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Transient electroluminescence from rubrene light-emitting diodes using double voltage pulse
Synthetic Metals 106 Ž1999. 85–88 www.elsevier.comrlocatersynmet
Transient electroluminescence from rubrene light-emitting diodes using double voltage pulse A. Chowdhury, A.J. Pal
)
Department of Solid State Physics, Indian Association for the CultiÕation of Science, JadaÕpur, Calcutta, 700 032, India Received 7 September 1998; accepted 19 December 1998
Abstract Light-emitting diodes using a dye-insulating matrix blend of rubrene and arachidic acid have been fabricated using Langmuir–Blodgett film deposition technique. Transient electroluminescence ŽEL. measurements were performed by applying two consecutive square-wave voltage pulses separated by a short delay. This enabled us to study the effect of accumulated charges during the first pulse separated from that of the injected carriers, which are predominant during the second pulse. During the first voltage pulse, intrinsically accumulated charges recombine to generate an instantaneous EL peak, which is absent during the second pulse. A constant EL was observed during both the pulses. We also have observed an overshoot in EL in the falling edges of mainly negative voltage pulses. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Electroluminescence; Interface phenomena; Light-emitting diodes; Overshoot effect; Rubrene; Transient electroluminescence
1. Introduction Conjugated polymers and related organic semiconductors are specific topics of interest in the field of optoelectronics in the present decade, because they offer novel solutions for light-emitting devices ŽLEDs. w1x. To understand the operation mechanism and response characteristics of LEDs, transient electroluminescence ŽEL. is often studied w2–5x, where a single square-wave voltage pulse is applied across the device. The time lag between the sharp rise of voltage pulse and the first appearance of EL signal has often been attributed to the transit time of the more mobile charge carriers through the emitting layer. More recent work on transient EL manifested that charge accumulation plays a major role in device operation w6,7x. Spin casting and vacuum evaporation methods are most frequently used techniques to fabricate organic LEDs. Devices with organized emitting materials have been fabricated by layer-by-layer self-assembly techniques w8x. The possibility of controlling the fabrication of LEDs in the molecular scale is offered by the Langmuir–Blodgett ŽLB. film deposition technique w9–13x. Most of the transient EL measurements, published so far, have been obtained by single or repetitive square-wave )
Corresponding author. E-mail: [email protected]
voltage pulse operation. In this article, we report our results on transient EL by applying two consecutive square-wave voltage pulses separated by a short delay. This enabled us to separate out the effects of intrinsically accumulated charges from that of the injected carriers. Rubrene as a dopant w14x and its LB films w15x have earlier been used as emitting layer of LEDs. Here, we present our work on the transient EL characteristics of LEDs using rubrene LB films.
2. Experimental Organic dye material rubrene Ž5, 6, 11, 12-tetraphenylnaphthacene. was purchased from Aldrich Chemical and was used as received. To fabricate LEDs, LB films of rubrene and arachidic acid Žmolar ratio 60%:40%. were deposited on patterned indium tin oxide ŽITO. coated glass substrates at a surface pressure of 20 mNrm. ITO had a sheet resistance of less than 10 VrI. Seven LB layers were deposited and thickness of each monolayer was approximately 3 nm. Aluminium ŽAl. top electrodes were thermally evaporated at a pressure below 10y5 Torr. A mechanical shutter was used to protect LB films from excess heat before and after the metal evaporation. The active area of about 10 mm2 Žthe overlap of ITO and Al.
0379-6779r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 Ž 9 9 . 0 0 1 1 3 - 7
86
A. Chowdhury, A.J. Pal r Synthetic Metals 106 (1999) 85–88
was defined by a mask. All the devices were measured in nitrogen atmosphere at room temperature. The transient EL measurements were performed by applying two consecutive square-wave voltage pulses Žrise and fall time less than 800 ns. generated by Yokogawa 7651 dc source coupled with PC controlled fast switching transistors. Both the pulses generally had widths less than 100 ms and were separated by 30 ms. Light emission from the LED structures was detected by a Electron Tubes 9256B photomultiplier. The voltage pulses and the EL response were simultaneously digitized and stored by a Hewlett-Packard HP54600B two-channel real-time storage oscilloscope. All the instruments were operated by a PC via GPIB interface.
3. Results and discussion We have studied the EL response of rubrene LEDs operated under two consecutive square-wave drive voltage pulses of different amplitudes and widths. Fig. 1Ža. shows typical EL from a device along with the applied pulse signal of 3 V amplitude and 60 ms width. During the first voltage pulse, we have observed an instantaneous EL peak,
Fig. 1. Voltage pulse and transient EL response of LEDs with 7 layers of rubrene LB films. The q3 V Ža. and q8 V Žb. voltage pulses had a width of 60 ms and separation between the two pulses was 30 ms.
Fig. 2. Voltage pulse and transient EL response of LEDs with 7 layers of rubrene LB films. The y5 V voltage pulse had the same width and separation as in Fig. 1.
which decayed to a steady-state EL level. The EL peak was absent during the second voltage pulse, while the steady-state EL part had an equal intensity as during the first pulse. When the pulses were separated by longer time, the instantaneous EL peak started to appear also during the second pulse. When more than two pulses were applied, the EL peak was observed only during the first pulse and the steady state EL was observed during all the pulses. When the devices were operated under a pulse voltage of amplitude more than 6.5 V, in addition to the instantaneous EL peak and the steady state component, an overshoot in EL was observed at the falling edges of both the voltage pulses. A typical figure ŽFig. 1Žb.. shows the EL response with the overshoot effect when operated under q8 V pulse. The I–V characteristics of the structures are symmetric and EL was obtained under both bias directions w15x. This was reflected in the transient response of EL operated under two consecutive negative voltage pulses and Fig. 2. shows a typical response for y5 V. Instantaneous EL peak, steady-state EL, and overshoot effect at the falling edges of voltage pulses have been observed in the entire voltage range of our study Žy3 V to y8 V.. Overshoot effect in EL has recently been observed in certain bilayer LEDs w16x. In transient measurements, the occurrence of instantaneous EL peak at the beginning of first voltage pulse can be interpreted in terms of charge accumulation in organic LEDs w7,17x. By applying two consecutive voltage pulses, we have separated out the effect of accumulated charges from that of the injected carriers. The LEDs under study can be considered as p-type organic semiconductor sandwiched between Al and ITO electrodes. Due to the presence of different metal electrodes, there will be charge transfer process in the device to align the fermi level within the device before any bias is applied. Finally, equilibrium will be reached with holes and electrons accumulated at the two opposite metal–semiconductor inter-
A. Chowdhury, A.J. Pal r Synthetic Metals 106 (1999) 85–88
faces. If a square-wave positive voltage pulse is applied across the device ŽITO positive., the accumulated holes at the ITOrrubrene interface, which have a higher mobility, will drift towards the cathode electrode. The holes will then recombine with accumulated electrons which are already present there and result in an instantaneous EL component. Considering the hole mobility values in rubrene Ž; 10y4 cm2rVs w18x., the transit times for the holes through the emitting layer Ž21 nm thick. will be of the order of nanoseconds. We therefore observe an instantaneous EL peak at the beginning of the first voltage pulse in Fig. 1Ža. and 1Žb.. Since the number of holes near the ITO electrode is expected to be larger than the number of electrons near the Al electrode, the excess holes will build up a space charge when a voltage is applied. These space charges will assist in electron injection and result in steady state EL. During the second pulse on the other hand, we have not observed the instantaneous EL peak, since all the accumulated charges have recombined during the first pulse. Fig. 2 shows that we have observed such instantaneous EL peak during the first voltage pulse also when two square-wave negative voltage pulses were applied across the device ŽITO negative.. Here the accumulated electrons near the Al electrode modify the barrier for the holes, so that holes can be injected easily from the Al electrode. Similarly, the holes accumulated near the ITO electrode assist in electron injection. However, due to higher barrier height for the electrons than that of the holes, the instantaneous EL peak should mainly be contributed from the recombination of injected holes and the accumulated electrons near Al electrode. Once all the accumulated electrons have recombined, the steady-state EL remains mainly limited by the injection of electrons from the ITO electrode. As expected, the instantaneous and the steady state EL increase with amplitude of the applied voltage. Their ratio, however, decreases with increasing voltage for both the
Fig. 3. The ratio between instantaneous EL peak and steady-state EL intensities vs. applied pulse voltage. Results from different structures are included in the figure.
87
Fig. 4. Instantaneous EL peak intensity vs. applied pulse voltage for forward and reverse bias case. Results from different structures are included in the figure.
biases, as shown in Fig. 3. This behaviour reflects that the effect of injected charge carriers becomes predominant than that of the accumulated ones as the voltage increases. At higher voltage, the accumulated holes might also accelerate to yield hole current instead of recombining with the electrons near the Al electrode. Finally, only steady-state EL due to injected carriers remains observable at high enough voltage. In Fig. 4, we have plotted the instantaneous EL peak intensity vs. applied voltage for the forward and reverse bias cases. For the reverse bias case, the instantaneous EL intensity increases monotonically, since higher voltage allows more holes to be injected which recombine with the accumulated electrons present near the Al electrode. The intensity during the forward-bias pulse, however, shows a maximum at about 6.5 V. Initially, an increase in voltage will prompt more holes to recombine with electrons. At higher voltage, the accumulated holes might accelerate to yield hole current instead of recombining with the electrons near the Al electrode. This will lower the instantaneous EL at higher voltage.
Fig. 5. Overshoot in EL intensity vs. applied pulse voltage for forward Žcircles. and reverse Žup-triangle. bias case.
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A. Chowdhury, A.J. Pal r Synthetic Metals 106 (1999) 85–88
To understand the appearance of overshoot in EL ŽFig. 1Žb. and Fig. 2., we have plotted the overshoot EL intensity vs. applied voltage for both forward and reverse bias cases ŽFig. 5.. It varies approximately linearly with applied bias. On the other hand, the ratio between the overshoot intensity and the constant part depends on the applied voltage. This suggests that the overshoot effect is not solely related to the charge injection process. As suggested by Nikitenko et al. w16x, the EL overshoot could be due to recombination of stored space charges. When the bias is turned off, the space charges of opposite kinds will form excitons under the action of internal field. Due to higher mobility of the holes than that of the electrons in rubrene, the holes will move towards Al electrode when a forwardbias square-wave voltage pulse is switched off. As aluminium is a quencher of excitons, overshoot in EL is not observed when the applied bias is low. At higher forward bias, the space charges due to electrons extend inside the device and thus the resultant excitons are not totally quenched by aluminium. We therefore observe an overshoot in EL only at higher forward bias ŽFig. 1Žb... At the falling edges of reverse-bias square-wave voltage pulse, the recombination takes place near the ITO electrode and therefore we observe overshoot effect in EL even at low negative bias.
4. Conclusions We have studied transient EL response of LEDs fabricated with rubrene LB films. We have applied two consecutive square-wave voltage pulses to separate out the effect of accumulated charges from that of the injected carriers. We have observed an instantaneous EL peak during the first 10–20 ms of the first voltage pulse, which was generated from the recombination of intrinsically accumulated holes Žfor positive pulses. or injected holes Žfor negative pulses. with intrinsically accumulated electrons. Such an instantaneous EL peak was absent during the second pulse, since all the accumulated charges have already been recombined during the first pulse. The injected charge carriers however recombine to generate constant EL during all the pulses. We have observed an overshoot in EL intensity during the falling edges of mainly reverse
voltage pulses, which has been explained in terms of recombination of stored charges under the internal field. Acknowledgements The authors thank the Department of Atomic Energy, Government of India ŽProject no. 37r16r96-R& D-II., and Department of Science and Technology, Govt. of India ŽProject No. SPrS2rM-11r94. for providing financial assistance. References w1x D.R. Baigent, N.C. Greenham, J. Gruner, R.N. Marks, R.H. Friend, ¨ S.C. Moratti, A.B. Holmes, Synth. Met. 67 Ž1994. 3, and references therein. w2x C. Hosokawa, H. Tokailin, H. Higashi, T. Kusumoto, Appl. Phys. Lett. 60 Ž1992. 1220. w3x S. Karg, V. Dyakonov, M. Meier, W. Riess, G. Paasch, Synth. Met. 67 Ž1994. 165. w4x P. Delannoy, G. Horowitz, H. Bouchriha, F. Deloffre, J.-L. Fave, F. Garnier, R. Hajlaoui, M. Heyman, F. Kouki, J.-L. Monge, P. Valat, V. Wintgens, A. Yassar, Synth. Met. 67 Ž1994. 197. w5x Y.H. Tak, J. Pommerehne, H. Vestweber, R. Sander, H. Bassler, ¨ H.-H. Horhold, Appl. Phys. Lett. 69 Ž1996. 1291. ¨ w6x J. Pommerehne, H. Vestweber, Y.H. Tak, H. Bassler, Synth. Met. 76 ¨ Ž1996. 67. ¨ ¨ w7x A.J. Pal, T. Ostergard, J. Paloheimo, H. Stubb, IEEE ˚ R. Osterbacka, J. Sel. Top. Quantum Electron. 4 Ž1998. 137. w8x J.-K. Lee, D.S. Yoo, E.S. Handy, M.F. Rubner, Appl. Phys. Lett. 69 Ž1996. 1686. w9x G.G. Roberts, M. McGinnity, W.A. Barlow, P.S. Vincett, Solid State Commun. 32 Ž1979. 683. w10x A. Wu, M. Jikei, M. Kakimoto, Y. Imai, S. Ukishima, Y. Takahashi, Chem. Lett. Ž1994. 2319. w11x A.J. Pal, J. Paloheimo, H. Stubb, Appl. Phys. Lett. 67 Ž1995. 3909. w12x V. Cimrova, M. Remmers, D. Neher, G. Wegner, Adv. Mater. 8 Ž1996. 146. ¨ w13x A.J. Pal, T. Ostergard, ˚ J. Paloheimo, H. Stubb, Appl. Phys. Lett. 69 Ž1996. 1137. w14x Y. Hamada, T. Sano, K. Shibata, K. Kuroki, Jpn. J. Appl. Phys. 34 Ž1995. 824. ¨ w15x T. Ostergard, ˚ A.J. Pal, H. Stubb, Thin Solid Films 712 Ž1998. 327–329. w16x V.R. Nikitenko, V.I. Arkhipov, Y.-H Tak, J. Pommerehne, H. Bassler, H.-H. Horhold, J. Appl. Phys. 81 Ž1997. 7514. ¨ ¨ ¨ w17x T. Ostergard, ˚ A.J. Pal, H. Stubb, J. Appl. Phys. 83 Ž1998. 2338. w18x H. Murata, C.D. Merritt, Z.H. Kafafi, H. Tokuhisa, T. Tsutsui, SPIE Proc. 3148 Ž1997. 401.
Transient electroluminescence from rubrene light-emitting diodes using double voltage pulse A. Chowdhury, A.J. Pal
)
Department of Solid State Physics, Indian Association for the CultiÕation of Science, JadaÕpur, Calcutta, 700 032, India Received 7 September 1998; accepted 19 December 1998
Abstract Light-emitting diodes using a dye-insulating matrix blend of rubrene and arachidic acid have been fabricated using Langmuir–Blodgett film deposition technique. Transient electroluminescence ŽEL. measurements were performed by applying two consecutive square-wave voltage pulses separated by a short delay. This enabled us to study the effect of accumulated charges during the first pulse separated from that of the injected carriers, which are predominant during the second pulse. During the first voltage pulse, intrinsically accumulated charges recombine to generate an instantaneous EL peak, which is absent during the second pulse. A constant EL was observed during both the pulses. We also have observed an overshoot in EL in the falling edges of mainly negative voltage pulses. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Electroluminescence; Interface phenomena; Light-emitting diodes; Overshoot effect; Rubrene; Transient electroluminescence
1. Introduction Conjugated polymers and related organic semiconductors are specific topics of interest in the field of optoelectronics in the present decade, because they offer novel solutions for light-emitting devices ŽLEDs. w1x. To understand the operation mechanism and response characteristics of LEDs, transient electroluminescence ŽEL. is often studied w2–5x, where a single square-wave voltage pulse is applied across the device. The time lag between the sharp rise of voltage pulse and the first appearance of EL signal has often been attributed to the transit time of the more mobile charge carriers through the emitting layer. More recent work on transient EL manifested that charge accumulation plays a major role in device operation w6,7x. Spin casting and vacuum evaporation methods are most frequently used techniques to fabricate organic LEDs. Devices with organized emitting materials have been fabricated by layer-by-layer self-assembly techniques w8x. The possibility of controlling the fabrication of LEDs in the molecular scale is offered by the Langmuir–Blodgett ŽLB. film deposition technique w9–13x. Most of the transient EL measurements, published so far, have been obtained by single or repetitive square-wave )
Corresponding author. E-mail: [email protected]
voltage pulse operation. In this article, we report our results on transient EL by applying two consecutive square-wave voltage pulses separated by a short delay. This enabled us to separate out the effects of intrinsically accumulated charges from that of the injected carriers. Rubrene as a dopant w14x and its LB films w15x have earlier been used as emitting layer of LEDs. Here, we present our work on the transient EL characteristics of LEDs using rubrene LB films.
2. Experimental Organic dye material rubrene Ž5, 6, 11, 12-tetraphenylnaphthacene. was purchased from Aldrich Chemical and was used as received. To fabricate LEDs, LB films of rubrene and arachidic acid Žmolar ratio 60%:40%. were deposited on patterned indium tin oxide ŽITO. coated glass substrates at a surface pressure of 20 mNrm. ITO had a sheet resistance of less than 10 VrI. Seven LB layers were deposited and thickness of each monolayer was approximately 3 nm. Aluminium ŽAl. top electrodes were thermally evaporated at a pressure below 10y5 Torr. A mechanical shutter was used to protect LB films from excess heat before and after the metal evaporation. The active area of about 10 mm2 Žthe overlap of ITO and Al.
0379-6779r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 Ž 9 9 . 0 0 1 1 3 - 7
86
A. Chowdhury, A.J. Pal r Synthetic Metals 106 (1999) 85–88
was defined by a mask. All the devices were measured in nitrogen atmosphere at room temperature. The transient EL measurements were performed by applying two consecutive square-wave voltage pulses Žrise and fall time less than 800 ns. generated by Yokogawa 7651 dc source coupled with PC controlled fast switching transistors. Both the pulses generally had widths less than 100 ms and were separated by 30 ms. Light emission from the LED structures was detected by a Electron Tubes 9256B photomultiplier. The voltage pulses and the EL response were simultaneously digitized and stored by a Hewlett-Packard HP54600B two-channel real-time storage oscilloscope. All the instruments were operated by a PC via GPIB interface.
3. Results and discussion We have studied the EL response of rubrene LEDs operated under two consecutive square-wave drive voltage pulses of different amplitudes and widths. Fig. 1Ža. shows typical EL from a device along with the applied pulse signal of 3 V amplitude and 60 ms width. During the first voltage pulse, we have observed an instantaneous EL peak,
Fig. 1. Voltage pulse and transient EL response of LEDs with 7 layers of rubrene LB films. The q3 V Ža. and q8 V Žb. voltage pulses had a width of 60 ms and separation between the two pulses was 30 ms.
Fig. 2. Voltage pulse and transient EL response of LEDs with 7 layers of rubrene LB films. The y5 V voltage pulse had the same width and separation as in Fig. 1.
which decayed to a steady-state EL level. The EL peak was absent during the second voltage pulse, while the steady-state EL part had an equal intensity as during the first pulse. When the pulses were separated by longer time, the instantaneous EL peak started to appear also during the second pulse. When more than two pulses were applied, the EL peak was observed only during the first pulse and the steady state EL was observed during all the pulses. When the devices were operated under a pulse voltage of amplitude more than 6.5 V, in addition to the instantaneous EL peak and the steady state component, an overshoot in EL was observed at the falling edges of both the voltage pulses. A typical figure ŽFig. 1Žb.. shows the EL response with the overshoot effect when operated under q8 V pulse. The I–V characteristics of the structures are symmetric and EL was obtained under both bias directions w15x. This was reflected in the transient response of EL operated under two consecutive negative voltage pulses and Fig. 2. shows a typical response for y5 V. Instantaneous EL peak, steady-state EL, and overshoot effect at the falling edges of voltage pulses have been observed in the entire voltage range of our study Žy3 V to y8 V.. Overshoot effect in EL has recently been observed in certain bilayer LEDs w16x. In transient measurements, the occurrence of instantaneous EL peak at the beginning of first voltage pulse can be interpreted in terms of charge accumulation in organic LEDs w7,17x. By applying two consecutive voltage pulses, we have separated out the effect of accumulated charges from that of the injected carriers. The LEDs under study can be considered as p-type organic semiconductor sandwiched between Al and ITO electrodes. Due to the presence of different metal electrodes, there will be charge transfer process in the device to align the fermi level within the device before any bias is applied. Finally, equilibrium will be reached with holes and electrons accumulated at the two opposite metal–semiconductor inter-
A. Chowdhury, A.J. Pal r Synthetic Metals 106 (1999) 85–88
faces. If a square-wave positive voltage pulse is applied across the device ŽITO positive., the accumulated holes at the ITOrrubrene interface, which have a higher mobility, will drift towards the cathode electrode. The holes will then recombine with accumulated electrons which are already present there and result in an instantaneous EL component. Considering the hole mobility values in rubrene Ž; 10y4 cm2rVs w18x., the transit times for the holes through the emitting layer Ž21 nm thick. will be of the order of nanoseconds. We therefore observe an instantaneous EL peak at the beginning of the first voltage pulse in Fig. 1Ža. and 1Žb.. Since the number of holes near the ITO electrode is expected to be larger than the number of electrons near the Al electrode, the excess holes will build up a space charge when a voltage is applied. These space charges will assist in electron injection and result in steady state EL. During the second pulse on the other hand, we have not observed the instantaneous EL peak, since all the accumulated charges have recombined during the first pulse. Fig. 2 shows that we have observed such instantaneous EL peak during the first voltage pulse also when two square-wave negative voltage pulses were applied across the device ŽITO negative.. Here the accumulated electrons near the Al electrode modify the barrier for the holes, so that holes can be injected easily from the Al electrode. Similarly, the holes accumulated near the ITO electrode assist in electron injection. However, due to higher barrier height for the electrons than that of the holes, the instantaneous EL peak should mainly be contributed from the recombination of injected holes and the accumulated electrons near Al electrode. Once all the accumulated electrons have recombined, the steady-state EL remains mainly limited by the injection of electrons from the ITO electrode. As expected, the instantaneous and the steady state EL increase with amplitude of the applied voltage. Their ratio, however, decreases with increasing voltage for both the
Fig. 3. The ratio between instantaneous EL peak and steady-state EL intensities vs. applied pulse voltage. Results from different structures are included in the figure.
87
Fig. 4. Instantaneous EL peak intensity vs. applied pulse voltage for forward and reverse bias case. Results from different structures are included in the figure.
biases, as shown in Fig. 3. This behaviour reflects that the effect of injected charge carriers becomes predominant than that of the accumulated ones as the voltage increases. At higher voltage, the accumulated holes might also accelerate to yield hole current instead of recombining with the electrons near the Al electrode. Finally, only steady-state EL due to injected carriers remains observable at high enough voltage. In Fig. 4, we have plotted the instantaneous EL peak intensity vs. applied voltage for the forward and reverse bias cases. For the reverse bias case, the instantaneous EL intensity increases monotonically, since higher voltage allows more holes to be injected which recombine with the accumulated electrons present near the Al electrode. The intensity during the forward-bias pulse, however, shows a maximum at about 6.5 V. Initially, an increase in voltage will prompt more holes to recombine with electrons. At higher voltage, the accumulated holes might accelerate to yield hole current instead of recombining with the electrons near the Al electrode. This will lower the instantaneous EL at higher voltage.
Fig. 5. Overshoot in EL intensity vs. applied pulse voltage for forward Žcircles. and reverse Žup-triangle. bias case.
88
A. Chowdhury, A.J. Pal r Synthetic Metals 106 (1999) 85–88
To understand the appearance of overshoot in EL ŽFig. 1Žb. and Fig. 2., we have plotted the overshoot EL intensity vs. applied voltage for both forward and reverse bias cases ŽFig. 5.. It varies approximately linearly with applied bias. On the other hand, the ratio between the overshoot intensity and the constant part depends on the applied voltage. This suggests that the overshoot effect is not solely related to the charge injection process. As suggested by Nikitenko et al. w16x, the EL overshoot could be due to recombination of stored space charges. When the bias is turned off, the space charges of opposite kinds will form excitons under the action of internal field. Due to higher mobility of the holes than that of the electrons in rubrene, the holes will move towards Al electrode when a forwardbias square-wave voltage pulse is switched off. As aluminium is a quencher of excitons, overshoot in EL is not observed when the applied bias is low. At higher forward bias, the space charges due to electrons extend inside the device and thus the resultant excitons are not totally quenched by aluminium. We therefore observe an overshoot in EL only at higher forward bias ŽFig. 1Žb... At the falling edges of reverse-bias square-wave voltage pulse, the recombination takes place near the ITO electrode and therefore we observe overshoot effect in EL even at low negative bias.
4. Conclusions We have studied transient EL response of LEDs fabricated with rubrene LB films. We have applied two consecutive square-wave voltage pulses to separate out the effect of accumulated charges from that of the injected carriers. We have observed an instantaneous EL peak during the first 10–20 ms of the first voltage pulse, which was generated from the recombination of intrinsically accumulated holes Žfor positive pulses. or injected holes Žfor negative pulses. with intrinsically accumulated electrons. Such an instantaneous EL peak was absent during the second pulse, since all the accumulated charges have already been recombined during the first pulse. The injected charge carriers however recombine to generate constant EL during all the pulses. We have observed an overshoot in EL intensity during the falling edges of mainly reverse
voltage pulses, which has been explained in terms of recombination of stored charges under the internal field. Acknowledgements The authors thank the Department of Atomic Energy, Government of India ŽProject no. 37r16r96-R& D-II., and Department of Science and Technology, Govt. of India ŽProject No. SPrS2rM-11r94. for providing financial assistance. References w1x D.R. Baigent, N.C. Greenham, J. Gruner, R.N. Marks, R.H. Friend, ¨ S.C. Moratti, A.B. Holmes, Synth. Met. 67 Ž1994. 3, and references therein. w2x C. Hosokawa, H. Tokailin, H. Higashi, T. Kusumoto, Appl. Phys. Lett. 60 Ž1992. 1220. w3x S. Karg, V. Dyakonov, M. Meier, W. Riess, G. Paasch, Synth. Met. 67 Ž1994. 165. w4x P. Delannoy, G. Horowitz, H. Bouchriha, F. Deloffre, J.-L. Fave, F. Garnier, R. Hajlaoui, M. Heyman, F. Kouki, J.-L. Monge, P. Valat, V. Wintgens, A. Yassar, Synth. Met. 67 Ž1994. 197. w5x Y.H. Tak, J. Pommerehne, H. Vestweber, R. Sander, H. Bassler, ¨ H.-H. Horhold, Appl. Phys. Lett. 69 Ž1996. 1291. ¨ w6x J. Pommerehne, H. Vestweber, Y.H. Tak, H. Bassler, Synth. Met. 76 ¨ Ž1996. 67. ¨ ¨ w7x A.J. Pal, T. Ostergard, J. Paloheimo, H. Stubb, IEEE ˚ R. Osterbacka, J. Sel. Top. Quantum Electron. 4 Ž1998. 137. w8x J.-K. Lee, D.S. Yoo, E.S. Handy, M.F. Rubner, Appl. Phys. Lett. 69 Ž1996. 1686. w9x G.G. Roberts, M. McGinnity, W.A. Barlow, P.S. Vincett, Solid State Commun. 32 Ž1979. 683. w10x A. Wu, M. Jikei, M. Kakimoto, Y. Imai, S. Ukishima, Y. Takahashi, Chem. Lett. Ž1994. 2319. w11x A.J. Pal, J. Paloheimo, H. Stubb, Appl. Phys. Lett. 67 Ž1995. 3909. w12x V. Cimrova, M. Remmers, D. Neher, G. Wegner, Adv. Mater. 8 Ž1996. 146. ¨ w13x A.J. Pal, T. Ostergard, ˚ J. Paloheimo, H. Stubb, Appl. Phys. Lett. 69 Ž1996. 1137. w14x Y. Hamada, T. Sano, K. Shibata, K. Kuroki, Jpn. J. Appl. Phys. 34 Ž1995. 824. ¨ w15x T. Ostergard, ˚ A.J. Pal, H. Stubb, Thin Solid Films 712 Ž1998. 327–329. w16x V.R. Nikitenko, V.I. Arkhipov, Y.-H Tak, J. Pommerehne, H. Bassler, H.-H. Horhold, J. Appl. Phys. 81 Ž1997. 7514. ¨ ¨ ¨ w17x T. Ostergard, ˚ A.J. Pal, H. Stubb, J. Appl. Phys. 83 Ž1998. 2338. w18x H. Murata, C.D. Merritt, Z.H. Kafafi, H. Tokuhisa, T. Tsutsui, SPIE Proc. 3148 Ž1997. 401.