass substrate, a poly-acrylic acid (PAA) layer was formed using a bar coater and the layer was cured at 60 C for 3 min. The PAA layer was utilized as an optically clear adhesive (OCA). A green-yellow Sr2SiO4:Eu2þ phosphor was evenly spread on the adherent PAA surface. Non-bonded phosphor particles to the PAA surface were removed by nitrogen blowing. In this way, a monolayer of the phosphors was formed on the PAA surface. In order to put another phosphor layer, the same processes were repeated to coat another PAA layer and Sr2SiO4:Eu2þ phosphor layer. The SiO2 nanoparticle layer was coated using the same process for the phosphor layer. For the inclusion of an organic dye into our CCL structures, a red organic dye (ATTO 590, Sigma Aldrich) was dispersed with the poly-acrylic acid (PAA) at the concentration of 0.1 wt.%. The hybrid CCL structures were fabricated with the same processes using ATTO 590-dispersed PAA instead of the pure PAA. 3. Results and discussion We prepared 6 different CCL structures using Sr2SiO4:Eu2þ phosphor, ATTO 590 organic dye, and SiO2 scattering nanoparticles as shown in Fig. 1. The CCL 1 and 2 structures have one and two monolayers of the phosphor particles, respectively. The phosphor particles were directly bonded with an OCA as a monolayer. The OCA was a PAA layer. As shown in Fig. 2, the size of the phosphor
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particles ranges from 5 to 10 mm and the OCA thickness is around 20 mm. We controlled the amount of the phosphors in CCLs by increasing the number of monolayer. The CCL 3 structure has an additional monolayer of SiO2 nanoparticles on the CCL 2 surface in order to find out the scattering effect of the SiO2 nanoparticles. The structures CCL 4e6 are the same as CCL 1e3 except the inclusion of ATTO 590 organic dye in each OCA layer. Therefore, we could control the amount of the organic dye by increasing the number of OCA layer. Fig. 2 shows the actual cross-sectional scanning-electron-microscopy (SEM) images of all the CCL structures fabricated in this study. The CCL thickness was as high as 60 mm for the three story CCL 3 and 6 structures. In the insets of CCL 3 and 6 structures, we showed the enlarged SEM images of the SiO2 scattering nanoparticles of which the diameter was 300 nm. Fig. 3 shows the emission or absorption spectra of the blueemission OLED, Sr2SiO4:Eu2þ phosphor, and ATTO 590 organic dye. The blue electroluminescence (EL) spectrum has a peak wavelength at 460 nm wavelength with a small shoulder peak at 480 nm wavelength. The full-width-half-maximum of the spectrum was 60 nm. As shown in the figure, the EL spectrum overlaps well the absorption spectrum of the Sr2SiO4:Eu2þ phosphor in the range from 425 to 530 nm. We expect the efficient absorption of blue emission from the OLED by the Sr2SiO4:Eu2þ phosphors. The green-yellow emission centered at 570 nm from the excited Sr2SiO4:Eu2þ phosphors can be effectively absorbed by the ATTO 590 organic dye in the wavelength region of overlap between the emission and absorption spectra of the Sr2SiO4:Eu2þ phosphor and ATTO 590 dye, respectively. The excited ATTO 590 dye can then generate a red emission centered at 624 nm wavelength. The ATTO 590 belongs to the class of rhodamine dye. It shows strong absorption and high fluorescence at 593 nm and 622 nm wavelengths with a quantum yield of 80%. The organic dye is effective to achieve white emission from blue OLEDs since it does not absorb the light in blue-wavelength region but selectively absorb the light in yelloworange wavelength region unlike common inorganic red phosphors. Because the 6 different CCL structures are attached to the face of the blue OLEDs with a same structure, the current-voltage curves are identical for all the devices as shown in Fig. 4(a). The luminance of the CCL-attached white OLEDs was always higher than that of the bare blue OLED. It is due to the difference in the emission spectra caused by the different color conversion. The highest luminance was obtained with the CCL 2 and 3. In Table 1, we listed the performance of the bare blue OLED and down-converted white OLEDs. Luminous efficacy of radiation (LER) measures the fraction of electromagnetic power which is useful for lighting. It is obtained by dividing the luminous flux by the radiant flux. Light with wavelengths outside the visible spectrum reduces LER because it contributes to the radiant flux while the luminous flux of such light is zero. The response of a typical human eye to light is the strongest at a wavelength of 555 nm and diminishes gradually to the longer and shorter wavelengths. As shown in Fig. 4(c)-(d), and Table 1, the emission spectra obtained with the CCL 2 and 3 resemble the most to the photopic response function among all the measured spectra and thus show the highest LER values. The luminance and power efficiency (PE) of down-converted white OLEDs with CCLs showed the same order as the LER values listed in Table 1. The external quantum efficiency (EQE) shows a little different behavior since it is related with the color conversion efficiency of Sr2SiO4:Eu2þ phosphor and ATTO 590 organic dye. The highest EQE was obtained with the CCL 1 and 4 which were the simplest CCL structure in all the CCLs. More complex CCLs attached to the bare blue OLED result in more loss in EQE. The performance of the bare blue OLED and down-converted white OLEDs was summarized in Table 1. The bare fluorescent blue OLED has the LER of 200 and PE of 5.85 lm/W at
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Fig. 1. Schematics of six different CCL structures fabricated in this study. The OCA and phosphor particle were a poly-acrylic acid and Sr2SiO4:Eu2þ phosphor layer, respectively. The ATTO denotes the ATTO 590 organic dye dispersed in the OCA.
Fig. 2. Cross-sectional SEM images of six different CCL structures. The CCL 1e3 and CCL 4e6 structures are structurally the same except the ATTO 590 dispersed in the OCA layers in CCL 4e6 structures.
1000 cd/m2 luminance. The CCL 2 attachment enhanced the PE to 16.97 lm/W at the luminance. The utilization of SiO2 nanoparticle layer further enhanced the PE to 17.47 lm/W, which was almost
three times higher than that of the bare blue OLED. Without ATTO 590 organic dye, the CRI was always less than 70 since there was no red emission. With two layers of ATTO 590 dye, the CRI increased
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Table 1 CRI, LER, PE, EQE, and the enhancement ratios of the blue OLED and down-converted white OLEDs in the normal direction at 1000 cd/m2 luminance. Sample
Blue OLED CCL 1 CCL 2 CCL 3 CCL 4 CCL 5 CCL 6
Fig. 3. Normalized EL spectrum of a blue OLED and absorption/photoluminescence spectra of the Sr2SiO4:Eu2þ phosphor and ATTO 590 dye. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
up to 93 even though the PE and PE enhancement ratio dropped to 11.06 lm/W and 1.82 times, respectively. Fig. 4(c) and (d) shows the emission spectra from the bare blue OLED and down-converted white OLEDs, which are measured in
CRI
68 60 61 76 93 93
LER
200 341 399 390 311 298 297
1000 cd/m2
Enhancement ratio
lm/W
EQE
lm/W
EQE
LER
5.85 16.05 16.97 17.47 14.39 10.66 11.06
5.73 8.45 7.79 8.23 8.42 7.29 7.50
1.00 2.74 2.90 2.99 2.46 1.82 1.89
1.00 1.47 1.36 1.44 1.47 1.27 1.31
1.00 1.71 2.00 1.95 1.56 1.49 1.49
the direction normal to the OLED surface and averaged over all viewing directions at the luminance of 1000 cd/m2, respectively. The emission spectrum varies as the viewing angle changes. In Fig. 4(c), the blue emission intensity from the device with CCL 4 was stronger than that with CCL 1, even though the blue emission intensity with CCL 4 should be similar to or lower than that with CCL 1. We think that this experimental inconsistency comes from the random nature of phosphor particles on the CCL surface as can be seen in Fig. 2(a) and (d). The experimental inconsistency seems reduced as the CCL thickness increases. In Fig. 5, we showed the angular dependence of the emission spectra of OLEDs with different CCLs at the luminance of 1000 cd/ m2. The measurement was carried out by changing the viewing angle from 0 to 80 in 10 intervals. As the viewing angle increases,
Fig. 4. (a) Current density-luminance-applied voltage curves; (b) EQE and PE with respect to the luminance for the bare blue OLED and down-converted white OLEDs; (c) EL spectra of the OLEDs measured in the direction normal to the surface; (d) averaged EL spectra in all direction angles at 1000 cd/m2 luminance. (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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Fig. 5. Emission spectra of the down-converted white OLEDs at different viewing angles from 0 to 80 in 10 intervals. Insets show the emission positions in the International Commission on Illumination (CIE) coordinate and the relative positions to the Planckian locus.
the light travels longer path through the CCLs by experiencing stronger scattering with the Sr2SiO4:Eu2þ phosphor particles. As the result, the stronger light conversion occurs at the higher viewing angle. Fig. 5 shows the angularly dependent emission spectra of all the down-converted white OLEDs along with the positions in the color coordinate of the emissions. A single layer of Sr2SiO4:Eu2þ phosphor and ATTO 590 organic dye was not enough to convert blue and green-yellow emission as shown in Fig. 5(a) and (d), respectively. The CCL 2 and CCL 5 showed a balanced color conversion from the blue OLED to Sr2SiO4:Eu2þ phosphor or from blue OLED to ATTO 590 organic dye via Sr2SiO4:Eu2þ phosphor. The CCL 3 and 6 showed similar emission spectra to the CCL 2 and 5 but reduced variation of the spectra with the viewing angle. The positions in the color coordinates of the emissions look more stationary than those of other emissions. It seems to be due to the effective scattering of light by the SiO2 nanoparticles. Therefore, we can conclude that the inclusion of the SiO2 nanoparticle layer did not only increase PE but also reduce the emission spectrum variation to the viewing angle. The solid curves in the inset figures are the Planckian locus representing the color temperatures of a black body. The inset figure of Fig. 5(f) shows that the OLED with CCL 6 emits white emission with
an approximate color temperature of 3500 K at the color coordinate of (0.4, 0.4). In order to see the effect of SiO2 nanoparticle layer on the angular dependence more quantitatively, the changes in x and y components of CIE coordinate are shown in Fig. 6 when the viewing angle varies from 0 to 80 . The angular dependence of emission from device with CCL 2 and CCL 5 is similar to or a little weaker than that with CCL 1 and CCL 4. However, the changes in CIE coordinate of emission from device with CCL 3 and CCL 6 have been significantly lowered as shown in the figure. This observation clearly shows that the inclusion of SiO2 scattering nanoparticles reduces the emission spectrum variation to the viewing angle. In summary, we developed hybrid color-conversion layers to convert a blue OLED emission to a high CRI white emission. With a fluorescent blue emission centered at 460 nm wavelength, a white emission with CRI of 93 CRI and color temperature of 3500 K was obtained. The PE of the down-converted white OLED was 11 lm/W, which was enhanced by 1.9 times from that of the bare blue OLED. The relatively low PE is because we utilized a fluorescent blue OLED. We expect that our optimized CCL can produce a high PE and good quality white emission if highly efficient phosphorescent or thermallyactivated-delayed-fluorescent blue materials are available.
Fig. 6. Changes in the CIE coordinate when the viewing angle varies from 0 to 80 .
S.-H. Lee et al. / Current Applied Physics 17 (2017) 1108e1113
4. Conclusions In this study, we demonstrated a white OLED by the down conversion of a fluorescent blue emission. The down conversion was achieved by the Sr2SiO4:Eu2þ yellow-green phosphor and ATTO 590 red organic dye. In order to obtain a balanced white emission with a high CRI, the amount of the color converters was controlled by increasing the number of the phosphor monolayer using an adhesive layer containing the organic dye. A high CRI of 93 was obtained with the CCL 5 and 6 structures which had two layers of the organic dye. We showed that the SiO2 scattering nanoparticle was effective not only to enhance the PE of the white OLEDs but also to reduce the angular dependence of the emission. Acknowledgements This work was supported by the MSIP (Ministry of Science, ICT and Future Planning)under the ITRC (Information Technology Research Center)support program (IITP-2016-H8501-16-1009)
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supervised by the IITP (Institute for Information & communications Technology Promotion), Korea.
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