Extraction to Optimize

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Organic Electronic Devices

Engineering Asymmetric Charge Injection/Extraction to Optimize Organic Transistor Performances Tonnah Kwesi Rockson, Seolhee Baek, Hayeong Jang, Giheon Choi, Seungtaek Oh, Jaehan Kim, Hyewon Cho, Se Hyun Kim, and Hwa Sung Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b01658 • Publication Date (Web): 20 Feb 2019 Downloaded from http://pubs.acs.org on February 26, 2019

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ACS Applied Materials & Interfaces

Engineering Asymmetric Charge Injection/Extraction to Optimize Organic Transistor Performances Tonnah Kwesi Rockson1†, Seolhee Baek1†, Hayeong Jang1, Giheon Choi1, Seungtaek Oh1, Jaehan Kim1, Hyewon Cho1, Se Hyun Kim2*, Hwa Sung Lee1* 1Department

of Chemical & Biological Engineering, Hanbat National University, Daejeon 34158, Republic

of Korea 2School

†T.

of Chemical Engineering, Yeungnam University, Gyeongsan 38541, Republic of Korea

K. Rockson and S. Baek contributed equally to this work.

*Corresponding author. E-mail: [email protected] (S. H. Kim), [email protected] (H. S. Lee)

Abstract The introduction of an appropriate functionality on the electrode/active layer interface has been found to be an efficient methodology to enhance the electrical performances of organic field-effect transistors (OFETs). Herein, we efficiently optimized the charge injection/extraction characteristics of source/drain (S/D) electrodes by applying an asymmetric functionalization at each individual electrode/organic semiconductor (OSC) interface. To more clarify the functionalizing effects of the electrode/OSC interface, we systematically designed five different OFETs, one with pristine S/D electrodes (denoted as pristine S/D), and the remaining ones made by symmetrically or asymmetrically functionalizing the S/D electrodes with up to two different self-assembled monolayers (SAMs) based on thiolated molecules: the strongly electrondonating thiophenol (TP) and electron-withdrawing 2,3,4,5-pentafluorobenzenethiol (PFBT). Both the S and D electrodes were functionalized with TP (and denoted as TP-S/D) in one of the two symmetric cases and with PFBT in the other (PFBT-S/D). In each of the two asymmetric cases, one of the S/D electrodes was functionalized with TP and the other with PFBT (to produce PFBT-S/TP-D and TP-S/PFBT-D OFETs). The vapor-deposited p-type dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT) was used as the OSC active layer. The PFBT-S/TP-D case exhibited a field-effect mobility (μFET) of 0.86±0.23 cm2V-1s-1, about three times better than that of the pristine S/D case (0.31±0.12 cm2V-1s-1). On the other hand, the μFET of the TP-S/PFBT-D case (0.18±0.10 cm2V-1s-1) was significantly lower than that of the pristine case, and even lower than those of the TP-S/D (0.23±0.07 cm2V-1s-1) and PFBT-S/D (0.58±0.19 cm2V-1s-1) cases. These results were clearly correlated with the additional hole density, surface potential, and effective work function. In addition, the contact resistance (RC) for the asymmetric PFBT-S/TP-D case was ten-fold less than that for the TP-S/PFBT-D case, and more than five times lower than that for the pristine case. The results contribute a meaningful step forward in improving the electrical performances of various organic electronics such as OFETs, inverters, solar cells, and sensors. Keywords. Charge injection; charge extraction; Asymmetric functionalization; self-assembled monolayer; field-effect transistor; contact resistance.

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Introduction Organic field-effect transistors (OFETs) have, over the past few years, been investigated for and used in a variety of potential applications due to the rapid development of organic material and device processing. These applications have been revolutionary in their fields and have included organic electronic products such as flat panel displays,1-3 memory devices,4-6 complementary circuits,7-10 chemical sensors,11-13 and biological sensors.14-15 Most studies that have sought to develop OFETs with outstanding electrical characteristics have focused on improving the charge transport in the organic semiconductors (OSCs) either by designing new molecules or by enhancing the film-microstructure near the gate dielectrics through surface or interface engineering.16-20 Another commonly used approach to enhancing OFET performance has involved facilitating the injection of charges from the electrode to the active layer so as to enhance the overall charge driving capability of the OFETs.21-23 In particular, controlling the interface between the electrode and active layer plays a critical role in determining the charge injection or extraction properties and hence the electrical properties of OFETs, since these electrical properties are influenced by the heterogeneous nature of the metal/OSC contacts.24,25 Insertion of metal oxides,26,27 ionic interlayers,28 organic buffer layers,29 or graphene30 at the electrode/OSC interface are some of the common techniques that have been used to vary the contact resistance affecting the charge injection or extraction properties. Among these methods, the formation of self-assembled monolayers (SAMs) has been applied extensively as an adaptable methodology to control the interface characteristics of electrode/OSC bilayers.31,32 As the SAMs is introduced onto the surface of the source/drain (S/D) electrodes, two important effects occur. First, the surface modifications controlling surface energy and chemistry influence the crystallization process of the active layer, including the grain-boundary and molecular packing structures within the crystalline film.20,33 Second, they induce a shift of the Fermi level by chemisorbing on the surface of the S/D electrodes by introducing a dipole facing the metal surface. This effect contributes the increase or decrease of the work function on the electrode surface according to the shift direction and the dipole orientation.32,34 As a result, controlling work function (WF) of an electrode is possible to set the energy levels of the highest occupied molecular orbital (HOMO) (or lowest unoccupied molecular orbital, LUMO) of an organic semiconductor, inducing ohmic contact behavior at the OSC/electrode interfaces of the device. Therefore, the SAM modification of the electrode surface can control the charge injection/extraction properties at the OSC/electrode interface. The direction of the WF shift is influenced by the character of the functionalizing groups of the SAM molecules (especially whether they are electron-withdrawing or electron-donating groups). For examples, the WFs of alkane- and halogen-terminated thiols are shifted to lower and higher directions, respectively.35,36 Although the effect of the insertion of SAMs at metal/semiconductor interfaces on the WF has been widely studied, most of the studies has spotlighted on


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the symmetric functionalization of the S/D electrodes using with one kind of the electrode functionalization exposed equally on the electrode surfaces. In view of the above-mentioned electrode/OSC interface engineering, an asymmetric functionalization of each electrode can yield a more efficient injection/extraction behavior of holes or electrons. Although the surface functionalization of the electrodes was not applied, the similar studies have been reported by the Jackson group that improved the charge injection/extraction performances with the asymmetric electrodes.37,38 Among surface functionalization methods, a thiol-based SAM deposition may be regarded as one of most efficient methods for asymmetric surface functionalization of each electrode. The usage of vacuum-deposited metal oxide and organic buffer layer through a shadow mask can cause the contamination problem at the channel between S/D electrodes due to the diffusion of atoms or molecules.26,29 On the other hand, the dipping of thiol-terminated SAM materials enabled selective deposition onto the metal electrodes; however, it requires additional steps (e.g., photolithography) for asymmetric functionalization of S/D electrode to deposit different SAM onto each one. Recently, the Jurchescu group recently reported the systematic tuning of contact resistance of the S/D electrodes using different fluorinated thiol SAMs and the impact of these different SAMs on the film morphology of the organic semiconductors.39 They observed that addition of a SAM to the system increased the field-effect mobility by up to 10-fold and decreased the contact resistance by four-fold compared to the pristine Au electrode. Lee group studied thiol SAM modification of an inkjet-printed Ag electrode to enhance the charge injection in OFETs.40 The WFs of the electrodes were controlled to be from 4.90 eV to 4.66 eV or 5.24 eV by phenol- or fluorobenzene-terminated SAMs, respectively, inducing significant shifts of the threshold voltage depending on the thiol SAM polarity. The Samori group also reported a bottom-up technique for asymmetric functionalization of the electrodes with two kinds of thiol SAMs.41,42 They also introduced different electrode metals to enhance the extraction of holes or electrons for applications in various organic electronics, such as photovoltaic devices, photodetectors, etc. In view of the increasing importance of studies on asymmetric electrodes to improve the OFET performance, we studied on tuning the charge carrier densities from the electrode (or OSC) to the OSC (or electrode) by injection/extraction modulations. In our system, vapor-deposited modifications of thiolterminated SAMs were used to exclude the unwanted anchoring of the SAM molecules on each electrode in solution-dipping method due to the solution flow, and prototypical p-type dinaphtho[2,3-b:2′,3′f]thieno[3,2-b]thiophene (DNTT) was used as the OSC material. OFETs with a bottom-contact top-gate structure were used as a test platform, as such a structure is the one of the most commonly used in commercialized devices. The Au S/D electrodes were functionalized by two kinds of thiol SAMs with different dipole directions: thiophenol (TP) and 2,3,4,5,6-pentafluorobenzenethiol (PFBT). In particular, we systematically adjusted the direction of the TP- and PFBT-functionalized S/D electrodes to compare 3 ACS Paragon Plus Environment


precisely their charge injection/extraction characteristics. From the obtained results, we clarified the effect of charge density modulations on OFET performance by controlling the degree of chargeinjection/extraction at the interfaces between the two electrodes and the OSC layer. The changes of the injection/extraction properties by introducing the thiol SAMs were confirmed using ultraviolet photoelectron spectroscopy (UPS). Furthermore, we also analyzed the variations of the contact resistance (RC) at the electrode/OSC interface according to the injection/extraction barrier heights.

Experimental Materials and sample preparation. A glass slide was sequentially cleaned in acetone, isopropyl alcohol, and deionized water for 10 min, respectively, according to a conventional glass washing process, and then used as a substrate. A 3 nm-thick layer of Cr and then 70 nm-thick layer of Au were deposited on the glass substrate as, respectively, the adhesion layer and S/D electrodes by carrying out thermal evaporation through a shadow mask at a rate of 3.0 Å/s. The channel length (L) was varied to 50, 100, 150, 200 µm, and the channel width (W) was fixed at 1000 µm, for the OFET operation. To functionalize the TP or PFBT SAMs (Sigma-Aldrich) on each S/D electrodes, a polymeric film laminated with glue on one side was used as a protective mask (3M) to avoid the unintentional adsorption of molecules in the wrong location. The protective mask was placed on one of the two electrodes completely covering the electrode. The mask was manually placed with help of a long-focal microscope. The sample covered with a protective mask was located over the cylindrical bottle with the undiluted TP or PFBT SAM sprayed well on the floor for 2 hr in a fume hood under ambient conditions. After then, the functionalized S/D electrodes were thermally annealed in a vacuum oven at 90 °C for 1 hr. This annealing process aimed to remove the SAM molecules that have not formed chemical bonds with the Au surface and to increase the binding properties of the bonded SAM molecules on the surface. The protective mask was removed, and the other electrode was then modified with the TP or PFBT SAM through the same process as above. These samples in which the S/D electrodes were modified with TP or PFBT SAM were rinsed with ethanol (Samchun Chemical) and toluene (Sigma-Aldrich) carefully several times to remove the physisorbed or unreacted thiol SAMs and the glue from a protective mask. The sample was again sonicated in toluene for 5 min and dried in a stream of nitrogen gas (99.9%) to completely remove the mask residues such as glue. A 50 nm-thick OSC active layer of DNTT (Sigma-Aldrich, no purification) was thermally deposited from a quartz crucible using an organic molecular beam deposition (OMBD) system onto the SAM-functionalized S/D electrodes and the glass substrate (room temperature) under a base pressure of approximately 10-7 Torr at a deposition rate of 0.2 Å/s. The deposition information such as a deposition rate or thickness and a substrate temperature were used as displayed on the monitor. As the polymer gate dielectric layer, a 600 nm-thick parylene-C layer 4 ACS Paragon Plus Environment

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was produced using a chemical vapor deposition (CVD) system. Finally, 70 nm-thick Au gate electrodes were deposited on the parylene-C dielectric layer through a shadow mask at a rate of 3.0 Å/s. Characterization. The surface energies (SEs) of pristine, TP-functionalized, and PFBT-functionalized electrodes were obtained by measuring the contact angle, θ, values of two probe liquids, deionized water and diiodomethane, using a contact angle analyzer (Phoenix 300A, SEO Co., Inc.). The γs values were obtained by the Owens–Wendt equation (1)

1  cos  lv  2

 sd lvd  2  sp lvp

(1)

,

where γs and γlv are the SEs of the electrodes and the probe liquids, respectively, and the superscripts d and p mean the dispersive and polar (nondispersive) terms, respectively. The topographies of the DNTT active layers were visualized by measuring atomic force microscopy (AFM, Digital Instruments Multimode) via ex-situ tapping-mode AFM. The SiNx cantilever or Si tips used have 42 N/m and 320 kHz or tip radius of 10 nm, respectively. AFM data was analyzed using Nanoscope 5.30 software. To investigate the crystalline structures of the DNTT active layer, X-ray diffraction (XRD) using a wavelength of 1.02 Å was performed with the 50 nm-thick films. The surface potentials of the pristine, TP-functionalized, and PFBTfunctionalized Au electrodes were characterized by measuring the UPS secondary electron emission. These XRD and UPS studies were carried out in the Pohang Accelerator Laboratory (PAL), Korea. The device characteristics of the DNTT OFETs were measured at room temperature, in a dark environment, and under ambient conditions using an HP4156A instrument. To operate the p-type DNTT OFETs, the device was operated under a negative gate voltage with the grounded S electrode and the negatively biased D electrode. Agilent 4284 precision LCR meter was used to yield Ci values of 3.70±0.2 nFcm-2 for the 600 nm-thick parylene-C gate dielectric layer.

Results and Discussion



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Figure 1. Illustrations and chemical structures of thiol SAMs and OFETs. (a) Chemical structures of TP and PFBT SAMs and illustrations of the S/D Au electrodes functionalized with them. (b) OFET structure used in this system, with this structure including a 50 nm-thick layer of DNTT, a 600 nm-thick parylene-C gate dielectric, and 70 nm-thick Au gate electrode layers.

In this work, to modulate the charge injection/extraction properties from the electrode (or OSC) to the OSC (or electrode), we symmetrically or asymmetrically functionalized the S/D electrodes using SAMs based on thiolated molecules. TP and PFBT SAMs were chosen as the two kinds of thiol SAMs to modify the bottom-contact S/D electrodes, as shown in Figure 1(a). This choice resulted in a substantial difference between the energy levels of the two Au electrodes. The introduction of these two SAM materials led to opposite changes of the surface energy of the electrodes due the functionalized atoms exposed on the surface: relative to the thiol head, the phenyl group of TP increased the surface energy whereas the halogenated phenyl group strongly decreased it.38 This difference yielded a strong asymmetry of the surface dynamic behavior of OSC molecules. The results for the surface energy are summarized in Table 1. To clarify the effects of TP and PFBT SAMs on the charge injection/extraction properties, we systematically designed five different OFET samples: a symmetrical OFET with both the S/D electrodes functionalized with TP (denoted as TP-S/D), another symmetrical one functionalized with PFBT (PFBT-S/D), two asymmetrical OFETS (PFBT-S/TP-D and TP-S/PFBT-D), and the pristine case (pristine S/D). Figure 1(b) shows the OFET structure with a bottom-contact, top-gate architecture on a glass substrate. The 600 nm-thick parylene-C layer was used as the polymer gate dielectric and the DNTT OSC encapsulation layer to exclude additional interactions with moisture or oxygen.

Table 1. Surface characteristics (contact angles and surface energy values) of pristine, TP-functionalized, and PFBT-functionalized Au electrodes.

Electrode

Contact angles (o)

Surface energy (mJm-2)

Water

Diiodomethane

Polar

Dispersion

Total

Pristine Au

68

12

6.79

44.19

50.98

TP-functionalized Au

96

56

0.88

30.08

30.96

PFBT-functionalized Au

100

52

0.13

33.80

33.93


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Figure 2. (a-e) Output and (f-j) transfer characteristics of DNTT FETs fabricated with various S/D Au electrodes. (a,f) pristine S/D, (b,g) TP-S/D, (c,h) PFBT-S/D, (d,i) PFBT-S/TP-D, and (e,j) TP-S/PFBT-D. The output and transfer characteristics of each FET were measured at a fixed VD of -30 V and with a stepped VG of -10 V, respectively.

To determine the relationship between the symmetrical or asymmetrical functionalizations of the S/D electrode surfaces and the electrical performances in the DNTT OFETs, we obtained their output characteristics, i.e., drain current (ID) – drain voltage (VD) curves (Figure 2(a-e)), and transfer characteristics, i.e., ID – gate voltage (VG) curves (Figure 2(f-j)) using a bottom-contact top-gate device structure employing the variously functionalized S/D electrodes, namely the symmetric (pristine S/D, TPS/D and PFBT-S/D) and the asymmetric (PFBT-S/TP-D and TP-S/PFBT-D) devices. All devices have been found to be well-behaved p-type FETs with the definite linear and saturation properties of ID curves at low VD regimes and at VD regimes above VG, respectively. The high levels of linearity at low VD values indicated approximately ohmic-type contacts.43-45 Among the DNTT OFETs, the lowest saturated ID at the VD of -30 V was observed for the TP-S/PFBT-D device. The DNTT OFETs containing PFBT-S/TP-D showed the highest ID, followed by the PFBT-S/D, pristine S/D, and TP-S/D cases. Considering the operating principle of the p-type OFETs, the hole charge carrier transport in the active channel region from the grounded S electrode to the negatively biased D electrode leads, under negatively biased VG, to injection of hole charges 7 ACS Paragon Plus Environment


from the S electrode into the DNTT active layer and its extraction to the D electrode. That is, the relatively low RC for PFBT-S/TP-D could be attributed to the favorable behaviors of the injection/extraction at the interface. Based on these results and conclusions, we would also expect the TP-S/PFBT-D to have the highest RC at the electrode/active layer interface. From the saturation regimes of the transfer characteristic curves of the DNTT OFETs with the symmetrically or asymmetrically functionalized S/D electrodes (Figure 2(f-j)), we calculated device parameters such as field-effect mobility (μFET), on/off current ratio, and threshold voltage (Vth). The on/off current ratio was calculated from a logarithmic scale of ID–VG curves. The ID0.5–VG curves (open circles) for extracting Vth and μFET exhibited high linearity for our devices, meaning a VG-independent μFET performance with high device reliability for all samples. The Vth and the μFET values were calculated from linear fits of the ID0.5–VG plots at the on-state, corresponding to the VG-axis intercept (ID = 0) and a square of the slope proportional to μFET, respectively, according to the equation μFET = 2L/(CiW)∙(∂ID0.5/∂VG)2. The electrical parameters extracted from these OFETs are listed in Table 2. The μFET for the pristine S/D case was determined to be 0.31±0.12 cm2V-1s-1, i.e., somewhat lower than the values previously reported,46,47 which may have resulted from the bottom-contact, top-gate device structure. For the symmetrically functionalized S/D cases, the μFET for PFBT-S/D was determined to be 0.58±0.19 cm2V-1s-1, about twice the 0.23±0.07 cm2V-1s-1 value obtained for TP-S/D. These results suggested that the PFBT SAM modification of the S/D electrodes was relatively efficient at activating the charge injection/extraction or transport in the OFET device, whereas the TP SAM modification interfered with the kinetics of the charge carriers. It is interesting to note that the asymmetrically functionalized PFBT-S/TP-D case yielded a μFET of 0.86±0.23 cm2V-1s-1, about three times better than the value obtained for the pristine S/D case. On the other hand, a μFET of 0.18±0.10 cm2V-1s-1 was measured for the TP-S/PFBT-D device, significantly lower than that of the pristine case, and also a bit lower than that of the TP-S/D case. The results indicated the asymmetric modification of the S/D electrodes to be a much more efficient way of improving OFET electrical performance than the symmetric one, which is reasonable considering the nature of the charge injection/extraction between the S/D electrode and the DNTT active layer. Furthermore, disregardable hysteresis behavior was found in all of the devices, except for the TP-S/PFBT-D case. In general, to account for the cause of the hysteresis in the transfer curves, charge trapping at the interface is often associated with a chemical molecular structure, such as a hydroxyl group on the gate dielectric or the electrode surface where the charge carrier can be caught.48,49 In this study, however, the surfaces of the S/D electrodes were covered with TP or PTFE SAMs, and the gate dielectrics (parylene-C) were the same, and this configuration made it difficult to interpret the presence of hysteresis in terms of surface trapping. Therefore, in order to account for the presence of the


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hysteresis in the TP-S/PFBT-D case, another analysis that is more directly related to the nature of the charge injection/extraction would be needed.

Parameter

Pristine S/D

TP-S/D

PFBT-S/D

PFBT-S/TP-D

TP-S/PFBT-D

µFET (cm2V-1s-1)

0.31±0.12

0.23±0.07

0.58±0.19

1.10±0.35

0.18±0.10

Vth (V)

-0.92±0.94

-3.72±2.01

-0.27±0.64

0.07±0.01

-2.59±1.21

On/off current ratio

~104

~104

~104

~104

~103

Table 2. Summary of the performances of the DNTT OFETs prepared with symmetric or asymmetric S/D modifications.

Figure 3. (a) Secondary electron emission spectra of the pristine (black), TP-functionalized (blue), and PFBT-functionalized (red) Au electrode surfaces. (b) Energy diagrams of the charge injection/extraction at the interface between DNTT and Au S/D electrodes with/without SAM modifications, extracted from the spectra of the studied system shown in panel a. To determine the relationship between μFET and surface potential for the TP- and PFBT-functionalized Au electrodes, we acquired secondary electron emission spectra of their surfaces (Figure 3). The onset of secondary electrons, which corresponded to the surface potential of the SAM-functionalized surface, was determined by extrapolating two solid lines from the background and the straight onset from each spectrum. As shown in Figure 3, kinetic energies corresponding to the onset of secondary electrons followed the order 9 ACS Paragon Plus Environment


PFBT-functionalized > pristine > TP-functionalized Au surfaces. The difference between the kinetic energy of the onset for the Au functionalized with a PFBT SAM and that for TP SAM exceeded 0.9 eV. Such a marked difference in the kinetic energy was expected to yield a strong difference in the performances of their OFETs. We measured the effective WFs of the TP and pristine devices to be 4.5 and 4.9 eV, respectively. In contrast the PFBT case exhibited an effective WF value of 5.4 eV, with this relatively high value expected due to the presence of the perfluoro-terminated moieties. A higher kinetic energy indicated that the surface produced a higher potential; that is, the surface was strongly electron withdrawing.40,50-52 The high electronegativity of fluorine most likely contributed to the high potential of the PFBT-SAM. By contrast, the surface potential of the TP-functionalized Au electrode being lower than the surface potentials of the PFBT or pristine cases, despite plentiful electron density of the phenyl group, may have been due to the relatively electron-donating character of the benzene ring structure in the TP SAM.50-54 These results, in particular the WF results, indicated that the TP-S electrode presented, with respect to the DNTT active layer, a barrier to the injection of hole carriers, whereas a much lower injection barrier should occur for the PFBT-S case.40,50-52 At the D electrode/DNTT interface, however, the changes in the effective WF could produce different results. The low WF of the TP-D electrode apparently reduced the barrier of the extraction of hole carriers from the D electrode to the DNTT active layer, while the WF of the PFBT-D case rather induced its increase. Figure 3(b) depicts the energy diagrams of the charge injection/extraction at the interface between the S/D electrode and DNTT active layer in our system. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) values were from previous reports.53,54 As described in Figure 3(b), the LUMO and HOMO were determined to be located on the DNTT and to have energy values of 2.4 and 5.4 eV, respectively. The hole injection/extraction barriers could be presumed by the Mott-Schottky model that explains the injection barrier as the difference between the effective WF of the electrode and the HOMO level of the DNTT.55 In the absence of the SAM modification, i.e., in the pristine Au case, an injection barrier of 0.5 eV from the S electrode to the p-type DNTT active layer was obtained. After the TP modification of the S electrode, the effective WF was found to be decreased, to 4.5 eV, resulting in a higher barrier, of 0.9 eV, to the injection of holes into the DNTT layer. Conversely, the PFBT modification of the S electrode led to a decreased WF because of a direction of the dipole of the PFBT SAM being opposite that of the TP SAM, yielding a further decrease of the hole injection barrier, to 0 eV (i.e., no injection barrier). With regards to the D electrode, however, its modification with PFBT increased the barrier to the extraction of holes from the DNTT active layer to this electrode more so than did the modification of the D electrode with TP. This induced a different electronic response on the S/D electrode part with respect to the PFBT or TP modification, showing that the characteristics of the charge injection/extraction at the interfaces between the S/D electrodes and the active layer can be optimized by an asymmetric surface functionalization using the TP- and PFBT-SAMs. Moreover, the results provided 10 ACS Paragon Plus Environment

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clear evidence to explain that the OFET electrical performance of the PFBT-S/TP-D case were more than 8 times better than that of the TP-S/PFBT-D case, as shown in Figure 2. We also considered the variation of Vth, which is one of the major parameters of OFET performance, in connection with the WF values of the TP- or PFBT-functionalized S/D electrodes. Vth was affected by the intrinsic electric field produced by the permanent dipole of the SAM layers and by the electrochemical reaction between the surface functional groups and the semiconductor molecules.56,57 Early studies of electrode surface chemistry demonstrated that surfaces having a strong affinity for electrons preferentially attract electrons at the semiconductor side, leading to the injection of extra holes that balance the charge and positively shift the Vth. Therefore, adjusting the surface potential of an electrode is critical to modulating the charge and electrical performance of the device. As observed from the secondary electron emission spectra of our SAM-functionalized electrodes, the trend in the surface potential agreed with the trend in Vth obtained from the corresponding the DNTT OFETs, as shown in Figure 2 and Table 2. The positive shift of the Vth values to 0.07±0.01 and -0.27±0.64 V in the PFBT-S/TP-D and PFBT-S/D cases, respectively, relative to the other samples arose from the high electronegativity of the fluorine atoms. On the other hand, significant shifts of Vth in the negative direction were observed for the cases in which the S electrode was functionalized with TP, specifically for TP-S/D (-3.72±2.01V) and TP-S/PFBT-D (-2.59±1.21 V). These results, the positive or negative shift of the Vth values, indicated that the charge injection was principally determined by the surface characters of the S electrode. Here, the PFBT SAM formed on the S electrode facilitated the charge injection, and the TP SAM acted as an electronic barrier to the injection of the hole charge carrier into the DNTT active layer. That is, the TP-functionalized S cases could be necessary the negatively higher Vth to form the sufficient channel region at the interface due to the increase energy barrier. For a more quantitative investigation of the Vth shifts of the OFETs, we estimated the additional hole density (∆nh) modulated by SAM-functionalization of the S/D electrodes by applying the parallel capacitance model.40,58 This model guides a derivation of the relationship between ∆nh and Vth according to the equation Δnh = CiΔVth/e, where Ci is the gate dielectric capacitance (3.70 nF/cm2) of parylene-C, ΔVth is the difference between Vth values of the OFET devices, and e is the elementary charge (e = 1.602 x 10−19 C). The ∆nh obtained by the lowering of the Schottky barrier for the symmetric PFBT-S/D was calculated to be 4.32 x 1010 cm−2. In contrast, for the symmetric TP-S/D case, ∆nh was only 3.35 x 1010 cm−2 because of the increased Schottky barrier for charge injection in this case, which decreased the ease of saturation of charge carriers in the conduction channel. For the asymmetric PFBT-S/TP-D case, ∆nh was determined to be 5.10 x 1010 cm-2, whereas the ∆nh of the TP-S/PFBT-D case was only 7.39 x 109 cm-2. The positive shift of the Vth of the PFBT-S/TP-D OFETs implied an easier induction of the conducting channel. Conversely, the large negative shift noted in the asymmetric TP-S/PFBT-D case could have arisen from the increased 11 ACS Paragon Plus Environment


Schottky barrier to the injection of charges from the S electrode to the DNTT active layer, inducing a depletion of charge carriers at the channel region.

Figure 4. AFM morphologies of 50 nm-thick DNTT films deposited on pristine, PFBT-functionalized, and TP-functionalized Au surfaces: (a-c) height and (d-f) phase images of the DNTT films. All scale bars indicate 500 nm. (g,h) XRD patterns (θ-2θ mode) of 50 nm-thick DNTT films deposited on pristine (black), PFBT-functionalized (red), and TP-functionalized (blue) Au surfaces: (g) out of-plane and (h) in-plane XRD scan directions.

Since charge carriers migrate through the semiconducting layers, the change observed in the field-effect mobilities can be explained by the morphology and crystallinity of the DNTT active layer, which is highly dependent on certain parameters characterizing the organic interlayers such as PFBT and TP SAMs.50,59 For


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this reason, we performed the morphological and crystalline analysis of the DNTT layers. Figure 4 shows AFM height (a-c) and phase (d-f) images of 50 nm-thick DNTT layers deposited on the pristine, PFBTfunctionalized, and TP-functionalized Au surfaces. We confirmed that the DNTT films exhibited polycrystalline structures with well-grown grains and their boundaries. The grains fundamentally displayed island-like structures, but were affected by the presence of the SAM organic interlayers. For the pristine case without the SAM interlayer, the grains of the DNTT film showed round features with diameters between 100 and 250 nm, whereas for the PFBT- and TP-functionalized cases, the grains of their DNTT films showed relatively rod-like shapes with lengths of 350 ± 30 nm and 100 ± 20 nm, respectively. These morphological results were indicated by both the height (a-c) and phase (d-f) analyses shown in Figure 4. The growth behaviors of admolecular DNTT ﬁlm, including the nucleation and surface diffusion of the molecules, were significantly influenced by the surface properties, most importantly the surface energy, inducing changes in the morphologies of the OSC films.50,59 We speculated that, for the pristine Au case, a strong interaction between the DNTT molecule and the substrate with the high 50.98 mJm-2 surface energy, led to the formation of perpendicular granular structures in the films, resulting in the growth of isolated round grains as observed in the images of the DNTT films. On the other hand, the lower 33.93 and 30.96 mJm-2 surface energies, respectively, of the PFBT- and TP-Au cases was speculated to induce a stronger attraction between the DNTT molecules than that for the admolecule-substrate case, and to hence result in the well-defined rod-like grain structures due to the π-π interactions inherently formed by the DNTT molecules. The crystalline natures of the organic semiconducting film have been previously shown to significantly affect the electrical performances of OFET devices, so they have been considered to constitute one of the main factors leading to variations in device performance.50,59 For such a reason, we took one-dimensional synchrotron X-ray diffraction (XRD) measurements in the θ–2θ scan mode from the DNTT films prepared on the various organic interlayers to understand the relation between the electrode surface properties and the crystalline natures of the ad-deposited DNTT active layers. Figure 4(g) and (h) show the out-of-plane and in-plane XRD patterns of the 50 nm-thick DNTT films deposited on the pristine, PFBT-functionalized, and TP-functionalized Au electrodes, respectively. The out-of-plane XRD patterns for the PFBT- and TPAu cases showed only (00l) diffraction, i.e., no other (hkl) diffraction, indicating that the DNTT films had crystalline nature with the (00l) plane parallel to the dielectric surface. The (00l) diffractions of the DNTT films is consistent to a d001-spacing of 16.1 Å.60,61 Interestingly, the film containing PFBT-Au yielded highly ordered and intense (00l) diffraction peaks, but that containing TP-Au case yielded only a weak (001) reflection, indicating better crystallinity including larger crystals for the DNTT active layer combined with PFBT-Au than for that with TP-Au. These results showed that the similar morphologies of the PFBT- and



TP-containing DNTT films did not translate to similar crystalline natures. On the other hand, the film with the pristine Au case showed a completely different diffraction pattern, with several distinct (00l), (110), (020), and (120) peaks along the qz (out-of-plane) axis. These features indicated that the DNTT molecules were here unfavorably oriented in the film in such a manner that their backbones were stacked along a direction horizontal to the pristine Au surface.60,61 This stacking may have been induced by the formation of stable interactions between the π-orbitals of the DNTT molecules and the pristine Au surface with the high electron density, surface energy, and surface rms roughness (> 3.6 nm). In the in-plane diffraction, shown in Figure 4(h), the pristine Au case showed highly ordered (00l) diffraction peaks and peaks from other crystal planes related to the lateral growth direction to the surface. The PFBT-Au case showed crystalline natures grown parallel to the surface in the in-plane direction. The results are reasonable considering the diffractions shown in out-of-plane direction, shown in Figure 4(g). In contrast, the TP-Au case did not show any diffraction peak along the in-plane direction (Figure 4(g)), which is likely to be related to its low crystallinities. The morphological and crystallographic analysis of the DNTT active layers with different organic interlayers did not clearly explain the differences between their OFET performance measures, especially between their μFET values. The μFET value provides a characterization of charge transport and thus was closely related to the degree of interconnectivity and aligned π−π stacking between the DNTT molecules. The charge carriers mainly flowed through the channel region that formed at the interface between the DNTT and the parylene-C dielectric layers in the OFETs. The crystalline morphology of the first few DNTT layers situated on the dielectric surface could determine the charge transport characteristics in the device. Considering the above discussion, the DNTT active layers formed on the electrodes could not significantly affect the charge transport characteristics in the channel region, and their influences were mainly limited to the characteristics of charge injection/extraction between the active layer and electrodes. Although the film morphology and crystalline natures of the DNTT active layer on the electrodes, therefore, influence the OFET electrical performances, we considered that these effects could be excluded here as an important variable in explaining the results shown in Figure 2.


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Figure 5. (a) μFET values of the DNTT OFETs consisting of pristine S/D electrodes and those whose surfaces were functionalized in various ways with PFBT and/or TP . (b) Channel width-normalized Rtotal, extracted from the DNTT OFETs fabricated with various functionalized S/D electrodes as a function of the channel length at VG = -30 V. (c) Channel width-normalized RC, obtained from the DNTT OFETs prepared with various functionalized S/D electrodes as a function of gate overdrive voltage (VG - Vth). To understand the relationship between the functionalized ways of the S/D electrodes and the DNTT OFET electrical performance, we plotted the mean and deviation of the μFET values of the DNTT OFETs consisting of pristine S/D electrodes and those whose surfaces were functionalized in various symmetric or asymmetric ways with PFBT and TP SAMs (Figure 5(a)). (The μFET values shown in Figure 2 and Table 2 were used here.) The PFBT-S/TP-D case showed the best μFET performance, with maximum and average μFET values of 1.45 and 1.10 cm2V-1s-1, respectively; and the TP-S/PFBT-D case showed the lowest μFET values, with maximum and average values of 0.28 and 0.18 cm2V-1s-1, respectively. Compared with the asymmetric TP-S/PFBT-D case, the symmetric PFBT-S/D case showed higher μFET values, with maximum and average values of 0.77 and 0.58 cm2V-1s-1, respectively. The results implied that the OFET performances could not be improved simply by asymmetrically functionalizing the S/D surfaces but be optimized only by carrying out the surface functionalizations in accordance with the operating roles of the S or D electrode in the OFET device. The differences between the μFET values of the different devices were closely related to the positive/negative shifts of their effective WFs shown in Figure 3. To more accurately understand these results, however, it was necessary to investigate the electrical properties at the interface between the DNTT active layer and the S/D electrodes. For this reason, the variations of Rtotal or RC at the electrode/DNTT active layer interface were apprehended by analyzing the dependency of the resistance on the channel length at small voltage regimes of the I-V characteristics, as shown in Figure 5(b) and Figure 5(c). The channel width-normalized total resistance (Rtotal·W) was fitted as a function of the channel length (50, 100, 150, and 200 µm) for each of the DNTT OFETs prepared with the various types of PFBT/TP15 ACS Paragon Plus Environment


functionalized S/D electrodes using the transmission line method (TLM) method,31,62 shown in Figure 5(b). Rtotal was calculated using the equation Rtotal = RC + RCH, where RC and RCH are the contact resistance and channel resistance, respectively. RCH was changed with the L/W dimension and VG, followed by the equation RCH = L/W[μiCi(VG − Vth,i)], where μi and Vth,i are the intrinsic μFET and intrinsic Vth, respectively. Rtotal was extracted from the inverse slope of the output curves in the linear region, and RC was calculated from the L = 0 intersection of the device resistance measured at each VG of each prepared DNTT OFET. (Note that we showed the Rtotal·W at VG = -30 V as a representative result and omitted the other Rtotal·W results obtained from VGs of -10 and -20 V.) The width-normalized RC (RC·W) as a function of gate overdrive voltage, VOV, (VG - Vth) was plotted in Figure 5(c). It is because the RC is a strong function of the charge-carrier density.63,64 The RC showed a strong dependence on the VOVs for all OFET devices due to VG can tune injecting/extracting efficiency at electrode/OSC junctions through modulating Fermi level and carrier density of OSC layer.63,64 Higher VOV could lead to higher carrier density in the DNTT OSC layer underneath the contact and thinner the Schottky barrier of junction, inducing a smaller RC.63,65 The RC·W for the asymmetric PFBT-S/TP-D case decreased from 2.80 to 0.21 MΩ⋅cm with increasing the VOV. This resistance was impressively low, especially when compared to the values for the other cases including the asymmetric TP-S/PFBT-D case and the symmetric TP-S/D and PFBT-S/D cases. In addition, the value of RC·W for the TP-S/PFBT-D case varied from 14.9 to 2.24 MΩ⋅cm, as the VOV was increased, which was significantly higher than that for the PFBT-S/TP-D case. Although the RC·W of the symmetrically functionalized TP-S/D case was over three times greater than that of the PFBT-S/D case, the RC·W of the asymmetric TP-S/PFBT-D case was nearly five times greater than that of the PFBT-S/TP-D case. As shown in the results, the functionalization of PFBT or TP on the Au electrode do not simply signify the low or high RC values but should introduce to cooperate with the roles of the S and D electrodes. To minimize the RC in OFETs, the S electrode had to be functionalized using a strong electron-withdrawing (or holedonating) element such as the F present of the organic interlayer, which induced a large internal dipole and a downward shift of the effective WF, and the D electrode had to be functionalized using the electrondonating (or hole-withdrawing) character of the organic interlayer, which induced an upward shift of the WF, as shown in Figure 3. The product of the surface functionalizations of the S/D electrodes that adhered to these concepts was PFBT-S/TP-D, which yielded the lowest RC and dramatically improved OFET performances.

Conclusions In summary, we have presented an efficient control of the charge carrier densities in the bottom-contact top-gate structured OFET devices by modulating their injection/extraction properties. To achieve this goal, we systematically functionalized the S and D electrodes with two different SAMs based on thiolated 16 ACS Paragon Plus Environment

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molecules, PFBT and TP, in symmetric and asymmetric ways, and compared the resulting TP-S/D, PFBTS/D, PFBT-S/TP-D, and TP-S/PFBT-D devices. According to the generally accepted operating principles of p-type OFETs, hole charges in our devices were injected from the S electrode to the DNTT active layer and then extracted to the D electrode. The PFBT-S electrode, having a strong electron-withdrawing (or hole-donating) character, induced the formation of a large internal dipole and a decrease of the effective WF; and the TP-D electrode, being electron donating (or hole withdrawing), showed an increase in the effective WF. These concepts were best manifested in the surface functionalization of the S/D electrodes that produced the PFBT-S/TP-D device, which showed the lowest RC and a dramatically improved OFET performance. This device showed a μFET of 0.86±0.23 cm2V-1s-1, about three times better than the 0.31±0.12 cm2V-1s-1 value measured for the pristine S/D case. On the other hand, the μFET of the TP-S/PFBT-D case (0.18±0.10 cm2V-1s-1) was significantly less than that of the pristine case, and even less than the values for the TP-S/D (0.23±0.07 cm2V-1s-1) and PFBT-S/D (0.58±0.19 cm2V-1s-1) cases. The ∆nh of the PFBT-S/TP-D device showed a higher value of 5.10 x 1010 cm-2 than those of the other cases. These results for the DNTT OFET devices were clearly correlated with the surface potential variation and the effective WF shift. Furthermore, the RC for the asymmetric PFBT-S/TP-D case was confirmed to be 10 times lower than that for the TP-S/PFBT-D case, and more than 5 times lower than that for the pristine case. We expect our asymmetric electrode functionalization method to be used as an efficient way for improving the organic electronic performances in practical applications such as OFETs, inverters, solar cells, and sensors. ASSOCIATED CONTENT

Supporting Information. AFM images of pristine-, PFBT-, and TP-Au electrodes

Acknowledgements. This work was supported by Basic Science Research Program through the National Research

Foundation

of

Korea

(NRF)

funded

by

the

2016R1D1A1B03936094).


Ministry

of

Education

(NRF-


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