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Novel 4-(2,2-diphenyl-vinyl)-phenyl substituted pyrene derivatives as efficient emitters for organic light-emitting diodes
Organic Electronics 13 (2012) 2898–2904
Contents lists available at SciVerse ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Novel 4-(2,2-diphenyl-vinyl)-phenyl substituted pyrene derivatives as efficient emitters for organic light-emitting diodes Zuo-Qin Liang a,b, Zeng-Ze Chu c, De-Chun Zou c, Xiao-Mei Wang b, Xu-Tang Tao a,⇑ a
State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, PR China Jiangsu Key Laboratory for Environment Functional Materials, College of Chemical and Biological Engineering, Suzhou University of Science and Technology, Suzhou 215009, PR China c Department of Polymer Science and Engineering, College of Chemistry, Peking University, Beijing 100871, PR China b
a r t i c l e
i n f o
Article history: Received 28 June 2012 Received in revised form 26 August 2012 Accepted 30 August 2012 Available online 12 September 2012 Keywords: Pyrene Solid-state quantum yields Thermal stability Electron injection Electroluminescent
a b s t r a c t Two pyrene derivatives (PVPP and TPVPP) bearing 4-(2,2-diphenyl-vinyl)-phenyl side unit were developed for organic light-emitting diodes. Their photophysical, thermal, electrochemical, and electroluminescent properties as well as the film morphologies have been investigated in detail. Both of them exhibit high solid-state quantum yields, good thermal stability, and high glass-transition temperatures in the range of 127–150 °C. In particular, it is found that multiple side units can significantly affect the electrochemical properties to improve the electron injection. The LUMO energy level of TPVPP is 2.76 eV, which is very close to that of commonly used electron transport materials. The maximum luminance of OLED devices using TPVPP as an emitter layer is 103835 cd/m2 with a maximum current efficiency of 5.19 cd/A and a maximum power efficiency of 3.38 lm/W. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Organic light-emitting diodes (OLEDs) are of considerable interest for their distinctive advantages and potential applications to flat-panel displays and solid-state lightings. Since the first efficient OLEDs reported by Tang and Van Slyke [1], impressive scientific and technological progress has been achieved in this field [2]. However, on the route to applications, there are still challenges remaining. For example, the efficiency and operational lifetime of OLEDs are considered to be the main hurdles for commercial applications [3]. And these strongly depend on the luminescent efficiency and thermal stability of emitting materials. Thus, the development of new solid emitters is of paramount importance. Among various emitting materials, pyrene is a potentially good candidate for OLED applications due to its high photoluminescence (PL) quantum yield, excellent thermal
⇑ Corresponding author. Tel./fax: +86 531 88364963. E-mail address: [email protected] (X.-T. Tao). 1566-1199/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.orgel.2012.08.041
stability, and high charge carrier mobility [4,5]. However, planar pyrene molecule shows a great propensity to form p aggregates/excimers in the solid state, resulting to long-wavelength p aggregates/excimers emission with low fluorescence quantum yield [6]. To circumvent these problems, many new materials with hindered structures have been carried out [7–11], involving polypyrene molecules [8], tetraarylpyrene molecules [9], as well as pyrene–fluorene [10] and pyrene–carbazole [11] systems. Recently, two new pyrene derivatives, 1-[4-(2,2-diphenylvinyl)phenyl]pyrene (PVPP) and 1,3,6,8-tetrakis[4(2,2-diphenylvinyl)phenyl]pyrene (TPVPP), have been synthesized by linking 4-(2,2-diphenyl-vinyl)-phenyl to the pyrene ring [12]. Their structures are shown in Fig. 1. Interestingly, PVPP successfully suppresses the fluorescence quenching of pyrene units, displaying aggregation-induced enhanced emission (AIEE). Though TPVPP is AIEE-inactive, it shows strong solid-state fluorescence. In its crystal structure, there are no intermolecular p–p interactions. Encouraged by their excellent PL properties, we fabricated multilayered OLEDs using PVPP and TPVPP as emitting
Z.-Q. Liang et al. / Organic Electronics 13 (2012) 2898–2904
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Fig. 1. Chemical structures of PVPP and TPVPP.
layers and investigated their electroluminescent (EL) properties in this paper. The devices give high efficiency with a maximum luminance of 103835 cd/m2, a maximum current efficiency of 5.19 cd/A and a maximum power efficiency of 3.38 lm/W. Besides, their photophysical, electrochemical, and thermal properties as well as the film morphologies were studied. The compounds exhibit high glass transition temperatures of 127 °C for PVPP and 150 °C for TPVPP. In particular, it is found that multiple side units can significantly affect the electrochemical properties to improve the electron injection. 2. Results and discussion 2.1. Photophysical properties To investigate the photophysical properties, the absorption and PL spectra of PVPP and TPVPP were measured in THF and in thin film, as shown in Fig. 2. All the corresponding spectral data are summarized in Table 1. In THF, PVPP exhibits a main absorption band at about 354 nm. For TPVPP, two prominent absorption bands are observed: a high-energy band at 339 nm and a low-energy band at 407 nm. The low-energy bands of PVPP and TPVPP originate from the p–p⁄ transitions of the conjugated system. When transfer from solution to film state, the low-energy absorption bands of the two compounds are red-shifted by 11 nm for PVPP and 14 nm for TPVPP, respectively. The PL emission spectra of both compounds are also shown in Fig. 2. PVPP shows a blue emission at 445 nm in THF with a modest full width at half-maximum (FWHM) value of 68 nm. The PL emission of TPVPP is centered at 469 nm and has a FWHM value of 56 nm. In thin film state, the PL spectrum of PVPP represents a 23 nm bathochromic shift relative to its solution state. The film state PL emission spectrum of TPVPP is shifted into the green region with a band centered at 500 nm. The longer red-shift of PL spectra for TPVPP is due to the multiple intermolecular C–H p interactions [12]. In addition, the PL quantum yields of PVPP and TPVPP in THF solution were measured by using quinine sulfate as standard. Their solid-state quantum yields were determined by an integrating sphere. PVPP displays a weak emission in THF solution with a PL quantum yield of 14%, while it emits strongly in film state with a PL quantum yield of 51%. As for TPVPP, it shows a ‘‘normal’’
Fig. 2. Normalized UV–vis absorption and PL spectra of PVPP and TPVPP in THF (1.0 106 M) and in thin film.
fluorescent behavior with a high fluorescence quantum yield of 80% in THF solution and a quantum yield of 32% in thin film state. The totally different fluorescence behavior between PVPP and TPVPP has been explained in our previous work [12].
2.2. Thermal properties and morphology The primary degradation of OLEDs is caused by morphological changes resulting from thermal instability of amorphous organic layer. Therefore, high glass transition temperature and decomposition temperature are prerequisites for organic materials in the practical applications of
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Table 1 Physical properties of two pyrene derivatives. Compound
PVPP TPVPP
Tg/Tm/Td (°C)
127/182/406 150/286/525
kem (nm)b
kmax (nm)a ab Solution
Film
Solution
Film
354 407
365 421
445 469
468 500
Eopt (eV) g
HOMO (eV)c
LUMO (eV)c
3.06 2.66
5.61 5.42
2.55 2.76
a
Peak position of the low-energy absorption band. Peak position of PL, excited at the absorption maximum. ox ox c HOMO calculated from cyclic voltammetry potentials using ferrocene as standard HOMO ¼ ð4:80 þ Eox onset EFc . Eonset Þ is the onset oxidation potential + + versus Ag/Ag+. Eox Fc stands for the onset oxidation potential of ferrocene versus Ag/Ag . It is assumed that the redox potential of Fc/Fc has an absolute energy level of 4.80 eV to vacuum. b
OLEDs. The thermal properties of PVPP and TPVPP were evaluated by differential scanning calorimetry (DSC) (Fig. 3) and thermal gravimetric analysis (TGA) (Fig. 4) experiments. In the first-heating DSC scan of PVPP, a sharp endothermic peak due to melting was observed at 182 °C (Tm), but this behavior no longer existed upon subsequent cooling or heating scans; only a glass phase transition step (Tg) occurred at 127 °C. A similar thermal transition behavior was obtained for TPVPP with a Tm at 286 °C and a Tg at 150 °C. These results indicate that PVPP and TPVPP more favorably form amorphous film rather than crystalline morphology upon thermal treatment. The two compounds also exhibit high thermal stability. Their decomposition temperatures (Td) were found to be 406 °C for PVPP and 525 °C for TPVPP. Key thermal data of the two compounds are listed in Table 1. It shows that the Tg, Tm, and Td of the two derivatives increase with the number of substitutions. Furthermore, the decomposition temperature of TPVPP is significantly higher than those previously reported for other materials containing pyrenyl moieties [7a– 7c,9,10b,10c,11]. The morphology of thin film is a key factor related to the performance of organic thin-film devices. Thus, the amorphous characteristics of PVPP and TPVPP in the film state were further validated by X-ray diffraction (XRD) analysis. As shown in Fig. 5, only a weak and broad peak appears on their XRD patterns. It clearly indicates that the two compounds are amorphous in thin film state. After
storage for four months under ambient conditions, the films of the two compounds are still amorphous, which means that their amorphous films are morphological stable. 2.3. Electrochemical properties For PVPP and TPVPP used in OLEDs, it is important to know the energy levels of their highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). Cyclic voltammetry analysis was carried out to identify their electrochemical behaviors and estimate their orbital energies. The orbital energies and band gaps are
Fig. 4. TGA thermograms of PVPP and TPVPP.
Fig. 3. DSC thermograms of PVPP and TPVPP (recorded during the second heating scan).
Fig. 5. X-ray diffraction patterns of PVPP and TPVPP in thin vacuum sublimated films.
Z.-Q. Liang et al. / Organic Electronics 13 (2012) 2898–2904
also listed in Table 1. Both PVPP and TPVPP exhibit irreversible oxidation peaks, as shown in Fig. 6. The first oxidation peaks are recorded at 0.97 and 0.78 V with the onset potentials at 0.88 and 0.69 V for PVPP and TPVPP, respectively. TPVPP has a lower oxidation potential than PVPP, which might be cause by its enlarged p-conjugated system. Their HOMO energy levels are estimated from the onset oxidation potentials to be 5.61 eV for PVPP and 5.42 eV for TPVPP, using ferrocene as internal standard. Accordingly, the LUMO energy levels of PVPP and TPVPP are calculated to be 2.55 and 2.76 eV, respectively, by combining the HOMO energy levels together with the optical band gaps. The LUMO level of TPVPP is lower than that of PVPP, and the value is very close to the reported LUMO level of commonly used electron transport material such as 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBI) (HOMO = 6.20 eV, LUMO = 2.70 eV) [13]. It indicates that the introduction of multiple 4-(2,2-diphenyl-vinyl)phenyl groups is beneficial to decrease the LUMO energy level. 2.4. Electroluminescence properties The devices performance based on PVPP and TPVPP were investigated by utilizing them as emissive layers. Both pyrene derivatives were sublimed under vacuum to construct multilayer devices. The device configuration is indium tin oxide (ITO)/4,40 -bis[N-(1-naphthyl)-N-phenylamino]-biphenyl (NPB) (50 nm)/PVPP or TPVPP (50 nm)/ TPBI (15 nm)/LiF (1 nm)/Al (80 nm), where NPB (HOMO =
Fig. 6. Cyclic voltammetry of PVPP and TPVPP in dichloromethane.
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Fig. 7. HOMO/LUMO energy levels of the materials used in OLEDs.
5.70 eV, LUMO = 2.60 eV) [14] is used as a hole-transporting layer, TPBI as an electron-transporting layer and a hole-blocking layer, and LiF:Al as the composite cathode. The energy level diagrams of the devices based on PVPP and TPVPP are shown in Fig. 7. According to the diagrams, the energy barriers at PVPP/TPBI and TPVPP/TPBI interfaces for the electron transport are 0.15 and 0.06 eV, respectively. It indicates that the electron can be easily injected to the emitting layers. Fig. 8 shows the normalized EL spectra of PVPP and TPVPP in three-layer devices and their PL spectra. It can be seen that the EL spectra of PVPP matches approximately its PL spectrum. The device based on TPVPP shows a main EL peak (545 nm) and a shoulder peak (489 nm) at 6 V, which is obvious different from its PL spectrum. The emission band at 489 nm in the EL spectra displays a 11 nm blue shift relative to its PL spectrum, implying that they originate from the same excited species. The small deviation between the EL and PL might be due to the device interference phenomenon [15]. The appearance of new emissions in EL spectra usually indicates there is a certain degree of excimer, exciplexe, electromer, or electroplexe formation. To exclude the deduction of exciplex formation, the PL spectra of the spin-coated films of TPVPP/NPB and TPVPP/TPBI were measured [16]. No new emissions were observed in the long wavelength region. It excludes the possibility that the emission at 545 nm originates from the exciplex at the interface between TPVPP and NPB or TPVPP and TPBI. In the crystals, the structure TPVPP is significantly distorted and the twisted geometry successfully prevents the tight intermolecular packing [12]. Therefore, excimers cannot form in the solid states. Moreover, the derivatives based on 4-(2,2-diphenylvinyl)phenyl group have recently been reported by several groups in the context of organic EL applications. They pointed out that two nonplanar phenyl rings located at the end of the molecule can prevent the formation of excimer [17]. To examine the origin of the emission at 545 nm, a single-layer device (ITO/TPVPP (80 nm)/LiF (1 nm)/Al (100 nm)) was fabricated. The device displays two EL peaks at about 490 and 544 nm as shown in Fig. 9. The similarity of EL spectra both in single- and three-layer devices suggests that TPVPP is the single component responsible for the emission at 545 nm. Therefore, the emission at 545 nm is assigned to the electric-field-induced singlet complex electromers (M+/M) [18]. The electromer could be regarded as an excited state of a pair of TPVPP molecules (TPVPP+/TPVPP)⁄ under electric excitation, in which one carried an excess electron while the other carried a hole.
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Fig. 8. EL spectra of PVPP and TPVPP in three-layer devices and their PL spectra.
Fig. 10. Current density–bias voltage–luminance characteristics of the three-layer devices containing PVPP and TPVPP.
Fig. 9. EL spectra of TPVPP in single-layer device.
The current density–bias voltage–luminance and the current efficiency–current density–power efficiency characteristics of the three-layer devices based on PVPP and TPVPP are shown in Figs. 10 and 11, respectively. The essential device performances are collected in Table 2. Both devices exhibit low turn-on voltage of about 3.50 V. The device made from PVPP achieves a maximum luminance of 30996 cd/m2 at 18.5 V, as well as maximum current and power efficiencies of 2.06 cd/A and 1.06 lm/W, respectively. Moreover, the current efficiency shows only a mild decrease as the current density increases. The performance suggests that AIEE-active PVPP is a promising blue-emitting material for the preparation of highly
Fig. 11. Current efficiency–current density–power efficiency characteristics of the three-layer devices containing PVPP and TPVPP.
Z.-Q. Liang et al. / Organic Electronics 13 (2012) 2898–2904 Table 2 Summary of three-layer devices performances.
a b c d e f g h
Compound
PVPP
TPVPP
Turn-on voltage (V)a Maximal luminance (cd/m2) Maximal current efficiency (cd/A) Maximal power efficiency (lm/W) kmax (nm)h EL CIE (x, y)h
3.50 30,996b 2.06c 1.06d 485 (0.16, 0.21)
3.51 103,835e 5.19f 3.38g 545 (0.38, 0.53)
Recorded Recorded Recorded Recorded Recorded Recorded Recorded Recorded
at at at at at at at at
1 cd/m2. 18.5 V. 6.75 V, 60.97 mA/cm2. 6.0 V, 25.87 mA/cm2. 19.25 V. 6.5 V, 10.64 mA/cm2. 3.75 V, 0.044 mA/cm2. 6 V.
efficient blue devices. The device containing TPVPP exhibits a maximum luminance of 103835 cd/m2 at 19.25 V with a maximum current efficiency of 5.19 cd/A and a maximum power efficiency of 3.38 lm/W. Similarly, its current efficiency still stands at a high value (about 2.83 cd/A) as the device reaches a high current density. Compared to the device using PVPP as an emitting layer, the higher efficiency of the device based on TPVPP may be attributed to the low LUMO energy levels, which is near to that of TPBI. The low LUMO energy level facilitates electron injection from the electron transport layer to the emitting layer, while the good electron-injecting ability can assure more efficient hole–electron recombination in the emitting layer. 3. Conclusion In conclusion, the photophysical, thermal, electrochemical and electroluminescent properties as well as the film morphologies of PVPP and TPVPP have been studied. PVPP and TPVPP behave as high brightness solid-state light emitters with excellent thermal and morphological stabilities. In particular, TPVPP exhibits electron transport property, which can assure efficient hole–electron recombination in the emitting layer. The OLEDs made using these compounds as the host emitter show high efficiency. The results of this work indicate that the pyrene derivatives, which have rigid and bulky 4-(2,2-diphenyl-vinyl)-phenyl side units, are promising host-emitter materials with high efficiency and stability. 4. Experimental section 4.1. Chemicals and instruments All solvents used for the measurements were further purified. Dry dichloromethane was freshly distilled over calcium hydride prior to use. UV–vis absorption spectra were recorded on Varian Cary 50 spectrophotometer. Fluorescence measurements were carried out with a Hitachi F4500 fluorescence spectrometer equipped with a 150 W Xe lamp. The fluorescence quantum yields in thin film state were determined by an integrating sphere connected to a spectrophotometer. TGA and DSC measurements were car-
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ried out under a nitrogen atmosphere with a Perkin-Elmer Diamond thermogravimetric analyzer and a Perkin-Elmer Diamond DSC thermal analysis system, respectively. The XRD measurements were investigated with a Bruker D8 advanced diffractometer equipped with CuKa radiation (k = 1.5406 Å). The data were collected using a Ni-filtered Cu target tube at room temperature in the 2h range from 3° to 28°. Cyclic voltammetry was performed on a CHI660B electrochemical work station in dry dichloromethane using Bu4NClO4 (0.1 M) as supporting electrolyte with a scan rate of 50 mV/s at room temperature under argon. A platinum wire was used as a counter electrode, glassy carbon as a working electrode, and Ag/Ag+ (0.1 M of AgNO3 in acetonitrile) as a reference electrode. The Ag/Ag+ reference electrode was calibrated by running cyclic voltammetry on ferrocene as the internal standard. 4.2. OLED fabrication and characterization The devices were fabricated with the configurations ITO/NPB (50 nm)/PVPP or TPVPP (50 nm)/TPBI (15 nm)/ LiF (1 nm)/Al (80 nm) and ITO/TPVPP (80 nm)/LiF (1 nm)/ Al (100 nm). The patterned ITO substrate was carefully cleaned sequentially by detergent, deionized water and acetone, and finally treated with UV–ozone for about 25 min. The organic films and metal electrode were deposited on the ITO substrate by thermal evaporation under a vacuum of 106 Torr. The deposition rates were 2–3 Å/s for organic compounds, and 5–7 Å/s for the metal cathode. The emitting area was 2 2 mm. The EL spectra were measured with a spectrofluorometer FP-6200 (JASCO). A source-measure unit R6145 (Advantest), multimeter 2000 (Keithley), and luminance meter LS-110 (Minolta) were used for current density–bias voltage–luminance measurements. Relative luminance was directly detected by using a multifunctional optical meter. Acknowledgments This work was supported by National Natural Science Foundation of China (Grant Nos. 51021062, 50990061), the 973 Program of the People’s Republic of China (Grant No. 2010CB630702), NSFC (21001068, 20520120221), Shandong Province Science Foundation for Excellent Youths (BS2009CL054), Project of Person with Ability of Jiangsu Province (2010-xcl-015). We thank Dr. Wenming Su for his help with the OLED fabrication. References [1] C.W. Tang, S.A. VanSlyke, Appl. Phys. Lett. 51 (1987) 913–915. [2] (a) S.R. Forrest, Nature 428 (2004) 911–918; (b) C.-T. Chen, Chem. Mater. 16 (2004) 4389–4400; (c) C.A. Zuniga, S. Barlow, S.R. Marder, Chem. Mater. 23 (2011) 658– 681; (d) L. Duan, L.D. Hou, T.-W. Lee, J. Qiao, D.Q. Zhang, G.F. Dong, L.D. Wang, Y. Qiu, J. Mater. Chem. 20 (2010) 6392–6407. [3] C.H. Huang, F.Y. Li, W. Huang, Introduction to Organic Light Emitting Materials and Devices, Fudan University Press, Shanghai, 2005. [4] J.R. Lakowicz, Principles of Fluorescence Spectroscopy, Plenum Press, New York, 1999. [5] (a) N. Karl, Synth. Met. 133–134 (2003) 649–657; (b) L. Zöphel, D. Beckmann, V. Enkelmann, D. Chercka, R. Rieger, K. Müllen, Chem. Commun. 47 (2011) 6960–6962.
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Contents lists available at SciVerse ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Novel 4-(2,2-diphenyl-vinyl)-phenyl substituted pyrene derivatives as efficient emitters for organic light-emitting diodes Zuo-Qin Liang a,b, Zeng-Ze Chu c, De-Chun Zou c, Xiao-Mei Wang b, Xu-Tang Tao a,⇑ a
State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, PR China Jiangsu Key Laboratory for Environment Functional Materials, College of Chemical and Biological Engineering, Suzhou University of Science and Technology, Suzhou 215009, PR China c Department of Polymer Science and Engineering, College of Chemistry, Peking University, Beijing 100871, PR China b
a r t i c l e
i n f o
Article history: Received 28 June 2012 Received in revised form 26 August 2012 Accepted 30 August 2012 Available online 12 September 2012 Keywords: Pyrene Solid-state quantum yields Thermal stability Electron injection Electroluminescent
a b s t r a c t Two pyrene derivatives (PVPP and TPVPP) bearing 4-(2,2-diphenyl-vinyl)-phenyl side unit were developed for organic light-emitting diodes. Their photophysical, thermal, electrochemical, and electroluminescent properties as well as the film morphologies have been investigated in detail. Both of them exhibit high solid-state quantum yields, good thermal stability, and high glass-transition temperatures in the range of 127–150 °C. In particular, it is found that multiple side units can significantly affect the electrochemical properties to improve the electron injection. The LUMO energy level of TPVPP is 2.76 eV, which is very close to that of commonly used electron transport materials. The maximum luminance of OLED devices using TPVPP as an emitter layer is 103835 cd/m2 with a maximum current efficiency of 5.19 cd/A and a maximum power efficiency of 3.38 lm/W. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Organic light-emitting diodes (OLEDs) are of considerable interest for their distinctive advantages and potential applications to flat-panel displays and solid-state lightings. Since the first efficient OLEDs reported by Tang and Van Slyke [1], impressive scientific and technological progress has been achieved in this field [2]. However, on the route to applications, there are still challenges remaining. For example, the efficiency and operational lifetime of OLEDs are considered to be the main hurdles for commercial applications [3]. And these strongly depend on the luminescent efficiency and thermal stability of emitting materials. Thus, the development of new solid emitters is of paramount importance. Among various emitting materials, pyrene is a potentially good candidate for OLED applications due to its high photoluminescence (PL) quantum yield, excellent thermal
⇑ Corresponding author. Tel./fax: +86 531 88364963. E-mail address: [email protected] (X.-T. Tao). 1566-1199/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.orgel.2012.08.041
stability, and high charge carrier mobility [4,5]. However, planar pyrene molecule shows a great propensity to form p aggregates/excimers in the solid state, resulting to long-wavelength p aggregates/excimers emission with low fluorescence quantum yield [6]. To circumvent these problems, many new materials with hindered structures have been carried out [7–11], involving polypyrene molecules [8], tetraarylpyrene molecules [9], as well as pyrene–fluorene [10] and pyrene–carbazole [11] systems. Recently, two new pyrene derivatives, 1-[4-(2,2-diphenylvinyl)phenyl]pyrene (PVPP) and 1,3,6,8-tetrakis[4(2,2-diphenylvinyl)phenyl]pyrene (TPVPP), have been synthesized by linking 4-(2,2-diphenyl-vinyl)-phenyl to the pyrene ring [12]. Their structures are shown in Fig. 1. Interestingly, PVPP successfully suppresses the fluorescence quenching of pyrene units, displaying aggregation-induced enhanced emission (AIEE). Though TPVPP is AIEE-inactive, it shows strong solid-state fluorescence. In its crystal structure, there are no intermolecular p–p interactions. Encouraged by their excellent PL properties, we fabricated multilayered OLEDs using PVPP and TPVPP as emitting
Z.-Q. Liang et al. / Organic Electronics 13 (2012) 2898–2904
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Fig. 1. Chemical structures of PVPP and TPVPP.
layers and investigated their electroluminescent (EL) properties in this paper. The devices give high efficiency with a maximum luminance of 103835 cd/m2, a maximum current efficiency of 5.19 cd/A and a maximum power efficiency of 3.38 lm/W. Besides, their photophysical, electrochemical, and thermal properties as well as the film morphologies were studied. The compounds exhibit high glass transition temperatures of 127 °C for PVPP and 150 °C for TPVPP. In particular, it is found that multiple side units can significantly affect the electrochemical properties to improve the electron injection. 2. Results and discussion 2.1. Photophysical properties To investigate the photophysical properties, the absorption and PL spectra of PVPP and TPVPP were measured in THF and in thin film, as shown in Fig. 2. All the corresponding spectral data are summarized in Table 1. In THF, PVPP exhibits a main absorption band at about 354 nm. For TPVPP, two prominent absorption bands are observed: a high-energy band at 339 nm and a low-energy band at 407 nm. The low-energy bands of PVPP and TPVPP originate from the p–p⁄ transitions of the conjugated system. When transfer from solution to film state, the low-energy absorption bands of the two compounds are red-shifted by 11 nm for PVPP and 14 nm for TPVPP, respectively. The PL emission spectra of both compounds are also shown in Fig. 2. PVPP shows a blue emission at 445 nm in THF with a modest full width at half-maximum (FWHM) value of 68 nm. The PL emission of TPVPP is centered at 469 nm and has a FWHM value of 56 nm. In thin film state, the PL spectrum of PVPP represents a 23 nm bathochromic shift relative to its solution state. The film state PL emission spectrum of TPVPP is shifted into the green region with a band centered at 500 nm. The longer red-shift of PL spectra for TPVPP is due to the multiple intermolecular C–H p interactions [12]. In addition, the PL quantum yields of PVPP and TPVPP in THF solution were measured by using quinine sulfate as standard. Their solid-state quantum yields were determined by an integrating sphere. PVPP displays a weak emission in THF solution with a PL quantum yield of 14%, while it emits strongly in film state with a PL quantum yield of 51%. As for TPVPP, it shows a ‘‘normal’’
Fig. 2. Normalized UV–vis absorption and PL spectra of PVPP and TPVPP in THF (1.0 106 M) and in thin film.
fluorescent behavior with a high fluorescence quantum yield of 80% in THF solution and a quantum yield of 32% in thin film state. The totally different fluorescence behavior between PVPP and TPVPP has been explained in our previous work [12].
2.2. Thermal properties and morphology The primary degradation of OLEDs is caused by morphological changes resulting from thermal instability of amorphous organic layer. Therefore, high glass transition temperature and decomposition temperature are prerequisites for organic materials in the practical applications of
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Table 1 Physical properties of two pyrene derivatives. Compound
PVPP TPVPP
Tg/Tm/Td (°C)
127/182/406 150/286/525
kem (nm)b
kmax (nm)a ab Solution
Film
Solution
Film
354 407
365 421
445 469
468 500
Eopt (eV) g
HOMO (eV)c
LUMO (eV)c
3.06 2.66
5.61 5.42
2.55 2.76
a
Peak position of the low-energy absorption band. Peak position of PL, excited at the absorption maximum. ox ox c HOMO calculated from cyclic voltammetry potentials using ferrocene as standard HOMO ¼ ð4:80 þ Eox onset EFc . Eonset Þ is the onset oxidation potential + + versus Ag/Ag+. Eox Fc stands for the onset oxidation potential of ferrocene versus Ag/Ag . It is assumed that the redox potential of Fc/Fc has an absolute energy level of 4.80 eV to vacuum. b
OLEDs. The thermal properties of PVPP and TPVPP were evaluated by differential scanning calorimetry (DSC) (Fig. 3) and thermal gravimetric analysis (TGA) (Fig. 4) experiments. In the first-heating DSC scan of PVPP, a sharp endothermic peak due to melting was observed at 182 °C (Tm), but this behavior no longer existed upon subsequent cooling or heating scans; only a glass phase transition step (Tg) occurred at 127 °C. A similar thermal transition behavior was obtained for TPVPP with a Tm at 286 °C and a Tg at 150 °C. These results indicate that PVPP and TPVPP more favorably form amorphous film rather than crystalline morphology upon thermal treatment. The two compounds also exhibit high thermal stability. Their decomposition temperatures (Td) were found to be 406 °C for PVPP and 525 °C for TPVPP. Key thermal data of the two compounds are listed in Table 1. It shows that the Tg, Tm, and Td of the two derivatives increase with the number of substitutions. Furthermore, the decomposition temperature of TPVPP is significantly higher than those previously reported for other materials containing pyrenyl moieties [7a– 7c,9,10b,10c,11]. The morphology of thin film is a key factor related to the performance of organic thin-film devices. Thus, the amorphous characteristics of PVPP and TPVPP in the film state were further validated by X-ray diffraction (XRD) analysis. As shown in Fig. 5, only a weak and broad peak appears on their XRD patterns. It clearly indicates that the two compounds are amorphous in thin film state. After
storage for four months under ambient conditions, the films of the two compounds are still amorphous, which means that their amorphous films are morphological stable. 2.3. Electrochemical properties For PVPP and TPVPP used in OLEDs, it is important to know the energy levels of their highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). Cyclic voltammetry analysis was carried out to identify their electrochemical behaviors and estimate their orbital energies. The orbital energies and band gaps are
Fig. 4. TGA thermograms of PVPP and TPVPP.
Fig. 3. DSC thermograms of PVPP and TPVPP (recorded during the second heating scan).
Fig. 5. X-ray diffraction patterns of PVPP and TPVPP in thin vacuum sublimated films.
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also listed in Table 1. Both PVPP and TPVPP exhibit irreversible oxidation peaks, as shown in Fig. 6. The first oxidation peaks are recorded at 0.97 and 0.78 V with the onset potentials at 0.88 and 0.69 V for PVPP and TPVPP, respectively. TPVPP has a lower oxidation potential than PVPP, which might be cause by its enlarged p-conjugated system. Their HOMO energy levels are estimated from the onset oxidation potentials to be 5.61 eV for PVPP and 5.42 eV for TPVPP, using ferrocene as internal standard. Accordingly, the LUMO energy levels of PVPP and TPVPP are calculated to be 2.55 and 2.76 eV, respectively, by combining the HOMO energy levels together with the optical band gaps. The LUMO level of TPVPP is lower than that of PVPP, and the value is very close to the reported LUMO level of commonly used electron transport material such as 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBI) (HOMO = 6.20 eV, LUMO = 2.70 eV) [13]. It indicates that the introduction of multiple 4-(2,2-diphenyl-vinyl)phenyl groups is beneficial to decrease the LUMO energy level. 2.4. Electroluminescence properties The devices performance based on PVPP and TPVPP were investigated by utilizing them as emissive layers. Both pyrene derivatives were sublimed under vacuum to construct multilayer devices. The device configuration is indium tin oxide (ITO)/4,40 -bis[N-(1-naphthyl)-N-phenylamino]-biphenyl (NPB) (50 nm)/PVPP or TPVPP (50 nm)/ TPBI (15 nm)/LiF (1 nm)/Al (80 nm), where NPB (HOMO =
Fig. 6. Cyclic voltammetry of PVPP and TPVPP in dichloromethane.
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Fig. 7. HOMO/LUMO energy levels of the materials used in OLEDs.
5.70 eV, LUMO = 2.60 eV) [14] is used as a hole-transporting layer, TPBI as an electron-transporting layer and a hole-blocking layer, and LiF:Al as the composite cathode. The energy level diagrams of the devices based on PVPP and TPVPP are shown in Fig. 7. According to the diagrams, the energy barriers at PVPP/TPBI and TPVPP/TPBI interfaces for the electron transport are 0.15 and 0.06 eV, respectively. It indicates that the electron can be easily injected to the emitting layers. Fig. 8 shows the normalized EL spectra of PVPP and TPVPP in three-layer devices and their PL spectra. It can be seen that the EL spectra of PVPP matches approximately its PL spectrum. The device based on TPVPP shows a main EL peak (545 nm) and a shoulder peak (489 nm) at 6 V, which is obvious different from its PL spectrum. The emission band at 489 nm in the EL spectra displays a 11 nm blue shift relative to its PL spectrum, implying that they originate from the same excited species. The small deviation between the EL and PL might be due to the device interference phenomenon [15]. The appearance of new emissions in EL spectra usually indicates there is a certain degree of excimer, exciplexe, electromer, or electroplexe formation. To exclude the deduction of exciplex formation, the PL spectra of the spin-coated films of TPVPP/NPB and TPVPP/TPBI were measured [16]. No new emissions were observed in the long wavelength region. It excludes the possibility that the emission at 545 nm originates from the exciplex at the interface between TPVPP and NPB or TPVPP and TPBI. In the crystals, the structure TPVPP is significantly distorted and the twisted geometry successfully prevents the tight intermolecular packing [12]. Therefore, excimers cannot form in the solid states. Moreover, the derivatives based on 4-(2,2-diphenylvinyl)phenyl group have recently been reported by several groups in the context of organic EL applications. They pointed out that two nonplanar phenyl rings located at the end of the molecule can prevent the formation of excimer [17]. To examine the origin of the emission at 545 nm, a single-layer device (ITO/TPVPP (80 nm)/LiF (1 nm)/Al (100 nm)) was fabricated. The device displays two EL peaks at about 490 and 544 nm as shown in Fig. 9. The similarity of EL spectra both in single- and three-layer devices suggests that TPVPP is the single component responsible for the emission at 545 nm. Therefore, the emission at 545 nm is assigned to the electric-field-induced singlet complex electromers (M+/M) [18]. The electromer could be regarded as an excited state of a pair of TPVPP molecules (TPVPP+/TPVPP)⁄ under electric excitation, in which one carried an excess electron while the other carried a hole.
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Fig. 8. EL spectra of PVPP and TPVPP in three-layer devices and their PL spectra.
Fig. 10. Current density–bias voltage–luminance characteristics of the three-layer devices containing PVPP and TPVPP.
Fig. 9. EL spectra of TPVPP in single-layer device.
The current density–bias voltage–luminance and the current efficiency–current density–power efficiency characteristics of the three-layer devices based on PVPP and TPVPP are shown in Figs. 10 and 11, respectively. The essential device performances are collected in Table 2. Both devices exhibit low turn-on voltage of about 3.50 V. The device made from PVPP achieves a maximum luminance of 30996 cd/m2 at 18.5 V, as well as maximum current and power efficiencies of 2.06 cd/A and 1.06 lm/W, respectively. Moreover, the current efficiency shows only a mild decrease as the current density increases. The performance suggests that AIEE-active PVPP is a promising blue-emitting material for the preparation of highly
Fig. 11. Current efficiency–current density–power efficiency characteristics of the three-layer devices containing PVPP and TPVPP.
Z.-Q. Liang et al. / Organic Electronics 13 (2012) 2898–2904 Table 2 Summary of three-layer devices performances.
a b c d e f g h
Compound
PVPP
TPVPP
Turn-on voltage (V)a Maximal luminance (cd/m2) Maximal current efficiency (cd/A) Maximal power efficiency (lm/W) kmax (nm)h EL CIE (x, y)h
3.50 30,996b 2.06c 1.06d 485 (0.16, 0.21)
3.51 103,835e 5.19f 3.38g 545 (0.38, 0.53)
Recorded Recorded Recorded Recorded Recorded Recorded Recorded Recorded
at at at at at at at at
1 cd/m2. 18.5 V. 6.75 V, 60.97 mA/cm2. 6.0 V, 25.87 mA/cm2. 19.25 V. 6.5 V, 10.64 mA/cm2. 3.75 V, 0.044 mA/cm2. 6 V.
efficient blue devices. The device containing TPVPP exhibits a maximum luminance of 103835 cd/m2 at 19.25 V with a maximum current efficiency of 5.19 cd/A and a maximum power efficiency of 3.38 lm/W. Similarly, its current efficiency still stands at a high value (about 2.83 cd/A) as the device reaches a high current density. Compared to the device using PVPP as an emitting layer, the higher efficiency of the device based on TPVPP may be attributed to the low LUMO energy levels, which is near to that of TPBI. The low LUMO energy level facilitates electron injection from the electron transport layer to the emitting layer, while the good electron-injecting ability can assure more efficient hole–electron recombination in the emitting layer. 3. Conclusion In conclusion, the photophysical, thermal, electrochemical and electroluminescent properties as well as the film morphologies of PVPP and TPVPP have been studied. PVPP and TPVPP behave as high brightness solid-state light emitters with excellent thermal and morphological stabilities. In particular, TPVPP exhibits electron transport property, which can assure efficient hole–electron recombination in the emitting layer. The OLEDs made using these compounds as the host emitter show high efficiency. The results of this work indicate that the pyrene derivatives, which have rigid and bulky 4-(2,2-diphenyl-vinyl)-phenyl side units, are promising host-emitter materials with high efficiency and stability. 4. Experimental section 4.1. Chemicals and instruments All solvents used for the measurements were further purified. Dry dichloromethane was freshly distilled over calcium hydride prior to use. UV–vis absorption spectra were recorded on Varian Cary 50 spectrophotometer. Fluorescence measurements were carried out with a Hitachi F4500 fluorescence spectrometer equipped with a 150 W Xe lamp. The fluorescence quantum yields in thin film state were determined by an integrating sphere connected to a spectrophotometer. TGA and DSC measurements were car-
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ried out under a nitrogen atmosphere with a Perkin-Elmer Diamond thermogravimetric analyzer and a Perkin-Elmer Diamond DSC thermal analysis system, respectively. The XRD measurements were investigated with a Bruker D8 advanced diffractometer equipped with CuKa radiation (k = 1.5406 Å). The data were collected using a Ni-filtered Cu target tube at room temperature in the 2h range from 3° to 28°. Cyclic voltammetry was performed on a CHI660B electrochemical work station in dry dichloromethane using Bu4NClO4 (0.1 M) as supporting electrolyte with a scan rate of 50 mV/s at room temperature under argon. A platinum wire was used as a counter electrode, glassy carbon as a working electrode, and Ag/Ag+ (0.1 M of AgNO3 in acetonitrile) as a reference electrode. The Ag/Ag+ reference electrode was calibrated by running cyclic voltammetry on ferrocene as the internal standard. 4.2. OLED fabrication and characterization The devices were fabricated with the configurations ITO/NPB (50 nm)/PVPP or TPVPP (50 nm)/TPBI (15 nm)/ LiF (1 nm)/Al (80 nm) and ITO/TPVPP (80 nm)/LiF (1 nm)/ Al (100 nm). The patterned ITO substrate was carefully cleaned sequentially by detergent, deionized water and acetone, and finally treated with UV–ozone for about 25 min. The organic films and metal electrode were deposited on the ITO substrate by thermal evaporation under a vacuum of 106 Torr. The deposition rates were 2–3 Å/s for organic compounds, and 5–7 Å/s for the metal cathode. The emitting area was 2 2 mm. The EL spectra were measured with a spectrofluorometer FP-6200 (JASCO). A source-measure unit R6145 (Advantest), multimeter 2000 (Keithley), and luminance meter LS-110 (Minolta) were used for current density–bias voltage–luminance measurements. Relative luminance was directly detected by using a multifunctional optical meter. Acknowledgments This work was supported by National Natural Science Foundation of China (Grant Nos. 51021062, 50990061), the 973 Program of the People’s Republic of China (Grant No. 2010CB630702), NSFC (21001068, 20520120221), Shandong Province Science Foundation for Excellent Youths (BS2009CL054), Project of Person with Ability of Jiangsu Province (2010-xcl-015). We thank Dr. Wenming Su for his help with the OLED fabrication. References [1] C.W. Tang, S.A. VanSlyke, Appl. Phys. Lett. 51 (1987) 913–915. [2] (a) S.R. Forrest, Nature 428 (2004) 911–918; (b) C.-T. Chen, Chem. Mater. 16 (2004) 4389–4400; (c) C.A. Zuniga, S. Barlow, S.R. Marder, Chem. Mater. 23 (2011) 658– 681; (d) L. Duan, L.D. Hou, T.-W. Lee, J. Qiao, D.Q. Zhang, G.F. Dong, L.D. Wang, Y. Qiu, J. Mater. Chem. 20 (2010) 6392–6407. [3] C.H. Huang, F.Y. Li, W. Huang, Introduction to Organic Light Emitting Materials and Devices, Fudan University Press, Shanghai, 2005. [4] J.R. Lakowicz, Principles of Fluorescence Spectroscopy, Plenum Press, New York, 1999. [5] (a) N. Karl, Synth. Met. 133–134 (2003) 649–657; (b) L. Zöphel, D. Beckmann, V. Enkelmann, D. Chercka, R. Rieger, K. Müllen, Chem. Commun. 47 (2011) 6960–6962.
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