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Optical properties of substituted poly(paraphenylene)
ELSEVIER
Synthetic
Metals
84 (1997)
641642
Optical properties of substituted poly(paraphenylene) P. A. Lanea, M. Lie&, Z. V. Vardenya, M. Hamaguchib, M. Ozakib, and K. Yoshinob aUniversib of Utah, Salt Lake Cify, UT 84103 , USA bOsaka Universiv, 2-1 Yamada-Oka, Suita, Osaka 565, Japan
Abstract
The optical properties of substitutedPPP are described.The absorptionand electroabsorptionspectracontain a number of features, due to even and odd parity singlet excitons. The photoinducedabsorptionspectrumcontainsthree bands,which were attributedto chargedpolaronsandtriplet excitons,respectively. Optical absorption and emission spectroscopy,Electroabsorption,Photoinduced absorption spectroscopy, Other conjugatedand/orconductingpolymers Keywords:
1. Introduction
The high photoluminescencequantumyield of n-conjugated polymers has led to intensive investigations of conductingpolymerswith respectto their applicationsasactive elements in LEDs.l Poly(p-phenylene) (PPP) has drawn particular interest as blue electroluminescencehas been demonstrated2from PPP-baseddiodes. The band gap of unsubstitutedPPPhasbeenreportedto be between2.73 and3.0 eV.4 ChemicallysubstitutedPPPhasan advantagecomparedto unsubstitutedPPP as it is solublein organic solvents,which enableseasy fabrication of electronic devices. Interestingly, chemical substitutionalso resultsin an increaseof the band gap,to 3.4-3.5 eV.2J We have investigatedthe optical propertiesof poly(2,5dihelptyloxy-1,4-phenylene) (substitutedPPP) by absorption, electroabsorption(EA), and photoinduced absorption (PA) spectroscopies.The dominant excited states have been identified andare attributedto chargedpolaronsandsingletand triplet excitons. The PA spectrumof polaronsis distinguished from that of triplet excitonsby comparingthe PA of pure and C60-dopedPPP (C6O:PPP).We assignbandsat 0.75 and 2.5 eV, respectively, to polaronsand a band at 2 eV to triplet excitons. For EA spectroscopy,the sampleis depositedonto a transparentsubstratewith a seriesof interpenetratingmetal electrodes.An AC electric field is applied,and lo&in-detected changesin transmissionthroughthe samplearemonitoredat 2f, where f is the electric field modulation frequency. 037%‘-6?79/971$17.00
0 1997
PII SO379-6779(96)04087-8
Elsevier
Science
S.A. All rights
reserved
Photoinduced absorption (PA) spectroscopyuses standard phase-sensitive techniqueswith a modulatedAr+ laserbeamas a pump.PhotoinducedchangesAT in the sampletransmissionT are recordedto obtain the normalizedchangesin transmission (-AT / T e Aad, where d is the samplethicknessand ad its optical density). The use of lo&in-amplificationpermitssignal resolution T/T of betterthan 10’6. By useof multipledetectors and diffraction gratings,PA can be measuredin the spectral rangeof 0.1 to 2.4 eV.6 Both the pump andprobebeamswere modulatedto obtainthe PA spectrumof pure PPPabove2 eV. This schemewas used to avoid any thermal effects of the probebeamon the PL from affecting the spectrum.
2. Results
The absorptionandEA spectraof PPPare shownin Figure 1. Thereis an onsetof absorptionat about 2.7 eV and the fast dominant absorption peak occurs at 3.68 eV. Similar to substitutedPPVS,~the absorptionspectrumexhibits structureat higher energies,with featuresat appr. 4.5, 5.3 and 6 eV. The EA spectrum[Fig. l(b)] hasa sharpfeature at 3.6 eV, which is similar to the first derivative of the absorption spectrum shown as a dashedline in Fig. l(b). We W(ad)lW, thereforeattributethis featureto a Stark shift of the lowest odd parity (lB,) exciton. There is also a feature at 4.6 eV, which cannot be found by a derivative analysis of the absorption spectrumand must therefore be due to an even parity (mAg) exciton.
642
PA. Lane et al. /SyntheticA4etals
84 (1993
641-642
10’(a)PPP
P’
8 6 4 2 -
03
0 l,,,“,,,,““,“““,,‘,‘,,,: 3 l(b) C,,:PPP (x10)
1
21 0 d-Y
2 -
, id ,
--
1 -1 0 2
3
4
5
6
Energy (eV) Figure 1. (a) Absorption spectrum at 300K of substituted PPP film. (b) Electroabsorption spectrum at 8OK (solid line) and first derivative of absorption spectrum (dashed line).
The PA spectrum of pure PPP, shown in Fig. 2(a), consists of three broad PA bands at 0.8,2, and 2.5 eV, respectively. The different PA bands have fairly similar dependence on the modulation frequency and intensity, so it was not possible establish which bands were correlated with the same kinds of excitations. However, charge transfer in fullerene-polymer complexes has been well established8 and permits us to determine which excitations are associated with charged excitations. A solution of PPP was doped by C,, (25% by weight or 12 mol%) and evaporated onto a quartz substrate. The PA spectrum of C,,:PPP has a sharp feature at ml.15 eV due to C60-, indicating that charge transfer takes place in the composite sample. The PA band at 2.5 is enhanced by C,, doping. We therefore assign this band to the high energy PA band of charged polarons. As the PA spectroscopy of other conjugated polymers has shown that polarons have two transitions,g we therefore assign the PA band at 0.75 eV to the low energy PA band of polarons. Triplet excitons have been found to contribute to the PA of other conjugated polymers, leading us to suggest that the PA band at 2 eV may be due to triplet excitons.
/ 1111"'1"1111'1111'1111"111 0 0.5 1 1.5
2
2.5
Probe Energy (eV) Figure 2. PA spectra at SOK of (a) PPP and (b) Cso-doped PPP. In summary, we have characterized the optical properties of substituted PPP. Three PA bands were found, two of which were attributed to charged polarons and the other to triplet excitons. The absorption and EA spectra were explained as due to a variety of even and odd parity singlet excitons. Acknowledgements. The work at Utah was supported by the Dept. of Energy and ONK grant no. N00014-9-1-0853. l.J. H. Burroughes et al., Nutwe 347,539 (1990). 2M. Hamaguchiand K. Yoshino, Japan. J. Appl. Phys. (in press). 3G. Grem,G. Leditzky, B. Ullrich, andG. Leising,Adv. Muter. 4, 36 (1992). 4M. Tabata,M. Satoh,K. Kaneto, andK. Yoshino, J. P/I~s. C: SolidState Phys. 19,LlOl (1986). 5G.GremandG. Leising,Synth.Met. 55-57,4105(1993). 62. V. Vardeny and X. Wei, in “Handbook of Conducting PolymersII”, Marie1Dekker(in press). 7M. Chandross,S. Mazumdar, S. Jeglinski,X. Wei, T. Miller andZ. V. Vardeny,Phys. Rev. Lett. 50,14702 (1994). 8Y. Wang, Nature 356, 585 (1992); N. S. Sariciftci, L. Smilowitz, A. J. HeegerandF. Wudl, Science258,1474(1992). gP. A. Lane, X. Wei, and Z. V. Vardeny, Phys. Rev. Lett. (in press)andelsewhere in theseproceedings.
Synthetic
Metals
84 (1997)
641642
Optical properties of substituted poly(paraphenylene) P. A. Lanea, M. Lie&, Z. V. Vardenya, M. Hamaguchib, M. Ozakib, and K. Yoshinob aUniversib of Utah, Salt Lake Cify, UT 84103 , USA bOsaka Universiv, 2-1 Yamada-Oka, Suita, Osaka 565, Japan
Abstract
The optical properties of substitutedPPP are described.The absorptionand electroabsorptionspectracontain a number of features, due to even and odd parity singlet excitons. The photoinducedabsorptionspectrumcontainsthree bands,which were attributedto chargedpolaronsandtriplet excitons,respectively. Optical absorption and emission spectroscopy,Electroabsorption,Photoinduced absorption spectroscopy, Other conjugatedand/orconductingpolymers Keywords:
1. Introduction
The high photoluminescencequantumyield of n-conjugated polymers has led to intensive investigations of conductingpolymerswith respectto their applicationsasactive elements in LEDs.l Poly(p-phenylene) (PPP) has drawn particular interest as blue electroluminescencehas been demonstrated2from PPP-baseddiodes. The band gap of unsubstitutedPPPhasbeenreportedto be between2.73 and3.0 eV.4 ChemicallysubstitutedPPPhasan advantagecomparedto unsubstitutedPPP as it is solublein organic solvents,which enableseasy fabrication of electronic devices. Interestingly, chemical substitutionalso resultsin an increaseof the band gap,to 3.4-3.5 eV.2J We have investigatedthe optical propertiesof poly(2,5dihelptyloxy-1,4-phenylene) (substitutedPPP) by absorption, electroabsorption(EA), and photoinduced absorption (PA) spectroscopies.The dominant excited states have been identified andare attributedto chargedpolaronsandsingletand triplet excitons. The PA spectrumof polaronsis distinguished from that of triplet excitonsby comparingthe PA of pure and C60-dopedPPP (C6O:PPP).We assignbandsat 0.75 and 2.5 eV, respectively, to polaronsand a band at 2 eV to triplet excitons. For EA spectroscopy,the sampleis depositedonto a transparentsubstratewith a seriesof interpenetratingmetal electrodes.An AC electric field is applied,and lo&in-detected changesin transmissionthroughthe samplearemonitoredat 2f, where f is the electric field modulation frequency. 037%‘-6?79/971$17.00
0 1997
PII SO379-6779(96)04087-8
Elsevier
Science
S.A. All rights
reserved
Photoinduced absorption (PA) spectroscopyuses standard phase-sensitive techniqueswith a modulatedAr+ laserbeamas a pump.PhotoinducedchangesAT in the sampletransmissionT are recordedto obtain the normalizedchangesin transmission (-AT / T e Aad, where d is the samplethicknessand ad its optical density). The use of lo&in-amplificationpermitssignal resolution T/T of betterthan 10’6. By useof multipledetectors and diffraction gratings,PA can be measuredin the spectral rangeof 0.1 to 2.4 eV.6 Both the pump andprobebeamswere modulatedto obtainthe PA spectrumof pure PPPabove2 eV. This schemewas used to avoid any thermal effects of the probebeamon the PL from affecting the spectrum.
2. Results
The absorptionandEA spectraof PPPare shownin Figure 1. Thereis an onsetof absorptionat about 2.7 eV and the fast dominant absorption peak occurs at 3.68 eV. Similar to substitutedPPVS,~the absorptionspectrumexhibits structureat higher energies,with featuresat appr. 4.5, 5.3 and 6 eV. The EA spectrum[Fig. l(b)] hasa sharpfeature at 3.6 eV, which is similar to the first derivative of the absorption spectrum shown as a dashedline in Fig. l(b). We W(ad)lW, thereforeattributethis featureto a Stark shift of the lowest odd parity (lB,) exciton. There is also a feature at 4.6 eV, which cannot be found by a derivative analysis of the absorption spectrumand must therefore be due to an even parity (mAg) exciton.
642
PA. Lane et al. /SyntheticA4etals
84 (1993
641-642
10’(a)PPP
P’
8 6 4 2 -
03
0 l,,,“,,,,““,“““,,‘,‘,,,: 3 l(b) C,,:PPP (x10)
1
21 0 d-Y
2 -
, id ,
--
1 -1 0 2
3
4
5
6
Energy (eV) Figure 1. (a) Absorption spectrum at 300K of substituted PPP film. (b) Electroabsorption spectrum at 8OK (solid line) and first derivative of absorption spectrum (dashed line).
The PA spectrum of pure PPP, shown in Fig. 2(a), consists of three broad PA bands at 0.8,2, and 2.5 eV, respectively. The different PA bands have fairly similar dependence on the modulation frequency and intensity, so it was not possible establish which bands were correlated with the same kinds of excitations. However, charge transfer in fullerene-polymer complexes has been well established8 and permits us to determine which excitations are associated with charged excitations. A solution of PPP was doped by C,, (25% by weight or 12 mol%) and evaporated onto a quartz substrate. The PA spectrum of C,,:PPP has a sharp feature at ml.15 eV due to C60-, indicating that charge transfer takes place in the composite sample. The PA band at 2.5 is enhanced by C,, doping. We therefore assign this band to the high energy PA band of charged polarons. As the PA spectroscopy of other conjugated polymers has shown that polarons have two transitions,g we therefore assign the PA band at 0.75 eV to the low energy PA band of polarons. Triplet excitons have been found to contribute to the PA of other conjugated polymers, leading us to suggest that the PA band at 2 eV may be due to triplet excitons.
/ 1111"'1"1111'1111'1111"111 0 0.5 1 1.5
2
2.5
Probe Energy (eV) Figure 2. PA spectra at SOK of (a) PPP and (b) Cso-doped PPP. In summary, we have characterized the optical properties of substituted PPP. Three PA bands were found, two of which were attributed to charged polarons and the other to triplet excitons. The absorption and EA spectra were explained as due to a variety of even and odd parity singlet excitons. Acknowledgements. The work at Utah was supported by the Dept. of Energy and ONK grant no. N00014-9-1-0853. l.J. H. Burroughes et al., Nutwe 347,539 (1990). 2M. Hamaguchiand K. Yoshino, Japan. J. Appl. Phys. (in press). 3G. Grem,G. Leditzky, B. Ullrich, andG. Leising,Adv. Muter. 4, 36 (1992). 4M. Tabata,M. Satoh,K. Kaneto, andK. Yoshino, J. P/I~s. C: SolidState Phys. 19,LlOl (1986). 5G.GremandG. Leising,Synth.Met. 55-57,4105(1993). 62. V. Vardeny and X. Wei, in “Handbook of Conducting PolymersII”, Marie1Dekker(in press). 7M. Chandross,S. Mazumdar, S. Jeglinski,X. Wei, T. Miller andZ. V. Vardeny,Phys. Rev. Lett. 50,14702 (1994). 8Y. Wang, Nature 356, 585 (1992); N. S. Sariciftci, L. Smilowitz, A. J. HeegerandF. Wudl, Science258,1474(1992). gP. A. Lane, X. Wei, and Z. V. Vardeny, Phys. Rev. Lett. (in press)andelsewhere in theseproceedings.