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Light-emitting electrochemical cells based on poly(2-methoxy-5-triethoxy-1,4-phenylene vinylene)
Displays 21 (2000) 69–72 www.elsevier.nl/locate/displa
Light-emitting electrochemical cells based on poly(2-methoxy-5-triethoxy-1,4-phenylene vinylene) Gufeng He, Chunhe Yang, Rongqiu Wang, Yongfang Li* Laboratory of Organic Solids, Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China
Abstract A luminescent polymer with a PEO-like side chain, poly(2-methoxy-5-triethoxy-1,4-phenylene vinylene)(MTEO-PPV), was synthesized and characterized by UV–Vis spectra, photoluminescence (PL) spectra and cyclic voltammetry. The relatively high molecular weight Mn 77; 000 permitted the fabrication of good quality films. Its PL relative quantum yield can reach 38% in chloroform solution. Single layer light-emitting electrochemical cell (LEC) devices were fabricated using MTEO-PPV and lithium triflate as the emissive layer, ITO and Al as the electrodes. The devices were turned on at about 2.8 V, while that without lithium triflate was turned on at about 9 V, and the brightness increased significantly in contrast to the latter. 䉷 2000 Elsevier Science B.V. All rights reserved. Keywords: MTEO-PPV; Luminescence; Light-emitting electrochemical cell
1. Introduction Light-emitting electrochemical cells (LECs) [1,2] were reported first in 1995 using the blends of poly(1,4-phenylene vinylene)(PPV) and poly(ethylene oxide)(PEO) complexed with lithium trifluoromethanesulfonate as the active materials. PPV is a luminescent polymer that can be both n- and p-type electrochemically doped, while PEO complexed with lithium salt is a polymer electrolyte with relatively high ionic conductivity. When a voltage greater than Eg/e, where Eg is the p–p ⴱ energy gap of the polymer and e is the electronic charge, is applied, PPV near the anode is oxidized and p-type carriers (holes) are introduced, whereas PPV near the cathode is reduced and n-type carriers (electrons) are introduced. Counterions from the electrolytes move to compensate the charges on the oxidized and reduced polymer chains. Under the influence of the applied voltage, the holes in the PPV propagate from anode towards the cathode, and the electrons propagate from the cathode towards the anode. These holes and electrons meet in the compensated region between the n- and p-type-doped regions. Holes and electrons recombine to give out light. According to this mechanism, LECs have some advantages [3] such as low operating voltage, high quantum efficiency, high power efficiency, and so on. Because the metal/polymer contacts in the LEC are ohmic, they are * Corresponding author. Tel.: ⫹ 86-10-6256-3063; fax: ⫹ 86-10-62569564. E-mail address: [email protected] (Y. Li).
not sensitive to the work function of the metal electrodes. So air-stable electrodes can be used in LECs. However, in the active layer, the PEO–salt complex is very polar and its dissolution requires the use of a polar solvent, while most of the soluble luminescent conjugated polymers such as PPV, MEH-PPV are non-polar. Hence, it is difficult to find a solvent to dissolve these polymers simultaneously, so that phase separation often occurs. This can degrade the device performance significantly. In order to solve the phase separation problem, we synthesized a new PPV derivative with PEO-like units in phenylene moiety. These side groups can not only reduce or completely eliminate the use of PEO but also increase the solubility of the polymer in common organic solvents.
2. Experimental The synthesis of poly(2-methoxy-5-triethoxy-1,4-phenylene vinylene) (MTEO-PPV) is outlined in Fig. 1. The PEOlike units were attached to the benzene ring via Williamson ether formation between the triethoxy-p-toluenesulfonate and 4-methoxy phenol. Chloromethylation of the aromatic ethers with HCl in the presence of formaldehyde led to the bis-chloromethylated compounds [1,2]. Polymerization of the monomer by base-promoted dehydrohalogenation [4] using NaH in DMF gave polymers [1–3] with reasonable molecular weights, which were soluble in common organic solvents. Gel permeation chromatography (GPC) was recorded on
0141-9382/00/$ - see front matter 䉷 2000 Elsevier Science B.V. All rights reserved. PII: S0141-938 2(00)00042-1
70
G. He et al. / Displays 21 (2000) 69–72
Fig. 1. Synthetic route to MTEO-PPV.
a Waters Associates GPC system, using THF as the eluent and a low-dispersity polystyrene standards. The number average molecular weight is about 77,000, which corresponds to 290 repeating units per polymer chain. Ultra-
violet–Visible (UV–Vis) measurements were made on Hitachi U-3010 spectrophotometer. PL spectroscopy was performed on Hitachi F-4500 fluorescence spectrophotometer. PL efficiency in CHCl3 was determined with 9,10diphenylanthracene as in Ref. [5]. Cyclic voltammetry were measured on EG & G PARC 370 electrochemical system, using a Pt disk as working electrode (0.785 mm 2), a Pt wire as a counter electrode and an Ag wire as quasi-reference electrode. The MTEO-PPV/working electrode was produced by simply casting its solution in chloroform on the electrode and then letting the solvent to evaporate to leave a thin film. 0.1 M tetrabutylammonium hexafluorophosphate (Bu4NPF6) dissolved in acetonitrile was employed as the supporting electrolyte and the entire system was degassed with argon for 10 min. Single layer LEC devices were fabricated on indium–tin oxide (ITO) coated glass substrate. Thin films were spin cast from a mixture solution of MTEO-PPV and lithium triflate using chloroform as the solvent. The weight ratio of lithium salt to MTEO-PPV was 0.18. Then these thin films were heated at 60–70⬚C for a few hours on a hot plate to remove the residual solvent completely. An aluminum layer was evaporated onto the film at pressure around 6 × 10⫺6 Torr and used as a back electrode. Typical active areas were between 10 and 14 mm 2. The I–V characteristics were recorded with an Advantest R8340 ultra high resistance meter, and a calibrated photodiode was used for light intensity measurements. All measurements were controlled by an NEC computer and were performed in air at room temperature.
3. Results and discussion
Fig. 2. (a) UV–Vis spectra of solution (dot line) and film (solid line) of MTEO-PPV. (b) PL spectra of solution (dot line) and film (solid line) of MTEO-PPV.
Fig. 2a shows the optical absorption of a solution of MTEO-PPV in chloroform (1 mg/100 ml) and a thin film spin-cast from a concentrated solution (10 mg/ml). The absorption spectrum of the dilute solution has a peak at
G. He et al. / Displays 21 (2000) 69–72
Fig. 3. Cyclic voltammogram of a cast film of MTEO-PPV on Pt disk (0.1 M Bu4PF6/CH3CN) at a scan rate of 20 mV/s.
463 nm, with an onset of absorption at about 540 nm. The spectrum of the thin film has the same peak wavelength, but the peak is broader. This means there is no solvatochromism for MTEO-PPV. In contrast, large solvatochromism has been observed for some soluble conjugated polymers such as poly(3-alkylthiophene) [6]. It is well known that an absorption spectrum of a conjugated polymer depends on its molecular conformation due to changing conjugation length of the polymer. No solvatochroism implies that backbone structure of MTEO-PPV is more rigid than that of poly(3-alkylthiophene) due to good coplanarity of a phenyl ring and a vinylene group. Hence MTEO-PPV have the similar conformation in both the solution and the film. The photoluminescent (PL), spectra of the dilute solution and the film of MTEO-PPV pumped by UV light l 365 nm are displayed in Fig. 2b. The relative quantum yield was measured to be 38%, comparable to many conjugated polymers in dilute solution. There is a large red shift
71
in the PL spectra from solution (540 nm) to film (580 nm) on the contrary to no shift for absorption spectra. The shift may be not attributed to the change of conjugation length but the difference in energy transfer process between the film and the solution [6]. The energy-transfer process takes place more easily in the film than the solution and exciton migrates to the emission center having smaller energy with longer conjugation sequence in the molecules. Fig. 3 shows the cyclic voltammogram of MTEO-PPV. It is seen in the figure that the polymer has good redox reversibility. The polymer can be repeatedly ‘doped–dedoped’ for many cycles without significant changes in its oxidation and reduction peak characteristics. The onset potentials of oxidation and reduction of MTEO-PPV have been determined as ⫹0.67 and ⫺1.40 V vs. Ag wire, respectively. The oxidation process corresponds to the removal of charge from the HOMO energy level whereas the reduction cycle corresponds to the electron addition to the LUMO. So the difference between the two onset potentials can give the bandgap of the polymer [7]. The separation between the two onset potentials of MTEO-PPV is 2.07 V. This value is comparable to the energy corresponding to the onset of the optical absorption of MTEO-PPV film cast from its solution. The LEC was fabricated by sandwiching a thin film of a blend of MTEO-PPV and lithium triflate between an ITO coated glass and a vacuum-evaporated aluminum film. In contrast, an LED device with the same structure was fabricated too, but the lithium triflate was removed from the active layer. For convenience, ITO was connected to the anode and Al was connected to the cathode though LEC has nearly antisymmetric I–V characteristics. When an external voltage above the threshold was applied to the LEC device, orange light emitted from the polymer layer. Fig. 4 shows the I–V and L–V characteristics of an ITO/ MTEO-PPV ⫹ LiCF3SO3/Al device under forward bias. The threshold voltage for current is about 2.4 V but for light intensity it is about 3 V. This hysteresis should be attributed to the sensitivity of the luminescence meter. In fact, faint light could be seen with naked eyes at the bias of 2.6 V. However, the onset potential is up to about 9 V (Fig. 5) for the single layer LED having the same structure. The decrease in the turn-on voltage in the LEC is considered as due to the presence of Li salt. The PEO-like units on the phenylene moiety can supply enough ionic conductivity necessary for the operation of LECs.
4. Conclusions
Fig. 4. Current–voltage and light–voltage characteristics of an ITO/ MTEO-PPV ⫹ LiCF3SO3/Al LEC device.
A highly soluble polymer, MTEO-PPV, was synthesized using the dehydrochlorination route. The PEO-like units on the PPV skeleton increased its solubility and promoted its ionic conductivity. The LEC using MTEO-PPV and Li triflate blend as active material without any extra supporting polymer electrolyte (e.g. poly(ethylene oxide), PEO) turned
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G. He et al. / Displays 21 (2000) 69–72
Acknowledgements This work was supported by NSFC (No. 29673051, 29992530) and the Chinese Academy of Sciences (No. KJ951-A1-501).
References
Fig. 5. Current–voltage and light–voltage characteristics of an ITO/ MTEO-PPV/Al single layer LED device.
on at a bias about 2.8 V, which was much lower than that of an LED using MTEO-PPV as active material only.
[1] Q. Pei, G. Yu, C. Zhang, Y. Yang, A.J. Heeger, Science 269 (1995) 1086–1088. [2] Q. Pei, Y. Yang, G. Yu, C. Zhang, A.J. Heeger, J. Am. Chem. Soc. 118 (1996) 3922–3929. [3] Y. Yang, Q. Pei, J. Appl. Phys. 81 (1997) 3294–3298. [4] R.O. Garay, B. Mayer, F.E. Karasz, R.W. Lenz, J. Polym. Sci., Part A: Polym. Chem. 33 (1995) 525–531. [5] I.B. Berlman, Handbook of Fluorescence Spectra of Aromatic Molecules, Academic Press, New York, 1971. [6] S. Doi, M. Kuwabara, T. Noguchi, T. Ohnishi, Synth. Met. 55–57 (1996) 4174–4179. [7] Y. Li, Y. Cao, J. Gao, D. Wang, G. Yu, A.J. Heeger, Synth. Met. 99 (1999) 243–248.
Light-emitting electrochemical cells based on poly(2-methoxy-5-triethoxy-1,4-phenylene vinylene) Gufeng He, Chunhe Yang, Rongqiu Wang, Yongfang Li* Laboratory of Organic Solids, Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China
Abstract A luminescent polymer with a PEO-like side chain, poly(2-methoxy-5-triethoxy-1,4-phenylene vinylene)(MTEO-PPV), was synthesized and characterized by UV–Vis spectra, photoluminescence (PL) spectra and cyclic voltammetry. The relatively high molecular weight Mn 77; 000 permitted the fabrication of good quality films. Its PL relative quantum yield can reach 38% in chloroform solution. Single layer light-emitting electrochemical cell (LEC) devices were fabricated using MTEO-PPV and lithium triflate as the emissive layer, ITO and Al as the electrodes. The devices were turned on at about 2.8 V, while that without lithium triflate was turned on at about 9 V, and the brightness increased significantly in contrast to the latter. 䉷 2000 Elsevier Science B.V. All rights reserved. Keywords: MTEO-PPV; Luminescence; Light-emitting electrochemical cell
1. Introduction Light-emitting electrochemical cells (LECs) [1,2] were reported first in 1995 using the blends of poly(1,4-phenylene vinylene)(PPV) and poly(ethylene oxide)(PEO) complexed with lithium trifluoromethanesulfonate as the active materials. PPV is a luminescent polymer that can be both n- and p-type electrochemically doped, while PEO complexed with lithium salt is a polymer electrolyte with relatively high ionic conductivity. When a voltage greater than Eg/e, where Eg is the p–p ⴱ energy gap of the polymer and e is the electronic charge, is applied, PPV near the anode is oxidized and p-type carriers (holes) are introduced, whereas PPV near the cathode is reduced and n-type carriers (electrons) are introduced. Counterions from the electrolytes move to compensate the charges on the oxidized and reduced polymer chains. Under the influence of the applied voltage, the holes in the PPV propagate from anode towards the cathode, and the electrons propagate from the cathode towards the anode. These holes and electrons meet in the compensated region between the n- and p-type-doped regions. Holes and electrons recombine to give out light. According to this mechanism, LECs have some advantages [3] such as low operating voltage, high quantum efficiency, high power efficiency, and so on. Because the metal/polymer contacts in the LEC are ohmic, they are * Corresponding author. Tel.: ⫹ 86-10-6256-3063; fax: ⫹ 86-10-62569564. E-mail address: [email protected] (Y. Li).
not sensitive to the work function of the metal electrodes. So air-stable electrodes can be used in LECs. However, in the active layer, the PEO–salt complex is very polar and its dissolution requires the use of a polar solvent, while most of the soluble luminescent conjugated polymers such as PPV, MEH-PPV are non-polar. Hence, it is difficult to find a solvent to dissolve these polymers simultaneously, so that phase separation often occurs. This can degrade the device performance significantly. In order to solve the phase separation problem, we synthesized a new PPV derivative with PEO-like units in phenylene moiety. These side groups can not only reduce or completely eliminate the use of PEO but also increase the solubility of the polymer in common organic solvents.
2. Experimental The synthesis of poly(2-methoxy-5-triethoxy-1,4-phenylene vinylene) (MTEO-PPV) is outlined in Fig. 1. The PEOlike units were attached to the benzene ring via Williamson ether formation between the triethoxy-p-toluenesulfonate and 4-methoxy phenol. Chloromethylation of the aromatic ethers with HCl in the presence of formaldehyde led to the bis-chloromethylated compounds [1,2]. Polymerization of the monomer by base-promoted dehydrohalogenation [4] using NaH in DMF gave polymers [1–3] with reasonable molecular weights, which were soluble in common organic solvents. Gel permeation chromatography (GPC) was recorded on
0141-9382/00/$ - see front matter 䉷 2000 Elsevier Science B.V. All rights reserved. PII: S0141-938 2(00)00042-1
70
G. He et al. / Displays 21 (2000) 69–72
Fig. 1. Synthetic route to MTEO-PPV.
a Waters Associates GPC system, using THF as the eluent and a low-dispersity polystyrene standards. The number average molecular weight is about 77,000, which corresponds to 290 repeating units per polymer chain. Ultra-
violet–Visible (UV–Vis) measurements were made on Hitachi U-3010 spectrophotometer. PL spectroscopy was performed on Hitachi F-4500 fluorescence spectrophotometer. PL efficiency in CHCl3 was determined with 9,10diphenylanthracene as in Ref. [5]. Cyclic voltammetry were measured on EG & G PARC 370 electrochemical system, using a Pt disk as working electrode (0.785 mm 2), a Pt wire as a counter electrode and an Ag wire as quasi-reference electrode. The MTEO-PPV/working electrode was produced by simply casting its solution in chloroform on the electrode and then letting the solvent to evaporate to leave a thin film. 0.1 M tetrabutylammonium hexafluorophosphate (Bu4NPF6) dissolved in acetonitrile was employed as the supporting electrolyte and the entire system was degassed with argon for 10 min. Single layer LEC devices were fabricated on indium–tin oxide (ITO) coated glass substrate. Thin films were spin cast from a mixture solution of MTEO-PPV and lithium triflate using chloroform as the solvent. The weight ratio of lithium salt to MTEO-PPV was 0.18. Then these thin films were heated at 60–70⬚C for a few hours on a hot plate to remove the residual solvent completely. An aluminum layer was evaporated onto the film at pressure around 6 × 10⫺6 Torr and used as a back electrode. Typical active areas were between 10 and 14 mm 2. The I–V characteristics were recorded with an Advantest R8340 ultra high resistance meter, and a calibrated photodiode was used for light intensity measurements. All measurements were controlled by an NEC computer and were performed in air at room temperature.
3. Results and discussion
Fig. 2. (a) UV–Vis spectra of solution (dot line) and film (solid line) of MTEO-PPV. (b) PL spectra of solution (dot line) and film (solid line) of MTEO-PPV.
Fig. 2a shows the optical absorption of a solution of MTEO-PPV in chloroform (1 mg/100 ml) and a thin film spin-cast from a concentrated solution (10 mg/ml). The absorption spectrum of the dilute solution has a peak at
G. He et al. / Displays 21 (2000) 69–72
Fig. 3. Cyclic voltammogram of a cast film of MTEO-PPV on Pt disk (0.1 M Bu4PF6/CH3CN) at a scan rate of 20 mV/s.
463 nm, with an onset of absorption at about 540 nm. The spectrum of the thin film has the same peak wavelength, but the peak is broader. This means there is no solvatochromism for MTEO-PPV. In contrast, large solvatochromism has been observed for some soluble conjugated polymers such as poly(3-alkylthiophene) [6]. It is well known that an absorption spectrum of a conjugated polymer depends on its molecular conformation due to changing conjugation length of the polymer. No solvatochroism implies that backbone structure of MTEO-PPV is more rigid than that of poly(3-alkylthiophene) due to good coplanarity of a phenyl ring and a vinylene group. Hence MTEO-PPV have the similar conformation in both the solution and the film. The photoluminescent (PL), spectra of the dilute solution and the film of MTEO-PPV pumped by UV light l 365 nm are displayed in Fig. 2b. The relative quantum yield was measured to be 38%, comparable to many conjugated polymers in dilute solution. There is a large red shift
71
in the PL spectra from solution (540 nm) to film (580 nm) on the contrary to no shift for absorption spectra. The shift may be not attributed to the change of conjugation length but the difference in energy transfer process between the film and the solution [6]. The energy-transfer process takes place more easily in the film than the solution and exciton migrates to the emission center having smaller energy with longer conjugation sequence in the molecules. Fig. 3 shows the cyclic voltammogram of MTEO-PPV. It is seen in the figure that the polymer has good redox reversibility. The polymer can be repeatedly ‘doped–dedoped’ for many cycles without significant changes in its oxidation and reduction peak characteristics. The onset potentials of oxidation and reduction of MTEO-PPV have been determined as ⫹0.67 and ⫺1.40 V vs. Ag wire, respectively. The oxidation process corresponds to the removal of charge from the HOMO energy level whereas the reduction cycle corresponds to the electron addition to the LUMO. So the difference between the two onset potentials can give the bandgap of the polymer [7]. The separation between the two onset potentials of MTEO-PPV is 2.07 V. This value is comparable to the energy corresponding to the onset of the optical absorption of MTEO-PPV film cast from its solution. The LEC was fabricated by sandwiching a thin film of a blend of MTEO-PPV and lithium triflate between an ITO coated glass and a vacuum-evaporated aluminum film. In contrast, an LED device with the same structure was fabricated too, but the lithium triflate was removed from the active layer. For convenience, ITO was connected to the anode and Al was connected to the cathode though LEC has nearly antisymmetric I–V characteristics. When an external voltage above the threshold was applied to the LEC device, orange light emitted from the polymer layer. Fig. 4 shows the I–V and L–V characteristics of an ITO/ MTEO-PPV ⫹ LiCF3SO3/Al device under forward bias. The threshold voltage for current is about 2.4 V but for light intensity it is about 3 V. This hysteresis should be attributed to the sensitivity of the luminescence meter. In fact, faint light could be seen with naked eyes at the bias of 2.6 V. However, the onset potential is up to about 9 V (Fig. 5) for the single layer LED having the same structure. The decrease in the turn-on voltage in the LEC is considered as due to the presence of Li salt. The PEO-like units on the phenylene moiety can supply enough ionic conductivity necessary for the operation of LECs.
4. Conclusions
Fig. 4. Current–voltage and light–voltage characteristics of an ITO/ MTEO-PPV ⫹ LiCF3SO3/Al LEC device.
A highly soluble polymer, MTEO-PPV, was synthesized using the dehydrochlorination route. The PEO-like units on the PPV skeleton increased its solubility and promoted its ionic conductivity. The LEC using MTEO-PPV and Li triflate blend as active material without any extra supporting polymer electrolyte (e.g. poly(ethylene oxide), PEO) turned
72
G. He et al. / Displays 21 (2000) 69–72
Acknowledgements This work was supported by NSFC (No. 29673051, 29992530) and the Chinese Academy of Sciences (No. KJ951-A1-501).
References
Fig. 5. Current–voltage and light–voltage characteristics of an ITO/ MTEO-PPV/Al single layer LED device.
on at a bias about 2.8 V, which was much lower than that of an LED using MTEO-PPV as active material only.
[1] Q. Pei, G. Yu, C. Zhang, Y. Yang, A.J. Heeger, Science 269 (1995) 1086–1088. [2] Q. Pei, Y. Yang, G. Yu, C. Zhang, A.J. Heeger, J. Am. Chem. Soc. 118 (1996) 3922–3929. [3] Y. Yang, Q. Pei, J. Appl. Phys. 81 (1997) 3294–3298. [4] R.O. Garay, B. Mayer, F.E. Karasz, R.W. Lenz, J. Polym. Sci., Part A: Polym. Chem. 33 (1995) 525–531. [5] I.B. Berlman, Handbook of Fluorescence Spectra of Aromatic Molecules, Academic Press, New York, 1971. [6] S. Doi, M. Kuwabara, T. Noguchi, T. Ohnishi, Synth. Met. 55–57 (1996) 4174–4179. [7] Y. Li, Y. Cao, J. Gao, D. Wang, G. Yu, A.J. Heeger, Synth. Met. 99 (1999) 243–248.