Dipolar SAMs Reduce Charge Carrier Injection Barriers in n-Channel

Article pubs.acs.org/Langmuir

Dipolar SAMs Reduce Charge Carrier Injection Barriers in n‑Channel Organic Field Effect Transistors Malte Jesper,*,†,‡ Milan Alt,‡,§,∥ Janusz Schinke,‡,⊥ Sabina Hillebrandt,‡,# Iva Angelova,‡,∇ Valentina Rohnacher,‡,# Annemarie Pucci,‡,#,• Uli Lemmer,‡,§,+ Wolfram Jaegermann,‡,¶ Wolfgang Kowalsky,‡,⊥,# Tobias Glaser,‡,# Eric Mankel,‡,¶ Robert Lovrincic,‡,⊥ Florian Golling,†,‡ Manuel Hamburger,*,† and Uwe H. F. Bunz*,†,‡,• †

Organisch-Chemisches Institut, Universität Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg, Germany InnovationLab GmbH, Speyerer Straße 4, 69115 Heidelberg, Germany § Light Technology Institute, Karlsruhe Institute of Technology, Engesserstraße 13, 76131 Karlsruhe, Germany ∥ Merck KGaA, Frankfurter Straße 250, 64293 Darmstadt, Germany ⊥ Institut für Hochfrequenztechnik, Technische Universität Braunschweig, Schleinitzstraße 22, 38106 Braunschweig, Germany # Kirchhoff Institute for Physics, Heidelberg University, Im Neuenheimer Feld 227, 69120 Heidelberg, Germany ∇ BASF SE, Quantum Chemistry Group, GMC/MQ B009, 67056 Ludwigshafen, Germany • Center for Advanced Materials, Heidelberg University, Im Neuenheimer Feld 225, 69120 Heidelberg, Germany + Institut für Mikrostrukturtechnik, Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany ¶ Materials Science Department, Surface Science Division, Technische Universität Darmstadt, Jovanka-Bontschits-Straße 2, 64287 Darmstadt, Germany

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‡

S Supporting Information *

ABSTRACT: In this work we examine small conjugated molecules bearing a thiol headgroup as self assembled monolayers (SAM). Functional groups in the SAM-active molecule shift the work function of gold to n-channel semiconductor regimes and improve the wettability of the surface. We examine the effect of the presence of methylene linkers on the orientation of the molecule within the SAM. 3,4,5-Trimethoxythiophenol (TMP-SH) and 3,4,5-trimethoxybenzylthiol (TMP-CH2-SH) were first subjected to computational analysis, predicting work function shifts of −430 and −310 meV. Contact angle measurements show an increase in the wetting envelope compared to that of pristine gold. Infrared (IR) measurements show tilt angles of 22 and 63°, with the methylene-linked molecule (TMP-CH2-SH) attaining a flatter orientation. The actual work function shift as measured with photoemission spectroscopy (XPS/UPS) is even larger, −600 and −430 meV, respectively. The contact resistance between gold electrodes and poly[N,N′-bis(2-octyldodecyl)-naphthalene-1,4:5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene) (Polyera Aktive Ink, N2200) in n-type OFETs is demonstrated to decrease by 3 orders of magnitude due to the use of TMP-SH and TMP-CH2-SH. The effective mobility was enhanced by two orders of magnitude, significantly decreasing the contact resistance to match the mobilities reported for N2200 with optimized electrodes.

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INTRODUCTION Organic electronics is a rapidly growing field of interest as it provides possibilities for efficient and inexpensive manufacturing of electronic devices such as integrated circuits based on organic field effect transistors (OFETs). In organic electronic devices, interface properties play a decisive role, particularly the electrode−semiconductor interface. Matching the energy levels of the electrode material and that of the semiconductor is crucial for efficient device operation.1−5 Especially for n-type© XXXX American Chemical Society

semiconductors, this imposes a challenge. The lowest unoccupied molecular orbital (LUMO) energies for n-channel semiconductor materials typically range between −3.5 and −4.5 eV.6−9 Electrode materials matching this energy range (e.g. calcium (−2.86 eV), aluminum (−4.24 eV))10 undergo fast Received: June 24, 2015 Revised: August 17, 2015

A

DOI: 10.1021/acs.langmuir.5b02316 Langmuir XXXX, XXX, XXX−XXX

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Figure 1. Schematic illustration of the scope of this work with target molecules 3,4,5-trimethoxythiophenol (TMP-SH) and 3,4,5trimethoxybenzylthiol (TMP-CH2-SH).

polar groups are attached, creating a dipolar molecule. The dipole moment is generated by adding methoxy substituents in para and meta positions to the aromatic core. Choosing methoxy groups as substituents should also increase the wettability of the surface due to their polar nature. A sulfur moiety was chosen as an anchor group to target gold and silver electrodes. As the tilt angle of substituted thiols on gold is about 30° (to the surface normal),14 we reasoned that inserting a methylene linker between the sulfur and the benzene ring would allow the molecule to straighten, decrease the tilt angle, and increase the effective dipole moment.28 Therefore, we chose to investigate 3,4,5-trimethoxy-thiophenol (TMP-SH) and 3,4,5-trimethoxybenzylthiol (TMP-CH2-SH) (Figure 1). The similar and commercially available p-methoxythiophenol (monomethoxythiophenol, MMP-SH) has already been studied, and its properties are described in the literature.24−27

oxidation under ambient conditions. Using air-stable, noble metals with higher work functions, on the other hand, imposes a significant energy offset and erects a charge-injection barrier. One solution to this challenge is to alter the work function of a noble metal electrode to match the energy range of n-channel materials. The work function of a bulk material can be altered by applying a dipole to its surface, most commonly achieved by the deposition of thin polar films (e.g., polyethylenimine (PEI) or ethoxylated polyethylenimine (PEIE))11 or dipolar monolayers. With thin PEIE films, the work function can be reasonably adapted; however, its weak surface bond and its electrically insulating nature are drawbacks. The weak surface bond might allow for diffusion into layers deposited on top (particularly in print applications),12 whereas its insulating nature poses an unnecessary obstacle to the charge injection at the interface. Self-assembled monolayers (SAMs), on the other hand, exhibit a discrete and rather strong surface binding, preventing the aforementioned diffusion. The interactions during SAM formation are well studied and understood.13−16 One of the best-studied systems is that of thiol SAMs on gold surfaces. The theoretical aspects of gold dipole−SAM junctions are well described by Heimel and co-workers.17−19 In addition to the theoretical studies, practical studies show the positive effects of dipolar SAMs on work functions.20−30 Although this topic has already been intensely studied, most of the research focuses on energy-level tailoring, but disregards other parameters crucial to efficient device preparation and performance, such as wetting behavior and interface morphology. In the case of crystalline semiconductors, the crystalline order at the interface plays an important role, which in turn may be influenced by the SAM.31 With respect to print applications, a critical point is the wetting behavior of the resulting surface. The approach described here combines energetic tailoring of the work function while at the same time improving the wettability of the surface. The molecular motif consists of a small aromatic core to allow for a free flow of charge through the interface, to which

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EXPERIMENTAL SECTION

Material Synthesis. Detailed experimental procedures for the synthesis of the noncommercial compounds in this study are given in the SI. SAM Formation. The gold substrates were immersed in solutions of SAM compounds in ethanol (anhydrous ethanol >99.8%, VWR) at various concentrations. After a given time the samples were washed with pure ethanol, dried under nitrogen, and immediately transferred under ambient conditions to the respective measurement technique. SAM Characterization. The contact angle determinations were carried out as a static contact angle. In order to determine the wetting envelopes, four different liquids were used: water, diiodomethylene, glycerol, and ethylene glycole. The photoelectron spectroscopy (PES) experiments were performed under ultrahigh vacuum conditions (base pressure about 10−9 mbar) using a Physical Electronics VersaProbe II spectrometer equipped with a monochromatized Al Kα X-ray (hν = 1486 eV) source and an Omicron HIS 13 helium discharge lamp (HeI: hν = 21.22 eV) as excitation sources for X-ray photoelectron spectroscopy (XPS) and UV photoelectron spectroscopy (UPS), respectively. All spectra are referenced in binding energy with respect to the Fermi edge and the core level lines of sputter-cleaned metal foils (Au, Ag, and Cu). B


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Langmuir Table 1. Calculated Molecule Propertiesa

Gas-phase dipole moment (μ), tilt angle (θ), effective dipole moment (μ⊥), and work function shifts (ΔΦ). b)Experimental values, taken from refs 24−27. Downloaded by UNIV OF NEBRASKA-LINCOLN on September 13, 2015 | http://pubs.acs.org Publication Date (Web): September 8, 2015 | doi: 10.1021/acs.langmuir.5b02316

a

Scheme 1. Synthesis of Trimethoxyphenylthiol (TMP-SH)

Scheme 2. Synthesis Routes for 3,4,5-Trimethoxybenzylthiol (TMP-CH2-SH)

Figure 2. (a) Wetting envelopes for pristine and SAM-treated gold surfaces (TMP-SH and TMP-CH2-SH). Additionally, dipersive/polar values for some common solvents were added. (b) Tilt angle of the molecules as a function of the immersion time as derived from IR measurements. Dashed lines mark the calculated tilt angle values. (Polyera ActivInk)32 was spin coated from 10 mg/mL solution in chlorobenzene on top of the SAM-treated electrodes. Cytop (CTL809M, Asahi Glass, TKY, Japan) dielectric layers with a thickness of 600 nm were prepared via spin coating. Gate electrodes were prepared by thermal evaporation of 80 nm Ag via shadow mask structuring. OFET Characterization. OFET characterization was carried out under ambient conditions in a three-probe setup using an Agilent 4155C semiconductor parameter analyzer (Agilent Technologies, CA, USA). Transfer characteristics were measured by operating OFETs in

To determine the tilt angle of the molecules, infrared (IR) spectroscopy was carried out in reflection−absorption (IRRAS) on SAMs and attenuated total reflection (ATR) geometry on bulk samples. (See SI for further details.) The tilt angle analysis was carried out as described previously.20 OFET Preparation. OFET devices were prepared in staggered bottom contact top gate (bctg) architecture on glass substrates using gold source and drain electrodes with a 10, 20, or 50 μm channel length by 1000 μm channel width. The N2200 n-type semiconductor C


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Langmuir accumulation mode. Onset voltages were determined as the voltage where the current excess was 1 × 10−11 A. Contact resistances were determined from the transfer characteristics using the transfer-line method (TLM). Computational Studies. The SAM properties were extracted from DFT-based calculations as described in the literature.33,34 The pure gold(111) surface was modeled by a periodic slab composed of five metal layers and a vacuum of 12 Å. Adsorption was modeled on only one side of the slab. During the geometry optimization the three bottom layers were kept fixed at their optimized bulk truncated geometry for the gold(111) surface. The organic layer and the two outermost atomic metal layers were allowed to relax without any further constraints. The atomic positions were relaxed until the force on the unconstrained atoms was less than 0.003 eV/A. To model the TMP-SH and TMP-CH2-SH monolayer on the gold(111) surface, we studied two molecules per unit cell arranged in a herringbone-like pattern. (See the SI for details.) The calculated work function shifts are assessed as the difference between the work function of the SAMcovered Au(111) surface and that of the clean Au(111) surface (5.24 eV).

group, decreases the tile angle. The calculated dipole moments of the TMP derivatives are considerably lower than that of MMP-SH (1.48/1.42 Db vs 2.42 Db). The effective dipole moment is even lower for the benzylic species (0.97 Db vs 1.04 Db) although it was designed to enhance the effective dipole moment. Interestingly, the TMP derivatives should display a work function shift similar to that of MMP-SH, despite their significantly lower dipole moments. Prompted by the calculations, we prepared these compounds and studied their properties. Synthesis. To investigate the impact of the aforementioned structures on gold surfaces, the different TMP derivatives were synthesized. 3,4,5-Trimethoxythiophenol was obtained starting from 3,4,5-trimethoxyaniline in a Sandmeyer-type reaction.35 Diazotization and reaction with potassium ethylxanthate followed by basic hydrolysis gave the desired compound in good yield (82%, Scheme 1). The methylene-linked trimethoxybenzyl derivative is accessible via nucleophilic substitution of 3,4,5-trimethoxybenzyl chloride (Scheme 2). While a thioacetate nucleophilic substitution/hydrolysis route36,37 gave higher yields, direct conversion into the thiol with sodium hydrogen sulfide38 proved superior due to easier purification. Even though the NMR spectra of the two obtained products were identical, the hydrogen sulfide route furnished a colorless solid, whereas the product isolated from the thioacetate route retained a slightly brownish color which could not be removed. SAM Formation and Characterization. As the next step, the SAM characteristics of the molecules were examined. Therefore, the compounds were cast on gold substrates (cf. Experimental Section), the static contact angle was measured for water, diiodomethane, glycerol, and ethylene glycol, and the surface energy was calculated (SI). From these values, the wetting envelopes of the SAM-treated surfaces were obtained (Figure 2a). [The area under the curve shows the range of solvents, which exhibit complete wetting. Each solvent has a pair of numbers SFT (p) and SFT (d). The area inside the wetting envelope shows all solvents that completely wet the surface (Figure 2)]. The contact angle for water decreased from 74° for the bare gold surface to 58° for the treated surfaces (TMP-SH and TMP-CH2-SH alike), indicating the formation of a SAM. The calculated wetting envelopes show an increase in both the polar and dispersive parts, which results in an overall increase in the wettability of the surface. TMP-CH2-SH shows a significantly larger impact on the dispersive factor than TMPSH. This can be explained by a larger tilt angle of the molecules within the TMP-CH2-SH-SAM. Assuming almost flat-lying TMP-CH2 molecules, the π systems can interact more strongly with nonpolar solvent molecules and thus increase the dispersive part of the wetting envelope. To examine this hypothesis, IR measurements (see SI) were performed to determine the tilt angle of the molecules shown as a function of the immersion time (Figure 2b). TMP-SH is shown to adapt a tilt angle of (36 ± 1)°, meeting the expectations and the calculated value (23°). TMP-CH2-SH, however, differs from the calculated values (12°) by showing a tilt angle of (63 ± 3)°. This heavily tilted orientation is in agreement with the findings of the contact angle measurements. This result can be explained by taking the affinity between gold and oxygen into account. The three methoxy groups exhibit a strong enough attraction to the gold surface to hinder the molecule from standing up. This is supported by the development of the tilt angle over time. While the more rigid system of TMP-SH shows a shift of its tilt

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RESULTS AND DISCUSSION Computational Analysis. To predict structural properties of our molecules, we conducted a computational analysis. The

Figure 3. Representative selection of PES measurements on pristine gold substrates and substrates treated with TMP-SH and TMP-CH2SH. From left to right, detailed spectra of C 1s, O 1s core levels, and the SE cutoff with the absolute work function values are presented (top and bottom).

gas-phase dipole moments (μ) were calculated for our compounds as well as for MMP-SH. In the case of TMP-SH and TMP-CH2-SH, the tilt angle (θ), the effective dipole moment on the surface (μ⊥), and the expected work function shift (ΔΦ) were determined. Work function shifts of MMP-SH were taken from the literature (Table 1).24−27 The molecules form a (2 × 4) herringbone structure with 2 molecules per unit cell occupying fcc-shifted bridge positions. The calculated tilt angle fits the expectation with 23° for the phenyl junction and 12° for the benzyl junction, supporting our prediction that a methylene bridge, extending the anchoring D


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Figure 4. Left: transistor architecture and molecular formula of the semiconductor used. Middle: transfer characteristics for untreated (black/square) and SAM-treated (TMP-SH, green/asterik; TMP-CH2-SH, blue/triangle) transistors at 60 V (top) and 10 V (bottom) drain voltages. Right: TLM measurements for untreated (black/square) and SAM-treated (TMP-SH, green/asterik; TMP-CH2-SH, blue/triangle) transistors at 60 V (top) and 10 V (bottom) gate voltages.

On the basis of these findings, the chemical and electronic effects of the SAMs on gold surfaces were studied using XPS and UPS measurements. In Figure 3, a representative selection of photoelectron spectroscopy core-level lines as well as the secondary electron cutoff is presented for the bare gold surfaces and SAM-treated substrates. Detailed spectra of the carbon (C 1s) and the oxygen (O 1s) core level are presented. After 5 s of immersion time, the specific chemical signature of the TMP C 1s core level is visible, and an effect of the SAM on the work function is apparent. With longer immersion times, a slight additional decrease in the work function shift is observed, which is related to the tilt-angle relaxation as shown in the IR measurement section of this paper. TMP-SH shows a maximum work function shift of −600 meV (compared to that of pristine gold), whereas TMP-CH2-SH shows a slightly lower shift of −430 meV. These values are surprisingly high compared to the calculated shift (−430 meV and −310 meV, respectively) and also even higher than the shift resulting from MMP, despite its higher calculated dipole moment (Table 1). In the case of TMP-CH2-SH, the actual work function shift could even be a little higher as the gold reference shows minute carbon impurities (due to contamination during transfer of the cleaned gold substrate to the XPS) and therefore a slightly diminished work function (Figure 3, bottom, black line). This impurity, however, shows the potency of thiol-gold-SAMs as it

Table 2. Performance Characteristics (Single Device Performance) of OFET Devices with Pristine Gold Electrodes and SAM-Treated Electrodes

pristine gold TMP-SH TMPCH2-SH

drain voltage (V)

μmax(sat) [cm2/(V s)]

10 60 10 60 10 60

6 × 10−4 5 × 10−3 0.10 0.10 0.06 0.07

contact resistance [Ohm*cm]

onset voltage [V]

× × × × × ×

6.5 6.4 2.7 4.2 4.0 4.5

3.2 2.4 9.6 4.2 2.4 1.7

1010 108 106 105 107 106

angle from 22 to 36°, indicating a relaxation of the SAM, the tilt angle of TMP-CH2-SH stays almost the same. This is due to the additional affinity of the methoxy groups for the gold surface on combination with an additional degree of freedom, stemming from the methylene linker. To provide additional evidence for the orientation of the surface molecules, we reinvestigated the parameters of the calculation. This time, however, the calculations were started from a flat-lying TMPCH2-SH molecule instead of an upright orientation. With this alteration the calculated tilt angle settled at 73°, which is in good agreement with the experimental findings. E


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effective mobility by two orders of magnitude. TMP-CH2-SH (3,4,5-trimethoxybenzylthiol) has a lower impact on the work function with a shift of −430 meV, attributed to its higher tilt angle of 63°. Nevertheless, OFET characteristics improved for the use of this SAM in a similar way. Both compounds showed a significantly larger wetting envelope than the pristine gold surface. The initial computational analysis of the compounds showed corresponding trends with regard to work function shifts and tilt angles. However, after optimizing parameters and embedding experimental values into the calculations, a systematical deviation of the computed work functions by roughly 200 meV was observed. Overall, this is an attractive yet simple concept in improving the overall performance of nchannel OFETs based on N2200.

is replaced after 5 s by the SAM, solely leaving the characteristic C 1s double peaks of the TMP moiety. This behavior can also be observed for other SAMs containing thiol anchor groups.20 The observed work function shifts are somewhat higher than the initially calculated values (−430 and −310 meV for TMPSH and TMP-CH2-SH, respectively). These values, however, still work under the assumption of an upright standing TMPCH2-SH, so in a new calculation we corrected the tilt angles by freezing them at the values found from the IR measurements, which gave us shifts of −420 and −170 meV, respectively. These calculated shifts are still less than the actual shifts. In contrast to the previous results, computed results based on experimental tilt angles now systematically underestimate the impact on the work function by roughly 200 meV. OFET Devices. The surprisingly high work function shifts prompted us to fabricate OFET devices to examine the compounds in actual working devices and to determine their contact resistances. A schematic cross section of the applied device architecture is presented in Figure 4. Transfer characteristics of OFETs with pristine and SAM-treated source-drain contacts, operated at drain voltages of 60 and 10 V, are shown in Figure 4. The derived contact resistance, obtained via the transfer-line method (TLM),39 is also shown. OFET performance characteristics are summarized in Table 2. Both the 60 and the 10 V measurements show significantly improved transfer characteristics for the SAM-treated electrodes. The effective mobilities derived from the transistor measurements increased by 2 to 3 orders of magnitude. The resulting values are independent of the drain voltage and in good agreement with previously reported mobilities for N2200.11,32 The increase in effective mobilities can be attributed to a decrease in the contact resistance. Although the determined absolute contact resistance at a drain voltage of 10 V is generally higher with respect to 60 V, a relative decrease in contact resistance by 3 orders of magnitude is observed in both cases. Additionally, the onset voltage is decreased significantly, supporting the assumption of a decreased contact resistance. Overall, the TMP-CH2-SH-treated devices show a somewhat smaller change in OFET performance than do the TMP-SH-treated devices, which correlates with the findings from UPS- and IR measurements alike. Considering the frontier orbital levels of N2200 (−5.9 eV for HOMO and −4.3 eV for LUMO),40 the reduced electrode work function now matches the LUMO level (−4.4 eV for TMP-SH), resulting in a reduced injection barrier. Therefore, the mobility of the semiconductor material can be fully exploited. Even when shifting the work function further using PEIE, the mobility is not improved any further.11

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b02316. Detailed synthesis information, detailed IR and XPS/ UPS information, detailed computational information, contact angles and measurement details, and OFET architecture and fabrication details (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Present Address

(M.H.) Merck KGaA, Frankfurter Str. 250, 64293 Darmstadt, Germany. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The Federal Ministry for Education and Research (BMBF) is acknowledged for funding under contract no. 13N11701 (project MORPHEUS).

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REFERENCES

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CONCLUSIONS We have examined small conjugated molecules comprising thiol headgroups for SAM formation and dipole moments fit for shifting the work function of gold in regimes matching nchannel semiconductors as well as improving the wettability of the surface. The impact of a methylene linker on the orientation of the molecules within the SAM was examined. TMP-SH (3,4,5-trimethoxythiophenol) was shown to induce work function shifts of −600 meV, surpassing the known experimental values for MMP by almost 100 meV. The tilt angle toward the surface normal was determined to be 36°. TMP-SH improves n-type OFET characteristics by adapting energy levels of gold electrodes to the semiconductor energy levels, thus lowering the contact resistance and increasing the F


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