Electrical Contact Properties between the Accumulation Layer and

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Electrical Contact Properties between the Accumulation Layer and Metal Electrodes in Ultrathin Poly(3-hexylthiophene)(P3HT) Field Effect Transistors Byoungnam Park,*,† Avishek Aiyar,† Jung-il Hong,‡ and Elsa Reichmanis*,† † ‡

School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30126, United States ABSTRACT: Probing contact properties between an ultrathin conjugated polymer film and metal electrodes in field effect transistors (FETs) is crucial not only to understanding charge transport properties in the accumulation layer but also in building organic sensors with high sensitivity. We investigated the contact properties between gold electrodes and poly(3-hexylthiophene) (P3HT) as a function of film thickness using gated four-point sheet resistance measurements. In an FET with a 2 nm thick P3HT film, a large voltage drop of 1.9 V (VD =
3 V) corresponding to a contact resistance of 2.3  108 Ω was observed. An effective FET mobility of 1.4  10
3 cm2/(V s) was calculated when the voltage drop at the contacts was factored out, which is approximately a factor of 3 greater than the twocontact FET mobility of 5.5  10
4 cm2/(V s). A sharp decrease in the ratio of the contact resistance to the channel resistance was observed with increasing film thickness up to a thickness of approximately 6 nm, separating a contact limited regime from a charge transport limited regime. The origin of the large contact resistance observed in the device prepared with an ultrathin P3HT film is discussed in light of results from X-ray diffraction (XRD) and atomic force microscopy (AFM) studies. KEYWORDS: P3HT, contact resistance, field-effect mobility, contact-limited, charge transport, charge injection

’ INTRODUCTION To explore the charge transport of carriers in an individual molecule in molecular electronic devices, formation of contacts between those molecules and device electrodes is essential. These contacts can also create opportunities to observe a functional interface that can be incorporated into electronic devices by tuning the electrodes chemically and electrically.1,2 However, complexity in the nanoscale organic/inorganic contact arising from defects3
5 at the junctions and size dependence6,7 of the contact properties has limited the ability to understand charge transport in a single molecule, reducing flexibility in designing nanoelectronic devices.8 The challenges at the contact have been extended to flexible electronic devices using relatively thick conjugated organic semiconducting films such as organic field effect transistors (OFETs), organic light emitting diodes (OLEDs), and organic solar cells (OSCs), where understanding of the relative positions of the energy levels across the metal/organic semiconductor interface is complicated by the interface dipole caused by charge transfer and interface states.9
12 This has raised many difficulties in achieving an Ohmic contact crucial for device operation. To address the problem, numerous groups have tried to identify the metal/semiconductor contact barrier in OFETs.13
15 For example, large voltage drops at the metal/organic semiconductor contacts have been visualized by scanning Kelvin probe microscopy (SKPM), and the contact resistance has been determined r 2011 American Chemical Society

using the transmission line method and four-terminal measurements.16,17 The large voltage drop observed at the contact underestimated the FET mobility by more than 1 order of magnitude.13,18 Further, effects of the charge injection barrier, gate electric field, and charge carrier mobility near the contact region on the contact resistance have been investigated.15,19
22 The Schottky barrier height between poly(3-hexylthiophene) (P3HT) and Au has been estimated to be between 0.1 and 0.5 eV.15,21,23 In addition, the difference in the molecular orientation and structural ordering of P3HT on Au modified the interface energy level alignments, changing the injection barrier, as well.24 Compared with the intensive study of the contacts in FETs with thick conjugated polymer films, little is known about the contact properties of FETs fabricated with solution-processed ultrathin films, investigation of which is relevant to direct study of charge injection and extraction between the accumulation layer and metal electrode. In this paper, we probed contact formation between an ultrathin P3HT film and gold electrodes in a FET with multiple voltage probes along the conducting channel, enabling measurement of the voltage drop near the electrodes. The origin of a large contact resistance in the FET fabricated with an ultrathin film is explored by systematically varying P3HT film Received: February 2, 2011 Accepted: April 25, 2011 Published: April 25, 2011 1574

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channel region with a length and width of up to 165 μm and 2.8 mm was defined by removing the P3HT spin-coated outside the channel region using a sharp brush. This procedure was carried out in order to prevent leakage current through the gate dielectric. All the FET devices were measured in an ambient atmosphere, and the structural properties of an ultrathin P3HT film spin-coated onto an SiO2 substrate was examined by XRD (Panalytical X’Pert Pro system equipped with a Cu X-ray source operating at 45 kV and 40 mA) and atomic force microscopy (AFM; Veeco Digital Instruments Dimension 3100 scanning probe microscope in tapping mode with a silicon tip). P3HT FETs were analyzed in the linear regime of transistor operation by applying a small drain voltage of
3 V. We differentiate the effective FET mobility, μeff, acquired by the gated sheet conductance measurement from the conventional two-contact FET mobility, μ, from the drain current-gate voltage transfer characteristic curve. The values of μeff were obtained by plotting sheet conductance as a function of gate voltage. The slope of a linear fit of the plot is proportional to the effective FET mobility, independent of the contact resistance between Au
Cr and P3HT, as described in the following equation, μeff = (1/Ci)/(dσ0/dVG), where Ci is the capacitance of the gate dielectric and σ0 is the sheet conductance.

Figure 1. (a) Plot of effective FET mobility as a function of P3HT thickness. The inset shows a schematic diagram of an FET device with voltage probes along the P3HT channel between source and drain electrodes. (b) Plot of sheet conductance as a function of gate voltage for three different P3HT thicknesses. Threshold voltages were 3.4, 14, and 27 V for the FETs with 2, 8, and 15 nm thick P3HT films, respectively.

thickness. The effect of the channel resistance on the contact resistance is discussed as well.

’ EXPERIMENTAL SECTION P3HT and anhydrous chloroform were purchased from Sigma Aldrich and used without further purification. The P3HT had an Mn of 24 kD and Mw of 47.7 kD, as obtained from gel permeation chromatography (GPC) in tetrahydrofuran calibrated with polystyrene standards and a head to tail regioregularity of 92
94% (as estimated from the 1H NMR spectrum). Solutions were prepared with concentrations up to 10 mg/mL affording spin coated P3HT film thicknesses ranging from 2 to 60 nm. To ensure thickness control for films less than ∼10 nm thick, a P3HT solution with a concentration of ∼2 mg/mL was systemically diluted by adding a chloroform solvent. The thickness of each film was measured using ellipsometry (M-2000 VASE Ellipsometer, J.A. Woollam Co. Inc.). The thicknesses were measured by fitting the spectrscopic data from 700 to 1000 nm using the Cauchy model. We fabricated field effect devices in which voltage probes are spaced evenly between source and drain electrodes [Au (50 nm)/Cr (5 nm)] on 200 nm thick SiO2 gate dielectrics, as displayed in Figure 1a inset. A highly p-doped silicon substrate served as a gate electrode. The FET devices were piranha-cleaned followed by ultrasonication in acetone, methanol, and deionized water, for several minutes in each bath. The subsequent P3HT spin-coating process was carried out at 1500 rpm for 60 s for each P3HT solution. After P3HT film formation, a P3HT

’ RESULTS AND DISCUSSION For FET devices fabricated with spin-coated P3HT, varying the thickness of the semiconductor film alters the values of the effective FET mobility and threshold voltage, as shown in Figure 1. As seen in Figure 1a, a small thickness change in the ultrathin film region, below ∼5 nm, dramatically affects charge transport properties within the FET channel. In the thickness range we studied here, the effective mobility spanned approximately 2 orders of magnitude from 1.7  10
5 to 1.9  10
3 cm2/(V s). With an increase in thickness past 6 nm, the mobility varied within the relatively narrow range between 1.5  10
3 and 7.5  10
3 cm2/(V s), except for the mobility of 0.015 cm2/(V s) observed for P3HT films with a thickness of 13 nm. Interestingly, we reproducibly observed a very high mobility, μeff = 0.017 ( 0.006 cm2/(V s), for devices prepared with approximately 13 nm thick films of the polymeric semiconductor deposited from a solution having a concentration of ∼3 mg/mL. Given that the two-dimensional charge transport sheet created by charge carriers accumulated at the P3HT/SiO2 interface governs the effective FET mobility, we suggest that the high mobility arose from enhanced structural ordering of the P3HT close to the SiO2 gate dielectric. This improved structural ordering is believed to take place near the particular P3HT solution concentration of 3 mg/mL. However, information of the spatial distribution of the structural properties of P3HT films depending on P3HT concentration is not yet available. Figure 1b shows the thickness dependence of the threshold voltage in the semiconductor devices studied here. Sheet conductance is plotted as a function of gate voltage for the FETs fabricated with P3HT films ranging between 2 and 15 nm in thickness. As the gate voltage increased, the sheet conductance increased because a more negative gate voltage induced more hole carriers in the P3HT semiconductor layer near the gate dielectric as expressed by n = [Ci(VG
VT)]/q, where n is the two-dimensional charge carrier density and VT is the threshold voltage. It is found that the threshold voltage shifted to a more positive value when the thickness increased. The value of the threshold voltage was 5.4 ( 2.9 V for the FETs prepared with 2 to 4 nm thick P3HT films and 20 ( 5.8 V for devices with 1575

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The gated sheet conductance measurement allowed for the study of electrical properties of an ultrathin P3HT film, independent of the contact resistance. Figure 2a shows the plots of sheet conductance and drain current as a function of gate voltage for an FET prepared with a ∼2 nm thick P3HT film. The twocontact and effective FET hole mobilities were 5.5  10
4 and 1.4  10
3 cm2/(V s), respectively. The difference in mobility by a factor of 2.5 results from the voltage drops at the Au
Cr/ P3HT contacts, as seen in Figure 2b in which the potential within the P3HT channel between the electrodes is plotted as a function of distance from the source electrode at different gate voltages. The observed voltage drops at the source and drain electrodes were 0.14 and 1.71 V, respectively, at a gate voltage of
30 V. This significant voltage drop at the contacts led to the 2.5 fold higher effective FET mobility μeff than the FET mobility μ in eq 1, which is derived from the metal oxide semiconductor fieldeffect transistor description in which the drain voltage VD is assumed to be equal to the voltage drop across the channel. The magnitude difference between μeff and μ is consistent with the ratio of the applied drain voltage, 3.0 V, to the voltage drop across the channel, 1.2 V. Id ¼

Figure 2. (a) Plot of sheet conductance and drain current as a function of gate voltage for a FET fabricated with an ∼2 nm thick P3HT film. (b) Potential variations along the conducting channel of the P3HT FET with 2 nm (filled circles) and 13 nm (empty circles) thick P3HT films at different gate voltages. Gate voltage for the thick film (13 nm) was varied from
10 to
40 V by 10 V. The inset shows an optical micrograph of an FET with multiple voltage probes. The channel length and width are 165 μm and 1.2 mm, respectively, and the voltage probes are spaced by 25 μm along the channel.

semiconductor film thicknesses in the range of 5 to 51 nm. As noted in previous studies,25,26 the observed thickness dependence of the threshold voltage can be attributed to a rise in the bulk conductance with increased film thickness. The P3HT channel and contact resistances between the Au
Cr electrodes and the semiconductor were extracted from the potential values measured at the voltage probes along the channel in a P3HT FET in the accumulation mode. The channel resistance, Rch, was determined by Rch = R0(L/Z), where R0 is the channel sheet resistance, R0 = (|V1
V2|/Id)(Z/d), measured from the FET geometry as shown in Figure 1a inset. The contact resistance was calculated by deducting the channel resistance obtained using the sheet resistance measurement from the total resistance acquired from the drain-source current
voltage characteristic curve at a particular gate voltage. The contact resistance at the source and drain electrodes was approximated by dividing the voltage drop at each electrode by the drain current. The voltage drops at the source and drain contacts, ΔVs and ΔVd, are expressed by ΔVs = V1
[(L1(V2
V1)/(L2
L1))] and ΔVd = Vd
[V2 þ ((L
L2)(V2
V1)/(L2
L1))], respectively.

Z μCi ðVG
VT ÞVD L

ð1Þ

In order to validate the results from the gated sheet conductance measurements using multiple voltage probes, we compared the width normalized contact resistance values obtained from our experiments with methods used by other researchers such as noncontact SKPM15 and the transfer line method (TLM).27 With the gated sheet conductance measurements, the width normalized contact resistance of a device with a 13 nm thick P3HT film decreased with increasing gate voltage producing values in the range of 104
105 Ω cm. These values are comparable with the measurement of ∼5  104 Ω cm from SKPM and that of ∼4  104 Ω cm from TLM where the contact resistance is determined from the resistance value at zero channel length in the plot of resistance vs channel length.28 This clearly suggests that the voltage probes in the channel area do not influence the transport properties and thus were not a critical factor in probing the contact properties. Further, homogeneous structural properties of the 2 nm thick P3HT film are supported through comparison of the effective FET mobilities measured with three different spacings (25, 50, and 100 μm) between voltage probes (probe 3
4, 2
4, and 1
5) in Figure 2b. The effective FET mobilities for these three spacings were 1.5  10
3, 1.4  10
3 and 1.5  10
3 cm2/(V s), respectively, producing a very small standard deviation of 1.5  10
3 ( 5.8  10
5 cm2/(V s). The close correspondence between these mobility values suggests that the morphology and the degree of molecular ordering in the P3HT films are similar over the entire channel area. It is worth noting that the gated four-probe measurements reflect the charge transport properties in the accumulation layer near the SiO2 gate dielectric modulated by the gate voltage, as inferred from the large voltage drop near the drain electrode at zero gate voltage due to a depletion region near the drain contact in Figure 2b. The depletion region arises because the applied drain voltage of 3 V is close to the pinch off voltage, 3 V, represented by |VD,sat| = |VG
VT|. The large contact resistance found in a FET device with an ultrathin film was highly gate voltage dependent which is in contrast to to that observed for the FET fabricated with a 1576

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ACS Applied Materials & Interfaces relatively thick semiconductor film (13 nm) (Figure 2b). This observation is consistent with previous studies.13,28 The voltage drop at the drain electrode of the device with an ultrathin (∼2 nm) P3HT layer increased with a decrease in the gate voltage that causes decreased charge carrier density. The corresponding contact resistance increased from 2.3  108 to 1.6  109 Ω with a change in gate voltage from
30 to 0 V. This is in sharp contrast to the slight increase from 1.7  106 to 2.5  106 Ω found for the FET fabricated with a 13 nm thick P3HT film for the same change in gate voltage. The contact resistance for the ultrathin P3HT FET was clearly much higher than that for the thicker film device. The total voltage drop in the FET with a relatively thick semiconductor film (13 nm) was 0.27 V at a gate voltage of
30 V. To gain further insight into the origin of the large contact resistance in the ultrathin film FET device, we compared how the contact and channel resistances change with P3HT film thickness under the same carrier concentration of 3.2  1012 cm
2, corresponding to VG
VT = 30 V. This treatment eliminates changes in the charge injection and extraction energy barriers13,29 that could arise from differences in charge carrier density due to the variation of threshold voltage with P3HT thickness, as observed earlier. Further, we compared the width normalized contact resistance at different thicknesses. Homogeneous transport properties over the channel area, confirmed by measurements of the effective FET mobility at different channel lengths using pairs of voltage probes, eliminated dependence of mobility on channel length, as observed by Wang et al.30 These experiments enabled a comparison of the channel and contact properties, independent of the channel dimensions. The thickness dependence of the width normalized contact and channel resistances is shown in Figures 3a and b, respectively. The magnitude of both resistances sharply decreased with increasing thickness from 1.1  109 Ω cm and 5.8  1010 Ω/0 for devices fabricated with 2 nm thick P3HT, to 2.5  107 Ω cm and 4.4  109 Ω/0 for those devices with a 4 nm thick polymer semiconducting layer. With an increase in P3HT film thickness from 4 to 6 nm, the channel sheet resistance varied in a narrow range from 4.4  109 to 3.7  109 Ω/0, while the width normalized contact resistance decreased by more than a factor of 3, from 2.5  107 to 7.0  106 Ω cm. When the P3HT film thickness was increased to 13 nm from 6 nm (approximately a factor of 2), the width normalized contact and channel sheet resistances dropped sharply from 7.0  106 to 1.3  105 Ω cm and 3.7  109 to 1.0  108 Ω/0, respectively, which represents more than 1 order of magnitude. With a further increase in semiconductor film thickness (up to 55 nm), no further change was observed in both resistances values which seem to level off at values higher than those observed for the 13 nm device, where a valley can be seen in the plot. The relative variations of the contact and channel resistances with thickness in Figure 3c which are based on the results depicted in Figures 3a and b, lead to two distinct regimes in P3HT based FET device operation. The ratio of the contact resistance to the channel resistance decreased from 1.1 ( 0.7 for devices with 2.3 ( 0.4 nm thick P3HT films, to 0.2 ( 0.2 for devices with 6.3 ( 0.9 nm thick layers of the semiconductor. After the crossover, the ratio was stabilized. This result shows that a critical polymer semiconductor film thickness separates a charge transport limited regime from a contact limited regime in polymer based FET devices.

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Figure 3. Plots of (a) width normalized contact resistance, (b) channel sheet resistance, and (c) the ratio of the contact resistance to the channel resistance as a function of P3HT thickness in FETs.

In the contact limited regime, a conducting channel is formed through percolation of P3HT chains generating conduction pathways. For several devices with P3HT films at a thickness of ∼2 nm, no drain-source current was observed indicating that a 1577

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ACS Applied Materials & Interfaces continuous conducting pathway through geometric percolation had not been established. However, a significant proportion of the 2 nm structures exhibited effective FET mobilities spanning more than 2 orders of magnitude (10
6 to 10
3 cm2/(V s)). These results suggest that at ∼2 nm, the polymer film is at a critical point where formation of a two-dimensional continuous conducting channel vs a discontinuous channel becomes allowed. This observation is consistent with a previous study31 demonstrating that a two-dimensional continuous conducting channel is formed by an increase of the area coverage of 2 nm thick P3HT domains through repeated dip coating. Therefore, it is believed that the low FET mobility of ∼10
6 cm2/(V s) is due to poor connectivity between P3HT domains after percolation, while the higher mobility of 1.4  10
3 cm2/(V s) (Figure 2b) can be explained by formation of a two-dimensional continuous charge transport sheet over the entire channel area, as evidenced by the similar mobilities acquired with different spacings between the voltage probes. The FET mobility of 1.4  10
3 cm2/(V s) is comparable to those we obtained in our experiments with devices prepared using semiconductor films over 50 nm thick, implying that the hole accumulation region near the SiO2 gate dielectric is complete. The possible origins of the observed contact limited regime are addressed here. For the devices fabricated from P3HT films less than ∼6 nm in thickness, development of intimate contact between the Au
Cr electrode and the semiconductor is hampered by issues arising from either geometry of the electrodes or morphology. Reports by Singh et al., on monolayer films of P3HT, suggest that the nonplanar geometry of conventional bottom source/drain contacts leads to significant distortion of the film morphology, consequently increasing the contact resistance.32 The morphology of an ∼2 nm thick P3HT film was examined by AFM and is shown in Figure 4a, while its crystallinity is confirmed by the X-ray diffraction data in Figure 4b where a weak (100) diffraction peak at 2θ = ∼4° was observed.33,34 From the AFM results of the ultrathin P3HT film, a relatively large root-mean-square (rms) roughness of 0.5 nm, compared with a thickness of 2 nm, is obtained which is due to a number of P3HT mounds (bright regions) and valleys (dark regions) as seen in the profile of the cross section image, preventing formation of a complete Au
Cr/P3HT contact plane. In Figure 3, it is seen that the width normalized contact resistance decreased from 2.5  107 to 7.0  106 Ω cm with a P3HT thickness increase from 4 to 6 nm, while the channel sheet resistance was relatively stable at approximately 4  109 Ω/0. These results demonstrate that only the contact properties of the materials in question were improved upon increasing the semiconductor thickness, compensating for contact area loss likely due to impurities such as photoresist residue near the bottom edge of metal electrodes resulting from the photolithography process and/or a large rms roughness associated with the P3HT film. With increased semiconductor film thickness, a current path in addition to that associated with the P3HT layer near the bottom edge of electrodes is produced, enhancing charge injection and extraction by offering alternate routes for the charge carriers. In addition, for P3HT films below ∼6 nm, which is comparable to the thickness of Cr adhesion layer, an increase of the injection energy barrier, characterized by the energy difference between the highest occupied molecular orbital of P3HT and the work function of Cr, should be considered in discussing the origin of the contact limited regime. On the basis of the Schottky
Mott Rule, we estimate that a decreased Schottky

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Figure 4. (a) Tapping-mode AFM height image of an ultrathin P3HT film (∼2 nm) formed on an SiO2 substrate. (b) Out-of-plane X-ray diffraction data for the P3HT film.

barrier between P3HT and Au due to the higher work function of Au (5.2 eV), compared with Cr (4.5 eV), led to a factor of 1010 larger thermionic emission current than in a similar contact between P3HT and Cr. Another possible explanation for the existence of a contact limited regime is found in the strong correlation between the effective FET mobility and contact resistance observed in our experiments. As shown in Figure 5, in which the width normalized contact resistance is seen to decrease with an increase in the effective FET mobility, enhanced mobility of P3HT with increased thickness contributed to the reduced contact resistance (Figure 5). In the very low mobility region between 10
5 and 10
3 cm2/(V s) in Figure 5, the large width normalized contact resistance could be due to geometry and/or morphology effects as mentioned earlier. After completing channel formation, however, the decrease of the width normalized contact resistance with increasing mobility from 10
3 to 10
2 cm2/(V s) represents dependence of the contact resistance on the effective FET 1578

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Figure 5. Plot of width normalized contact resistance as a function of effective FET mobility for P3HT FETs.

mobility. Indeed, the high mobilities found at the P3HT thickness of ∼13 nm (Figure 1a) can explain the presence of a valley with the minimum value in the width normalized contact resistance (Figure 3a). In organic materials with low mobility, the net injection current has been known to be determined by the balance between the injection current from metal to organic material and the diffusion current from organic material to metal. Emtage and O’Dwyer35 proposed that the net injection current density, J, is proportional to mobility, μ, according to the equation, J = N exp(
ejB/kT)eμF where N is a constant, e is the elementary charge, jB is the injection energy barrier, k is Boltzmann constant, T is temperature, and F is the electric field applied. Scott and Malliaras36 derived a relationship for the net injection current as a result of the competition between the injecting current and recombination current driven by field enhanced diffusion from metal to organic material, adding μ as a variable that proportionally increases J. The dependence J on charge carrier mobility was experimentally demonstrated by Shen et al.37 In Figure 5, the relation between μ and J exhibited J ∼ μ1.4 which is in reasonable agreement with the above models. Interestingly, we found a large voltage drop at the drain electrode for an FET fabricated with an ultrathin P3HT film (Figure 2). More importantly, the large contact resistance at the drain electrode was reproducibly observed in devices with ultrathin P3HT films (less than ∼6 nm), in contrast to thick films where the contact resistance at the drain contact was negligible. This is puzzling because of the Schottky diode configuration in which a reverse bias in injecting hole carriers occurs due to the deep highest occupied molecular orbital (HOMO) level of P3HT. One might think that the large voltage drop at the drain electrode originated from a depletion region near that electrode induced by a reduced threshold voltage with decreasing P3HT film thickness, as seen in Figure 2b. This possibility is valid at zero gate voltage at which the value of the pinch-off voltage is comparable to the applied drain voltage. Under application of a gate voltage of more than
10 V, however, a large voltage drop at the drain electrode continued to be observed, indicating that this possibility can be ruled out because the entire channel region is in the accumulation mode under those bias conditions. Alternatively, the large voltage drop

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at the drain electrode can be linked to degradation of the device in air. In our experiments, where the electrical measurements were carried out in air, a voltage drop at the source electrode was observed instantly upon film formation and increased with increased exposure to air. Unlike the source contact, the voltage drop at the drain electrode appeared only after exposure to air and the time of exposure required scaled with P3HT thickness. In other words, a large voltage drop at the drain electrode was observed instantly upon film formation for devices prepared with 2 nm thick P3HT films. For films thicker than 6 nm, the time scale needed to observe a voltage drop was on the order of hours, implying that the large contact resistance at the drain electrode observed for the ultrathin film P3HT devices could be due to degradation of the polymer charge accumulation layer near the drain electrode thereby affecting extraction of the charge carriers injected into P3HT. The presence of a high contact resistance at the drain electrode has been reported in Li et al.38 where a significant increase of the voltage drop at the drain contact was observed in a simulation using a bottom contact P3HT FET with a low mobility region near the metal/P3HT contact. To elucidate the origin of the high contact resistance at the drain contact, systematic experiments under controlled environments are necessary. To summarize, the width normalized contact resistance for P3HT based FET devices decreased with P3HT thickness up to a thickness of 13 nm. Device geometry and P3HT morphology in a bottom-contact FET combined with a high charge injection barrier due to a Cr
P3HT contact are believed to cause a large contact resistance in devices fabricated with a semiconducting polymer film less than ∼6 nm. Increasing thickness in this range was crucial to decreasing contact resistance in that this provided an additional path through which holes could be injected through Au. When the film thickness reached 13 nm, enhanced carrier mobility in the P3HT is believed to contribute to a significant drop of the contact resistance based on the diffusion limited thermionic emission model.

’ CONCLUSIONS In conclusion, two distinct regimes govern the electrical properties of P3HT based FETs. With devices fabricated from ultrathin P3HT films (thickness less than 6 nm), the FETs exhibited a contact limited regime where the contact resistance decreases with increasing thickness. The large contact resistance in the ultrathin P3HT FET worked as a bottleneck for charge transport, hindering measurement of the electrical properties of ultrathin film P3HT devices. The large contact resistance would be detrimental to the use of such materials as the active layer in devices such as chemical sensors. This issue can be overcome either through engineering of the metal/semiconductor contacts; or through identification of an optimum thickness, enabling device operation in a charge transport limited regime. Notably, at the crossover thickness of 6 nm, low contact resistance and high channel conductivity are observed. With the above approaches, high sensitivity organic/polymer semiconductor FET sensors should be feasible. It is anticipated that the effects related to metal/semiconductor contacts in FET devices fabricated with ultrathin P3HT observed here would be universally applicable to all solution-processed conjugated polymer systems. 1579

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’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (E.R.) and metalpbn@ gmail.com (B.P.).

’ ACKNOWLEDGMENT This research was funded in part by the Materials and Devices for Information Technology Research (MDITR) STC Program of the National Science Foundation (DMR-0120967), the Georgia Tech Center for Organic Photonics and Electronics (COPE), and by Georgia Tech. ’ REFERENCES (1) Cahen, D.; Hodes, G. Adv. Mater. 2002, 14, 789–798. (2) Joachim, C.; Gimzewski, J. K.; Aviram, A. Nature 2000, 408, 541–548. (3) Derosa, P. A.; Seminario, J. M. J. Phys. Chem. B 2001, 105, 471–481. (4) Margaritondo, G. Rep. Prog. Phys. 1999, 62, 765–808. (5) Peressi, M.; Binggeli, N.; Baldereschi, A. J. Phys. D Appl. Phys. 1998, 31, 1273–1299. (6) Leonard, F.; Talin, A. A. Phys. Rev. Lett. 2006, 97, 026804. (7) Chen, Z. H.; Appenzeller, J.; Knoch, J.; Lin, Y. M.; Avouris, P. Nano Lett. 2005, 5, 1497–1502. (8) Nitzan, A.; Ratner, M. A. Science 2003, 300, 1384–1389. (9) Gunes, S.; Neugebauer, H.; Sariciftci, N. S. Chem. Rev. 2007, 107, 1324–1338. (10) Armstrong, N. R.; Wang, W. N.; Alloway, D. M.; Placencia, D.; Ratcliff, E.; Brumbach, M. Macromol. Rapid Commun. 2009, 30, 717–731. (11) Cicoira, F.; Santato, C. Adv. Funct. Mater. 2007, 17, 3421–3434. (12) Hamadani, B. H.; Ding, H.; Gao, Y.; Natelson, D. Phys. Rev. B 2005, 72, 235302. (13) Seshadri, K.; Frisbie, C. D. Appl. Phys. Lett. 2001, 78, 993–995. (14) Zaumseil, J.; Baldwin, K. W.; Rogers, J. A. J. Appl. Phys. 2003, 93, 6117–6124. (15) Burgi, L.; Richards, T. J.; Friend, R. H.; Sirringhaus, H. J. Appl. Phys. 2003, 94, 6129–6137. (16) Nichols, J. A.; Gundlach, D. J.; Jackson, T. N. Appl. Phys. Lett. 2003, 83, 2366–2368. (17) Yagi, I.; Tsukagoshi, K.; Aoyagi, Y. Appl. Phys. Lett. 2004, 84, 813–815. (18) Park, B. N.; Seo, S.; Evans, P. G. J. Phys. D Appl. Phys. 2007, 40, 3506–3511. (19) Shibata, K.; Wada, H.; Ishikawa, K.; Takezoe, H.; Mori, T. Appl. Phys. Lett. 2007, 90, 193509. (20) Chesterfield, R. J.; McKeen, J. C.; Newman, C. R.; Frisbie, C. D.; Ewbank, P. C.; Mann, K. R.; Miller, L. L. J. Appl. Phys. 2004, 95, 6396–6405. (21) Diao, L.; Frisbie, C. D.; Schroepfer, D. D.; Ruden, P. P. J. Appl. Phys. 2007, 101, 0141510. (22) Nakamura, M.; Goto, N.; Ohashi, N.; Sakai, M.; Kudo, K. Appl. Phys. Lett. 2005, 86, 122112. (23) Thakur, A. K.; Mukherjee, A. K.; Preethichandra, D. M. G.; Takashima, W.; Kaneto, K. J. Appl. Phys. 2007, 101, 104508. (24) Park, Y. D.; Cho, J. H.; Kim, D. H.; Jang, Y.; Lee, H. S.; Ihm, K.; Kang, T. H.; Cho, K. Electrochem. Solid- State Lett. 2006, 9, G317–G319. (25) Jia, H. P.; Gowrisanker, S.; Pant, G. K.; Wallace, R. M.; Gnade, B. E. J. Vac. Sci. Technol. A 2006, 24, 1228–1232. (26) Deen, A. J.; Kazemeini, M. H.; Haddlara, Y. M.; Yu, H. F.; Vamvounis, G.; Holdcroft, S.; Woods, W. IEEE Trans. Electron Devices 2004, 51, 1892–1901. (27) Sirringhaus, H.; Tessler, N.; Thomas, D. S.; Brown, P. J.; Friend, R. H. Adv. Solid State Phys. 1999, 39, 101–110. (28) Pesavento, P. V.; Chesterfield, R. J.; Newman, C. R.; Frisbie, C. D. J. Appl. Phys. 2004, 96, 7312–7324.

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