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Electrical conductivity and oxygen doping of vapour-deposited oligothiophene films
ELSEVIER
Synthetic Metals 76 (1996) 133-136
Electrical conductivity and oxygen doping of vapour-deposited oligothiophene films C. Vgterlein a,B. Ziegler ‘, W. Gebauer a,H. Neureiter a,M. Stoldt a, M.S. Weaver b, P. Btiuerle ‘, M. Sokolowski a3*,D.D.C. Bradley b, E. Umbach a a Institutfiir
Experirnentelle Physik II, Universith’t Wiirzburg, Am Hubland, D-97074 Wiirzburg, Germany b Physics Department, Hounsfeld Road, University of Shefield, ShefJield S3 7RH, UK ’ Institutflr Organ&he Chemie, Universitiit Wiirzburg, Am Hubland, D-97074 Wiirzburg, Germany
Abstract We have measured current-voltage (I-V) characteristicsof vapour-deposited films of various oligothiophenes in the direction parallel to the substrateusing a two-point probe technique with symmetric contacts.For applied field strengthsup to 2 X lo4 V cm- ‘, the I-Vcharacteristics are linear. The conductivity ((+) of freshly prepared films is very low (below IO-” S cm- ’ for EC6T, an oligothiophene with end-substituted 4,5,6,7-tetrahydrobenzo groups (‘end caps’) and with six thiophene units), but can be increasedby four ordersof magnitude on doping with oxygen (g = 4 X 10m7S cm- ‘), the doping processbeing strongly promoted by light and/or applied current. Using capacitance-voltage (CV) spectroscopya charge carrier density of 1017cmm3 wasmeasured.The temperature dependenceof r (140-320 K) can best be fitted by an exponential law (exp( crT)). Keywords:
Electrical conductivity; Doping; Vapour deposition; Oligothiophene; Films
1. Introduction Conjugated polymers and oligomers are presently regarded as promising materials for the development of electronic devices based on organic materials, e.g. light-emitting devices (LEDs) [l-3] or field-effect transistors (FETs) [4,5]. An important key to the understanding and optimization of such devices is the mechanism of charge carrier transport. Compared to polymers, oligomeric materials are especially attractive for investigating this aspect, since their molecular structure is well defined and (poly ) crystallinity is achieved in many cases. In addition, ultrapure (free of solvent residues) and homogeneous thin films can be prepared by thermal vapour deposition. This is of concern as the charge carrier density (NA) in these (p-doped) materials is mainly due to chemical impurities acting as electron acceptors. Thus, vapour-deposited oligomeric films are well suited to study the intrinsic conductivity as well as the deliberate variation of the charge carrier concentration by chemical doping [ 6,7]. In our previous work [ 31 we have shown that the current transport properties of the active organic layer considerably determine the characteristics of LEDs based on oligothiophe* Corresponding author. Tel,: f49 931 8885127; fax: +49 931 888 5158; e-mail: [email protected]. 0379-6779/96/$15.00
0 1996 Elsevier Science S.A. All rights reserved
nes. In this work we report on electrical transport measurements in thin vapour-deposited films of oligothiophenes. In order to give a comprehensive picture, we include data taken for three slightly different oligothiophenes, i.e., sexithiophene (o16T) and two oligothiophenes with end-substituted 4,5,6,7-tetrahydrobenzo groups (‘end caps’) and with six or seven thiophene units (EC6T, EC7T) [ 81. We demonstrate that the intrinsic conductivity of the films is very low, and that the conductivity of films which had been exposed to air is mainly due to doping with ambient oxygen. Moreover, the conductivity shows a strong temperature dependence which cannot be explained by existing models, and the mobilities which are estimated from the conductivity and the carrier density NA are found to be very small.
2. Experimental Details of the synthesis of the ECnT are reported in [ 81. For the conductivity measurements, we used a Pt finger structure [ 91 on sapphire with 400 nm high Pt banks and a groove width of 100 p,rn (EC6T, cr6T) or a Au finger structure with a 1 mm wide groove [ 61 covered by a film of 100 nm (EC7T). The capacitance versus voltage (C-V) measure-
134
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et al. /Synthetic
“2 10.3 ii,@ 0 E .5 8 10 Lc F 10’6 a 10" IO" 2
5
lo2
2
5 lo3
Electrical field strength
2
5
IO4
2
(V/cm)
Fig. 1. Log-log plots of the I-V characteristics of a Pt/EC6T/Pt finger strncture after various subsequent doping procedures at room temperature (a)-(d). Curve (e) represents the I-V curve several days after the doping experiment. Note the ohmic characteristic (slope 1).
ments were performed on LEDs with a sandwich structure Mg/EC6T on IT0 (indium-tin oxide)-covered glass [ 31. In all cases, the vapour deposition of the organic materials onto the substrates was carried out under vacuum (p < 1 X 10M6 mbar) from Knudsen cells at deposition rates of about 0.1 nm s-l . Measurements were also performed under high vacuum (below 10m6 mbar), partly without intermediate exposure of the sample to air, and in all cases with the sample shielded from external light.
3. Results and discussion 3.1. Conductivity and doping
For a freshly evaporated EC6T film, the current wasbelow the detection limit (below 10 PA). Hence, the upper limit for the conductivity is lo-” S cm-‘. After doping of the sample, symmetric and linear (ohmic) I-Vcurves could be measured (Fig. 1). The doping procedure was the following: the vacuum chamber was flooded with about 1 bar of dry oxygen for 30 min and then evacuated for 3 h to a pressure of 1 X 10m6 mbar before the Z-V curves were recorded. Four cycles (a)(d) with subsequently increasing doping conditions were performed: for (b) and (c) the sample was illuminated with (white) light of different intensities during the doping process; for (d) intense light and a current (about 1O-3 A cmm2) were applied during exposure. Due to the doping the conductivity of the EC6T film was increased by up to four orders of magnitude to 3 X 10m7 S cm-’ (Fig. 1). The photochemical reaction of O2 with thiophenes is known to proceed via the optically excited triplet state and yields positively charged thiophene radicals and 02- ions [ lo] ; the former will contribute to the density of mobile charge carriers (polarons), leading to the increase of conductivity. As demonstrated by curve (d) the reaction is apparently supported by the presence of positive polarons due to an externally driven current. The possibility of doping oligothiophenes with 0, after evaporation is remarkable, since the source material was stored in
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air before being vapour-deposited in the vacuum chamber. This indicates that the material is purified during the process of vapour deposition, Fig. 1(e) shows that the doping which was reached in curve (d) is not stable. When the sample was left under ambient atmosphere for some days, the conductivity decreased significantly. This dedoping process was investigated more carefully for a EC7T sample. As the morphology and the electrical properties of EC6T and EC7T are similar, we are sure that the doping and dedoping processes are also similar. The time profile of the doping and dedoping process is shown in Fig. 2. After evacuation, the conductivity of the air-doped EC7T sample drops by one order of magnitude in the first few hours and decreases very slowly during the following 30 days. By redoping with air, the same high conductivity was reached. As a consequence of this very large time constant of the second step of the dedoping process, the conductivity of samples which were exposed to air between preparation and measurements generally has to be attributed to unintentional O2 doping. Finally, we note that a similar doping behaviour, with about the same conductivities, was reported for (r6T [ 41. 3.2. Temperaturedependenceof the conductivity
For the understanding of the microscopic transport mechanism the temperature dependence of the conductivity is of interest [ 111. As the conductivity of an undoped EC6T layer is below the detection limit, we used (r6T for the temperaturedependent measurements. Fig. 3 displays the conductivity of an undoped c~6T film (triangles) in the temperature range of 170 to 320 K. The Z-V curves of this sample were found to be linear at all temperatures with no indications for space charge limited (SCL) currents. The conductivity obtained at room temperature (a(cl6T) = 3.3 X 10-s S cm- ‘) is of the same order of magnitude as the value estimated by Oeter et al. [7], and one order of magnitude smaller than the value measured by Horowitz et al. [ 121, For (r6T, the temperature dependence of cr was previously fitted to an Arrhenius law yielding an activation energy AE
““-5
0
5
10
15 20 25 Time (days)
30
35
40
45
Fig. 2. Conductivity changes induced by (I) doping in ambient air, (II) subsequent dedoping of the film under high vacuum and (III) redoping in air, as measured for a Au/EC7T/Au structure.
C. Viiterlein
lo4
I
100 150 Temperature
200 (K)
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t
I
I 250
1
1 300
Fig. 3. Semilogarithmic plots of the current at fixed voltage as a function of temperature for an c16T finger structure (triangles), an a6T LED for U= 5 V (circles), and an EC6T LED sample for U=5 V (squares). The solid lines represent fits according to an exponential function ((T - exp (crT) ) ; the dotted line represents a best fit to an Arrhenius law (r - exp( - A.EIkB7’) ).
of 0.28 eV [ 41. This energy was assigned to a level of shallow traps 0.3 eV above the valence band edge E,, Using an Arrhenius fit, we obtain about the same value for A E (0.3 rt: 0.05 eV) (Fig. 3, dotted curve) but the agreement of experimental results and fit is poor at low temperatures. However, a significantly better fit with a value of x2 reduced by a factor of 10 is obtained with an exponential function of the form: c==,,exp(cuT) withavalueforaof4.5X10-2K-1.Lfthe same sample is doped the temperature dependence does not change significantly (a= 3.8 X 10m2 K-l). This dependence appears to be a more general behaviour. To check this we repeated the experiment with LED samples for which currents could be measured down to 40 K because of the higher possible field strength (factor 100) and because of the different geometry [ 131. The results for cx6T and EC6T LEDs (of different thickness), respectively, are also displayed in Fig. 3. It is remarkable that the same exp(aZJ behaviour with Q = 3.7 and 3.2, respectively, is obtained, whereas fits using the Arrhenius law failed. To our knowledge no microscopic model exists for an exponential temperature dependence of the carrier mobility. We can think of at least two possible explanations: either a carrier transport mechanism with such an exponential temperature dependence actually exists but has not been found yet, or the temperature dependence of charge carrier generation dominates the conductivity in the present cases [ 111. As we have not yet been able to determine the temperature dependence of the carrier density, we cannot decide at present which alternative is more likely.
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135
The LED was exposed to air and light for several days after preparation. The obtained impedance spectra plotted in the complex plane are similar to those observed for other LEDs based on organic films [ 15,161; they clearly show two distinct semicircles. These spectra can be interpreted using an equivalent circuit composed of two RC networks in series, the first representing the Schottky junction at the Mg/EC6T interface and the second the undepleted EC6T bulk. Based on this interpretation CB2 versus bias voltage Vplots can be derived. For an ideal Schottky junction with a constant doping profile, a linear variation of CM2 as a function of V can be expected [14]:
(1) where A is the device area, 4 the electron charge, V, the diffusion voltage, and NA the concentration of ionized acceptor dopants. Fig. 4 demonstrates that a linear Ce2-Vdependence is indeed observed for our LEDs for negative bias voltages of between - 3.5 and 0 V. This result indicates that the Mg/EC6T contact can be described by a Schottky junction to a p-doped material with an approximately constant concentration profile. (The deviations at positive voltages can be explained well by the increasing contribution of the bulk semicircle which prevents unambiguous determination of C from the complex impedance.) From linear fits to several C-V spectra according to Eq. ( 1) (Fig. 4) an acceptor density of NA= (2.0 +0.5) X 1017 cme3 was derived. The unknown dielectric constant ofEC6T, 4, was approximated by or= 2, the value for c~6T [ 121. A diffusion voltage V, of 1.3 +0.2 eV is obtained (Fig. 4)) which is rather large compared to typical values of 0.23-1.0 V reported for other organic LEDs [ 15-171. The reason is not quite understood yet and may be due to charged states at the interface [ 181, The measured acceptor density NA is compatible with the fact that no SCL currents were observed in the above experiments. In combination with the conductivity crnCeTmeasured for an air-doped film (Fig. 1 (e) ) , a hole mobility of &c6T =5X lo-’ cm2 V-l s-l is formally 0.45,
I
,
I
,
I
3.3. Charge carrier density
The charge carrier density of organic materials (NA) can be determined from capacitance versus bias voltage measurements at the Schottky junction between the organic material and a low work function metal [ 141. For this purpose we used a LED with a 200 nm thick EC6T film and a Mg cathode.
-5 -4
-3
-2
-1 0 Bias Voltage
1 (V)
2
3
4
5
Fig. 4. Plot of C-’ vs. bias voltage V for a Mg/EC6T (200 nm) /ITO LED at 100 Hz at room temperature (device area 6 mm*). The slope of the curve is used for deriving the charge carrier density NA.
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derived for room temperature (p = (T(eN,) - ‘) . Compared to typical values for organic materials of 10e4 to 10e6 [ 191, this value for p is rather small. For CYST,it has been argued that the effective concentration of charge carriers contributing to the current is only a small fraction of the total acceptor concentration NA which is measured by C-V spectroscopy, the true mobility thus being considerably larger (e.g. of the order of 1 cm* V-’ s-‘) [ 191. Of course, this could also be the reason in the present case. We hope to clarify this discrepancy from temperature-dependent C-V spectroscopy, which is in preparation. 4. Conclusions We have demonstrated that the intrinsic conductivity of vacuum-deposited oligothiophene films is very low, and that the conductivity of devices which have been exposed to air is mainly due to unintentional doping of the organic material by ambient oxygen. This doping may depend on the morphology of the films and should be considered when the relationship between morphological and electrical properties of an organic material is investigated. Since the reaction is highly promoted by current and/or light it may also explain the fast degradation observed for oligothiophene LEDs when operated in air. We have also investigated the temperature dependence of the conductivity of undoped and doped a6T films and of a6T and EC6T LEDs. We emphasize the finding of an exponential relation, cr N exp( 057), which cannot be explained by a simple hopping model for carrier transport. Finally, we determined the hole carrier density by C-V spectroscopy yielding NA = 2.0 X 10” cme3. However, the mobilities thus derived are very low, leading to speculation that different carrier densities are observed in transport and C-V experiments, respectively. Acknowledgements The cu6T material was kindly supplied by Dr Naarmann (BASF AG) . We are grateful to Ch. Ziegler for making the
Pt finger structure available to us. This work was supported by the Bundesministerium fiir Forschung und Technologie (Project 05 BM 25012). MS. acknowledges a scholarship from the Jubil8umsstiftung of the University of Wiirzburg. E.U. acknowledges support by the Fond der Chemischen Industrie.
References [l] D.R. Baignet, NC. Greenham, J. Grilner, R.N. Marks, R.H. Friend, SC. Moratti and A.B. Holmes, Synrh. Met., 67 (1994) 3. [2] K. Uchiyama, H. Akimichi, S. Hotta, H. Noge and H. Sakaki, Syntlt. Met., 63 (1994) 51. [3] F. Geiger, M. Stoldt, H. Schweizer, P. BBuerle and E. Umbach, Adv. Mater., 5 (1993) 922; H. Neureiter, W. Gebauer, C, Vtiterlein, M. Sokolowski, P. BIuerle and E. Umbach, Synth. Met,, 67 (1994) 173. [4] G. Horowitz, X. Peng, D. Fichou and F. Gamier, J. Appl. Phys., 67 (1990) 528. [5] F. Gamier, A. Yassar, G. Horowitz, F. Deloffre, B. Servet, S. Ries and P. Alnot, J. Am. Chem. Sot., 115 (1993) 8716. [6] M. Stoldt, P. Bluerle, H. Schweizer and E. Umbach, Mol. Cryst. Liq. Cryst.,240 (1994) 127. [7] D. Oeter, Ch. Ziegler, W. GGpel and H. Naarmann, Synth. Met., 67 (1994) 267. [8] P. Bherle, Adv. Mater., 4 (1992) 102. [9] K. D. Schierbaum, S. Vaihinger, W. GBpel, H.H. van den Vlekkert, B. KIoeck and N.F. De Rooij, Sensors and Actuators, El (1990) 171. [lo] M.S.A. Abdou, F.P. Orfino, Z.W. Xie, M J. Deen and S. Holdcroft, Adv. Mater., 6 (1994) 838. [ 1l] G. Paasch,W. Riel3,S. Karg, M. Meierand M. Schwoerer, Synk Met., 67 (1994) 177. I:121 G. Horowitz, D. Fichou, X. Peng and P. Delannoy, J. Phys. France, 51 (1990) 1489, [ 131 C. Vlterlein, H. Neureiter, W. Gebauer, B. Ziegler, M. Sokolowski, P. BLerle and E. Umbach, in preparation. [14] E.H. Rhoderic and R.H. Williams, Metal Semicondmzor Contacts, Oxford University Press,London, 1986. [ 151 D.M. de Leeuw and E.J. Lous, Sy&. Met., 65 (1994) 45. [ 161 S. Karg, W. Rie!3, V. Dyakonov and M. Schwoerer, Synth. Met., 54 (1993) 427. [17] H. KoezukaandS.Etoh,J. Appl. Phys., 54 (1983) 2511. [18] H.C. Card and E.H. Rhode&k, J. PIzys.D: Appl. Phys., 4 (1971) 1589. [I91 F. Gamier, in W.R. Salaneck, I. LundstrGm and B. RBnby (eds.), Conjugated Polymers and RelatedMaterials, Oxford University Press, London, 1993, p. 273.
Synthetic Metals 76 (1996) 133-136
Electrical conductivity and oxygen doping of vapour-deposited oligothiophene films C. Vgterlein a,B. Ziegler ‘, W. Gebauer a,H. Neureiter a,M. Stoldt a, M.S. Weaver b, P. Btiuerle ‘, M. Sokolowski a3*,D.D.C. Bradley b, E. Umbach a a Institutfiir
Experirnentelle Physik II, Universith’t Wiirzburg, Am Hubland, D-97074 Wiirzburg, Germany b Physics Department, Hounsfeld Road, University of Shefield, ShefJield S3 7RH, UK ’ Institutflr Organ&he Chemie, Universitiit Wiirzburg, Am Hubland, D-97074 Wiirzburg, Germany
Abstract We have measured current-voltage (I-V) characteristicsof vapour-deposited films of various oligothiophenes in the direction parallel to the substrateusing a two-point probe technique with symmetric contacts.For applied field strengthsup to 2 X lo4 V cm- ‘, the I-Vcharacteristics are linear. The conductivity ((+) of freshly prepared films is very low (below IO-” S cm- ’ for EC6T, an oligothiophene with end-substituted 4,5,6,7-tetrahydrobenzo groups (‘end caps’) and with six thiophene units), but can be increasedby four ordersof magnitude on doping with oxygen (g = 4 X 10m7S cm- ‘), the doping processbeing strongly promoted by light and/or applied current. Using capacitance-voltage (CV) spectroscopya charge carrier density of 1017cmm3 wasmeasured.The temperature dependenceof r (140-320 K) can best be fitted by an exponential law (exp( crT)). Keywords:
Electrical conductivity; Doping; Vapour deposition; Oligothiophene; Films
1. Introduction Conjugated polymers and oligomers are presently regarded as promising materials for the development of electronic devices based on organic materials, e.g. light-emitting devices (LEDs) [l-3] or field-effect transistors (FETs) [4,5]. An important key to the understanding and optimization of such devices is the mechanism of charge carrier transport. Compared to polymers, oligomeric materials are especially attractive for investigating this aspect, since their molecular structure is well defined and (poly ) crystallinity is achieved in many cases. In addition, ultrapure (free of solvent residues) and homogeneous thin films can be prepared by thermal vapour deposition. This is of concern as the charge carrier density (NA) in these (p-doped) materials is mainly due to chemical impurities acting as electron acceptors. Thus, vapour-deposited oligomeric films are well suited to study the intrinsic conductivity as well as the deliberate variation of the charge carrier concentration by chemical doping [ 6,7]. In our previous work [ 31 we have shown that the current transport properties of the active organic layer considerably determine the characteristics of LEDs based on oligothiophe* Corresponding author. Tel,: f49 931 8885127; fax: +49 931 888 5158; e-mail: [email protected]. 0379-6779/96/$15.00
0 1996 Elsevier Science S.A. All rights reserved
nes. In this work we report on electrical transport measurements in thin vapour-deposited films of oligothiophenes. In order to give a comprehensive picture, we include data taken for three slightly different oligothiophenes, i.e., sexithiophene (o16T) and two oligothiophenes with end-substituted 4,5,6,7-tetrahydrobenzo groups (‘end caps’) and with six or seven thiophene units (EC6T, EC7T) [ 81. We demonstrate that the intrinsic conductivity of the films is very low, and that the conductivity of films which had been exposed to air is mainly due to doping with ambient oxygen. Moreover, the conductivity shows a strong temperature dependence which cannot be explained by existing models, and the mobilities which are estimated from the conductivity and the carrier density NA are found to be very small.
2. Experimental Details of the synthesis of the ECnT are reported in [ 81. For the conductivity measurements, we used a Pt finger structure [ 91 on sapphire with 400 nm high Pt banks and a groove width of 100 p,rn (EC6T, cr6T) or a Au finger structure with a 1 mm wide groove [ 61 covered by a film of 100 nm (EC7T). The capacitance versus voltage (C-V) measure-
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“2 10.3 ii,@ 0 E .5 8 10 Lc F 10’6 a 10" IO" 2
5
lo2
2
5 lo3
Electrical field strength
2
5
IO4
2
(V/cm)
Fig. 1. Log-log plots of the I-V characteristics of a Pt/EC6T/Pt finger strncture after various subsequent doping procedures at room temperature (a)-(d). Curve (e) represents the I-V curve several days after the doping experiment. Note the ohmic characteristic (slope 1).
ments were performed on LEDs with a sandwich structure Mg/EC6T on IT0 (indium-tin oxide)-covered glass [ 31. In all cases, the vapour deposition of the organic materials onto the substrates was carried out under vacuum (p < 1 X 10M6 mbar) from Knudsen cells at deposition rates of about 0.1 nm s-l . Measurements were also performed under high vacuum (below 10m6 mbar), partly without intermediate exposure of the sample to air, and in all cases with the sample shielded from external light.
3. Results and discussion 3.1. Conductivity and doping
For a freshly evaporated EC6T film, the current wasbelow the detection limit (below 10 PA). Hence, the upper limit for the conductivity is lo-” S cm-‘. After doping of the sample, symmetric and linear (ohmic) I-Vcurves could be measured (Fig. 1). The doping procedure was the following: the vacuum chamber was flooded with about 1 bar of dry oxygen for 30 min and then evacuated for 3 h to a pressure of 1 X 10m6 mbar before the Z-V curves were recorded. Four cycles (a)(d) with subsequently increasing doping conditions were performed: for (b) and (c) the sample was illuminated with (white) light of different intensities during the doping process; for (d) intense light and a current (about 1O-3 A cmm2) were applied during exposure. Due to the doping the conductivity of the EC6T film was increased by up to four orders of magnitude to 3 X 10m7 S cm-’ (Fig. 1). The photochemical reaction of O2 with thiophenes is known to proceed via the optically excited triplet state and yields positively charged thiophene radicals and 02- ions [ lo] ; the former will contribute to the density of mobile charge carriers (polarons), leading to the increase of conductivity. As demonstrated by curve (d) the reaction is apparently supported by the presence of positive polarons due to an externally driven current. The possibility of doping oligothiophenes with 0, after evaporation is remarkable, since the source material was stored in
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air before being vapour-deposited in the vacuum chamber. This indicates that the material is purified during the process of vapour deposition, Fig. 1(e) shows that the doping which was reached in curve (d) is not stable. When the sample was left under ambient atmosphere for some days, the conductivity decreased significantly. This dedoping process was investigated more carefully for a EC7T sample. As the morphology and the electrical properties of EC6T and EC7T are similar, we are sure that the doping and dedoping processes are also similar. The time profile of the doping and dedoping process is shown in Fig. 2. After evacuation, the conductivity of the air-doped EC7T sample drops by one order of magnitude in the first few hours and decreases very slowly during the following 30 days. By redoping with air, the same high conductivity was reached. As a consequence of this very large time constant of the second step of the dedoping process, the conductivity of samples which were exposed to air between preparation and measurements generally has to be attributed to unintentional O2 doping. Finally, we note that a similar doping behaviour, with about the same conductivities, was reported for (r6T [ 41. 3.2. Temperaturedependenceof the conductivity
For the understanding of the microscopic transport mechanism the temperature dependence of the conductivity is of interest [ 111. As the conductivity of an undoped EC6T layer is below the detection limit, we used (r6T for the temperaturedependent measurements. Fig. 3 displays the conductivity of an undoped c~6T film (triangles) in the temperature range of 170 to 320 K. The Z-V curves of this sample were found to be linear at all temperatures with no indications for space charge limited (SCL) currents. The conductivity obtained at room temperature (a(cl6T) = 3.3 X 10-s S cm- ‘) is of the same order of magnitude as the value estimated by Oeter et al. [7], and one order of magnitude smaller than the value measured by Horowitz et al. [ 121, For (r6T, the temperature dependence of cr was previously fitted to an Arrhenius law yielding an activation energy AE
““-5
0
5
10
15 20 25 Time (days)
30
35
40
45
Fig. 2. Conductivity changes induced by (I) doping in ambient air, (II) subsequent dedoping of the film under high vacuum and (III) redoping in air, as measured for a Au/EC7T/Au structure.
C. Viiterlein
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1 300
Fig. 3. Semilogarithmic plots of the current at fixed voltage as a function of temperature for an c16T finger structure (triangles), an a6T LED for U= 5 V (circles), and an EC6T LED sample for U=5 V (squares). The solid lines represent fits according to an exponential function ((T - exp (crT) ) ; the dotted line represents a best fit to an Arrhenius law (r - exp( - A.EIkB7’) ).
of 0.28 eV [ 41. This energy was assigned to a level of shallow traps 0.3 eV above the valence band edge E,, Using an Arrhenius fit, we obtain about the same value for A E (0.3 rt: 0.05 eV) (Fig. 3, dotted curve) but the agreement of experimental results and fit is poor at low temperatures. However, a significantly better fit with a value of x2 reduced by a factor of 10 is obtained with an exponential function of the form: c==,,exp(cuT) withavalueforaof4.5X10-2K-1.Lfthe same sample is doped the temperature dependence does not change significantly (a= 3.8 X 10m2 K-l). This dependence appears to be a more general behaviour. To check this we repeated the experiment with LED samples for which currents could be measured down to 40 K because of the higher possible field strength (factor 100) and because of the different geometry [ 131. The results for cx6T and EC6T LEDs (of different thickness), respectively, are also displayed in Fig. 3. It is remarkable that the same exp(aZJ behaviour with Q = 3.7 and 3.2, respectively, is obtained, whereas fits using the Arrhenius law failed. To our knowledge no microscopic model exists for an exponential temperature dependence of the carrier mobility. We can think of at least two possible explanations: either a carrier transport mechanism with such an exponential temperature dependence actually exists but has not been found yet, or the temperature dependence of charge carrier generation dominates the conductivity in the present cases [ 111. As we have not yet been able to determine the temperature dependence of the carrier density, we cannot decide at present which alternative is more likely.
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135
The LED was exposed to air and light for several days after preparation. The obtained impedance spectra plotted in the complex plane are similar to those observed for other LEDs based on organic films [ 15,161; they clearly show two distinct semicircles. These spectra can be interpreted using an equivalent circuit composed of two RC networks in series, the first representing the Schottky junction at the Mg/EC6T interface and the second the undepleted EC6T bulk. Based on this interpretation CB2 versus bias voltage Vplots can be derived. For an ideal Schottky junction with a constant doping profile, a linear variation of CM2 as a function of V can be expected [14]:
(1) where A is the device area, 4 the electron charge, V, the diffusion voltage, and NA the concentration of ionized acceptor dopants. Fig. 4 demonstrates that a linear Ce2-Vdependence is indeed observed for our LEDs for negative bias voltages of between - 3.5 and 0 V. This result indicates that the Mg/EC6T contact can be described by a Schottky junction to a p-doped material with an approximately constant concentration profile. (The deviations at positive voltages can be explained well by the increasing contribution of the bulk semicircle which prevents unambiguous determination of C from the complex impedance.) From linear fits to several C-V spectra according to Eq. ( 1) (Fig. 4) an acceptor density of NA= (2.0 +0.5) X 1017 cme3 was derived. The unknown dielectric constant ofEC6T, 4, was approximated by or= 2, the value for c~6T [ 121. A diffusion voltage V, of 1.3 +0.2 eV is obtained (Fig. 4)) which is rather large compared to typical values of 0.23-1.0 V reported for other organic LEDs [ 15-171. The reason is not quite understood yet and may be due to charged states at the interface [ 181, The measured acceptor density NA is compatible with the fact that no SCL currents were observed in the above experiments. In combination with the conductivity crnCeTmeasured for an air-doped film (Fig. 1 (e) ) , a hole mobility of &c6T =5X lo-’ cm2 V-l s-l is formally 0.45,
I
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,
I
3.3. Charge carrier density
The charge carrier density of organic materials (NA) can be determined from capacitance versus bias voltage measurements at the Schottky junction between the organic material and a low work function metal [ 141. For this purpose we used a LED with a 200 nm thick EC6T film and a Mg cathode.
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-3
-2
-1 0 Bias Voltage
1 (V)
2
3
4
5
Fig. 4. Plot of C-’ vs. bias voltage V for a Mg/EC6T (200 nm) /ITO LED at 100 Hz at room temperature (device area 6 mm*). The slope of the curve is used for deriving the charge carrier density NA.
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C. Viiterlein et al. /Synthetic Metals 76 (1996) 133-136
derived for room temperature (p = (T(eN,) - ‘) . Compared to typical values for organic materials of 10e4 to 10e6 [ 191, this value for p is rather small. For CYST,it has been argued that the effective concentration of charge carriers contributing to the current is only a small fraction of the total acceptor concentration NA which is measured by C-V spectroscopy, the true mobility thus being considerably larger (e.g. of the order of 1 cm* V-’ s-‘) [ 191. Of course, this could also be the reason in the present case. We hope to clarify this discrepancy from temperature-dependent C-V spectroscopy, which is in preparation. 4. Conclusions We have demonstrated that the intrinsic conductivity of vacuum-deposited oligothiophene films is very low, and that the conductivity of devices which have been exposed to air is mainly due to unintentional doping of the organic material by ambient oxygen. This doping may depend on the morphology of the films and should be considered when the relationship between morphological and electrical properties of an organic material is investigated. Since the reaction is highly promoted by current and/or light it may also explain the fast degradation observed for oligothiophene LEDs when operated in air. We have also investigated the temperature dependence of the conductivity of undoped and doped a6T films and of a6T and EC6T LEDs. We emphasize the finding of an exponential relation, cr N exp( 057), which cannot be explained by a simple hopping model for carrier transport. Finally, we determined the hole carrier density by C-V spectroscopy yielding NA = 2.0 X 10” cme3. However, the mobilities thus derived are very low, leading to speculation that different carrier densities are observed in transport and C-V experiments, respectively. Acknowledgements The cu6T material was kindly supplied by Dr Naarmann (BASF AG) . We are grateful to Ch. Ziegler for making the
Pt finger structure available to us. This work was supported by the Bundesministerium fiir Forschung und Technologie (Project 05 BM 25012). MS. acknowledges a scholarship from the Jubil8umsstiftung of the University of Wiirzburg. E.U. acknowledges support by the Fond der Chemischen Industrie.
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