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Green light-emitting devices based on soluble oligo(phenylenevinylenes)
Applied Surface Science 246 (2005) 458–463 www.elsevier.com/locate/apsusc
Green light-emitting devices based on soluble oligo(phenylenevinylenes) J.E. Wonga,*, H. Detertb, L. Brehmera, S. Schradera a
b
Institut fu¨r Physik, Universita¨t Potsdam, Am Neuen Palais 10, 14469 Potsdam, Germany Institut fu¨r Organische Chemie, Johannes Gutenberg-Universita¨t Mainz, Duesbergweg 10-14, 55099 Mainz, Germany Available online 1 January 2005
Abstract In this work, we report our investigations on the film-forming properties as well as the optical and electroluminescent characterisations of a series of lateral-substituted soluble oligo(phenylenevinylenes) of various conjugation length. Preliminary investigations show that these materials are potential candidates for use in organic light-emitting devices (OLEDs). Two types of OLEDs were fabricated: single layer (SL) and single heterostructure (SHS), with poly(p-phenylenevinylene) (PPV) as hole transporting layer. Our best results were obtained with single layer device emitting green light with a luminance of 0.18 cd m2 and 0.24 cd m2 at a driving voltage of 10 V. # 2004 Elsevier B.V. All rights reserved. Keywords: Oligo(phenylenevinylenes); Photoluminescence; Electroluminescence
1. Introduction Since Tang et al. [1,2] demonstrated the first organic light-emitting devices (OLEDs), there has been tremendous progress in this field. With an ever increasing interest in novel organic materials, both small molecules as well as polymers, together with a better understanding of the physics of the mechanism involved in the recombination of holes and electrons for bright efficient devices, the architecture of OLEDs has evolved from single layer (SL) to single * Corresponding author. Present address: Stranski-labor, Technische Universita¨t Berlin, Strasse des 17. Juni 112, 10623 Berlin, Germany. Tel.: +49 30 31424938; fax: +49 30 31426602. E-mail address: [email protected] (J.E. Wong).
heterostructure (SHS) or even double heterostructure (DHS) depending on the number of organic layers sandwiched between the electrodes [3–7]. A typical OLED structure can consist of an emission layer (EML), a hole-transport layer (HTL) and/or electrontransport layer (ETL). Most SHS and DHS OLEDs are prepared by vacuum deposition. Not only does this technique require costly equipment, but its use is also limited by the (rather low) molecular mass of the molecules being deposited. The materials studied in this work are oligo(phenylenevinylenes) [8,9] of varying conjugation length, and different substitution on the terminal benzene rings. We recently reported the filmforming properties of these materials [10] using techniques of deposition such as spin-coating and
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.11.049
J.E. Wong et al. / Applied Surface Science 246 (2005) 458–463
Langmuir–Blodgett. However, these techniques require that the molecules deposited be soluble in an organic solvent. One major inconvenience in preparing multilayer system is that pre-existing layer should not be soluble in the solvent from which the successive layer is being deposited from. Even if socalled ‘‘orthogonal’’ solvents are used, the surface is usually altered, resulting in a mixing of the two materials at the surface. In this work, using the spin-coating technique, two types of OLEDs were fabricated: SL and SHS devices. In a SL structure, the oligo(phenylenevinylenes) constitute the emissive layer (EML), while in a SHS device, the oligo(phenylenevinylenes) are spincoated on a layer of poly(p-phenylenevinylene) (PPV) which constitutes the hole-transport layer.
2. Experimental The synthesis of the oligo(phenylenevinylenes) and the poly(p-phenylenevinylene) are detailed in the literature [8,9] and Fig. 1 shows the molecular structures of the materials studied in this work. The thin films were obtained by spin-coating solutions of the oligo(phenylenevinylenes) of various concentrations at various speed (500–2000 rpm) to optimise the film thickness as well as the uniformity and homogeneity of the films on the substrate, which were either quartz, silicon or indium–tin–oxide (ITO)-coated glass substrate. The film thickness was obtained with a Dektak surface profiler, and null-ellipsometer. UV–vis absorption and photoluminescence spectra of the films were recorded using a Perkin-Elmer Lamda 16 spectrometer and Perkin-Elmer LS50 luminescence spectrometer, respectively. Single layer EL cells were fabricated by spincoating the organic materials onto an ITO-coated glass substrate, and an aluminium electrode was vacuumdeposited at a chamber pressure of 1 106 Torr onto the organic layer. The emitting area of EL cells is 2 mm 2 mm. The luminance–current–voltage characteristics were measured using a luminance spectrometer (Minolta LS-50), and a current–voltage (I–V) measuring unit (Keithley 2400 source meter and a Hewlett-Packard 3458A multimeter). On one substrate, six individual OLED samples were prepared
459
simultaneously. EL measurements were made under forward bias (ITO positive) and the emission measured in the forward direction through the transparent ITO.
3. Results and discussion 3.1. Thin film studies Fig. 2 shows UV–vis absorption spectra of thin films of oligo(phenylenevinylenes). They are similar to those obtained in solution [8,9] which is quite broad peaks of almost identical full width at half maximum (FWHM), between 100 and 150 nm. However, the spectra of the thin films are red-shifted compared to those obtained from the solution [10] and are characterised by a strong absorption at l = 380 nm, attributed to the p ! p* transition of the phenyl groups, with a shoulder at around l = 445 nm. Fig. 3 shows the photoluminescence spectrum of A7 with peaks at l = 490 nm and l = 513 nm, and a shoulder at l = 540 nm. 3.2. Single layer devices The device structures studied are as follows: SL1 ITO (130 nm)/A1/Al (75 nm) SL2 ITO (130 nm)/A12/Al (75 nm) SL3 ITO (130 nm)/A16/Al (75 nm) SL4 ITO (130 nm)/A17/Al (75 nm) The current (I)–voltage (V) characteristics for the device structures of SL2, SL3 and SL4 are shown in Fig. 4(a–c), respectively. All show clear rectification behaviour although, the somewhat erratic curve obtained for SL2 (Fig. 4a) and SL4 (Fig. 4c) is an indication of poor injection efficiencies, and is also probably caused by the use of aluminium as the cathode electrode. Devices SL2 and SL4 have a turn-on voltage of around 4 V, which is lower than that of device SL3 (around 6 V). We observed green light emission from devices SL3 and SL4 with a luminance of 0.18 cd m2 and 0.24 cd m2, respectively. However, we could only measure light emission from device SL3 (Fig. 4b (inset)).
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J.E. Wong et al. / Applied Surface Science 246 (2005) 458–463
Fig. 1. Molecular structure of the oligo(phenylenevinylenes) studied.
Fig. 2. Absorption spectra of thin films deposited by spin-coating on quartz.
Fig. 3. Photoluminescence spectrum of a thin film of A7.
J.E. Wong et al. / Applied Surface Science 246 (2005) 458–463
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holes can accumulate at the polymer/oligomer interface if there is an energy upset. This could decrease the turn-on voltage, and consequently improve the power efficiency. With conventional poly(p-phenylenevinylene) precursor, the precursor leaving group is usually tetrahydrothiophene and HCl. The latter can easily react with the ITO substrate forming InCl3, which in turn can diffuse into the PPV layer and oxidise (or dope) the polymer. To overcome this problem, we have used a PPV precursor [11,12], using a novel synthetic pathway so that, on polymerisation, the leaving group is no longer HCl. After spin-coating on ITO-coated glass substrate of this PPV precursor, the substrates were baked at 280 8C under high vacuum (106 Torr) for 8 h to convert the soluble precursor to the insoluble PPV. To built multilayer system devices, solutions of the different oligo(phenylenevinylenes) were then deposited by spin-coating onto the insoluble PPV layer. The device structures studied are as follows: SHS5 ITO(130 nm)/PPV/A1/Al (75 nm) SHS6 ITO(130 nm)/PPV/A12/Al (75 nm) SHS7 ITO(130 nm)/PPV/A16/Al (75 nm) SHS8 ITO(130 nm)/PPV/A17/Al (75 nm)
Fig. 4. I–V characteristics of single layer devices: (a) ITO/A12/Al; (b) ITO/A16/Al (the inset shows the brightness–voltage characteristics); (c) ITO/A17/Al.
3.3. Single heterostructure devices The PPV constitutes the hole-transport layer. A two-layer device that incorporates a hole-transport layer means that more holes are injected into the device due to a lower barrier to hole injection. Secondly, it can improve the electron injection as
The current (I)–voltage (V) characteristics for these device structures are shown in Fig. 5. All show clear rectification behaviour, especially in the case of SHS7 (Fig. 5b) and SHS9 (Fig. 5a). With this configuration, the turn-on voltage is about the same as that obtained for single layer devices, although in the case of SHS5 (Fig. 5d), a turn-on voltage of less than 2.5 V was recorded. There is no reason to suppose that this turnon voltage cannot be reduced. We also observed yellowish green light emission from SHS7 and SHS9 but the luminance were too weak to be detected. As a reference we will note the 0.06 cd m2 luminance of SHS5. It is interesting to point out that in the same batch (i.e., one substrate with six devices), our best results were obtained with devices fabricated around the central part of the substrate, and that most of the devices that did not work, were at the end of the substrate. This is probably due to the fact that the film is more homogeneous in the centre than at the end.
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J.E. Wong et al. / Applied Surface Science 246 (2005) 458–463
Fig. 5. I–V characteristics of single heterostructure devices: (a) ITO/PPV/A17/Al, (b) ITO/PPV/A16/Al, (c) ITO/PPV/A12/Al and (d) ITO/PPV/ A1/Al. The insets in (a) and (b) show the corresponding brightness–voltage characteristics.
This is typical for spin-coated film as the solution spins radially outwards to leave a thin film on the substrate. One may speculate that the rather poor quality of the polymerised films is mainly due to the large volume shrinkage, which usually takes place during polymerisation. The volume shrinkage in thin films can lead to the formation of microcracks, which could result in leak currents and ultimately cause electrical short circuits during operation.
4. Conclusions We have reported electroluminescent characteristics of both single layer and single heterostructure organic light-emitting devices made from these materials. All devices show clear rectification behaviour with a turn-on voltage around 4–6 V, and our most successful devices emit in the green and yellowgreen with a luminance of 0.24 cd m2 for a single
layer device. We have shown that soluble oligo(phenylenevinylenes) are very promising candidates for use in organic light-emitting devices. Current work is being undertaken to optimise the efficiency of the devices. Acknowledgements The authors would like to acknowledge support from the European Commission under the contract number FMRX-CT97-0106 (TMR-EUROLED). H. Detert would like to thank the Deutsche Forschungsgemeinschaft for financial support. References [1] C.W. Tang, S.A. VanSlyke, Appl. Phys. Lett. 51 (1987) 913. [2] C.W. Tang, S.A. VanSlyke, C.H. Chen, J. Appl. Phys. 65 (1989) 3610.
J.E. Wong et al. / Applied Surface Science 246 (2005) 458–463 [3] C. Adachi, S. Tokito, T. Tsutsui, S. Saito, Jpn. J. Appl. Phys. 27 (1988) 268. [4] C. Adachi, T. Tsutsui, S. Saito, Appl. Phys. Lett. 55 (1989) 1489. [5] C. Adachi, T. Tsutsui, S. Saito, Appl. Phys. Lett. 56 (1990) 799. [6] N.C. Greenham, S.C. Moratti, D.D.C. Bradley, R.H. Friend, A.B. Holmes, Nature 365 (1993) 628. [7] J. Kido, M. Kimura, K. Nagai, Science 267 (1995) 1332.
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[8] H. Detert, E. Sugiono, J. Prakt. Chem. 341 (1999) 358. [9] U. Stalmanch, H. Detert, J. Prakt. Chem. 342 (2000) 10. [10] J.E. Wong, S. Schrader, H. Detert, S. Katholy, L. Brehmer, Mater. Sci. Eng. C 22 (2003) 413. [11] P. Adriaensens, M. Van Der Borght, L. Hontis, A. Issaris, A. van Breemen, M. de Kok, D. Vanderzande, J. Gelan, Polymer 41 (2000) 7003. [12] B. Grimm, personal communication.
Green light-emitting devices based on soluble oligo(phenylenevinylenes) J.E. Wonga,*, H. Detertb, L. Brehmera, S. Schradera a
b
Institut fu¨r Physik, Universita¨t Potsdam, Am Neuen Palais 10, 14469 Potsdam, Germany Institut fu¨r Organische Chemie, Johannes Gutenberg-Universita¨t Mainz, Duesbergweg 10-14, 55099 Mainz, Germany Available online 1 January 2005
Abstract In this work, we report our investigations on the film-forming properties as well as the optical and electroluminescent characterisations of a series of lateral-substituted soluble oligo(phenylenevinylenes) of various conjugation length. Preliminary investigations show that these materials are potential candidates for use in organic light-emitting devices (OLEDs). Two types of OLEDs were fabricated: single layer (SL) and single heterostructure (SHS), with poly(p-phenylenevinylene) (PPV) as hole transporting layer. Our best results were obtained with single layer device emitting green light with a luminance of 0.18 cd m2 and 0.24 cd m2 at a driving voltage of 10 V. # 2004 Elsevier B.V. All rights reserved. Keywords: Oligo(phenylenevinylenes); Photoluminescence; Electroluminescence
1. Introduction Since Tang et al. [1,2] demonstrated the first organic light-emitting devices (OLEDs), there has been tremendous progress in this field. With an ever increasing interest in novel organic materials, both small molecules as well as polymers, together with a better understanding of the physics of the mechanism involved in the recombination of holes and electrons for bright efficient devices, the architecture of OLEDs has evolved from single layer (SL) to single * Corresponding author. Present address: Stranski-labor, Technische Universita¨t Berlin, Strasse des 17. Juni 112, 10623 Berlin, Germany. Tel.: +49 30 31424938; fax: +49 30 31426602. E-mail address: [email protected] (J.E. Wong).
heterostructure (SHS) or even double heterostructure (DHS) depending on the number of organic layers sandwiched between the electrodes [3–7]. A typical OLED structure can consist of an emission layer (EML), a hole-transport layer (HTL) and/or electrontransport layer (ETL). Most SHS and DHS OLEDs are prepared by vacuum deposition. Not only does this technique require costly equipment, but its use is also limited by the (rather low) molecular mass of the molecules being deposited. The materials studied in this work are oligo(phenylenevinylenes) [8,9] of varying conjugation length, and different substitution on the terminal benzene rings. We recently reported the filmforming properties of these materials [10] using techniques of deposition such as spin-coating and
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.11.049
J.E. Wong et al. / Applied Surface Science 246 (2005) 458–463
Langmuir–Blodgett. However, these techniques require that the molecules deposited be soluble in an organic solvent. One major inconvenience in preparing multilayer system is that pre-existing layer should not be soluble in the solvent from which the successive layer is being deposited from. Even if socalled ‘‘orthogonal’’ solvents are used, the surface is usually altered, resulting in a mixing of the two materials at the surface. In this work, using the spin-coating technique, two types of OLEDs were fabricated: SL and SHS devices. In a SL structure, the oligo(phenylenevinylenes) constitute the emissive layer (EML), while in a SHS device, the oligo(phenylenevinylenes) are spincoated on a layer of poly(p-phenylenevinylene) (PPV) which constitutes the hole-transport layer.
2. Experimental The synthesis of the oligo(phenylenevinylenes) and the poly(p-phenylenevinylene) are detailed in the literature [8,9] and Fig. 1 shows the molecular structures of the materials studied in this work. The thin films were obtained by spin-coating solutions of the oligo(phenylenevinylenes) of various concentrations at various speed (500–2000 rpm) to optimise the film thickness as well as the uniformity and homogeneity of the films on the substrate, which were either quartz, silicon or indium–tin–oxide (ITO)-coated glass substrate. The film thickness was obtained with a Dektak surface profiler, and null-ellipsometer. UV–vis absorption and photoluminescence spectra of the films were recorded using a Perkin-Elmer Lamda 16 spectrometer and Perkin-Elmer LS50 luminescence spectrometer, respectively. Single layer EL cells were fabricated by spincoating the organic materials onto an ITO-coated glass substrate, and an aluminium electrode was vacuumdeposited at a chamber pressure of 1 106 Torr onto the organic layer. The emitting area of EL cells is 2 mm 2 mm. The luminance–current–voltage characteristics were measured using a luminance spectrometer (Minolta LS-50), and a current–voltage (I–V) measuring unit (Keithley 2400 source meter and a Hewlett-Packard 3458A multimeter). On one substrate, six individual OLED samples were prepared
459
simultaneously. EL measurements were made under forward bias (ITO positive) and the emission measured in the forward direction through the transparent ITO.
3. Results and discussion 3.1. Thin film studies Fig. 2 shows UV–vis absorption spectra of thin films of oligo(phenylenevinylenes). They are similar to those obtained in solution [8,9] which is quite broad peaks of almost identical full width at half maximum (FWHM), between 100 and 150 nm. However, the spectra of the thin films are red-shifted compared to those obtained from the solution [10] and are characterised by a strong absorption at l = 380 nm, attributed to the p ! p* transition of the phenyl groups, with a shoulder at around l = 445 nm. Fig. 3 shows the photoluminescence spectrum of A7 with peaks at l = 490 nm and l = 513 nm, and a shoulder at l = 540 nm. 3.2. Single layer devices The device structures studied are as follows: SL1 ITO (130 nm)/A1/Al (75 nm) SL2 ITO (130 nm)/A12/Al (75 nm) SL3 ITO (130 nm)/A16/Al (75 nm) SL4 ITO (130 nm)/A17/Al (75 nm) The current (I)–voltage (V) characteristics for the device structures of SL2, SL3 and SL4 are shown in Fig. 4(a–c), respectively. All show clear rectification behaviour although, the somewhat erratic curve obtained for SL2 (Fig. 4a) and SL4 (Fig. 4c) is an indication of poor injection efficiencies, and is also probably caused by the use of aluminium as the cathode electrode. Devices SL2 and SL4 have a turn-on voltage of around 4 V, which is lower than that of device SL3 (around 6 V). We observed green light emission from devices SL3 and SL4 with a luminance of 0.18 cd m2 and 0.24 cd m2, respectively. However, we could only measure light emission from device SL3 (Fig. 4b (inset)).
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J.E. Wong et al. / Applied Surface Science 246 (2005) 458–463
Fig. 1. Molecular structure of the oligo(phenylenevinylenes) studied.
Fig. 2. Absorption spectra of thin films deposited by spin-coating on quartz.
Fig. 3. Photoluminescence spectrum of a thin film of A7.
J.E. Wong et al. / Applied Surface Science 246 (2005) 458–463
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holes can accumulate at the polymer/oligomer interface if there is an energy upset. This could decrease the turn-on voltage, and consequently improve the power efficiency. With conventional poly(p-phenylenevinylene) precursor, the precursor leaving group is usually tetrahydrothiophene and HCl. The latter can easily react with the ITO substrate forming InCl3, which in turn can diffuse into the PPV layer and oxidise (or dope) the polymer. To overcome this problem, we have used a PPV precursor [11,12], using a novel synthetic pathway so that, on polymerisation, the leaving group is no longer HCl. After spin-coating on ITO-coated glass substrate of this PPV precursor, the substrates were baked at 280 8C under high vacuum (106 Torr) for 8 h to convert the soluble precursor to the insoluble PPV. To built multilayer system devices, solutions of the different oligo(phenylenevinylenes) were then deposited by spin-coating onto the insoluble PPV layer. The device structures studied are as follows: SHS5 ITO(130 nm)/PPV/A1/Al (75 nm) SHS6 ITO(130 nm)/PPV/A12/Al (75 nm) SHS7 ITO(130 nm)/PPV/A16/Al (75 nm) SHS8 ITO(130 nm)/PPV/A17/Al (75 nm)
Fig. 4. I–V characteristics of single layer devices: (a) ITO/A12/Al; (b) ITO/A16/Al (the inset shows the brightness–voltage characteristics); (c) ITO/A17/Al.
3.3. Single heterostructure devices The PPV constitutes the hole-transport layer. A two-layer device that incorporates a hole-transport layer means that more holes are injected into the device due to a lower barrier to hole injection. Secondly, it can improve the electron injection as
The current (I)–voltage (V) characteristics for these device structures are shown in Fig. 5. All show clear rectification behaviour, especially in the case of SHS7 (Fig. 5b) and SHS9 (Fig. 5a). With this configuration, the turn-on voltage is about the same as that obtained for single layer devices, although in the case of SHS5 (Fig. 5d), a turn-on voltage of less than 2.5 V was recorded. There is no reason to suppose that this turnon voltage cannot be reduced. We also observed yellowish green light emission from SHS7 and SHS9 but the luminance were too weak to be detected. As a reference we will note the 0.06 cd m2 luminance of SHS5. It is interesting to point out that in the same batch (i.e., one substrate with six devices), our best results were obtained with devices fabricated around the central part of the substrate, and that most of the devices that did not work, were at the end of the substrate. This is probably due to the fact that the film is more homogeneous in the centre than at the end.
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J.E. Wong et al. / Applied Surface Science 246 (2005) 458–463
Fig. 5. I–V characteristics of single heterostructure devices: (a) ITO/PPV/A17/Al, (b) ITO/PPV/A16/Al, (c) ITO/PPV/A12/Al and (d) ITO/PPV/ A1/Al. The insets in (a) and (b) show the corresponding brightness–voltage characteristics.
This is typical for spin-coated film as the solution spins radially outwards to leave a thin film on the substrate. One may speculate that the rather poor quality of the polymerised films is mainly due to the large volume shrinkage, which usually takes place during polymerisation. The volume shrinkage in thin films can lead to the formation of microcracks, which could result in leak currents and ultimately cause electrical short circuits during operation.
4. Conclusions We have reported electroluminescent characteristics of both single layer and single heterostructure organic light-emitting devices made from these materials. All devices show clear rectification behaviour with a turn-on voltage around 4–6 V, and our most successful devices emit in the green and yellowgreen with a luminance of 0.24 cd m2 for a single
layer device. We have shown that soluble oligo(phenylenevinylenes) are very promising candidates for use in organic light-emitting devices. Current work is being undertaken to optimise the efficiency of the devices. Acknowledgements The authors would like to acknowledge support from the European Commission under the contract number FMRX-CT97-0106 (TMR-EUROLED). H. Detert would like to thank the Deutsche Forschungsgemeinschaft for financial support. References [1] C.W. Tang, S.A. VanSlyke, Appl. Phys. Lett. 51 (1987) 913. [2] C.W. Tang, S.A. VanSlyke, C.H. Chen, J. Appl. Phys. 65 (1989) 3610.
J.E. Wong et al. / Applied Surface Science 246 (2005) 458–463 [3] C. Adachi, S. Tokito, T. Tsutsui, S. Saito, Jpn. J. Appl. Phys. 27 (1988) 268. [4] C. Adachi, T. Tsutsui, S. Saito, Appl. Phys. Lett. 55 (1989) 1489. [5] C. Adachi, T. Tsutsui, S. Saito, Appl. Phys. Lett. 56 (1990) 799. [6] N.C. Greenham, S.C. Moratti, D.D.C. Bradley, R.H. Friend, A.B. Holmes, Nature 365 (1993) 628. [7] J. Kido, M. Kimura, K. Nagai, Science 267 (1995) 1332.
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[8] H. Detert, E. Sugiono, J. Prakt. Chem. 341 (1999) 358. [9] U. Stalmanch, H. Detert, J. Prakt. Chem. 342 (2000) 10. [10] J.E. Wong, S. Schrader, H. Detert, S. Katholy, L. Brehmer, Mater. Sci. Eng. C 22 (2003) 413. [11] P. Adriaensens, M. Van Der Borght, L. Hontis, A. Issaris, A. van Breemen, M. de Kok, D. Vanderzande, J. Gelan, Polymer 41 (2000) 7003. [12] B. Grimm, personal communication.