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White light emitting diodes using polymer blends
Optical Materials 21 (2002) 205–210 www.elsevier.com/locate/optmat
White light emitting diodes using polymer blends Jeong-Ik Lee b
a,*
, Hye Yong Chu a, Seong Hyun Kim a, Lee-Mi Do a, Taehyoung Zyung a, Do-Hoon Hwang b
a Basic Research Laboratory, ETRI, Taejon 305-350, South Korea Department of Applied Chemistry, Kumoh National University of Technology, Kumi 730-701, South Korea
Abstract White light emitting devices were fabricated using the blends of poly(fluorine) (PF) and poly(1,4-phenylenevinylene) (PPV) derivatives. Thin films were obtained by spin-coating on fused quartz wafers to investigate UV–vis absorption and fluorescence properties. An inefficient energy transfer between PF and PPV derivatives was observed which was a good advantage to obtain white light emitting device. Light emitting device was fabricated using the polymer blends and their characteristics were investigated. The emissions of each polymer were observed in electroluminescence spectra and white light emission was achieved. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 71.20.Rv; 72.80.Le; 73.61.Ph; 78.60.Fi; 78.40.Me Keywords: Electroluminescence; Polymers; Blends; Energy transfer
1. Introduction Conjugated polymers have attracted much research interest in science and technology in the past few decades as electroactive materials for diverse applications such as batteries, molecular electronic devices, and light emitting diodes (LEDs), etc. Especially, electroluminescence (EL) has proven to be a promising application of semiconducting polymers. Poly(fluorene)s (PFs) and poly(1,4-phenylenevinylene)s (PPVs) are promising candidates for polymer light emitting diodes (PLED) [1–3]. PPV derivatives are normally green and red emitting materials and introduction of electron donating or *
Corresponding author. Tel.: +82-42-860-1166; fax: +82-42860-5202. E-mail address: [email protected] (J.-I. Lee).
electron withdrawing group can change emitting color. Although PFs are blue emitting materials, color tuning is easily obtained by copolymerization with low band gap comonomer [4,5] or blend with dyes [6]. White LEDs have been studied in various groups as back light of LCD, lighting devices and full color LED application using color filters. The doping method has been widely used to obtain white light. For small molecule device, red emitting material is co-deposited with blue and/or green emitting materials [7]. In solution processed polymer devices, Kido et al. reported composites of blue (B), green (G), and red (R) emitting dyes and poly(vinylcarbazole) emitted white light [8]. In both vacuum deposited small molecule devices and solution processed polymer devices, control of energy transfer between R, G, B dyes is essential
0925-3467/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 3 4 6 7 ( 0 2 ) 0 0 1 3 7 - 4
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Fig. 1. Structures and HOMO and LUMO levels of the polymers used in this work (R ¼ 2-ethylhexyl, R0 ¼ 3,7-dimethyloctyl).
and it usually needs very low level (<103 %) doping controls. Recently, we have studied color tuning of PLED based on PFs. Composites or blends of PF and green and/or red emitting dyes exhibited green or red emission from blue emitting PF [6]. From this study, we found that there is an inefficient energy transfer between poly(9,9-bis(2-ethylhexyl)fluorine-2,7-diyl) (BEHF) and poly(2,5-bis(dimethyloctylsilyl)-1,4-phenylenevinylene) (BDMOS-PPV). Therefore, there is an opportunity to obtain white light emission from the blend of PF and PPV derivatives in controllable blend ratio. In this paper, energy transfer from PF to BDMOS-PPV and fabrication of white light emitting devices will be described.
2. Experimental BEHF, BDMOS-PPV, poly(2-(20 -ethylhexyloxy)-5-methoxy-1,4-phenylene vinylene) (MEHPPV) and poly(2-(30 ,70 -dimethyloctyloxy)-5-methoxy-1,4-phenylene-1-cyanovinylene) (CN-PPV)
were prepared according to the previous papers [1,9]. The structures of the polymers are shown in Fig. 1. Thin films of the polymers were obtained by spin-coating from polymer solutions in p-xylene on fused quartz plates. All the photophysical measurements were performed at room temperature. The absorption spectra were measured by a Hitachi spectrophotometer model U-3501 and steady-state photoluminescence spectra were recorded on a Spex FL3-11. Single-layered LEDs utilized the polymer blend of BEHF, BDMOS-PPV and MEH-PPV or CNPPV as an emitting layer with ITO anodes and aluminum (Al) cathodes. PEDT:PSS purchased from Bayer (Baytron P 4083) was used as a buffer layer on ITO. Blend ratio of BEHF, BDMOS-PPV and MEH-PPV or CN-PPV was 95, 4, 1 based on weight and blending was performed by dissolving three polymers in p-xylene. The polymer layer was spin-coated to a typical thickness of 100 nm. A patterned Al cathode (100 nm) was then deposited by thermal evaporation. EL spectra were recorded using Minolta CS-1000 and current–voltage (I–V ) and luminance–voltage characteristics of the de-
J.-I. Lee et al. / Optical Materials 21 (2002) 205–210
vices were simultaneously measured with a Keithley 2400 Source Measure Unit and a Minolta LS-100.
3. Results and discussion The polymer blends were obtained by dissolving BEHF and BDMOS-PPV in p-xylene and thin films were made by spin-coating on fused quartz for spectroscopic measurements. Fig. 2 shows the absorption spectra of the thin films. There are two absorptions from BEHF and green emitting material, BDMOS-PPV. The bigger absorption centered at about 380 nm is originated from BEHF and the shoulder like absorption at 450 nm is from BDMOS-PPV. With increase of BDMOS-PPV contents in the polymer blends, the shoulder like absorption increases which also leads to the redshift of BEHF absorption maximum. As for 50/50 blend of BEHF and BDMOS-PPV, the absorption intensity of BEHF is about two times bigger than that of BDMOS-PPV, which indicates that the absorption coefficient of BEHF based on weight is higher than that of BDMOS-PPV. As shown in Fig. 3, two emissions from BEHF and BDMOS-PPV were observed regardless of
207
blend ratios. The emission of BDMOS-PPV increases with increase of BDMOS-PPV blend ratio. The emission of BEHF did not disappeared even in 50/50 blend, which might indicate that there was an inefficient energy transfer between BEHF and BDMOS-PPV. As the spectral overlap of the emission of BEHF and the absorption of BDMOSPPV is large enough to make energy transfer, one possible explanation is that there might be inhomogeneous mixing in the polymer blends and the domain sizes of BDMOS-PPV are bigger than the energy transfer radius. Therefore, we are now under investigation on the above possibility. Anyway, we can take advantage of such an inefficient energy transfer to obtain white light emission in PLED. To enhance red emission, orange or red light emitting material (MEH-PPV or CN-PPV) was added to the above BEHF and BDMOS-PPV blends. The blending ratio of BEHF, BDMOSPPV and MEH-PPV or CN-PPV is 95, 4, and 1. Fig. 4 displays the absorption and the emission spectra of the BEHF/BDMOS-PPV/MEH-PPV and BEHF/BDMOS-PPV/CN-PPV. The absorption and the emission spectra of BEHF were also shown for comparison. Little difference between BEHF and the blended polymers was observed except slight increase of absorption at the
Fig. 2. Absorption spectra of BEHF and BDMOS-PPV blends.
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Fig. 3. Emission spectra of BEHF and BDMOS-PPV blends. Excitation wavelength was 380 nm.
Fig. 4. Absorption and emission spectra of BEHF/BDMOS-PPV/MEH-PPV and BEHF/BDMOS-PPV/CN-PPV blends.
absorption edges. As for emissions excited by 380 nm light, the blue emission of BEHF and the emissions of PPVs were present simultaneously.
Fig. 5 shows the EL spectra of BEHF/BDMOSPPV/MEH-PPV and BEHF/BDMOS-PPV/CNPPV. As similar to fluorescence spectra, three
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209
Fig. 5. EL spectra of BEHF/BDMOS-PPV/MEH-PPV (solid line) and BEHF/BDMOS-PPV/CN-PPV (dotted line) blends. The device configuration was ITO/PEDT:PSS/Polymer/LiF/Al.
emission components from the polymers were observed. The emissions of BEHF and BDMOSPPV were strong while the emission of MEH-PPV was very weak. The inefficient energy transfer from BEHF to BDMOS-PPV accounts for the presence of both emissions and mismatches of HOMO and LUMO positions for MEH-PPV and BEHF or BDMOS-PPV explains the weak emission of MEH-PPV in EL device as shown in Fig. 1. The LUMO of MEH-PPV is lower than that of BEHF and the electron hardly moves to the LUMO of MEH-PPV. As for BEHF/BDMOS-PPV/CNPPV, red emission was greatly enhanced compared with BEHF/BDMOS-PPV/MEH-PPV. The fact that the HOMO and LUMO of CN-PPV are located between those of BEHF and BDMOS-PPV, explains the lager red emission of BEHF/BDMOSPPV/MEH-PPV than that of BEHF/BDMOSPPV/CN-PPV in EL [10]. The CIE coordinates of the ELs were 0.32, 0.28 and 0.40, 0.41 for BEHF/ BDMOS-PPV/MEH-PPV and BEHF/BDMOSPPV/CN-PPV, respectively. The CIE coordinate of EL from polymer blend can be further tuned by changing blend ratio. The results of the EL devices from the above polymer blends were preliminary and the optimizations of device and blend ratio is needed to obtain white light emission with a good efficiency, which are under investigation.
4. Conclusion The inefficient energy transfer from BEHF to BDMOS-PPV was observed, which resulted in the presence of both emissions in PL and EL. As the spectral overlap of the emission of BEHF and the absorption of BDMOS-PPV is large enough to make energy transfer, possible explanation should be inhomogeneous mixing of two polymers. Using the inefficient energy transfer, white light emitting polymer devices were fabricated. To enhance red emission, red emitting MEH-PPV or CN-PPV was added to BEHF and BDMOS-PPV blend and white light was obtained. Therefore, using the polymer blends and the inefficient energy transfer, one possible method to achieve white light emitting polymer device was suggested. Acknowledgements Funding from Ministry of Information and Communication (MIC) is acknowledged. References [1] A. Kraft, A.C. Grimsdale, A.B. Holmes, Angew. Chem. Int. Ed. 37 (1998) 403.
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J.-I. Lee et al. / Optical Materials 21 (2002) 205–210
[2] R.H. Friend, R.W. Gymer, A.B. Holmes, J.H. Burroughes, R.N. Marks, C. Taliani, D.D.C. Bradley, D.A. Dos Santos, J.L. Bredas, M. L€ ogdlund, W.R. Salaneck, Nature 397 (1999) 121. [3] G. Klaerner, M.H. Davey, W.D. Chen, J.C. Scott, R.D. Miller, Adv. Mater. 10 (1998) 993. [4] G. Klaerner, J.-I. Lee, M.H. Davey, R.D. Miller, Adv. Mater. 11 (1999) 115. [5] J.-I. Lee, G. Klaerner, M.H. Davey, R.D. Miller, Syn. Met. 103 (1999) 1087.
[6] J.-I. Lee, L.-M. Do, H.Y. Chu, S.H. Kim, T. Zyung, J. Inform. Display, IDMC 2000 Digest (2000) 251. [7] J. Shi, C.W. Tang, US patent 5683823, 1997. [8] J. Kido, H. Shionya, K. Nagai, Appl. Phys. Lett. 67 (1995) 2281. [9] M. Kreyenschmidt, G. Klaerner, T. Fuhrer, J. Ashenhurst, S. Karg, W.D. Chen, V.Y. Lee, J.C. Scott, R.D. Miller, Macromolecules 31 (1998) 1099. [10] V. Cleave, G. Yahioglu, P.L. Barny, D.-H. Hwang, A.B. Holmes, R.H. Friend, N. Tessler, Adv. Mater. 13 (2001) 44.
White light emitting diodes using polymer blends Jeong-Ik Lee b
a,*
, Hye Yong Chu a, Seong Hyun Kim a, Lee-Mi Do a, Taehyoung Zyung a, Do-Hoon Hwang b
a Basic Research Laboratory, ETRI, Taejon 305-350, South Korea Department of Applied Chemistry, Kumoh National University of Technology, Kumi 730-701, South Korea
Abstract White light emitting devices were fabricated using the blends of poly(fluorine) (PF) and poly(1,4-phenylenevinylene) (PPV) derivatives. Thin films were obtained by spin-coating on fused quartz wafers to investigate UV–vis absorption and fluorescence properties. An inefficient energy transfer between PF and PPV derivatives was observed which was a good advantage to obtain white light emitting device. Light emitting device was fabricated using the polymer blends and their characteristics were investigated. The emissions of each polymer were observed in electroluminescence spectra and white light emission was achieved. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 71.20.Rv; 72.80.Le; 73.61.Ph; 78.60.Fi; 78.40.Me Keywords: Electroluminescence; Polymers; Blends; Energy transfer
1. Introduction Conjugated polymers have attracted much research interest in science and technology in the past few decades as electroactive materials for diverse applications such as batteries, molecular electronic devices, and light emitting diodes (LEDs), etc. Especially, electroluminescence (EL) has proven to be a promising application of semiconducting polymers. Poly(fluorene)s (PFs) and poly(1,4-phenylenevinylene)s (PPVs) are promising candidates for polymer light emitting diodes (PLED) [1–3]. PPV derivatives are normally green and red emitting materials and introduction of electron donating or *
Corresponding author. Tel.: +82-42-860-1166; fax: +82-42860-5202. E-mail address: [email protected] (J.-I. Lee).
electron withdrawing group can change emitting color. Although PFs are blue emitting materials, color tuning is easily obtained by copolymerization with low band gap comonomer [4,5] or blend with dyes [6]. White LEDs have been studied in various groups as back light of LCD, lighting devices and full color LED application using color filters. The doping method has been widely used to obtain white light. For small molecule device, red emitting material is co-deposited with blue and/or green emitting materials [7]. In solution processed polymer devices, Kido et al. reported composites of blue (B), green (G), and red (R) emitting dyes and poly(vinylcarbazole) emitted white light [8]. In both vacuum deposited small molecule devices and solution processed polymer devices, control of energy transfer between R, G, B dyes is essential
0925-3467/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 3 4 6 7 ( 0 2 ) 0 0 1 3 7 - 4
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J.-I. Lee et al. / Optical Materials 21 (2002) 205–210
Fig. 1. Structures and HOMO and LUMO levels of the polymers used in this work (R ¼ 2-ethylhexyl, R0 ¼ 3,7-dimethyloctyl).
and it usually needs very low level (<103 %) doping controls. Recently, we have studied color tuning of PLED based on PFs. Composites or blends of PF and green and/or red emitting dyes exhibited green or red emission from blue emitting PF [6]. From this study, we found that there is an inefficient energy transfer between poly(9,9-bis(2-ethylhexyl)fluorine-2,7-diyl) (BEHF) and poly(2,5-bis(dimethyloctylsilyl)-1,4-phenylenevinylene) (BDMOS-PPV). Therefore, there is an opportunity to obtain white light emission from the blend of PF and PPV derivatives in controllable blend ratio. In this paper, energy transfer from PF to BDMOS-PPV and fabrication of white light emitting devices will be described.
2. Experimental BEHF, BDMOS-PPV, poly(2-(20 -ethylhexyloxy)-5-methoxy-1,4-phenylene vinylene) (MEHPPV) and poly(2-(30 ,70 -dimethyloctyloxy)-5-methoxy-1,4-phenylene-1-cyanovinylene) (CN-PPV)
were prepared according to the previous papers [1,9]. The structures of the polymers are shown in Fig. 1. Thin films of the polymers were obtained by spin-coating from polymer solutions in p-xylene on fused quartz plates. All the photophysical measurements were performed at room temperature. The absorption spectra were measured by a Hitachi spectrophotometer model U-3501 and steady-state photoluminescence spectra were recorded on a Spex FL3-11. Single-layered LEDs utilized the polymer blend of BEHF, BDMOS-PPV and MEH-PPV or CNPPV as an emitting layer with ITO anodes and aluminum (Al) cathodes. PEDT:PSS purchased from Bayer (Baytron P 4083) was used as a buffer layer on ITO. Blend ratio of BEHF, BDMOS-PPV and MEH-PPV or CN-PPV was 95, 4, 1 based on weight and blending was performed by dissolving three polymers in p-xylene. The polymer layer was spin-coated to a typical thickness of 100 nm. A patterned Al cathode (100 nm) was then deposited by thermal evaporation. EL spectra were recorded using Minolta CS-1000 and current–voltage (I–V ) and luminance–voltage characteristics of the de-
J.-I. Lee et al. / Optical Materials 21 (2002) 205–210
vices were simultaneously measured with a Keithley 2400 Source Measure Unit and a Minolta LS-100.
3. Results and discussion The polymer blends were obtained by dissolving BEHF and BDMOS-PPV in p-xylene and thin films were made by spin-coating on fused quartz for spectroscopic measurements. Fig. 2 shows the absorption spectra of the thin films. There are two absorptions from BEHF and green emitting material, BDMOS-PPV. The bigger absorption centered at about 380 nm is originated from BEHF and the shoulder like absorption at 450 nm is from BDMOS-PPV. With increase of BDMOS-PPV contents in the polymer blends, the shoulder like absorption increases which also leads to the redshift of BEHF absorption maximum. As for 50/50 blend of BEHF and BDMOS-PPV, the absorption intensity of BEHF is about two times bigger than that of BDMOS-PPV, which indicates that the absorption coefficient of BEHF based on weight is higher than that of BDMOS-PPV. As shown in Fig. 3, two emissions from BEHF and BDMOS-PPV were observed regardless of
207
blend ratios. The emission of BDMOS-PPV increases with increase of BDMOS-PPV blend ratio. The emission of BEHF did not disappeared even in 50/50 blend, which might indicate that there was an inefficient energy transfer between BEHF and BDMOS-PPV. As the spectral overlap of the emission of BEHF and the absorption of BDMOSPPV is large enough to make energy transfer, one possible explanation is that there might be inhomogeneous mixing in the polymer blends and the domain sizes of BDMOS-PPV are bigger than the energy transfer radius. Therefore, we are now under investigation on the above possibility. Anyway, we can take advantage of such an inefficient energy transfer to obtain white light emission in PLED. To enhance red emission, orange or red light emitting material (MEH-PPV or CN-PPV) was added to the above BEHF and BDMOS-PPV blends. The blending ratio of BEHF, BDMOSPPV and MEH-PPV or CN-PPV is 95, 4, and 1. Fig. 4 displays the absorption and the emission spectra of the BEHF/BDMOS-PPV/MEH-PPV and BEHF/BDMOS-PPV/CN-PPV. The absorption and the emission spectra of BEHF were also shown for comparison. Little difference between BEHF and the blended polymers was observed except slight increase of absorption at the
Fig. 2. Absorption spectra of BEHF and BDMOS-PPV blends.
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Fig. 3. Emission spectra of BEHF and BDMOS-PPV blends. Excitation wavelength was 380 nm.
Fig. 4. Absorption and emission spectra of BEHF/BDMOS-PPV/MEH-PPV and BEHF/BDMOS-PPV/CN-PPV blends.
absorption edges. As for emissions excited by 380 nm light, the blue emission of BEHF and the emissions of PPVs were present simultaneously.
Fig. 5 shows the EL spectra of BEHF/BDMOSPPV/MEH-PPV and BEHF/BDMOS-PPV/CNPPV. As similar to fluorescence spectra, three
J.-I. Lee et al. / Optical Materials 21 (2002) 205–210
209
Fig. 5. EL spectra of BEHF/BDMOS-PPV/MEH-PPV (solid line) and BEHF/BDMOS-PPV/CN-PPV (dotted line) blends. The device configuration was ITO/PEDT:PSS/Polymer/LiF/Al.
emission components from the polymers were observed. The emissions of BEHF and BDMOSPPV were strong while the emission of MEH-PPV was very weak. The inefficient energy transfer from BEHF to BDMOS-PPV accounts for the presence of both emissions and mismatches of HOMO and LUMO positions for MEH-PPV and BEHF or BDMOS-PPV explains the weak emission of MEH-PPV in EL device as shown in Fig. 1. The LUMO of MEH-PPV is lower than that of BEHF and the electron hardly moves to the LUMO of MEH-PPV. As for BEHF/BDMOS-PPV/CNPPV, red emission was greatly enhanced compared with BEHF/BDMOS-PPV/MEH-PPV. The fact that the HOMO and LUMO of CN-PPV are located between those of BEHF and BDMOS-PPV, explains the lager red emission of BEHF/BDMOSPPV/MEH-PPV than that of BEHF/BDMOSPPV/CN-PPV in EL [10]. The CIE coordinates of the ELs were 0.32, 0.28 and 0.40, 0.41 for BEHF/ BDMOS-PPV/MEH-PPV and BEHF/BDMOSPPV/CN-PPV, respectively. The CIE coordinate of EL from polymer blend can be further tuned by changing blend ratio. The results of the EL devices from the above polymer blends were preliminary and the optimizations of device and blend ratio is needed to obtain white light emission with a good efficiency, which are under investigation.
4. Conclusion The inefficient energy transfer from BEHF to BDMOS-PPV was observed, which resulted in the presence of both emissions in PL and EL. As the spectral overlap of the emission of BEHF and the absorption of BDMOS-PPV is large enough to make energy transfer, possible explanation should be inhomogeneous mixing of two polymers. Using the inefficient energy transfer, white light emitting polymer devices were fabricated. To enhance red emission, red emitting MEH-PPV or CN-PPV was added to BEHF and BDMOS-PPV blend and white light was obtained. Therefore, using the polymer blends and the inefficient energy transfer, one possible method to achieve white light emitting polymer device was suggested. Acknowledgements Funding from Ministry of Information and Communication (MIC) is acknowledged. References [1] A. Kraft, A.C. Grimsdale, A.B. Holmes, Angew. Chem. Int. Ed. 37 (1998) 403.
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J.-I. Lee et al. / Optical Materials 21 (2002) 205–210
[2] R.H. Friend, R.W. Gymer, A.B. Holmes, J.H. Burroughes, R.N. Marks, C. Taliani, D.D.C. Bradley, D.A. Dos Santos, J.L. Bredas, M. L€ ogdlund, W.R. Salaneck, Nature 397 (1999) 121. [3] G. Klaerner, M.H. Davey, W.D. Chen, J.C. Scott, R.D. Miller, Adv. Mater. 10 (1998) 993. [4] G. Klaerner, J.-I. Lee, M.H. Davey, R.D. Miller, Adv. Mater. 11 (1999) 115. [5] J.-I. Lee, G. Klaerner, M.H. Davey, R.D. Miller, Syn. Met. 103 (1999) 1087.
[6] J.-I. Lee, L.-M. Do, H.Y. Chu, S.H. Kim, T. Zyung, J. Inform. Display, IDMC 2000 Digest (2000) 251. [7] J. Shi, C.W. Tang, US patent 5683823, 1997. [8] J. Kido, H. Shionya, K. Nagai, Appl. Phys. Lett. 67 (1995) 2281. [9] M. Kreyenschmidt, G. Klaerner, T. Fuhrer, J. Ashenhurst, S. Karg, W.D. Chen, V.Y. Lee, J.C. Scott, R.D. Miller, Macromolecules 31 (1998) 1099. [10] V. Cleave, G. Yahioglu, P.L. Barny, D.-H. Hwang, A.B. Holmes, R.H. Friend, N. Tessler, Adv. Mater. 13 (2001) 44.