VOLUME 106, NUMBER 24, JUNE 20, 2002
© Copyright 2002 by the American Chemical Society
LETTERS n-Channel Field-Effect Transistors from Blends of Conjugated Polymers Amit Babel and Samson A. Jenekhe* Department of Chemical Engineering and Department of Chemistry, UniVersity of Washington, Box 351750, Seattle, Washington 98195-1750 ReceiVed: March 13, 2002; In Final Form: May 6, 2002
We report the first field-effect transistors from blends of conjugated polymers and the determination of the field-effect mobility of electrons as a function of blend composition. It is found that the field-effect electron mobility (µe) in a series of 12 binary blends of poly(benzobisimidazobenzophenanthroline) (BBL, µe ) 5 × 10-4 cm2/Vs) and poly(p-phenylene-2,6-benzobisthiazole) (PBZT, µe ) 2 × 10-7 cm2/Vs) has a novel “stair step” dependence on composition. The electron mobility is relatively high (5 × 10-5 cm2/Vs) and constant over a wide blend composition range (x ) 0.05-0.6, wt fraction of PBZT). At higher concentration of the lower-mobility component, the electron mobility falls exponentially with composition. The results suggest that thin film transistors with relatively high mobility may be realized in blends of conjugated polymers if one of the components has a sufficiently high mobility. The observed “stair step” electron mobility dependence on blend composition is not explained by current theories of hopping transport.
Blends of π-conjugated polymers have been extensively studied and used in electroluminescent diodes,1 photovoltaic cells,2 photodetectors,3,4 and electrophotographic imaging devices.4 Conjugated polymer blends have, to our knowledge, not been investigated as semiconductors in field-effect transistors. Current understanding is that maximum order is essential to achieving high carrier mobility in organic molecular and conjugated polymer semiconductors.5-11 Because of their inherent disorder, polymer blends would thus seem not ideal for achieving high carrier mobility in field-effect transistors. However, knowledge of charge transport in blends is of fundamental and practical interest. Blends of conjugated polymers can be thought of as “alloy semiconductors” that can facilitate not only the tuning of charge transport but also the integration of other properties such as light emission, photoresponse, or sensing into a transistor. Blend composition is a facile means of controlling the degree of disorder, “impurity”, and “traps” and their effects on charge transport and the field* To whom correspondence should be addressed.
effect mobility of carriers. Understanding of charge transport in conjugated polymer blends is also useful for improving such multicomponent materials for solar cells, light emitting diodes, and other applications. Here, we demonstrate field-effect transistors (FET) based on blends of π-conjugated polymers and use them to investigate charge transport in the multicomponent materials. The role of disorder in limiting the FET mobility of charge carriers in molecular and conjugated polymer semiconductors has been extensively studied in various systems.5-11 Among conjugated polymers, poly(3-hexylthiophene) with high (98.5%) regioregularity and ordered self-assembled supramolecular structure has the highest FET mobility (0.02-0.1 cm2/Vs).9 These mobility values are about 2-4 orders of magnitude higher than those of regiorandom poly(alkylthiophenes).5,9 The increased π-π stacking distance in regioregular poly(3-dodecylthiophene) is believed to account for its much lower mobility, 10-6 cm2/Vs, for example.5b Orders of magnitude difference in FET mobility values have also been observed due to spin coating versus solution casting of thin films, and this difference is
10.1021/jp020695l CCC: $22.00 © 2002 American Chemical Society Published on Web 05/25/2002
6130 J. Phys. Chem. B, Vol. 106, No. 24, 2002
Letters characterized in air. Prior to electrical measurements at ambient temperature, the FETs were heated at 100 °C for 5 min. Electrical characteristics of all devices were measured in ambient air at room temperature on a Hewlett-Packard (HP) 4155A semiconductor parameter analyzer. The electrical characteristics of the devices in the form of drain current (Id) as a function of drain voltage (Vd) at different gate voltages (Vg) showed the expected n-channel FET behavior. The observed slight decrease in the drain saturation current with increasing drain voltage may be a result of contact resistance at the blend/gold interface. These n-channel FETs are accumulation mode devices and thus the field-effect electron mobility µe was calculated by using the saturation region assumption for Vd > Vg - Vt (eq 1):5a,d,6
Id ) (W/2L)µeCo(Vg - Vt)2 Figure 1. Molecular structures of BBL and PBZT and a schematic of the polymer blend field-effect transistor.
understood in terms of the ordered self-assembled morphology obtained with the latter method.9 To shed light on the effects of disorder, impurity, and composition on charge transport in conjugated polymer semiconductors, we investigated electron transport in thin film transistors based on a series of binary blends. In a polymer blend, the composition controls the degree of disorder, morphology, and physical properties.12 The binary blend system we investigated consisted of two rigid-chain n-type conjugated polymers, BBL and PBZT,13,14 whose structures are shown in Figure 1. We recently found that n-channel thin film transistors fabricated from BBL had fieldeffect mobilities as high as 5 × 10-4 cm2/Vs.15 Although the n-type (electron accepting) nature of PBZT has been previously recognized in photoconductivity16 and electrochemical doping experiments17a and exploited in electroluminescent diodes17b,c and in electrophotographic imaging devices,16 its electron transport properties have heretofore not been measured. The morphology18 and optical and photoconductive properties19 of the same BBL/PBZT blend system have previously been investigated in our laboratory. It was concluded from transmission electron microscopy (TEM), electron diffraction, and wideangle X-ray diffraction (WAXD) studies on thin films (20-50 nm) and thick films (1-2 µm) that the BBL/PBZT blends were miscible throughout the entire composition and were largely amorphous down to the 2 nm scale.18 The n-type rigid-chain conjugated polymers BBL and PBZT were synthesized and characterized as previously reported.13,14 The experimental details of thin film processing from solutions of both BBL and PBZT have been reported.14-21 Twelve blend compositions between 5 and 95 wt % PBZT were prepared by mixing known amounts of a 0.09 wt % solution of each component. The insulated gate FETs based on BBL/PBZT blend thin films (Figure 1) were fabricated on an n-type doped silicon substrate that had a resistivity of 0.001 cm/S. The gate insulator was a 300 nm SiO2 layer. Patterned gold (Au) electrodes were used as the source and drain. The 90 nm gold electrodes were sputtered onto a 10 nm thick Ti:W adhesive layer. FETs were made with various channel width (W, µm) to channel length (L, µm) ratios of 10-50: W/L ) 2500/50, 1500/30, 1500/50, 1500/75, 500/25, 400/20, and 300/30. Thin films of BBL, PBZT, and a series of their blends were each spin coated from a 0.09 wt % solution in methanesulfonic acid (MSA) and treated in methanol for more than 6 h and then in water for 12 h to remove any remaining acid and dried in a vacuum oven at 60 °C for 12 h.15 The resulting polymer FETs were stored in air and
(1)
where Co is the capacitance per unit area of the dielectric (SiO2, 11 nF/cm2 for the 300-nm thickness) layer and Vt is the threshold voltage. The mobility was calculated from the slope of the FET transfer characteristics, i.e., (Id)1/2 versus Vg plot. Effect of the channel W/L ratio on the performance of the blend FETs was not observed in accord with prior findings on thin film transistors made from BBL homopolymer.15 n-channel BBL FETs fabricated under the same processing conditions as used for the series of blends gave a field-effect mobility of (2-5) × 10-4 cm2/Vs and an on/off current ratio of 300. These results are close to those recently reported for this polymer.15 A field-effect mobility could not be measured in similar PBZT thin film devices. The field-effect mobility of PBZT homopolymer was estimated to be 2 × 10-7 cm2/Vs by extrapolation of the composition-dependent electron mobility data for the blends presented below. All of the 12 blends in the series from x ) 0.05 to 0.95 showed air-stable n-channel FET characteristics and allowed electron mobility values to be determined. Representative FET output and transfer characteristics are shown in Figures 2-3 for blends of x ) 0.25 and 0.50 (wt fraction of PBZT), respectively. The FET mobility of electrons calculated from the saturation region of the 25 wt % blend (Figure 2) was 6.4 × 10-5 cm2/Vs, and the corresponding on/off current ratio was 1080. Similar FET results for the symmetric composition (x ) 0.50) in Figure 3 were 3.4 × 10-5 cm2/Vs and 400. At 80 wt % PBZT, the electron mobility and on/off current ratio obtained were 2.5 × 10-6 cm2/Vs and 150, respectively. The on/off current ratio of the blend FETs varied nonsystematically from 1700 in the 5 wt % blend to 100 in the 95 wt % blend. However, the on/off current ratios of all blend FETs in the 5-60 wt % composition range were generally larger than that of the BBL homopolymer FETs. The composition-dependent field-effect mobility of electrons in the BBL/PBZT blend system is shown in Figure 4. The µe(x) data have the appearance of a “stair step” with three different features corresponding to three blend composition regions: (i) 0 < x e 0.05, (ii) 0.05 e x e 0.6, and 0.6 e x < 1. Compared to the electron mobility in the BBL homopolymer, which is the higher-mobility component, there is a factor of about 4-10 reduction in the mobility when only 5 wt % PBZT is present in the matrix of BBL. This represents a large effect on charge transport of what is essentially an “impurity” amount of another conjugated polymer. Because the electron affinity of the host BBL (EA ) 4.0 eV)20 is 1.3 eV larger than that of PBZT (EA ) 2.7 eV),17a the observed reduction in electron mobility in going from pure BBL to the 5% blend cannot be due to effects of electron trapping. If electron hopping in pure BBL occurs in
Letters
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Figure 4. Blend composition dependence of the field-effect mobility of electrons in BBL/PBZT blends. The solid line in the 0.6 e x< 1 region is an exponential fit, see text.
Figure 2. Drain current-voltage output (a) and transfer (b) characteristics of a 25% PBZT blend FET.
Figure 3. Drain current-voltage output (a) and transfer (b) characteristics of a 50% PBZT blend FET.
a manifold of nearly isoenergetic states, introduction of 5% PBZT represents a disorder that can reduce the hopping rate and hence the electron mobility.5d,11 A second feature of the composition-dependent electron mobility data in Figure 4 is the nearly constant value of the mobility (ca. 5 × 10-5 cm2/Vs) as the concentration of PBZT goes from 5 to 60%. This means that the expected large increase in disorder in this concentration range has only a very small effect on electron transport. This surprising result currently lacks
a theoretical explanation but may be a consequence of the molecular miscibility of the blend system as previous studies have established.18 This phenomenon of high field-effect mobility in blends with relatively high concentrations of a much lower mobility component may make blend FETs useful for incorporating components carrying other functions such as light emission, photoconduction, and sensing while retaining good transistor performance. The electron mobility in the region where BBL is a minor component dispersed in PBZT matrix, i.e., 0.6 e x < 1, was found to be well described by an empirical expression: µe(x) ) µo exp(-12x), where µo ) 0.033 cm2/Vs. From this expression, we extrapolated to x ) 1 to obtain 2 × 10-7 cm2/ Vs for the electron mobility in pure PBZT homopolymer. We note that the measured mobility of the 95% PBZT blend is 3.6 × 10-7 cm2/Vs, which is already very close to the extrapolated value for the pure PBZT. Achievement of n-channel FET characteristics in the 95% blend but not in the 100% pure PBZT is likely a result of charge injection difficulties. In this regard, a binary blend thus represents a useful method of estimating the field-effect mobility of a conjugated polymer not otherwise active in a FET because of injection problems. The charge carrier mobilities in well-known “molecularly doped” polymers, which are insulating polymers blended with charge-transport small molecules as exemplified by tris(ptolylamine) (TTA) in polycarbonate, are known to have strong concentration dependencies.22 The concentration dependence of carrier mobility in such systems has been interpreted by lattice gas and percolation models of hopping transport.11b,22 However, these models appear not to explain our observed “stair step” concentration dependence of electron mobility, especially in the region of almost constant mobility (0.05 e x e 0.6). This “stair step” electron mobility dependence on blend composition may be an effect arising from the disorder introduced into the twodimensional electron system in the channel of the FET by blending.23 To critically evaluate this and the applicable models of electron transport in these blends of conjugated polymer semiconductors, additional experimental data such as the temperature-dependent mobility will be needed.11b,22 In summary, binary blends of π-conjugated polymers have been used as semiconductors in n-channel field-effect transistors. The field-effect mobility of electrons in many of the blends was fairly high, considering their highly disordered and amorphous nature, compared to the lower-mobility component. Over a wide composition range (5-60% PBZT), the blend FETs had higher on/off current ratios (350-1700) than either of the parent
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