Poly(p-phenylene ethynylene)-based light-emitting devices

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Synthetic Metals 97 (1998) 123-126

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Poly (p-phenylene ethynylene)-based light-emitting devices Andrea Montali, Paul Smith, Christoph Weder * DepartTnent of Materials, Institute of Polymers, ETH Ziirich, CH-8092 Zurich, Switzerland

Received 2 July 1998; accepted9 July i998

Abstract

We here report polymer light-emitting diodes with substituted poly(p-phenylene ethynylenes) as the emissive layer. Yellow-green electroluminescence was observed for different poly(p-phenylene ethynylene) derivatives. Surprisingly, and importantly in view of stabi~ty issues, devices with an aluminum cathode were found to have a higher external quantum efficiency (0.035%) and a lower onset voltage for etectroluminescence ( 10.8 V) than those with a calcium cathode. These results are explained in terms of a lower energy barrier for electron injection than for hole injection, consistent with the ionization potential of poly(p-phenylene ethynylenes) which was determined to be 6.3 eV below vacuum level with ultraviolet photoelectron spectroscopy and 5.8 eV with cyclovoltalnmetry. © 1998 Elsevier Science S.A. All rights reserved. Ke)~c,ords: Poly(p-phenyleneethynylene);Electroluminescence;Light-emittingdiodes

1. Introduction

Since the discovery of electroluminescence (EL) from polymeric light-emitting diodes (LEDs) [1], the performance and availability of colors have significantly improved by making multilayer devices and using a variety of lightemitting polymers [2,3]. Poly(p-phenylene vinylene) (PPV) and its derivatives have hereto been the material of choice for EL applications [1-3]. Only few groups have studied the photophysical properties of poly(p-phenylene ethynylene)s (PPEs), which feature a triple rather than a double bond in the conjugated backbone [4-16]. EL properties of PPE-based LEDs were studied by Shinar and coworkers [4-7], and PPE is generally not considered to be a promising material for LED applications [3,7]. Here, we report the fabrication and characterization of single-layer LEDs based on two different PPE derivatives, confirming preliminary experiments carried out in our group [ 12], and, thus, demonstrate the principal suitability, and correct misconceptions [3,7] with respect to the use of PPEs as useful emitting layer in polymer LEDs. Surprisingly, and importantly in view of stability issues, devices with an A1 cathode were found to have a higher external quantum efficiency and a lower onset voltage for EL than those with a Ca cathode. These results are explained in terms of a lower energy barrier * Corresponding author. TeL: +41 1 632 3337; fax: +41 1 632 1178; e-mall: [email protected]

for electron injection than for hole injection, which is consistent with the ionization potential of PPEs which was determined to be 6.3 eV below vacuum level with ultraviolet photoelectron spectroscopy (UPS) and 5.8 eV with cyclovoltammetry (CV).

2. Experimental The PPE derivatives selected for this work are O-OPPE, substituted with only linear alkyloxy side chains, and EHOOPPE, derivatized with linear and sterically hindered alkytoxy groups in an alternating pattern (Fig. 1 ). Both polymers were prepared according to the procedures previously described [ 10] and had number-average molecular weights Mn of about 10 000 g mo1-1. Single-layer EL devices were produced by spin-coating filtered solutions of the PPEs (1 wt.% in toluene) onto indium-tin oxide (ITO)-coated (20 fl/sq.) glass substrates. The thickness of the films was determined to be about O-OPPE: tR -- 0 ~ / ~ A ~

~

EHO-OPPE:

R-- 0 / ' - / / ' ' - / ' ~

~

'--'

Fig. I. Chemical structures of the substituted poly(p-phenyleneethynylene)s used in this work.

0379-6779/98/$ - see front matter © 1998ElsevierScience S.A. All rights reserved. PLIS0379-6779(98)00120-9

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A. Montati et al. / Synthetic Metals 97 (1998) 123-126

124

100 + 10 nm with a Tencor Instruments c~-step profilometer. An A1 ( 100 nm), Ca (30 nm) or Cr ( 100 nm) cathode, and in the case of Ca an additional Au protection layer (70 rim), was deposited onto the PPE films with a Baltec MED 020 coating system at pressures of about 2.0 × 10 - 5 mbar, to yield pixilated structures with active areas of 3 × 3 mm 2. Devices were operated and characterized in a glove box under inert Na atmosphere at room temperature. The currentvoltage (I-V) characteristics and light intensities were simultaneously measured with a Keithley 237 SMU and a calibrated Si photodiode. Quantum efficiencies were derived according to the method described by Greenham et al. [ 17]. Emission spectra were recorded using a SPEX-Fluorolog 3 spectrometer. The absotute brightness was measured using a Minolta LS 100 luminance meter, fitted with a close-up lens 110. The valence band edge was determined on thin films (d < 30 nm) spin-coated from toluene onto ITO-coated glass slides by UPS using a monochromatized He I radiation source ( h v = 21.2 eV). CV measurements were carried out in acetonitrile/0.1 M TBAPF6 solutions at a glassy carbon electrode versus Ag/AgNO3 as reference electrode.

characteristics were observed for LEDs based on O-OPPE and EHO-OPPE, in agreement with previous studies, which revealed similar photophysical properties for amorphous films of these polymers [ 10]. A typical I - V curve, showing current density and luminance versus voltage, for an A1/O-OPPE/ITO LED is shown in Fig. 2, the inset illustrating the dependence of light output on the injected current density for devices with different electron-injecting contacts. In contrast to results reported for PPV-based LEDs [2,3], the nature of the cathode material, when comparing A1 and Ca, was found to influence only marginally the characteristics of the devices investigated here. The EL threshold voltage (defined as the voltage at which the photodiode signal starts to increase monotonically) of devices with an A1 rather than a Ca cathode, in fact, was slightly lower (10.8 versus 14.5 V) and external quantum efficiencies of A1 devices were higher than the latter (0.035 versus 0.02%). Devices comprising a Cr cathode, however, exhibit a clearly higher EL threshold voltage ( 19.7 V) and lower quantum efficiencies (0.015%). All devices were found to emit yellow-green light, with an emission maximum at 535 nm. The brightness measured on devices with an A1 cathode at a driving voltage of 22.0 V was 80 c d / m 2. Highest current densities and, thus, maximum brightness were obtained for devices comprising A1 cathodes.

3. R e s u l t s a n d d i s c u s s i o n

Typical values for relevant parameters of the investigated PPE-based devices are given in Table 1. Essentially identical

Table 1 EL characteristics of substituted poly(p-phenylene ethynylene), EHO-OPPE and O-OPPE, single-layer LEDs Emitting layer

EHO-OPPE

EHO-OPPE

EHO-OPPE

O-OPPE

O-OPPE

Cathode material

A1

Ca

Cr

A1

Ca

EL threshold voltage (V) External quantum efficiency (%) Max. brightness ( c d / m 2)

10.8 0.035 80

14.5 0.023 38

19.7 0.015 33

ii.0 0.032 80

14.2 0.020 35

6

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Fig. 2. Typical current density vs. voltage (solid line) and EL intensity vs. voltage (dotted line) curves for an AI/O-OPPE/ITO LED. Inset: Luminancecurrent density characteristics of O-OPPE single-layer LEDs with different cathode materials; the three curves are shown on the same scale.

i25

A. Montali et al. / Symhetic Metals 97 (1998) 123-126

Absorbance, photolurninescence (PL) and EL emission spectra are shown in Fig. 3 for the O-OPPE single-layer LEDs. The EL spectra essentially match the PL spectra but additionally show a distinctive red-shifted band. This feature is in accordance with data of other groups and can be attributed to defect-induced trap states, which can block charge carriers and, thus, yield lower energy EL emission [4,5]. The fact that the nature of the cathode - - when comparing A1 and C a - - barely influences the characteristics of the PPEbased devices contrasts the situation observed for many single-layer devices based on PPE [4] and PPV derivatives. In the latter case, a change from an A1 to a Ca cathode typically leads to a lower EL threshold voltage and an improved quantum efficiency [2], due to the enhanced electron injection from Ca, which has a significantly lower work function than A1 (2.9 and 4.3 eV, respectively) [ 18]. As our internal reference, and to control the quality of our Ca layers, singlelayer devices with unsubstituted PPV as the emitting layer were produced as well [ 19], and, as expected, a significant increase in quantum efficiency was observed when using Ca rather than A1 as the cathode. The behaviour of our PPEbased devices is, however, consistent with the potentials of the conduction and valence band edges of these materials. The upper edge of the valence band was determined with UPS to be 6.3 eV below the vacuum level. The bandgap was determined to be 2.4 eV for both EHO-OPPE and O-OPPE from the onset of emission in fluorescence excitation scans. From these data, the lower edge of the conduction band was calculated to be 3.9 eV below vacuum level. Valence and conduction band edges of EHO-OPPE determined with CV measurements were found to be 5.8 and 3.6 eV below vacuum level, respectively, slightly higher than the values calculated for unsubstituted PPE [ 20], 5.6 and 3.4 eV, respectively. The discrepancy of a few tenths of eV between these values and the UPS results is due to intrinsic differences between measurements in solution and in the solid state [21]. According to these data, a significantly lower energy barrier has to be overcome by the electrons being injected from the metal

Ca: 2.9 eV 3.9 eV LUMO

AI: 4.3 eV Cr: 4.5 eV

5 . 0 eV

PPE

ITO 6.3 eV 1

HOMO

I

Fig. 4. Schematic representation of energy levels ]n a PPE-based singlelayer LED, as determined by UPS.

cathode into the conduction band of the investigated PPEs than by the holes injected from the ITO anode, the valence band edge of which we determined with UPS to be at 5.0 eV below vacuum. This is in contrast to PPV derivatives, where hole injection and transport are facilitated and electron injection is the limiting factor [2]. The EL characteristics, which also demonstrate electron injection and light emission with Cr as the cathode, and the results of the UPS and CV measurements, suggest good electron-transporting properties for the investigated PPEs. The higher efficiency observed when using A1 as a cathode rather than Ca is due to a more balanced charge injection, since electron injection from Ca occurs without having to overcome an energy barrier (Fig. 4). The discrepancy between the results published by Swanson et aI. [ 6] for PPE-based LEDs and the present work could be related to the use of different dialkoxy-PPE derivatives. In addition, we omitted to bake the devices at elevated temperatures (150°C) in order to retain the device performance, as we have demonstrated earlier [ 11 ] that at least the dialkoxy-PPE derivatives investigated here undergo irreversible chemical crosslinking under the conditions denoted.

4. Conclusions

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300

r

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500

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600

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700

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800

Wavelength [nm] Fig. 3. Absorbance (solid line), PL (dotted line), and EL (dashed line) emission spectrafor an A1/O-OPPE/ITOLED.

In summary, we have demonstrated that dialkoxy-substituted PPEs can effectively be used as the emitting layer in polymeric LEDs, as can be clearly seen in Fig. 5. While no efforts were made to optimize the device characteristics with respect to brightness and efficiency, yellow-green electroluminescence with a brightness of up to 80 cd/m 2 at external quantum efficiencies of up to 0.035% was observed for the PPE-based single-layer devices. The positions of the valence and conduction band edges suggest that in the case of PPEs the hole injection barrier is a limiting factor, while the injection of electrons from the low work function electrode is facilitated. The results obtained for PPE-based LEDs comprising different cathode materials confirm this assumption. Consequently, these devices favorably comprise a cathode with moderate work function (i.e. A1 rather than Ca) and,

thus, exhibit enhanced oxidation stability.

126

A. MomaIi et al./ Synthetic Metals 97 (1998) 123-126

Fig. 5, ITO/EHO-OPPE/A1 LED in operation. The size of the device was 9 mine; the picture was taken in daylight.

Acknowledgements The authors are deeply indebted to Dr Ye Tao, Institute for Quantum Electronics, ETH Zfirich, for UPS studies, to Dr Mukundan Thelakkat, Makromotekulare Chemie I, University of Bayreuth, for CV studies, and to Dr Andreas Greiner and Michael Ishaque, Institut fiir Physikalische Chemie-Polymere, Philipps Universitiit Marburg, for preparation of the PPV layers.

References [t] J.H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Mackay, R.H. Friend, P.L. Burn, A.B. Holmes, Nature 347 (1990) 539. [2] D. Bradley, Curr. Opinion Solid State Mater. Sci. 1 (1996) 789 and Refs. therein. [3] A. Kraft, A.C. Grimsdale, A.B. Holmes, Angew. Chem., Int. Ed. Engl. 37 (1998) 403. [4] L.S. Swanson, F. Lu, J. Shinar, Y.W. Ding, T.J. Barton, Proc. SPIE 1910 (1993) 101. [5] J. Shinar, L.S. Swanson, Proc. SPIE 1910 (I993) 147. [6] L.S. Swanson, J. Shinar, Y.W. Ding, T.J. Barton, Synth. Met. 55-57 (1993) 1. [7] W. Chen, S. ijada-Maghsoodi, T.J. Barton, T. Cerkvenik, J. Shinar, Polym. Prepr. (Am. Chem. Soc., Div. Polym, Chem.) 36 (1995) 495.

[8] T.M. Swager, C.G. Git, M.S. Wrighton, J. Phys. Chem. 99 (1995) 4886. [9] D. Ofer, T.M. Swager, M.S. Wrighton, Chem. Mater, 7 (1995) 418. [ 10] Ch. Weder, M.S. ~ 1righton, MacromoIecules 29 (I996) 5t57. [11] D. Steiger, Ch. Weder, P. Smith, Macromol. Rapid Commun. [8 (1997) 643. [ 12] A. Montali, Ch. Weder, P. Smith, Proc. SPIE 3148 (1997) 298. [ 13] Ch. Weder, C. Sarwa, C. Bastiaansen, P. Smith, Adv. Mater. 9 (1997) 1035. [ 14] Ch. Weder, C. Sarwa, A. Montali, C. Bastiaansen, P. Smith, Science 279 (1998) 835. [ 15] A. Montali, C. Bastiaansen, P. Smith, Ch. Weder, Nature 392 (1998-) 261. [16] For a recent review, see: R. Giesa, J.M.S. Rev. Macromol. Chem. Phys. C36 (1996) 631. [ 17] N.C. Greenham, R.H. Friend, D.D.C. Bradley, Adv. Mater. 6 (1994) 491. [ 18] CRC Handbook of Chemistry and Physics, CRC Press, Boca Raton, FL, 76th edn., 1995. [ 19] PPV layers were produced according to the procedures described by O.Sehfifer, J. Pommerehne, W. Guss, H. Veatweber, H.Y. Tak, H. B~issler, G. Ltissem, B. Schartek C, Schmidt, V. Sttimpflen, J.H. Wendorff, S. Spiegel, C. M{511er,A. Greiner, Synth. Met. 82 (1996) 1. [20] J.L. Br~das, R.R. Chance, R.H. Baughman, R. Silbey, J. Chem. Phys. 76 (1982) 3673. [21] M. Thelakkat, R. Fink, P. Pdsch, J. Ring, H.-W. Schmidt, Polym. Prepr. 38 (1997) 394.