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Photoluminescent and electroluminescent properties of phenol-pyridine beryllium and carbonyl polypyridyl Re(I) complexes codeposited films
Synthetic Metals 118 (2001) 175±179
Photoluminescent and electroluminescent properties of phenol-pyridine beryllium and carbonyl polypyridyl Re(I) complexes codeposited ®lms Yanqin Li, Yu Liu, Jianhua Guo, Fang Wu, Wenjing Tian, Bofu Li, Yue Wang* Key Laboratory for Supramolecular Structure and Spectroscopy, Jilin University, Changchun 120023, PR China Received 28 April 2000; received in revised form 16 August 2000; accepted 16 August 2000
Abstract The codeposited ®lms of BePP2 (PP 2-(2-hydroxyphenyl)-pyridine) and (Butbpy)Re(CO)3Cl (But bpy 4,40 -bi(tert-butyl)-2,20 bipyridine) were prepared by vacuum evaporation deposition. The photoluminescent and electroluminescent properties of the ®lms were investigated. The ®lms with different (Butbpy)Re(CO)3Cl concentration were employed to study the energy transfer properties between BePP2 and (Butbpy)Re(CO)3Cl. Experimental results showed that energy transfer from BePP2 to (Butbpy)Re(CO)3Cl took place in the codeposited ®lms. The ®lms which were excited optically, exhibited complete energy transfer if (Butbpy)Re(CO)3Cl concentrations were equal or higher than 9%. Under electrical excitation, complete energy transfer were observed when (Butbpy)Re(CO)3Cl concentrations Ê )/BePP2:(Butbpy)Re(CO)3Cl (500 A Ê )/Al (2000 A Ê )) which contained the were equal or higher than 3%. The devices of (ITO/TPD (500 A codeposited ®lm with (Butbpy)Re(CO)3Cl concentration of 50%, showed maximum EL ef®ciency value of 1.6 lm/W and maximum lightness of 6500 cd/m2. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Organic electroluminescence; Organic electroluminescent material; Re(I)-polypyridine complexes; Metal-ligand charge transfer
1. Introduction During the past 10 years, a wide range of electroluminescent small molecular compounds have been made which are suitable for use in organic light emitting devices (OLEDs) [1±14]. Alternation of their chemical structures by synthetic design has stimulated the preparation of molecule base devices emitting in any part of the visible spectrum. Among the wide variety of electroluminescent small molecular compounds, only few have been extensively studied. A number of task still remain to be performed before the apparent bright prospects of small molecular organic electroluminescent compounds can be realized. Traditionally, most organic electroluminescent materials are ¯uorescent compounds. For a ¯uorescent emitter (singlet emission material), the upper limit of internal quantum ef®ciency of OLEDs is 25%. Forrest and coworks have demonstrated that upper limit of internal quantum ef®ciency of OLEDs can be dramatically improved by use of phosphorescent emitter (triplet emission materials) and the energy transfer between ¯uorescent material and phosphorescent material [15]. A large number of experimental and *
Corresponding author. Tel.: 86-431-8922331; fax: 86-431-8923907. E-mail address: [email protected] (Y. Wang).
theoretical studies showed that many luminescent transitionmetal complexes can be used as ef®cient energy transfer donor or acceptor [16]. Therefore, luminescent transitionmetal complexes possess the potential for the development of high ef®ciency electroluminescent devices. In this report, we present the phtoluminescent and electroluminescent properties of codeposited ®lms of ¯uorescent material, phenol-pyridine beryllium complex (BePP2) and metal to ligand charge transfer (MLCT) luminescent material, carbonyl polypyridine Re(I) complex (Butbpy)Re(CO)3Cl) (Fig. 1). We demonstrate that energy transfer phenomenon occurs from BePP2 to (Butbpy)Re(CO)3Cl. 2. Experiment 2.1. Materials The Re(CO)5Cl was purchased from Aldrich and used as received. The 4,40 -bi(tert-butyl)-2,20 -bipyridine(Butbpy) was synthesized according to literature [17]. (Butbpy)Re(CO)3Cl was prepared analogously to methods described in the literature [18]. To a 250 ml ¯ask ®tted with magnetic stirrer, 0.361 g (1.0 mmol) Re(CO)3Cl and 0.268 g (1.0 mmol) (Butbpy) ligand was added to 60 ml methanol. The mixture solution was heated to re¯ux for 24 h
0379-6779/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 ( 0 0 ) 0 0 4 5 4 - 9
176
Y. Li et al. / Synthetic Metals 118 (2001) 175±179
Fig. 1. The molecular structures of TPD, BePP2 and (Butbpy)Re(CO)3Cl.
and then cooled to room temperature. Bright yellow solid precipitated from the solution. It was then ®ltered and washed with methanol. The bright yellow solid (Butbpy)Re(CO)3Cl was dried in air. Before (Butbpy)Re(CO)3Cl was used for spectroscopic measurement and devices fabrication, it was puri®ed by recrystalization in CHCl3 and sublimation methods. 2.2. The film deposition The sample layers were fabricated by vacuum evaporation Ê /s in a vacuum chamber with base at nominal rate, 3±5 A ÿ6 pressure of 4 10 Torr. The deposition sources BePP2 and (Butbpy)Re(CO)3Cl were set in separate quartz crucibles whose temperatures were independently controlled by coil heaters. In present study, (Butbpy)Re(CO)3Cl concentration has been systematically varied between 1.5 and 95%. The quartz glass and glass coated with indium-tin-oxide (ITO) were used as the substrates for PL measurements and EL characterizations, respectively. Besides the single layer, BePP2:(Butbpy)Re(CO)3Cl codeposited ®lm for PL characterization, shown in Fig. 2(a), several EL devices were fabricated. The EL devices structure consists of (ITO/TPD Ê )/Al (2000 A Ê )) Ê )/BePP2: (Butbpy)Re(CO)3Cl (500 A (500 A (Fig. 2(b)). The emitting area was 2 mm 2 mm. The codeposited ®lms with the same (Butbpy)Re(CO)3Cl concentration for PL and EL characterizations were prepared at the same time in a chamber. The PL and EL spectra were recorded with a Shimadzu RF-5301PC spectrometer.
3. Results and discussions Recently, we have developed new blue light emitting material, BePP2 which can be used to fabricate blue light electroluminescent devices. The synthesis and EL property of BePP2 will be report in elsewhere. Carbonyl polypyridine Re(I) complexes have been employed as EL materials [18]. Figs. 1 and 2 show the molecular structures of used material and the single BePP2:(Butbpy)Re(CO)3Cl vacuum codeposited layer for photoluminescent characterization together with the devices con®guration for electroluminescent study, respectively. Fig. 3 presidents the energy diagrams of TPD, BePP2 and (Butbpy)Re(CO)3Cl which were obtained by standard electrochemical analysis method. The maxima of photoluminescent excitation (PLE) and photoluminescent for BePP2 are 350 and 450 nm, respectively (Fig. 4). (Butbpy)Re(CO)3Cl exhibit peaks of PLE and photoluminescence at 385 and 540 nm, respectively. There are partly overlap between the emission spectrum of BePP2 and the absorption spectrum of (Butbpy)Re(CO)3Cl, although they do not correspond exactly. This make the possible energy transfer from BePP2 to (Butbpy)Re(CO)3Cl. The vacuum deposition ®lms of pure BePP2 and (Butbpy)Re(CO)3Cl show emission peaks at 450 and 540 nm, respectively (excitation at 360 nm). In order to investigate energy transfer property of BePP2:(Butbpy)Re(CO)3Cl system, ®lms consisting (Butbpy)Re(CO)3Cl of 3±97% by weight of (Butbpy)Re(CO)3Cl in BePP2 were excited optically (Fig. 5). If the concentrations of (Butbpy)Re(CO)3Cl is
Fig. 2. Sample structure with the BrPP2: (Butbpy)Re(CO)3Cl codeposited layer for (a) PL measurements and (b) EL measurements.
Y. Li et al. / Synthetic Metals 118 (2001) 175±179
177
Fig. 3. Energy diagrams of TPD, BePP2 and (Butbpy)Re(CO)3Cl.
Fig. 6. The EL spectra of the devices containing the film with different (Butbpy)Re(CO)3Cl concentrations: (a) 5% (b) 9% (c) 50% (d) 90%.
Fig. 4. The PLE spectrum of (Butbpy)Re(CO)3Cl film and the PL spectrum of BePP2.
Fig. 5. The PL spectra of the films with different (Butbpy)Re(CO)3Cl concentrations: (a) 5% (b) 9% (c) 50% (d) 90%.
higher than 9%, only one emission peak at around 540 nm was observed which is attributed to (Butbpy)Re(CO)3Cl. This suggests that energy transfer take place from BePP2 to (Butbpy)Re(CO)3Cl. When the concentration of (Butbpy)Re(CO)3Cl is equal or lower than 9%, a emission peak at 450 nm appears which is due to BePP2. This means that incomplete energy transfer occurs from BePP2 to (Butbpy)Re(CO)3Cl. It is seen that the emission maximum of (Butbpy)Re(CO)3Cl shifted gradually toward shorter wavelength region with decreasing concentration of (Butbpy)Re(CO)3Cl. These phenomena might be attributed to that the large changes in blend stoichiometry unequally affect absolute energies of the HOMO and LUMO levels. This in turn change optical gaps thus cause the observed shifts. The EL spectra of the devices which have a BePP2: (Butbpy)Re(CO)3Cl codeposited layer as an emitting layer are shown in Fig. 6. The EL emission around at range of 535±560 nm from (Butbpy)Re(CO)3Cl was dominant for all samples. With the decrease in (Butbpy)Re(CO)3Cl concentration, the EL peak from (Butbpy)Re(CO)3Cl shifts to higher energy, similar to the phenomena observed for the PL spectra of single layer ®lms. The EL emission peaks together with EL ef®ciencies of BePP2:(Butbpy)Re(CO)3Cl codeposition ®lms with different (Butbpy)Re(CO)3Cl concentration are summarized in Table 1. It is worth to mention that the concentration dependence of the EL spectra for BePP2:(Butbpy)Re(CO)3Cl system exhibits important difference compared with that of PL spectra. Only one EL emission peak which is due to (Butbpy)Re(CO)3Cl was observed during the concentration changing from 100 to 5%. This suggested that at this concentration range complete energy transfer from BePP2 to (Butbpy)Re(CO)3Cl take place. The devices with (Butbpy)Re(CO)3Cl concentration
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Y. Li et al. / Synthetic Metals 118 (2001) 175±179
Table 1 The EL performances of the devices containing BePP2:(Butbpy)Re(CO)3Cl codeposited film with different (Butbpy)Re(CO)3Cl concentrations (Butbpy)Re(CO)3Cl (wt.%) EL emission peak (nm) Efficient (lm/W) Luminescent (cd/m2)
100 560 0.4 1609
3.0 535 0.4 2959
5.0 535 0.5 2182
9.0 535 1.1 2365
13.0 535 1.0 4280
of 3% exhibited a very weak emission around 450 nm that is due to BePP2. It means that the energy transfer from BePP2 to (Butbpy)Re(CO)3Cl occurs incompletely in the case of lower (Butbpy)Re(CO)3Cl concentration. The attempt to prepare the BePP2:(Butbpy)Re(CO)3Cl ®lm with lower (Butbpy)Re(CO)3Cl concentration than 3% was unsuccessful due to the limitation of thickness monitor. The obviously different threshold (Butbpy)Re(CO)3Cl concentration for complete photo-induced energy transfer and electro-induced energy transfer is not clearly understood. For electroluminescence, the ratio of production of singlet and triplet excited state is 1:3 whereas for photoluminscence 100% of excited molecules are singlet excitons. When BePP2: (Butbpy)Re(CO)3Cl was excited optically, it is easy to reach the saturation situation of energy transfer from BePP2 to (Butbpy)Re(CO)3Cl then singlet BePP2 excitons are less likely to transfer to (Butbpy)Re(CO)3Cl, corresponding to an increase in the probability of BePP2 emission. Under the condition of (Butbpy)Re(CO)3Cl concentration <10% or >95%, the devices show lower EL ef®ciencies. When (Butbpy)Re(CO)3Cl concentration varies from 10 to 90%, devices exhibit higher EL ef®ciencies. The EL ef®ciency reaches a maximum value of 1.6 lm/W at (Butbpy)Re(CO)3Cl concentration of 50% and a maximum lightness of 6450 cd/m2. The energy diagrams (Fig. 3) of TPD, BePP2 and (Butbpy)Re(CO)3Cl which were obtained by measurements of UV absorption and standard electrochemical analysis suggest that the hole injection barrier from TPD to BePP2:(Butbpy)Re(CO)3Cl layer will becomes dif®cult due to the decrease in the fraction of BePP2 and the electron injection will be more obstructed for codeposited layers with lower (Butbpy)Re(CO)3Cl concentration. Therefore, balanced injection requires that BePP2 concentration and (Butbpy)Re(CO)3Cl concentration must reach a certain level at the same time. For common dopant systems, the dopant molecules were only used as energy acceptor and have not the functions of carrier injection and transport. Frequently, the devices with dopped emitting layer show highest EL ef®ciency at lower dopant concentration. The BePP2: (Butbpy)Re(CO)3Cl codeposition system is different from classical dopant systems. In this system, (Butbpy)Re(CO)3Cl can play a role of energy acceptor as well as electron traps. It is worth to note that we have obtained the higher performance OLEDs using air-stable aluminum as the cathode electrode. The use of a stable metal cathode is potentially bene®cial to the development of high stable OLEDs. It is not certain that the energy transfer from the host to the gust occur from the ligand of donor to the ligand
50.0 544 1.6 6448
75.0 547 1.0 2300
83.5 547 1.2 2880
87.5 550 1.1 4448
90.0 550 1.1 4255
95.0 550 1.1 2808
97.0 560 0.4 3283
of the acceptor, or Re metal to ligand transition is directly involved. We expect that Dexter energy transfer could take place from BePP2 to (Butbpy)Re(CO)3Cl. But FoÈrster energy transfer also is possible in this system. At present, we are unable to con®rm the energy transfer mechanism of this system, further investigations are necessary to solve the problem. Some concerning experiments are in progress. 4. Conclusions In conclusion, energy transfer from BePP2 to (Butbpy)Re(CO)3Cl in BePP2:(Butbpy)Re(CO)3Cl codeposited ®lms was demonstrated by PL and EL investigation. For codeposited ®lms, the PL and EL emission peak shifts, compared to pure (Butbpy)Re(CO)3Cl ®lm were observed. For EL devices using BePP2:(Butbpy)Re(CO)3Cl codeposited ®lm as the emitting layer, (Butbpy)Re(CO)3Cl played a role of energy acceptor as well as electron traps. This property leaded to the fact that higher EL ef®ciency of BePP2: (Butbpy)Re(CO)3Cl need higher (Butbpy)Re(CO)3Cl concentration. We demonstrated that higher performance BePP2:(Butbpy)Re(CO)3Cl-based devices using air-stable Al as cathode material can be obtained. Acknowledgements We are grateful for the National Natural Science Foundation of China no. 597905006. References [1] C.H. Che, J. Shi, C.W. Tang, Macromol. Symp. 125 (1997) 1. [2] T.A. Hopkins, K. Meerholz, B. Kippelen, A.B. Padias, H.K. Hall Jr., N. Peyghambarian, N.R. Armstrong, Chem. Mater. 8 (1996) 344. [3] H. Antoniadis, M. Inbasekaran, E.P. Woo, Appl. Phys. Lett. 73 (1998) 3055. [4] C. Hoskawa, H. Higashi, H. Nakamura, T. Kusumoto, Appl. Phys. Lett. 67 (1995) 3853. [5] T. Noda, H. Ogawa, Y. Shirota, Adv. Mater. 11 (1999) 283. [6] C.H. Chen, C.W. Tang, J. Shi, K.P. Klubek, Macromol. Symp. 125 (1997) 49. [7] Z. Shen, P.E. Burrows, V. Bulovic, S.R. Forrest, M.E. Thompson, Science 276 (1997) 2009. [8] Z.Y. Xie, J.S. Huang, C.N. Li, S.Y. Liu, Y. Wang, Y.Q. Li, J.C. Shen, Appl. Phys. Lett. 74 (1999) 641. [9] J. Kido, Y. Lizumi, Appl. Phys. Lett. 73 (1998) 2721. [10] S.A. VanSlyke, C.H. Chen, C.W. Tang, Appl. Phys. Lett. 69 (1996) 2160.
Y. Li et al. / Synthetic Metals 118 (2001) 175±179 [11] J. Shi, C.W. Tang, Appl. Phys. Lett. 70 (1997) 1665. [12] C.W. Tang, S.A. VanSlyke, C.H. Chen, J. Appl. Phys. 65 (1989) 3610. [13] C.W. Tang, S.A. VanSlyke, Appl. Phys. Lett. 51 (1987) 913. [14] Y. Hamada, T. Sano, M. Fujita, T. Fujii, Y. Nishio, K. Shibata, Chem. lett. (1993) 905.
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[15] M.A. Baldo, D.F. O'Brien, Y. You, A. Shoustikov, S. Sibley, M.E. Thompson, S.R. Forrest, Nature 359 (1998) 151. [16] V. Balzani, F. Scandola, Supramolecular Photochemistry, Ellis Horwood, England, 1991, p. 72. [17] T.B. Hadda, H.L. Bozec, Polyhedron 7 (1988) 575. [18] Y. Li, Y. Wang, Y. Wu, J.C. Shen, Synth. Met. 99 (1999) 257.
Photoluminescent and electroluminescent properties of phenol-pyridine beryllium and carbonyl polypyridyl Re(I) complexes codeposited ®lms Yanqin Li, Yu Liu, Jianhua Guo, Fang Wu, Wenjing Tian, Bofu Li, Yue Wang* Key Laboratory for Supramolecular Structure and Spectroscopy, Jilin University, Changchun 120023, PR China Received 28 April 2000; received in revised form 16 August 2000; accepted 16 August 2000
Abstract The codeposited ®lms of BePP2 (PP 2-(2-hydroxyphenyl)-pyridine) and (Butbpy)Re(CO)3Cl (But bpy 4,40 -bi(tert-butyl)-2,20 bipyridine) were prepared by vacuum evaporation deposition. The photoluminescent and electroluminescent properties of the ®lms were investigated. The ®lms with different (Butbpy)Re(CO)3Cl concentration were employed to study the energy transfer properties between BePP2 and (Butbpy)Re(CO)3Cl. Experimental results showed that energy transfer from BePP2 to (Butbpy)Re(CO)3Cl took place in the codeposited ®lms. The ®lms which were excited optically, exhibited complete energy transfer if (Butbpy)Re(CO)3Cl concentrations were equal or higher than 9%. Under electrical excitation, complete energy transfer were observed when (Butbpy)Re(CO)3Cl concentrations Ê )/BePP2:(Butbpy)Re(CO)3Cl (500 A Ê )/Al (2000 A Ê )) which contained the were equal or higher than 3%. The devices of (ITO/TPD (500 A codeposited ®lm with (Butbpy)Re(CO)3Cl concentration of 50%, showed maximum EL ef®ciency value of 1.6 lm/W and maximum lightness of 6500 cd/m2. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Organic electroluminescence; Organic electroluminescent material; Re(I)-polypyridine complexes; Metal-ligand charge transfer
1. Introduction During the past 10 years, a wide range of electroluminescent small molecular compounds have been made which are suitable for use in organic light emitting devices (OLEDs) [1±14]. Alternation of their chemical structures by synthetic design has stimulated the preparation of molecule base devices emitting in any part of the visible spectrum. Among the wide variety of electroluminescent small molecular compounds, only few have been extensively studied. A number of task still remain to be performed before the apparent bright prospects of small molecular organic electroluminescent compounds can be realized. Traditionally, most organic electroluminescent materials are ¯uorescent compounds. For a ¯uorescent emitter (singlet emission material), the upper limit of internal quantum ef®ciency of OLEDs is 25%. Forrest and coworks have demonstrated that upper limit of internal quantum ef®ciency of OLEDs can be dramatically improved by use of phosphorescent emitter (triplet emission materials) and the energy transfer between ¯uorescent material and phosphorescent material [15]. A large number of experimental and *
Corresponding author. Tel.: 86-431-8922331; fax: 86-431-8923907. E-mail address: [email protected] (Y. Wang).
theoretical studies showed that many luminescent transitionmetal complexes can be used as ef®cient energy transfer donor or acceptor [16]. Therefore, luminescent transitionmetal complexes possess the potential for the development of high ef®ciency electroluminescent devices. In this report, we present the phtoluminescent and electroluminescent properties of codeposited ®lms of ¯uorescent material, phenol-pyridine beryllium complex (BePP2) and metal to ligand charge transfer (MLCT) luminescent material, carbonyl polypyridine Re(I) complex (Butbpy)Re(CO)3Cl) (Fig. 1). We demonstrate that energy transfer phenomenon occurs from BePP2 to (Butbpy)Re(CO)3Cl. 2. Experiment 2.1. Materials The Re(CO)5Cl was purchased from Aldrich and used as received. The 4,40 -bi(tert-butyl)-2,20 -bipyridine(Butbpy) was synthesized according to literature [17]. (Butbpy)Re(CO)3Cl was prepared analogously to methods described in the literature [18]. To a 250 ml ¯ask ®tted with magnetic stirrer, 0.361 g (1.0 mmol) Re(CO)3Cl and 0.268 g (1.0 mmol) (Butbpy) ligand was added to 60 ml methanol. The mixture solution was heated to re¯ux for 24 h
0379-6779/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 ( 0 0 ) 0 0 4 5 4 - 9
176
Y. Li et al. / Synthetic Metals 118 (2001) 175±179
Fig. 1. The molecular structures of TPD, BePP2 and (Butbpy)Re(CO)3Cl.
and then cooled to room temperature. Bright yellow solid precipitated from the solution. It was then ®ltered and washed with methanol. The bright yellow solid (Butbpy)Re(CO)3Cl was dried in air. Before (Butbpy)Re(CO)3Cl was used for spectroscopic measurement and devices fabrication, it was puri®ed by recrystalization in CHCl3 and sublimation methods. 2.2. The film deposition The sample layers were fabricated by vacuum evaporation Ê /s in a vacuum chamber with base at nominal rate, 3±5 A ÿ6 pressure of 4 10 Torr. The deposition sources BePP2 and (Butbpy)Re(CO)3Cl were set in separate quartz crucibles whose temperatures were independently controlled by coil heaters. In present study, (Butbpy)Re(CO)3Cl concentration has been systematically varied between 1.5 and 95%. The quartz glass and glass coated with indium-tin-oxide (ITO) were used as the substrates for PL measurements and EL characterizations, respectively. Besides the single layer, BePP2:(Butbpy)Re(CO)3Cl codeposited ®lm for PL characterization, shown in Fig. 2(a), several EL devices were fabricated. The EL devices structure consists of (ITO/TPD Ê )/Al (2000 A Ê )) Ê )/BePP2: (Butbpy)Re(CO)3Cl (500 A (500 A (Fig. 2(b)). The emitting area was 2 mm 2 mm. The codeposited ®lms with the same (Butbpy)Re(CO)3Cl concentration for PL and EL characterizations were prepared at the same time in a chamber. The PL and EL spectra were recorded with a Shimadzu RF-5301PC spectrometer.
3. Results and discussions Recently, we have developed new blue light emitting material, BePP2 which can be used to fabricate blue light electroluminescent devices. The synthesis and EL property of BePP2 will be report in elsewhere. Carbonyl polypyridine Re(I) complexes have been employed as EL materials [18]. Figs. 1 and 2 show the molecular structures of used material and the single BePP2:(Butbpy)Re(CO)3Cl vacuum codeposited layer for photoluminescent characterization together with the devices con®guration for electroluminescent study, respectively. Fig. 3 presidents the energy diagrams of TPD, BePP2 and (Butbpy)Re(CO)3Cl which were obtained by standard electrochemical analysis method. The maxima of photoluminescent excitation (PLE) and photoluminescent for BePP2 are 350 and 450 nm, respectively (Fig. 4). (Butbpy)Re(CO)3Cl exhibit peaks of PLE and photoluminescence at 385 and 540 nm, respectively. There are partly overlap between the emission spectrum of BePP2 and the absorption spectrum of (Butbpy)Re(CO)3Cl, although they do not correspond exactly. This make the possible energy transfer from BePP2 to (Butbpy)Re(CO)3Cl. The vacuum deposition ®lms of pure BePP2 and (Butbpy)Re(CO)3Cl show emission peaks at 450 and 540 nm, respectively (excitation at 360 nm). In order to investigate energy transfer property of BePP2:(Butbpy)Re(CO)3Cl system, ®lms consisting (Butbpy)Re(CO)3Cl of 3±97% by weight of (Butbpy)Re(CO)3Cl in BePP2 were excited optically (Fig. 5). If the concentrations of (Butbpy)Re(CO)3Cl is
Fig. 2. Sample structure with the BrPP2: (Butbpy)Re(CO)3Cl codeposited layer for (a) PL measurements and (b) EL measurements.
Y. Li et al. / Synthetic Metals 118 (2001) 175±179
177
Fig. 3. Energy diagrams of TPD, BePP2 and (Butbpy)Re(CO)3Cl.
Fig. 6. The EL spectra of the devices containing the film with different (Butbpy)Re(CO)3Cl concentrations: (a) 5% (b) 9% (c) 50% (d) 90%.
Fig. 4. The PLE spectrum of (Butbpy)Re(CO)3Cl film and the PL spectrum of BePP2.
Fig. 5. The PL spectra of the films with different (Butbpy)Re(CO)3Cl concentrations: (a) 5% (b) 9% (c) 50% (d) 90%.
higher than 9%, only one emission peak at around 540 nm was observed which is attributed to (Butbpy)Re(CO)3Cl. This suggests that energy transfer take place from BePP2 to (Butbpy)Re(CO)3Cl. When the concentration of (Butbpy)Re(CO)3Cl is equal or lower than 9%, a emission peak at 450 nm appears which is due to BePP2. This means that incomplete energy transfer occurs from BePP2 to (Butbpy)Re(CO)3Cl. It is seen that the emission maximum of (Butbpy)Re(CO)3Cl shifted gradually toward shorter wavelength region with decreasing concentration of (Butbpy)Re(CO)3Cl. These phenomena might be attributed to that the large changes in blend stoichiometry unequally affect absolute energies of the HOMO and LUMO levels. This in turn change optical gaps thus cause the observed shifts. The EL spectra of the devices which have a BePP2: (Butbpy)Re(CO)3Cl codeposited layer as an emitting layer are shown in Fig. 6. The EL emission around at range of 535±560 nm from (Butbpy)Re(CO)3Cl was dominant for all samples. With the decrease in (Butbpy)Re(CO)3Cl concentration, the EL peak from (Butbpy)Re(CO)3Cl shifts to higher energy, similar to the phenomena observed for the PL spectra of single layer ®lms. The EL emission peaks together with EL ef®ciencies of BePP2:(Butbpy)Re(CO)3Cl codeposition ®lms with different (Butbpy)Re(CO)3Cl concentration are summarized in Table 1. It is worth to mention that the concentration dependence of the EL spectra for BePP2:(Butbpy)Re(CO)3Cl system exhibits important difference compared with that of PL spectra. Only one EL emission peak which is due to (Butbpy)Re(CO)3Cl was observed during the concentration changing from 100 to 5%. This suggested that at this concentration range complete energy transfer from BePP2 to (Butbpy)Re(CO)3Cl take place. The devices with (Butbpy)Re(CO)3Cl concentration
178
Y. Li et al. / Synthetic Metals 118 (2001) 175±179
Table 1 The EL performances of the devices containing BePP2:(Butbpy)Re(CO)3Cl codeposited film with different (Butbpy)Re(CO)3Cl concentrations (Butbpy)Re(CO)3Cl (wt.%) EL emission peak (nm) Efficient (lm/W) Luminescent (cd/m2)
100 560 0.4 1609
3.0 535 0.4 2959
5.0 535 0.5 2182
9.0 535 1.1 2365
13.0 535 1.0 4280
of 3% exhibited a very weak emission around 450 nm that is due to BePP2. It means that the energy transfer from BePP2 to (Butbpy)Re(CO)3Cl occurs incompletely in the case of lower (Butbpy)Re(CO)3Cl concentration. The attempt to prepare the BePP2:(Butbpy)Re(CO)3Cl ®lm with lower (Butbpy)Re(CO)3Cl concentration than 3% was unsuccessful due to the limitation of thickness monitor. The obviously different threshold (Butbpy)Re(CO)3Cl concentration for complete photo-induced energy transfer and electro-induced energy transfer is not clearly understood. For electroluminescence, the ratio of production of singlet and triplet excited state is 1:3 whereas for photoluminscence 100% of excited molecules are singlet excitons. When BePP2: (Butbpy)Re(CO)3Cl was excited optically, it is easy to reach the saturation situation of energy transfer from BePP2 to (Butbpy)Re(CO)3Cl then singlet BePP2 excitons are less likely to transfer to (Butbpy)Re(CO)3Cl, corresponding to an increase in the probability of BePP2 emission. Under the condition of (Butbpy)Re(CO)3Cl concentration <10% or >95%, the devices show lower EL ef®ciencies. When (Butbpy)Re(CO)3Cl concentration varies from 10 to 90%, devices exhibit higher EL ef®ciencies. The EL ef®ciency reaches a maximum value of 1.6 lm/W at (Butbpy)Re(CO)3Cl concentration of 50% and a maximum lightness of 6450 cd/m2. The energy diagrams (Fig. 3) of TPD, BePP2 and (Butbpy)Re(CO)3Cl which were obtained by measurements of UV absorption and standard electrochemical analysis suggest that the hole injection barrier from TPD to BePP2:(Butbpy)Re(CO)3Cl layer will becomes dif®cult due to the decrease in the fraction of BePP2 and the electron injection will be more obstructed for codeposited layers with lower (Butbpy)Re(CO)3Cl concentration. Therefore, balanced injection requires that BePP2 concentration and (Butbpy)Re(CO)3Cl concentration must reach a certain level at the same time. For common dopant systems, the dopant molecules were only used as energy acceptor and have not the functions of carrier injection and transport. Frequently, the devices with dopped emitting layer show highest EL ef®ciency at lower dopant concentration. The BePP2: (Butbpy)Re(CO)3Cl codeposition system is different from classical dopant systems. In this system, (Butbpy)Re(CO)3Cl can play a role of energy acceptor as well as electron traps. It is worth to note that we have obtained the higher performance OLEDs using air-stable aluminum as the cathode electrode. The use of a stable metal cathode is potentially bene®cial to the development of high stable OLEDs. It is not certain that the energy transfer from the host to the gust occur from the ligand of donor to the ligand
50.0 544 1.6 6448
75.0 547 1.0 2300
83.5 547 1.2 2880
87.5 550 1.1 4448
90.0 550 1.1 4255
95.0 550 1.1 2808
97.0 560 0.4 3283
of the acceptor, or Re metal to ligand transition is directly involved. We expect that Dexter energy transfer could take place from BePP2 to (Butbpy)Re(CO)3Cl. But FoÈrster energy transfer also is possible in this system. At present, we are unable to con®rm the energy transfer mechanism of this system, further investigations are necessary to solve the problem. Some concerning experiments are in progress. 4. Conclusions In conclusion, energy transfer from BePP2 to (Butbpy)Re(CO)3Cl in BePP2:(Butbpy)Re(CO)3Cl codeposited ®lms was demonstrated by PL and EL investigation. For codeposited ®lms, the PL and EL emission peak shifts, compared to pure (Butbpy)Re(CO)3Cl ®lm were observed. For EL devices using BePP2:(Butbpy)Re(CO)3Cl codeposited ®lm as the emitting layer, (Butbpy)Re(CO)3Cl played a role of energy acceptor as well as electron traps. This property leaded to the fact that higher EL ef®ciency of BePP2: (Butbpy)Re(CO)3Cl need higher (Butbpy)Re(CO)3Cl concentration. We demonstrated that higher performance BePP2:(Butbpy)Re(CO)3Cl-based devices using air-stable Al as cathode material can be obtained. Acknowledgements We are grateful for the National Natural Science Foundation of China no. 597905006. References [1] C.H. Che, J. Shi, C.W. Tang, Macromol. Symp. 125 (1997) 1. [2] T.A. Hopkins, K. Meerholz, B. Kippelen, A.B. Padias, H.K. Hall Jr., N. Peyghambarian, N.R. Armstrong, Chem. Mater. 8 (1996) 344. [3] H. Antoniadis, M. Inbasekaran, E.P. Woo, Appl. Phys. Lett. 73 (1998) 3055. [4] C. Hoskawa, H. Higashi, H. Nakamura, T. Kusumoto, Appl. Phys. Lett. 67 (1995) 3853. [5] T. Noda, H. Ogawa, Y. Shirota, Adv. Mater. 11 (1999) 283. [6] C.H. Chen, C.W. Tang, J. Shi, K.P. Klubek, Macromol. Symp. 125 (1997) 49. [7] Z. Shen, P.E. Burrows, V. Bulovic, S.R. Forrest, M.E. Thompson, Science 276 (1997) 2009. [8] Z.Y. Xie, J.S. Huang, C.N. Li, S.Y. Liu, Y. Wang, Y.Q. Li, J.C. Shen, Appl. Phys. Lett. 74 (1999) 641. [9] J. Kido, Y. Lizumi, Appl. Phys. Lett. 73 (1998) 2721. [10] S.A. VanSlyke, C.H. Chen, C.W. Tang, Appl. Phys. Lett. 69 (1996) 2160.
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