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Electroluminescent devices with LB films of amphiphilic 8-hydroxyquinoline complexes as emitter
Thin Solid Films 363 (2000) 130±133 www.elsevier.com/locate/tsf
Electroluminescent devices with LB ®lms of amphiphilic 8-hydroxyquinoline complexes as emitter Jian-Ming Ouyang a,*, Zhi-Ming Zhang a, Xiao-Le Zhang a, Qiu-Jin Chao a, Chang-Jiang Ye b, Hong-Wei Di b a
Department of Chemistry, Jinan University, Guangzhou 510632, People's Republic of China b Department of Physics, Jinan University, Guangzhou 510632, People's Republic of China
Abstract Eleven amphiphilic complexes of 2-(N-hexadecylcarbamoyl)-8-hydroxy quinoline (HL), ML2 (M Mg, Ca, Mn, Co, Ni, Zn, Cd, Pb), ML2Cl (M La, Er) and AlL2OH form a stable monolayer on pure water subphase and can be easily fabricated as highly ordered LB ®lms. Absorption and ¯uorescent spectra showed that the LB ®lms were J-aggregates. Low-angle X-ray diffraction measurements showed these LB ®lms had good homogeneity with a bilayer spacing of ca. 4.86±5.13 nm. These LB ®lms exhibited intense ¯uorescence and were used as emitting material of electroluminescent devices. The deposited surface pressure of the LB ®lms had a strong in¯uence on the EL intensity. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Langmuir±Blodgett ®lms; Electroluminescent device; 8-Hydroxyquinoline derivatives
1. Introduction Organic electroluminescent (EL) devices have recently received much attention because they can potentially produce emissions of many colors throughout the visible spectrum and because of their possible application as large-area light-emitting displays [1±3]. The most important milestone led to the development of a stable organic EL device was the discovery of tris(8-hydroxyquinolinato)aluminum (Alq3) [4]. After that, many other metal chelates derived from 8-hydroxyquinoline, such as Znq2, Beq2, Mgq2, Zn(mq)2, Be(mq)2 and Al(prq)2 (mq and prq are 2methyl-8-hydroxyquinoline and 7-propyl-8-hydroxyquinoline) were used as emitting elements in electroluminescent devices [5]. In these devices, high external quantum ef®ciency and brightness are achievable at a driving voltage below 10 V. LB ®lm technique makes it possible to prepare organic functional ultrathin ®lms with a controlled thickness at a molecular size and with-de®ned molecular orientation, therefore, if some amphiphilic complexes with 8-hydroxyquinoline can be synthesized and further be incorporated in LB ®lms, these LB ®lms may be used as the emitting layer of EL devices. The highly ordered arrangement of emitting
molecules may be able to increase the luminous ef®ciency of EL devices. 2. Experimental The synthesis of the amphiphilic ligand, 2-(N-hexadecylcarbamoyl)-8-hydroxy quinoline (HL) (its molecular structure is shown in the inset in Fig. 1) was carried out by a previously reported method [6]. The HL complexes, ML2 (M Mg, Ca, Mn, Co, Ni, Zn, Cd, Pb), ML2Cl (M La, Er) and AlL2OH were prepared by coordinating the metal salts with HL in boiling aqueous methanol solution and details of this procedure will be reported elsewhere. Formation of air±water monolayers and the deposition of the LB ®lms followed the previous paper [7]. The EL device had a single layer of organic material sandwiched between two injecting electrodes: ITO/emitting layer/Al. The emitting layer was a LB ®lm of amphiphilic complex and the emitting area was 2 £ 3 mm 2. 3. Results and discussion 3.1. Formation of monolayer All the 11 amphiphilic complexes can form stable and condensed monolayers at the air±water interface. Fig. 2 shows typical p±A isotherms of the complexes MgL2 and
0040-6090/00/$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S00 40-6090(99)0101 8-4
J.-M. Ouyang et al. / Thin Solid Films 363 (2000) 130±133
131
ence of the structures of these three complexes from those of the other eight complexes. The hybrid orbitals of the central metal ions in complexes ML2 (M Mn, Co, Ni, Zn, Cd, Pb) and ML2Cl (M La, Er) are sp 3d 2 or more superior. HL is coordinated to the central metal ion as a tridentate ligand (with coordinating atoms: heterocyclic N, phenolic O, and carbonyl O) [8]. However, the hybrid orbitals of the central metal ions in complexes MgL2, CaL2 and AlL2OH are sp 3. HL is coordinated as a bidentate (with coordinating atoms heterocyclic N and phenolic O). Upon compression, the steric strain of the molecular interaction in the complex with sp 3 hybrid orbitals of the central metal ion is larger than that in the complex with sp 3d 2 hybrid orbitals. It makes the angle between the quinoline ring planes in the former be larger and results in an increase of the limiting areas. 3.2. UV-visible spectra of LB ®lms Fig. 1. Spectrum of electroluminescence of an Al / ErL2Cl LB ®lm / ITO EL device. The inset is the molecular structure of HL.
ErL2Cl with high collapse surface pressures of ca. 56 and 54 mN/m, respectively. As a result of the change of the molecular packing, the isotherm of ErL2Cl consists of two segments, (a) and (b), and two limiting areas, 1.23 (A0(a)) and 0.70 (A0(b)) nm 2 per ErL2Cl molecule, can be obtained by extrapolating (a) and (b) to zero surface pressure. It suggested that the ring planes change from a `lie ¯at on' to an `edge-on' structure in the monolayer upon compression (see the insets in Fig. 2). The p±A isotherms of complexes MnL2, CoL2, NiL2, ZnL2, CdL2, PbL2 and LaL2Cl showed similar character to ErL2Cl. However, the complexes MgL2, CaL2 and AlL2OH only showed one limiting area. It may be due to the differ-
The absorption peaks of ErL2Cl in LB ®lms occurred at ca. 208, 271, 356 and 433 nm. The bands at 271 and 356 nm are ascribed to 1Bb, and 1La bands [8] and the band at 433 nm is assigned to an electron transfer transition from HL to metallic ion (LMCT) band. In comparison with the DMF solution, the absorption spectra of the complex in LB ®lms showed considerable line broadening and red shift. Table 1 lists the UV-visible spectra of all the complexes in LB ®lms. The absorption maxima of the LMCT bands in the LB ®lms bathochromically shifted ca. 4±17 nm compared with those in DMF solution. Besides, a strong absorption band at ca. 205 nm appeared in the LB ®lms. This band corresponds to the n ! p transition of alkylcarbamoyl group in C-2 position of quinoline ring, and not be seen in DMF solutions due to the absorption of solvent. The difference in UV-visible spectra in LB ®lms from DMF solutions was due to the ordered orientation of complex molecules and the strong interaction between the molecules in the LB ®lms. As these wavelengths remain the same for ®lms with differing numbers of layers, this implies that the interaction exists within rather than between the layers. Since J-aggregates are characterized by an intense narrow absorption band with a bathochromical shift, relative to the monomeric band [9], the red-shifted intense absorption of the complexes in LB ®lms suggests that J-aggregates were formed in the complexes LB ®lms. Plotting the absorbance vs. the number of layers of ErL2Cl LB ®lms, we obtain a linear relationship through the origin. It implies that the monolayer of ErL2Cl at the air±water interface is uniform and the monolayer deposition is reproducible and the ®lms have vertical homogeneity. 3.3. Fluorescence
Fig. 2. Surface pressure±area isotherms. (X) MgL2,; (B) ErL2Cl. The insets show the orientational cahnge of ErL2Cl molecules in monolayer at selected pressure.
In general, the complexes of 8-quinolinol or 5-sulfo-8quinolinol with some metal ions such as Mn(II), Co(II), Ni(II), Cu(II) and Fe(II,III) are non-¯uorescent at room temperature [10]. However, all the complexes of HL emit ¯uorescence at room temperature. The magnitudes of the
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J.-M. Ouyang et al. / Thin Solid Films 363 (2000) 130±133
Table 1 UV-visible spectra data of amphiphilic complexes LB ®lms LB ®lms
MgL2 CaL2 AlL2OH MnL2 CoL2 NiL2 ZnL2 CdL2 PbL2 LaL2Cl ErL2Cl
Charge
Transfer 1
Bands
Ligand ®eld band
1
(n ! p)
( Bb)
( La)
201 199 204 201 207 205 265 200 202 205 208
270 269 281 273 276 268 283 277 271 274 271
353 352 354 352 354 355 354 354 360 355 356
450 416 405 435 506 466 453 467 463 410 433
¯uorescent strength of these complexes (1.0 £ 10 25 mol/l) follow the decreasing order: Cd (32) . Al (23) . Mg (9.8) . Zn (8.2), Ca (8.2) . Er (7.6) . La (5.6) . Mn (5.5) . Ni (4.3), Pb (4.3) . Co (3.8). Obvious ¯uorescence emissions, even with only one layer of ErL2Cl, could be detected. One of the probable reasons for the strong ¯uorescence is that microcrystals of this complex are generated at the substrate±®lm interface. 3.4. Low-angle X-ray diffraction The thickness of the LB ®lms has been measured by lowangle X-ray diffraction measurement. From the position of the Bragg peaks, a thickness of ca. 4.86±5.13 nm per bilayer can be calculated, in agreement with the values estimated by the CPK model (2 £ 2.5 nm). It further indicates that these amphiphilic complexes were fabricated as highly ordered LB ®lms. A slight effect of the deposition pressure to the thickness of LB ®lms was observed. The thickness of ErL2Cl LB ®lms prepared at 10, 20, 30, 40, 50 mN/m are approximately constant at 5.13 nm except for p 10 mN/m (Table 2). The decrease of thickness of ErL2Cl LB ®lms deposited at 10 mN/m was caused by the decrease of the contribution of quinoline rings of ErL2Cl molecule to the height of LB ®lms because quinoline rings oriented with faces touching water in low surface pressure (the inset in Fig. 2). In high surface pressure, the fact that the thickness remains nearly constant can be ascribed to that the p±A isotherm for ErL2Cl shows a rapid increase in surface pressure with decreasing surface area. Table 2 The thickness of 21-layer ErL2Cl LB ®lms built up at different deposition pressure p (mN/m) Thickness (nm)
10 4.76
20 5.10
30 5.13
40 5.11
50 5.13
Fig. 3. Current-voltage (I±V) characteristics of EL devices with 21-layer ErL2Cl LB ®lms deposited at different surface pressures.
3.5. EL devices with LB ®lms as emitter The intensive ¯uorescence properties of the LB ®lms of these complexes make them possible be used as emitting layer of EL devices. Fig. 3 shows the current-voltage (I±V) characteristics of EL devices with 21-layer LB ®lms deposited at a surface pressures of 10, 20 and 30 mN/m, respectively. The deposited surface pressure of the LB ®lms strongly in¯uenced the current-voltage (I±V) characteristics and the electroluminescence of the organic EL diode. When the LB ®lms were prepared at surface pressure of 20 mN/m, the current density crossing this kind of diodes had stronger nonlinear I±V characteristics and the organic diode had higher EL intensity than another diode prepared at lower surface pressure (10 mN/m). At lower surface pressure, there may be some point defects or pinholes in the LB ®lm, in which the high tunnel current would increase exponentially with the ®eld, leading to an increase in the current density crossing the EL device and a decrease in the recombination probability of electrons and holes at higher voltage. When the surface pressure is higher than 20 mN/m the I±V characteristics of the devices are similar. It indicates there may be fewer point defects or pinholes in these LB ®lms and the difference in quality of these ®lms are small. That is, the LB ®lms with surface pressures higher than 20 mN/m can be important in raising the ErL2Cl EL intensity and ef®ciency. Fig. 1 shows EL spectrum of the EL devices with 21-layer ErL2Cl LB ®lms deposited at 30 mN/m as the emitter. The EL emission spectrum was independent of the driving voltage and current. This result indicates that the radiative recombination of injected electrons and holes takes place in the ErL2Cl LB ®lms [11].
J.-M. Ouyang et al. / Thin Solid Films 363 (2000) 130±133
Acknowledgements This research work was supported by Natural Science Foundation of Guangdong Province and the Foundation of National Key Laboratory of Organic Solid of Institute of Chemistry of the Chinese Academy of Sciences. References [1] C.H. Chen, J.-M. Shi, Coord. Chem. Rev. 171 (1998) 161. [2] A. Kraft, A.C. Grimsdale, A.B. Holmes, Angew. Chem. Int. Ed. 37 (1998) 402.
133
[3] S. Dirr, S. Wiese, H.-H. Johannes, Synth. Met. 91 (1997) 53. [4] C.W. Tang, S.A. Vanslyke, C.H. Chen, Appl. Phys. Lett. 51 (1987) 913. [5] Y. Hamada, T. Sano, M. Fujita, T. Fujita, Y. Nishio, K. Shibata, Jpn. J. Appl. Phys. 32 (1993) L514. [6] J.-M. Ouyang, Z.-H. Tai, C.-Y. Jiang, Spectrosc. Lett. 29 (1996) 763. [7] J.-M. Ouyang, C. Li, Y.-Q. Li, Thin Solid Films 348 (1999) 242. [8] J.-M. Ouyang, Z.-H. Tai, Spectrosc. Lett. 31 (1998) 1001. [9] A.K. Dutta, A.J. Pal, T.N. Misra, Bull. Chem. Soc. Jpn. 66 (1993) 3575. [10] D.C. Bhatnagar, L.S. Forster, Spectrochim. Acta 21 (1965) 1803. [11] J.-M. Ouyang, L. Li, Z.-H. Tai, G.-M. Wang, Z.-H. Lu, Chem. Commun. (1997) 815.
Electroluminescent devices with LB ®lms of amphiphilic 8-hydroxyquinoline complexes as emitter Jian-Ming Ouyang a,*, Zhi-Ming Zhang a, Xiao-Le Zhang a, Qiu-Jin Chao a, Chang-Jiang Ye b, Hong-Wei Di b a
Department of Chemistry, Jinan University, Guangzhou 510632, People's Republic of China b Department of Physics, Jinan University, Guangzhou 510632, People's Republic of China
Abstract Eleven amphiphilic complexes of 2-(N-hexadecylcarbamoyl)-8-hydroxy quinoline (HL), ML2 (M Mg, Ca, Mn, Co, Ni, Zn, Cd, Pb), ML2Cl (M La, Er) and AlL2OH form a stable monolayer on pure water subphase and can be easily fabricated as highly ordered LB ®lms. Absorption and ¯uorescent spectra showed that the LB ®lms were J-aggregates. Low-angle X-ray diffraction measurements showed these LB ®lms had good homogeneity with a bilayer spacing of ca. 4.86±5.13 nm. These LB ®lms exhibited intense ¯uorescence and were used as emitting material of electroluminescent devices. The deposited surface pressure of the LB ®lms had a strong in¯uence on the EL intensity. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Langmuir±Blodgett ®lms; Electroluminescent device; 8-Hydroxyquinoline derivatives
1. Introduction Organic electroluminescent (EL) devices have recently received much attention because they can potentially produce emissions of many colors throughout the visible spectrum and because of their possible application as large-area light-emitting displays [1±3]. The most important milestone led to the development of a stable organic EL device was the discovery of tris(8-hydroxyquinolinato)aluminum (Alq3) [4]. After that, many other metal chelates derived from 8-hydroxyquinoline, such as Znq2, Beq2, Mgq2, Zn(mq)2, Be(mq)2 and Al(prq)2 (mq and prq are 2methyl-8-hydroxyquinoline and 7-propyl-8-hydroxyquinoline) were used as emitting elements in electroluminescent devices [5]. In these devices, high external quantum ef®ciency and brightness are achievable at a driving voltage below 10 V. LB ®lm technique makes it possible to prepare organic functional ultrathin ®lms with a controlled thickness at a molecular size and with-de®ned molecular orientation, therefore, if some amphiphilic complexes with 8-hydroxyquinoline can be synthesized and further be incorporated in LB ®lms, these LB ®lms may be used as the emitting layer of EL devices. The highly ordered arrangement of emitting
molecules may be able to increase the luminous ef®ciency of EL devices. 2. Experimental The synthesis of the amphiphilic ligand, 2-(N-hexadecylcarbamoyl)-8-hydroxy quinoline (HL) (its molecular structure is shown in the inset in Fig. 1) was carried out by a previously reported method [6]. The HL complexes, ML2 (M Mg, Ca, Mn, Co, Ni, Zn, Cd, Pb), ML2Cl (M La, Er) and AlL2OH were prepared by coordinating the metal salts with HL in boiling aqueous methanol solution and details of this procedure will be reported elsewhere. Formation of air±water monolayers and the deposition of the LB ®lms followed the previous paper [7]. The EL device had a single layer of organic material sandwiched between two injecting electrodes: ITO/emitting layer/Al. The emitting layer was a LB ®lm of amphiphilic complex and the emitting area was 2 £ 3 mm 2. 3. Results and discussion 3.1. Formation of monolayer All the 11 amphiphilic complexes can form stable and condensed monolayers at the air±water interface. Fig. 2 shows typical p±A isotherms of the complexes MgL2 and
0040-6090/00/$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S00 40-6090(99)0101 8-4
J.-M. Ouyang et al. / Thin Solid Films 363 (2000) 130±133
131
ence of the structures of these three complexes from those of the other eight complexes. The hybrid orbitals of the central metal ions in complexes ML2 (M Mn, Co, Ni, Zn, Cd, Pb) and ML2Cl (M La, Er) are sp 3d 2 or more superior. HL is coordinated to the central metal ion as a tridentate ligand (with coordinating atoms: heterocyclic N, phenolic O, and carbonyl O) [8]. However, the hybrid orbitals of the central metal ions in complexes MgL2, CaL2 and AlL2OH are sp 3. HL is coordinated as a bidentate (with coordinating atoms heterocyclic N and phenolic O). Upon compression, the steric strain of the molecular interaction in the complex with sp 3 hybrid orbitals of the central metal ion is larger than that in the complex with sp 3d 2 hybrid orbitals. It makes the angle between the quinoline ring planes in the former be larger and results in an increase of the limiting areas. 3.2. UV-visible spectra of LB ®lms Fig. 1. Spectrum of electroluminescence of an Al / ErL2Cl LB ®lm / ITO EL device. The inset is the molecular structure of HL.
ErL2Cl with high collapse surface pressures of ca. 56 and 54 mN/m, respectively. As a result of the change of the molecular packing, the isotherm of ErL2Cl consists of two segments, (a) and (b), and two limiting areas, 1.23 (A0(a)) and 0.70 (A0(b)) nm 2 per ErL2Cl molecule, can be obtained by extrapolating (a) and (b) to zero surface pressure. It suggested that the ring planes change from a `lie ¯at on' to an `edge-on' structure in the monolayer upon compression (see the insets in Fig. 2). The p±A isotherms of complexes MnL2, CoL2, NiL2, ZnL2, CdL2, PbL2 and LaL2Cl showed similar character to ErL2Cl. However, the complexes MgL2, CaL2 and AlL2OH only showed one limiting area. It may be due to the differ-
The absorption peaks of ErL2Cl in LB ®lms occurred at ca. 208, 271, 356 and 433 nm. The bands at 271 and 356 nm are ascribed to 1Bb, and 1La bands [8] and the band at 433 nm is assigned to an electron transfer transition from HL to metallic ion (LMCT) band. In comparison with the DMF solution, the absorption spectra of the complex in LB ®lms showed considerable line broadening and red shift. Table 1 lists the UV-visible spectra of all the complexes in LB ®lms. The absorption maxima of the LMCT bands in the LB ®lms bathochromically shifted ca. 4±17 nm compared with those in DMF solution. Besides, a strong absorption band at ca. 205 nm appeared in the LB ®lms. This band corresponds to the n ! p transition of alkylcarbamoyl group in C-2 position of quinoline ring, and not be seen in DMF solutions due to the absorption of solvent. The difference in UV-visible spectra in LB ®lms from DMF solutions was due to the ordered orientation of complex molecules and the strong interaction between the molecules in the LB ®lms. As these wavelengths remain the same for ®lms with differing numbers of layers, this implies that the interaction exists within rather than between the layers. Since J-aggregates are characterized by an intense narrow absorption band with a bathochromical shift, relative to the monomeric band [9], the red-shifted intense absorption of the complexes in LB ®lms suggests that J-aggregates were formed in the complexes LB ®lms. Plotting the absorbance vs. the number of layers of ErL2Cl LB ®lms, we obtain a linear relationship through the origin. It implies that the monolayer of ErL2Cl at the air±water interface is uniform and the monolayer deposition is reproducible and the ®lms have vertical homogeneity. 3.3. Fluorescence
Fig. 2. Surface pressure±area isotherms. (X) MgL2,; (B) ErL2Cl. The insets show the orientational cahnge of ErL2Cl molecules in monolayer at selected pressure.
In general, the complexes of 8-quinolinol or 5-sulfo-8quinolinol with some metal ions such as Mn(II), Co(II), Ni(II), Cu(II) and Fe(II,III) are non-¯uorescent at room temperature [10]. However, all the complexes of HL emit ¯uorescence at room temperature. The magnitudes of the
132
J.-M. Ouyang et al. / Thin Solid Films 363 (2000) 130±133
Table 1 UV-visible spectra data of amphiphilic complexes LB ®lms LB ®lms
MgL2 CaL2 AlL2OH MnL2 CoL2 NiL2 ZnL2 CdL2 PbL2 LaL2Cl ErL2Cl
Charge
Transfer 1
Bands
Ligand ®eld band
1
(n ! p)
( Bb)
( La)
201 199 204 201 207 205 265 200 202 205 208
270 269 281 273 276 268 283 277 271 274 271
353 352 354 352 354 355 354 354 360 355 356
450 416 405 435 506 466 453 467 463 410 433
¯uorescent strength of these complexes (1.0 £ 10 25 mol/l) follow the decreasing order: Cd (32) . Al (23) . Mg (9.8) . Zn (8.2), Ca (8.2) . Er (7.6) . La (5.6) . Mn (5.5) . Ni (4.3), Pb (4.3) . Co (3.8). Obvious ¯uorescence emissions, even with only one layer of ErL2Cl, could be detected. One of the probable reasons for the strong ¯uorescence is that microcrystals of this complex are generated at the substrate±®lm interface. 3.4. Low-angle X-ray diffraction The thickness of the LB ®lms has been measured by lowangle X-ray diffraction measurement. From the position of the Bragg peaks, a thickness of ca. 4.86±5.13 nm per bilayer can be calculated, in agreement with the values estimated by the CPK model (2 £ 2.5 nm). It further indicates that these amphiphilic complexes were fabricated as highly ordered LB ®lms. A slight effect of the deposition pressure to the thickness of LB ®lms was observed. The thickness of ErL2Cl LB ®lms prepared at 10, 20, 30, 40, 50 mN/m are approximately constant at 5.13 nm except for p 10 mN/m (Table 2). The decrease of thickness of ErL2Cl LB ®lms deposited at 10 mN/m was caused by the decrease of the contribution of quinoline rings of ErL2Cl molecule to the height of LB ®lms because quinoline rings oriented with faces touching water in low surface pressure (the inset in Fig. 2). In high surface pressure, the fact that the thickness remains nearly constant can be ascribed to that the p±A isotherm for ErL2Cl shows a rapid increase in surface pressure with decreasing surface area. Table 2 The thickness of 21-layer ErL2Cl LB ®lms built up at different deposition pressure p (mN/m) Thickness (nm)
10 4.76
20 5.10
30 5.13
40 5.11
50 5.13
Fig. 3. Current-voltage (I±V) characteristics of EL devices with 21-layer ErL2Cl LB ®lms deposited at different surface pressures.
3.5. EL devices with LB ®lms as emitter The intensive ¯uorescence properties of the LB ®lms of these complexes make them possible be used as emitting layer of EL devices. Fig. 3 shows the current-voltage (I±V) characteristics of EL devices with 21-layer LB ®lms deposited at a surface pressures of 10, 20 and 30 mN/m, respectively. The deposited surface pressure of the LB ®lms strongly in¯uenced the current-voltage (I±V) characteristics and the electroluminescence of the organic EL diode. When the LB ®lms were prepared at surface pressure of 20 mN/m, the current density crossing this kind of diodes had stronger nonlinear I±V characteristics and the organic diode had higher EL intensity than another diode prepared at lower surface pressure (10 mN/m). At lower surface pressure, there may be some point defects or pinholes in the LB ®lm, in which the high tunnel current would increase exponentially with the ®eld, leading to an increase in the current density crossing the EL device and a decrease in the recombination probability of electrons and holes at higher voltage. When the surface pressure is higher than 20 mN/m the I±V characteristics of the devices are similar. It indicates there may be fewer point defects or pinholes in these LB ®lms and the difference in quality of these ®lms are small. That is, the LB ®lms with surface pressures higher than 20 mN/m can be important in raising the ErL2Cl EL intensity and ef®ciency. Fig. 1 shows EL spectrum of the EL devices with 21-layer ErL2Cl LB ®lms deposited at 30 mN/m as the emitter. The EL emission spectrum was independent of the driving voltage and current. This result indicates that the radiative recombination of injected electrons and holes takes place in the ErL2Cl LB ®lms [11].
J.-M. Ouyang et al. / Thin Solid Films 363 (2000) 130±133
Acknowledgements This research work was supported by Natural Science Foundation of Guangdong Province and the Foundation of National Key Laboratory of Organic Solid of Institute of Chemistry of the Chinese Academy of Sciences. References [1] C.H. Chen, J.-M. Shi, Coord. Chem. Rev. 171 (1998) 161. [2] A. Kraft, A.C. Grimsdale, A.B. Holmes, Angew. Chem. Int. Ed. 37 (1998) 402.
133
[3] S. Dirr, S. Wiese, H.-H. Johannes, Synth. Met. 91 (1997) 53. [4] C.W. Tang, S.A. Vanslyke, C.H. Chen, Appl. Phys. Lett. 51 (1987) 913. [5] Y. Hamada, T. Sano, M. Fujita, T. Fujita, Y. Nishio, K. Shibata, Jpn. J. Appl. Phys. 32 (1993) L514. [6] J.-M. Ouyang, Z.-H. Tai, C.-Y. Jiang, Spectrosc. Lett. 29 (1996) 763. [7] J.-M. Ouyang, C. Li, Y.-Q. Li, Thin Solid Films 348 (1999) 242. [8] J.-M. Ouyang, Z.-H. Tai, Spectrosc. Lett. 31 (1998) 1001. [9] A.K. Dutta, A.J. Pal, T.N. Misra, Bull. Chem. Soc. Jpn. 66 (1993) 3575. [10] D.C. Bhatnagar, L.S. Forster, Spectrochim. Acta 21 (1965) 1803. [11] J.-M. Ouyang, L. Li, Z.-H. Tai, G.-M. Wang, Z.-H. Lu, Chem. Commun. (1997) 815.