A Study on Reducing Contact Resistance in Solution-Processed

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A Study on Reducing Contact Resistance in Solution-Processed Organic Field-Effect Transistors Sangmoo Choi, Canek Fuentes-Hernandez, Cheng-Yin Wang, Talha Mansur Khan, Felipe A. Larraín, Yadong Zhang, Stephen Barlow, Seth R. Marder, and Bernard Kippelen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b07029 • Publication Date (Web): 31 Aug 2016 Downloaded from http://pubs.acs.org on September 4, 2016

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ACS Applied Materials & Interfaces

A Study on Reducing Contact Resistance in Solution-Processed Organic Field-Effect Transistors 1

Sangmoo Choi, 1Canek Fuentes-Hernandez, 1Cheng-Yin Wang, 1Talha M. Khan, 1Felipe A. Larrain, 2Yadong Zhang, 2Stephen Barlow, 2Seth R. Marder, and 1*Bernard Kippelen

1

Center for Organic Photonics and Electronics (COPE), School of Electrical and Computer

Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0250, United States 2

Center for Organic Photonics and Electronics (COPE), School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, United States

KEYWORDS: organic field-effect transistors, TIPS-pentacene/PTAA, contact resistance, contact doping, Molybdenum tris-[1,2-bis(trifluoromethyl)ethane-1,2-dithiolene], Molybdenum trioxide, top-gate geometry, Fermi-level pinning

ABSTRACT: We report on the reduction of contact resistance in solution-processed TIPSpentacene (6,13-bis(triisopropylsilylethynyl)pentacene) and PTAA (poly[bis(4-phenyl)(2,4,6trimethylphenyl)amine]) top-gate bottom-contact organic field-effect transistors (OFETs) by using different contact-modification strategies. The study compares the contact resistance values in devices that comprise Au source/drain electrodes either treated with 2,3,4,5,6pentafluorothiophenol (PFBT), or modified with an evaporated thin layer of the metal-organic molecular dopant molybdenum tris-[1,2-bis(trifluoromethyl)ethane-1,2-dithiolene] (Mo(tfd)3), or

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modified with a thin layer of the oxide MoO3. An improved performance is observed in devices modified with Mo(tfd)3 or MoO3 compared to devices in which Au electrodes are modified with PFBT. We discuss the origin of the decrease in contact resistance in terms of increase of the work function of the modified Au electrodes, Fermi-level pinning effects, and decrease of bulk resistance by electrically doping the organic semiconductor films in the vicinity of the source/drain electrodes.

INTRODUCTION In recent years, field-effect mobility values displayed by organic field-effect transistors (OFETs) have increased significantly1-3 and their operating voltage reduced to a few volts.4-5 As the channel resistance is reduced due to improved charge transport, the parasitic effects caused by the contact resistance at the metal-semiconductor interfaces become a bottleneck for the optimization of the performance of an OFET.6-20 The contact resistance in OFETs has two components; one associated with the energy barrier for injection of charge carriers from the electrodes into the semiconductor and the other associated with a resistive voltage loss for the injected charge carriers during transport from the metal-semiconductor interface to the channel of the OFETs; the latter is related to the electrical conductivity of the bulk of the semiconductor. Although many studies have discussed parasitic contact resistance effects in terms of the injection barrier height at the metal-semiconductor interface,17,

21-31

a study of the effects of

systematically varying the work function of the source/drain electrodes on the contact resistance and the Fermi-level pinning has only been reported scarcely.16, 32-33 Furthermore, the influence of the bulk resistance of a semiconductor film near the contacts has not been studied beyond contact doping. Contact doping is a common method for decreasing the bulk resistance near the contacts of an OFET through the use of dopants that selectively increase


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the carrier concentration at the source/drain electrodes.19,

27, 34-55

An increased carrier

concentration decreases the bulk resistance and, in addition, enhances the tunneling injection of charge carriers at the metal-semiconductor interface.18-19 For p-channel OFETs, contact doping has been implemented primarily by thermally evaporating/co-evaporating

dopants

such

as

2,3,5,6-tetrafluoro-7,7,8,8-

tetracyanoquinodimethane (F4TCNQ),45-50 transition metal-oxides such as MoO3,41,

56-57

molybdenum tris-[1,2-bis(trifluoromethyl)ethane-1,2-dithiolene] (Mo(tfd)3),27, 34-35 and FeCl319, 52-53

. More generally, these dopants (including MoO3 and Mo(tfd)3) have led to the electrical

doping of a wide range of organic semiconductors based on building blocks such as acene, amine, phatolocyanine, thiophene, etc. to name a few. For n-channel OFETs, ethoxylated polyethylenimine (PEIE) and rhodocene dimer have been used to n-dope electron-transport materials near the contacts.54, 58 However, the mechanism of the improvement in the contact resistance by contact doping is not well defined, and the effects of contact doping on top-gate OFETs, particularly those being fabricated by solution processing of the semiconductor films, have not yet been demonstrated. Here, we first use a contact modification approach to conduct an evaluation of the change in the injection barrier height at metal-semiconductor interfaces by systematically varying the work function of an electrode and, in addition, investigate the effects that this approach has on increasing carrier concentration (decreasing bulk resistance) in an organic semiconductor film near the metal-semiconductor interface. Second, we demonstrate that contact modification using a thin layer of Mo(tfd)3 or MoO3, deposited on the source and drain contacts of an OFET prior to solution-processing the organic semiconductor layer, results in contact doping of bottom-contact top-gate TIPS-pentacene/PTAA OFETs having a bilayer gate dielectric comprised of an


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amorphous fluoropolymer layer (CYTOP) and an Al2O3 layer by atomic layer deposition (ALD).59-63 As we have reported in the past, this device geometry based on smallmolecule/polymer semiconductor blends, has been found to yield top-gate OFETs with highly reproducible performance parameters as well as high operational and environmental stability.59-63 We should note that the use of Mo(tfd)3 in the present work differs from that in previous reports where dopants were used for contact doping of bottom-gate OFETs. In such studies, Mo(tfd)3 was co-evaporated with a small-molecule organic semiconductor, which is not a viable approach when polymeric semiconductors are used. Using the approach described here, contact-doped topgate OFETs yield contact resistance values that are five times lower than those found in OFETs having PFBT-treated Au electrodes, which are the most widely used electrodes for p-channel OFETs.

EXPERIMENTAL METHODS Fabrication of OFETs Top-gate OFETs with bare Au, PFBT-treated Au, Mo(tfd)3-doped Au, or MoO3-doped Au source/drain electrodes were fabricated as follows: Glass substrates (Corning® Eagle2000TM) were cleaned by sonication in sequential baths of acetone, deionized water, and isopropanol for 5 min each. A 50 nm-thick gold film was deposited on the substrates through a shadow mask using a thermal evaporator at a deposition rate of 1 Å/s under 5 × 10-7 Torr at room temperature for source/drain electrodes of OFETs. Source/drain electrodes were PFBT-treated or contact-doped prior to the deposition of semiconductor films as follows. 1.5 nm-thick Mo(tfd)3 or 10 nm-thick MoO3, were thermally deposited at a base pressure of < 5 × 10-7 Torr onto source and drain gold electrodes, without venting the deposition chamber, through the same shadow mask used to


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pattern the electrodes. For PFBT-treatment of Au electrodes, substrates having gold source and drain electrodes were immersed into a 10 mM PFBT solution in ethanol for 15 min, and then rinsed in pure ethanol for 1 min followed by annealing at 60 °C on a hot plate for 5 min in a N2filled glove box. A 70 nm-thick TIPS-pentacene/PTAA layer was deposited by spin-coating 30 mg/mL of 1:1 weight ratio TIPS-pentacene/PTAA solution in tetralin at 500 rpm for 10 s with 500 rpm/s acceleration and 2000 rpm for 20 s with 1000 rpm/s acceleration, followed by annealing at 100 ºC for 15 min on a hot plate in a N2-filled glove box. CYTOP (ASAHI GLASS, CTL-809M) diluted with a solvent (ASAHI GLASS, CT-SOLV180) (1:3.5 volume ratio) was spin-coated on top of the semiconductor layer at 3000 rpm for 60 s (acceleration of 10000 rpm/s) producing a 35 nm-thick film. Samples were annealed at 100 ºC for 10 min on a hot plate inside the glove box. As the top layer of the bilayer gate dielectrics, a 40 nm-thick Al2O3 film was grown at 110 ºC by atomic layer deposition (Savannah 100 ALD system, Cambridge Nanotech). This bilayer gate dielectric geometry has been shown to display a high gate capacitance density ca. 32 nF/cm2 as well as a low gate leakage current, typically below 100 nA/cm2 at electric fields below 1 MV/cm2.59-63 Finally, 100 nm-thick Ag gate electrodes were deposited by thermal evaporation through a shadow mask at a base pressure of < 5 × 10-7 Torr. Electrical and Optical Characterizations The thickness of thin films was measured by modeling spectroscopic ellipsometric data (J.A. Woollam M-2000UI) acquired at three angles of incidence (65°, 70° and 75°) using the software CompleEASE®. The work function of electrodes was measured at four points on a substrate with a Kelvin Probe (Besocke Delta Phi) in a N2-filled glove box, and then averaged. Water contact angle of gold electrodes was measured with a contact angle analyzer (Phoenix300, SEO) in ambient air. The material composition of the TIPS-pentacene/PTAA film on the electrodes


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was analyzed by x-ray photoelectron spectroscopy (Thermo K-Alpha, Thermo Scientific, Inc., Waltham, MA, USA) using incident photons of 1486.6 eV from an Al Kα monochromatic source. Polarized-light microscopy (Olympus BX51) was employed to visually capture film morphologies. Current-voltage (I-V) characteristics of OFETs were measured using an Agilent E5272A source/monitor unit in a N2-filled glove box (O2 and H2O < 0.1 ppm, 25°C), and the lateral contact resistance of the TIPS-pentacene/PTAA film was simulated by an HSPICE simulator (Synopsys).

RESULTS AND DISCUSSION Four types of electrodes were used as source and drain electrodes in top-gate OFETs described in this study: bare Au, PFBT-treated Au (Au/PFBT), Au/Mo(tfd)3, and Au/MoO3, which, on average exhibited work function values, as measured using a Kelvin probe inside a N2-filled glove box, of 4.80 ± 0.17, 5.30 ± 0.24, 5.40 ± 0.13 and 6.00 ± 0.48 eV, respectively (Figure 1). The work function of Au/Mo(tfd)3 was also measured after spin-coating tetralin, the solvent which was used to process the TIPS-pentacene/PTAA layers, in order to investigate the potential removal of Mo(tfd)3 upon processing of the organic semiconductor layer, and was found to be unchanged, suggesting that the organic layer processing does not remove Mo(tfd)3. This was further confirmed through X-ray photoelectron spectroscopy (XPS) by conducting element composition analysis on the surface of Au/Mo(tfd)3 before and after spin-coating tetralin, as shown in Figure S1. Therefore, we can conclude that the thin-layer of Mo(tfd)3 deposited on top of the source/drain electrodes remains on the electrodes after the solution-based processing of the organic semiconductor layer.


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According to the Schottky-Mott theory, the energy barrier height for the injection of hole carriers at the metal-p-channel semiconductor interface is the energy level difference between the ionization energy (IE) of the semiconductor and the work function of the electrode. Hence, according to this theory, the energy barrier height must decrease as the work function of an electrode approaches the IE of the semiconductor and reduce to zero when their values coincide. However, it is well known that an energy exists beyond which an increase of the work function of an electrode does not further decrease the energetic barrier height at the contact but instead, induces a shift of the vacuum level at the interface,16, 32-33 an effect described as Fermi-level pinning. Here, we studied the Fermi-level pining on PTAA-blended TIPS-pentacene films deposited on conductive substrates, having work function values ranging from 4.6 eV to 6.9 eV, by using a Kelvin probe in a N2-filled glove box. The work function values of the TIPSpentacene/PTAA films were found to be pinned at 4.89 ± 0.09 eV as shown in Figure 1(e). This pinning level value is in good agreement with the previously reported value of 4.83 eV for pure TIPS-pentacene.13 Since the IE values of films of pure TIPS-pentacene and PTAA films are reported to be equal, 5.2 eV,13, 64 it is thus reasonable to find that the pining level of the blend is similar to that of pure TIPS-pentacene. Hence, considering the effect of Fermi-level pinning at metal-semiconductor interfaces, the barrier height for hole injection from Au/PFBT, Au/Mo(tfd)3, and Au/MoO3 into TIPS-pentacene/PTAA films can be expected to be similar. Furthermore, if the barrier height were the only contribution to the contact resistance, it would be reasonable to hypothesize that the contact resistance would be similar in OFETs using these three types of source and drain electrodes. To test this hypothesis, we fabricated OFETs having bare Au, Au/PFBT, Au/Mo(tfd)3, and Au/MoO3 source and drain electrodes as shown in Figure 2(a). The output characteristics


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measured at the gate-source voltage bias (VGS) of -6 V shown in Figure 2(b) display improvements in the contact resistance when using Au/Mo(tfd)3, and Au/MoO3 source and drain electrodes, as revealed by the linear increase of the drain current (ID) when the drain-source voltage bias (VDS) is close to 0 V in contrast to the s-shaped increase of ID for OFETs having bare Au and Au/PFBT electrodes. These observations are also supported by comparing the shape of the transfer characteristics displayed in Figure 2(c)-(f), which clearly show that the characteristics of the OFETs with Au/Mo(tfd)3 and Au/MoO3 source and drain electrodes are better described by the theoretical square law model of ideal field-effect transistors in the saturation regime than those displayed by OFETs having bare Au and Au/PFBT source and drain electrodes. The transfer characteristics also reveal that OFETs with Au/Mo(tfd)3, and Au/MoO3 source and drain electrodes display smaller values of the threshold voltage (VTH) (by ca. 2 V) than those with bare Au or Au/PFBT OFETs, as shown in Figure 3 and Figure S2(a). It should be noted that the smaller VTH values contribute to a decrease of the contact resistance at a given VG condition by increasing the effective gate bias, |VGS – VTH|. All field-effect mobility values were then calculated in the saturation regime (i.e. |VGS– VTH|< |VDS|) by fitting a MOSFET model to the transfer characteristics in the range wherein ID1/2 displays a linear dependence on VGS. For these calculations and for each batch of devices, the gate dielectric capacitance density, 32 ± 1.9 nF/cm2 on average (out of 13 batches), was measured on capacitors fabricated alongside OFET devices. The field-effect mobility (µ) values in the saturation regime for OFETs with a channel length of 165 µm were extracted and found to be 0.6 ± 0.16 cm2/Vs with bare Au (out of 10 devices), 0.7 ± 0.30 cm2/Vs with Au/PFBT (out of 138 devices), 0.8 ± 0.38 cm2/Vs with Au/Mo(tfd)3 (out of 61 devices), and 0.6 ± 0.18 cm2/Vs with Au/MoO3 (out of 51 devices) electrodes, as depicted in Figure S2(b). As shown in Figure 4, only the OFETs having bare Au


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electrodes exhibited a significant and systematic decrease of the field-effect mobility when the geometrical channel length decreased, which is an indication of high contact resistance. Here, it should be noted that because the field-effect mobility values are extracted in the saturation regime, the contact resistance has less impact than what would be obtained if derived in the linear regime. To quantify the values of the contact resistance in this set of top-gate OFETs, the widthnormalized contact resistance, RCW, was extracted from the linear regime (i.e. |VDS |< |VGS– VTH|) in the output characteristics of OFETs having channel lengths of 60, 85, 115, 165, or 215 µm and a channel width of 2,000 µm at three different gate voltages, -4, -6, or -8 V, by using transmission-line methods (TLMs) as shown in Figure 5(a). The devices having Au/PFBT, Au/Mo(tfd)3, and Au/MoO3 electrodes respectively exhibited on average 6, 35, and 33 times smaller contact resistance values than the device having bare Au electrodes at a gate voltage of 6 V. Figure 5(b) compares the work function values of the different electrode architectures with the contact resistance values estimated from TLM measurements for OFETs fabricated on the same batches of modified electrodes. Given the number of samples analyzed and the limited batch-tobatch variability, it is unlikely that a correlation exists between the contact resistance of the devices and the electrode’s work function in the region beyond the Fermi-level pinning energy (i.e. for work function values > 4.89 eV). For instance, the contact resistance values on OFETs with Au/Mo(tfd)3 electrodes are consistently much smaller than those measured on OFETs having Au/PFBT electrodes, despite the fact that former have similar or even slightly smaller work function values. Since the injection barrier height at the metal-semiconductor interfaces remains the same after pinning of the Fermi-level energy, differences in the contact resistance


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values between OFETs having different electrode architectures can then be attributed to differences in the vertical bulk resistance around the source and drain electrodes. To investigate differences in the bulk resistance of top-gate OFETs having different electrode architectures, we measured the resistance of surface-modified TIPS-pentacene/PTAA thin-film resistors with the architecture described in the inset of Figure 6(a) and Figure S4, and independently extracted the bulk resistance from the measured contact/channel resistance using HSPICE simulations. In the first set of measurements, the surface-modified TIPSpentacene/PTAA thin-film resistors incorporating Mo(tfd)3 or MoO3 displayed about three orders or one order of magnitude lower resistance values, respectively, than those of a pristine TIPSpentacene/PTAA thin-film, shown in Figure S4(b). The resistance values of thin layers of Mo(tfd)3 and MoO3 used in the surface modified resistors, but without the TIPSpentacene/PTAA film on top, were measured to be higher than those found on a TIPSpentacene/PTAA thin-film resistor, by a factors of ca. 5 or 100, respectively. Therefore, the decrease in resistance on the surface-modified thin-film resistors is likely attributable to p-doping of the semiconductor film. Here, the sheet resistance of a Mo(tfd)3-doped TIPS-pentacene/PTAA thin-film resistor was found to be 1.8 × 106 kΩ/square by a TLM. However, the sheet resistance the MoO3-doped and pristine TIPS-pentacene/PTAA thin-film resistors could not be extracted reliably because of the wide spread in values, as shown in Figure S4. Hence, this surface-doping approach results in a significant decrease of the sheet resistance of an organic semiconductor film, similar to the one observed in previous studies in which the bulk of the organic semiconductor was p-doped by thermal co-evaporation with Mo(tfd)3. However, it should be noted that the current approach is well-suited for devices (i.e. OFETs) wherein the organic


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semiconductor is solution-processed and requires area-selective doping around the source/drain contact regions of the device. To further investigate the origin of the increase of the bulk conductivity in surface-doped resistors, depth profile XPS studies on the fluorine 1s peak were conducted across a TIPSpentacene/PTAA film on Au/Mo(tfd)3. Figure S5 shows no indication of significant diffusion of Mo(tfd)3 into the semiconductor’s bulk, which is consistent with a previous report by Qi et al.34 of the limited diffusion of Mo(tfd)3 in host materials because of it large molecular size. In addition, by using the sheet resistance value of the TIPS-pentacene/PTAA film surface-doped with Mo(tfd)3 and approximate electrical/physical parameters of the film (Table S1), the hole concentration and the Fermi-level energy of the doped film were calculated as ca. 1 × 1015 cm-3 and 4.84 eV, respectively; the calculated Fermi-level energy is in good agreement with the Fermi level pinning energy shown in Figure 1(e). The calculated hole density suggests that either a very small fraction (below the limit of detection on XPS) of diffused Mo(tfd)3 molecules would be needed to explain the increase of bulk conductivity, or alternatively, as reported by Zhao et al.,36 Mo(tfd)3 could be inducing remote doping of charge carriers in the bulk. It should be noted that in top-gate OFETs, the aggregate contact resistance evaluated by TLMs is a combination of the injection resistance, RINJ, the vertical bulk resistance, RBLK1, and the horizontal bulk resistance, RBLK2, as illustrated in Figure S6. To gain insight into the vertical bulk resistance component, HSPICE simulations were implemented by using the measured RC and RBLK2, and by assuming that: RBLK2 has the same value of channel sheet resistance; and RINJ is identical and negligible for all the OFETs having electrodes with a work function higher than the Fermi level pinning energy. In the simulation, the aggregate contact resistance in an OFET having W/L = 2,000 µm/165 µm, shown in Figure S6(b), was modeled as a grid of 80 × 2,000 ×


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2 resistors as illustrated in Figure S6(c). The unit vertical bulk resistance in the grid, rBLK1, that best fitted the measured RC and RBLK2 at VGS = -6 V, was extracted for the four types of OFETs, and is presented in Figure 6(b). This result suggests that contact doping of the surface of electrodes either by Mo(tfd)3 or by MoO3 significantly reduces the vertical bulk resistance. Finally, we would like to point out that although detailed morphological studies have not been conducted, the contact angle of a surface is known to be a key parameter influencing the morphology of a film deposited onto such surface. As shown in Figure S3, the water contact angle on bare Au, Au/PFBT and Au/Mo(tfd)3 electrodes are nearly identical within statistical errors, with values in the range between ca. 70-75°. In contrast, Au/MoO3 electrodes display an average contact angle value of 13°. However, despite large differences in the contact angle measured for different electrodes, the performance and bulk conductivity of OFETs having Au/MoO3 and Mo(tfd)3 electrodes is nonetheless very similar. In contrast, even if their electrodes display nearly identical contact angles, OFETs with Mo(tfd)3 electrodes display much improved performance and higher bulk conductivity compared to OFETs having bare Au or Au/PFBT electrodes. Furthermore, depth profile XPS measurements on a TIPS-pentacene/PTAA film deposited on Au/Mo(tfd)3 electrodes (Figure S5) indicate that the top surface of the film is TIPSpentacene-rich, as revealed by the strong signal of silicon atoms located at its surface, consistent with the vertical phase segregation profiles we have previously reported on TIPSpentacene/PTAA films deposited on glass.62 Taking all the collective evidence into consideration, we can state that even if we cannot rule out that morphological differences exist between the films formed on different electrodes, the lack of correlation between the contact angle of such electrodes, vertical phase segregation profiles and the device performance, strongly


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suggests that such differences may be playing a minor role on the increased bulk conductivity observed between films deposited on different electrodes, and ultimately on device performance.

CONCLUSIONS This study demonstrated the fabrication of solution-processed top-gate TIPS-pentacene/PTAA OFETs having contact-doped electrodes with Mo(tfd)3 and MoO3. A significant decrease in contact resistance in the OFETs was observed. The Fermi-level pining energy of TIPSpentacene/PTAA films was found to be 4.89 eV. The systematic analysis of the OFET contact resistance and the correlation with the work function values of the source/drain electrodes offered in this paper, allow us to conclude that the substantial decrease in contact resistance is attributed to the reduction of vertical bulk resistance in contacts of the OFETs. The evaluation of the vertical bulk resistance provides further evidence that surface-doping using a thin layer of Mo(tfd)3 and MoO3 at metal-semiconductor interfaces, can be effective at increasing the electrical conductivity of the bulk film near the contact even if the dopants are not co-deposited or blended with the organic semiconductor. As such, this work offers not only the first demonstration of the use of the p-dopant materials, Mo(tfd)3, in the context of bottom-contact top-gate OFETs, but also clear evidence that the use of high work function contacts such as MoO3 can offer an alternative approach which appears as effective as the use of dopants in reducing parasitic contact resistance effects in OFETs. Further detailed investigations on the change of film morphology/microstructure and the creation of gap states at the metalsemiconductor interfaces as a function of surface modification should provide an improved physical understanding of the mechanism for the decrease of contact resistance, and remained as future work. However, the results here presented offer compelling evidence of surface-doping


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effects and their importance in reducing the contact resistance in top-gate OFETs. Hence, nextgeneration electrodes or electrode-modifications for top-gate OFETs should combine the selectivity and processability offered by solution-processed PFBT, while at the same time, offering the possibility of producing electrodes that could electrically dope the bulk of the semiconductor film in the vicinity of the contact.

Supporting Information XPS spectra on the surface of Mo(tfd)3-deposited metal electrodes, statistics in VTH and µ of the top-gate OFETs, dependence of µ on channel length, thin-film resistance of pristine and MoO3-doped TIPS-pentacene/PTAA films, XPS depth profile of a Mo(tfd)3-doped TIPSpentacene/PTAA film, and a schematic diagram of a TIPS-pentacene/PTAA film for the HSPICE simulation. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENTS


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S. C. received funding from the Academic Training Program of Samsung Display, and C.F.-H., T.M.K., Y.Z. S.B., S.R.M. and B.K received funding in part from the Department of the Navy, Office of Naval Research under Award No. N00014-14-1-0580 and N00014-14-1-0126, C.-Y.W. received funding from Mitsubishi Chemicals under award number AGR DTD 09/01/14 and from the Renewable Bioproducts Institute at Georgia Tech, and USDA under award number 13-JV11111129-079. B.K. acknowledges a Visiting Professorship from the University of Cologne, Germany.

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(a)

(b)

(c)

(d)

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(e) Figure 1. Histogram of work function values of (a) bare Au, (b) PFBT-treated Au, (c) Mo(tfd)3doped Au, and (d) MoO3-doped Au; and (e) work function of TIPS-pentacene/PTAA films on conducting substrates as a function of the work function of the substrates. The structure shown for MoO3 represents that presents in the gas phase during sublimation.


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Gate Al2O3 CYTOP TIPS-pentacene/PTAA S

Au Au/PFBT Au/Mo(tfd)3 Au/MoO3

D

Glass substrate

(a)

(b)

(c)

(d)

(e)

(f)

Figure 2. (a) Cross-sectional view of fabricated top-gate OFETs, (b) a comparison of output characteristics of the OFETs at VG = -6 V, and transfer characteristics when the source/drain electrodes of the OFETs are (c) bare Au, (d) Au/PFBT, (e) Au/Mo(tfd)3, and (f) Au/MoO3. (The source voltage bias, VS, is 0 V for all measurement.)


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(a)

(b)

(c)

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Figure 3. Histogram of threshold voltage values of top-gate TIPS-pentacene/PTAA OFETs with (a) bare Au, (b) Au/PFBT, (c) Au/Mo(tfd)3, and (d) Au/MoO3 source/drain electrodes.


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Figure 4. Dependence of field-effect mobility on channel length in top-gate OFETs with (a) bare Au, (b) Au/PFBT, (c) Au/Mo(tfd)3, and (d) Au/MoO3 source/drain electrodes in the same batch.

(a)

(b)

Figure 5. Width-normalized contact resistance (a) as a function of gate bias and (b) as a function of work function values of electrodes. (VS = 0 V)


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(a)

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(b)

Figure 6. Evaluation of vertical bulk resistance in contacts: (a) Extraction of sheet resistance of Mo(tfd)3-doped films and (b) a comparison of vertical bulk resistance values extracted by HSPICE simulation based on measured RC and RCH.


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Graphical abstract


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A Study on Reducing Contact Resistance in Solution-Processed

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