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Green electroluminescent characteristics of poly (2-dimethyloctylsilyl-1,4-phenylenevinylene)
Synthetic Metals 84 (1997) 655656
ELSEVIER
Green electroluminescent characteristics of poly(2-dimethyloctylsilyl1,4-phenylenevinylene) ST. Kimfayb, D.H. Hwangc, A.B. Holmesa*c, R.H. Friendb and H.K. Shimd Laboratory for Polymer Synthesis, Department of Chemistv, Pembroke Street, Cambridge CB2 3RA, UK bCavendish Laboratory, Department of Physics, Madingley Road, Cambridge CB3 OHE, UK ‘University Chemical Laboratory, Department of Chemistry Lens$eld Road, Cambridge CB2 IEW, UK dDepartment of Chemistry, Korea Advanced Institute of Science and Technology, Taejon 305-701, South Korea
aMelville
Abstract We report a novel solution-processible polymer, poly(2-dimethyloctylsilyl-1,4-phenylenevinylene) (DMOS-PPV) which shows a very high photoluminescence efficiency (60% for thin solid films). With the use of aluminium as cathode, an internal quantum efficiency as high as 2% has been obtained from bilayer diodes. The photoluminescence excitation measurement on a pristine film suggests constant efficiency for singlet intrachain exciton generation. Keywords:
1.
poly(phenylenevinylene),
light sources, electroluminescence,
Introduction
We report here the use of a new derivative of poly@phenylenevinylene) (PPV) [l], poly(2-dimethyloctylsilyl1,4-phenylenevinylene) (DMOS-PPV) which is soluble in common organic solvents. Single-layer light emitting diodes (LEDs) using DMOS-PPV yielded a quantum efficiency as high as 0.2 % when made with an air-stable electrode of aluminium as cathode. A number of previous reports have confirmed the enhancement of polymer emission efficiency through the incorporation of an electron conducting and hole blocking (ECHB) material [2]. In this work, we have fabricated bilayer devices using the emissive layer of DMOS-PPV and the ECHB layer of 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4oxadiazole (PBD). Photo-oxidation of DMOS-PPV has been studied using photoluminescence excitation (PLE) and ultraviolet-visible (UV-vis) absorption. 2.
silyl
was measured with a commercial Perkin-Elmer 19 spectrometer. The data were not corrected for reflectivity. DMOS-PPV was dissolved in chloroform. PBD was dispersed in polystyrene (PS) in chloroform. A PBD solution without PS was also prepared. For the fabrication of bilayer LEDs, the DMOS-PPV layer (70-100 nm thick) was spin-cast on top of the anodic electrode, iridium tin oxide (ITO) on glass, and spin-casting of PBD (30-40 run) followed. The active emissive area defined by cathode was about 4 mm2. 3.
Results
and
Discussion
Films of DMOS-PPV were found to be highly fluorescent, with the PL quantum efficiency exceeding 60 %.
Experimental
The photo-oxidation of a DMOS-PPV film was studied by PLE and UV-VIS absorption. Spectra of PLE and UV-VIS absorption were measured at ambient temperature first on a pristine DMOS-PPV film on quartz and then on the same film oxidised for progressively longer time. The photo-oxidation was performed by exposing the film, in air, to the Hg line of 440 run at the intensity of about 30 mW/cm2. PLE spectra were measured with a monochromated Xenon lamp as the excitation source. The incident light of 150 W was mechanically chopped and the signal was detected with a photodiode and a lock-in amplifier. An optical filter was used to block stray excitation and only PL wavelengths longer than 550 nm were detected. The PLE results were corrected for incident photon flux, but not for reflection. Instead, a film thicker than 2 l.trn was used (optical density greater than 2 over the whole excitation range). The UV-VIS absorption spectrum 0379-6779/97/$17.00 0 1997 Else&r Science S.A. All rights reserved PII
photoluminescence,
s0379-6779(96)04094-5
2
2.6
3
3.5
Energy
4
4.5
5
(eV)
Fig. 1. PLE spectra of a thick DMOS-PPV
film
The EL spectrum for the 2-layer diode basically duplicated the PL spectrum of a DMOS-PPV film. The PL spectra taken as a function of temperature showed a red shift as the temperature decreased from 300 K to 23 K. The red shift seems to be associated with the suppression of torsional modes which tend
656
XT. Kim et al. /Synthetic
to reduce the coplanarity of the phenylene rings along the chain and hence decrease the x and n* bandwidths [3].
t...........
2
2.5
3
..s....*....i
3.5 4 Energy (eV)
4.5
5
Fig. 2. UV-vis absorption spectra of a thick DMOS-PPV
film
As seen in Fig. 1, the PLE spectrum of a pristine DMOSPPV film is broadly flat after the absorption onset at about 2.5 eV. The flat spectrum, also observed in PPV4 and cyanoderivative of PPV [5], indicates that the generation efficiency for singlet excitations is constant for the given excitation range and accordingly the branching ratio for producing intrachain singlet excitations from higher energy photoexcited states is also constant over this excitation range. The intensity and shape of the spectrum change for different levels of oxidation, which suggests that the PL efficiency of a photo-oxidised sample varies as the excitation energy. As seen in Fig. 2, the optical absorption of a DMOSPPV film was progressively bleached out in the range 2.5-4 eV upon each exposure, but negligibly changed beyond 4 eV. As for PPV, photo-generated carbonyl defects are most probably responsible for the spectral change observed. While the overall trend of spectral changes looks similar for both PPV and DMOS-PPV, the latter appears to be bleached more easily than the former. The PL spectra of DMOS-PPV films exhibited wellresolved vibronic features, which we considered might be attributed to possible structural order in the film. Transmission electron microscopy and X-ray diffraction analysis did not reveal any noticeable crystalline features on a spin-cast film, possibly due to the random orientation inherent in a spin-cast film. Features well-aligned along the flow direction were however observed, by polarised optical microscopy, on a thin film prepared by the drop-casting and subsequent downward-flow of DMOS-PPV solution on a glass substrate. The tendency to ordering is likely to be manifested as a short-range order in a spin-cast film, and we conjecture
2 1 z P E 0 0.1 E E 0, -I 0.01
. ITODUOS-PPVIPBDIAI
n=
..
,PD
.
.
Metals
4.
Acknowledgements We thank the Engineering and Physical Sciences Research. Council (UK), the Korean Science and Engineering Foundation, the British Council (DHH), the European Commission (ESPRIT project 8013 ‘LEDFOS’, Brite-Euram BRE2-CT93-0592 ‘PolyLED’) and LG Electronics (STK) for financial support, We thank Dr S C Moratti, N.T. Harrison, D.S. Thomas and A. Koch for their interest in this work, and J.W. Lee, J.S. Lee, H.J. Kwon and Y.W. Jung at the Analytical Laboratory of LG-ERC for the surface and structural characterisations. References [l] [2]
l
’ ITO/DMOS-PPWAI
[3]
. 0.001 Currant
0.01 density
(rrdz2)
Fig. 3. Light intensity vs. current characteristic that the relatively orderly molecular arrangement of DMOSPPV is partly responsible for a high PL efficiency of 60 % and
Conclusions
We report here the properties of a new, solutionprocessible derivative of PPV, which is remarkable in that it shows a very high photoluminescence efficiency of 60%. We find that this polymer can be used to make highly efficient green electroluminescent diodes; single-layer diodes made with aluminium cathodes show efficiencies of 0.2 %. Efficiencies are further improved by the introduction of an oxadiazole-based hole-blocking layer. The PLE measurement on a pristine DMOS-PPV film suggests constant efficiency for singlet intrachain exciton generation.
.
D
655-656
a high EL efficiency observed. In addition, the high EL efficiency could be attributed to a good morphology as observed by atomic force microscopy. The efficiency of the single-layer DMOS-PPV devices was increased by incorporating a thin layer of PBD(PS) between the emissive DMOS-PPV layer and the cathode. With an Al cathode, an internal efficiency of 0.5-0.8 % has been reproducibly obtained, and we have obtained higher efficiencies of up to 2% from the LEDs fabricated with the PBD solution without PS. We have previously shown [2] that the efficiency of an ITOIPPVICa device can be substantially enhanced with the addition of a PBD layer, and have accounted for this behaviour in terms of the blocking of hole current at the PPV/PBD interface. The luminance vs. current characteristic presented in Fig. 3 shows clearly that the higher emission efficiency for ITO/DMOS-PPV/PBD(PS)/AL over ITO/DMOS-PPV/Al is mainly due to the reduction of current density. We consider that better charge injection and transport capability appear to be responsible for higher efficiency obtained with the pure PBD layer over the PBD(PS) layer.
l
.
84 (I997/
[4] [5]
J. H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Mackay, R.H. Friend, P.L. Burn, and A.B. Holmes, Nature , 347 (1990) 539. A.R. Brown, D.D.C. Bradley, J.H. Burroughes, R.H. Friend, NC. Greenham, P.L. Burn, A.B. Holmes and A. Kraft, Appl. Phys. Left., 61 (1992) 2793. K. Pichler, D.A. Halliday, D.D.C. Bradley, P.L. Burn, R.H. Friend and A.B. Holmes, J. Phys: Condens. Matter, 5 (1993) 7155. N.T. Harrison, G.R. Hayes, R.T. Phillips and R.H. Friend (manuscript in preparation). N.T. Harrison, D.R. Baigent, J.J. M. Halls, K. Pichler and R.H. Friend, Synth. Met., 76 (1996) 43.
TPermanent address: LG Electronics Research Center, Woomyeon-dong, Seocho-gu, Seoul 137-140, Korea
16
ELSEVIER
Green electroluminescent characteristics of poly(2-dimethyloctylsilyl1,4-phenylenevinylene) ST. Kimfayb, D.H. Hwangc, A.B. Holmesa*c, R.H. Friendb and H.K. Shimd Laboratory for Polymer Synthesis, Department of Chemistv, Pembroke Street, Cambridge CB2 3RA, UK bCavendish Laboratory, Department of Physics, Madingley Road, Cambridge CB3 OHE, UK ‘University Chemical Laboratory, Department of Chemistry Lens$eld Road, Cambridge CB2 IEW, UK dDepartment of Chemistry, Korea Advanced Institute of Science and Technology, Taejon 305-701, South Korea
aMelville
Abstract We report a novel solution-processible polymer, poly(2-dimethyloctylsilyl-1,4-phenylenevinylene) (DMOS-PPV) which shows a very high photoluminescence efficiency (60% for thin solid films). With the use of aluminium as cathode, an internal quantum efficiency as high as 2% has been obtained from bilayer diodes. The photoluminescence excitation measurement on a pristine film suggests constant efficiency for singlet intrachain exciton generation. Keywords:
1.
poly(phenylenevinylene),
light sources, electroluminescence,
Introduction
We report here the use of a new derivative of poly@phenylenevinylene) (PPV) [l], poly(2-dimethyloctylsilyl1,4-phenylenevinylene) (DMOS-PPV) which is soluble in common organic solvents. Single-layer light emitting diodes (LEDs) using DMOS-PPV yielded a quantum efficiency as high as 0.2 % when made with an air-stable electrode of aluminium as cathode. A number of previous reports have confirmed the enhancement of polymer emission efficiency through the incorporation of an electron conducting and hole blocking (ECHB) material [2]. In this work, we have fabricated bilayer devices using the emissive layer of DMOS-PPV and the ECHB layer of 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4oxadiazole (PBD). Photo-oxidation of DMOS-PPV has been studied using photoluminescence excitation (PLE) and ultraviolet-visible (UV-vis) absorption. 2.
silyl
was measured with a commercial Perkin-Elmer 19 spectrometer. The data were not corrected for reflectivity. DMOS-PPV was dissolved in chloroform. PBD was dispersed in polystyrene (PS) in chloroform. A PBD solution without PS was also prepared. For the fabrication of bilayer LEDs, the DMOS-PPV layer (70-100 nm thick) was spin-cast on top of the anodic electrode, iridium tin oxide (ITO) on glass, and spin-casting of PBD (30-40 run) followed. The active emissive area defined by cathode was about 4 mm2. 3.
Results
and
Discussion
Films of DMOS-PPV were found to be highly fluorescent, with the PL quantum efficiency exceeding 60 %.
Experimental
The photo-oxidation of a DMOS-PPV film was studied by PLE and UV-VIS absorption. Spectra of PLE and UV-VIS absorption were measured at ambient temperature first on a pristine DMOS-PPV film on quartz and then on the same film oxidised for progressively longer time. The photo-oxidation was performed by exposing the film, in air, to the Hg line of 440 run at the intensity of about 30 mW/cm2. PLE spectra were measured with a monochromated Xenon lamp as the excitation source. The incident light of 150 W was mechanically chopped and the signal was detected with a photodiode and a lock-in amplifier. An optical filter was used to block stray excitation and only PL wavelengths longer than 550 nm were detected. The PLE results were corrected for incident photon flux, but not for reflection. Instead, a film thicker than 2 l.trn was used (optical density greater than 2 over the whole excitation range). The UV-VIS absorption spectrum 0379-6779/97/$17.00 0 1997 Else&r Science S.A. All rights reserved PII
photoluminescence,
s0379-6779(96)04094-5
2
2.6
3
3.5
Energy
4
4.5
5
(eV)
Fig. 1. PLE spectra of a thick DMOS-PPV
film
The EL spectrum for the 2-layer diode basically duplicated the PL spectrum of a DMOS-PPV film. The PL spectra taken as a function of temperature showed a red shift as the temperature decreased from 300 K to 23 K. The red shift seems to be associated with the suppression of torsional modes which tend
656
XT. Kim et al. /Synthetic
to reduce the coplanarity of the phenylene rings along the chain and hence decrease the x and n* bandwidths [3].
t...........
2
2.5
3
..s....*....i
3.5 4 Energy (eV)
4.5
5
Fig. 2. UV-vis absorption spectra of a thick DMOS-PPV
film
As seen in Fig. 1, the PLE spectrum of a pristine DMOSPPV film is broadly flat after the absorption onset at about 2.5 eV. The flat spectrum, also observed in PPV4 and cyanoderivative of PPV [5], indicates that the generation efficiency for singlet excitations is constant for the given excitation range and accordingly the branching ratio for producing intrachain singlet excitations from higher energy photoexcited states is also constant over this excitation range. The intensity and shape of the spectrum change for different levels of oxidation, which suggests that the PL efficiency of a photo-oxidised sample varies as the excitation energy. As seen in Fig. 2, the optical absorption of a DMOSPPV film was progressively bleached out in the range 2.5-4 eV upon each exposure, but negligibly changed beyond 4 eV. As for PPV, photo-generated carbonyl defects are most probably responsible for the spectral change observed. While the overall trend of spectral changes looks similar for both PPV and DMOS-PPV, the latter appears to be bleached more easily than the former. The PL spectra of DMOS-PPV films exhibited wellresolved vibronic features, which we considered might be attributed to possible structural order in the film. Transmission electron microscopy and X-ray diffraction analysis did not reveal any noticeable crystalline features on a spin-cast film, possibly due to the random orientation inherent in a spin-cast film. Features well-aligned along the flow direction were however observed, by polarised optical microscopy, on a thin film prepared by the drop-casting and subsequent downward-flow of DMOS-PPV solution on a glass substrate. The tendency to ordering is likely to be manifested as a short-range order in a spin-cast film, and we conjecture
2 1 z P E 0 0.1 E E 0, -I 0.01
. ITODUOS-PPVIPBDIAI
n=
..
,PD
.
.
Metals
4.
Acknowledgements We thank the Engineering and Physical Sciences Research. Council (UK), the Korean Science and Engineering Foundation, the British Council (DHH), the European Commission (ESPRIT project 8013 ‘LEDFOS’, Brite-Euram BRE2-CT93-0592 ‘PolyLED’) and LG Electronics (STK) for financial support, We thank Dr S C Moratti, N.T. Harrison, D.S. Thomas and A. Koch for their interest in this work, and J.W. Lee, J.S. Lee, H.J. Kwon and Y.W. Jung at the Analytical Laboratory of LG-ERC for the surface and structural characterisations. References [l] [2]
l
’ ITO/DMOS-PPWAI
[3]
. 0.001 Currant
0.01 density
(rrdz2)
Fig. 3. Light intensity vs. current characteristic that the relatively orderly molecular arrangement of DMOSPPV is partly responsible for a high PL efficiency of 60 % and
Conclusions
We report here the properties of a new, solutionprocessible derivative of PPV, which is remarkable in that it shows a very high photoluminescence efficiency of 60%. We find that this polymer can be used to make highly efficient green electroluminescent diodes; single-layer diodes made with aluminium cathodes show efficiencies of 0.2 %. Efficiencies are further improved by the introduction of an oxadiazole-based hole-blocking layer. The PLE measurement on a pristine DMOS-PPV film suggests constant efficiency for singlet intrachain exciton generation.
.
D
655-656
a high EL efficiency observed. In addition, the high EL efficiency could be attributed to a good morphology as observed by atomic force microscopy. The efficiency of the single-layer DMOS-PPV devices was increased by incorporating a thin layer of PBD(PS) between the emissive DMOS-PPV layer and the cathode. With an Al cathode, an internal efficiency of 0.5-0.8 % has been reproducibly obtained, and we have obtained higher efficiencies of up to 2% from the LEDs fabricated with the PBD solution without PS. We have previously shown [2] that the efficiency of an ITOIPPVICa device can be substantially enhanced with the addition of a PBD layer, and have accounted for this behaviour in terms of the blocking of hole current at the PPV/PBD interface. The luminance vs. current characteristic presented in Fig. 3 shows clearly that the higher emission efficiency for ITO/DMOS-PPV/PBD(PS)/AL over ITO/DMOS-PPV/Al is mainly due to the reduction of current density. We consider that better charge injection and transport capability appear to be responsible for higher efficiency obtained with the pure PBD layer over the PBD(PS) layer.
l
.
84 (I997/
[4] [5]
J. H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Mackay, R.H. Friend, P.L. Burn, and A.B. Holmes, Nature , 347 (1990) 539. A.R. Brown, D.D.C. Bradley, J.H. Burroughes, R.H. Friend, NC. Greenham, P.L. Burn, A.B. Holmes and A. Kraft, Appl. Phys. Left., 61 (1992) 2793. K. Pichler, D.A. Halliday, D.D.C. Bradley, P.L. Burn, R.H. Friend and A.B. Holmes, J. Phys: Condens. Matter, 5 (1993) 7155. N.T. Harrison, G.R. Hayes, R.T. Phillips and R.H. Friend (manuscript in preparation). N.T. Harrison, D.R. Baigent, J.J. M. Halls, K. Pichler and R.H. Friend, Synth. Met., 76 (1996) 43.
TPermanent address: LG Electronics Research Center, Woomyeon-dong, Seocho-gu, Seoul 137-140, Korea
16