Application of polyaniline (emeraldine base, EB) in polymer light-emitting devices

Synthetic Metals 78 (1996) 33-37

Application of polyaniline (emeraldine base, EB) in polymer light-emitting devices H.L. Wang a, A.G. MacDiarmid a, Y.Z. Wang b, D.D. Gebler b, A.J. Epstein b a Department of Chemistry, Vniversity b Departments of Physics and Chemistnj,

of Pennsylvania, Philadelphia, PA 19104-6323, USA Ohio State Vniver&, Columbus, OH 43210-1106, USA

Received 18 September 1995; accepted 28 September 1995

Abstract We report the fabricationof severalmultilayerlight-emitting-diode (LED) devicesbasedon a novel conjugatedpolymer,poly(2,5dihexadecanoxy phenylenevinylenepyridyl vinylene) (PPV-PPyV), involvingtheuseof polyaniline(emeraldine base,EB) asaninsulating layerbetweentheemissivepolymerlayerandtheelectrodes. In all theabovedeviceswith variousconfigurations (‘3-layers’,‘6layers-l’, ‘4layers-2’and‘5layers’) ,only thesymmetricallyconfiguredac. light-emitting(SCALE) (‘5-layers’)deviceshowsthecapabilityof operating in both forwardandreversebiasmodesandin an a.c. mode.The SCALEdeviceshavea typical turn-onvoltageof about4-6 V andwork well underboth forwardandreversebiasmodes.It is importantto notethat the total resistance (R= V/I) of the four devicesat anygiven appliedpotentialdecreases asthenumberof insulating polymer layersincreases,suggestingthat the nature of the electrode/polymer interface playsa criticalrolein determiningthecharacteristics of thedevices. Keywords:

Diodes; Polyaniline; SCALE devices;Luminescence

1. Introduction

Conjugated polymers have been studied extensively for their electrical conductivity, interesting optical and, more recently, electroluminescent properties [ l-71. Conjugated polymer light-emitting diodes (CPLEDs) were first reported in 1990 using poly( phenylene vinylene) (PPV) as the lightemitting material [ 1] . PPV thus became the model compound for the study of CPLED properties. Even today PPV and its derivatives remain the most popular polymers in fabricating CPLED devices. Previously, we have reported the electroluminescent properties of a copolymer of a derivative of PPV and the nitrogen analog of PPV, poly(2,5-dihexadecanoxy phenylene vinylene pyridyl vinylene) (PPV-PPyV) [ 81, as shown’ in Scheme 1.

‘p-Q&-y R= c1 6H33 Scheme 1. Molecular structure of PPV-PPyV. 0379-6779/96/515.00 0 1996 Elsevier Science S.A. All rights reserved SSDlO379-6779(95)03569-6

The polymer backbone is isoelectronic with that of PPV and its derivatives, The nitrogen of this polymer and of related nitrogen-containing polymers can also be protonated and alkylated [ g-111, raising the possibility for fine tuning of its photoluminescence [ 111 and electroluminescence [ 121 spectra. This polymer presents a distinct advantage as compared to PPV in fabricating CPLED devices since it is soluble in a variety of organic solvents including CHCls, xylene, THF, etc., and is somewhat soluble in 98% HCOOH. In this paper we report the electroluminescent properties of PPVPPyV in a variety of different device configurations, some of which can be operated in forward and reverse bias modes between aluminum and indium-tin oxide (ITO) glasselectrodes, thus enabling operation in an a.c. mode. Some of these involve a five-layered symmetrically configured ac. lightemitting (SCALE) configuration [ 131. The use of doped polyaniline as the hole injection electrode in an electroluminescent device employing poly (2-methoxy, 5- (2’-ethylhexyloxy ) - 1,4-phenylene vinylene) (MEHPPV) as the light-emitting polymer and calcium as the electron injection electrode has been described previously [ 141. It was reported that the use of doped polyaniline instead of an IT0 glass substrate resulted in significant reduction of the operating potential [ 151. We have previously extended these results to PPV-PPyV instead of MEH-PPV and have also

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found a significant reduction in turn-on potential [ 81. In this communication we employ non-doped polyaniline (emeraldine base, EB) [ 8,13,16] not as an electrode material but rather as an insulating polymer layer in four- and five-layer devices. In the past, CPLED devices have usually been operated in the forward bias mode only, the IT0 glass being attached to the positive terminal of the dc. power supply. More recently several reports have been published involving operation of these devices in the reverse bias mode [ 12,13,17-191 although such operation, at least in certain cases [ 12,171, results in rapid breakdown of the device. Both symmetric and asymmetric I-V curves have been reported, but even in the almost symmetrical I-Vcurve the electroluminescence intensity while operating in an a.c. mode may differ in the forward and reverse bias modes [ 181.

2. Experimental Polyaniline (emeraldine base, EB) was synthesized by the oxidative polymerization of aniline by the standard procedure [ 201. Deprotonation of the initially precipitated emeraldineHCl was accomplished using (0.1 M) ammonium hydroxide. Anal. Calc.: C, 79.53; H, 5.01; N, 15.46. Found: C, 78.70; H, 4.74; N, 15.66; total, 99.1%. The W-Vis spectrum of the polyaniline (emeraldine base, EB) in NMP solution has absorptions at 328 and 633 nm. These values are essentially identical to published data [ 211 in the same solvent. The synthesis and characterization of the PPV-PPyV copolymer have been reported earlier [ 91. All polymer solutions were filtered through a 0.2 p,rn Spartan filter and were spin-cast from about 1 wt.% PPV-PPyV solution in CHC&. An IT0 glass substrate with a sheet resistance of about 100 ohm/square (Delta Technology Inc.) was cleaned by ultrasonication in a mixture of isopropanol and water ( 1: 1) [ 221. Aluminum was vacuum-deposited on top of the polymer under 2 X 10m6 Torr, the rate of deposition being l-2 A/s to give an emitting area of 4 mm2, A typical CPLED device had an emissive polymer layer thickness of about 1500 A and an aluminum layer of about 2000 A. Thickness measurements were carried out with a Tencor Co. profilometer. Photoluminescence (PL) spectra were obtained by with a PTI fluorescence spectrometer in air. The excitation wavelengths used were the absorption maxima in the W-Vis spectra. Electroluminescence (EL) spectra were measured using a PTI spectrometer and a Lambda LPD-421A-FM d-c. power supply. An a.c. sinusoidal voltige was generated by a Tektronix CFG 250 function generator, connected to a Hewlett Packard 6827A bipolar power supply/amplifier. W-Vis spectra of polymer films were obtained using a Perkin-Elmer Lambda 9 W-Vis-NIR spectrometer. The turn-on potential for the CPLED devices described below was taken as the potential at which a non-zero current is visually clearly apparent from a linear plot of the current voltage curves. All fab-

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rication of devices and all measurements were performed in air.

3. Results 3.1. Al/PPV-PPyV/ITO/glass

(‘3~layers’) device

The device was fabricated by spin-casting PPV-PPyV from CHCl, solution on an IT0 glass electrode. An aluminum layer (about 2000 A) was later vacuum-deposited. Fig. 1 shows the W-Vis and PL spectra of a PPV-PPyV film and the EL spectrum from the above device operating in a forward bias mode. The forward bias current was obtained when the IT0 electrode was positively biased and the Al electrode was negatively biased. The I-V characteristics are given in Fig. 2 (‘3-layers’). The turn-on voltage was about 13.5 V. The device did not operate in a reverse bias mode up to about 2.5 V at which value no significant current was observed. Visible light was observed in the forward bias mode in a dimly lit room at a current density of about 0.45 mA/mm2. 3.2. Al/EB/PPV-PPyV/ITO/glass

(‘4-layers-2’) device

This device wasfabricated according to the general method described in the Experimental Section. After depositing the PPV-PPyV layer from CHCl, solution, a layer of polyaniline emeraldine base (EB) wasspin-cast from 1 wt,% EB solution in NMP on top of the PPV-PPyV layer, upon which the aluminum layer was then deposited. The Z-V characteristics are given in Fig. 2 (‘4-layers-2’). The turn-on potential was about 10.0 V. The device did not operate in a reverse bias mode up to about 25.0 V at which value no significant current was observed. Visible light was observed in the forward bias mode in a dimly lit room at a current density of about 0.30 mA/mm2. 1

350

I

450

I

I

I

550

Wavelength

I

650

I

I

750

(r-m)

Fig. 1. (a) UV-Vis spectrum of a film of PPV-PPyV, (b) EL spectrum of an Al/PPV-PPyV/ITO device and (c) PL spectrum of a film of PPVPPyV.

H.L. Wang et al. /Synthetic

-1.9 -2.5 -25

I

, -20

I

/, I

, -15

, -5

-10

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I 5

8

8

I 10

1 15

35

8

I 20

*

Bias

Fig. 2. I-Vcharacteristics of ‘3-layers’, ‘4-layers-l’, ‘4-layers-2’ and ‘5-layers’ devices using Al as the metal electrode.

3.3. AWPPV-PPyV/EB/ITO/glass (‘4~layers-l’) device

This device was constructed in an analogous manner to the ‘4-layers-2’ device, except that the PPV-PPyV and EB layers were reversed. The I-Vcharacteristics are given in Fig. 2 ( ‘4layers-l’). The turn-on potential was about 5.0 V. The device did not operate in a reverse bias mode up to about 15.0 V at which value no significant current was observed. Visible light was observed in the forward bias mode in a dimly lit room at a current density of about 0.50 mA/mm*. 3.4. AUEWPPV-PPyV/EB/ITO/glass device

(‘Slayers SCALE)

This device was constructed in an similar manner to the ‘4-layers-2’ and ‘4-layers-l’ devices, except that the number and order of polymer layers were modified as indicated. The I-V characteristics are given in Fig. 2 (‘5-layers’). This device operated in both forward and reverse bias modes at approximately the same turn-on potentials (about + 5.0 V) . It also operated in an ac. mode at 1 to 60 Hz, as shown in X F R FRF

A A’

O+'j'l

RF

I

I

I

I

I

I

I

I

I

2

3

4

5

6

7

8

9

10

‘Ilme (seconds)

Fig. 3. EL intensity as a function of time for an Al/EB/PPV-PPyV/EB/ IT0 device driven by a 1 Hz sinusoidal voltage operating from + 8 to - 8 V.

Fig. 3. It should be noted that the intensity of the emitted light and the current density were more intense in the forward bias mode than in the reverse bias mode at the same applied potentials ( f 8 V). Visible light was observed in the forward bias mode in a dimly lit room at a current density of about 0.18 mA/mm*.

4. Discussion Excellent agreement between PL and EL spectra in the Al/ PPV-PPyV/ITO device indicates that the PL and EL have the same emission centers. Of all the above devices with various configurations (‘3-layers’, ‘4-layers-l’, ‘4-layers-2’ and ‘5 layers’), only the SCALE ( ‘5-layers’) device shows the capability of operating in both forward and reverse bias modes and in an a.c. mode. In all cases where a significant current was observed, visible light was also observed. In the ‘5-layers’ device only, both holes and electrons can be injected from both IT0 glass and from Al electrodes. It can be noted from Fig. 3 that the intensity of the luminescence at f 8 V is smaller in the reverse bias mode than in the forward bias mode. This experiment does not indicate whether the smaller intensity in the reverse bias mode as compared to the forward bias mode is due to a lower ability to inject electrons from the IT0 glass electrode or from a lower ability to inject holes from the Al electrode, or from both these effects operating simultaneously. Regardless of which of the above controlling factors predominate, the relative values of the work functions of IT0 and Al (4.7 and 4.2 eV, respectively) favor a smaller intensity in the reverse bias mode [ 181, It is most important to note the unexpected electrical properties of the above devices as distinct from their EL properties. For example, as shown in Fig. 2, the total resistance (R = VI Z) of the four devices shown at any given applied potential decreases as the number of insulating polymer layers increases from one ( ‘3-layers’ device) to two ( ‘4-layers-l’ and ‘4-layers-2’) to three ( ‘5-layers’ device). Although the exact thickness of the various polymers layers has not yet been accurately determined, each polymer layer is certainly

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approximately the same thickness. We believe that the nature of the electrode/polymer interface is of very great importance as illustrated, for example, by the ‘4-layers- 1’ device (EB in contact with ITO) and the ‘4-layers-2’ device (EB in contact with Al). In both devices the EB is in contact with PPVPPyV. The difference in the nature of the polymer/IT0 and polymer/Al interfaces obviously has an extremely large effect on the overall total resistance of the devices. Also, a much greater reduction in total resistance of the device is observed when the EB is in contact with the IT0 (‘4-layersl’), as compared to when it is in contact with the Al (‘4layers-2’). Therefore, the EB/electrode, especially the EB/ IT0 interface, may be primarily responsible for the reduction in the total resistance, since a high total resistance is observed in the ‘3-layers’ device in which the PPV-PPyV is in contact with both electrodes. It is noteworthy that qualitatively identical Z-V characteristics in the forward bias mode have been observed in a separate laboratory using poly(p-pyridine), (-C&H,-)., (PPy) [ 16b], instead of PPV-PPyV in the four different configurations given above. When the causesof these unusual electrical phenomena have been elucidated, a better understanding of the associated EL phenomenon will undoubtedly evolve. It is important to distinguish clearly between the observation of current in a given mode and the observation of light emission, The devices in this communication using PPVPPyV, although behaving similarly to those using PPy in forward bias mode as discussed above, show some differences in the reverse bias mode: ‘3-layers’ device, ITO/polymer/Al: When the polymer is PPV-PPyV and PPy, no reverse current is observed [ 16b]. ‘4-layers-l’ device, ITO/EB/polymer/Al: When the polymer is PPV-PPyV, no reverse current is observed up to 15 V; however, when the polymer is PPy, a reverse bias current and emitted light are observed [ 16b]. ‘4-layers-2’ device, ITO/polymer/EB/Al: When the polymer is PPV-PPyV, no reverse current is observed up to 25 V; however, when the polymer is PPy, a reverse bias current is observed, but no light is detected [ 16b]. ‘5-layers’ device, ITO/EB /polymer/EB /Al: Both reverse bias current and emitted light are observed for the polymers PPV-PPyV and PPy. In summary, when the polymer is PPy areverse bias current (but not necessarily light emission) is observed in all the above configurations incorporating an EB layer. However, when the polymer is PPV-PPyV a reverse bias current and accompanying emitted light are observed only in the ‘5-layers’ configuration. The reason for the above differences between PPy and PPV-PPyV in certain of the above configurations is not clear. However, it should be stressed that it is not yet known whether the EB, PPV-PPyV and PPy are p-doped, n-doped or ‘non-doped’ and to what extent doping, if present, occurs. We have previously ‘shown that ‘as-synthesized’, pristine, ‘non-doped’ trans- (CH), is actually slightly p-doped; it

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forms Schottky junctions with relatively low electronegative metals (Na, Ba, In) and it forms ohmic contacts with highly electronegative metals (Cu, Au, Pt) [23]. We have also shown that heterojunctions are formed from pristine p-type (CH), and p-doped poly (N-methylpyrrole) [ 241. The possibility of similar types of junction behavior in the present systems must therefore be considered. Slight differences in the synthesis and handling of the polymers may drastically affect their doping and, hence, their Fermi levels and associated characteristics of the devices. Unless the Fermi levels of the emeraldine base (EB) and PPV-PPyV are identical, which is most unlikely, band bending will occur at the EB/ PPV-PPyV interface with the likely formation of a nonohmic junction in either the forward or reverse bias mode. A similar argument applies to the EB /PPy interface. Moreover, since it is most unlikely that PPV-PPyV and PPy will have identical Fermi levels, identical behavior in forward and reverse bias modes is not necessarily expected. The possibility must be considered that, under appropriate conditions, EB might act both as a good electron-transporting and as a good hole-transporting material. It may be concluded that reduction in injection barriers for electrons or holes may possibly be optimized by judicious matching of electrode material, which interacts favorably with the polymer with which it is in contact and that the nature of the polymer/polymer interface may also play a critical role. Acknowledgements The authors gratefully acknowledge the kind donation of the PPV-PPyV sample used in these studies by Professor T.M. Swager and Dr D.K. Fu. Financial support by the Materials Research Laboratory, University of Pennsylvania, by NSF (Grant No. DMR-91-20668) and by the Office of Naval Research (A.G.M. and A.J.E.) is gratefully acknowledged. References [l] J.H. Burroughes, D.D. Bradley, A.R. Brown, R.N. Marks, K. Mackay, R.H. Friend, P.L. Bum and A.B. Holmes, Nature, 347 (1990) 539. [2] Q.Pei,G.Yu,C.Zhang,Y.YangandA.J.Heegcr,Scierrce, 269( 1995) 1086 and Refs. therein, [3] D.R. Baigent, NC. Greenham, J. Gruner, R.N. Marks, R,H. Friend, SC. Moratti and A,B. Holmes, Synri~.Met.,67 (1994) 3 and Refs. therein. [4] I. Sokolik, Z. Yang, F.E. Karasz and DC. Morton, J. Appl. Phyys., 74 (1993) 3584. [5] D.D. Gebler, Y.Z. Wang, J.W. Blatchford, SW. Jessen,L.B. Lin,T.L. Gustafson,A.J. Epstein, H.L Wang, T. Swager and A.G. MacDiarmid, J. Appl. Phys., (1995) in press. [6] Y.Z. Wang, D.D. Gebler, L.B. Lin, T.L. Gustafson, A,J. Epstein, H.L Wang, T. Swager and A.G. MacDiarmid, Bull. Am, Phys. Sot., 40 (1995) 228. [7] H.L. Wang, M.J. Marsella, D.K. Fu, T,M, Swager, A.G. MacDiarmid and A.J. Epstein, Polym. Mater. Sci. Eng., 73 (1995) 473. [S] H.L. Wang, J.W. Park, M.J. Marsella, D.K. Fu, T.M. Swager, A.G. MacDiarmid Y.Z. Wang, D.D. Geblerand A.J. Epstein, Polym Prepr., 36 (1995) 45.

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