Direct Effect of Dielectric Surface Energy on Carrier Transport in

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Organic Electronic Devices

Direct Effect of Dielectric Surface Energy on Carrier Transport in Organic Field-Effect Transistors Shujun Zhou, Qingxin Tang, Hongkun Tian, Xiaoli Zhao, Yanhong Tong, Stephen Barlow, Seth R. Marder, and Yichun Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02304 • Publication Date (Web): 24 Apr 2018 Downloaded from http://pubs.acs.org on April 24, 2018

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ACS Applied Materials & Interfaces

Direct Effect of Dielectric Surface Energy on Carrier Transport in Organic Field-Effect Transistors Shujun Zhou,1 Qingxin Tang,1* Hongkun Tian,2 Xiaoli Zhao,1 Yanhong Tong,1 Stephen Barlow,3 Seth R. Marder3* and Yichun Liu1* †

Center for Advanced Optoelectronic Functional Materials Research, and Key Lab of UV-

Emitting Materials and Technology of Ministry of Education, Northeast Normal University, 5268 Renmin Street, Changchun 130024, China ‡

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied

Chemistry, Chinese Academy of Sciences, Changchun 130022, China §

School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics,

Georgia Institute of Technology, Atlanta, Georgia 30332-0400, United States KEYWORDS: organic single crystal, field-effect transistor, mobility, dielectric, surface energy, polar component, dispersive component.

ABSTRACT: The understanding of the characteristics of gate dielectric that leads to optimized carrier transport remains controversial, and the conventional studies applied organic semiconductor thin films, which introduces the effect of dielectric on growth of the deposited semiconductor thin films and hence only can explore the indirect effects. Here, we introduce pregrown organic single crystals to eliminate the indirect effect (semiconductor growth) in the conventional studies, and to undertake an investigation of direct effect of dielectric on carrier

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transport. It is shown that the matched polar and dispersive components of surface energy between semiconductor and dielectric are favorable for higher mobility. This new empirical finding may provide the direct relation between dielectric and carrier transport for optimized mobility of OFETs, and hence shows a promising potential for the development of nextgeneration high-performance organic electronic devices.

1. INTRODUCTION The tremendous opportunities afforded by flexible and stretchable electronics, from conformal displays to wearable health monitoring devices, have stimulated substantial research effort to develop high-mobility semiconductors for organic field-effect transistors (OFETs).1-6 In addition to the intrinsic properties of the semiconductor, the gate dielectric plays a critical role in determining the OFET mobility.7-10 Despite this, the effect of the gate dielectric material on carrier transport cannot in general be reliably predicted, and thus robust guidelines for dielectric selection leading to optimized device performance remain elusive. Most studies to date exploring the influence of the gate dielectric on transport in OFETs have focused on organic semiconductor thin films grown directly on the dielectrics.10-13 Inevitably, the dielectric affects the growth of the deposited semiconductor thin film, and thus characteristics of the grown thin films, such as crystallinity, grain size, grain boundaries, and packing orientation, further affect the carrier transport.14-16 Some researchers have suggested that the surface roughness of the dielectric determines the morphology and crystalline structure of the deposited semiconductor thin film, and, therefore, affects the carrier mobility,17-20 whereas others have observed that nonpolar dielectric surfaces led to the growth of higher-quality higher-mobility semiconductor thin films.20-22 Generally the dielectrics with a lower total surface energy (γtot) tend to result in


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semiconductor thin films with smaller grain sizes; however, some studies have found higher mobility for smaller grains,23-31 whereas another have found higher mobility for the larger grains grown on the dielectric surface with higher γtot.32 Other investigations have concluded that larger grain sizes, a more perfect first monolayer of organic semiconductor thin film, with higher mobility are preferentially grown when γtot of the dielectric is matched to that of the semiconductor.33-37 Based on these facts, the previously reported dielectric effect on carrier transport has been mainly ascribed to the growth of semiconductor thin films, i.e., the effect of the organic semiconductor thin film on the carrier transport. In contrast, transferring the pre-grown organic single crystals onto the target dielectrics rather than directly growing organic semiconductor thin films on the dielectrics, can effectively separate the direct effect of dielectric properties on carrier transport from the effect of organic semiconductor growth on carrier transport. Currently, only a few reports have investigated the dielectric effect based on the pre-grown organic single crystal. Among them, the similar conclusions to those based on organic thin films, for example, nonpolar dielectric for higher mobility,38 higher and lower dielectric constant respectively for higher mobility,39-41 have been obtained. Further studies are requisite to further address the direct effect of the dielectric material on mobility change with dielectric properties. The lack of an understanding of the direct effect of the dielectric properties on charge-carrier mobility, together with the lack of consistent findings in studies of its indirect influence, and the general absence of clear guidelines for the selection of dielectrics prompt us to undertake further investigation of the direct effect of characteristics of the gate dielectric on the carrier transport. Here, we use pre-grown dinaphtho[3,4-d:3',4'-d']benzo[1,2-b:4,5-b'] dithiophene (Ph5T2) single crystals to show that closely matching both polar (γp) and dispersive (γd) components of the


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surface energy of the dielectric with those of the semiconductor surface energy are favorable for the high OFET mobility. This empirical finding is different from previous reports, and hence possibly gives a new perspective in the effect of dielectric on carrier transport. 2. Experiment section 2.1 Materials. Poly(methyl methacrylate) (PMMA, Mw = 550 kDa, Alfa Aesar), polystyrene (PS, Mw = 280 kDa, Sigma-Aldrich), divinyltetramethyldisiloxane-bis(benzocyclobutene) (BCB, 46%, Dow Chemicals), (γ-mercaptopropyl)trimethoxysilane (MPT, 95%, Sigma-Aldrich), octadecyltrichlorosilane (OTS, 95%, Alfa Aesar), para-sexiphenyl (p-6P, 95%, Aladdin), were used

without

further

purification.

Dinaphtho[3,4-d:3',4'-d']benzo[1,2-b:4,5-b']dithiophene

(Ph5T2) and 2,7-bis(4-biphenylyl)phenanthrene (BPPh) were synthesized according to the previous reports.42,43 Pentacene (99.99%, Sigma-Aldrich) and zinc phthalocyanine (ZnPc, 97%, Sigma-Aldrich) were used with further purification. 2.2 Single Crystal Growth. Physical vapor transport was used to grow single crystals of Ph5T2, pentacene and ZnPc in a horizontal tube furnace. Ph5T2, pentacene and ZnPc powder were vaporized at 240 oC for 10 min, 310 oC for 1 h, and 290 oC for 3 h, respectively. The chamber pressure kept at 25 Pa for Ph5T2, an atmosphere for pentacene, and 30 Pa for ZnPc during the whole growth process, respectively. 2.3 Dielectric Fabrication. PMMA (60 mg mL-1) and PS (50 mg mL-1) were respectively dissolved in anisole and toluene, and then deposited by spin-coating on Si substrates. They were spin coated at 4000 rpm for 40 s followed by thermal annealing at 100 oC for 5 min. Silicon dioxide (SiO2, Ci = 10 nF cm-2) were used as the substrates. The substrates were cleaned by etching with piranha (7: 3 H2SO4: H2O2 by volume) at room temperature for 30 min, followed


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thoroughly washed with deionized water several times. The SiO2 substrates were exposed to MPT (30 µL) under vacuum for 20 min at room temperature. The OTS modification was carried out by dipping the SiO2 substrates into a solution of OTS: n-heptane (1:1000 by volume) for 3 h.44 For the BCB treatment,45 BCB thin-film was spin-coated from a mesitylene (Fluka) solution (VBCB/Vmesitylene = 1:30) and thermally cross-linked on a hotplate at 290 oC. The p-6P and BPPh were deposited on the cleaned SiO2 by vacuum evaporation. During the deposition, the pressure was at 10-4-10-5 Pa, the deposition rate was at 0.16 Ås-1, and deposition thickness of p-6P and BPPh of 5 and 25 nm were deposited, with substrate temperature were kept at 80 and 100 oC, respectively. 2.4 Semiconductor and Dielectric Characterization. The morphology of single crystals and devices were obtained with a field-emission scanning electron microscopy (SEM). The surface morphology of semiconductors and dielectrics were measured by atomic force microscopy (AFM). AFM images were obtained in ScanAsyst mode using a Dimension Icon produced by Bruker. Out-of-plane and in-plane X-ray diffraction (XRD) were measured on the Bruker D8 Discover thin-film diffractometer and Rigaku Smart Lab with CuKα radiation (λ = 1.54056 Å), respectively. Contact angles were measured by Drop Shape Analyzer and analyzed

by Drop shape Analysis. The polar component ( ) and dispersive component ( ) of surface energy were calculated to solve two simultaneous equations, from the sum of these components, the total surface energy (γ) was calculated.46 1 + cos =

/ ( )/ ( )

+

( )/ ( )/

(1)

Typical liquids DI water and diiodomethane (DIM) were used as test liquids.


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2.5 Device Fabrication and Characterization. The Ph5T2 single-crystal OFETs were fabricated on 8 kinds of dielectrics (SiO2, MPT/SiO2, PMMA, BCB/SiO2, OTS/SiO2, PS, p6P/SiO2 and BPPh/SiO2) by the “gold film stamping” method.47 The fabrication process of single-crystal device is that the pre-grown Ph5T2 single crystal was mechanically transferred onto the dielectric with a probe, and then two pieces of gold films were transferred onto the two sides of the single crystal as source and drain electrodes. The pentacene and ZnPc single-crystal devices were fabricated on SiO2, OTS/SiO2, and p-6P/SiO2 dielectrics using the same technique. The charge carrier mobility was calculated in the saturation regime using the following equation:7

= (

!

−

#)

(2)

where ISD is the source-drain current, W and L are respectively the width and length of channel, µ is the mobility, Ci is the capacitance per unit area of the dielectric, VG and VT are the gate voltage and threshold voltage, respectively. 3. RESULTS AND DISCUSSION 3.1 OFET Fabrication and Characterization. Ph5T2 single crystals were selected as the semiconductor material for our initial investigation due to the thin thickness, which favors enhanced carrier injection,48 hence weakening the effect of the vertical bulk resistance on the device performance,49 and highlighting the intrinsic effects of the dielectric on the device performance. Typical SEM images of the Ph5T2 single crystals show regular rhombic shapes (Supporting Information, Figures S1a,b). Optical microscope images and polarized optical microscope images (Supporting Information, Figures S1d,e), combined with our previously reported XRD results, and selected area electron diffraction pattern of the Ph5T2 microplates,48


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confirm the single crystal nature of Ph5T2 microplates. As shown in Figure 1a, Ph5T2 singlecrystal OFETs were fabricated by mechanically transferring the single crystal onto the dielectric with a probe. And then the 100 nm-thick thin Au films were glued onto the Ph5T2 microplates by van der Waals forces with the help of the mechanical probes and functioned as source/drain electrodes (Supporting Information, Figures S1c,f). Until now, the similar transferring methods have been extensively reported to fabricate high-mobility single-crystal OFETs by a few leading research groups.50-54 The achievement of high mobility suggests the weak mechanical damage in transferring process of crystals. Our recent investigation has shown that the possible mechanical damage via transferring process is negligible.55 Eight dielectrics or dielectric surface modification materials were used: silicon dioxide (SiO2), (γ-mercaptopropyl) trimethoxysilane (MPT), poly(methyl methacrylate) (PMMA), divinyltetramethyldisiloxane-bis(benzocyclobutene) (BCB), octadecyltrichlorosilane (OTS), polystyrene (PS), para-sexiphenyl (p-6P), and 2,7-bis(4biphenylyl)-phenanthrene (BPPh). Their corresponding molecular structures are shown in Figure 1b. The crystals in all devices present the uniform color under the observation of optical microscopy (Supporting Information, Figure S1f), ensuring the good contact between crystal and dielectric. Figure 1c presents typical transfer curves for Ph5T2 single-crystal OFETs on 8 kinds of dielectrics. The multi-measured results show the good gate-bias stability of Ph5T2 single-crystal organic OFETs on eight kinds of dielectrics (Supporting Information, Figures S2). The obvious linear region at low source-drain bias of typical output curve suggests good electrode contact (Supporting Information, Figure S3). The mobilities were calculated from the saturation regime of transfer curves based on the measured capacitance values (Supporting Information, Figure S4). In order to eliminate the influence of crystal thickness and anisotropy on experimental results, we chose crystals with similar color,48 and applied statistical methods to


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obtain results from dozens of devices (Table 1). Both the average and the highest mobility values show the same trends with dielectric. Next, we applied a general method in this field to see what determines the mobility difference. The extracted mobilities from transfer curves in different dielectrics were first compared with the extensively reported possible effect factors, like surface roughness, surface polarity, and surface energy. 3.2 Effect of Surface Roughness. The surface roughness of the dielectric has been shown to be one of important parameters that affects the field-effect performance in thin-film OFETs.1013,17-20

The surface roughness of dielectric affects the growth of the semiconductor thin film, and,

thus, potentially also the device performance. Several previous studies of thin-film OFETs found higher mobility on dielectric surfaces with lower root mean square surface roughness (Rrms); this effect has been attributed to enhanced molecular order, decreased charge traps, and fewer nucleation sites leading to increased grain size and fewer grain boundaries in the semiconductor thin film.18-20 However, Bao et al. have reported higher mobility for pentacene thin-film OFETs on rougher dielectrics (3.4 cm2V-1s-1 on a 0.5 nm Rrms dielectric, compared with 0.5 cm2V-1s-1 on a 0.1 nm Rrms dielectric).17 Some other reports have found surface roughness of the dielectric to have little influence on the mobility in thin-film OFETs.17,23,25,29 These contradictory findings make it difficult to understand the relation between the surface roughness of the dielectric and mobility. Here, different from the previous conventional reports, we applied the pre-grown organic crystals that were transferred on the dielectrics, rather than directly grew the organic thin film on the dielectrics. To investigate what, if any, influence the gate-dielectric surface roughness has on the mobility of our single-crystal devices, we conducted AFM measurements on the 8 kinds of dielectrics (Figure 2). In our experiments, the mobility of Ph5T2 single-crystal OFETs (Table 1)


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shows no obvious correlation with the surface roughness of the dielectric as determined by AFM (Figure 2). For example, the Rrms values of PMMA and PS surfaces are identical (0.32 nm), while the corresponding mobilities differ significantly (0.31 and 0.51 cm2V-1s-1, respectively). Furthermore, the Rrms values of p-6P/SiO2 and BPPh/SiO2 (0.99 and 1.09 nm, respectively) are higher than for the other dielectric layers, but so are the corresponding mobility values (1.47 and 1.87 cm2V-1s-1, respectively). 3.3 Effect of Surface Polarity. Previous reports have shown that the polarity of the dielectric can significantly affect the electrical performance in thin film and single crystal OFETs.20-22,38,41 The nonpolar dielectrics have been found to be favorable for growth of the high-quality thin films exhibiting higher mobility values.20-22 For example, Wünsche et al. found that the nonpolar PS/SiO2 surface is favorable for the formation of the flat and well coalesced island-like tetracene film compared with other polar dielectrics such as PMMA/SiO2, HMDS/SiO2, and PARYC/SiO2.21 In addition, Gomez et al. have addressed that the polar groups of the dielectric near the surface can cause energetic disorder at interface which will contribute to charge traps.41 Although many previous reports have come to the conclusion that the mobility on nonpolar dielectric layer is higher than that of polar, until now it is challenging to further understand the mobility difference on dielectrics with similar polarity. In our experiments, the nonpolar dielectrics (OTS/SiO2, PS, p-6P/SiO2, and BPPh/SiO2) give higher mobility than the polar ones (SiO2, MPT/SiO2, PMMA) and weak polar one (BCB/SiO2) in Ph5T2 single-crystal OFETs (Table 1), in good agreement with the previous general investigation on polarity effect.20-22,38,41 However, the mobility shows obvious differences on dielectrics with similar polarity, for example, mobility on nonpolar p-6P/SiO2 dielectric is much higher than that on the nonpolar OTS/SiO2 (1.47 and 0.39 cm2V-1s-1, respectively). This


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phenomenon has been extensively found in previous reports and cannot be explained by the polarity effect. Therefore, further efforts should be tried to explore other possible factors. 3.4 Effect of Total Surface Energy. The total surface energy (γtot) has also been considered to be a crucial parameter that affects the mobility in thin-film OFETs.23-37 According to previous reports, dielectrics with lower γtot tend to result in higher mobility than those with higher γtot.23-31 Some researchers attributed the increase of the mobility to the smaller grain sizes within the compact semiconductor thin film grown on lower-γtot dielectric. For instance, Yang et al. have studied pentacene thin-film OFETs fabricated on poly(imide-siloxane) dielectrics. They found that smaller grains were obtained with a lower-γtot dielectric, on which the higher performance was obtained. They ascribed the higher performance to the enhanced interconnectivity between small pentacene grains on the lower-γtot dielectric.23 However, Bae and coworkers have found higher mobility on the higher-γtot dielectric,32 where larger pentacene grains were obtained. They suggested that carriers in the pentacene thin-film OFFTs can move more freely due to the low grain-boundary density. In addition, some groups have reached a different conclusion, i.e., that matching γtot of the dielectric to that of the semiconductor is favorable for the improved mobility.33-37 For example, Chou and coworkers have found that the pentacene thin films have similar morphology on both the photosensitive polyimide (PSPI) and the SiO2 dielectric, whereas the transistor with the two dielectric showed different mobility (2.05 and 0.11 cm2V-1s-1, respectively).33 They ascribed the improved mobility to matching of γtot of the PSPI (38.2 mJ m-2) to that of pentacene (38 mJ m-2). These inconsistent reports show the relation between the total surface energy of dielectric and carrier transport is still unclear. In addition, as with many attempts to correlate mobility with other dielectric characteristics, these studies mainly focused on the indirect effects on mobility through semiconductor thin film growth.


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In our experiments, to investigate the direct relationship between the mobility and total surface energy of the dielectric, we calculated the surface energy by measuring the contact angles of two liquids of different polarity (deionized water and diiodomethane) on dielectrics (Supporting Information, Figure S5). Table 1 gives the total surface energy γtot. The surface energy of the Ph5T2 material was determined by a series of optimization process (Supporting Information, Figures S6-S8 and Table S1). The corresponding mobility values are also summarized. As shown in Table 1, the highest mobility of Ph5T2 single-crystal OFETs is obtained on the BPPh/SiO2, γtot of which is similar to that of Ph5T2 (37.65 mJ m-2), in good agreement with the previously reported studies of organic thin-film OFETs.33-37 However, BCB/SiO2 and p-6P/SiO2 also have similar γtot (37.47 and 38.72 mJ m-2, respectively), but lead to significantly different mobility of single-crystal OFETs (0.36 and 1.47 cm2V-1s-1, respectively). Figure S9 (Supporting Information) clearly shows that the relationship between the total surface energy of dielectrics and the mobility of Ph5T2 single-crystal devices is not regular. Therefore, our experimental results show that the total surface energy of the dielectric is not the decisive factor affecting the mobility in Ph5T2 single-crystal OFETs. The extra results for p-6P and BPPh are shown in Figures S10-S13 (Supporting Information) to exclude the other possible effects. 3.5 Effect of polar and dispersive components of surface energy. Interestingly, by comparing the measured surface energy and the measured mobility for different dielectrics, our experiment results show a novel discovery where the matched polar and dispersive components of the dielectric surface energy to that of semiconductor correspond to the highest mobility of single-crystal OFETs. As we know, few literature has reported this experiment result. Table 1 gives the total surface energy γtot and its polar and dispersive components (γp, γd), for the dielectrics and Ph5T2. The relation of the mobility and the surface energy (γtot, γp, γd) is further


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presented in Figure 3. For polar dielectrics like SiO2, MPT/SiO2, PMMA, and weakly polar BCB/SiO2, their γd components of surface energy are close to the corresponding value of Ph5T2. In this case, the dielectric with more matched γp component of surface energy shows the higher mobility. For nonpolar dielectrics like OTS/SiO2, PS, p-6P/SiO2, and BPPh/SiO2, their γp components of surface energy are close to the corresponding value of Ph5T2. In this case, the dielectric with more matched γd component of surface energy shows the higher mobility. That is to say, the matched polar and dispersive components of surface energy between semiconductor and dielectric are favorable for high mobility. Therefore, our experimental results show that the polar and dispersive components jointly affect the mobility in Ph5T2 single-crystal OFETs. Here, to understand the effect of the surface energy on carrier transport in the organic semiconductor, the interfacial tension between two adjacent substances is given as: $ =

%&$

−

& '

+

%&$

−

& '

(3)

where γ12 is the interfacial tension, $ and are the polar components of two substances, $ and are the dispersive components of two substances, respectively.

46,56,57

Obviously the interfacial

tension depends on the difference of the polar and dispersive values of the two substances. According to Equation (3), the smaller the difference of the polar and dispersive components between the two contacted substances is, the smaller the interfacial tension between them will be. The presence of the interfacial tension between semiconductor and dielectric may introduce the extra interfacial traps, which can effectively decrease the system energy. Here, more matched surface energy components reflect smaller interfacial tension. Therefore, highest mobility is presented in organic single-crystal devices with the best matched polar and dispersive components of surface energy between semiconductor and dielectric.


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Further, the trap-related electrical properties were characterized by the forward/reverse sweeping transfer curves and the temperature-dependence mobility. The obvious hysteresis behavior for the devices on the different dielectrics shows the presence of traps (Supporting Information, Figure S14), and the increased mobility with the increased temperature in the range of 100-290 K shows that the transport in our device is dominated by the multiple trapping and releasing of carriers by shallow traps (Supporting Information, Figure S15). Podzorov et al has proposed that the threshold voltage was affected by deep traps while the mobility is related to shallow traps.58 Here, since we focus on the dielectric effect on the mobility, the density of shallow trap states N is further characterized by the sub-threshold slope SS with the equation as follows:59-61 (( =

)* +

ln10 /1 +

+0 1

2

(4)

where k is Boltzmann’s constant, T is temperature, q is electronic charge, and C is capacitance. According to the extensively accepted theory on the carrier transport of organic semiconductor, the effective mobility is related to the shallow traps at the interface between semiconductor and dielectric.62-64 Based on the measured transfer curves with 8 kinds of dielectric layers, the average value of shallow trap density is extracted from the statistical results and shown in Figure 4. From comparison, the mobility values are also shown here. The matched surface energy components between semiconductor and dielectric are favorable for the decreased average value of shallow trap density, resulting in the increased mobility. 3.6 Generalization to other semiconductors. To test the generality of our findings with Ph5T2 described above, two other organic semiconductor materials, pentacene and ZnPc, were used to fabricate single-crystal OFETs on three dielectrics (SiO2, OTS/SiO2 and p-6P/SiO2) with different γp, γd and γtot. The molecular structures of pentacene and ZnPc, and typical transfer


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curves on three different dielectrics are shown in Figure 5, along with SEM images of typical single-crystal OFETs. The values of the polar and dispersive components of the surface energy of pentacene (Supporting Information, $ = 0.06 mJ m-2, $ = 34.55 mJ m-2, see Figure S16 and Table S2) best match those of the p-6P/SiO2 dielectric (γp = 0.29 mJ m-2, γd = 38.43 mJ m-2), while those of ZnPc (Supporting Information, = 0.33 mJ m-2, = 20.55 mJ m-2, see Figures S17-S19 and Table S3) are closely approach those of the OTS/SiO2 dielectric (γp = 0.40 mJ m-2, γd = 17.99 mJ m-2). Fully consistent with our hypothesis and with our observations for Ph5T2, the highest mobilities for pentacene and ZnPc single-crystal OFETs are indeed obtained on p6P/SiO2 and OTS/SiO2, respectively (Figures. 5c,f, see Tables S4,S5, Supporting Information). Further, it is found that our hypothesis is also consistent with some previous reported results on thin-film OFETs.23,24,28,31-33,65-68 For example, Bae and coworkers obtained similar carrier mobility (1.09 and 1.11 cm2V-1s-1) for pentacene grown on MTF0 and MTF1 dielectrics, for which the grain sizes were very different, and much lower mobilities (0.23 and 0.12 cm2V-1s-1) on MTF5 and MTF10.31 The similar mobility seen for the first two dielectrics was attributed a trade-off between low-density of grain boundaries of large-size grains for MTF0 and more uniformity and better connection of small-size grains for MTF1. However, the findings can be explained using our model: values of γp of pentacene, MTF0, and MTF1 are 3.0, 6.8 and 2.7 mJ m-2, and those for γd are 35.3, 36.7 and 22.9 mJ m-2, respectively.32 Thus, γp of MTF1 and γd of MTF0 is similar to that of pentacene, respectively, whereas for MTF5 and MTF10, neither component matches well with that for pentacene. In addition, as we mentioned above, the polarity is an extensively considered factor for mobility. It has been reported that the polar component value of the surface energy of the dielectrics suggests the surface polarity strength of the dielectric.21,69 On the other hand, the current studies on dielectric effect generally apply the


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nonpolar semiconductors like pentacene,14,15,17-20,22-29,32,33,35-37 tetracene,16,21 and DNTT.30,31 Therefore, the matched polar surface energy component of dielectric to semiconductor for high mobility in our results also means the low polar component of the dielectric for high mobility, in good agreement with the previous reports.20-22,38,41 However, there are still some challenges, for example, interfacial defect produce process under surface tension and possible local polarization, that need to be further explored. 4. CONCLUSION In conclusion, using pre-grown organic single crystals as the semiconductor active layer, we eliminate the indirect influence of the dielectric on carrier transport via semiconductor structure and morphology, and present its direct influence on carrier transport in OFETs. The possible effect factors such as surface roughness, total surface energy, surface polarity, were first discussed and excluded. It is experimentally found that matching both the polar and dispersive components of the dielectric surface energy to the corresponding values of the semiconductor is favorable to obtain the high mobility, which may effectively decrease the interfacial shallow traps. This matching may also play a significant role in affecting charge-carrier mobilities in OFETs. Our findings may provide a new consideration for the design of gate dielectric and fabrication of high-performance organic electronic devices.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI:


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Results of transfer curves, output curves and hysteresis loops measurements; the relationship between total surface energy and mobility; the temperature-dependence mobility; the results of SEM, optical microscopes, contact angle, XRD and AFM (PDF) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Q.T.). *E-mail: [email protected] (S.R.M.). *E-mail: [email protected] (Y.L.). ACKNOWLEDGEMENT This work is supported by NSFC (51322305, 61574032, 91233204), 111 Project (B13013). The authors thank Dr. Keqiang He for out-of-plane and in-plane XRD measurements, and thank Prof. Haiyang Xu for temperature-dependence mobility experiments. S.R.M. thanks Georgia Power for their generous support of this work. REFERENCES (1)

Kim, D.-H.; Ghaffari, R.; Lu, N.; Rogers, J. A. Flexible and Stretchable Electronics for Biointegrated Devices. Annu. Rev. Biomed. Eng. 2012, 14, 113-128.

(2)

Salvatore, G. A.; Münzenrieder, N.; Kinkeldei, T.; Petti, L.; Zysset, C.; Strebel, I.; Büthe, L.; Tröster, G. Wafer-Scale Design of Lightweight and Transparent Electronics that Wraps Around Hairs. Nat. Commun. 2014, 5, 2982.

(3)

Takei, K.; Honda, W.; Harada, S.; Arie, T.; Akita, S. Toward Flexible and Wearable Human-Interactive Health-Monitoring Devices. Adv. Healthcare Mater. 2015, 4, 487-500.


16

Page 17 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(4)

Yao, Y.; Dong, H.; Hu, W. Charge Transport in Organic and Polymeric Semiconductors for Flexible and Stretchable Devices. Adv. Mater. 2016, 28, 4513-4523.

(5)

Yang, J.; Wang, H.; Chen, J.; Huang, J.; Jiang, Y.; Zhang, J.; Shi, L.; Sun, Y.; Wei, Z.; Yu, G.; Guo, Y.; Wang, S.; Liu, Y. Bis-Diketopyrrolopyrrole Moiety as a Promising Building Block to Enable Balanced Ambipolar Polymers for Flexible Transistors. Adv. Mater. 2017, 29, 1606162.

(6)

Fei, Z.; Han, Y.; Gann, E.; Hodsden, T.; Chesman, A. S. R.; McNeill, C. R.; Anthopoulos, T. D.; Heeney, M. Alkylated Selenophene-Based Ladder-Type Monomers via a Facile Route for High-Performance Thin-Film Transistor Applications. J. Am. Chem. Soc. 2017, 139, 8552-8561.

(7)

Horowitz, G. Organic Field-Effect Transistors. Adv. Mater. 1998, 10, 365-377.

(8)

Veres, J.; Ogier, S. D.; Leeming, S. W.; Cupertino, D. C.; Khaffaf, S. M. Low-k insulators as the choice of dielectrics in organic field-effect transistors. Adv. Funct. Mater. 2003, 13, 199-204.

(9)

Di, C.-a.; Liu, Y.; Yu, G.; Zhu, D. Interface Engineering: An Effective Approach toward High-Performance Organic Field-Effect Transistors. Acc. Chem. Res. 2009, 42, 15731583.

(10)

Park, Y. D.; Lim, J. A.; Lee, H. S.; Cho, K. Interface Engineering in Organic Transistors. Mater. Today 2007, 10, 46-54.

(11)

Sun, X.; Di, C.-a.; Liu, Y. Engineering of the Dielectric–Semiconductor Interface in Organic Field-Effect Transistors. J. Mater. Chem. 2010, 20, 2599-2611.

(12)

Shao, W.; Dong, H.; Jiang, L.; Hu, W. Morphology Control f or High Performance Organic Thin Film Transistors. Chem. Sci. 2011, 2, 590-600.


17


(13)

Page 18 of 32

Dong, H.; Jiang, L.; Hu, W.; Interface Engineering for High-Performance Organic FieldEffect Transistors. Phys. Chem. Chem. Phys. 2012, 14, 14165-14180.

(14)

Shtein, M.; Mapel, J.; Benziger, J. B.; Forrest, S. R. Effects of Film Morphology and Gate Dielectric Surface Preparation on the Electrical Characteristics of Organic-VaporPhase-Deposited Pentacene Thin-Flm Transistors. Appl. Phys. Lett. 2002, 81, 268-270.

(15)

Virkar, A. A.; Mannsfeld, S.; Bao, Z.; Stingelin, N. Organic Semiconductor Growth and Morphology Considerations for Organic Thin-Film Transistors. Adv. Mater. 2010, 22, 3857-3875.

(16)

Cicoira, F.; Santato, C.; Dinelli, F.; Murgia, M.; Loi, M. A.; Biscarini, F.; Zamboni, R.; Heremans, P.; Muccini, M. Correlation Between Morphology and Field-Effect-Transistor Mobility in Tetracene Thin Films. Adv. Funct. Mater. 2005, 15, 375-380.

(17)

Yang, H.; Shin, T. J.; Ling, M.-M.; Cho, K.; Ryu, C. Y.; Bao, Z. Conducting AFM and 2D GIXD Studies on Pentacene Thin Films. J. Am. Chem. Soc. 2005, 127, 11542-11543.

(18)

Steudel, S.; Vusser, S. D.; Jonge, S. D.; Janssen, D.; Verlaak, S.; Genoe, J.; Heremans, P. Influence of the Dielectric Roughness on the Performance of Pentacene Transistors. Appl. Phys. Lett. 2004, 85, 4400-4402.

(19)

Knipp, D.; Street, R. A.; Völkel, A. R. Morphology and Electronic Transport of Polycrystalline Pentacene Thin-Film Transistors. Appl. Phys. Lett. 2003, 82, 3907-3909.

(20)

Shin, K.; Yang, S. Y.; Yang, C.; Jeon, H.; Park, C. E. Effects of Polar Functional Groups and Roughness Topography of Polymer Gate Dielectric Layers on Pentacene Field-Effect Transistors. Org. Electron. 2007, 8, 336-342.

(21)

Wünsche, J.; Tarabella, G.; Bertolazzi, S.; Bocoum, M.; Coppede, N.; Barba, L.; Arrighetti, G.; Lutterotti, L.; Iannotta, S.; Cicoirae, F.; Santato, C. The Correlation


18

Page 19 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Between Gate Dielectric, Film Growth, and Charge Transport in Organic Thin Film Transistors: the Case of Vacuum-Sublimed Tetracene Thin Films. J. Mater. Chem. C 2013, 1, 967-976. (22)

Lu, Y.; Lee, W. H.; Lee, H. S.; Jang, Y.; Cho, K. Low-Voltage Organic Transistors with Titanium Oxide/Polystyrene Bilayer Dielectrics. Appl. Phys. Lett. 2009, 94, 113303.

(23)

Yang, S. Y.; Shin, K.; Park, C. E. The Effect of Gate-Dielectric Surface Energy on Pentacene Morphology and Organic Field-Effect Transistor Characteristics. Adv. Funct. Mater. 2005, 15, 1806-1814.

(24)

Lim, S. C.; Kim, S. H.; Lee, J. H.; Kim, M. K.; Kim, D. J.; Zyung, T. Surface-Treatment Effects on Organic Thin-Film Transistors. Synth. Met. 2005, 148, 75-79.

(25)

Yang, H.; Kim, S. H.; Yang, L.; Yang, S. Y.; Park, C. E. Pentacene Nanostructures on Surface-Hydrophobicity-Controlled Polymer/SiO2 Bilayer Gate-Dielectrics. Adv. Mater. 2007, 19, 2868-2872.

(26)

Miskiewicz, P.; Kotarba, S.; Jung, J.; Marszalek, T.; Mas-Torrent, M.; Gomar-Nadal, E.; Amabilino, D. B.; Rovira, C.; Veciana, J.; Maniukiewicz, W.; Ulanski, J. Influence of Surface Energy on the Performance of Organic Field Effect Transistors based on Highly Oriented, Zone-Cast Layers of a Tetrathiafulvalene Derivative. J. Appl. Phys. 2008, 104, 054509.

(27)

Umeda, T.; Kumaki, D.; Tokito, S. Surface-Energy-Dependent Field-Effect Mobilities up to 1 cm2/Vs for Polymer Thin-Film Transistor. J. Appl. Phys. 2009, 105, 024516.

(28)

Nayak, P. K.; Kim, J.; Cho, J.; Lee, C.; Hong, Y. Effect of Cadmium Arachidate Layers on the Growth of Pentacene and the Performance of Pentacene-Based Thin Film Transistors. Langmuir 2009, 25, 6565-6569.


19


(29)

Page 20 of 32

He, W.; Xu, W.; Peng, Q.; Liu, C.; Zhou, G.; Wu, S.; Zeng, M.; Zhang, Z.; Gao, J.; Gao, X.; Lu, X.; Liu, J.-M. Surface Modification on Solution Processable ZrO2 High-k Dielectrics for Low Voltage Operations of Organic Thin Film Transistors. J. Phys. Chem. C 2016, 120, 9949-9957.

(30)

Yoo, S.; Yi, M. H.; Kim, Y. H.; Jang, K.-S. One-Pot Surface Modification of Poly(ethylene-alt-maleic anhydride) Gate Insulators for Low-Voltage DNTT Thin-Film Transistors. Org. Electron. 2016, 33, 263-268.

(31)

Prisawong, P.; Zalar, P.; Reuveny, A.; Matsuhisa, N.; Lee, W.; Yokota, T.; Someya, T. Vacuum Ultraviolet Treatment of Self-Assembled Monolayers: A Tool for Understanding Growth and Tuning Charge Transport in Organic Field-Effect Transistors. Adv. Mater. 2016, 28, 2049-2054.

(32)

Kwak, S.-Y.; Choi, C. G.; Bae, B.-S. Effect of Surface Energy on Pentacene Growth and Characteristics of Organic Thin-Film Transistors. Electrochem. Solid-State Lett. 2009, 12, G37- G39.

(33)

Chou, W-Y.; Kuo, C.-W.; Cheng, H.-L.; Chen, Y.-R.; Tang, F.-C.; Yang, F.-Y.; Shu, D.Y.; Liao, C.-C. Effect of Surface Free Energy in Gate Dielectric in Pentacene Thin-Film Transistors. Appl. Phys. Lett. 2006, 89, 112126.

(34)

Gao, J.; Asadi, K.; Xu, J. B.; An, J. Controlling of the Surface Energy of the Gate Dielectric in Organic Field-Effect Transistors by Polymer blend. Appl. Phys. Lett. 2009, 94, 093302.

(35)

Chou, W.-Y.; Kuo, C.-W.; Chang, C.-W.; Yeh, B.-L.; Chang, M.-H. Tuning Surface Properties in Photosensitive Polyimide. Material Design for High Performance Organic Thin-Film Transistors. J. Mater. Chem. 2010, 20, 5474-5480.


20

Page 21 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(36)

Wei, C.-Y.; Kuo, S.-H.; Hung, Y.-M.; Huang, W.-C.; Adriyanto, F.; Wang, Y.-H. HighMobility Pentacene-Based Thin-Film Transistors With a Solution-Processed Barium Titanate Insulator. IEEE Electron Device Lett. 2011, 32, 90-92.

(37)

Liu, C.; Zhu, Q.; Jin, W.; Gu, W.; Wang, J. The Ultraviolet-Ozone Effects on Organic Thin-Film Transistors with Double Polymeric Dielectric Layers. Synth. Met. 2011, 161, 1635-1639.

(38)

Islam, M. M.; Pola, S.; Tao, Y.-T. Effect of Interfacial Structure on the Transistor Properties: Probing the Role of Surface Modification of Gate Dielectrics with SelfAssembled Monolayer Using Organic Single-Crystal Field-Effect Transistors. ACS Appl. Mater. Interfaces 2011, 3, 2136-2141.

(39)

Stassen, A. F.; De Boer, R. W. I.; Iosad, N. N.; Morpurgo, A. F. Influence of the Gate Dielectric on the Mobility of Rubrene Single-Crystal Field-Effect Transistors. Appl. Phys. Lett. 2004, 85, 3899-3901.

(40)

Hulea, I. N.; Fratini, S.; Xie, H.; Mulder, C. L.; Iossad, N. N.; Rastelli, G.; Ciuchi, S.; Morpurgo, A. F. Tunable Fröhlich Polarons in Organic Single-Crystal Transistors. Nat. Mater. 2006, 5, 982-986.

(41)

Adhikari, J. M.; Gadinski, M. R.; Li, Q.; Sun, K. G.; Reyes-Martinez, M. A.; Iagodkine, E.; Briseno, A. L.; Jackson, T. N.; Wang, Q.; Gomez, E. D. Controlling Chain Conformations of High-k Fluoropolymer Dielectrics to Enhance Charge Mobilities in Rubrene Single-Crystal Field-Effect Transistors. Adv. Mater. 2016, 28, 10095-10102.

(42)

Chen, Y.; Chang, H.; Tian, H.; Bao, C.; Li, W.; Yan, D.; Geng, Y.; Wang, F. An Easily Made Thienoacene Comprising Seven Fused Rings for Ambient-Stable Organic Thin Film Transistors. Org. Electron. 2012, 13, 3268-3275.


21


(43)

Page 22 of 32

Huang, L.; Liu, C.; Qiao, X.; Tian, H.; Geng, Y.; Yan, D. Tunable Field-Effect Mobility Utilizing Mixed Crystals of Organic Molecules. Adv. Mater. 2011, 23, 3455-3459.

(44)

Zhao, X.; Tong, Y.; Tang, Q.; Liu, Y. Wafer-Scale Coplanar Electrodes for 3D Conformal Organic Single-Crystal Circuits. Adv. Electron. Mater. 2015, 1, 1500239.

(45)

Chua, L.-L.; Zaumseil, J.; Chang, J.-F.; Ou, E. C.-W.; Ho, P. K.-H.; Sirringhaus, H.; Friend, R. H. General Observation of n-Type Field-Effect Behaviour in Organic Semiconductors. Nature 2005, 434, 194-199.

(46)

Owens, D. K.; Wendt, R. C. Estimation of the Surface Free Energy of Polymers. J. Appl. Polym. Sci. 1969, 13, 1741-1747.

(47)

Tang, Q.; Tong, Y.; Li, H.; Hu, W. Air/Vacuum Dielectric Organic Single Crystalline Transistors of Copperhexadecafluorophthalocyanine Ribbons.Appl. Phys. Lett. 2008, 92, 083309.

(48)

Zhao, X.; Pei, T.; Cai, B.; Zhou, S.; Tang, Q.; Tong, Y.; Tian, H.; Geng, Y.; Liu, Y. High ON/OFF Ratio Single Crystal Transistors based on Ultrathin Thienoacene Microplates. J. Mater. Chem. C 2014, 2, 5382-5388.

(49)

Zhang, Y.; Dong, H.; Tang, Q.; He, Y.; Hu, W. Mobility Dependence on the Conducting Channel Dimension of Organic Field-Effect Transistors based on Single-Crystalline Nanoribbons. J. Mater. Chem. 2010, 20,7029-7033.

(50)

Tang, Q.; Tong, Y.; Li, H.; Ji, Z.; Li, L.; Hu, W.; Liu, Y.; Zhu, D. High-Performance AirStable Bipolar Field-Effect Transistors of Organic Single-Crystalline Ribbons with an Air-Gap Dielectric. Adv. Mater. 2008, 20, 1511-1515.

(51)

Jiang, L.; Dong, H.; Hu, W. Organic Single Crystal Field-Effect Transistors: Advances and Perspectives. J. Mater. Chem. 2010, 20, 4994-5007.


22

Page 23 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(52)

Zhang, S.; Guo, Y.; Zhang, Y.; Liu, R.; Li, Q.; Zhan, X.; Liu, Y.; Hu, W. Synthesis, SelfAssembly, and Solution-Processed Nanoribbon Field-Effect Transistor of a Fused-NineRing Thienoacene. Chem. Commun. 2010, 46, 2841-2843.

(53)

He, Y,; Dong, H.; Meng, Q.; Jiang, L.; Shao, W.; He, L.; Hu, W. Mica, a Potential TwoDimensional-Crystal Gate Insulator for Organic Field-Effect Transistors. Adv. Mater. 2011, 23, 5502-5507.

(54)

Lv, A.; Puniredd, S. R.; Zhang, J.; Li, Z.; Zhu, H.; Jiang, W.; Dong, H.; He, Y.; Jiang, L.; Li, Y.; Pisula, W.; Meng, Q.; Hu, W.; Wang, Z. High Mobility, Air Stable, Organic Single Crystal Transistors of an n-Type Diperylene Bisimide. Adv. Mater. 2012, 24, 2626-2630.

(55)

Zhao, X.; Ding, X.; Tang, Q.; Tong, Y.; Liu, Y.; Photolithography-Compatible Conformal Electrodes for High-Performance Bottom-Contact Organic Single-Crystal Transistor. J. Mater. Chem. C 2017, 5, 12699-12706.

(56)

Girifalco, L. A.; Good, R. J. A Theory for the Estimation of Surface and Interfacial Energies. I. Derivation and Application to Interfacial Tension. J. Phys. Chem. 1956, 61, 904-909.

(57)

Fowkes, F. M. Attractive Forces at Interfaces. Ind. Eng. Chem. 1964, 56, 40-52.

(58)

Podzorov, V.; Menard, E.; Borissov, A.; Kiryukhin, V.; Rogers, J. A.; Gershenson, M. E. Intrinsic Charge Transport on the Surface of Organic Semiconductors. Phys. Rev. Lett., 2004, 93, 086602.

(59)

Sze, S. M.; Ng, K. K. Physics of Semiconductor Devices, John Wiley & Sons, Inc., Hoboken, New Jersey, 2006, p, 314-316.

(60)

Klauk, H. Organic Thin-Film Transistors. Chem. Soc. Rev. 2010, 39, 2643-2666.


23


(61)

Page 24 of 32

Shaymurat, T.; Tang, Q.; Tong, Y.; Dong, L.; Liu, Y. Gas Dielectric Transistor of CuPc Single Crystalline Nanowire for SO2 Detection Down to Sub-ppm Levels at Room Temperature. Adv. Mater. 2013, 25, 2269-2273.

(62)

Zilker, S. J.; Detcheverry, C.; Cantatore, E.; de Leeuw, D. M. Bias Stress in Organic Thin-Film Transistors and Logic Gates. Appl. Phys. Lett. 2001, 79, 1124-1126.

(63)

Horowitz, G. Organic Thin Film Transistors: From Theory to Real Devices. J. Mater. Res. 2004, 19, 1946-1962.

(64)

Chen, Y.; Podzorov, V. Bias Stress Effect in “Air-Gap” Organic Field-Effect Transistors. Adv. Mater. 2012, 24, 2679-2684.

(65)

Hutchins, D. O.; Weidner, T.; Baio, J.; Polishak, B.; Acton, O.; Cernetic, N.; Ma, H.; Jen, A. K.-Y. Effects of Self-Assembled Monolayer Structural Order, Surface Homogeneity and Surface Energy on Pentacene Morphology and Thin Film Transistor Device Performance. J. Mater. Chem. C 2013, 1, 101-113.

(66)

Ding, Z.; Abbas, G. A.; Assender, H. E.; Morrison, J. J.; Sanchez-Romaguera, V.; Yeates, S. G.; Taylor, D. M. Improving the Performance of Organic Thin Film Transistors Formed on a Vacuum Flashevaporated Acrylate Insulator. Appl. Phys. Lett. 2013, 103, 233301.

(67)

Kim, S. H.; Lee, J.; Park, N.; Min, H.; Park, H. W.; Kim, D. H.; Lee, H. S. Impact of Energetically Engineered Dielectrics on Charge Transport in Vacuum-Deposited Bis(triisopropylsilylethynyl)pentacene. J. Phys. Chem. C 2015, 119, 28819- 28827.

(68)

Kim, K.; Song, H. W.; Shin, K.; Kim, S. H.; Park, C. E. Photo-Cross-Linkable OrganicInorganic Hybrid Gate Dielectric for High Performance Organic Thin Film Transistors. J. Phys. Chem. C, 2016, 120, 5790-5796.


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(69)

Kim, J. S.; Friend, R. H.; Cacialli, F. Surface Wetting Properties of Treated Indium Tin Oxide Anodes for Polymer Light-Emitting Diodes. Synth. Met. 2000, 111, 369-372.


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Page 26 of 32

Figure 1. Ph5T2 single-crystal OFETs and characteristics on different dielectrics. a) Schematic of single-crystal device fabrication: The single crystal was mechanically transferred onto the dielectric with a probe, and then two pieces of gold films were transferred onto the two sides of the crystal as source and drain electrodes. b) Molecular structure of the dielectrics and dielectric modification materials: MPT, PMMA, BCB, OTS, PS, p-6P and BPPh. c) Typical transfer curves for Ph5T2 single-crystal devices on 8 kinds of dielectrics. The insets show the corresponding obtained highest mobility, VSD = - 30 V.


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Figure 2. AFM images (5 µm×5 µm) illustrating the surface morphologies of the dielectrics. a) SiO2. b) (γ-mercaptopropyl) trimethoxysilane (MPT) modified SiO2 (MPT/SiO2). c) poly(methyl methacrylate) (PMMA). d) divinyltetramethyldisiloxane-bis(benzocyclobutene) (BCB) modified SiO2 (BCB/SiO2). e) octadecyltrichlorosilane (OTS) modified SiO2 (OTS/SiO2). f) polystyrene (PS).

g)

para-sexiphenyl

(p-6P)

modified

SiO2

(p-6P/SiO2)

and

h)

2,7-bis(4-

biphenylyl)phenanthrene (BPPh) modified SiO2 (BPPh/SiO2). The corresponding surface roughness (Rrms) were also shown.


27


Page 28 of 32

Table 1. Surface energy of dielectrics and Ph5T2, and mobility of Ph5T2 single-crystal OFETs on the dielectrics. Contact angle (deg) avg. ± avg. dev DIM

γp (mJ m-2) avg. ± avg. dev

γd (mJ m-2) avg. ± avg. dev

γ (mJ m-2) avg. ± avg. dev

Materials DI Water

µ (cm2V-1s-1) avg. ± avg. dev

SiO2

50.94±0.87

40.04±0.48

17.60±0.42

39.59±0.24

57.19±0.62

(0.21) 0.13±0.04

MPT/SiO2

62.82±0.48

44.32±1.02

11.76±0.35

37.37±0.54

49.13±0.39

(0.37) 0.26±0.08

PMMA

73.92±0.27

35.92±0.38

5.25±0.09

41.60±0.18

46.85±0.22

(0.40) 0.31±0.08

BCB/SiO2

89.30±0.92

47.00±0.78

1.54±0.16

35.93±0.42

37.47±0.56

(0.49) 0.36±0.12

OTS/SiO2

109.38±0.67

79.04±0.29

0.40±0.07

17.99±0.15

18.39±0.16

(0.51) 0.39±0.09

PS

99.02±0.51

38.04±0.55

0.02±0.01

40.58±0.27

40.60±0.27

(0.61) 0.51±0.05

p-6P/SiO2

95.70±0.32

42.30±1.07

0.29±0.04

38.43±0.56

38.72±0.53

(1.65) 1.47±0.21

BPPh/SiO2

101.04±0.68

43.82±0.65

0.02±0.01

37.64±0.34

37.65±0.34

(2.15) 1.87±0.31

Ph5T2

101.74±0.09

40.28±0.86

0.00±0.01

37.65±0.27

37.65±0.27

——

Materials, dielectrics and semiconductor; deg., degree; DI Water, deionized water; DIM, diiodomethane. γp and γd are the polar and dispersive components of the surface energy; γtot is the total of surface energy. γtot = γp + γd. µ is the mobility of the Ph5T2 single-crystal field-effect transistors, highest, the highest mobility of devices. SiO2, silicon dioxide; MPT/SiO2, (γ-mercaptopropyl) trimethoxysilane (MPT) modified SiO2; PMMA, poly(methyl methacrylate); BCB/SiO2, divinyltetramethyldisiloxane-bis(benzocyclobutene) (BCB) modified SiO2; OTS/SiO2, octadecyltrichlorosilane (OTS) modified SiO2; PS, polystyrene; p-6P/SiO2, para-sexiphenyl (p-6P) modified SiO2; BPPh/SiO2, 2,7-bis(4-biphenylyl)phenanthrene (BPPh) modified SiO2. Ph5T2, dinaphtho[3,4-d:3',4'-d']benzo[1,2-b:4,5-b']dithiophene.


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Figure 3. Relationship between the surface energy of dielectric and the mobility of Ph5T2 single-crystal OFETs on different dielectrics. The total surface energies (γtot) are broken into their polar (γp) and dispersive (γd) components. The black line is drawn between the polar and dispersive components of surface energy of Ph5T2 (the polar component for which is too small to be clearly shown on this figure). The mobility values of Ph5T2 single-crystal OFETs on different dielectrics are plotted in red (right axis) with error bars that represent the standard deviation from average values. The highest mobility of the Ph5T2 single-crystal devices is reached when both components of surface energy of the dielectric are close to the corresponding values for Ph5T2, i.e., for BPPh/SiO2.


29


Figure

4.

Relationship

between

the

average

value

Page 30 of 32

of

shallow

trap

density

in

semiconductor/dielectric interface and mobility of Ph5T2 single-crystal OFETs on different dielectric layers. The mobility increases with the decrease of the average value of shallow trap density. The highest mobility was obtained on the BPPh/SiO2 with the lowest average value of shallow trap density, and the dielectric is more matched surface energy components to Ph5T2 material.


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Figure 5. Generalization to pentacene and ZnPc semiconductors. Molecular structure, electrical properties of single-crystal OFETs on the dielectrics with different surface energy (SiO2, OTS/SiO2 and p-6P/SiO2), and relationship between the surface energy of dielectric and the mobility of single-crystal devices on different dielectrics. a-c) Pentacene. d-f) ZnPc. The insets are the SEM images of corresponding single-crystal devices. Scale bar, 20 µm. Error bars in c) and f) represent the standard deviation from average values. The source-drain bias (VSD) are 40 V and - 25 V for pentacene and ZnPc, respectively.


31


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Table of Contents Graphic (TOC)


32

Direct Effect of Dielectric Surface Energy on Carrier Transport in

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