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Electrical properties of polymeric light-emitting diodes
Journal of Non-Crystalline Solids 338–340 (2004) 590–594 www.elsevier.com/locate/jnoncrysol
Electrical properties of polymeric light-emitting diodes L.F. Santos *, R.F. Bianchi, R.M. Faria Instituto de Fısica de S~ao Carlos, Universidade de S~ao Paulo, Av. Trabalhador S~ao Carlense, 400, C.P. 369, 13560-970, S~ao Carlos, SP, Brazil Available online 8 May 2004
Abstract In this work, the electrical properties of polymeric light-emitting diodes (LEDs) based on poly(2-methoxy-5-(20 -ethyl-hexyloxy)1,4-phenylene vinylene), MEH-PPV, were studied by dc current–voltage (I–V ) curves and impedance/admittance spectroscopy, at different temperatures. ITO/polymer/metal (Al and Au) structures were used to study the effect of the barrier height on the charge injection at the polymer/metal interfaces. For ITO/MEH-PPV/Al devices, strong temperature influence on the dc conductivity was observed in the forward direction whereas, in the reverse direction, the conductivity was almost temperature independent. On the other hand, ITO/MEH-PPV/Au devices presented temperature dependent conductivities for both polarities. This indicates that, for low barrier height at the interfaces (<0.4 eV), the carrier injection is a thermally activated process while, for higher barrier heights, the injection becomes dominated by a tunneling-type process. Application of a dc bias voltage superimposed to the alternating voltage in the impedance measurements also causes a diminution in the activation energy of the conductivity, resembling Schottky barrier lowering at the interfaces or Poole–Frenkel type effects in the bulk. 2004 Elsevier B.V. All rights reserved. PACS: 72.20.)i; 85.30.)z
1. Introduction Conjugated polymers have been considered promising materials for opto-electronic applications since the discovery of electroluminescence in poly(p-phenylene vinylene), PPV [1]. The most interesting application of such materials is the fabrication of polymeric lightemitting diodes (PLEDs) for large area applications, due to the ease of processing from solution, high electroluminescence efficiency and relative low operation voltages. Improvements in the device overall performance in the latest years made possible the development of structures which will become commercial in a near future. However, despite this advance, some basic concepts concerning the electrical characteristics of PLEDs are still not completely understood. Charge injection through the electrode/polymer interfaces and bulk transport through the polymeric layer are among the most important processes that determine the difference for good device performance. The identification of the mechanisms that dominate these processes in conjugated *
Corresponding author. Tel.: +55-16 273 9825; fax: +55-16 271 5365. E-mail address: [email protected] (L.F. Santos). 0022-3093/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2004.03.048
polymer devices may provide the necessary background for the development of new device structures or the improvement of the existing ones. In PLEDs, the current flow is mainly determined by charge carriers injected from the electrodes. Consequently, when the metal/polymer interface resistance is high in comparison with the bulk resistance, the current is said to be injection limited. On the other hand, when the energy barriers at the electrode/polymer interface are low enough to consider the contacts practically ohmic, the current is limited by the bulk properties of the polymeric layer. To account for the current–voltage behavior in PLEDs, earlier models were proposed based on Schottky barrier effect [2], Fowler–Nordheim (FN) carrier tunneling [3] as well as space charge limited conduction [4]. Better descriptions of the experimental results can be obtained by the combination of these effects [5] and/or the inclusion of effects like charge diffusion, trapping and recombination [6]. Some alternate models considering mobility-dependent charge injection due to surface recombination [7] and charge injection from a metal into a random hopping system [8] have also been quite successfully employed. In addition to dc current–voltage measurements, impedance spectroscopy has also been used to study the
L.F. Santos et al. / Journal of Non-Crystalline Solids 338–340 (2004) 590–594
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Current (mA)
frequency dependence of the conductivity on the conduction processes in organic devices. Models based on equivalent parallel/series circuits and on dielectric/conductive response functions are commonly considered to obtain information about charge carrier mobility, charged defect concentrations and space-charge distributions at the interfaces and in the bulk [4,9–11]. We propose here a simple method of analysis of the experimental results obtained from conductivity (dc and ac) measurements that makes possible the determination of the parameters that characterize the charge injection and/or transport in conjugated polymers light-emitting diodes.
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The temperature influence on the I–V curves for two different device structures (ITO/MEH-PPV/Al and ITO/ MEH-PPV/Au) is shown in Fig. 1(a) and (b). Here, and hereafter, in the scope of this paper, positive voltages are assigned by positively biasing the ITO electrode. The thickness of the MEH-PPV layer, measured by atomic force microscopy, was 420 ± 20 nm for the ITO/MEHPPV/Al device and 250 ± 15 nm for the ITO/MEH-PPV/ Au device. For temperatures close to the ambient, the ITO/ MEH-PPV/Al device (Fig. 1(a)) exhibits a significant
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3. Results and discussion
ITO/MEH-PPV/Al
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Uniform thin MEH-PPV films (200–500 nm thick) were obtained by spin casting chloroform solutions onto appropriately patterned ITO coated glass substrates (Delta Technologies, 8–12 X/h). The films were treated at 80 C under vacuum conditions (102 Torr) during 4 h to eliminate any residual solvents. Afterwards, metallic top electrodes (Al or Au) were deposited over the polymer layer by thermal evaporation through appropriately designed mechanical masks in high vacuum conditions (106 Torr). The area of the sandwichtype devices was about 16.0 ± 0.5 mm2 . For the electrical measurements, the samples were placed inside a closed-cycle cryostat (APD Cryogenics model CCS-202), under vacuum, enabling temperature control from 10 K up to 350 K. Current–voltage measurements (I–V ) were carried out using a Keithley 238 source-measurement unit while impedance/admittance vs. frequency curves were obtained by a Frequency Response Analyzer (Solartron Instruments model 1260) in the 1 Hz–10 MHz range. To study the influence of the charge injection on the ac conductivity, a dc voltage (Vb ), which can vary from )41 to 41 V, was superimposed to the alternating voltage (Vac ), which amplitude was typically 0.5 V.
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Voltage (V) Fig. 1. Current–voltage curves for (a) a ITO/MEH-PPV/Al device (L ¼ 420 nm) and (b) a ITO/MEH-PPV/Au device (L ¼ 250 nm) at different temperatures.
rectifying factor, presenting a forward current (ITO biased positively) almost three orders of magnitude higher than the reverse current at the same voltage (30 V). By lowering the temperature, the forward current decreases while the reverse current is practically temperature independent. Furthermore, for temperatures below 150 K, the I–V curve becomes practically symmetric, showing that, for low temperatures, the same mechanism determines the current for both polarities. Taking into account the work-function values of the metallic electrodes (/ITO ¼ 4:6–4:8 eV and /Al ¼ 4:1–4:3 eV) and the HOMO and LUMO energy values for MEH-PPV (EHOMO 5:0 eV and ELUMO 2:8 eV) we conclude that the lowest barrier for charge carrier (holes) injection into the polymer is placed at the ITO/ MEH-PPV interface (0.2–0.4 eV). This barrier height allows charge injection by thermionic emission, as remarked above for forward bias, giving rise to temperature dependent currents. In reverse bias, however, the energy barriers for electron injection from the ITO and hole injection from Al are too high for thermionic emission, being tunneling-type effects more probable to occur.
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For the ITO/MEH-PPV/Au device (Fig. 1(b)), the I–V curves show that the current reaches relatively high values for both polarities and that both directions of conduction present a thermally activated behavior. Nevertheless, the I–V curves are not symmetric, presenting higher values of current for negative voltages. This is reasonable because the gold work-function (5.0– 5.2 eV) allows a practically ohmic contact for positive carrier injection into the HOMO level of the MEH-PPV (EHOMO 5:0 eV), providing a better hole-injecting interface than the ITO/polymer interface. Fig. 2 shows the frequency dependence of the real part, r0 ðf Þ, of the complex conductivity for both ITO/ MEH-PPV/Al and ITO/MEH-PPV/Au devices, at nearly room temperature, obtained with different superimposed bias voltages (Vb ). For frequencies below a characteristic frequency fd , r0 ðf Þ tends to become frequency independent, defining a plateau which value is coincident with the dc conductivity (gdc ) that can be obtained from the slope of the I–V curves (Fig. 1(a) and (b)). One observes that gdc and fd are straightly dependent on both the amplitude and polarity of Vb . Since the
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V b = +10 V V b = +20 V V b = +25 V V b = +30 V
Vb = 0 V V b = -20 V V b = -30 V
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equivalent sample capacitance was practically frequency and voltage independent (except for high negative voltages in the Au device), the shorter relaxation times observed at higher values of Vb are caused by the increase in the charge injection (and consequent increase in gdc ) promoted by the bias voltage (Fig. 2(a) and (b)). At high frequencies, the ac conductivity follows approximately a frequency squared (x2 ) dependence, which may due to the resistance of the contacts. The temperature influence on r0 ðf Þ is depicted in Fig. 3(a) and (b) for the Al and the Au devices at fixed bias voltages of +20 and +10 V, respectively. As could be expected for a thermal activated injection process, the value of gdc is strongly dependent with the temperature for temperatures above 150 K. This observation reinforces the hypothesis of the presence of Schottky barrier at the ITO/MEH-PPV interface. Since the values of gdc obtained either from the I–V curves or from the extrapolation r0 ðf ! 0Þ are temperature dependent for the Al device in forward bias and for the Au device in both polarities, we consider the assumption that the dc conductivity can be described by
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Vb= 0 V V b = +8 V V b = +10 V V b = +15 V
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Frequency (Hz) Fig. 2. Behavior of the real component (r0 ) of the complex conductivity with different superimposed dc bias voltages (Vb ) for a ITO/MEH-PPV/ Al PLED (a) and for a ITO/MEH-PPV/Au device (b) at 300 K.
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Frequency (Hz) Fig. 3. Temperature dependence of the real component (r0 ) of the complex conductivity for (a) a ITO/MEH-PPV/Al PLED with Vb ¼ þ20 V; and (b) a ITO/MEH-PPV/Au device with Vb ¼ þ10 V.
L.F. Santos et al. / Journal of Non-Crystalline Solids 338–340 (2004) 590–594
a thermal activated process which obeys the following equation: D gdc ¼ G0 exp ; ð1Þ kB T where D and G0 , which can depend on the applied electric field, are the activation energy of the process and a temperature-independent pre-factor, respectively. Fig. 4 shows the Arrhenius plots of the dc conductivity (obtained either from the I–V curves or from the low-frequency value of r0 ) of the Al and Au devices at different voltages. The results for ITO/MEH-PPV/Al
Temperature (K) 300
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device under reverse voltages are not exhibited, since gdc practically does not vary with the temperature. One observes that reasonably good fittings can be obtained using Eq. (1) in the considered temperature and dc voltage ranges. The adjusting parameters D and G0 , obtained for each voltage value, revealed to be, as expected, voltage dependent. This dependence is shown in Fig. 5. We observe that, for ITO/MEH-PPV/Al, G0 increases with the bias up to approximately 10 V, becoming nearly constant above this voltage (Fig. 5(a)). The activation energy D also increases for voltages below 10 V, reaches a maximum and then decays with the dc voltage; this decay was fitted by a field dependence of the type u aE1=2
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Voltage (V) Fig. 4. Determination of the activation energy values of the dc conductivity at different voltages for (a) a ITO/MEH-PPV/Al PLED in the forward bias; and a ITO/MEH-PPV/Au device biased positively (b) and negatively (c). The dashed lines represent the fittings for an exponential dependence of the conductivity with the reciprocal of the temperature.
Fig. 5. Voltage dependence of the activation energy (D) and the dc conductivity pre-factor (G0 ) obtained from the fittings according to Eq. (1) for the ITO/MEH-PPV/Al device (a) and the ITO/MEH-PPV/ Au device ((b) and (c)). The dashed lines are given by Schottky-type barrier lowering functions.
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Table 1 Parameters obtained from the dc conductivity fittings for the studied devices Device
u (eV)
G0 (S cm1 )
Vc (V)
ITO/MEH-PPV/Al (forward bias) ITO/MEH-PPV/Au (forward bias) ITO/MEH-PPV/Au (reverse bias)
0.29
(1.8 ± 0.8) · 108
10 ± 2
0.38
(6 ± 2) · 108
0.22
–
6±1 –
which resembles the field lowering dependence of a Schottky barrier. Differently from the previous results, for the ITO/MEH-PPV/Au device at forward bias (Fig. 5(b)), G0 initially decreases with voltage, but also reaches a constant value above a characteristic voltage (Vc 6 V). The activation energy obeys the same field functional dependence expressed by Eq. (2), which indicates a Schottky barrier lowering effect in the ITO/ MEH-PPV interface. The small difference in the barrier height at the ITO/polymer interface obtained for the Al and the Au devices may be due to impurities, or even to slight variation in the ITO work-function, which cannot be easily controlled during the deposition process. For the ITO/MEH-PPV/Au device operating at reverse bias, considering that the MEH-PPV/Au interface forms an almost ohmic contact, the field influence on the activation energy D may not be attributed to the lowering of the Schottky barrier at the interface. These results can be interpreted as owing to the bulk transport properties of the polymeric layer. Therefore, the decay of D with the electric field (Fig. 5(c)) may be associated to the Poole–Frenkel effect for volume charged localized states, which has a similar functional dependence given by Eqs. (1) and (2). Additionally, it is worthwhile to note that, in this case, G0 does not assume a constant value, continuously decreasing with the applied voltage, which is not yet well understood. Table 1 summarizes the obtained parameters from the fitting results of the dc conductivity data.
4. Conclusions In this paper, current–voltage and electric impedance/ admittance measurements were used to characterize polymeric light-emitting diodes at different structures and temperatures. At low temperatures and high energy
barriers at the interfaces, the current revealed to be dominated by a temperature-independent injection process which has a functional dependence similar to the previewed by Fowler–Nordheim tunneling whereas, for temperatures in the vicinity of the ambient temperature and barrier heights below 0.4 eV, a clear thermally activated process occurs. The influence of the temperature and the applied electric field on dc conductivity of the studied devices showed that, above a certain characteristic voltage, the injection mechanism is equivalent to Schottky barrier thermionic emission. For devices that present quasi-ohmic contacts, however, the conductivity behavior can be associated to bulk transport properties. Analysis of the sample equivalent capacitance with the dc applied voltage, which will be explored in a future work, indicates that space-charge effects might be responsible for the deviation from the expected behavior.
Acknowledgements The authors would like to acknowledge Fapesp and Millenium Science Initiative (sponsored by CNPq/Brazil) for supporting this work.
References [1] J.H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Mackay, R.H. Friend, P.L. Burns, A.B. Holmes, Nature 347 (1990) 539. [2] I.H. Campbell, P.S. Davids, D.L. Smith, N.N. Barashkov, J.P. Ferraris, Appl. Phys. Lett. 72 (1998) 1863. [3] P.S. Davids, Sh.M. Kogan, I.D. Parker, D.L. Smith, Appl. Phys. Lett. 69 (1996) 2270. [4] A.J. Campbell, D.D.C. Bradley, D.G. Lidzey, J. Appl. Phys. 82 (1997) 6326. [5] P.S. Davids, I.H. Campbell, D.L. Smith, J. Appl. Phys. 82 (1997) 6319. [6] J.C. Scott, P.J. Brock, J.R. Salem, S. Ramos, G.G. Malliaras, S.A. Carter, L. Bozano, Synth. Met. 111&112 (2000) 289. [7] J.C. Scott, G.G. Malliaras, Chem. Phys. Lett. 299 (1999) 115. [8] V.I. Arkhipov, E.V. Emelianova, Y.H. Tak, H. B€assler, J. Appl. Phys. 84 (1998) 848. [9] H.C.F. Marteens, H.B. Brom, P.W.M. Blom, Phys. Rev. B 60 (1999) R8489. [10] J. Scherbel, P.H. Nguyen, G. Paasch, W. Br€ utting, M. Schwoerer, J. Appl. Phys. 83 (1998) 5046. [11] M. Meier, S. Karg, W. Riess, J. Appl. Phys. 82 (1997) 1961.
Electrical properties of polymeric light-emitting diodes L.F. Santos *, R.F. Bianchi, R.M. Faria Instituto de Fısica de S~ao Carlos, Universidade de S~ao Paulo, Av. Trabalhador S~ao Carlense, 400, C.P. 369, 13560-970, S~ao Carlos, SP, Brazil Available online 8 May 2004
Abstract In this work, the electrical properties of polymeric light-emitting diodes (LEDs) based on poly(2-methoxy-5-(20 -ethyl-hexyloxy)1,4-phenylene vinylene), MEH-PPV, were studied by dc current–voltage (I–V ) curves and impedance/admittance spectroscopy, at different temperatures. ITO/polymer/metal (Al and Au) structures were used to study the effect of the barrier height on the charge injection at the polymer/metal interfaces. For ITO/MEH-PPV/Al devices, strong temperature influence on the dc conductivity was observed in the forward direction whereas, in the reverse direction, the conductivity was almost temperature independent. On the other hand, ITO/MEH-PPV/Au devices presented temperature dependent conductivities for both polarities. This indicates that, for low barrier height at the interfaces (<0.4 eV), the carrier injection is a thermally activated process while, for higher barrier heights, the injection becomes dominated by a tunneling-type process. Application of a dc bias voltage superimposed to the alternating voltage in the impedance measurements also causes a diminution in the activation energy of the conductivity, resembling Schottky barrier lowering at the interfaces or Poole–Frenkel type effects in the bulk. 2004 Elsevier B.V. All rights reserved. PACS: 72.20.)i; 85.30.)z
1. Introduction Conjugated polymers have been considered promising materials for opto-electronic applications since the discovery of electroluminescence in poly(p-phenylene vinylene), PPV [1]. The most interesting application of such materials is the fabrication of polymeric lightemitting diodes (PLEDs) for large area applications, due to the ease of processing from solution, high electroluminescence efficiency and relative low operation voltages. Improvements in the device overall performance in the latest years made possible the development of structures which will become commercial in a near future. However, despite this advance, some basic concepts concerning the electrical characteristics of PLEDs are still not completely understood. Charge injection through the electrode/polymer interfaces and bulk transport through the polymeric layer are among the most important processes that determine the difference for good device performance. The identification of the mechanisms that dominate these processes in conjugated *
Corresponding author. Tel.: +55-16 273 9825; fax: +55-16 271 5365. E-mail address: [email protected] (L.F. Santos). 0022-3093/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2004.03.048
polymer devices may provide the necessary background for the development of new device structures or the improvement of the existing ones. In PLEDs, the current flow is mainly determined by charge carriers injected from the electrodes. Consequently, when the metal/polymer interface resistance is high in comparison with the bulk resistance, the current is said to be injection limited. On the other hand, when the energy barriers at the electrode/polymer interface are low enough to consider the contacts practically ohmic, the current is limited by the bulk properties of the polymeric layer. To account for the current–voltage behavior in PLEDs, earlier models were proposed based on Schottky barrier effect [2], Fowler–Nordheim (FN) carrier tunneling [3] as well as space charge limited conduction [4]. Better descriptions of the experimental results can be obtained by the combination of these effects [5] and/or the inclusion of effects like charge diffusion, trapping and recombination [6]. Some alternate models considering mobility-dependent charge injection due to surface recombination [7] and charge injection from a metal into a random hopping system [8] have also been quite successfully employed. In addition to dc current–voltage measurements, impedance spectroscopy has also been used to study the
L.F. Santos et al. / Journal of Non-Crystalline Solids 338–340 (2004) 590–594
-1
300 K 250 K 200 K 150 K 100 K 50 K
10
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10
Current (mA)
frequency dependence of the conductivity on the conduction processes in organic devices. Models based on equivalent parallel/series circuits and on dielectric/conductive response functions are commonly considered to obtain information about charge carrier mobility, charged defect concentrations and space-charge distributions at the interfaces and in the bulk [4,9–11]. We propose here a simple method of analysis of the experimental results obtained from conductivity (dc and ac) measurements that makes possible the determination of the parameters that characterize the charge injection and/or transport in conjugated polymers light-emitting diodes.
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The temperature influence on the I–V curves for two different device structures (ITO/MEH-PPV/Al and ITO/ MEH-PPV/Au) is shown in Fig. 1(a) and (b). Here, and hereafter, in the scope of this paper, positive voltages are assigned by positively biasing the ITO electrode. The thickness of the MEH-PPV layer, measured by atomic force microscopy, was 420 ± 20 nm for the ITO/MEHPPV/Al device and 250 ± 15 nm for the ITO/MEH-PPV/ Au device. For temperatures close to the ambient, the ITO/ MEH-PPV/Al device (Fig. 1(a)) exhibits a significant
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3. Results and discussion
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2. Experimental details
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Uniform thin MEH-PPV films (200–500 nm thick) were obtained by spin casting chloroform solutions onto appropriately patterned ITO coated glass substrates (Delta Technologies, 8–12 X/h). The films were treated at 80 C under vacuum conditions (102 Torr) during 4 h to eliminate any residual solvents. Afterwards, metallic top electrodes (Al or Au) were deposited over the polymer layer by thermal evaporation through appropriately designed mechanical masks in high vacuum conditions (106 Torr). The area of the sandwichtype devices was about 16.0 ± 0.5 mm2 . For the electrical measurements, the samples were placed inside a closed-cycle cryostat (APD Cryogenics model CCS-202), under vacuum, enabling temperature control from 10 K up to 350 K. Current–voltage measurements (I–V ) were carried out using a Keithley 238 source-measurement unit while impedance/admittance vs. frequency curves were obtained by a Frequency Response Analyzer (Solartron Instruments model 1260) in the 1 Hz–10 MHz range. To study the influence of the charge injection on the ac conductivity, a dc voltage (Vb ), which can vary from )41 to 41 V, was superimposed to the alternating voltage (Vac ), which amplitude was typically 0.5 V.
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Voltage (V) Fig. 1. Current–voltage curves for (a) a ITO/MEH-PPV/Al device (L ¼ 420 nm) and (b) a ITO/MEH-PPV/Au device (L ¼ 250 nm) at different temperatures.
rectifying factor, presenting a forward current (ITO biased positively) almost three orders of magnitude higher than the reverse current at the same voltage (30 V). By lowering the temperature, the forward current decreases while the reverse current is practically temperature independent. Furthermore, for temperatures below 150 K, the I–V curve becomes practically symmetric, showing that, for low temperatures, the same mechanism determines the current for both polarities. Taking into account the work-function values of the metallic electrodes (/ITO ¼ 4:6–4:8 eV and /Al ¼ 4:1–4:3 eV) and the HOMO and LUMO energy values for MEH-PPV (EHOMO 5:0 eV and ELUMO 2:8 eV) we conclude that the lowest barrier for charge carrier (holes) injection into the polymer is placed at the ITO/ MEH-PPV interface (0.2–0.4 eV). This barrier height allows charge injection by thermionic emission, as remarked above for forward bias, giving rise to temperature dependent currents. In reverse bias, however, the energy barriers for electron injection from the ITO and hole injection from Al are too high for thermionic emission, being tunneling-type effects more probable to occur.
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L.F. Santos et al. / Journal of Non-Crystalline Solids 338–340 (2004) 590–594
For the ITO/MEH-PPV/Au device (Fig. 1(b)), the I–V curves show that the current reaches relatively high values for both polarities and that both directions of conduction present a thermally activated behavior. Nevertheless, the I–V curves are not symmetric, presenting higher values of current for negative voltages. This is reasonable because the gold work-function (5.0– 5.2 eV) allows a practically ohmic contact for positive carrier injection into the HOMO level of the MEH-PPV (EHOMO 5:0 eV), providing a better hole-injecting interface than the ITO/polymer interface. Fig. 2 shows the frequency dependence of the real part, r0 ðf Þ, of the complex conductivity for both ITO/ MEH-PPV/Al and ITO/MEH-PPV/Au devices, at nearly room temperature, obtained with different superimposed bias voltages (Vb ). For frequencies below a characteristic frequency fd , r0 ðf Þ tends to become frequency independent, defining a plateau which value is coincident with the dc conductivity (gdc ) that can be obtained from the slope of the I–V curves (Fig. 1(a) and (b)). One observes that gdc and fd are straightly dependent on both the amplitude and polarity of Vb . Since the
10
σ ' (S.cm-1)
10 10
-6
-6
10
ITO/MEH-PPV/Au
-7
V b = +10 V V b = +20 V V b = +25 V V b = +30 V
Vb = 0 V V b = -20 V V b = -30 V
-8
-7
-9
10
σ' (S.cm-1)
10
equivalent sample capacitance was practically frequency and voltage independent (except for high negative voltages in the Au device), the shorter relaxation times observed at higher values of Vb are caused by the increase in the charge injection (and consequent increase in gdc ) promoted by the bias voltage (Fig. 2(a) and (b)). At high frequencies, the ac conductivity follows approximately a frequency squared (x2 ) dependence, which may due to the resistance of the contacts. The temperature influence on r0 ðf Þ is depicted in Fig. 3(a) and (b) for the Al and the Au devices at fixed bias voltages of +20 and +10 V, respectively. As could be expected for a thermal activated injection process, the value of gdc is strongly dependent with the temperature for temperatures above 150 K. This observation reinforces the hypothesis of the presence of Schottky barrier at the ITO/MEH-PPV interface. Since the values of gdc obtained either from the I–V curves or from the extrapolation r0 ðf ! 0Þ are temperature dependent for the Al device in forward bias and for the Au device in both polarities, we consider the assumption that the dc conductivity can be described by
-10
10
-8
10
-9
10
ITO/MEH-PPV/Al
300 K 250 K 200 K 150 K 100 K 50 K 12 K
Vac = 0.5 V Vb = +20 V
-10
10 -11
10
-11
10
V ac = 0.5 V T = 300 K
-12
10
(a)
10
10
σ'(S.cm-1)
10 10 10
-6
ITO/MEH-PPV/Au
-5
V b = -2 V V b = -4 V V b = -8 V V b = -10 V
Vb= 0 V V b = +8 V V b = +10 V V b = +15 V
-6 -7 -8
10
-7
10
σ' (S.cm-1)
10
(a)
-12
10
-13
-9
-8
10
-9
10
ITO/MEH-PPV/Au
310 K 280 K 250 K 230 K 210 K 180 K
Vac = 0.5 V Vb = +10 V
-10
10
-10
10
-11
10
-12
-11
V ac = 0.5 V T = 300 K
10
-13
10
10
(b)
(b)
-12
10 1
10
2
10
3
10
4
10
5
10
6
10
Frequency (Hz) Fig. 2. Behavior of the real component (r0 ) of the complex conductivity with different superimposed dc bias voltages (Vb ) for a ITO/MEH-PPV/ Al PLED (a) and for a ITO/MEH-PPV/Au device (b) at 300 K.
1
10
2
10
3
10
4
10
5
10
6
10
Frequency (Hz) Fig. 3. Temperature dependence of the real component (r0 ) of the complex conductivity for (a) a ITO/MEH-PPV/Al PLED with Vb ¼ þ20 V; and (b) a ITO/MEH-PPV/Au device with Vb ¼ þ10 V.
L.F. Santos et al. / Journal of Non-Crystalline Solids 338–340 (2004) 590–594
a thermal activated process which obeys the following equation: D gdc ¼ G0 exp ; ð1Þ kB T where D and G0 , which can depend on the applied electric field, are the activation energy of the process and a temperature-independent pre-factor, respectively. Fig. 4 shows the Arrhenius plots of the dc conductivity (obtained either from the I–V curves or from the low-frequency value of r0 ) of the Al and Au devices at different voltages. The results for ITO/MEH-PPV/Al
Temperature (K) 300
270
240
210
593
device under reverse voltages are not exhibited, since gdc practically does not vary with the temperature. One observes that reasonably good fittings can be obtained using Eq. (1) in the considered temperature and dc voltage ranges. The adjusting parameters D and G0 , obtained for each voltage value, revealed to be, as expected, voltage dependent. This dependence is shown in Fig. 5. We observe that, for ITO/MEH-PPV/Al, G0 increases with the bias up to approximately 10 V, becoming nearly constant above this voltage (Fig. 5(a)). The activation energy D also increases for voltages below 10 V, reaches a maximum and then decays with the dc voltage; this decay was fitted by a field dependence of the type u aE1=2
ð2Þ
180
ITO/MEH-PPV/Al (Forward bias)
-9
10
0.3
-11
+ 20 V
10 10
-12
+ 10 V
-13
+ 5V
10 10
10
0.2
∆ (eV)
+ 30 V
(a) G0 (E) ∆ (E)
0.1
-14
10
(a)
-8
+3V
10
-9
G0 (S.cm-1)
gdc (S.cm-1)
ITO/MEH-PPV/Al (Forward bias) -10
( ϕ - aE ) 1/2
Voltage (V) 0.0
-8
10
0
ITO/MEH-PPV/Au (positive bias at ITO)
10
20
30
-9
+18 V
10
-11
10
-12
10
0.4
+12 V +10 V +8 V
0.3
+6 V
-13
10
-14
+15 V
∆ (eV)
-10
+2 V
(b)
+4 V
(b)
10
10
0.2
G0 0.1
10
10
ITO/MEH-PPV/Au (Positive bias at ITO)
10 1/2
10
-9
-10
ITO/MEH-PPV/Au (negetive bias at ITO)
-8
10
10
ITO/MEH-PPV/Au (Negative bias at ITO)
-9
0.2
10
-10
10
-11
10
-12
(c)
∆ (eV)
-12 V -10 V -8 V -6 V
10 0.1
G0
-4 V -2 V
(c)
∆
4.0
4 .5
5.0
5.5
10
(ϕ - aE )
-13
10
3 .5
10
6.0
1000/T (K)-1
1/2
10
-6
-7
-8
G0 (S.cm-1)
gdc (S.cm-1)
-8
0.0
10
10
-7
∆
( ϕ - aE ) -7
-6
G0 (S.cm-1)
gdc (S.cm-1)
10
-9
-10
0.0 0
5
10
15
Voltage (V) Fig. 4. Determination of the activation energy values of the dc conductivity at different voltages for (a) a ITO/MEH-PPV/Al PLED in the forward bias; and a ITO/MEH-PPV/Au device biased positively (b) and negatively (c). The dashed lines represent the fittings for an exponential dependence of the conductivity with the reciprocal of the temperature.
Fig. 5. Voltage dependence of the activation energy (D) and the dc conductivity pre-factor (G0 ) obtained from the fittings according to Eq. (1) for the ITO/MEH-PPV/Al device (a) and the ITO/MEH-PPV/ Au device ((b) and (c)). The dashed lines are given by Schottky-type barrier lowering functions.
594
L.F. Santos et al. / Journal of Non-Crystalline Solids 338–340 (2004) 590–594
Table 1 Parameters obtained from the dc conductivity fittings for the studied devices Device
u (eV)
G0 (S cm1 )
Vc (V)
ITO/MEH-PPV/Al (forward bias) ITO/MEH-PPV/Au (forward bias) ITO/MEH-PPV/Au (reverse bias)
0.29
(1.8 ± 0.8) · 108
10 ± 2
0.38
(6 ± 2) · 108
0.22
–
6±1 –
which resembles the field lowering dependence of a Schottky barrier. Differently from the previous results, for the ITO/MEH-PPV/Au device at forward bias (Fig. 5(b)), G0 initially decreases with voltage, but also reaches a constant value above a characteristic voltage (Vc 6 V). The activation energy obeys the same field functional dependence expressed by Eq. (2), which indicates a Schottky barrier lowering effect in the ITO/ MEH-PPV interface. The small difference in the barrier height at the ITO/polymer interface obtained for the Al and the Au devices may be due to impurities, or even to slight variation in the ITO work-function, which cannot be easily controlled during the deposition process. For the ITO/MEH-PPV/Au device operating at reverse bias, considering that the MEH-PPV/Au interface forms an almost ohmic contact, the field influence on the activation energy D may not be attributed to the lowering of the Schottky barrier at the interface. These results can be interpreted as owing to the bulk transport properties of the polymeric layer. Therefore, the decay of D with the electric field (Fig. 5(c)) may be associated to the Poole–Frenkel effect for volume charged localized states, which has a similar functional dependence given by Eqs. (1) and (2). Additionally, it is worthwhile to note that, in this case, G0 does not assume a constant value, continuously decreasing with the applied voltage, which is not yet well understood. Table 1 summarizes the obtained parameters from the fitting results of the dc conductivity data.
4. Conclusions In this paper, current–voltage and electric impedance/ admittance measurements were used to characterize polymeric light-emitting diodes at different structures and temperatures. At low temperatures and high energy
barriers at the interfaces, the current revealed to be dominated by a temperature-independent injection process which has a functional dependence similar to the previewed by Fowler–Nordheim tunneling whereas, for temperatures in the vicinity of the ambient temperature and barrier heights below 0.4 eV, a clear thermally activated process occurs. The influence of the temperature and the applied electric field on dc conductivity of the studied devices showed that, above a certain characteristic voltage, the injection mechanism is equivalent to Schottky barrier thermionic emission. For devices that present quasi-ohmic contacts, however, the conductivity behavior can be associated to bulk transport properties. Analysis of the sample equivalent capacitance with the dc applied voltage, which will be explored in a future work, indicates that space-charge effects might be responsible for the deviation from the expected behavior.
Acknowledgements The authors would like to acknowledge Fapesp and Millenium Science Initiative (sponsored by CNPq/Brazil) for supporting this work.
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