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UV and white electroluminescence in AVB polymer
Synthetic Metals 139 (2003) 245–249
UV and white electroluminescence in AVB polymer S. Romdhane a,b , F. Marai a,c,∗ , L. Hassine a , J.L. Fave d , J. Roussel d , M. Majdoub e , H. Bouchriha a a
Laboratoire de Physique de la Matière Condensée, Faculté des Sciences de Tunis, Campus Universitaire, Tunis 1060, Tunisia b Faculté des Sciences de Bizerte, Jarzouna 7021, Bizerte, Tunisia c Institut National de Recherche Scientifique et Technique, Hammam-Lif 2050, Tunisia d Groupe de Physique des Solides, UMR CNRS 75-88 Universités Paris 6 et 7, 75251 Paris Cedex 05, France e Laboratoire de Chimie des Polymères, Faculté des Sciences de Monastir, Monastir 5000, Tunisia Received 30 January 2003; received in revised form 17 February 2003; accepted 19 February 2003
Abstract UV-Vis absorption, photoluminescence (PL), and electroluminescence (EL) from heterostructure thin films made of 1,4-bis-(9-anthrylvinyl)-benzene (AVB) are investigated. UV-Vis absorption data are used to determine the band gap of the material which is estimated to be 2.28 eV. It is observed that the AVB electroluminescence spectrum is broad, ranging from 400 to 850 nm. The EL is obtained at a low voltage (less than 6 V) at which the current density is 3 mA/cm2 . The current–voltage–electroluminescence (I–V–EL) characteristics are systematically studied in the ITO/AVB/Al device. The diode shows a power-law I–V dependence and a space-charge-limited current with traps (SCLC). © 2003 Elsevier Science B.V. All rights reserved. Keywords: UV and white electroluminescence; AVB polymer; Organic light emitting diodes
1. Introduction Since electroluminescent phenomenon in organic materials was discovered, there have been extensive research efforts on organic electroluminescent devices with the aim of application for flat panel display. Organic light emitting diodes (OLEDs) have also attracted much interest because of their particular low driving voltage, convenient design of the device structures, possible control of emission band, high brightness, low power consumption, large viewing angles, fast response time and low fabrication cost compared with any other devices [1–4]. These attractive attributes lead to much commercial interest in the OLEDs as a new display technology. In the past few years, green-emitting OLEDs as well as full-color dot matrix OLEDs displays for use in automobiles and cellular phones have been introduced in the market place [5]. UV and white OLEDs of high performance are desired, since they can play the important role of a white light source, as other colors can be obtained from the blue color via various filtering or color conversion schemes. Generally, white light can be produced by mixing emission of the red, green, and blue colors. Kido and ∗ Corresponding author. Tel.: +216-71-872-600; fax: +216-71-885-073. E-mail address: [email protected] (F. Marai).
coworkers [6–8] have obtained white EL emission by using a multi-emission layers structure in which the three basic colors were emitted from different organic layers. Other researchers used emitting materials with broad emission spectrum covered the whole or a large part of the visible range [9]. In this paper, firstly, we report a successful fabrication of UV and white electroluminescent devices using the vacuum-deposited 1,4-bis-(9-anthrylvinyl)-benzene (AVB) film. Secondly, we give the absorption, photoluminescence (PL) and electroluminescence (EL) measurements on the AVB film and we discussed our results.
2. Experiments The AVB, represented in Fig. 1, is a conjugated organic compound of yellow color. It precipitates in the reaction medium and separated by filtration and then washed several times with water and methanol. The powder is dried in vacuum. The study of the thermal behaviour shows that this organic compound is very stable and its melting point is 320 ◦ C [10]. The AVB is insoluble in the usual organic solvents at ambient temperature. PL and EL spectra of the film are recorded at 20 K on a Jobin Yvon spectrometer coupled to a nitrogen cooled CCD
0379-6779/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0379-6779(03)00128-0
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S. Romdhane et al. / Synthetic Metals 139 (2003) 245–249
negative biasing. With this in mind, a driving waveform with a positive voltage source and a negative voltage source (amplitude approximately equal to −2 to −3 V) is suitable to improve lifetime and brightness characteristics of OLEDs.
3. Results and discussion Fig. 1. Chemical formulae of 1,4-bis-(9-anthrylvinyl)-benzene (AVB).
3.1. Absorption camera. Sample was excited with a pulsed nitrogen laser line 317 nm for PL spectrum and a pulsed 15 V forward bias voltage for EL spectrum measurements. Diodes have the typical sandwich configuration ITO/AVB/ Al. The 250 nm organic film and the Al cathode were evaporated successively by conventional vacuum vapour deposition at pressure below 266.6 × 10−6 Pa on a precleaned ITO glass. All measurements are carried out using pulsed voltage measurements. We used a fast pulse generator (Tektronix PG 2012) with a nominal rise time lower than 2 ns and typical pulse widths varying from 1 to 10 ms. EL intensity was measured by a Hamamatsu H5783 photomultiplier, giving a time resolution comparable to the diode voltage time constant. The photomultiplier was connected to a 2 GHz digital storage oscilloscope Tektronix TDS620. The measurements were performed in repetitive mode with few seconds allowed between pulses to avoid the heating of the samples. In order to minimize the instrumental time constant and to observe the intrinsic response, the current and the EL intensity were measured across a 50 and 1 k loads, respectively. Also, we show that aging characteristics of OLEDs are improved if the OLED is subjected to periodic
The AVB film shows a characteristic UV-Vis absorption band at 430 nm (Fig. 2) corresponding to the photon energy of 2.9 eV, which is originated from the → ∗ transition. The band gap energy of this polymer is estimated to be (2.28 ± 0.05) eV from absorption edge analysis of (αhν)2 versus hν, where α, h and ν are the absorption coefficient, Plank’s constant and light frequency, respectively. This band gap is comparable with that of PPV film and it is a direct gap material [11]. 3.2. Photoluminescence (PL) Using the facilities and procedures explained in the experimental section, the PL spectrum of the polymer was measured. As shown in Fig. 3, PL spectrum of AVB is broad and presents a significant peak at 543 nm and two less intense humps at 520 and 570 nm. The PL is originated from the radiative recombination of the singlet polaron–exciton formed by intrachain excitation [12,13]. The peak wavelength 543 nm corresponds well to the 2.28 eV gap calculated previously.
Fig. 2. Optical absorption spectrum of vacuum-deposited AVB thin layer.
S. Romdhane et al. / Synthetic Metals 139 (2003) 245–249
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Fig. 3. PL spectrum of the AVB thin layer.
3.3. Electrical measurements The forward bias current is obtained when the ITO electrode is positively biased and the Al electrode grounded. The I–V characteristic (Fig. 4) shows typical profiles similar to what we reported elsewhere [14]. The forward current increases with increasing forward bias voltage and shows a typical diode behaviour. The voltage dependence of current appears to follow the power law I ∝ Vn , where n vary
from: 1 at the ohmic region, 2 at trapped charge-limited region and 6 at filled trap space-charge-limited region with a further increase [10,15–17]. The turn-on voltage of the device is about 2 V that means that substantial charge injection into an organic emitting layer occurs. A maximum current density of 23 mA/cm2 is obtained at 15.5 V. As shown in Fig. 5, the EL intensity increases upon increasing the voltage, just like current density. The curve shows clearly the existence of EL threshold of 6 V bias
Fig. 4. Measured I–V characteristics for ITO/AVB/Al device in forward bias mode.
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Fig. 5. Measured EL–V characteristic of the ITO/AVB/Al device.
voltage which corresponds to 3 mA/cm2 threshold forward current density. The difference between charge injection (2 V) and the EL turn-on voltage (6 V) is due to a non radiative process and an unreliable exciton generation process [18]. Fig. 6 shows the EL intensity as a function of the density of current flow under increasing forward voltage. Beyond the threshold EL intensity increases approximately linearly with increasing injected current.
EL spectrum (Fig. 7) is very broad and cover all UV and visible regions with peaks at 412 and 567 and 660 nm. EL emission is generated by recombination of electrons and holes injected from opposite electrodes of the device. That is, electrons and holes are injected in the conduction and valence bands, respectively. Some of them should become negative and positive polarons. When they collide, polaron excitons will be formed. The EL is considered to be due to radiative recombinations [19].
Fig. 6. EL–current density characteristic of the ITO/AVB/Al device.
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Fig. 7. EL spectrum of the OLED at a forward bias of 15 V.
4. Conclusion In summary, an organic light-emitting device with UV and white light emission was obtained from AVB polymer. We have also presented a comprehensive study of the electrical and optical characterization of an UV and white OLED fabricated with the vacuum-deposited AVB thin film. The OLED performance was studied by measuring the I–V–EL characteristics, optical absorption, PL, and EL spectra. The band gap energy is estimated to be 2.28 eV. The ITO/AVB/Al OLED turned on at approximately 6 V. The I–V dependence of the devices appears to follow the power law I–Vn , characteristics of a space-charge-limited current with traps (SCLC).
Acknowledgements The authors would like to thank Dr. R. Chtourou (IPEST, Tunisie) to allow them to use the equipments of absorption measurements.
References [1] C.W. Tang, S.A. van Slyke, Appl. Phys. Lett. 61 (1987) 913. [2] T. Tsutsui, E. Aminaka, Y. Fujita, Y. Hamada, S. Saito, Synth. Met. 55 (1993) 4157.
[3] D. Braun, A.J. Heeger, Appl. Phys. Lett. 58 (1991) 1982. [4] C.H. Chen, J. Shi, C.W. Tang, Macromol. Symp. 125 (1997) 1. [5] Y.Y. Fukuda, S. Miyaguchi, S. Ishzuka, Organic LED full-color passive-matrix display, in: J. Morreale (Ed.), Proceedings of SID International Symposium (Digest of technical papers, SID 1999), San Jose, CA, 1999, pp. 430–433. [6] J. Kido, M. Kimura, K. Nagai, Science 267 (1995) 1332. [7] J. Kido, H. Shionaya, K. Nagai, Appl. Phys. Lett. 67 (1995) 2281. [8] J. Kido, W. Ikeda, M. Kimura, K. Nagai, Jpn. J. Appl. Phys. 35 (1996) L394. [9] Y. Hamada, T. Sano, H. Fujii, Y. Nishi, H. Takahashi, K. Shibata, Jpn. J. Appl. Phys. 35 (1996) L1339. [10] F. Marai, S. Romdhane, L. Hassine, M. Majdoub, H. Bouchriha, Synth. Met. 132 (2003) 117–122. [11] R. Cervini, X.C. Li, G.W.C. Spencer, A.B. Holmes, S.C. Moratti, R.H. Friend, Synth. Met. 84 (1997) 359. [12] R.H. Friend, D.D.C. Bradley, P.D. Townsend, J. Phys. D20 (1987) 1367–1384. [13] D.D.C. Bradley, R.H. Friend, J. Phys. Condensed Matter 1 (1989) 3671–3678. [14] F. Marai, S. Romdhane, A. Romdhane, R. Bourguigua, N. Loussaief, J.L. Fave, M. Majdoub, H. Bouchriha, Synth. Met. 114 (2000) 255– 259. [15] D.D. Gebler, et al., J. Appl. Phys. 78 (6) (1995) 4264–4266. [16] P.E. Burrows, S.R. Forrest, Appl. Phys. Lett. 64 (1994) 2285. [17] G. Horowitz, D. Ficher, X.Z. Peng, P. Delannoy, J. Phys. (Paris) 51 (1990) 1489. [18] H. Lim, Thin Solid Films 363 (2000) 152–155. [19] M. Onoda, J. Appl. Phys. Lett. 58 (1995) 2.
UV and white electroluminescence in AVB polymer S. Romdhane a,b , F. Marai a,c,∗ , L. Hassine a , J.L. Fave d , J. Roussel d , M. Majdoub e , H. Bouchriha a a
Laboratoire de Physique de la Matière Condensée, Faculté des Sciences de Tunis, Campus Universitaire, Tunis 1060, Tunisia b Faculté des Sciences de Bizerte, Jarzouna 7021, Bizerte, Tunisia c Institut National de Recherche Scientifique et Technique, Hammam-Lif 2050, Tunisia d Groupe de Physique des Solides, UMR CNRS 75-88 Universités Paris 6 et 7, 75251 Paris Cedex 05, France e Laboratoire de Chimie des Polymères, Faculté des Sciences de Monastir, Monastir 5000, Tunisia Received 30 January 2003; received in revised form 17 February 2003; accepted 19 February 2003
Abstract UV-Vis absorption, photoluminescence (PL), and electroluminescence (EL) from heterostructure thin films made of 1,4-bis-(9-anthrylvinyl)-benzene (AVB) are investigated. UV-Vis absorption data are used to determine the band gap of the material which is estimated to be 2.28 eV. It is observed that the AVB electroluminescence spectrum is broad, ranging from 400 to 850 nm. The EL is obtained at a low voltage (less than 6 V) at which the current density is 3 mA/cm2 . The current–voltage–electroluminescence (I–V–EL) characteristics are systematically studied in the ITO/AVB/Al device. The diode shows a power-law I–V dependence and a space-charge-limited current with traps (SCLC). © 2003 Elsevier Science B.V. All rights reserved. Keywords: UV and white electroluminescence; AVB polymer; Organic light emitting diodes
1. Introduction Since electroluminescent phenomenon in organic materials was discovered, there have been extensive research efforts on organic electroluminescent devices with the aim of application for flat panel display. Organic light emitting diodes (OLEDs) have also attracted much interest because of their particular low driving voltage, convenient design of the device structures, possible control of emission band, high brightness, low power consumption, large viewing angles, fast response time and low fabrication cost compared with any other devices [1–4]. These attractive attributes lead to much commercial interest in the OLEDs as a new display technology. In the past few years, green-emitting OLEDs as well as full-color dot matrix OLEDs displays for use in automobiles and cellular phones have been introduced in the market place [5]. UV and white OLEDs of high performance are desired, since they can play the important role of a white light source, as other colors can be obtained from the blue color via various filtering or color conversion schemes. Generally, white light can be produced by mixing emission of the red, green, and blue colors. Kido and ∗ Corresponding author. Tel.: +216-71-872-600; fax: +216-71-885-073. E-mail address: [email protected] (F. Marai).
coworkers [6–8] have obtained white EL emission by using a multi-emission layers structure in which the three basic colors were emitted from different organic layers. Other researchers used emitting materials with broad emission spectrum covered the whole or a large part of the visible range [9]. In this paper, firstly, we report a successful fabrication of UV and white electroluminescent devices using the vacuum-deposited 1,4-bis-(9-anthrylvinyl)-benzene (AVB) film. Secondly, we give the absorption, photoluminescence (PL) and electroluminescence (EL) measurements on the AVB film and we discussed our results.
2. Experiments The AVB, represented in Fig. 1, is a conjugated organic compound of yellow color. It precipitates in the reaction medium and separated by filtration and then washed several times with water and methanol. The powder is dried in vacuum. The study of the thermal behaviour shows that this organic compound is very stable and its melting point is 320 ◦ C [10]. The AVB is insoluble in the usual organic solvents at ambient temperature. PL and EL spectra of the film are recorded at 20 K on a Jobin Yvon spectrometer coupled to a nitrogen cooled CCD
0379-6779/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0379-6779(03)00128-0
246
S. Romdhane et al. / Synthetic Metals 139 (2003) 245–249
negative biasing. With this in mind, a driving waveform with a positive voltage source and a negative voltage source (amplitude approximately equal to −2 to −3 V) is suitable to improve lifetime and brightness characteristics of OLEDs.
3. Results and discussion Fig. 1. Chemical formulae of 1,4-bis-(9-anthrylvinyl)-benzene (AVB).
3.1. Absorption camera. Sample was excited with a pulsed nitrogen laser line 317 nm for PL spectrum and a pulsed 15 V forward bias voltage for EL spectrum measurements. Diodes have the typical sandwich configuration ITO/AVB/ Al. The 250 nm organic film and the Al cathode were evaporated successively by conventional vacuum vapour deposition at pressure below 266.6 × 10−6 Pa on a precleaned ITO glass. All measurements are carried out using pulsed voltage measurements. We used a fast pulse generator (Tektronix PG 2012) with a nominal rise time lower than 2 ns and typical pulse widths varying from 1 to 10 ms. EL intensity was measured by a Hamamatsu H5783 photomultiplier, giving a time resolution comparable to the diode voltage time constant. The photomultiplier was connected to a 2 GHz digital storage oscilloscope Tektronix TDS620. The measurements were performed in repetitive mode with few seconds allowed between pulses to avoid the heating of the samples. In order to minimize the instrumental time constant and to observe the intrinsic response, the current and the EL intensity were measured across a 50 and 1 k loads, respectively. Also, we show that aging characteristics of OLEDs are improved if the OLED is subjected to periodic
The AVB film shows a characteristic UV-Vis absorption band at 430 nm (Fig. 2) corresponding to the photon energy of 2.9 eV, which is originated from the → ∗ transition. The band gap energy of this polymer is estimated to be (2.28 ± 0.05) eV from absorption edge analysis of (αhν)2 versus hν, where α, h and ν are the absorption coefficient, Plank’s constant and light frequency, respectively. This band gap is comparable with that of PPV film and it is a direct gap material [11]. 3.2. Photoluminescence (PL) Using the facilities and procedures explained in the experimental section, the PL spectrum of the polymer was measured. As shown in Fig. 3, PL spectrum of AVB is broad and presents a significant peak at 543 nm and two less intense humps at 520 and 570 nm. The PL is originated from the radiative recombination of the singlet polaron–exciton formed by intrachain excitation [12,13]. The peak wavelength 543 nm corresponds well to the 2.28 eV gap calculated previously.
Fig. 2. Optical absorption spectrum of vacuum-deposited AVB thin layer.
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Fig. 3. PL spectrum of the AVB thin layer.
3.3. Electrical measurements The forward bias current is obtained when the ITO electrode is positively biased and the Al electrode grounded. The I–V characteristic (Fig. 4) shows typical profiles similar to what we reported elsewhere [14]. The forward current increases with increasing forward bias voltage and shows a typical diode behaviour. The voltage dependence of current appears to follow the power law I ∝ Vn , where n vary
from: 1 at the ohmic region, 2 at trapped charge-limited region and 6 at filled trap space-charge-limited region with a further increase [10,15–17]. The turn-on voltage of the device is about 2 V that means that substantial charge injection into an organic emitting layer occurs. A maximum current density of 23 mA/cm2 is obtained at 15.5 V. As shown in Fig. 5, the EL intensity increases upon increasing the voltage, just like current density. The curve shows clearly the existence of EL threshold of 6 V bias
Fig. 4. Measured I–V characteristics for ITO/AVB/Al device in forward bias mode.
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Fig. 5. Measured EL–V characteristic of the ITO/AVB/Al device.
voltage which corresponds to 3 mA/cm2 threshold forward current density. The difference between charge injection (2 V) and the EL turn-on voltage (6 V) is due to a non radiative process and an unreliable exciton generation process [18]. Fig. 6 shows the EL intensity as a function of the density of current flow under increasing forward voltage. Beyond the threshold EL intensity increases approximately linearly with increasing injected current.
EL spectrum (Fig. 7) is very broad and cover all UV and visible regions with peaks at 412 and 567 and 660 nm. EL emission is generated by recombination of electrons and holes injected from opposite electrodes of the device. That is, electrons and holes are injected in the conduction and valence bands, respectively. Some of them should become negative and positive polarons. When they collide, polaron excitons will be formed. The EL is considered to be due to radiative recombinations [19].
Fig. 6. EL–current density characteristic of the ITO/AVB/Al device.
S. Romdhane et al. / Synthetic Metals 139 (2003) 245–249
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Fig. 7. EL spectrum of the OLED at a forward bias of 15 V.
4. Conclusion In summary, an organic light-emitting device with UV and white light emission was obtained from AVB polymer. We have also presented a comprehensive study of the electrical and optical characterization of an UV and white OLED fabricated with the vacuum-deposited AVB thin film. The OLED performance was studied by measuring the I–V–EL characteristics, optical absorption, PL, and EL spectra. The band gap energy is estimated to be 2.28 eV. The ITO/AVB/Al OLED turned on at approximately 6 V. The I–V dependence of the devices appears to follow the power law I–Vn , characteristics of a space-charge-limited current with traps (SCLC).
Acknowledgements The authors would like to thank Dr. R. Chtourou (IPEST, Tunisie) to allow them to use the equipments of absorption measurements.
References [1] C.W. Tang, S.A. van Slyke, Appl. Phys. Lett. 61 (1987) 913. [2] T. Tsutsui, E. Aminaka, Y. Fujita, Y. Hamada, S. Saito, Synth. Met. 55 (1993) 4157.
[3] D. Braun, A.J. Heeger, Appl. Phys. Lett. 58 (1991) 1982. [4] C.H. Chen, J. Shi, C.W. Tang, Macromol. Symp. 125 (1997) 1. [5] Y.Y. Fukuda, S. Miyaguchi, S. Ishzuka, Organic LED full-color passive-matrix display, in: J. Morreale (Ed.), Proceedings of SID International Symposium (Digest of technical papers, SID 1999), San Jose, CA, 1999, pp. 430–433. [6] J. Kido, M. Kimura, K. Nagai, Science 267 (1995) 1332. [7] J. Kido, H. Shionaya, K. Nagai, Appl. Phys. Lett. 67 (1995) 2281. [8] J. Kido, W. Ikeda, M. Kimura, K. Nagai, Jpn. J. Appl. Phys. 35 (1996) L394. [9] Y. Hamada, T. Sano, H. Fujii, Y. Nishi, H. Takahashi, K. Shibata, Jpn. J. Appl. Phys. 35 (1996) L1339. [10] F. Marai, S. Romdhane, L. Hassine, M. Majdoub, H. Bouchriha, Synth. Met. 132 (2003) 117–122. [11] R. Cervini, X.C. Li, G.W.C. Spencer, A.B. Holmes, S.C. Moratti, R.H. Friend, Synth. Met. 84 (1997) 359. [12] R.H. Friend, D.D.C. Bradley, P.D. Townsend, J. Phys. D20 (1987) 1367–1384. [13] D.D.C. Bradley, R.H. Friend, J. Phys. Condensed Matter 1 (1989) 3671–3678. [14] F. Marai, S. Romdhane, A. Romdhane, R. Bourguigua, N. Loussaief, J.L. Fave, M. Majdoub, H. Bouchriha, Synth. Met. 114 (2000) 255– 259. [15] D.D. Gebler, et al., J. Appl. Phys. 78 (6) (1995) 4264–4266. [16] P.E. Burrows, S.R. Forrest, Appl. Phys. Lett. 64 (1994) 2285. [17] G. Horowitz, D. Ficher, X.Z. Peng, P. Delannoy, J. Phys. (Paris) 51 (1990) 1489. [18] H. Lim, Thin Solid Films 363 (2000) 152–155. [19] M. Onoda, J. Appl. Phys. Lett. 58 (1995) 2.