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Photoluminescence and electroluminescence investigations in PEPPV and its derivatives
Synthetic Metals 114 Ž2000. 255–259 www.elsevier.comrlocatersynmet
Photoluminescence and electroluminescence investigations in PEPPV and its derivatives F. Marai a , S. Romdhane a,b, A. Romdhane a , R. Bourguigua a,b, N. Loussaief a , J.L. Fave a , M. Majdoub d , H. Bouchriha a,c,) a
Laboratoire de Physique de la Matiere ` Condensee, ´ Faculte´ des Sciences de Tunis-Campus UniÕer., Tunis 1002, Tunisia b Faculte´ des Sciences de Bizerte, 7021 Jarzouna, Bizerte, Tunisia c Groupe de Physique des Solides, UMR CNRS 75-88 and UniÕersites ´ Paris 6 and 7, 75251 ParisCedex 05, France d Laboratoire de Chimie des Polymeres, ` Faculte´ des Sciences de Monastir, 5000 Monastir, Tunisia Received 5 January 2000; accepted 6 March 2000
Abstract We report investigations of photoluminescence ŽPL. and electroluminescence ŽEL. in several derivatives of polyether–polyphenylenevinylene copolymer ŽPEPPV.. PL spectra depend on the conjugated block lengths on the end group, chlorine or bromine. Electroluminescent diodes were built in the classical sandwich structure, by evaporating PEPPV and aluminium layer under vacuum on an indium–tin oxide-covered glass slide. EL is obtained at a low voltage Žless than 4 V. and the diode quantum efficiency depends strongly on the applied voltage. Transient behaviour of PEPPV thin films devices was also investigated. Between the application of a short rectangular voltage pulse and the onset of EL, a time lag exists that is attributed to the mobility of the charge carriers. q 2000 Elsevier Science S.A. All rights reserved. Keywords: PEPPV; Electroluminescence; Photoluminescence
1. Introduction Electroluminescence ŽEL. devices using conjugated polymers layers as emitting medium have become a subject of great interest, since the first report of EL in polyŽ p-phenylenevinylene. ŽPPV. in 1990 w1x, due to their potential for use in large flat display. In addition to their relatively simple structure and the ability to make such devices on flexible substrates w2x, the colour of the light emission can be varied by using different conjugated compounds w1,3,4x or by using copolymers and polymer blends w5x. The efficiency Žphotons emitted per injected electron. of first devices was 0.01% in a sandwich configuration of aluminiumrpolymerrindium–tin oxide w1x; recently, a significant increase of the efficiency was observed for a polymer alternating conjugated and non-conjugated sequences w6x, or by using evaporated calcium as an
)
Corresponding author.
electron-injection contact w7x, or by building diodes with a bilayer structure diode w8x. However, in all cases, the efficiency remains less than 1%. We present in this paper our first investigations on photoluminescence ŽPL. and EL of a new copolymer polyether–polyphenylenevinylene ŽPEPPV., where conjugated phenylenevinylene and ether short segments alternate. The polymer can be synthesized with various total chain lengths and terminal atoms, chlorine or bromine.
2. Experimental 2.1. Synthesis and description The PEPPV copolymer is synthesized by homopolycondensation of para-dihalogenoxylene in heterogeneous medium through a phase transfer catalysis route using potassium hydroxide solution Ž50%. as base. The PEPPV that precipitates in the reaction medium is separated by filtration. The polymer obtained ŽFig. 1. is formed by the
0379-6779r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 Ž 0 0 . 0 0 2 4 6 - 0
F. Marai et al.r Synthetic Metals 114 (2000) 255–259
256
Fig. 1. Structure of the investigated PEPPV.
alternation of two oligomers with the average lengths x and y, for the phenylenevinylene and ether parts, respectively; n represents the arithmetic average taken on all the polymer lengths. Chlorine or bromine terminal atoms are obtained according to the reaction medium w9x. Three kinds of PEPPV have been investigated, for which x s 3 and y s 6, as determined by elemental analysis. PEPPV1 and PEPPV2 have the same statistical length Ž n s 10. and different end atoms Ž1 for chlorine and 2 for bromine.; PEPPV3 differs from PEPPV1 by a shorter statistical length Ž n s 3. and is soluble. 2.2. PL and EL PL spectra of solid samples at 2 and 10 K are recorded on a Jobin-Yvon U1000 spectrometer, through a cooled photomultiplier. Low power excitation at various wave˚ . from an argon ion lengths Žranging from 4579 to 5145 A laser Coherent Innova 90 are used. The diodes have the typical sandwich configuration ITOrPEPPVrAl. The 250-nm organic film and the Al cathode were evaporated successively by conventional vacuum vapour deposition at pressure below 2 = 10y6 Torr on the ITO glass. For pulsed voltage measurements, we used a Hewlett-Packard HP 33120A pulse generator with a nominal rise time lower than 20 ns and typical pulse widths varying from 1 to 10 ms. EL intensity was measured by an EMI photomultiplier connected to a 500-MHz digital storage oscilloscope HP 54503A. Measurements are performed in repetitive mode with few seconds allowed between pulses to avoid the heating of the samples. To measure the luminescence time response, we use single pulses; both the transient current and EL are recorded simultaneously on the oscilloscope. In order to minimize the instrumental time constant and to observe the intrinsic response, the current is measured via a 50-V load and the EL via 1 k V. All EL measurements were carried out at room temperature under ambient conditions and the presented results have been reproduced on several samples.
3. Results and discussions 3.1. Photoluminescence The polymer exhibits a strong luminescence, so it is possible to use weak excitation intensities and to record
spectra with a negligible instrumental broadening. Fig. 2 gives the PL spectra of the different polymers at different excitation wavelengths. PEPPV2 ŽFig. 2b. and PEPPV3 ŽFig. 2c. show three prominent peaks at 2.37, 2.20 and 2.05 eV. The analogous peaks for PEPPV1 ŽFig. 2a. are obviously blue-shifted and more dispersed around 2.42, 2.27 and 2.10 eV. At first sight, and for each sample, the position of the O–O origin and the vibronic progression Žf 1400 cmy1 . of the fluorescence spectra are fully consistent with the value x s 3 determined from elemental analysis for the conjugated part in the copolymers. Direct comparison indeed can be made with PPV oligomers, such as the so-called P2 oligoŽphenylenevinylene.; indeed the O–O line appears at 2.35 eV in aggregates fluorescence spectrum w10x. Moreover, the details of the spectra, and of their evolution versus the excitation energies ranging from 2.5 to 2.7 eV we have used, can give extra pieces of information on these PEPPV. Clearly for PEPPV1 ŽFig. 2a., the various excitations take place in the tail of a broad inhomogeneous absorption band; the displacement towards the low energies allows the excitation of a lesser number of distinct states, giving rise to narrower and more structured emission bands. In contrast, for PEPPV2 and particularly for PEPPV3, this photoselection by different laser lines seems less efficient. This could mean all the energy levels of these compounds are located significantly below the corresponding ones of PEPPV1 Žthat appear effectively blue-shifted.. In the different PEPPV, the line positions are sensitive to the effective conjugation length, i.e. the conjugated part of the copolymer can be dispersed in length or be out of the strict planarity; in addition, structural disorder and excitation transfer to emitting traps also play an essential role and can modify spectra w10x. At this point, we cannot distinguish between these effects, but in this sense, the shorter and soluble PEPPV3 ŽFig. 2c. seems to have the more homogeneous length and surrounding distribution and to be the best defined compound. The spectra for the films evaporated from PEPPV1 have been published previously; the deposition process breaks the copolymer in shorter subchains and seems to leave behind preferably the largest conjugated segments w11x.
3.2. Electroluminescence The forward bias current is obtained when the ITO electrode is positively biased and the A1 electrode grounded. Fig. 3 presents the current and light intensity versus voltage characteristics of a typical device measured with a 8000 V sy1 voltage ramp. The EL threshold is 3.5 V for a current density of 2.7 mArcm2 , then current and EL intensity increase drastically with increasing electric field.
F. Marai et al.r Synthetic Metals 114 (2000) 255–259
257
Fig. 2. Emission spectra of PEPPV1, PEPPV2 and PEPPV3 for various excitation wavelengths at T s 2 K.
The mobility of the charge carrier is calculated by m s L2rTtr V w8x, where L is the thickness of the organic layer, V the applied voltage and Ttr the time lag between
the application of a rectangular voltage and the onset of EL. A trivial mistake in the interpretation of this kind of experiment would be to neglect the time constant due to
F. Marai et al.r Synthetic Metals 114 (2000) 255–259
258
the transit time of the fastest carriers across the device. A typical value of t tr is 30 ms ŽFig. 4. Žfor Vmax s 4 V., and the estimated mobility of charge carriers is 5 = 10y6 cm2 Vy1 sy1 , which is comparable to the mobility of positive charge carriers in PPV 3 = 10y6 cm2 Vy1 sy1 w12x. This low value of transit mobility in PEPPV is correlated to the hole charge. However, in the EL process, both of signs charge carriers are involved and the charge transport in organic compounds is known to be limited by traps. In the light-emitting diode ŽLED., the organic film is sandwiched between two asymmetric electrodes. At equilibrium, the Fermi level is flat and most of the traps are empty. After establishing the voltage, the occupancy of the traps is not immediately changed, and the Fermi level remains pinned to the electrode levels. The hole mobility is then considerably reduced by a high density of empty, hence active, traps w8x. The internal quantum efficiency in organic LEDs is determined by three major factors w13,14x
F EL s hphrhel s ghrF PL
Fig. 3. Current density and EL intensity–voltage characteristics for a diode having a PEPPV1 film 250-nm thick and active area of 10 mm2 .
the measurement circuits. Indeed the voltage on the diode terminals reaches the threshold voltage Vth at time tth . t th s RC ln
Vmax Vmax y Vth
where Vmax is the height of the rectangular voltage pulse. Here the time constant RC stems from the unavoidable load resistors Ž) 100 V . and from the diode capacitance Ž- 5 nF.; so at the very warts, t th could be few microseconds, and then the delay time can be safely attributed to
In this equation, F PL is the PL quantum efficiency, hr the fraction of neutral excited states formed as singlet excitons via the electrical current and g the balancing factor between holes and electrons. We note that F PL and hr are intrinsic properties of the polymer and are independent of the applied voltage. The low efficiency of these devices is a consequence of the Schottky-like behavior described elsewhere w15x. Schottky diodes are majority carrier devices Žin the case of PEPPV, holes are the majority carriers., while for highly efficient EL, a well-balanced injection of both types of charge carriers is essential. The experimentally measured current corresponds mainly to the majority carrier, but includes components of the minority carrier current and of the recombination current. When the voltage is increased, the increase of the quantum efficiency
Fig. 4. Current density and EL intensity versus time for a rectangular applied voltage with Vma x s 4 V.
F. Marai et al.r Synthetic Metals 114 (2000) 255–259 Table 1 Quantum efficiency of the electroluminescent diodes versus voltage Voltage ŽV.
4
6
8
10
Quantum efficiency Ž%.
5=10y4
4=10y3
10y2
8=10y2
259
la Recherche Scientifique Tunisie. and the contact CMCU code 96F13012.
References shows that the injection of the electrons becomes relatively more efficient than the hole injection, increasing the balancing factor g . At this time, we cannot completely exclude other possible mechanisms to explain the applied voltage-dependent efficiency ŽTable 1..
4. Conclusion We have demonstrated the fabrication of a LED using a new conjugated copolymer, PEPPV, as an emitting layer. From elemental analysis and PL, the conjugated segments of this copolymer are rather similar to PPV oligomers, connected by ether parts, and emit a green–yellow light. The results of the experimental EL study show that the EL diodes have a low turn-on voltage and a low current density. However, in order to achieve high EL efficiency, it would be necessary to better balance electrons and hole injections from opposite contacts into the LED device.
Acknowledgements We are grateful to Dr. Francis Garnier ŽDirector of Laboratoire des Materiaux Moleculaires, Thiais Paris, ´ ´ where diodes where realized. for helpful discussions. This work is supported by the DGRST ŽDirection Gen’erale de ´
w1x J.H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Mackay, R.H. Friend, P.L. Burns, A.B. Holmes, Nature 347 Ž1990. 539. w2x G. Gustafsson, Y. Cao, G.M. Treacy, F. Klavetter, N. Colaneri, A.J. Heeger, Nature 357 Ž1992. 477. w3x D. Braun, A.J. Heeger, Appl. Phys. Lett. 58 Ž1991. 1982. w4x C. Zhang, S. Hoger, K. Pakbaz, F. Wuld, A.J. Heeger, J. Electron. ¨ Mater. 22 Ž1993. 413. w5x C. Zhang, H. von Seggern, K. Pakbaz, B. Kraabel, H.W. Schmidt, A.J. Heeger, Synth. Met. 62 Ž1994. 35. w6x D.D.C. Bradley, P.L. Burn, R.H. Friend, A.B. Holmes, A. Kraft, Electronic properties of conjugated polymers, in: H. Kuzmany ŽEd.., Springer Series on Solid Sate Sciences, Springer, New York, 1993. w7x C. Zhang, D. Braun, A.J. Heeger, J. Appl. Phys. 73 Ž1993. 5177. w8x G. Horowitz, P. Delannoy, H. Bouchriha, F. Deloffre, J.L. Fave, F. Garnier, R. Hajlaoui, M. Heyman, F. Kouki, P. Valat, V. Wintgens, A. Yassar, Adv. Mater. 6 Ž1994. 752. w9x Ch. Dridi, A. Chaieb, F. Hassen, M. Majdoub, M. Gamoudi, Synth. Met. 90 Ž1997. 233. w10x H.-J. Egelhaaf, J. Gierschnerm, D. Oelkrug, Synth. Met. 83 Ž1986. 221. w11x R. Kalfat, N. Ghardi, S. Romdhane, J.L. Fave, A. Chaıeb, M. ¨ Majdoub, J. Sol-Gel Sci. Technol. 13 Ž1998. 439. w12x S. Karg, V. Daykonov, M. Meier, W. Riess, G. Paasch, Synth. Met. 67 Ž1994. 165. w13x T. Tsutsui, S. Saito, in: M. Aldissi ŽEd.., Intrinsically Conducting Polymers: An Emerging Technology, Kluwer Academic Publishing, Netherlands, 1993, p. 123. w14x E. Buchwald, M. Meier, S. Karg, P. Posh, H.W. Schmidt, P. ¨ Strohriegl, W. Reiss, M. Schwoerer, Adv. Mater. 7 Ž1995. 839. w15x S. Karg, W. Riess, M. Meier, M. Schwoerer, Synth. Met 55–57 Ž1993. 4186.
Photoluminescence and electroluminescence investigations in PEPPV and its derivatives F. Marai a , S. Romdhane a,b, A. Romdhane a , R. Bourguigua a,b, N. Loussaief a , J.L. Fave a , M. Majdoub d , H. Bouchriha a,c,) a
Laboratoire de Physique de la Matiere ` Condensee, ´ Faculte´ des Sciences de Tunis-Campus UniÕer., Tunis 1002, Tunisia b Faculte´ des Sciences de Bizerte, 7021 Jarzouna, Bizerte, Tunisia c Groupe de Physique des Solides, UMR CNRS 75-88 and UniÕersites ´ Paris 6 and 7, 75251 ParisCedex 05, France d Laboratoire de Chimie des Polymeres, ` Faculte´ des Sciences de Monastir, 5000 Monastir, Tunisia Received 5 January 2000; accepted 6 March 2000
Abstract We report investigations of photoluminescence ŽPL. and electroluminescence ŽEL. in several derivatives of polyether–polyphenylenevinylene copolymer ŽPEPPV.. PL spectra depend on the conjugated block lengths on the end group, chlorine or bromine. Electroluminescent diodes were built in the classical sandwich structure, by evaporating PEPPV and aluminium layer under vacuum on an indium–tin oxide-covered glass slide. EL is obtained at a low voltage Žless than 4 V. and the diode quantum efficiency depends strongly on the applied voltage. Transient behaviour of PEPPV thin films devices was also investigated. Between the application of a short rectangular voltage pulse and the onset of EL, a time lag exists that is attributed to the mobility of the charge carriers. q 2000 Elsevier Science S.A. All rights reserved. Keywords: PEPPV; Electroluminescence; Photoluminescence
1. Introduction Electroluminescence ŽEL. devices using conjugated polymers layers as emitting medium have become a subject of great interest, since the first report of EL in polyŽ p-phenylenevinylene. ŽPPV. in 1990 w1x, due to their potential for use in large flat display. In addition to their relatively simple structure and the ability to make such devices on flexible substrates w2x, the colour of the light emission can be varied by using different conjugated compounds w1,3,4x or by using copolymers and polymer blends w5x. The efficiency Žphotons emitted per injected electron. of first devices was 0.01% in a sandwich configuration of aluminiumrpolymerrindium–tin oxide w1x; recently, a significant increase of the efficiency was observed for a polymer alternating conjugated and non-conjugated sequences w6x, or by using evaporated calcium as an
)
Corresponding author.
electron-injection contact w7x, or by building diodes with a bilayer structure diode w8x. However, in all cases, the efficiency remains less than 1%. We present in this paper our first investigations on photoluminescence ŽPL. and EL of a new copolymer polyether–polyphenylenevinylene ŽPEPPV., where conjugated phenylenevinylene and ether short segments alternate. The polymer can be synthesized with various total chain lengths and terminal atoms, chlorine or bromine.
2. Experimental 2.1. Synthesis and description The PEPPV copolymer is synthesized by homopolycondensation of para-dihalogenoxylene in heterogeneous medium through a phase transfer catalysis route using potassium hydroxide solution Ž50%. as base. The PEPPV that precipitates in the reaction medium is separated by filtration. The polymer obtained ŽFig. 1. is formed by the
0379-6779r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 Ž 0 0 . 0 0 2 4 6 - 0
F. Marai et al.r Synthetic Metals 114 (2000) 255–259
256
Fig. 1. Structure of the investigated PEPPV.
alternation of two oligomers with the average lengths x and y, for the phenylenevinylene and ether parts, respectively; n represents the arithmetic average taken on all the polymer lengths. Chlorine or bromine terminal atoms are obtained according to the reaction medium w9x. Three kinds of PEPPV have been investigated, for which x s 3 and y s 6, as determined by elemental analysis. PEPPV1 and PEPPV2 have the same statistical length Ž n s 10. and different end atoms Ž1 for chlorine and 2 for bromine.; PEPPV3 differs from PEPPV1 by a shorter statistical length Ž n s 3. and is soluble. 2.2. PL and EL PL spectra of solid samples at 2 and 10 K are recorded on a Jobin-Yvon U1000 spectrometer, through a cooled photomultiplier. Low power excitation at various wave˚ . from an argon ion lengths Žranging from 4579 to 5145 A laser Coherent Innova 90 are used. The diodes have the typical sandwich configuration ITOrPEPPVrAl. The 250-nm organic film and the Al cathode were evaporated successively by conventional vacuum vapour deposition at pressure below 2 = 10y6 Torr on the ITO glass. For pulsed voltage measurements, we used a Hewlett-Packard HP 33120A pulse generator with a nominal rise time lower than 20 ns and typical pulse widths varying from 1 to 10 ms. EL intensity was measured by an EMI photomultiplier connected to a 500-MHz digital storage oscilloscope HP 54503A. Measurements are performed in repetitive mode with few seconds allowed between pulses to avoid the heating of the samples. To measure the luminescence time response, we use single pulses; both the transient current and EL are recorded simultaneously on the oscilloscope. In order to minimize the instrumental time constant and to observe the intrinsic response, the current is measured via a 50-V load and the EL via 1 k V. All EL measurements were carried out at room temperature under ambient conditions and the presented results have been reproduced on several samples.
3. Results and discussions 3.1. Photoluminescence The polymer exhibits a strong luminescence, so it is possible to use weak excitation intensities and to record
spectra with a negligible instrumental broadening. Fig. 2 gives the PL spectra of the different polymers at different excitation wavelengths. PEPPV2 ŽFig. 2b. and PEPPV3 ŽFig. 2c. show three prominent peaks at 2.37, 2.20 and 2.05 eV. The analogous peaks for PEPPV1 ŽFig. 2a. are obviously blue-shifted and more dispersed around 2.42, 2.27 and 2.10 eV. At first sight, and for each sample, the position of the O–O origin and the vibronic progression Žf 1400 cmy1 . of the fluorescence spectra are fully consistent with the value x s 3 determined from elemental analysis for the conjugated part in the copolymers. Direct comparison indeed can be made with PPV oligomers, such as the so-called P2 oligoŽphenylenevinylene.; indeed the O–O line appears at 2.35 eV in aggregates fluorescence spectrum w10x. Moreover, the details of the spectra, and of their evolution versus the excitation energies ranging from 2.5 to 2.7 eV we have used, can give extra pieces of information on these PEPPV. Clearly for PEPPV1 ŽFig. 2a., the various excitations take place in the tail of a broad inhomogeneous absorption band; the displacement towards the low energies allows the excitation of a lesser number of distinct states, giving rise to narrower and more structured emission bands. In contrast, for PEPPV2 and particularly for PEPPV3, this photoselection by different laser lines seems less efficient. This could mean all the energy levels of these compounds are located significantly below the corresponding ones of PEPPV1 Žthat appear effectively blue-shifted.. In the different PEPPV, the line positions are sensitive to the effective conjugation length, i.e. the conjugated part of the copolymer can be dispersed in length or be out of the strict planarity; in addition, structural disorder and excitation transfer to emitting traps also play an essential role and can modify spectra w10x. At this point, we cannot distinguish between these effects, but in this sense, the shorter and soluble PEPPV3 ŽFig. 2c. seems to have the more homogeneous length and surrounding distribution and to be the best defined compound. The spectra for the films evaporated from PEPPV1 have been published previously; the deposition process breaks the copolymer in shorter subchains and seems to leave behind preferably the largest conjugated segments w11x.
3.2. Electroluminescence The forward bias current is obtained when the ITO electrode is positively biased and the A1 electrode grounded. Fig. 3 presents the current and light intensity versus voltage characteristics of a typical device measured with a 8000 V sy1 voltage ramp. The EL threshold is 3.5 V for a current density of 2.7 mArcm2 , then current and EL intensity increase drastically with increasing electric field.
F. Marai et al.r Synthetic Metals 114 (2000) 255–259
257
Fig. 2. Emission spectra of PEPPV1, PEPPV2 and PEPPV3 for various excitation wavelengths at T s 2 K.
The mobility of the charge carrier is calculated by m s L2rTtr V w8x, where L is the thickness of the organic layer, V the applied voltage and Ttr the time lag between
the application of a rectangular voltage and the onset of EL. A trivial mistake in the interpretation of this kind of experiment would be to neglect the time constant due to
F. Marai et al.r Synthetic Metals 114 (2000) 255–259
258
the transit time of the fastest carriers across the device. A typical value of t tr is 30 ms ŽFig. 4. Žfor Vmax s 4 V., and the estimated mobility of charge carriers is 5 = 10y6 cm2 Vy1 sy1 , which is comparable to the mobility of positive charge carriers in PPV 3 = 10y6 cm2 Vy1 sy1 w12x. This low value of transit mobility in PEPPV is correlated to the hole charge. However, in the EL process, both of signs charge carriers are involved and the charge transport in organic compounds is known to be limited by traps. In the light-emitting diode ŽLED., the organic film is sandwiched between two asymmetric electrodes. At equilibrium, the Fermi level is flat and most of the traps are empty. After establishing the voltage, the occupancy of the traps is not immediately changed, and the Fermi level remains pinned to the electrode levels. The hole mobility is then considerably reduced by a high density of empty, hence active, traps w8x. The internal quantum efficiency in organic LEDs is determined by three major factors w13,14x
F EL s hphrhel s ghrF PL
Fig. 3. Current density and EL intensity–voltage characteristics for a diode having a PEPPV1 film 250-nm thick and active area of 10 mm2 .
the measurement circuits. Indeed the voltage on the diode terminals reaches the threshold voltage Vth at time tth . t th s RC ln
Vmax Vmax y Vth
where Vmax is the height of the rectangular voltage pulse. Here the time constant RC stems from the unavoidable load resistors Ž) 100 V . and from the diode capacitance Ž- 5 nF.; so at the very warts, t th could be few microseconds, and then the delay time can be safely attributed to
In this equation, F PL is the PL quantum efficiency, hr the fraction of neutral excited states formed as singlet excitons via the electrical current and g the balancing factor between holes and electrons. We note that F PL and hr are intrinsic properties of the polymer and are independent of the applied voltage. The low efficiency of these devices is a consequence of the Schottky-like behavior described elsewhere w15x. Schottky diodes are majority carrier devices Žin the case of PEPPV, holes are the majority carriers., while for highly efficient EL, a well-balanced injection of both types of charge carriers is essential. The experimentally measured current corresponds mainly to the majority carrier, but includes components of the minority carrier current and of the recombination current. When the voltage is increased, the increase of the quantum efficiency
Fig. 4. Current density and EL intensity versus time for a rectangular applied voltage with Vma x s 4 V.
F. Marai et al.r Synthetic Metals 114 (2000) 255–259 Table 1 Quantum efficiency of the electroluminescent diodes versus voltage Voltage ŽV.
4
6
8
10
Quantum efficiency Ž%.
5=10y4
4=10y3
10y2
8=10y2
259
la Recherche Scientifique Tunisie. and the contact CMCU code 96F13012.
References shows that the injection of the electrons becomes relatively more efficient than the hole injection, increasing the balancing factor g . At this time, we cannot completely exclude other possible mechanisms to explain the applied voltage-dependent efficiency ŽTable 1..
4. Conclusion We have demonstrated the fabrication of a LED using a new conjugated copolymer, PEPPV, as an emitting layer. From elemental analysis and PL, the conjugated segments of this copolymer are rather similar to PPV oligomers, connected by ether parts, and emit a green–yellow light. The results of the experimental EL study show that the EL diodes have a low turn-on voltage and a low current density. However, in order to achieve high EL efficiency, it would be necessary to better balance electrons and hole injections from opposite contacts into the LED device.
Acknowledgements We are grateful to Dr. Francis Garnier ŽDirector of Laboratoire des Materiaux Moleculaires, Thiais Paris, ´ ´ where diodes where realized. for helpful discussions. This work is supported by the DGRST ŽDirection Gen’erale de ´
w1x J.H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Mackay, R.H. Friend, P.L. Burns, A.B. Holmes, Nature 347 Ž1990. 539. w2x G. Gustafsson, Y. Cao, G.M. Treacy, F. Klavetter, N. Colaneri, A.J. Heeger, Nature 357 Ž1992. 477. w3x D. Braun, A.J. Heeger, Appl. Phys. Lett. 58 Ž1991. 1982. w4x C. Zhang, S. Hoger, K. Pakbaz, F. Wuld, A.J. Heeger, J. Electron. ¨ Mater. 22 Ž1993. 413. w5x C. Zhang, H. von Seggern, K. Pakbaz, B. Kraabel, H.W. Schmidt, A.J. Heeger, Synth. Met. 62 Ž1994. 35. w6x D.D.C. Bradley, P.L. Burn, R.H. Friend, A.B. Holmes, A. Kraft, Electronic properties of conjugated polymers, in: H. Kuzmany ŽEd.., Springer Series on Solid Sate Sciences, Springer, New York, 1993. w7x C. Zhang, D. Braun, A.J. Heeger, J. Appl. Phys. 73 Ž1993. 5177. w8x G. Horowitz, P. Delannoy, H. Bouchriha, F. Deloffre, J.L. Fave, F. Garnier, R. Hajlaoui, M. Heyman, F. Kouki, P. Valat, V. Wintgens, A. Yassar, Adv. Mater. 6 Ž1994. 752. w9x Ch. Dridi, A. Chaieb, F. Hassen, M. Majdoub, M. Gamoudi, Synth. Met. 90 Ž1997. 233. w10x H.-J. Egelhaaf, J. Gierschnerm, D. Oelkrug, Synth. Met. 83 Ž1986. 221. w11x R. Kalfat, N. Ghardi, S. Romdhane, J.L. Fave, A. Chaıeb, M. ¨ Majdoub, J. Sol-Gel Sci. Technol. 13 Ž1998. 439. w12x S. Karg, V. Daykonov, M. Meier, W. Riess, G. Paasch, Synth. Met. 67 Ž1994. 165. w13x T. Tsutsui, S. Saito, in: M. Aldissi ŽEd.., Intrinsically Conducting Polymers: An Emerging Technology, Kluwer Academic Publishing, Netherlands, 1993, p. 123. w14x E. Buchwald, M. Meier, S. Karg, P. Posh, H.W. Schmidt, P. ¨ Strohriegl, W. Reiss, M. Schwoerer, Adv. Mater. 7 Ž1995. 839. w15x S. Karg, W. Riess, M. Meier, M. Schwoerer, Synth. Met 55–57 Ž1993. 4186.