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Novel poly(arylene vinylene)s carrying donor and acceptor substituents
ELSEVIER
Synthetic Metals 76 (1996) 165-167
Novel poly ( arylene vinylene) s carrying donor and acceptor substituents Andrew C. Grimsdale a~b , Franc0 Cacialli ‘, Johannes Griiner ‘, Xiao-Chang Li a, Andrew B. Holmes aPb7*,Stephen C. Moratti b, Richard H. Friend ’ a University Chemical Laboratory, Department of Chemistyv, University of Cambridge, Lensfield Road, Cambridge CB2 IEF~ UK ’ Melville Laboratory for Polymer Synthesis, Department of Chemistry, University of Cambridge, Pembroke Street, Cambridge CB2 3R.4, UK ’ Cavendish Laboratory, Department of Physics, University of Cambridge, Madingley Road, Cambridge CB3 OHE, UK
Abstract New electroluminescent poly(arylene vinylene)s 1 containingelectron-withdrawing/donating substituents havebeensynthesizedby the precursorroute outlinedbelow.The synthesisof two novel polymersandthe evaluationof their photoluminescent andelectroluminescent propertiesin polymerlight-emittingdiodes(LEDs) are reported. Keywords:
Poly(arylene vinylene) ; Electroluminescence; Diodes
1. Introduction Poly (phenylene vinylene) (PPV) and its derivatives have been the subject of much investigation into their use as emissive layers in light-emitting diodes (LEDs) [ 11. A major problem with PPV is that due to its high LUMO energy it is a poor electron acceptor, and construction of an efficient device thus requires the use of a metal with a low work function such as calcium for electron injection [ 21, which is disadvantageous due to its high susceptibility to atmospheric degradation. The use of a more stable higher work function metal such as aluminium is highly desirable especially for commercial applications, but the internal quantum efficiency of PPV devices with indium-tin oxide (ITO) hole-injecting and aluminium electron-injecting electrodes is no more than 10w2% [ 3,4]. Calculations [ 51 have shown that introduction of an electron-withdrawing group onto either the aryl ring or the vinyl group of PPV would lower the HOMO and LUMO energies of the polymer, thus permitting the construction of a device utilizing a higher work function metal. We have used a PPV derivative with a cyano substituent on the vinyl group to make a bilayer device with PPV as a hole-transporting layer which shows an efficiency of 4% with aluminium as electrode [ 61. To date, although a number of PPV derivatives have been prepared with electron-withdrawing halide [7-91 and cyano [ lo] substituents on the aryl ring, there has been no report of their electroluminescence *Corresponding author. Tel.: +44 1223 334370; fax: t44 334 866; e-mail: [email protected].
1223
0379-6779/96/$15.000 1996ElsevierScienceS.A.All rights reserved
(EL). We now report the synthesis by a novel route of two PPV derivatives with an electron-withdrawing trifluoromethy1 group on the aryl ring, and an evaluation of their photoluminescent and electroluminescent properties in polymer LEDs.
2. Discussion Bis(bromomethyI)trifluoromethylbenzenes were prepared from bromo-xylenes 2 as shown in Scheme 1 and were treated with one equivalent of base to give the soluble (chloroform, THF, toluene) bromo-precursor polymers 3, which could be spin-coated onto substrates and thermally converted in vacuum ( 180 “C at 10m5 mbar for 4 h) to give films of the insoluble conjugated polymers 1. The best films were obtained by spin-coating (2000 rpm) from toluene. The weight-averaged molar masses (M,,,) obtained by gel permeation chromatography (gpc, against polystyrene standards) for the precursor polymers were 84 kDa (A&,/ I& = 5.3) and 252 kDa (MJM, = 3.5) for 3a and 3b, respectively. This is the first time that a bromo-precursor route has been used to prepare PPV derivatives, though a similar chloro-precursor route has been used [ 11,121 to make alkyland aryl-substituted PPV derivatives, with thermal conversion of the precursors to the conjugated polymers at 290-300 “C. The polymer la has been previously prepared with low molar mass (J/l, = 3.5 kDa) by Heck coupling of 2-trifluoromethyl-1,4-dibromobenzene with ethylene. The polymer
166
A.C. Grirnsdale
et al. /Synthetic
R
Metals
76 (1996)
165467
R
R CFGOo’Na+,
cd,
NSS, ccl,,
NMP, 170 -2
II/
Bw4
Me
Br
CF3 WRXH
CF3
2a; Re H b; R 3 CCrHn
b;R=CCHn
3a; RaH b;R.OCHv
lr;R=H b;RIOClHl7
Scheme 1.
1.5 IO4
0.6
1.0 104
0.4
5.0 10’
02
0
0.0 100
2
1
3
4
5
6
7
Energy (eV)
Fig. 1. PL and absorption spectra of the trifluoromethyl-substituted PPV la and lb. 14
1
1
-1.4
1.6
1.6
2.0
2.2
2.4
2.6
2.6
Energy (eV)
Fig. 2. EL spectrum of a double-layer device ITO/PPV/lb/Ca.
was reported to be slightly soluble (less than 1 wt.%) in Nmethylpyrrolidinone at 150 “C [ 131. The optical properties of thin films of the polymers la and lb are reported in Fig. 1. We observe that the alkoxy group causes a small red shift of the absorption edge and of the emission features, consistent with the general behaviour observed in other PPV derivatives [ 2,14,1.5]. The absorption edges, at about 2.6 eV for lb and 2.8 eV for la, are at slightly higher energies with respect to fully converted para-linked PPV prepared from a tetrahydrothiophenium leaving-group precursor [ 11, for which the edge is at about 2.5 eV. Photoluminescence (PL) spectra, on the other hand, show the peaks of the emission at 2.39 and 2.21 eV for la and lb, respectively, with no evident sign of vibronic structure. Absolute PL quantum efficiencies, measured with an integrated
sphere [ 161 and excitation at 364 nm, were found to be 5 F 1% and 7 rt 1% for thin films of la and lb, respectively. Single-layer electroluminescent cells were prepared by spin-coating the precursors onto ITO-coated substrates and then converting as described above. Calcium or aluminium electrodes were thermally evaporated under vacuum. Doublelayer LEDs were prepared in the same way but by spincoating the precursors on the top of unsubstituted PPV, prepared according to the standard route [ 11, Both polymers la and lb gave non-uniform emission in single-layer structures, whereas we observed uniform emission in double-layer devices with an EL internal quantum efficiency of up to lo-‘% for the ITO/PPV/lb/Al structures, In Fig. 2 we report the emission spectrum of a doublelayer ITO/PPV/lb/Ca device where we observe that the maximum of the emission (about 2.1 eV) is slightly redshifted with respect to the PL. Replacement of aluminium with calcium did not produce significant increase in the efficiency of the cells, suggesting that the trifluoromethyl electron-withdrawing groups are effective in improving electron injection, The equivalent devices prepared by using polymer la in place of lb (alkoxy-substituted) gave poorer results, with a maximum efficiency of the order of 10w3%. Although the absolute figures for the EL efficiencies are in the same range (aluminium cathodes) or even an order of magnitude lower (calcium cathodes) than those reported for single-layer PPV structures [ 3,4], we observe that the PL efficiencies are much lower than those measured in unsubstituted PPV [ 161 where they are of the order of 27%. This indicates that charge-carrier balance in the double-layer device is improved compared to an ITO/PPV/Al structure and, therefore, an improvement of the performance with respect to PPV single-layer LEDs has to be expected upon optimization of the PL fluorescence yield of the trifluoromethyl polymers.
3. Conclusions The substitution of an electron-withdrawing trifluoromethy1 group onto the aryl ring of PPV gives fluorescent polymers with HOMO-LUMO gaps of size similar to that of PPV. These polymers are presumed to have greater electron affinity
A.C. Grimsdale
et al. /Synthetic
and therefore improve the electron injection in bilayer PPV LED devices with aluminium cathodes.
Acknowledgements
We thank the EPSRC and the European Commission (ESPRIT Project No. 8013 ‘LEDFOS’; Brite-EURAM Contract ‘POLYLED’, BRE2-CT93-0592) for financial support.
References [l] J.H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Mackay, R.H. Friend, P.L. Bum and A.B. Holmes,Narure, 347 ( 1990) 539. [2] D. Braun and A.J. Heeger, Appl. Phys. Lett., 58 (1991) 1982. [3] A.R. Brown, NC. Greenham, R.W. Gymer, K. Pichler, D.D.C. Bradley, R.H. Friend, P.L. Burn, A. Kraft and A.B. Holmes, in M. Aldissi (ed.), Intrinsically Conducting Polymers: An Emerging Technology, NATO ASI Series E, Kluwer, Dordrecht, 1993, pp. 87106.
Metals
76 (1996)
165-167
167
[4] D.D.C. Bradley, Synth. Met., 54, (1993) 401. [5] J.L. BrCdas and A.J. Heeger, Chem. Phys. Leti., 217 (1994) 507. [6] N.C. Greenham, S.C. Moratti, D.D.C. Bradley, R.H. Friend and A.B. Holmes, Nature, 365 (1993) 628. [7] R.K. McCoy, F.E. Karasz, A. Sarker and P.M. Lahti, Chetn. Mater., 3 (1991) 941. [8] C.J. Wung,K.-S. Lee, P.N.Prasad, J.-C.Kim, J.-I. JinandH.-K.Shim, Polymer, 33 (1992) 4145. [9] J.-I. Jin, J.-C. KimandH.-K. Shim, Macromolecules, 25 (1992) 5519. [lo] P.M. Lahti, A. Sarker, R.O. Garay, R.W. Lenz and F.E. Karasz, Polymer, 35 (1994) 1312. [ 111 W.J. Swatos and B. Gordon III, Polym. Prep., 31 (1990) 505. [ 121 B.R. Hsieh, H. Antoniadis, D.C. Bland and W.A. Feld, Adu. Mater., 7 (1995) 36. [ 131 A. Greiner, H. Martelock, A. Nell, N. Siegfried and W. Heitz, Polymer, 32 (1991) 1857. [14] D.A. Halliday, D.D.C. Bradley, P.L. Bum, R.H. Friend and A.B. Holmes, Synth. Met., 4143 (1991) 934. [15] P.L. Bum, A.B. Holmes, A. Kraft, D.D.C. Bradley, A.R. Brown, R.H. Friend and R.W. Gymer, Nature, 356 (1992) 47-49. [ 161 NC. Greenham, I.D.W. Samuel, G.R. Hayes, R.T. Phillips, Y.A.R.R. Kessener, S.C. Moratti, A.B. Holmes and R.H. Friend, Chem. Phys. Lett., 241 (1995) 89-96.
Synthetic Metals 76 (1996) 165-167
Novel poly ( arylene vinylene) s carrying donor and acceptor substituents Andrew C. Grimsdale a~b , Franc0 Cacialli ‘, Johannes Griiner ‘, Xiao-Chang Li a, Andrew B. Holmes aPb7*,Stephen C. Moratti b, Richard H. Friend ’ a University Chemical Laboratory, Department of Chemistyv, University of Cambridge, Lensfield Road, Cambridge CB2 IEF~ UK ’ Melville Laboratory for Polymer Synthesis, Department of Chemistry, University of Cambridge, Pembroke Street, Cambridge CB2 3R.4, UK ’ Cavendish Laboratory, Department of Physics, University of Cambridge, Madingley Road, Cambridge CB3 OHE, UK
Abstract New electroluminescent poly(arylene vinylene)s 1 containingelectron-withdrawing/donating substituents havebeensynthesizedby the precursorroute outlinedbelow.The synthesisof two novel polymersandthe evaluationof their photoluminescent andelectroluminescent propertiesin polymerlight-emittingdiodes(LEDs) are reported. Keywords:
Poly(arylene vinylene) ; Electroluminescence; Diodes
1. Introduction Poly (phenylene vinylene) (PPV) and its derivatives have been the subject of much investigation into their use as emissive layers in light-emitting diodes (LEDs) [ 11. A major problem with PPV is that due to its high LUMO energy it is a poor electron acceptor, and construction of an efficient device thus requires the use of a metal with a low work function such as calcium for electron injection [ 21, which is disadvantageous due to its high susceptibility to atmospheric degradation. The use of a more stable higher work function metal such as aluminium is highly desirable especially for commercial applications, but the internal quantum efficiency of PPV devices with indium-tin oxide (ITO) hole-injecting and aluminium electron-injecting electrodes is no more than 10w2% [ 3,4]. Calculations [ 51 have shown that introduction of an electron-withdrawing group onto either the aryl ring or the vinyl group of PPV would lower the HOMO and LUMO energies of the polymer, thus permitting the construction of a device utilizing a higher work function metal. We have used a PPV derivative with a cyano substituent on the vinyl group to make a bilayer device with PPV as a hole-transporting layer which shows an efficiency of 4% with aluminium as electrode [ 61. To date, although a number of PPV derivatives have been prepared with electron-withdrawing halide [7-91 and cyano [ lo] substituents on the aryl ring, there has been no report of their electroluminescence *Corresponding author. Tel.: +44 1223 334370; fax: t44 334 866; e-mail: [email protected].
1223
0379-6779/96/$15.000 1996ElsevierScienceS.A.All rights reserved
(EL). We now report the synthesis by a novel route of two PPV derivatives with an electron-withdrawing trifluoromethy1 group on the aryl ring, and an evaluation of their photoluminescent and electroluminescent properties in polymer LEDs.
2. Discussion Bis(bromomethyI)trifluoromethylbenzenes were prepared from bromo-xylenes 2 as shown in Scheme 1 and were treated with one equivalent of base to give the soluble (chloroform, THF, toluene) bromo-precursor polymers 3, which could be spin-coated onto substrates and thermally converted in vacuum ( 180 “C at 10m5 mbar for 4 h) to give films of the insoluble conjugated polymers 1. The best films were obtained by spin-coating (2000 rpm) from toluene. The weight-averaged molar masses (M,,,) obtained by gel permeation chromatography (gpc, against polystyrene standards) for the precursor polymers were 84 kDa (A&,/ I& = 5.3) and 252 kDa (MJM, = 3.5) for 3a and 3b, respectively. This is the first time that a bromo-precursor route has been used to prepare PPV derivatives, though a similar chloro-precursor route has been used [ 11,121 to make alkyland aryl-substituted PPV derivatives, with thermal conversion of the precursors to the conjugated polymers at 290-300 “C. The polymer la has been previously prepared with low molar mass (J/l, = 3.5 kDa) by Heck coupling of 2-trifluoromethyl-1,4-dibromobenzene with ethylene. The polymer
166
A.C. Grirnsdale
et al. /Synthetic
R
Metals
76 (1996)
165467
R
R CFGOo’Na+,
cd,
NSS, ccl,,
NMP, 170 -2
II/
Bw4
Me
Br
CF3 WRXH
CF3
2a; Re H b; R 3 CCrHn
b;R=CCHn
3a; RaH b;R.OCHv
lr;R=H b;RIOClHl7
Scheme 1.
1.5 IO4
0.6
1.0 104
0.4
5.0 10’
02
0
0.0 100
2
1
3
4
5
6
7
Energy (eV)
Fig. 1. PL and absorption spectra of the trifluoromethyl-substituted PPV la and lb. 14
1
1
-1.4
1.6
1.6
2.0
2.2
2.4
2.6
2.6
Energy (eV)
Fig. 2. EL spectrum of a double-layer device ITO/PPV/lb/Ca.
was reported to be slightly soluble (less than 1 wt.%) in Nmethylpyrrolidinone at 150 “C [ 131. The optical properties of thin films of the polymers la and lb are reported in Fig. 1. We observe that the alkoxy group causes a small red shift of the absorption edge and of the emission features, consistent with the general behaviour observed in other PPV derivatives [ 2,14,1.5]. The absorption edges, at about 2.6 eV for lb and 2.8 eV for la, are at slightly higher energies with respect to fully converted para-linked PPV prepared from a tetrahydrothiophenium leaving-group precursor [ 11, for which the edge is at about 2.5 eV. Photoluminescence (PL) spectra, on the other hand, show the peaks of the emission at 2.39 and 2.21 eV for la and lb, respectively, with no evident sign of vibronic structure. Absolute PL quantum efficiencies, measured with an integrated
sphere [ 161 and excitation at 364 nm, were found to be 5 F 1% and 7 rt 1% for thin films of la and lb, respectively. Single-layer electroluminescent cells were prepared by spin-coating the precursors onto ITO-coated substrates and then converting as described above. Calcium or aluminium electrodes were thermally evaporated under vacuum. Doublelayer LEDs were prepared in the same way but by spincoating the precursors on the top of unsubstituted PPV, prepared according to the standard route [ 11, Both polymers la and lb gave non-uniform emission in single-layer structures, whereas we observed uniform emission in double-layer devices with an EL internal quantum efficiency of up to lo-‘% for the ITO/PPV/lb/Al structures, In Fig. 2 we report the emission spectrum of a doublelayer ITO/PPV/lb/Ca device where we observe that the maximum of the emission (about 2.1 eV) is slightly redshifted with respect to the PL. Replacement of aluminium with calcium did not produce significant increase in the efficiency of the cells, suggesting that the trifluoromethyl electron-withdrawing groups are effective in improving electron injection, The equivalent devices prepared by using polymer la in place of lb (alkoxy-substituted) gave poorer results, with a maximum efficiency of the order of 10w3%. Although the absolute figures for the EL efficiencies are in the same range (aluminium cathodes) or even an order of magnitude lower (calcium cathodes) than those reported for single-layer PPV structures [ 3,4], we observe that the PL efficiencies are much lower than those measured in unsubstituted PPV [ 161 where they are of the order of 27%. This indicates that charge-carrier balance in the double-layer device is improved compared to an ITO/PPV/Al structure and, therefore, an improvement of the performance with respect to PPV single-layer LEDs has to be expected upon optimization of the PL fluorescence yield of the trifluoromethyl polymers.
3. Conclusions The substitution of an electron-withdrawing trifluoromethy1 group onto the aryl ring of PPV gives fluorescent polymers with HOMO-LUMO gaps of size similar to that of PPV. These polymers are presumed to have greater electron affinity
A.C. Grimsdale
et al. /Synthetic
and therefore improve the electron injection in bilayer PPV LED devices with aluminium cathodes.
Acknowledgements
We thank the EPSRC and the European Commission (ESPRIT Project No. 8013 ‘LEDFOS’; Brite-EURAM Contract ‘POLYLED’, BRE2-CT93-0592) for financial support.
References [l] J.H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Mackay, R.H. Friend, P.L. Bum and A.B. Holmes,Narure, 347 ( 1990) 539. [2] D. Braun and A.J. Heeger, Appl. Phys. Lett., 58 (1991) 1982. [3] A.R. Brown, NC. Greenham, R.W. Gymer, K. Pichler, D.D.C. Bradley, R.H. Friend, P.L. Burn, A. Kraft and A.B. Holmes, in M. Aldissi (ed.), Intrinsically Conducting Polymers: An Emerging Technology, NATO ASI Series E, Kluwer, Dordrecht, 1993, pp. 87106.
Metals
76 (1996)
165-167
167
[4] D.D.C. Bradley, Synth. Met., 54, (1993) 401. [5] J.L. BrCdas and A.J. Heeger, Chem. Phys. Leti., 217 (1994) 507. [6] N.C. Greenham, S.C. Moratti, D.D.C. Bradley, R.H. Friend and A.B. Holmes, Nature, 365 (1993) 628. [7] R.K. McCoy, F.E. Karasz, A. Sarker and P.M. Lahti, Chetn. Mater., 3 (1991) 941. [8] C.J. Wung,K.-S. Lee, P.N.Prasad, J.-C.Kim, J.-I. JinandH.-K.Shim, Polymer, 33 (1992) 4145. [9] J.-I. Jin, J.-C. KimandH.-K. Shim, Macromolecules, 25 (1992) 5519. [lo] P.M. Lahti, A. Sarker, R.O. Garay, R.W. Lenz and F.E. Karasz, Polymer, 35 (1994) 1312. [ 111 W.J. Swatos and B. Gordon III, Polym. Prep., 31 (1990) 505. [ 121 B.R. Hsieh, H. Antoniadis, D.C. Bland and W.A. Feld, Adu. Mater., 7 (1995) 36. [ 131 A. Greiner, H. Martelock, A. Nell, N. Siegfried and W. Heitz, Polymer, 32 (1991) 1857. [14] D.A. Halliday, D.D.C. Bradley, P.L. Bum, R.H. Friend and A.B. Holmes, Synth. Met., 4143 (1991) 934. [15] P.L. Bum, A.B. Holmes, A. Kraft, D.D.C. Bradley, A.R. Brown, R.H. Friend and R.W. Gymer, Nature, 356 (1992) 47-49. [ 161 NC. Greenham, I.D.W. Samuel, G.R. Hayes, R.T. Phillips, Y.A.R.R. Kessener, S.C. Moratti, A.B. Holmes and R.H. Friend, Chem. Phys. Lett., 241 (1995) 89-96.