Siloxane Side Chains: A Universal Tool for Practical Applications of

Article pubs.acs.org/Macromolecules

Siloxane Side Chains: A Universal Tool for Practical Applications of Organic Field-Effect Transistors A-Reum Han,† Junghoon Lee,∥,§ Hae Rang Lee,† Jungho Lee,∥ So-Huei Kang,∥ Hyungju Ahn,‡ Tae Joo Shin,⊥ Joon Hak Oh,*,† and Changduk Yang*,∥ †

Department of Chemical Engineering and ‡Pohang Accelerator Laboratory, Pohang University of Science and Technology (POSTECH), Pohang, Gyeongbuk 37673, South Korea ∥ Department of Energy Engineering, School of Energy and Chemical Engineering, and ⊥UNIST Central Research Facilities & School of Natural Science, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, South Korea § Materials Research Laboratory, University of California, Santa Barbara, Santa Barbara, California 93106, United States S Supporting Information *

ABSTRACT: To systematically address the intriguing question of how siloxane termini of the side chains relative to alkylterminal groups affect the various inherent properties of conjugated polymersincluding optical, electrical, and morphological characteristicswe have synthesized model polymers (PDPPTT-RTG and PDPPTT-SiTG, together with an accompanying reference PDPPTT-ref) containing an identical backbone yet possessing different terminal groups. In order to fairly compare the end functionalities by eliminating molecular weight (Mn) and polydispersity index (PDI) variations that may act as complicating factors, the polymers used in this study have been controlled to have similar Mn and PDI by carefully optimizing the catalyst system and reaction conditions. Although the molecular packing and orientation behaviors of PDPPTT-RTG and PDPPTT-SiTG are very different from each other, both polymers exhibit very high mobility exceeding 4.5 cm2 V−1 s−1. More meaningfully, organic field-effect transistors (OFETs) based on PDPPTT-SiTG are highly stable over extended periods in humid environments. Our findings provide new insights into the molecular design strategy aimed at the simultaneous enhancement of charge-carrier mobility and ambient stability, which is of great importance for practical OFET applications.

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INTRODUCTION Molecular packing and macroscopic order in the semiconducting layers of organic field-effect transistors (OFETs) are of the utmost importance in their electrical performance. With notably rare exceptions,1−11 state-of-the-art charge-carrier mobilities for either p-channel or n-channel OFET operation are usually achieved when semiconducting polymers form tightly packed molecular crystals with an outstanding structural order.12−17 Consequently, not unexpectedly, most research efforts have focused on “conjugated backbone” engineering via a reasonable molecular setup,6,18−22 because it was thought to be critical to directly promoting closer π−π stacking of the given systems, which can effectively induce charge transport through a hopping mechanism.14,17,23 On the other hand, recent studies have begun to emphasize the substantial impact of “side chains” far beyond the solubility issue for device fabrication, especially that on molecular ordering, packing, and thin-film morphology and hence OFET performance. For example, the pioneering work of Bao et al. reported that moving the side-chain branching point away from conjugated backbonesin which bulky siloxane © XXXX American Chemical Society

(Si−O−Si skeleton) blocks as part of the solubilizing groups were introduced at the end of the side chain to afford sufficient polymer solubilityhas crucial consequences, with related work on transistor devices showing improved charge-carrier mobilities.24 Using a similar concept, Pei et al. systematically elucidated the significant effect of the branching position of the regular branched alkyl side chain on the microcrystalline structure and OFET performance.25 Thereafter, a series of representative semiconductors with a variety of branching points followed, shedding light on their excellent charge transport properties.13,25−28 Independently, we also analyzed data from a broad set of high-performance polymers for OFETs and noted the specific effect of the branching position not only in the terminal siloxane groups but also in the conventional alkyl chains.29−31 To date, however, no report has clearly addressed the role of the siloxane-terminal groups in device performance and ambient stability in Received: January 29, 2016 Revised: April 24, 2016

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Figure 1. Chemical structures of PDPPTT-ref, PDPPTT-RTG, and PDPPTT-SiTG.

Figure 2. (a, b) UV−vis spectra of PDPPTT polymers (a) in chloroform solution and (b) as thin films. (c) UPS spectra of PDPPTT films.

from the siloxane groups, which is an essential requirement for the successful implementation of OFETs.

comparison with the alkyl-terminal analogue with the same branching point. With the aim of understanding the degree to which a subtle change in the terminal groups influences the nature of polymers, in this contribution, we report a detailed characterization of polymers (PDPPTT-RTG and PDPPTT-SiTG with an additional reference (PDPPTT-ref)) with essentially identical physical characteristics (i.e., structural units) except for the end functionalities of the side chains (siloxane- vs alkyl-terminal groups). We improve the OFET mobility of both the polymers (PDPPTT-RTG and PDPPTT-SiTG)which have a relatively larger distance between the terminal position and the conjugated coreto an extraordinarily high level. In addition, our comparative study reveals that the siloxane-terminal groups can effectively tune the intrinsic properties of polymers, including absorption, energetics, molecular packing, and charge transport. More importantly, we demonstrate the superior longterm stability of the PDPPTT-SiTG device in a harsh environment as a result of the high hydrophobicity derived

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RESULTS AND DISCUSSION Molecular Design, Synthesis, and Characterization. We chose a polymer (PDPPTT) based on diketopyrrolopyrrole (DPP) and thieno[3,2-b]thiophene (TT) units as the model system of the backbone, since this type is well-known as one of the best-performing semiconducting polymers. For an ideal test bench to compare the alkyl- versus siloxane-terminal groups, we designed and synthesized two PDPPTT polymers with an identical distance between the terminal point and the backbone core yet two different terminal groups (PDPPTT-RTG and PDPPTT-SiTG in Figure 1). Nonadecane with the branching point at C9 and 1,1,1,3,5,5,5-heptamethyltrisiloxane were selected as the terminal groups for alkyl and siloxane terminal groups, respectively, because they have been most frequently used for high-performance polymer semiconductors. We prepared the reference polymer (PDPPTT-ref) with the conventional 2-octyldodecyl side chains for the sake of B

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Figure 3. Morphology analyses of the polymer films. AFM height (top) and phase (bottom) images of annealed PDPPTT films at 260 °C: (a) PDPPTT-ref, (b) PDPPTT-RTG, and (c) PDPPTT-SiTG. The polymer films were prepared by drop-casting method on OTS-modified SiO2/Si substrates. More crystalline morphologies were observed in these annealed films compared with as-cast films. Scale bar = 400 nm.

PDPPTT-RTG, which showed relatively increased HOMO/ LUMO levels (−4.88/−3.62 eV). This observation is essentially consistent with the aforementioned behaviors based on the optical properties, unlike the previous results,24,30 in which the typical 2-octyldodecyl side chains led to the deeper HOMO level as a result of the slightly disrupted π−π interaction in the reference polymer. These results, together with the optical spectra of the polymers, indicate that optical properties and energetics can be affected by a combination of the structure backbone and the end functionalities of side chains. In addition, the computational studies using densityfunctional theory (DFT, B3LYP/6-31G(d,p)) revealed that the electron densities of both the HOMO and LUMO in the repeating units were well delocalized over the conjugated backbone regardless of the chosen side chains (Figure S1 and Table S1). Thin-Film Microstructure Analyses. To investigate the terminal group effects on the microstructure of PDPPTT films, tapping-mode atomic force microscopy (AFM) and grazing incidence X-ray diffraction (GIXD) analyses were performed on the polymer films. The PDPPTT films were prepared by drop-casting from chloroform solution (∼2 mg mL−1) on noctadecyltrimethoxysilane (OTS)-modified SiO2/Si substrates and annealing at an optimal temperature of 260 °C in a nitrogen atmosphere (vide inf ra). All polymer films formed densely interconnected nanofibrillar networks with relatively low surface roughness (less than 3.4 nm) (Figure 3 and Figure S2). Note that there were distinct differences in the thickness of the PDPPTT nanofibrillar aggregates depending on the types of side chains, indicating the existence of side-chain-dependent morphological features. We calculated the approximate dimensions of PDPPTT fibrillar aggregates based on their

comparison. Moreover, in order to eliminate their molecular weight (Mn)-dependent properties as a complicating variable,6,32−35 our synthetic effort was directed at achieving very similar Mn and polydispersity index (PDI) values of the studied polymers by optimizing the catalyst system and reaction conditions for the Stille polymerization: Mn = 48.1 kDa, PDI = 3.50 for PDPPTT-ref, Mn = 50.3 kDa, PDI = 3.02 for PDPPTT-RTG, and Mn = 49.3 kDa, PDI = 3.21 for PDPPTTSiTG. The synthetic details and complete characterization can be found in the Experimental Section. All polymers in both chloroform solution and as thin films exhibit strong and broad absorptions in the region of 350−1100 nm (Figure 2a,b). The spectra of all polymer films appear somewhat broadened and red-shifted when compared to those in solutions, which correlates with their solid-state packing. Very interestingly, we observe that both PDPPTT-ref and PDPPTT-SiTG feature remarkably similar patterns, whereas in the case of PDPPTT-RTG, the 0−1 vibrational transition relative to 0−0 is intensified. This behavior can be explained by the different types and degrees of aggregates that are formed by the planar and rigid DPP and TT moieties in conjugated backbones, which are highly dependent on their specific side chains. As yet, the clear reason for this behavior remains to be determined; nevertheless, the different aggregates that are formed in the each conjugated backbone are considered to be one of the factors. The frontier molecular orbital energies (the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels) were determined using ultraviolet photoelectron spectroscopy (UPS) (Figure 2c) and the absorption onsets from the UV−vis spectra. Both PDPPTT-ref and PDPPTT-SiTG were found to have the same HOMO/LUMO levels (−5.02/−3.77 eV) in contrast to C

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Figure 4. GIXD analyses of the PDPPTT films annealed at 260 °C. (a−c) 2D-GIXD images of (a) PDPPTT-ref, (b) PDPPTT-RTG, and (c) PDPPTT-SiTG. (d, e) The corresponding GIXD profiles of (d) in-plane and (e) out-of-plane direction. (f) 1/Lc−h2 plot extracted from the out-ofplane GIXD profile of PDPPTT films. (g) Pole figure for (010) diffraction of PDPPTT-SiTG (where χ is defined as the semicircular angle between the crystallite orientation and the surface normal).

phase images. PDPPTT-ref formed fine fibrillar structures with a thickness of 23 nm, whereas thicker fibrillary aggregates (51 nm) were observed in PDPPTT-RTG films, implying that the branching position affects the intermolecular interactions. Moreover, PDPPTT-SiTG formed the thickest fibrillary aggregates in the PDPPTT films (68 nm, the relative thickness ratio for ref:RTG:SiTG ≈ 1:2:3), which indicates that siloxaneterminal groups are useful tools to prepare high-quality nanofibrillar structures for efficient charge transport in polymer film. Furthermore, the area portions of PDPPTT fibrillar aggregates were estimated from quantitative image analysis and PDPPTT-SiTG showed the largest area portion of 76.5% compared with other PDPPTT films (44.3 and 66.7% for PDPPTT-ref and PDPPTT-RTG, respectively). These highly crystalline morphologies might be attributed to the comparatively inferior solubility of PDPPTT-SiTG caused by the siloxane-terminal groups in organic solvents (Figure S3), which formed larger fibrillar networks by the earlier generation of nuclei during solution-processing and longer grain growth period.36 The similar morphologies were observed for the PDPPTT-RTG and PDPPTT-SiTG films prepared using low molecular weight polymers (Mn of ∼20 kDa) in cyclobenzene and xylene solutions (Figure S4). This marginal solubility of PDPPTT-SiTG in several solvents may offer an opportunity to make the successive solution processing in device fabrication and enhance its stability in harsh environments. GIXD analyses were used to identify the crystalline nature and molecular orientation of the optimized PDPPTT films (Figure 4 and Table S2). All of the PDPPTT films exhibited dominant edge-on orientations with well-defined lamellar peaks up to the fourth order (Figure 4a−e). For lamellar diffractions, PDPPTT-RTG showed a relatively increased (100) layer distance of 25.09 Å compared to that of PDPPTT-ref (20.10 Å) due to the enlarged alkyl spacer length, but remarkably strong π−π stacking and alkyl chain interactions were observed in PDPPTT-RTG films at qxy ≈ 1.75 and 1.48 Å−1, respectively. On the other hand, PDPPTT-SiTG showed slightly shorter (100) layer distances of 23.03 Å than PDPPTT-RTG, presumably due to the relatively smaller terminal group size

in PDPPTT-SiTG despite the same alkyl spacer length as PDPPTT-RTG. The crystallite size and lattice distortion parameter for the (h00) plane were further investigated by plotting the 1/Lc−h2 curve (Lc and h mean the crystallite size and the order of diffractions, respectively) extracted from the out-of-plane GIXD profile of PDPPTT films (Figure 4f), which were derived from the y-intercept and slope (= g2π2/d, d is the domain spacing) of the straight line, respectively.37 For PDPPTT-ref, PDPPTT-RTG, and PDPPTT-SiTG, the calculated crystallite sizes were 204.1, 212.8, and 185.2 Å, respectively, and average numbers of the lamellar layers in the coherent domains (calculated from Lc/d) were 10.2, 8.5, and 8.0, respectively. PDPPTT-ref formed relatively denser packing with the larger average number of the (h00) plane compared with other PDPPTT films, whereas PDPPTT-RTG formed the largest crystallite size of 212.8 Å for out-of-plane direction. Moreover, the lattice distortion of PDPPTT-ref, PDPPTT-RTG, and PDPPTT-SiTG was estimated to 4.3, 3.9, and 4.0%, respectively, which indicates that PDPPTT-RTG formed the most effective edge-on orientations among the PDPPTT films. On the other hand, in PDPPTT-SiTG, the additional lamellar stacking and π−π stacking were observed in the in- and out-of-plane directions, respectively. Note that PDPPTT-SiTG featured the most advanced bimodal distributions in π-systems with a faceon portion of 21.5% and π−π stacking distance of 3.62 Å in both directions (Figure 4g). These results indicate that the relatively bulkier siloxane-terminal groups not only induced more tilted edge-on molecular orientations on substrates but also contributed to the formation of a larger amount of face-on crystallites in PDPPTT-SiTG films. In addition, these features indicate that crystalline orientations in polymer thin films are controlled by termini modification of side chain structures and siloxane-terminal groups are highly effective for forming edgeon dominant 3-D conduction channels in the entire film thickness. After thermal annealing, the PDPPTT films adopted thermodynamically preferred edge-on orientations with denser and enhanced edge-on molecular packing in the annealed films compared with as-cast films (Figure S5 and Table S2).38 D

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Figure 5. OFET and moisture stability characteristics obtained from optimized FET devices with PDPPTT films after annealing at 260 °C. (a) Schematic illustration of the bottom-gate/top-contact FET structure with gold electrodes (L = 50 μm and W = 1000 μm). (b−d) Transfer characteristics for (b) PDPPTT-ref, (c) PDPPTT-RTG, and (d) PDPPTT-SiTG at hole-enhancement operation with VDS = −100 V. (e) Comparison of the average mobilities and (f) changes in mobility ratio of PDPPTT-based FETs as a function of the exposure time to humid atmosphere. The μh,0 refers to the mobility before exposure to a saturated humidity atmosphere (∼100% RH) (the initial mobility measured in air). (g) Optical microscopy images of the polymer films exposed to humid environments. Scale bar = 100 μm.

Fabrication of Solution-Processable FETs and I−V Characterizations. Electrical Performances of SolutionProcessed Polymer FETs. To elucidate the charge transport properties modulated by terminal groups of side chains, we fabricated bottom-gate/top-contact OFETs based on PDPPTT films (Figure 5a). The device fabrication and measurement details are described in the Experimental Section. The optimal annealing temperature of 260 °C was traced by testing the electrical performance of OFETs based on the annealed films at various temperatures (Figure S6). All the polymers exhibited unipolar p-type field-effect behavior, most likely due to the welldelocalized HOMO levels and relatively lower injection barriers for holes with regard to gold electrodes (Figure 5b−d, Figure S7, and Table S3). PDPPTT-ref films exhibited high hole mobilities of up to 0.54 cm2 V−1 s−1 and current on-to-off ratios of 104−105 without thermal annealing treatments. This indicates the effects of the strong intermolecular interactions between the polymer chains through the efficient π−π intermolecular overlaps of DPP and TT moieties. The annealed PDPPTT-ref films showed significantly enhanced hole mobilities of up to 3.23 cm2 V−1 s−1 due to the formation of larger granular structures in the polymer films after thermal annealing. Interestingly, despite the aforementioned different molecular packing and orientation properties in the thin-film state, both PDPPTT-RTG and PDPPTT-SiTG films showed remarkably high hole mobilities of up to 4.96 and 4.55 cm2 V−1 s−1, respectively, confirming the significant effect of shifting the branching point away from the polymer backbone on FET mobility. It is noteworthy that in the case of PDPPTT-RTG the denser π−π stacking system with edge-on orientations and larger crystalline domains consisting of more lamellar layers

estimated from Lc(100)/d(100)which, in general, are considered favorable for charge transportwere largely responsible for the slightly higher mobility compared to that of PDPPTT-SiTG. These polymers exhibited highly air-stable FET performance with comparatively constant hole mobilities and threshold voltages after 1 month (Figure S8). Ambient Stability of Solution-Processed Polymer FETs. In order to determine the functional properties of PDPPTT polymers driven by their terminal groups, the PDPPTT films were exposed to a saturated humidity atmosphere (∼100% relative humidity, RH) to demonstrate how long they could survive in harsh conditions. We measured the electrical performance right after taking the devices out of highly humid conditions and compared their stability characteristics (Figure 5e−g and Figure S9). In comparison with PDPPTTRTG and PDPPTT-SiTG, the device performance of PDPPTTref decreased significantly upon exposure to the saturated humidity condition (mobility degradation of 78.2%), which indicates the shifted branching point might contribute to enhancement of device stability resulting from the formation of efficient pathways for charge-carrier transport. Intriguingly, PDPPTT-SiTG showed the most stable operations with mobility degradation of only 20.6%, which was 2 and 4 times lower value than that of PDPPTT-RTG and PDPPTT-ref, respectively. It might be due to the robust polymer films resulting from the intrinsic physical cross-linking and waterproof properties of the siloxane groups.24,39−41 We also assume that moisture stability is closely related to the kinetic factors. The relatively larger grains with lower trap densities in PDPPTT-SiTG films can more efficiently prevent the diffusion of ambient oxidants and other impurities into the channel region and lead to more stable E

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Macromolecules Table 1. Surface Energies of Optimized PDPPTT Films Calculated from the Contact Angle Measurement contact angle [deg]

a p γs

polymer

water

diiodomethane

γpsa [mJ m−2]

γdsa [mJ m−2]

γsb [mJ m−2]

PDPPTT-ref PDPPTT-RTG PDPPTT-SiTG

99.64 100.63 102.40

54.81 54.67 62.42

0.26 0.17 0.30

31.80 32.14 27.22

32.06 32.31 27.53

and γds refer to the polar and dispersion components of the surface energy, respectively. bγs is the surface energy; γs = γps + γds.

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FET performance than PDPPTT-RTG films.17,42,43 In addition, the surface properties of PDPPTT polymers were further investigated to evaluate their surface energy (γs) and the solvent wettability of the polymer films using contact angle measurements (Table 1). Similar surface properties were observed in PDPPTT-ref and PDPPTT-RTG films: hydrophobic surfaces with a high water contact angle of ∼100° and surface energy of ∼32.3 mJ m−2. On the other hand, the PDPPTT-SiTG film yielded more hydrophobic surfaces with a relatively lower surface energy of 27.5 mJ m−2 than the other polymer films, implying its poor compatibility with polar water molecules. This indicates that the terminal groups can modulate the surface properties of polymer films and derive solvent resistance for stable operations under harsh conditions. Therefore, we could clearly observe a meaningful role of the siloxane-terminal groups on the device stability, which was distinguishable from the branching positions in side chains. We further demonstrated the solvent-resistant PDPPTT-SiTG FETs by performing in situ stability tests of the devices under direct exposure to water and organic solvents (Figure 6). PDPPTT-based FETs showed doping effects under in situ water droplets on the channel, but the drain currents were immediately recovered when the droplets were removed. The device performances were maintained during repetitive exposures to water droplets. Note that the performance gap between pristine/recovered conditions was significantly smaller in PDPPTT-SiTG compared with that of PDPPTT-RTG, implying the superior reproducibility and stability of the devices against exposure to liquid water. In particular, under in situ organic solvent conditions, PDPPTT-SiTG FETs still retained the field-effect behaviors without serious damages to the channel and electrodes, whereas PDPPTT-RTG FET was vulnerable to solvents, resulting in the lifted-off films. These results demonstrate that modulating terminal group in side chain would be an important step forward to extend the application scope of organic electronics for practical uses in real life, such as direct detection of toxic liquid-phase chemicals (Figure S10).44 Solution Processability of Polymer FETs. As shown in Table 1, the lowest surface energy of PDPPTT-SiTG films allowed excellent solution processability on hydrophobic surfaces required for device optimization to form trap-free charge transport pathways. In particular, the well-balanced surface energies of PDPPTT-SiTG and the hydrophobic OTS-treated SiO2 dielectric layer (γs = 26.9 mJ m−2) led to favorable interactions between polymer and dielectric surfaces for excellent wettability and facile solution processing (Figure 7). These results shed light on a powerful strategy for realizing stable device operation and facile solution processing in terms of the hybrid side chain engineering of conjugated polymers while maintaining the good charge transport properties of conjugated systems.

CONCLUSION We have investigated how the siloxane- and alkyl-terminal groups in side chains affect the structure−property relationship and device performance to give new insights into the molecular design of conjugated polymers and the improvement of FET performance. Thus, we prepared PDPPTT-based polymers (PDPPTT-RTG and PDPPTT-SiTG together with an additional reference (PDPPTT-ref)) that have the same backbone structure with very similar Mn and PDI values but different terminal groups. By bringing together the data from comprehensive characterizations of the PDPPTT polymers in conjunction with their transport characteristics and thin-film morphologies, we determined the noticeable impacts of the terminal groups on the intrinsic properties of these polymers. In OFETs, both PDPPTT-RTG and PDPPTT-SiTG, despite having intrinsic differences in their nanostructural order and packing orientation, provide similarly high mobilities in excess of 4.5 cm2 V−1 s−1. More excitingly, in the devices fabricated from PDPPTT-SiTG, we found unexpected and remarkably high stability over an extended period in a harsh environment. Our investigation provides a new realm of key solutions for solving issues in processing, operational, and long-term stabilities while maintaining the excellent OFET properties that would be required for transitioning printed electronics from the laboratory to the marketplace.

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EXPERIMENTAL SECTION

Materials and Instruments. All starting materials were purchased either from Aldrich or Acros and used without further purification. All solvents are ACS grade unless otherwise noted. 3,6-Di(2-bromothien5-yl)-2,5-di(2-octyldodecyl)pyrrolo[3,4-c]pyrrole-1,4-dione, 3,6-di(2bromothien-5-yl)-2,5-bis[4-(1,1,1,3,5,5,5,-heptamethyltrisiloxan-3-yl)butyl]pyrrolo[3,4-c]pyrrole-1,4-dione, 3,6-di(2-bromothien-5-yl)-2,5di(5-octylpentadecyl)pyrrolo[3,4-c]pyrrole-1,4-dione, and 2,5-bis(trimethylstannyl)thieno[3,2-b]thiophene were prepared according to established literature procedures.29,31,45 1H NMR, 13C NMR, and 29 Si NMR spectra were recorded on a VNMRS 600 (Varian, USA) spectrophotometer using CDCl3 or C2D2Cl4 as solvent (Figures S11− S16) and tetramethylsilane (TMS) as the internal standard, and MALDI MS spectra were obtained from Ultraflex III (Bruker, Germany). UV−vis−NIR spectra were taken on Cary 5000 (Varian USA) spectrophotometer. DFT calculations were performed using the Gaussian 09 package with the nonlocal hybrid Becke three-parameter Lee−Yang−Parr (B3LYP) function and the 6-31G(d,p) basis set to elucidate the HOMO and LUMO levels after optimizing the geometry of PDPPTT polymers using the same method. Number-average (Mn) and weight-average (Mw) molecular weights and polydispersity index (PDI) of the polymer products were determined by gel-permeation chromatography (GPC) with a Waters 150C GPC using a series of monodisperse polystyrene as standards in tetrahydrofuran (THF) (HPLC grade). Ultraviolet photoelectron spectroscopy (UPS) was examined by AXIS-NOVA CJ109, Kratos. The polymer solutions were prepared in chloroform with 5 mg mL−1 for PDPPTT polymers and spin-coated on indium tin oxide (ITO) glass. Film preparation was done in a N2-atmosphere glovebox. The UPS analysis chamber was equipped with a hemispherical electron-energy analyzer (Kratos Ultra F

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Figure 6. In situ stability tests of PDPPTT-based FETs under direct exposure to water and organic solvents: (a−d) PDPPTT-RTG and (e−h) PDPPTT-SiTG. (a, e) In situ stability tests of OFETs upon exposure to water droplets. The FET performance was measured under pristine (before exposure), water (exposure to water droplets), and removal (removal of water droplet with a tissue paper) conditions. (b, f) Reproducible FET characteristics with repetitive exposures to water droplets. (c, g) In situ solvent resistivity tests of OFETs exposed to liquid-phase organic solvents: toluene and chloroform. (d, h) Optical microscopy images of the FETs directly exposed to organic solvents. Scale bar = 200 μm. spectrometer) and was maintained at 1.0 × 10−9 Torr. The UPS measurements were carried out using the He I (hν = 21.2 eV) source. Tapping-mode atomic force microscopy (AFM) measurements were performed using a MultiMode 8 scanning probe microscope (Veeco Instruments Inc.). Grazing incidence X-ray diffraction (GIXD) measurements were conducted at the PLS-II 9A U-SAXS beamline of Pohang Accelerator Laboratory in Korea. The X-rays coming from the in-vacuum undulator (IVU) were monochromated at 11.17 keV (wavelength, λ = 1.1099 Å) using a double crystal monochromator and

focused both horizontally and vertically (300 (H) × 30 (V) μm2 in fwhm at sample position) using K-B type mirrors. GIXD sample stage was equipped with a 7-axis motorized stage for the fine alignment of sample, and the incidence angle of X-ray beam was set to in the range of 0.11°, which was close to the critical angle of samples. GIXD patterns were recorded with a 2D CCD detector (Rayonix SX165). Diffraction angles were calibrated using a precalibrated sucrose (monoclinic, P21, a = 10.8631 Å, b = 8.7044 Å, c = 7.7624 Å, b = 102.938°), and the sample-to-detector distance was ∼224.4 mm. G

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Macromolecules

Figure 7. Wetting properties of the polymer solution. Contact angles of PDPPTT-ref, PDPPTT-RTG, and PDPPTT-SiTG solution droplets on OTSmodified SiO2/Si substrates. The high contact angle of water droplet indicated the hydrophobic surfaces of the substrates. PDPPTT-SiTG solution was more compatible with hydrophobic surfaces and favorable to solution-processing. where IDS is the drain-to-source current, μ is the mobility, and VG and VT are the gate voltage and threshold voltage, respectively. The in situ solvent stability tests were performed injecting a 20 μL of organic solvent into the polydimethylsiloxane (PDMS) based reservoir placed on the surface of the top-contact OFETs (L = 150 μm and W/L = 60). The stability performance was measured in the linear regime operation with VDS = −2 V. Surface Energy Calculations. The surface energies of the PDPPTT polymers were evaluated by measuring the contact angles of two test liquids, water and diiodomethane, on the optimized polymer films. The surface energy (γs) was calculated according to the equation

General Procedure for Polymerization and Polymer Purification. Typical procedure for Stille polymerization and polymer purification: Dibrominated DPP (0.20 mmol), distannylthienothiophene comonomer (0.20 mmol), tris(dibenzylidenacetone)dipalladium (0) (1.0 μmol), and anhydrous toluene (4 mL) were mixed in a Schlenk flask and purged with argon for 10 min. Tri(otolyl)phosphine (2.0 μmol) was added to the solution, and the reaction mixture was heated at 95 °C under vigorous stirring for 2 h. The crude product was poured into methanol (300 mL). The resulting solid was filtered off and subjected to sequential Soxhlet extraction with methanol (1 D), acetone (1 D), and hexane (1 D) to remove the low molecular weight fractions of the materials. The residue was extracted with chloroform in order to produce a product after precipitating again from methanol and drying in vacuo. PDPPTT-ref. Isolated yield = 80%. Mn = 48.1 kDa, Mw = 168.4 kDa, PDI = 3.50. 1H NMR (C2D2Cl4, 600 MHz, 353 K): δ ppm 8.54−8.04 (br, 2H), 6.82−6.00 (br, 4H), 3.51−3.13 (br, 4H), 1.42−1.20 (br, 2H), 1.06−0.40 (br, 64H), 0.36−0.12 (br, 12H). Anal. Calcd for C60H88N2O2S4: C, 72.24; H, 8.89; N, 2.81. Found: C, 71.31; H, 8.98; N, 2.52. PDPPTT-RTG. Isolated yield = 77%. Mn = 50.3 kDa, Mw = 151.9 kDa, PDI = 3.02. 1H NMR (C2D2Cl4, 600 MHz, 353 K): δ ppm 8.49−8.04 (br, 2H), 6.74−5.87 (br, 4H), 3.51−3.11 (br, 4H), 1.19−0.42 (br, 78H), 0.41−0.11 (br, 12H). Anal. Calcd for C66H100N2O2S2: C, 73.28; H, 9.32; N, 2.59. Found: C, 72.91; H, 8.97; N, 2.53. PDPPTT-SiTG. Isolated yield = 77%. Mn = 49.3 kDa, Mw = 158.3 kDa, PDI = 3.21. 1H NMR (CDCl3, 600 MHz, 293 K): δ ppm 9.10−8.80 (br, 2H), 7.10−6.45 (br, 4H), 4.90−4.78 (br, 4H), 2.61−2.31 (m, 4H), 1.02−0.88 (m, 4H), 0.90−0.77 (m, 4H), 0.40−0.23 (br, 36H), 0.13−(−0.18) (br, 6H). Anal. Calcd for C42H64N2O6S4Si6: C, 50.97; H, 6.52; N, 2.83. Found: C, 50.51; H, 6.79; N, 3.01. OFET Fabrication and Measurement. OFETs with bottomgate/top-contact configuration were prepared to characterize the electrical performance of PDPPTT polymers. A highly n-doped (100) Si wafer (