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An electroluminescent diode using liquid-crystalline conducting polymer
Thin Solid Films 363 (2000) 9±12 www.elsevier.com/locate/tsf
An electroluminescent diode using liquid-crystalline conducting polymer M. Onoda a, b,*, K. Tada a, M. Ozaki c, K. Yoshino c a
Graduate School of Engineering, Himeji Institute of Engineering, 2167 Shosha, Himeji, Hyogo 671-2201, Japan b Faculty of Science and Engineering, Saga University, 1 Honjyo, Saga 840-8502, Japan c Graduate School of Engineering, Osaka University, 2±1 Yamada-oka, Suita, Osaka 565-0871, Japan
Abstract Poly(p-phenylene vinylene) derivatives substituted with a long side chain containing alkoxybiphenyl mesogenic unit have been synthesized and its electrical and optical properties have been studied. A liquid-crystalline molecular alignment and layer structure have been con®rmed by optical microscopic observation and X-ray diffraction measurement, respectively. The dielectric constant changes in a stepwise manner at phase transition temperature. The bandgap of this new polymer in the liquid-crystalline phase has been evaluated to be about 2.3 eV, which monotonously decreases with decreasing temperature. It has been found that the emission spectra of electroluminescent diode using this new liquid-crystalline polymer as an emissive layer depend on the polarity of the applied voltage. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Electroluminescent diodes; Diquid-crystalline conducting polymers; Electrical and optical properties
1. Introduction Conducting polymers with a highly extended p-electron system in their polymer main chain have attracted much attention from both fundamental and practical viewpoints, because various new concepts such as the soliton, polaron and bipolaron have been introduced and also many types of applications utilizing these conducting polymers have been proposed [1]. In order to give full play functions of conducting polymers, their arrangement and control are indispensable. One of the methods used in orientation of conducting polymers is a mechanical elongation technique, but it is impossible to control the direction of orientation freely. On the other hand, because the liquid crystal orients itself, it is possible to control the direction of orientation using light, voltage, etc., easily. From the above stated standpoint, liquid-crystalline polymers having mesogenic units in the main chain or side chain have attracted considerable attention. Especially, since the discovery of ferroelectricity in the liquid-crystalline polymers with chiral smectic phases, a lot of liquid-crystalline side chain polymers have been synthesized and investigated. In these polymers, the main chain consists of ¯exible saturated bonds. However, only a few studies of a liquid-crystalline conducting polymer with an unsaturated conjugated main chain and with substituents containing mesogenic * Corresponding author.
side chain units have been reported [2,3]. Recently, we synthesized a novel poly(p-phenylene vinylene) derivative which has a long substituent containing alkoxybiphenyl mesogenic unit. In this article, liquid crystalline features in optical and electroluminescent (EL) properties of this new polymer are reported. 2. Experimental details Fig. 1 shows a schematic diagram of the synthetic route to poly(2-methoxy, 5-(4-decyloxy-biphenyl-4 0 -(1,10-dioxydecane))-p-phenylene vinylene), MDBD-PPV, used in this study. MDBD-PPV was soluble in common organic solvents such as chloroform, methylene chloride, etc. For texture observation and dielectric measurement, the polymer was sandwiched between ITO glass plates whose surfaces were coated with a polyimide and rubbed in direction parallel to each other. The texture of the polymer was observed using a polarizing optical microscope (Nikon, OPTIPHOT2-POL). The dielectric constant was measured with an impedance analyzer (YHTP, 4192A). For the absorption measurement, the polymer was cast on a quartz glass plate and absorption spectra were measured using a microscopic spectrometer system (Otsuka Electronics, IMUC7000G; Nikon, OPTIPHOT2) and a diode array spectrophotometer (HP, 3452A). The photoluminescence (PL) spectra were
0040-6090/00/$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0040-609 0(99)00971-2
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M. Onoda et al. / Thin Solid Films 363 (2000) 9±12
Fig. 2. Structure of EL device.
Fig. 1. Schematic diagram of synthetic route to MDBD-PPV.
measured using a Hitachi F-2000 ¯uorescent spectrometer. X-ray diffraction measurement was carried out using a Ê in waveRINT 1100 (RIGAKU) system with X-rays, 1.54 A length. An EL device structure fabricated using MDBD-PPV is shown in Fig. 2. A several hundred-nm thick ®lm of MDBD-PPV was fabricated by the spin-coating technique onto ITO-coated quartz substrate. A mixture of Mg and In metals was deposited onto the spin-coated MDBD-PPV ®lm. The electrode area of the EL device was about 4 m 2.
from the X-ray diffraction peak position. The layer spacing increases with decreasing temperature from the transition temperature. Generally, the spacing of the smectic layer in the Sm A phase gradually increases with decreasing temperature. This can be interpreted to be due to the change in the conformation of the side chain of the MDBD-PPV upon changing temperature. That is, the side chain becomes slightly shorter because of a trans-gauche conformational change. The layer Ê in MDBD-PPV evaluated from the Xspacing of about 30 A ray diffraction measurement is slightly short compared with the calculated length of the side chain moiety with all-trans Ê ). The twist of the main chain might also conformation (32 A contribute to the decrease in the layer spacing with respect to the calculated value. On the other hand, the dielectric constant increases in a stepwise manner at the phase transition from the isotropic to
3. Results and discussion MDBD-PPV was found to melt at 1108C and be in isotropic liquid phase above this temperature. By decreasing the temperature from this isotropic liquid phase, an optically anisotropic texture appeared below 1108C, suggesting the transition to the liquid-crystalline phase. Then, an extinction can be realized in a crossed polarizer geometry, which becomes bright by the rotation of the MDBD-PPV cell. A conspicuous X-ray diffraction peak was observed at around 38 of 2u below 1108C as shown in Fig. 3. This low-angle diffraction signal should originate in the smectic layer structure and correspond to the spacing of the smectic layers. The inset of Fig. 3 shows the layer spacing evaluated
Fig. 3. Typical X-ray diffraction spectrum of MDBD-PPV. The inset shows temperature dependence of layer spacing evaluated from the X-ray diffraction measurement.
M. Onoda et al. / Thin Solid Films 363 (2000) 9±12
Fig. 4. Absorption and photoluminescence spectra of MDBD-PPV.
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Fig. 6. EL intensity as a function of current ¯ow under increasing applied voltage for MDBD-PPV EL diode.
the smectic phases with decreasing temperature. It again decreases at the phase transition from the smectic to the solid phases. From the above mentioned experimental results, MDBDPPV has characteristics of liquid-crystalline material with Sm A phase. Fig. 4 shows the absorption and PL spectra of MDBDPPV ®lm. The peak wavelengths in the absorption spectrum are mainly found at 270 and 470 nm. The bandgap energy estimated from the absorption edge is about 2.3 eV. It should be noted that there is small increase in bandgap energy with increasing temperature. This result indicated that the effective conjugation length of the main chain is shortened by twisting and/or bending of the main chain at higher temperature. A similar thermochromic phenomena has been already reported for poly(3-alkylthiophene) [4]. As evident from Fig. 4, the PL spectrum shows a peak at 550 nm with an excitation wavelength of 468 nm and is also similar to that of the PPV derivatives. However, when MDBD-PPV is excited by higher energy light with 267 nm, additional features appeared in the PL spectrum in the higher energy range at around 400 nm. The peak at around 400 nm was also observed in the PL spectrum of 4,4 0 -didecyloxybiphenyl which corresponds to the mesogenic part of
the MDBD-PPV side chain. Therefore, it is assumed that the PL peak at around 400 nm of MDBD-PPV originates from the biphenyl in the mesogenic side chain. Fig. 5 shows the typical dark current-bias voltage characteristic of the EL device with MDBD-PPV as an emitter material, together with EL intensity-bias voltage characteristic of a MDBD-PPV EL diode. In the EL diode studied here, the forward bias is de®ned as the situation where the ITO electrode is positively biased against the Mg:In elec-
Fig. 5. Typical dark current-bias voltage characteristic of the MDBD-PPV EL device, together with EL intensity-bias voltage characteristic of the EL device with MDBD-PPV as an emitter material.
Fig. 7. EL spectra of the MDBD-PPV EL diode as a function of applied voltage. Positive voltage was applied to the ITO (a) and Mg:In (b) electrode.
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M. Onoda et al. / Thin Solid Films 363 (2000) 9±12
trode. Both the forward bias current and the reverse bias current increase non-linearly at bias voltages higher than about ^1.0 V, respectively, which is nearly identical to the threshold of light emission. This threshold voltage means that substantial charge injection into an emitting layer occurs. That is, it should be noted that the EL emission was detected both in the forward bias and the reverse bias. Fig. 6 shows the EL intensity as a function of current ¯ow under increasing both forward and reverse bias voltages. The EL intensity is approximately linear with injected current. Fig. 7a,b shows the EL spectra of the EL diode with MDBD-PPV at room temperature as a function of driving voltage with positive and negative polarities, respectively. Upon applying positive bias, a broad emission peak centered at 600 nm was observed as shown in Fig. 7a. The emission intensity drastically increases at above 5 V. This peak in EL spectra at around 600 nm might be associated with that in the PL spectrum shown in Fig. 4, which corresponds to the interband transition of the MDBD-PPV main chain. On the contrary, when negative bias was applied, relatively sharp peaks at around 400 nm were observed above 210 V in EL spectra. These peaks corresponds to the PL peak observed by the high-energy excitation as already mentioned above. In other words, when the Mg:In metal electrode was positively biased, the biphenyl mesogenic moiety in the side chain was excited, though the mechanism of this anomaly is not clear at this stage.
4. Conclusions A poly(p-phenylene vinylene) derivative, MDBD-PPV, substituted with long side chains containing mesogen units with biphenyl mesogenic moieties showed a typical microscopic texture to the liquid-crystalline phase and a sharp Xray diffraction peak due to the smectic layer structure. Then, the temperature dependence of the dielectric constant of MDBD-PPV re¯ected the characteristics of phase transitions and changes at phase transition temperature in a stepwise manner. In the EL diode using MDBD-PPV as an emitter material, the selective emission in the conjugated main chain and the mesogenic moiety of side chain were observed upon polarity of the applied voltage.
References [1] K. Yoshino, M. Onoda, Polymer Electronics, Corona, (1996), in Japanese. [2] S.-H. Jin, S.-H. Kim, H.-N. Cho, S.-K. Choi, Macromolecules 24 (1991) 6050. [3] S.-Y. Oh, K. Akagi, H. Shirakawa, K. Araya, Macromolecules 26 (1993) 6203. [4] K. Yoshino, D.-H. Park, B.-K. Park, M. Onoda, R. Sugimoto, Jpn. J. Appl. Phys. 37 (1988) L1612.
An electroluminescent diode using liquid-crystalline conducting polymer M. Onoda a, b,*, K. Tada a, M. Ozaki c, K. Yoshino c a
Graduate School of Engineering, Himeji Institute of Engineering, 2167 Shosha, Himeji, Hyogo 671-2201, Japan b Faculty of Science and Engineering, Saga University, 1 Honjyo, Saga 840-8502, Japan c Graduate School of Engineering, Osaka University, 2±1 Yamada-oka, Suita, Osaka 565-0871, Japan
Abstract Poly(p-phenylene vinylene) derivatives substituted with a long side chain containing alkoxybiphenyl mesogenic unit have been synthesized and its electrical and optical properties have been studied. A liquid-crystalline molecular alignment and layer structure have been con®rmed by optical microscopic observation and X-ray diffraction measurement, respectively. The dielectric constant changes in a stepwise manner at phase transition temperature. The bandgap of this new polymer in the liquid-crystalline phase has been evaluated to be about 2.3 eV, which monotonously decreases with decreasing temperature. It has been found that the emission spectra of electroluminescent diode using this new liquid-crystalline polymer as an emissive layer depend on the polarity of the applied voltage. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Electroluminescent diodes; Diquid-crystalline conducting polymers; Electrical and optical properties
1. Introduction Conducting polymers with a highly extended p-electron system in their polymer main chain have attracted much attention from both fundamental and practical viewpoints, because various new concepts such as the soliton, polaron and bipolaron have been introduced and also many types of applications utilizing these conducting polymers have been proposed [1]. In order to give full play functions of conducting polymers, their arrangement and control are indispensable. One of the methods used in orientation of conducting polymers is a mechanical elongation technique, but it is impossible to control the direction of orientation freely. On the other hand, because the liquid crystal orients itself, it is possible to control the direction of orientation using light, voltage, etc., easily. From the above stated standpoint, liquid-crystalline polymers having mesogenic units in the main chain or side chain have attracted considerable attention. Especially, since the discovery of ferroelectricity in the liquid-crystalline polymers with chiral smectic phases, a lot of liquid-crystalline side chain polymers have been synthesized and investigated. In these polymers, the main chain consists of ¯exible saturated bonds. However, only a few studies of a liquid-crystalline conducting polymer with an unsaturated conjugated main chain and with substituents containing mesogenic * Corresponding author.
side chain units have been reported [2,3]. Recently, we synthesized a novel poly(p-phenylene vinylene) derivative which has a long substituent containing alkoxybiphenyl mesogenic unit. In this article, liquid crystalline features in optical and electroluminescent (EL) properties of this new polymer are reported. 2. Experimental details Fig. 1 shows a schematic diagram of the synthetic route to poly(2-methoxy, 5-(4-decyloxy-biphenyl-4 0 -(1,10-dioxydecane))-p-phenylene vinylene), MDBD-PPV, used in this study. MDBD-PPV was soluble in common organic solvents such as chloroform, methylene chloride, etc. For texture observation and dielectric measurement, the polymer was sandwiched between ITO glass plates whose surfaces were coated with a polyimide and rubbed in direction parallel to each other. The texture of the polymer was observed using a polarizing optical microscope (Nikon, OPTIPHOT2-POL). The dielectric constant was measured with an impedance analyzer (YHTP, 4192A). For the absorption measurement, the polymer was cast on a quartz glass plate and absorption spectra were measured using a microscopic spectrometer system (Otsuka Electronics, IMUC7000G; Nikon, OPTIPHOT2) and a diode array spectrophotometer (HP, 3452A). The photoluminescence (PL) spectra were
0040-6090/00/$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0040-609 0(99)00971-2
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M. Onoda et al. / Thin Solid Films 363 (2000) 9±12
Fig. 2. Structure of EL device.
Fig. 1. Schematic diagram of synthetic route to MDBD-PPV.
measured using a Hitachi F-2000 ¯uorescent spectrometer. X-ray diffraction measurement was carried out using a Ê in waveRINT 1100 (RIGAKU) system with X-rays, 1.54 A length. An EL device structure fabricated using MDBD-PPV is shown in Fig. 2. A several hundred-nm thick ®lm of MDBD-PPV was fabricated by the spin-coating technique onto ITO-coated quartz substrate. A mixture of Mg and In metals was deposited onto the spin-coated MDBD-PPV ®lm. The electrode area of the EL device was about 4 m 2.
from the X-ray diffraction peak position. The layer spacing increases with decreasing temperature from the transition temperature. Generally, the spacing of the smectic layer in the Sm A phase gradually increases with decreasing temperature. This can be interpreted to be due to the change in the conformation of the side chain of the MDBD-PPV upon changing temperature. That is, the side chain becomes slightly shorter because of a trans-gauche conformational change. The layer Ê in MDBD-PPV evaluated from the Xspacing of about 30 A ray diffraction measurement is slightly short compared with the calculated length of the side chain moiety with all-trans Ê ). The twist of the main chain might also conformation (32 A contribute to the decrease in the layer spacing with respect to the calculated value. On the other hand, the dielectric constant increases in a stepwise manner at the phase transition from the isotropic to
3. Results and discussion MDBD-PPV was found to melt at 1108C and be in isotropic liquid phase above this temperature. By decreasing the temperature from this isotropic liquid phase, an optically anisotropic texture appeared below 1108C, suggesting the transition to the liquid-crystalline phase. Then, an extinction can be realized in a crossed polarizer geometry, which becomes bright by the rotation of the MDBD-PPV cell. A conspicuous X-ray diffraction peak was observed at around 38 of 2u below 1108C as shown in Fig. 3. This low-angle diffraction signal should originate in the smectic layer structure and correspond to the spacing of the smectic layers. The inset of Fig. 3 shows the layer spacing evaluated
Fig. 3. Typical X-ray diffraction spectrum of MDBD-PPV. The inset shows temperature dependence of layer spacing evaluated from the X-ray diffraction measurement.
M. Onoda et al. / Thin Solid Films 363 (2000) 9±12
Fig. 4. Absorption and photoluminescence spectra of MDBD-PPV.
11
Fig. 6. EL intensity as a function of current ¯ow under increasing applied voltage for MDBD-PPV EL diode.
the smectic phases with decreasing temperature. It again decreases at the phase transition from the smectic to the solid phases. From the above mentioned experimental results, MDBDPPV has characteristics of liquid-crystalline material with Sm A phase. Fig. 4 shows the absorption and PL spectra of MDBDPPV ®lm. The peak wavelengths in the absorption spectrum are mainly found at 270 and 470 nm. The bandgap energy estimated from the absorption edge is about 2.3 eV. It should be noted that there is small increase in bandgap energy with increasing temperature. This result indicated that the effective conjugation length of the main chain is shortened by twisting and/or bending of the main chain at higher temperature. A similar thermochromic phenomena has been already reported for poly(3-alkylthiophene) [4]. As evident from Fig. 4, the PL spectrum shows a peak at 550 nm with an excitation wavelength of 468 nm and is also similar to that of the PPV derivatives. However, when MDBD-PPV is excited by higher energy light with 267 nm, additional features appeared in the PL spectrum in the higher energy range at around 400 nm. The peak at around 400 nm was also observed in the PL spectrum of 4,4 0 -didecyloxybiphenyl which corresponds to the mesogenic part of
the MDBD-PPV side chain. Therefore, it is assumed that the PL peak at around 400 nm of MDBD-PPV originates from the biphenyl in the mesogenic side chain. Fig. 5 shows the typical dark current-bias voltage characteristic of the EL device with MDBD-PPV as an emitter material, together with EL intensity-bias voltage characteristic of a MDBD-PPV EL diode. In the EL diode studied here, the forward bias is de®ned as the situation where the ITO electrode is positively biased against the Mg:In elec-
Fig. 5. Typical dark current-bias voltage characteristic of the MDBD-PPV EL device, together with EL intensity-bias voltage characteristic of the EL device with MDBD-PPV as an emitter material.
Fig. 7. EL spectra of the MDBD-PPV EL diode as a function of applied voltage. Positive voltage was applied to the ITO (a) and Mg:In (b) electrode.
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M. Onoda et al. / Thin Solid Films 363 (2000) 9±12
trode. Both the forward bias current and the reverse bias current increase non-linearly at bias voltages higher than about ^1.0 V, respectively, which is nearly identical to the threshold of light emission. This threshold voltage means that substantial charge injection into an emitting layer occurs. That is, it should be noted that the EL emission was detected both in the forward bias and the reverse bias. Fig. 6 shows the EL intensity as a function of current ¯ow under increasing both forward and reverse bias voltages. The EL intensity is approximately linear with injected current. Fig. 7a,b shows the EL spectra of the EL diode with MDBD-PPV at room temperature as a function of driving voltage with positive and negative polarities, respectively. Upon applying positive bias, a broad emission peak centered at 600 nm was observed as shown in Fig. 7a. The emission intensity drastically increases at above 5 V. This peak in EL spectra at around 600 nm might be associated with that in the PL spectrum shown in Fig. 4, which corresponds to the interband transition of the MDBD-PPV main chain. On the contrary, when negative bias was applied, relatively sharp peaks at around 400 nm were observed above 210 V in EL spectra. These peaks corresponds to the PL peak observed by the high-energy excitation as already mentioned above. In other words, when the Mg:In metal electrode was positively biased, the biphenyl mesogenic moiety in the side chain was excited, though the mechanism of this anomaly is not clear at this stage.
4. Conclusions A poly(p-phenylene vinylene) derivative, MDBD-PPV, substituted with long side chains containing mesogen units with biphenyl mesogenic moieties showed a typical microscopic texture to the liquid-crystalline phase and a sharp Xray diffraction peak due to the smectic layer structure. Then, the temperature dependence of the dielectric constant of MDBD-PPV re¯ected the characteristics of phase transitions and changes at phase transition temperature in a stepwise manner. In the EL diode using MDBD-PPV as an emitter material, the selective emission in the conjugated main chain and the mesogenic moiety of side chain were observed upon polarity of the applied voltage.
References [1] K. Yoshino, M. Onoda, Polymer Electronics, Corona, (1996), in Japanese. [2] S.-H. Jin, S.-H. Kim, H.-N. Cho, S.-K. Choi, Macromolecules 24 (1991) 6050. [3] S.-Y. Oh, K. Akagi, H. Shirakawa, K. Araya, Macromolecules 26 (1993) 6203. [4] K. Yoshino, D.-H. Park, B.-K. Park, M. Onoda, R. Sugimoto, Jpn. J. Appl. Phys. 37 (1988) L1612.