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Application of impedance spectroscopy to the study of organic multilayer devices
Colloids and Surfaces A: Physicochemical and Engineering Aspects 171 (2000) 159 – 166 www.elsevier.nl/locate/colsurfa
Application of impedance spectroscopy to the study of organic multilayer devices M.C. Petty a,*, C. Pearson a, A.P. Monkman b, R. Casalini c, S. Capaccioli c, J. Nagel d a
School of Engineering and Centre for Molecular Electronics, Science Laboratories, Uni6ersity of Durham, South Road, Durham DH1 3LE, UK b Department of Physics and Centre for Molecular Electronics, Uni6ersity of Durham, South Road, Durham DH1 3LE, UK c INFM and Department of Physics, Uni6ersity of Pisa, 6ia F. Buonarroti 2, 56127 Pisa, Italy d Institut fu¨r Polymerforschung Dresden e.V., Abt. Oberfla¨chenmodifizierung, Hohe Str. 6, 01609 Dresden, Germany
Abstract The application of electrical impedance spectroscopy to gas sensors and light emitting devices (LEDs) based on Langmuir–Blodgett (LB) films is reviewed. The sensing material was a co-ordination polymer formed by reaction of the bifunctional amphiphilic ligand 5,5%-methylenebis (N-hexadecylsalicylideneamine)) (MBSH) and copper ions in an interfacial reaction at the water surface. Changes of the device capacitance and conductance during exposure to ethanol, acetonitrile and benzene were related to the polarity of the organic vapour. Impedance measurements on LEDs incorporating a substituted polypyridine derivative, poly(6-hexyl-2,5-pyridinediyl), provided an understanding of the equivalent circuit of the device structure. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Multilayer devices; Impedance spectroscopy; Light emitting devices
1. Introduction Electrical impedance, or admittance, spectroscopy may be used to study the dynamics of bound or mobile charge in the bulk or interfacial regions of any kind of solid or liquid material [1,2]. This is now proving a useful method to investigate the electrical properties of thin organic layers, such as self-assembled or Langmuir–Blod* Corresponding author. Tel.: + 44-191-3742389; fax: +44191-3747492. E-mail address: [email protected] (M.C. Petty)
gett (LB) films [3]. The general approach is to apply a known voltage to the electrodes and observe the response. The impedance is measured in the frequency domain by monitoring the phase shift and amplitude (or real and imaginary parts) of the resulting current. This procedure is then repeated at different frequencies to provide an impedance spectrum. In this paper, the technique of impedance spectroscopy is outlined and its application to two types of electronic device based on LB layers: gas sensors [4] and electroluminescent displays is discussed [5]. In the case of gas sensors, changes of
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the device capacitance and conductance during exposure to a vapour can be related to a change of the film permittivity (generally frequency dependent) or to a change of the geometrical capacitance (i.e. a change of the film thickness). Impedance measurements on organic light emitting devices (LEDs) can be used to gain an understanding of the equivalent circuit and
Fig. 3. Chemical structures of materials used in this work: (a) 5,5%-methylenebis (N-hexadecylsalicylideneamine)) (MBSH); (b) poly(6-hexyl-2,5-pyridinediyl) (PHPY).
consequently an insight into the electrical conduction processes. 2. Equivalent circuit and dielectric model
Fig. 1. Techniques for measuring electrical conductivity in organic multilayer film structures: (a) in-plane; (b) throughplane.
Fig. 2. Electrical equivalent circuit of a metal/Langmuir– Blodgett (LB) film/metal structure. RS, LS represents the series resistance and inductance due to contacts; and RLB, CLB are the resistance and capacitance of the LB film.
Fig. 1 shows two different configurations that are often used for multilayer film studies. The electrical conductivity can either be measured parallel to the substrate surface (i.e. in the film plane) or perpendicular to it (through-plane). Throughplane measurements are conveniently performed by first thermally evaporating a metal, such as aluminium, onto part of a glass substrate. The LB film is then coated over this. Top contacts are established by evaporating another metal through a contact mask. This process must be carried out very carefully to avoid damaging the organic film. To measure the in-plane conductivity, two metal electrodes are deposited onto an insulating (e.g. quartz) substrate and the LB film coated on top. The electrodes may also be deposited on top of the multilayer film. A system of interdigitated electrodes, which provide improved sensitivity, can also be used. One problem with such measurements is that the capacitance of the device is not a simple function of the film thickness and it is difficult to obtain the permittivity of the organic material directly from the experimental data [6]. In this work, electrical measurements perpendicular to the plane of the multilayer film have been undertaken.
M.C. Petty et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 171 (2000) 159–166
The equivalent circuit (Fig. 2) used for the interpretation of our results includes contributions from both the LB film and the electrodes.
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The multilayer film can be modelled as a capacitance CLB in parallel with a resistance RLB. The electrodes are assumed to contribute a resistance
Fig. 4. Capacitance vs. frequency for a poly(CuMBSH) Langmuir – Blodgett (LB) film in nitrogen ( ) and exposed to ethanol (3.3%) (). Organic film thickness = 83 nm. The inset shows the transient behaviour measured at a fixed frequency (1 kHz); the arrows indicate when the vapour was turned on and off.
Fig. 5. Conductance vs. frequency for a poly(CuMBSH) Langmuir – Blodgett (LB) film in nitrogen ( ) and exposed to ethanol (3.3%) (). Organic film thickness=83 nm. The inset shows the transient behaviour measured at a fixed frequency (1 kHz); the arrows indicate when the vapour was turned on and off.
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RS and inductance LS in series with the sample. Under certain conditions (e.g. a very thin organic layer) the capacitance of any oxide layer on the electrode(s) must also be taken into consideration. The total complex impedance Ztot of the equivalent circuit shown in Fig. 2 is given by −1 1 Ztot = jvLS +RS +(R − LB +jvCLB)
(RSCLB) − 1, neglecting the inductance effect, G (RS) − 1 and C (v 2CLBR 2S) − 1. In this region, the experimental data are dominated by contact effects and are no longer related to the electrical properties of the bulk device.
3. Experimental details
RLB −jvCLBR 2LB = jvLS +RS + 1 +(vCLBRLB)2
(1)
At low frequencies, when v (RLBCLB) and jvLS is negligible, Ztot RS +RLB =RT. The total resistance RT is therefore related to the dc resistance measured from the I– V characteristic. When v (RLBCLB) − 1 the admittance of the capacitor dominates the parallel RLB//CLB network, and Ztot RS +1/( jvCLB) +jvLS. Assuming that jvLS can be neglected, the real part Z% RS and the imaginary part Z%% varies as v − 1. As the frequency is increased, Z%% changes sign when the inductive element dominates. Another useful way to compare the experimental results with theory is to transform the data into values of parallel conductance G and capacitance C. This is accomplished by noting
3.1. Vapour sensors
−1
Y =1/Z
C = imag(Y/v)
G =real(Y)
(2) where Y is the complex admittance. Neglecting the inductance effects, an expression can be obtained for the total admittance Ytot for the device. At low frequencies Ytot $ (RT) − 1 +jvCLB G$(RT) − 1;
RLB 2 ; RT
C $CLB
RLB RT
2
(3)
At higher frequencies, i.e. for v (RLBCLB) − 1. Ytot $ G$
jvCLB ; 1+ jvCLBRS
(vCLB)2RS ; 1+(vCLBRS)2
C$
CLB 1 +(vCLBRS)2
The sensing material was a co-ordination polymer formed by reaction of the bifunctional amphiphilic ligand 5,5%-methylenebis (N-hexadecylsalicylideneamine)) (MBSH, Fig. 3(a)) and copper ions in an interfacial reaction at the water surface. Deposition of LB polymer layers (poly(CuMBSH)) from this material has been demonstrated in a previous paper [7]. The resulting thin films possess a relatively open structure (i.e. the hydrocarbon chains are not so tightly packed compared to a simple fatty acid LB layer), allowing rapid interaction with appropriate solvent molecules [8]. In this study, the film thickness was 83 nm, corresponding to 39 LB layers, each of thickness 2.14 nm. The substrates used for the film deposition were glass microscope slides. These were cleaned in an ultrasonic bath and dried with a flow of nitrogen gas. The bottom electrodes were 500 nm gold stripes (width 1.4 mm). A second strip (perpendicular to the first) on top of the LB film was used to obtain a sandwich configuration that allowed electrical measurements in the out-of-plane direction. This second strip was aluminium (width 0.6 mm, thickness 35 nm). In all cases, the metals were deposited by thermal evaporation under high-vacuum conditions ( 10 − 6 mbar). Details of the vapour generation equipment have already been given [8,9]. The samples were exposed at room temperature to the vapours of three different organic solvents: acetonitrile, ethanol and benzene, all of few percent concentrations.
(4)
When (RLBCLB) − 1 v (RSCLB) − 1, (for RS RLB), the conductance increases as v 2 while the capacitance remains constant. In the case of v
3.2. Electroluminescent de6ices A substituted polypyridine derivative, poly(6hexyl-2,5-pyridinediyl) (PHPY, Fig. 3(b)) was used
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in the LED study [10,11]. The devices were of a standard sandwich configuration, i.e. indium– tin–oxide (ITO)/polymeric LB film/aluminium. The ITO was purchased from Balzers, with a sheet resistance of 300 V square − 1. The polymeric LB layers were built-up on the ITO substrates using the LB technique. Ten dipping cycles were used for each sample, providing a total organic film thickness of 38 nm. The deposition details have been given previously [12]. The sandwich structure was completed by the thermal evaporation of an aluminium top electrode. The device contact area was approximately 3 mm2.
3.3. Admittance measurements Admittance measurements in the frequency range 5 Hz to 13 MHz were undertaken using a HP4192A impedance analyser. For the gas sensing measurements, the r.m.s. amplitude of the AC voltage was 1.1 V, no changes in the samples’ impedance were observed at lower voltages. In the case of the LED devices, experiments were undertaken by superimposing a dc bias voltage Vbias onto the smaller ac voltage. Depending on the sample, the range [− 15, + 15] V of Vbias was swept, with steps of 0.2 V. In this paper, we refer to forward bias when the ITO electrode was positive with respect to the aluminium. The ac voltage level was chosen in the range [10 – 100 mV] (r.m.s. amplitude) yielding a good signal/noise ratio, a linear response (without the applied bias) and a low signal level with respect to Vbias. For the LED work, the series impedance of the contacts was estimated by positioning both electrical probes on the same electrode. The contact resistance related to the ITO was found to be approximately 300 V. The contact resistance associated with the top Al contact was less than 1 V, which was considered negligible. For all these measurements the same series inductance LS = 4809 5 nH was found. The temperature of the LED samples was stabilised at 20.090.1°C using a Peltier thermoelectric pump, driven with a constant current, and mounted on a water-cooled heatsink. A platinum resistance thermometer measured the temperature.
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Impedance measurements were carried out in vacuum and in the dark.
4. Results and discussion
4.1. Vapour sensors The capacitance and conductance of a typical device at room temperature in nitrogen are shown in Figs. 4 and 5 (solid circles), respectively. It is evident that C decreases with increasing frequency with a small frequency dependence (fitting a power law of the form C8 v − 0.1). Using the geometrical capacitance, a value for the permittivity close to 2 (2.490.3 at low frequencies and 2.29 0.3 at high frequencies) was estimated; this compares well to that obtained using optical techniques [13]. In contrast, the conductance is characterised by two power laws, one for low frequency with an exponent close to unity and the other, above 105 Hz, with an exponent of two. The latter was considered related to the series resistance of the electrodes as predicted by Eq. (4). When exposed to different organic vapours at room temperature, transient effects were observed with rapid changes of both C and G that saturated after approximately 10 min. The insets in Figs. 4 and 5 show the transient of the responses to 3.3% of ethanol measured at 1 kHz. The observed changes were reversible when the samples were re-exposed to nitrogen. Admittance changes were also noted for both benzene and acetonitrile, with similar response and recovery times to those for ethanol. The admittance of the device was measured in the organic vapours after 15 min exposure (the time to complete a frequency sweep was less than a minute). Admittance data for a device exposed to 3.3% ethanol (open circles) are also given in Figs. 4 and 5. An increase (dependent on frequency) for both C and G was observed. Fig. 6 contrasts the average percentage changes in C and G divided by the vapour concentration for the different vapours at four measurement frequencies. Three concentrations, in the range 3–7%, were generated for each vapour and the results
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Fig. 7. Capacitance against frequency for light emitting devices (LEDs) incorporating poly(6-hexyl-2,5-pyridinediyl) (PHPY) Langmuir – Blodgett (LB) layers. Organic film thickness= 38 nm. Selected spectra: Vbias =0.0 V before first run ( ); Vbias = +3.6 V in the first run (); Vbias = +1.0 V in the second run ( ); Vbias =0.0 V after second run (). Solid lines are related to data from fitting Eq. (1). Dotted lines are from fitting with the additional constant phase element (CPE).
Fig. 6. Average changes in (a) capacitance DC and (b) conductance DG measured at four different frequencies for three organic vapours.
(for each vapour) averaged. The figure shows that the capacitance increases when the device is exposed to ethanol (dipole moment = 1.69 Debye) and acetonitrile (dipole moment= 3.92 Debye) with a greater fractional increase for the acetonitrile. In contrast, a decrease in capacitance is observed with the non-polar benzene. This is almost certainly associated with the swelling of the film. The changes in capacitance and conductance for the ethanol and acetonitrile may be a result of a change in the film permittivity due to the bulk dissolution of the vapour in the LB film. As the dielectric relaxations for the organic solvents are expected to occur at a very high frequency (beyond the maximum used in this work), this should produce changes in C and G that are constant over the frequency range measured, which is not evident in Fig. 6. An alternative explanation is that the admittance changes result from a variation in the polymer permittivity induced by the
interaction between the polymer and the vapour molecules. A detailed discussion has been presented elsewhere [8]. However, from the point of view of a practical sensor, it is clear that monitoring the admittance at different frequencies can provide a useful means to discriminate between the vapours studied.
4.2. Electroluminescent de6ices The impedance spectra for the LED structures were found to depend on the history of the dc bias applied to the samples, confirming previous observations of the dc characteristics [12]. However, a similar frequency response could be discerned for all the devices studied, irrespective of the applied bias voltage. The data followed the predicted behaviour for C and G (Eqs. (3) and (4)), as shown in Figs. 7 and 8 for different values of the applied bias. An almost constant capacitance is seen up to 100 kHz (Fig. 8). At higher frequencies, C8 v − 2 and is independent of applied bias voltages. At low frequencies, the values of conductance depend on RLB (i.e. on bias), as predicted by Eq. (3). On increasing the frequency, G increases according to G8v 2. The high fre-
M.C. Petty et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 171 (2000) 159–166
Fig. 8. Conductance vs. frequency for light emitting devices (LEDs) incorporating poly(6-hexyl-2,5-pyridinediyl) (PHPY) Langmuir – Blodgett (LB) layers. Organic film thickness =38 nm. Selected spectra: Vbias = 0.0 V before first run ( ); Vbias = +3.6 V in the first run (); Vbias = + 1.0 V in the second run ( ); Vbias = 0.0 V after second run (). Solid lines are related to data from fitting using Eq. (1). Dotted lines are from fitting with the additional constant phase element (CPE).
quency conductance values were found to be close 1 to R − S . Figs. 7 and 8 show good agreement between the experimental data (symbols) and the theory (Eq. (1), continuous lines). However, an improved fit could be obtained by slightly modifying the equivalent circuit. A constant phase element (CPE) YCPE =A( jv)N was added in parallel to the RLB// CLB element. The factor N was found to be in the range [0.85, 0.95] which agrees with the predictions of models describing hopping transport. The CPE element takes into account a slight variation of the capacitance and an increase of conductance with frequency below 1 kHz. The new function considerably improved the fit at low frequency (see dotted lines in Figs. 7 and 8). An alternative (but equivalent) representation of the electrical response can be obtained by showing the data in the form of a Nyquist plot, i.e. plotting the imaginary part against the real part of the measured impedance, Fig. 9. All the spectra exhibit a single semicircle or arc of circle, which decreased in size with increasing applied dc bias. Similar data have been presented by Bijnens
165
Fig. 9. Nyquist plot of (−Z%%) versus Z% for a light emitting device (LED) incorporating poly(6-hexyl-2,5-pyridinediyl) (PHPY) Langmuir – Blodgett (LB) layers. Organic film thickness = 38 nm. Data measured at Vbias =[0; + 1.0; + 2.0; +4.2] V (second run). Solid lines are from best fit of the experimental data to Eq. (1). Arrow shows direction of increasing frequency.
et al. [14] and Campbell et al. for ITO/PPV/Al devices [15] and by Campbell et al. for ITO/ poly(2-methoxy,5-(2%-ethyloxy)-p-phenylene vinylene)/Au devices [16]; these LED structures were fabricated by spin-coating the polymer layer. A single semicircle in a Nyquist plot suggests a single relaxation time and the data could be fitted using a parallel combination of a frequency independent resistor RLB and capacitor CLB. This represents the dominant mechanisms of charge transport (RLB) and polarisation (CLB) in the organic layer. The full lines in Fig. 9 were obtained using Eq. (1) (i.e. with a resistance RS and inductance LS in series with the parallel RLB/CLB combination). The excellent agreement between the theory and experimental data in Figs. 7–9, over six decades in frequency, suggests that, in spite of applying different bias voltages, no regions within the devices with different electrical properties could be detected. In summary, the low frequency resistance RT = RS + RLB, related to the dc resistance, was dominated by RLB (electrical transport in the polymer) at low bias voltages and by RS (ITO substrate resistance) at high bias voltages. In contrast, the capacitance CLB (polymer polarisation) was almost independent of applied bias.
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5. Conclusions Impedance measurements have been undertaken on LB film structures incorporating a co-ordination polymer (gas sensor) and a conductive polymer (electroluminescent device). This characterisation technique can provide an insight into the electrical equivalent circuits of the multilayer device. Contact effects dominate the impedance spectra at frequencies above 105 Hz and the experimental data are no longer related to the electrical properties of the organic film. At lower frequencies, information relating to the processes of electrical transport and polarisation in the LB film may be discerned. Changes in the admittance of the gas sensing structure were obtained on exposure to benzene, ethanol and acetonitrile. These could be related to the polarity of the organic vapours. In spite of effects due to different applied bias voltage the history of the dc bias, the ac electrical behaviour of the light emitting device was dominated by a single relaxation time.
References [1] J.R. MacDonald, Impedance Spectroscopy, Wiley, New York, 1987.
.
[2] A.K. Jonscher, Dielectric Relaxation in Solids, Chelsea Dielectrics, London, 1983. [3] M.C. Petty, Langmuir – Blodgett Films, Cambridge University Press, Cambridge, 1996. [4] C. Cornila, A. Hielemann, R. Lenggenhager, P. Malcovati, H. Baltes, G. Noetzl, U. Weimar, W. Go¨pel, Sensors and Actuators B 18 – 19 (1994) 448 – 452. [5] M. Meier, S. Karg, W. Riess, J. Appl. Phys. 82 (1997) 1961 – 1966. [6] H.E. Endres, S. Drost, Sensors and Actuators B 4 (1991) 95 – 98. [7] U. Oertel, J. Nagel, Thin Solid Films 284 – 285 (1996) 313 – 316. [8] R. Casalini, J. Nagel, U. Oertel, M.C. Petty, J. Phys. D Appl. Phys. 31 (1998) 3146 – 3153. [9] C. Granito, J.N. Wilde, M.C. Petty, S. Houghton, P.J. Iredale, Thin Solid Films 284 – 285 (1996) 98 – 101. [10] S. Dailey, M. Halim, E. Rebourt, I.D.W. Samuel, A.P. Monkman, SPIE Proc. 3148 (1997) 82 – 88. [11] S. Capaccioli, C. Pearson, A.P. Monkman, L.E. Horsburgh, P.A. Rolla, M.C. Petty, J. Phys. D Appl. Phys. (submitted). [12] C. Pearson, M. Kilitziraki, S. Dailey, L.E. Horsburgh, A.P. Monkman, I.D.W. Samuel, M.C Petty, in preparation. [13] J.N. Wilde, A.J. Wigman, J. Nagel, A. Beeby, B. Tanner, M.C. Petty, Acta Polym. 49 (1998) 294 – 300. [14] W. Bijnens, J. Manca, T. Wu, M. D’Olieslaeger, D. Vanderzande, J. Gelan, W. De Ceuninck, L De Schepper, L.M. Stals, Synthetic Met. 83 (1996) 261 – 265. [15] A.J. Campbell, D.D.C. Bradley, D.G. Ledzey, J. Appl. Phys. 82 (1997) 6326 – 6342. [16] I.H. Campbell, D.L. Smith, J.P. Ferraris, Appl. Phys. Lett. 66 (1995) 3030 – 3032.
Application of impedance spectroscopy to the study of organic multilayer devices M.C. Petty a,*, C. Pearson a, A.P. Monkman b, R. Casalini c, S. Capaccioli c, J. Nagel d a
School of Engineering and Centre for Molecular Electronics, Science Laboratories, Uni6ersity of Durham, South Road, Durham DH1 3LE, UK b Department of Physics and Centre for Molecular Electronics, Uni6ersity of Durham, South Road, Durham DH1 3LE, UK c INFM and Department of Physics, Uni6ersity of Pisa, 6ia F. Buonarroti 2, 56127 Pisa, Italy d Institut fu¨r Polymerforschung Dresden e.V., Abt. Oberfla¨chenmodifizierung, Hohe Str. 6, 01609 Dresden, Germany
Abstract The application of electrical impedance spectroscopy to gas sensors and light emitting devices (LEDs) based on Langmuir–Blodgett (LB) films is reviewed. The sensing material was a co-ordination polymer formed by reaction of the bifunctional amphiphilic ligand 5,5%-methylenebis (N-hexadecylsalicylideneamine)) (MBSH) and copper ions in an interfacial reaction at the water surface. Changes of the device capacitance and conductance during exposure to ethanol, acetonitrile and benzene were related to the polarity of the organic vapour. Impedance measurements on LEDs incorporating a substituted polypyridine derivative, poly(6-hexyl-2,5-pyridinediyl), provided an understanding of the equivalent circuit of the device structure. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Multilayer devices; Impedance spectroscopy; Light emitting devices
1. Introduction Electrical impedance, or admittance, spectroscopy may be used to study the dynamics of bound or mobile charge in the bulk or interfacial regions of any kind of solid or liquid material [1,2]. This is now proving a useful method to investigate the electrical properties of thin organic layers, such as self-assembled or Langmuir–Blod* Corresponding author. Tel.: + 44-191-3742389; fax: +44191-3747492. E-mail address: [email protected] (M.C. Petty)
gett (LB) films [3]. The general approach is to apply a known voltage to the electrodes and observe the response. The impedance is measured in the frequency domain by monitoring the phase shift and amplitude (or real and imaginary parts) of the resulting current. This procedure is then repeated at different frequencies to provide an impedance spectrum. In this paper, the technique of impedance spectroscopy is outlined and its application to two types of electronic device based on LB layers: gas sensors [4] and electroluminescent displays is discussed [5]. In the case of gas sensors, changes of
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the device capacitance and conductance during exposure to a vapour can be related to a change of the film permittivity (generally frequency dependent) or to a change of the geometrical capacitance (i.e. a change of the film thickness). Impedance measurements on organic light emitting devices (LEDs) can be used to gain an understanding of the equivalent circuit and
Fig. 3. Chemical structures of materials used in this work: (a) 5,5%-methylenebis (N-hexadecylsalicylideneamine)) (MBSH); (b) poly(6-hexyl-2,5-pyridinediyl) (PHPY).
consequently an insight into the electrical conduction processes. 2. Equivalent circuit and dielectric model
Fig. 1. Techniques for measuring electrical conductivity in organic multilayer film structures: (a) in-plane; (b) throughplane.
Fig. 2. Electrical equivalent circuit of a metal/Langmuir– Blodgett (LB) film/metal structure. RS, LS represents the series resistance and inductance due to contacts; and RLB, CLB are the resistance and capacitance of the LB film.
Fig. 1 shows two different configurations that are often used for multilayer film studies. The electrical conductivity can either be measured parallel to the substrate surface (i.e. in the film plane) or perpendicular to it (through-plane). Throughplane measurements are conveniently performed by first thermally evaporating a metal, such as aluminium, onto part of a glass substrate. The LB film is then coated over this. Top contacts are established by evaporating another metal through a contact mask. This process must be carried out very carefully to avoid damaging the organic film. To measure the in-plane conductivity, two metal electrodes are deposited onto an insulating (e.g. quartz) substrate and the LB film coated on top. The electrodes may also be deposited on top of the multilayer film. A system of interdigitated electrodes, which provide improved sensitivity, can also be used. One problem with such measurements is that the capacitance of the device is not a simple function of the film thickness and it is difficult to obtain the permittivity of the organic material directly from the experimental data [6]. In this work, electrical measurements perpendicular to the plane of the multilayer film have been undertaken.
M.C. Petty et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 171 (2000) 159–166
The equivalent circuit (Fig. 2) used for the interpretation of our results includes contributions from both the LB film and the electrodes.
161
The multilayer film can be modelled as a capacitance CLB in parallel with a resistance RLB. The electrodes are assumed to contribute a resistance
Fig. 4. Capacitance vs. frequency for a poly(CuMBSH) Langmuir – Blodgett (LB) film in nitrogen ( ) and exposed to ethanol (3.3%) (). Organic film thickness = 83 nm. The inset shows the transient behaviour measured at a fixed frequency (1 kHz); the arrows indicate when the vapour was turned on and off.
Fig. 5. Conductance vs. frequency for a poly(CuMBSH) Langmuir – Blodgett (LB) film in nitrogen ( ) and exposed to ethanol (3.3%) (). Organic film thickness=83 nm. The inset shows the transient behaviour measured at a fixed frequency (1 kHz); the arrows indicate when the vapour was turned on and off.
M.C. Petty et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 171 (2000) 159–166
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RS and inductance LS in series with the sample. Under certain conditions (e.g. a very thin organic layer) the capacitance of any oxide layer on the electrode(s) must also be taken into consideration. The total complex impedance Ztot of the equivalent circuit shown in Fig. 2 is given by −1 1 Ztot = jvLS +RS +(R − LB +jvCLB)
(RSCLB) − 1, neglecting the inductance effect, G (RS) − 1 and C (v 2CLBR 2S) − 1. In this region, the experimental data are dominated by contact effects and are no longer related to the electrical properties of the bulk device.
3. Experimental details
RLB −jvCLBR 2LB = jvLS +RS + 1 +(vCLBRLB)2
(1)
At low frequencies, when v (RLBCLB) and jvLS is negligible, Ztot RS +RLB =RT. The total resistance RT is therefore related to the dc resistance measured from the I– V characteristic. When v (RLBCLB) − 1 the admittance of the capacitor dominates the parallel RLB//CLB network, and Ztot RS +1/( jvCLB) +jvLS. Assuming that jvLS can be neglected, the real part Z% RS and the imaginary part Z%% varies as v − 1. As the frequency is increased, Z%% changes sign when the inductive element dominates. Another useful way to compare the experimental results with theory is to transform the data into values of parallel conductance G and capacitance C. This is accomplished by noting
3.1. Vapour sensors
−1
Y =1/Z
C = imag(Y/v)
G =real(Y)
(2) where Y is the complex admittance. Neglecting the inductance effects, an expression can be obtained for the total admittance Ytot for the device. At low frequencies Ytot $ (RT) − 1 +jvCLB G$(RT) − 1;
RLB 2 ; RT
C $CLB
RLB RT
2
(3)
At higher frequencies, i.e. for v (RLBCLB) − 1. Ytot $ G$
jvCLB ; 1+ jvCLBRS
(vCLB)2RS ; 1+(vCLBRS)2
C$
CLB 1 +(vCLBRS)2
The sensing material was a co-ordination polymer formed by reaction of the bifunctional amphiphilic ligand 5,5%-methylenebis (N-hexadecylsalicylideneamine)) (MBSH, Fig. 3(a)) and copper ions in an interfacial reaction at the water surface. Deposition of LB polymer layers (poly(CuMBSH)) from this material has been demonstrated in a previous paper [7]. The resulting thin films possess a relatively open structure (i.e. the hydrocarbon chains are not so tightly packed compared to a simple fatty acid LB layer), allowing rapid interaction with appropriate solvent molecules [8]. In this study, the film thickness was 83 nm, corresponding to 39 LB layers, each of thickness 2.14 nm. The substrates used for the film deposition were glass microscope slides. These were cleaned in an ultrasonic bath and dried with a flow of nitrogen gas. The bottom electrodes were 500 nm gold stripes (width 1.4 mm). A second strip (perpendicular to the first) on top of the LB film was used to obtain a sandwich configuration that allowed electrical measurements in the out-of-plane direction. This second strip was aluminium (width 0.6 mm, thickness 35 nm). In all cases, the metals were deposited by thermal evaporation under high-vacuum conditions ( 10 − 6 mbar). Details of the vapour generation equipment have already been given [8,9]. The samples were exposed at room temperature to the vapours of three different organic solvents: acetonitrile, ethanol and benzene, all of few percent concentrations.
(4)
When (RLBCLB) − 1 v (RSCLB) − 1, (for RS RLB), the conductance increases as v 2 while the capacitance remains constant. In the case of v
3.2. Electroluminescent de6ices A substituted polypyridine derivative, poly(6hexyl-2,5-pyridinediyl) (PHPY, Fig. 3(b)) was used
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in the LED study [10,11]. The devices were of a standard sandwich configuration, i.e. indium– tin–oxide (ITO)/polymeric LB film/aluminium. The ITO was purchased from Balzers, with a sheet resistance of 300 V square − 1. The polymeric LB layers were built-up on the ITO substrates using the LB technique. Ten dipping cycles were used for each sample, providing a total organic film thickness of 38 nm. The deposition details have been given previously [12]. The sandwich structure was completed by the thermal evaporation of an aluminium top electrode. The device contact area was approximately 3 mm2.
3.3. Admittance measurements Admittance measurements in the frequency range 5 Hz to 13 MHz were undertaken using a HP4192A impedance analyser. For the gas sensing measurements, the r.m.s. amplitude of the AC voltage was 1.1 V, no changes in the samples’ impedance were observed at lower voltages. In the case of the LED devices, experiments were undertaken by superimposing a dc bias voltage Vbias onto the smaller ac voltage. Depending on the sample, the range [− 15, + 15] V of Vbias was swept, with steps of 0.2 V. In this paper, we refer to forward bias when the ITO electrode was positive with respect to the aluminium. The ac voltage level was chosen in the range [10 – 100 mV] (r.m.s. amplitude) yielding a good signal/noise ratio, a linear response (without the applied bias) and a low signal level with respect to Vbias. For the LED work, the series impedance of the contacts was estimated by positioning both electrical probes on the same electrode. The contact resistance related to the ITO was found to be approximately 300 V. The contact resistance associated with the top Al contact was less than 1 V, which was considered negligible. For all these measurements the same series inductance LS = 4809 5 nH was found. The temperature of the LED samples was stabilised at 20.090.1°C using a Peltier thermoelectric pump, driven with a constant current, and mounted on a water-cooled heatsink. A platinum resistance thermometer measured the temperature.
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Impedance measurements were carried out in vacuum and in the dark.
4. Results and discussion
4.1. Vapour sensors The capacitance and conductance of a typical device at room temperature in nitrogen are shown in Figs. 4 and 5 (solid circles), respectively. It is evident that C decreases with increasing frequency with a small frequency dependence (fitting a power law of the form C8 v − 0.1). Using the geometrical capacitance, a value for the permittivity close to 2 (2.490.3 at low frequencies and 2.29 0.3 at high frequencies) was estimated; this compares well to that obtained using optical techniques [13]. In contrast, the conductance is characterised by two power laws, one for low frequency with an exponent close to unity and the other, above 105 Hz, with an exponent of two. The latter was considered related to the series resistance of the electrodes as predicted by Eq. (4). When exposed to different organic vapours at room temperature, transient effects were observed with rapid changes of both C and G that saturated after approximately 10 min. The insets in Figs. 4 and 5 show the transient of the responses to 3.3% of ethanol measured at 1 kHz. The observed changes were reversible when the samples were re-exposed to nitrogen. Admittance changes were also noted for both benzene and acetonitrile, with similar response and recovery times to those for ethanol. The admittance of the device was measured in the organic vapours after 15 min exposure (the time to complete a frequency sweep was less than a minute). Admittance data for a device exposed to 3.3% ethanol (open circles) are also given in Figs. 4 and 5. An increase (dependent on frequency) for both C and G was observed. Fig. 6 contrasts the average percentage changes in C and G divided by the vapour concentration for the different vapours at four measurement frequencies. Three concentrations, in the range 3–7%, were generated for each vapour and the results
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Fig. 7. Capacitance against frequency for light emitting devices (LEDs) incorporating poly(6-hexyl-2,5-pyridinediyl) (PHPY) Langmuir – Blodgett (LB) layers. Organic film thickness= 38 nm. Selected spectra: Vbias =0.0 V before first run ( ); Vbias = +3.6 V in the first run (); Vbias = +1.0 V in the second run ( ); Vbias =0.0 V after second run (). Solid lines are related to data from fitting Eq. (1). Dotted lines are from fitting with the additional constant phase element (CPE).
Fig. 6. Average changes in (a) capacitance DC and (b) conductance DG measured at four different frequencies for three organic vapours.
(for each vapour) averaged. The figure shows that the capacitance increases when the device is exposed to ethanol (dipole moment = 1.69 Debye) and acetonitrile (dipole moment= 3.92 Debye) with a greater fractional increase for the acetonitrile. In contrast, a decrease in capacitance is observed with the non-polar benzene. This is almost certainly associated with the swelling of the film. The changes in capacitance and conductance for the ethanol and acetonitrile may be a result of a change in the film permittivity due to the bulk dissolution of the vapour in the LB film. As the dielectric relaxations for the organic solvents are expected to occur at a very high frequency (beyond the maximum used in this work), this should produce changes in C and G that are constant over the frequency range measured, which is not evident in Fig. 6. An alternative explanation is that the admittance changes result from a variation in the polymer permittivity induced by the
interaction between the polymer and the vapour molecules. A detailed discussion has been presented elsewhere [8]. However, from the point of view of a practical sensor, it is clear that monitoring the admittance at different frequencies can provide a useful means to discriminate between the vapours studied.
4.2. Electroluminescent de6ices The impedance spectra for the LED structures were found to depend on the history of the dc bias applied to the samples, confirming previous observations of the dc characteristics [12]. However, a similar frequency response could be discerned for all the devices studied, irrespective of the applied bias voltage. The data followed the predicted behaviour for C and G (Eqs. (3) and (4)), as shown in Figs. 7 and 8 for different values of the applied bias. An almost constant capacitance is seen up to 100 kHz (Fig. 8). At higher frequencies, C8 v − 2 and is independent of applied bias voltages. At low frequencies, the values of conductance depend on RLB (i.e. on bias), as predicted by Eq. (3). On increasing the frequency, G increases according to G8v 2. The high fre-
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Fig. 8. Conductance vs. frequency for light emitting devices (LEDs) incorporating poly(6-hexyl-2,5-pyridinediyl) (PHPY) Langmuir – Blodgett (LB) layers. Organic film thickness =38 nm. Selected spectra: Vbias = 0.0 V before first run ( ); Vbias = +3.6 V in the first run (); Vbias = + 1.0 V in the second run ( ); Vbias = 0.0 V after second run (). Solid lines are related to data from fitting using Eq. (1). Dotted lines are from fitting with the additional constant phase element (CPE).
quency conductance values were found to be close 1 to R − S . Figs. 7 and 8 show good agreement between the experimental data (symbols) and the theory (Eq. (1), continuous lines). However, an improved fit could be obtained by slightly modifying the equivalent circuit. A constant phase element (CPE) YCPE =A( jv)N was added in parallel to the RLB// CLB element. The factor N was found to be in the range [0.85, 0.95] which agrees with the predictions of models describing hopping transport. The CPE element takes into account a slight variation of the capacitance and an increase of conductance with frequency below 1 kHz. The new function considerably improved the fit at low frequency (see dotted lines in Figs. 7 and 8). An alternative (but equivalent) representation of the electrical response can be obtained by showing the data in the form of a Nyquist plot, i.e. plotting the imaginary part against the real part of the measured impedance, Fig. 9. All the spectra exhibit a single semicircle or arc of circle, which decreased in size with increasing applied dc bias. Similar data have been presented by Bijnens
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Fig. 9. Nyquist plot of (−Z%%) versus Z% for a light emitting device (LED) incorporating poly(6-hexyl-2,5-pyridinediyl) (PHPY) Langmuir – Blodgett (LB) layers. Organic film thickness = 38 nm. Data measured at Vbias =[0; + 1.0; + 2.0; +4.2] V (second run). Solid lines are from best fit of the experimental data to Eq. (1). Arrow shows direction of increasing frequency.
et al. [14] and Campbell et al. for ITO/PPV/Al devices [15] and by Campbell et al. for ITO/ poly(2-methoxy,5-(2%-ethyloxy)-p-phenylene vinylene)/Au devices [16]; these LED structures were fabricated by spin-coating the polymer layer. A single semicircle in a Nyquist plot suggests a single relaxation time and the data could be fitted using a parallel combination of a frequency independent resistor RLB and capacitor CLB. This represents the dominant mechanisms of charge transport (RLB) and polarisation (CLB) in the organic layer. The full lines in Fig. 9 were obtained using Eq. (1) (i.e. with a resistance RS and inductance LS in series with the parallel RLB/CLB combination). The excellent agreement between the theory and experimental data in Figs. 7–9, over six decades in frequency, suggests that, in spite of applying different bias voltages, no regions within the devices with different electrical properties could be detected. In summary, the low frequency resistance RT = RS + RLB, related to the dc resistance, was dominated by RLB (electrical transport in the polymer) at low bias voltages and by RS (ITO substrate resistance) at high bias voltages. In contrast, the capacitance CLB (polymer polarisation) was almost independent of applied bias.
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5. Conclusions Impedance measurements have been undertaken on LB film structures incorporating a co-ordination polymer (gas sensor) and a conductive polymer (electroluminescent device). This characterisation technique can provide an insight into the electrical equivalent circuits of the multilayer device. Contact effects dominate the impedance spectra at frequencies above 105 Hz and the experimental data are no longer related to the electrical properties of the organic film. At lower frequencies, information relating to the processes of electrical transport and polarisation in the LB film may be discerned. Changes in the admittance of the gas sensing structure were obtained on exposure to benzene, ethanol and acetonitrile. These could be related to the polarity of the organic vapours. In spite of effects due to different applied bias voltage the history of the dc bias, the ac electrical behaviour of the light emitting device was dominated by a single relaxation time.
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