- Email: [email protected]
Tuning of chromaticity in organic multiple-quantum well white light emitting devices
Synthetic Metals 108 Ž2000. 81–84 www.elsevier.comrlocatersynmet
Letter
Tuning of chromaticity in organic multiple-quantum well white light emitting devices Z.Y. Xie b
a,)
, J. Feng a , J.S. Huang a , S.Y. Liu a , Y. Wang b, J.C. Shen
b
a State Laboratory of Integrated Optoelectronics, Jilin UniÕersity, Changchun 130023, China Key Laboratory of Superamolecular Structure and Spectroscopy, Jilin UniÕersity, Changchun 130023, China
Received 26 June 1999; received in revised form 26 July 1999; accepted 2 August 1999
Abstract We fabricate organic multiple-quantum wells structure ŽMQWs. white light-emitting diodes ŽLEDs., in which blue fluorescent phenylpyridine beryllium ŽBePP2 ., orange fluorescent rubrene doped green fluorescent aluminum complex ŽAlq 3 . act as MQWs light-emitting layers between potential barriers triphenyldiamine derivative ŽTPD.. Excitons are confined in different MQWs light-emitting layers due to higher bandgap Eg of TPD. The Commission Internationale de l’Eclairage ŽCIE. coordinates of the emitted light are tuned by changing the well number ratio of BePP2 to Alq:rubrene. The organic MQWs white LEDs with three BePP2 wells show excellent optical and electrical performance. The CIE coordinates of the emitted light vary from Ž0.42, 0.38. to Ž0.34, 0.34. when forward voltages change from 7 to 13 V, which are just near to the white-light equi-energy point Ž0.33, 0.33.. The brightness and luminous efficiency are 611 cdrm2 and 0.34 lmrW at 12 V, respectively. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Organic white LEDs; Multiple-quantum wells; Chromaticity
Organic white electroluminescent devices have attracted much attention due to their promising applications as illumination light sources and backlights for laptop computers w1–7x. So far, various device configurations, such as polymer blends, multi-layer structure, polymer doped with three primary color dyes and microcavity structure are used to produce white light w8–13x. Recently we have reported that white light emission is induced by confinement in organic multiple-quantum wells ŽMQWs. white light-emitting diodes ŽLEDs. w14x. The organic MQWs white LEDs have the following merits: Ž1. The confinement of charge carriers in light-emitting quantum wells can enhance the forming probability of excitons and the light-emitting efficiencies of the devices; Ž2. compared with organic multi-layer white LEDs, the influence of bandgap matching can be negligible and the positions of different color light-emitting quantum wells can be exchangeable; Ž3. easy tuning of Commission Internationale de l’Eclairage ŽCIE. chromaticity. The CIE coordinates can be tuned by changing the ratio of the number of
) Corresponding [email protected]
author.
Fax:
q 86-431-8964939;
e-mail:
different-color light-emitting potential wells. The configuration of organic MQWs white LEDs we reported previously is ITOrTPD Ž50 nm.rBePP2 Ž5 nm.rTPD Ž4 nm.r BePP2 :rubrene Ž5 nm.rTPD Ž4 nm.rAlq 3 Ž10 nm.rAl w14x. Triphenyldiamine derivative ŽTPD. is used as a holetransporting layer and potential barrier layers. Blue fluorescent phenylpyridine Beryllium ŽBePP2 ., orange fluorescent rubrene, and green fluorescent Aluminum complex ŽAlq 3 . are used as three primary color dyes. BePP2 and BePP2 doped with rubrene act as potential wells sandwiched between TPD barrier layers, in which excitons are confined. Alq 3 is used as an electron-transporting greencolor emitter. The injected carriers distribute in different quantum wells and form excitons. The formed excitons are confined in the different MQWs light-emitting layers by potential barrier layers because the bandgap of TPD is larger than those of BePP2 , rubrene and Alq 3 . The bandgap Eg , molecular ionization energy I and the electron affinity A of TPD, BePP2 , Alq 3 and rubrene are Ž3.52, y5.39 and y1.87 eV., Ž3.16, y5.73 and y2.57 eV., Ž2.97, y5.81 and y2.84 eV. and Ž1.92, y5.51 and y3.59 eV., respectively. These values are all measured by the cyclic voltammetry method. The formed excitons decay radiatively and emit different-color light in their own MQWs light-emit-
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 9 9 . 0 0 1 6 1 - 7
82
Z.Y. Xie et al.r Synthetic Metals 108 (2000) 81–84
Fig. 1. The materials used in this study and the configuration of organic MQWs white LEDs.
ting layers. The white light emission comes from a combination of these different-energy photons. But in this kind of device ITOrTPD Ž50 nm.rBePP2 Ž5 nm.rTPD Ž4 nm.rBePP2 :rubrene Ž5 nm.rTPD Ž4 nm.rAlq 3 Ž10 nm.rAl, the fraction of orange light intensity of rubrene is stronger than those of blue and green color due to the higher fluorescent efficiency of rubrene than those of Alq 3 and BePP2 . Especially the low fluorescent efficiency of BePP2 influences seriously the chromaticity of the emitted light and makes the CIE coordinates deviate to orange
region. One of the characteristics of this kind of organic MQWs white LEDs is of easy tuning of CIE chromaticity by increasing the number of quantum wells of low-efficient fluorescent dyes. In this paper, we will discuss the shift of CIE chromaticity when the organic MQWs white LEDs have different number of blue light-emitting potential wells. Fig. 1 shows the materials used in this experiment and the configuration of the organic MQWs white LEDs. TPD is used as a hole transporting layer Ž50 nm. and the
Fig. 2. Proposed mechanism for organic MQWs white LEDs at a forward bias.
Z.Y. Xie et al.r Synthetic Metals 108 (2000) 81–84
83
layers surrounding the light-emitting layers have a higher bandgap compared with the light-emitting layers. The bandgap difference helps to confine excitons to the MQWs light-emitting layers, where they decay radiatively to produce different-color light. Due to the low fluorescent efficiency of BePP2 , the fraction of blue light is small compared with that of orange light in organic MQWs white LEDs. The number of blue BePP2 light-emitting wells are increased in order to balance the intensity ratio of different-color light in the EL spectrum for this kind of white LEDs. Fig. 3 shows the shift of CIE coordinates in organic MQWs white LEDs with different number of BePP2 wells at various biases. The CIE coordinates change from Ž0.48, 0.45. to Ž0.41, 0.41. when the forward bias changes from 7 to 13 V for the organic MQWs white LEDs with one BePP2 wells. Increasing the number of BePP2 wells, the fraction of blue light from BePP2 becomes larger compared with that of orange light from rubrene in the EL spectra and the CIE coordinates of the emitted light shift blue. When the organic MQWs white LED has three BePP2 wells, the CIE coordinates of the emitted light change from Ž0.42, 0.38. to Ž0.34, 0.34. while the forward bias are increased from 7 to 13 V. The region is just near to the white-light equi-energy point Ž0.33, 0.33.. Table 1 shows the performance of organic MQWs white LEDs with different number of BePP2 wells at various biases. At 10 V bias, the CIE coordinates of organic MQWs white LEDs with one BePP2 light emitting well are Ž0.42, 0.47.. But when the organic MQWs white LEDs have two and three BePP2 light emitting wells, the CIE coordinates are Ž0.38, 0.40. and Ž0.37, 0.35., respectively. At the same bias, the CIE coordinates of the emitted light shift to the equi-energy white point Ž0.33, 0.33. when the number of BePP2 light-emitting wells is increased. The coordinates of the three kinds of organic MQWs white LEDs are Ž0.41, 0.45., Ž0.34, 0.45. and Ž0.35, 0.35. at 12 V, respectively. The results show that the chromaticity of organic MQWs white LEDs can be tuned by adding the number of BePP2 light emitting wells. In organic MQWs LEDs, the excitons are confined in narrow band-gap lightemitting layers, in which excitons decay radiatively to produce light. When the number of potential wells are small the excitons can be distributed in different wells uniformly. When increasing the number of BePP2 lightemitting wells, the fraction of excitons in BePP2 wells will
Fig. 3. The shift of CIE coordinates of organic MQWs white LEDs with different number of BePP2 wells at various voltages.
potential barrier layers Ž4 nm.. BePP2 layer Ž5 nm. and Alq 3 doped with Rubrene layer act as potential wells sandwiched between TPD barrier layers, respectively. The ratio of Alq 3 :rubrene is 100:1–3. Alq 3 is used as a green color emitter and electron-transporting layer Ž30 nm.. Thinner Alq 3 layer will result in a significant fraction of holes reaching the Al contacts before they can radiatively recombine with an electron w15x. Air-stable aluminum ŽAl. is deposited as an electron injection electrode. Compared with the device configuration reported by us previously w14x, Alq 3 is used to serve as a matrix of Rubrene due to sufficient energy transfer between them w16x. The configuration of organic MQWs white LEDs shown in Fig. 1 is ITOrTPDŽ50 nm.rBePP2 Ž5 nm.rTPDŽ4 nm.r PPP rAlq 3 :rubrene Ž5 nm.rTPD Ž4 nm.rAlq 3 Ž30 nm.rAl. A series of organic MQWs white LEDs with different number of blue BePP2 potential wells are fabricated. The fabrication of organic white LEDs just is the same as reported previously w14x. Fig. 2 shows the schematic diagram of the band gap model of the organic MQWs white LEDs at a forward bias. When a forward voltage is applied ŽITO and Al contacts are wired as an anode and cathode, respectively., holes and electrons are injected from the anode and cathode, respectively. Holes and electrons form a spatial distribution in different MQWs. The TPD potential barrier
Table 1 The performance of organic MQWs white LEDs with different BePP2 well numbers at various voltages No. of BePP2
1 2 3
10 V
12 V
Brightness Žcdrm2 .
Luminous efficiency ŽlmrW.
CIE coordinates x y
Brightness Žcdrm2 .
Luminous efficiency ŽlmrW.
CIE coordinates x y
131 122 142
– – 0.44
0.42 0.38 0.37
322 314 611
– – 0.34
0.41 0.34 0.35
0.47 0.40 0.35
0.45 0.45 0.35
84
Z.Y. Xie et al.r Synthetic Metals 108 (2000) 81–84
be enlarged compared with that of excitons in Alq 3 :rubrene well. The blue light from BePP2 and the orange light from rubrene are balanced though the efficiency of BePP2 is low compared with rubrene. The CIE coordinates shift blue and are near to white-light equi-energy point increasing the number of BePP2 light-emitting wells. At 12 V forward bias, the organic MQWs white LEDs with three BePP2 light-emitting wells show brightness of 611 cdrm2 and luminous efficiency of 0.34 lmrW, respectively. In summary, we fabricate organic MQWs white LEDs, in which blue fluorescent BePP2 , orange fluorescent rubrene, and green fluorescent Alq 3 are used as three primary color fluorescent dyes. The white light comes from the excitons confined in different the light-emitting potential wells. The CIE coordinates of the emitted light are tuned by increasing the number of blue BePP2 lightemitting potential wells. The organic MQWs white LEDs with three BePP2 wells show excellent optical and electrical performance. The region of CIE coordinates of the emitted light at various biases Ž7–13 V. is Ž0.42–0.34, 0.38–0.34., which are just adjacent to the white-light equi-energy point Ž0.33, 0.33..
Acknowledgements This work was supported by the National Natural Science Foundation of China wNo. 69637010x and w597905006x
and the National ‘‘863’’ project wNo. 307-12-04Ž02.x. Dr. Y. Zhao and C.N. Lee of Jilin University, China, are also acknowledged for their valuable discussions.
References w1x J. Kido, M. Kimura, K. Nagai, Science 267 Ž1995. 1332. w2x J. Kido, C. Ohtaki, K. Hongawa, K. Okuyama, K. Ngai, Jpn. J. Appl. Phys. 32 Ž1993. L917. w3x Y. Hammada, T. Sano, M. Fujita, T. Fujii, Y. Nishio, K. Shibata, Jpn. J. Appl. Phys. 32 Ž1993. L514. w4x J. Kido, Trends Polym. Sci. 10 Ž1994. 350. w5x J. Kido, W. Ikeda, M. Kimura, K. Nagai, Jpn. J. Appl. Phys. 35 Ž1996. L394. w6x S. Tokito, J. Sakata, Y. Taga, J. Appl. Phys. 77 Ž1995. 1985. w7x J. Kido, W. Ikeda, M. Kimura, K. Nagai, Jpn. J. Appl. Phys. 35 Ž1996. L394. w8x J. Kido, K. Hongawa, K. Okuyama, K. Nagai, Appl. Phys. Lett. 64 Ž1994. 815. w9x M. Strukeji, R.H. Jordan, A. Dodabalapur, J. Am. Chem. Soc. 118 Ž1996. 1213. w10x R.H. Jordan, A. Dodabalapur, M. Strukeji, T.M. Miller, Appl. Phys. Lett. 68 Ž1996. 1192. w11x M. Granstom, O. Inganas, Appl. Phys. Lett. 68 Ž1996. 147. w12x Y. Yang, Q. Pei, J. Appl. Phys. 81 Ž1997. 3294. w13x A. Dodabalapur, L.J. Rothberg, T.M. Miller, Appl. Phys. Lett. 65 Ž1994. 2308. w14x Z.Y. Xie, Y.Q. Li, J.S. Huang, S.Y. Liu et al., Syn. Met., in press. w15x P.E. Burrows, S.R. Forrest, Appl. Phys. Lett. 64 Ž1994. 2285. w16x J.S. Huang, K.X. Yang, Z.Y. Xie et al., Appl. Phys. Lett. 73 Ž1998. 3348.
Letter
Tuning of chromaticity in organic multiple-quantum well white light emitting devices Z.Y. Xie b
a,)
, J. Feng a , J.S. Huang a , S.Y. Liu a , Y. Wang b, J.C. Shen
b
a State Laboratory of Integrated Optoelectronics, Jilin UniÕersity, Changchun 130023, China Key Laboratory of Superamolecular Structure and Spectroscopy, Jilin UniÕersity, Changchun 130023, China
Received 26 June 1999; received in revised form 26 July 1999; accepted 2 August 1999
Abstract We fabricate organic multiple-quantum wells structure ŽMQWs. white light-emitting diodes ŽLEDs., in which blue fluorescent phenylpyridine beryllium ŽBePP2 ., orange fluorescent rubrene doped green fluorescent aluminum complex ŽAlq 3 . act as MQWs light-emitting layers between potential barriers triphenyldiamine derivative ŽTPD.. Excitons are confined in different MQWs light-emitting layers due to higher bandgap Eg of TPD. The Commission Internationale de l’Eclairage ŽCIE. coordinates of the emitted light are tuned by changing the well number ratio of BePP2 to Alq:rubrene. The organic MQWs white LEDs with three BePP2 wells show excellent optical and electrical performance. The CIE coordinates of the emitted light vary from Ž0.42, 0.38. to Ž0.34, 0.34. when forward voltages change from 7 to 13 V, which are just near to the white-light equi-energy point Ž0.33, 0.33.. The brightness and luminous efficiency are 611 cdrm2 and 0.34 lmrW at 12 V, respectively. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Organic white LEDs; Multiple-quantum wells; Chromaticity
Organic white electroluminescent devices have attracted much attention due to their promising applications as illumination light sources and backlights for laptop computers w1–7x. So far, various device configurations, such as polymer blends, multi-layer structure, polymer doped with three primary color dyes and microcavity structure are used to produce white light w8–13x. Recently we have reported that white light emission is induced by confinement in organic multiple-quantum wells ŽMQWs. white light-emitting diodes ŽLEDs. w14x. The organic MQWs white LEDs have the following merits: Ž1. The confinement of charge carriers in light-emitting quantum wells can enhance the forming probability of excitons and the light-emitting efficiencies of the devices; Ž2. compared with organic multi-layer white LEDs, the influence of bandgap matching can be negligible and the positions of different color light-emitting quantum wells can be exchangeable; Ž3. easy tuning of Commission Internationale de l’Eclairage ŽCIE. chromaticity. The CIE coordinates can be tuned by changing the ratio of the number of
) Corresponding [email protected]
author.
Fax:
q 86-431-8964939;
e-mail:
different-color light-emitting potential wells. The configuration of organic MQWs white LEDs we reported previously is ITOrTPD Ž50 nm.rBePP2 Ž5 nm.rTPD Ž4 nm.r BePP2 :rubrene Ž5 nm.rTPD Ž4 nm.rAlq 3 Ž10 nm.rAl w14x. Triphenyldiamine derivative ŽTPD. is used as a holetransporting layer and potential barrier layers. Blue fluorescent phenylpyridine Beryllium ŽBePP2 ., orange fluorescent rubrene, and green fluorescent Aluminum complex ŽAlq 3 . are used as three primary color dyes. BePP2 and BePP2 doped with rubrene act as potential wells sandwiched between TPD barrier layers, in which excitons are confined. Alq 3 is used as an electron-transporting greencolor emitter. The injected carriers distribute in different quantum wells and form excitons. The formed excitons are confined in the different MQWs light-emitting layers by potential barrier layers because the bandgap of TPD is larger than those of BePP2 , rubrene and Alq 3 . The bandgap Eg , molecular ionization energy I and the electron affinity A of TPD, BePP2 , Alq 3 and rubrene are Ž3.52, y5.39 and y1.87 eV., Ž3.16, y5.73 and y2.57 eV., Ž2.97, y5.81 and y2.84 eV. and Ž1.92, y5.51 and y3.59 eV., respectively. These values are all measured by the cyclic voltammetry method. The formed excitons decay radiatively and emit different-color light in their own MQWs light-emit-
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 9 9 . 0 0 1 6 1 - 7
82
Z.Y. Xie et al.r Synthetic Metals 108 (2000) 81–84
Fig. 1. The materials used in this study and the configuration of organic MQWs white LEDs.
ting layers. The white light emission comes from a combination of these different-energy photons. But in this kind of device ITOrTPD Ž50 nm.rBePP2 Ž5 nm.rTPD Ž4 nm.rBePP2 :rubrene Ž5 nm.rTPD Ž4 nm.rAlq 3 Ž10 nm.rAl, the fraction of orange light intensity of rubrene is stronger than those of blue and green color due to the higher fluorescent efficiency of rubrene than those of Alq 3 and BePP2 . Especially the low fluorescent efficiency of BePP2 influences seriously the chromaticity of the emitted light and makes the CIE coordinates deviate to orange
region. One of the characteristics of this kind of organic MQWs white LEDs is of easy tuning of CIE chromaticity by increasing the number of quantum wells of low-efficient fluorescent dyes. In this paper, we will discuss the shift of CIE chromaticity when the organic MQWs white LEDs have different number of blue light-emitting potential wells. Fig. 1 shows the materials used in this experiment and the configuration of the organic MQWs white LEDs. TPD is used as a hole transporting layer Ž50 nm. and the
Fig. 2. Proposed mechanism for organic MQWs white LEDs at a forward bias.
Z.Y. Xie et al.r Synthetic Metals 108 (2000) 81–84
83
layers surrounding the light-emitting layers have a higher bandgap compared with the light-emitting layers. The bandgap difference helps to confine excitons to the MQWs light-emitting layers, where they decay radiatively to produce different-color light. Due to the low fluorescent efficiency of BePP2 , the fraction of blue light is small compared with that of orange light in organic MQWs white LEDs. The number of blue BePP2 light-emitting wells are increased in order to balance the intensity ratio of different-color light in the EL spectrum for this kind of white LEDs. Fig. 3 shows the shift of CIE coordinates in organic MQWs white LEDs with different number of BePP2 wells at various biases. The CIE coordinates change from Ž0.48, 0.45. to Ž0.41, 0.41. when the forward bias changes from 7 to 13 V for the organic MQWs white LEDs with one BePP2 wells. Increasing the number of BePP2 wells, the fraction of blue light from BePP2 becomes larger compared with that of orange light from rubrene in the EL spectra and the CIE coordinates of the emitted light shift blue. When the organic MQWs white LED has three BePP2 wells, the CIE coordinates of the emitted light change from Ž0.42, 0.38. to Ž0.34, 0.34. while the forward bias are increased from 7 to 13 V. The region is just near to the white-light equi-energy point Ž0.33, 0.33.. Table 1 shows the performance of organic MQWs white LEDs with different number of BePP2 wells at various biases. At 10 V bias, the CIE coordinates of organic MQWs white LEDs with one BePP2 light emitting well are Ž0.42, 0.47.. But when the organic MQWs white LEDs have two and three BePP2 light emitting wells, the CIE coordinates are Ž0.38, 0.40. and Ž0.37, 0.35., respectively. At the same bias, the CIE coordinates of the emitted light shift to the equi-energy white point Ž0.33, 0.33. when the number of BePP2 light-emitting wells is increased. The coordinates of the three kinds of organic MQWs white LEDs are Ž0.41, 0.45., Ž0.34, 0.45. and Ž0.35, 0.35. at 12 V, respectively. The results show that the chromaticity of organic MQWs white LEDs can be tuned by adding the number of BePP2 light emitting wells. In organic MQWs LEDs, the excitons are confined in narrow band-gap lightemitting layers, in which excitons decay radiatively to produce light. When the number of potential wells are small the excitons can be distributed in different wells uniformly. When increasing the number of BePP2 lightemitting wells, the fraction of excitons in BePP2 wells will
Fig. 3. The shift of CIE coordinates of organic MQWs white LEDs with different number of BePP2 wells at various voltages.
potential barrier layers Ž4 nm.. BePP2 layer Ž5 nm. and Alq 3 doped with Rubrene layer act as potential wells sandwiched between TPD barrier layers, respectively. The ratio of Alq 3 :rubrene is 100:1–3. Alq 3 is used as a green color emitter and electron-transporting layer Ž30 nm.. Thinner Alq 3 layer will result in a significant fraction of holes reaching the Al contacts before they can radiatively recombine with an electron w15x. Air-stable aluminum ŽAl. is deposited as an electron injection electrode. Compared with the device configuration reported by us previously w14x, Alq 3 is used to serve as a matrix of Rubrene due to sufficient energy transfer between them w16x. The configuration of organic MQWs white LEDs shown in Fig. 1 is ITOrTPDŽ50 nm.rBePP2 Ž5 nm.rTPDŽ4 nm.r PPP rAlq 3 :rubrene Ž5 nm.rTPD Ž4 nm.rAlq 3 Ž30 nm.rAl. A series of organic MQWs white LEDs with different number of blue BePP2 potential wells are fabricated. The fabrication of organic white LEDs just is the same as reported previously w14x. Fig. 2 shows the schematic diagram of the band gap model of the organic MQWs white LEDs at a forward bias. When a forward voltage is applied ŽITO and Al contacts are wired as an anode and cathode, respectively., holes and electrons are injected from the anode and cathode, respectively. Holes and electrons form a spatial distribution in different MQWs. The TPD potential barrier
Table 1 The performance of organic MQWs white LEDs with different BePP2 well numbers at various voltages No. of BePP2
1 2 3
10 V
12 V
Brightness Žcdrm2 .
Luminous efficiency ŽlmrW.
CIE coordinates x y
Brightness Žcdrm2 .
Luminous efficiency ŽlmrW.
CIE coordinates x y
131 122 142
– – 0.44
0.42 0.38 0.37
322 314 611
– – 0.34
0.41 0.34 0.35
0.47 0.40 0.35
0.45 0.45 0.35
84
Z.Y. Xie et al.r Synthetic Metals 108 (2000) 81–84
be enlarged compared with that of excitons in Alq 3 :rubrene well. The blue light from BePP2 and the orange light from rubrene are balanced though the efficiency of BePP2 is low compared with rubrene. The CIE coordinates shift blue and are near to white-light equi-energy point increasing the number of BePP2 light-emitting wells. At 12 V forward bias, the organic MQWs white LEDs with three BePP2 light-emitting wells show brightness of 611 cdrm2 and luminous efficiency of 0.34 lmrW, respectively. In summary, we fabricate organic MQWs white LEDs, in which blue fluorescent BePP2 , orange fluorescent rubrene, and green fluorescent Alq 3 are used as three primary color fluorescent dyes. The white light comes from the excitons confined in different the light-emitting potential wells. The CIE coordinates of the emitted light are tuned by increasing the number of blue BePP2 lightemitting potential wells. The organic MQWs white LEDs with three BePP2 wells show excellent optical and electrical performance. The region of CIE coordinates of the emitted light at various biases Ž7–13 V. is Ž0.42–0.34, 0.38–0.34., which are just adjacent to the white-light equi-energy point Ž0.33, 0.33..
Acknowledgements This work was supported by the National Natural Science Foundation of China wNo. 69637010x and w597905006x
and the National ‘‘863’’ project wNo. 307-12-04Ž02.x. Dr. Y. Zhao and C.N. Lee of Jilin University, China, are also acknowledged for their valuable discussions.
References w1x J. Kido, M. Kimura, K. Nagai, Science 267 Ž1995. 1332. w2x J. Kido, C. Ohtaki, K. Hongawa, K. Okuyama, K. Ngai, Jpn. J. Appl. Phys. 32 Ž1993. L917. w3x Y. Hammada, T. Sano, M. Fujita, T. Fujii, Y. Nishio, K. Shibata, Jpn. J. Appl. Phys. 32 Ž1993. L514. w4x J. Kido, Trends Polym. Sci. 10 Ž1994. 350. w5x J. Kido, W. Ikeda, M. Kimura, K. Nagai, Jpn. J. Appl. Phys. 35 Ž1996. L394. w6x S. Tokito, J. Sakata, Y. Taga, J. Appl. Phys. 77 Ž1995. 1985. w7x J. Kido, W. Ikeda, M. Kimura, K. Nagai, Jpn. J. Appl. Phys. 35 Ž1996. L394. w8x J. Kido, K. Hongawa, K. Okuyama, K. Nagai, Appl. Phys. Lett. 64 Ž1994. 815. w9x M. Strukeji, R.H. Jordan, A. Dodabalapur, J. Am. Chem. Soc. 118 Ž1996. 1213. w10x R.H. Jordan, A. Dodabalapur, M. Strukeji, T.M. Miller, Appl. Phys. Lett. 68 Ž1996. 1192. w11x M. Granstom, O. Inganas, Appl. Phys. Lett. 68 Ž1996. 147. w12x Y. Yang, Q. Pei, J. Appl. Phys. 81 Ž1997. 3294. w13x A. Dodabalapur, L.J. Rothberg, T.M. Miller, Appl. Phys. Lett. 65 Ž1994. 2308. w14x Z.Y. Xie, Y.Q. Li, J.S. Huang, S.Y. Liu et al., Syn. Met., in press. w15x P.E. Burrows, S.R. Forrest, Appl. Phys. Lett. 64 Ž1994. 2285. w16x J.S. Huang, K.X. Yang, Z.Y. Xie et al., Appl. Phys. Lett. 73 Ž1998. 3348.