Surface Viscoelasticity of an Organic Interlayer Affects the Crystalline

Article pubs.acs.org/JPCC

Surface Viscoelasticity of an Organic Interlayer Affects the Crystalline Nanostructure of an Organic Semiconductor and Its Electrical Performance Hwa Sung Lee,†,∇ Moon Sung Kang,‡,∇ Sung Kyung Kang,§ Beom Joon Kim,∥ Youngjae Yoo,⊥ Ho Sun Lim,¶ Soong Ho Um,∥ Du Yeol Ryu,# Dong Ryeol Lee,*, ○ and Jeong Ho Cho*,∥ †

Department of Chemical & Biological Engineering, Hanbat National University, Daejeon 305-719, Republic of Korea Departments of ‡Chemical Engineering, §Organic Materials and Fiber Engineering, and ○Physics, Soongsil University, Seoul 156-743, Republic of Korea ∥ SKKU Advanced Institute of Nanotechnology (SAINT) and Center for Human Interface Nano Technology (HINT), School of Chemical Engineering, Sungkyunkwan University, Suwon 440-746, Korea ⊥ Information & Electronics Polymer Research Center, Korea Research Institute of Chemical Technology, Daejeon 305-600, Korea ¶ Electronic Materials and Device Research Center, Korea Electronics Technology Institute, Gyeonggi-do 463-816, Republic of Korea # Department of Chemical Engineering, Yonsei University, Seoul 120-749, Republic of Korea ABSTRACT: We demonstrated that the viscoelasticity of a dielectric surface affected the overlying pentacene crystalline nanostructures and the electrical performances of pentacenebased field-effect transistors (FETs). The surface viscoelasticities of the gate dielectrics were systematically controlled by varying the polymer chain lengths of polystyrene brushes (bPSs) and the substrate temperature during pentacene deposition. The b-PSs were chosen as a model surface because the glass−liquid transition affected neither the surface energy nor the surface roughness. Moreover, the glass−liquid transition temperature increased with increasing b-PS chain length. The liquid-like b-PS chains disturbed the surface arrangement of the pentacene molecules, which reduced the organization of the crystalline structures, yielding smaller grains during the early stages of pentacene growth. The dramatic changes in the film morphology and crystalline nanostructures above the b-PS glass−liquid transition resulted in noticeable changes in the OFET performance. The systematic investigation of the dielectric surface viscoelasticity presented here provides a significant step toward optimizing the nanostructures of organic semiconductors, and thereby, the device performance, by engineering the interfaces in the OFETs.

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INTRODUCTION

self-assembled monolayers (SAMs) and/or ultrathin polymer films on the inorganic dielectric surfaces has been used to induce the formation of well-ordered crystals comprising πconjugated organic semiconductors,15−18 as well as to minimize the formation of interfacial charge traps due to surface polar groups.19,20 Using this approach, Bao and others reported a dramatic improvement in OFET performance using octadecyltrichlorosilane (ODTS)-treated SiO2 as a gate dielectric.21−25 The surface roughness can hinder the movement of charge carriers because they are localized in roughness valleys at the dielectric surface.1,26−28 During thermal evaporation of the organic semiconductor, the surface roughness can lead to the formation of voids that disrupt the connectivity among grains,

Organic field-effect transistors (OFETs) have attracted considerable attention recently as a central component of low-cost flexible electronic devices.1−5 To improve OFET performances, it is important to understand the properties of the interface between the semiconducting layer and the gate dielectric because charge carrier transport takes place within a few molecular layers adjacent to the surface.2,6,7 The interfacial properties are, in turn, sensitive to the dielectric surface characteristics, which determine how the overlying semiconducting molecules are deposited and assembled on the surface.8−10 To control the dielectric surface properties and achieve favorable mesoscale/nanoscale ordering of the organic semiconductors, several research groups have explored the effects of the substrate characteristics, such as the surface energy,11,12 roughness,13,14 and phase states,8 on the OFET performance. For example, the introduction of hydrophobic © 2012 American Chemical Society

Received: June 13, 2012 Revised: August 23, 2012 Published: October 9, 2012 21673

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wt % toluene solutions and then heated at 170 °C for 48 h under vacuum to allow the hydroxyl end groups of the PS chains to react with the silanol groups in the SiO2 substrate. Tethered PS brush (b-PS)-modified substrates were rinsed with toluene to remove unreacted PS chains. The samples were annealed at 100 °C for 24 h under vacuum. Pentacene (Aldrich Chemical Co., no purification) films, 50 nm in thickness, were deposited from a quartz crucible onto the b-PS-modified substrates at a rate of 0.2 Å/s using an organic molecular beam deposition (OMBD) system. The devices were completed by evaporating gold through a shadow mask to define the source and drain contacts on the pentacene film, where the channel length and width were 100 and 800 μm, respectively. Characterization. The surface energies of the various dielectrics were determined by measuring the contact angles of distilled water and diiodomethane as probe liquids. The thicknesses of the b-PSs on Si wafers were measured using ellipsometry (M-2000 V, J. A. Woollam Co., Inc.) and synchrotron X-ray reflectivity (XRR) measurements. The root-mean-square (rms) roughness was determined by AFM (Digital Instruments Multimode). X-ray diffraction (XRD) measurements were carried out using the 5A beamline at the Pohang Accelerator Laboratory (PAL), Korea. The pentacene film morphology was examined by AFM operated in tapping mode. The current−voltage characteristics of the OFETs were measured at room temperature under ambient conditions in a dark environment using Keithley 2400 and 236 source/measure units.

which act as charge-trapping sites.26−29 The OFET performance therefore decreases with increasing dielectric surface roughness. Finally, the phase state (ordered or disordered) of the SAMs present on the semiconducting layer surfaces were recently reported to affected OFET performance.8,21 The Cho and Bao groups reported that the film growth of organic semiconductors is highly sensitive to the molecular ordering and packing density of a surface-layer SAM and that pentacene molecules deposited onto relatively highly ordered SAMs exhibited laterally well-ordered crystal structures and better OFET performances. The effects of the dielectric surface viscoelasticity on the pentacene growth and device performance have not been sufficiently examined until now. The Marks and Facchetti groups first examined the effects of the viscoelastic properties in polymer gate dielectrics on the OFET performance.30 They found that the surface glass transition temperature of the polymer dielectric layer was closely related to the microstructural and morphological changes in the semiconductor film and the field-effect mobility of the device; however, their studies insufficiently explained the relationship between the surface viscoelasticity and the OFET performance because the delamination or dewetting of the polymeric buffer layers above the glass transition temperature was not considered. The hydrophobic polymeric buffer layers such as polystyrene, poly(methylmethacrylate), and poly(4-tert-butylstyrene) coated onto the hydrophilic SiO2 can induce delamination and dewetting and introduce severe changes in the surface roughness above the glass transition;31−34 therefore, we used a polymer brush anchored covalently to the SiO2 surfaces as a model surface because it exhibited a glass−liquid transition without altering either the surface energy or the surface roughness.35−37 Here, we investigated the effects of the surface viscoelasticity of the dielectric on the crystalline nanostructures and film morphologies of pentacene and the field-effect mobilities of the pentacene FETs. The surface viscoelasticity of the dielectric was controlled using PS brushes with three different chain lengths which were realized by the different degrees (n) of polymerization, n = 14, 95, and 186 (hereafter, referred to as b-PS14, bPS95, and b-PS186, respectively). The OFETs prepared with bPS14 yielded the field-effect mobilities of OFETs that decreased dramatically as the substrate temperature increased from 30 to 45 °C. On the other hand, the field-effect mobilities of the OFETs prepared with the longer b-PS186 did not change significantly until reaching a substrate temperature of 45 °C. The mobilities then decreased rapidly above 45 °C. This abrupt drop in the field-effect mobility could be explained in terms of the surface viscoelasticity and the glass−liquid transition of the b-PS, which was analyzed based on the temperature-dependent polymer chain dynamics of the b-PS.

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RESULTS AND DISCUSSION The top-contact, bottom-gate pentacene OFETs were fabricated on the 300 nm thick SiO2 gate dielectrics modified with b-PS of three different lengths, as shown in Figure 1.

Figure 1. Schematic diagram of the pentacene OFETs based on b-PS organic interlayers with three different lengths.

Pentacene was selected as a model organic semiconductor due to its importance in organic electronics.30,38−41 The b-PS was used to modify the SiO2 gate dielectric surface by the “grafting to” method of hydroxyl-functionalized PS.37 Three hydroxylfunctionalized PSs with different molecular lengths, n = 14, 95, and 186 were used to systematically vary the length of the polymer brush. X-ray reflectivity measurements were used to measure the thicknesses of b-PSs, found to be 32.2 ± 1.5, 73.6 ± 0.8, and 115.6 ± 0.3 Å for b-PS14, b-PS95, and b-PS186, respectively, whereas their surface energies (40.0 ± 0.2 mJ/m2) and rms roughness (2.4 ± 0.3 Å) were equal within the error range. The effects of the surface energy and roughness on the pentacene nanostructure and film morphology could be excluded. The electrical performances of the pentacene FETs based on the b-PSs of three different lengths as a function of the substrate temperatures were investigated. The device characteristics were accurately estimated by fabricating and testing more than 10 devices under each condition. Figure 2a,b shows the

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EXPERIMENTAL SECTION Materials and Device Fabrication. OFETs were fabricated using a highly doped n-type Si wafer with a thermally grown 300 nm thick oxide layer as the substrate. The wafer served as the gate electrode and the oxide layer as the gate dielectric. Prior to treating the silicon oxide surface, the wafer was cleaned in a piranha solution for 30 min at 100 °C and then washed with copious amounts of distilled water. Hydroxyl endfunctionalized polystyrene with molecular weights Mn = 1.6 (n = 14), 10.0 (n = 95), and 19.5 (n = 186) kg/mol (Polymer Source Inc.) were spin-coated onto the SiO2 substrate from 0.5 21674

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Figure 2. Transfer characteristics of the pentacene OFETs based on pentacene films deposited onto b-PS14 (a) and b-PS186 (b) at various substrate temperatures: 30 (black), 45 (red), 60 (green), and 90 °C (blue). (c) Variations in the field-effect mobilities as a function of the substrate temperature.

drain current (ID) vs gate voltage (VG) plots of the pentacene FETs based on b-PS14 and b-PS186 at a drain voltage (VD) of −40 V. All devices were found to be well-behaved p-type transistors. The field-effect mobilities were calculated from the ID−VG plots in the saturation regime (VD = −40 V) using the relationship ID = CiμW(VG − Vth)2/2L, where W and L are the channel width and length, respectively, Ci is the specific capacitance of the gate dielectric, and μ is the field-effect mobility.42 The specific capacitances of the three gate dielectrics were similar to that of a 300 nm thick SiO2 dielectric (11 nF/cm2). The field-effect mobilities of the pentacene FETs based on the b-PS14, b-PS95, and b-PS186 layers as a function of the substrate temperature are summarized in Figure 2c. A clear difference was observed among the devices with b-PSs of different lengths. The fieldeffect mobilities of the pentacene FETs based on b-PS14 decreased dramatically at substrate temperatures between 30 and 45 °C, and the values were nearly constant above 60 °C. In the case of the pentacene FETs based on b-PS186, however, the field-effect mobility did not change until a substrate temperature of 45 °C. The field-effect mobility decreased abruptly above 60 °C. The abrupt decrease in the field-effect mobilities of the pentacene FETs based on b-PS95 were observed to be intermediate between those of the b-PS14 and b-PS186 devices. Note that the transition temperature of the field-effect mobility increased with the b-PS length. Importantly, b-PS186 FETs based on pentacene deposited at 30 °C showed excellent fieldeffect mobility of 0.82 cm2/(Vs) that is superior to that of devices using conventional dielectric surface treatments such as HMDS (0.32 cm2/(Vs)) and ODTS (0.21 cm2/(Vs)). The pentacene film growth model and assembly mechanism suggested that the surface characteristics, such as the surface energy, roughness, and gate dielectric viscoelasticity, affected the film morphologies and crystalline nanostructures of the overlying pentacene molecules, which played a crucial role in determining the OFET performance.8,43−47 The temperature dependence of the OFET’s electrical characteristics based on bPSs with different lengths could be explained in terms of the morphological and crystalline nanostructures of the pentacene films deposited on the b-PS, which was investigated using AFM and synchrotron XRD, respectively. Figure 3 shows the AFM images of 50 nm thick pentacene films deposited onto b-PS14 and b-PS186 at various substrate temperatures. A considerable difference between the pentacene films deposited onto b-PS14 and b-PS186 was observed in the

Figure 3. AFM images of the final morphologies of 50 nm thick pentacene films deposited onto b-PS14 and b-PS186 at various substrate temperatures of 30, 45, 60, and 90 °C.

grain size and variation as a function of the substrate temperature. The largest grains were observed in a pentacene film deposited at 30 °C. The pentacene grains on the b-PS186 were slightly larger than those on the b-PS14. As the substrate temperature increased in the b-PS14 case, the average grain size dramatically decreased from 440 nm at 30 °C to 270 nm at 45 °C. A b-PS14 pentacene film morphological transition was observed between 30 and 45 °C. After the transition, the typical terrace-like structure was not present in the pentacene films deposited at 45 and 60 °C, or 90 °C. The b-PS186 film, on the other hand, displayed an abrupt change in the pentacene grain size between 45 and 60 °C. The average size decreased from 580 nm at 45 °C to 360 nm at 60 °C and 320 nm at 90 °C. These results suggested that the morphological variations in the pentacene films as a function of the substrate temperature were closely correlated with the electrical performance of the pentacene-based FETs, as shown in Figure 2. The crystalline structures of the b-PS14 and b-PS186 pentacene films at various substrate temperatures were investigated by collecting synchrotron XRD measurements, as shown in Figure 4. The θ−2θ scans of the pentacene films on bPS14 and b-PS186 displayed only the (00L) reflection, indicating that the pentacene crystals were oriented with their (00L) planes parallel to the dielectric surface.48−50 Typically, pentacene crystals are composed of two distinct crystalline polymorphs, thin-f ilm and bulk phases, characterized by d(001) spacings of 15.5 ± 0.1 and 14.5 ± 0.1 Å, respectively.48−50 The pentacene films deposited onto the b-PS at 30 °C showed only the thin-film phase. The changes in the intensity of the (002) 21675

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angle made the region in reciprocal space inaccessible, as indicated by the gap in the scattering intensities near the qzaxis.51 Both pentacene films deposited at 30 °C displayed diffuse intensities around the (002)T diffraction peaks along the Debye rings, indicating that the pentacene films consisted of only the thin-film crystalline phase. As the substrate temperature increased, the (00L)B crystal reflections corresponding to the bulk phase were clearly observed in b-PS14 from the substrate temperature of 45 °C, whereas those for b-PS186 were not obvious until 60 °C. These results indicated that the substrate temperature differently affected the molecular assembly of pentacene on the PS brushes with different lengths. The pentacene nanostructure transition was observed at higher substrate temperatures for pentacene films deposited onto the longer b-PS. The reflection patterns along the qxy-axis were also analyzed, as shown in Figure 5b. In both pentacene films deposited at 30 °C, intense in-plane reflections corresponding to the thin-film phase were mainly observed vertically at a given qxy (>0) and were indexed to {1, ±1}, {0, 2}, and {1, ±2}, respectively.8,48−50 These vertical Bragg-rod reflections indicated that the pentacene films consisted of multistacked layers. As the substrate temperature increased, however, the bulk phase reflections and highly scattered patterns along the Debye rings for the b-PS14 increased distinctly, as indicated by the white arrows. On the other hand, the bulk phase reflections for b-PS186 were not observed until 45 °C, after which point they gradually became clearer at temperatures up to 60 °C. Interestingly, the precipitous fieldeffect mobility transition depicted in Figure 2 was strongly correlated with the observed dramatic changes in the crystalline nanostructures as well as the morphologies of the pentacene films.

Figure 4. XRD patterns of the 50 nm thick pentacene films deposited onto b-PS14 (a) and b-PS186 (b) at various substrate temperatures. The insets shows the enlarged (002) peak of the XRD patterns with identical y-scale for comparison.

Bragg reflection at qz = 0.815 Å−1 as a function of the substrate temperature were monitored in the inset of Figure 4. For a substrate temperature of 30 °C, the peak intensity of the bPS19.5k was slightly higher than that of the b-PS14. The peak intensity of the b-PS14 dramatically decreased above a substrate temperature of 30 °C. In the b-PS186 case, on the other hand, the peak intensity did not change until 45 °C, after which point the intensity decreased rapidly. The crystalline nanostructure of the pentacene films were characterized with more precision by collecting synchrotron grazing-incidence X-ray diffraction (GIXD) measurements. Figure 5a shows the magnified 2D GIXD patterns around the (002) peak of 50 nm thick pentacene films deposited onto bPS14 and b-PS186 at various substrate temperatures. GIXD measurements did not provide information about the diffraction peaks along the qz-axis because the fixed incidence

Figure 5. Magnified 2D GIXD patterns of the 50 nm thick pentacene films deposited onto b-PS along the (a) qz and (b) qxy directions, corresponding to the out-of-plane and in-plane directions, at various substrate temperatures, respectively. 21676

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that the liquid-like b-PS disturbed the surface arrangement process of the pentacene molecules in the film during the early stages of pentacene growth, which induced the growth of lessorganized crystalline structures and smaller grains during the early stages of pentacene film growth. The dramatic changes in the pentacene film structures above the b-PS glass−liquid transition resulted in a noticeable decrease in the OFET performance. This systematic investigation of the surface viscoelasticities of dielectrics provides a significant step toward optimizing the nanostructures of organic semiconductors, directly linked to device performance enhancement, by engineering the interfaces in OFETs.

Because both the surface energies and roughness values of the gate dielectrics were similar within a margin of error, the different morphological/crystalline nanostructures of the pentacene films on b-PS as a function of the substrate temperature were induced by the surface viscoelasticity of each dielectric. The polymer chain dynamics of the b-PS were investigated using XRR measurements as a function of the substrate temperature. Figure 6 shows the thickness variations

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]. Author Contributions ∇

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) grants (2009-0076010, 2009-0083540, and 20100026294), Republic of Korea.

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Figure 6. Thickness variations derived from the XRR data of the bPS14 and b-PS186 devices as a function of temperature. The inset shows the XRR curves for b-PS186 at various temperatures, 25, 30, 40, 50, 60, 70, 80, and 90 °C, from the top, respectively.

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of the b-PS as a function of the substrate temperature. These values were related to the surface viscoelasticity of the dielectrics. The b-PS14 exhibited a considerable decrease in thickness at temperatures of 25−60 °C. This decrease in the thickness was consistent with the surface glass−liquid transition behavior observed in ultrathin spin-coated PS films by the Orts group.52 The b-PS14 device above 30 °C displayed pentacene molecules that were deposited onto the liquid-like molten and, thus, mobile PS chains of the b-PS14. The liquid-like b-PS could disturb the surface diffusion of pentacene molecules, thereby inducing the formation of less-organized crystalline structures and smaller grains during the early stages of pentacene growth. On the other hand, the thickness of the bPS186 layer was nearly constant up to 40 °C, with a subsequent dramatic decrease above this temperature. Presumably, the pentacene molecules deposited below 45 °C onto the b-PS186 were deposited onto glassy PS chains. Overall, the glass−liquid transition temperatures of the b-PS14, b-PS95, and b-PS186 films significantly affected the film morphology and crystalline nanostructure of the overlying pentacene films, resulting in noticeable changes in the OFET performance.

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CONCLUSIONS In conclusion, the effects of the dielectric surface viscoelasticity on the pentacene film structures and electrical properties of the pentacene-based OFETs were investigated. The surface viscoelasticities of the dielectrics were systematically controlled using b-PSs with different lengths because the b-PSs exhibited a glass−liquid transition behavior without changing either the surface energy or the surface roughness. The glass−liquid transition temperatures increased with increasing b-PS chain length. The synchrotron XRD, AFM, and XRR data showed 21677

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