Tuning of the Stretchability and Charge Transport of Bis‐Diketopyrrolopyrrole and Carbazole‐Based Thermoplastic Soft Semiconductors by Modulating Soft Segment Contents

Polymer semiconductors are promising materials for stretchable, wearable, and implantable devices due to their intrinsic flexibility, facile functionalization, and solution processability at low temperatures. However, the crystalline domain of the conjugated structure for high charge carrier mobility in semiconducting polymers exhibits lower stretchability than that of the semi‐crystalline or amorphous domains. Herein, a set of thermoplastic soft semiconductors is synthesized with different ratios of diketopyrrolopyrrole–carbazole–diketopyrrolopyrrole (DPP‐Cz‐DPP)‐based hard segments and thiophene‐based aliphatic soft segments, having the similar structure of thermoplastic elastomers. The polymers exhibit decreased glassy temperatures with the increased content of the soft segments. The polymers show high crystallinity after copolymerization with a large‐sized DPP‐Cz‐DPP core and non‐conjugated segments due to an aggregation property of the conjugated core, still possessing a semi‐crystalline domain after annealing. The polymer films exhibit stretchability under strains of up to 60%. Organic field‐effect transistors fabricated using stretchable polymers show a mobility range of 0.125–0.005 cm2 V−1 s−1 with different proportions of the soft segment. The stretchability is improved significantly and the mobilities are decreased less when the content of the soft segment is increased. Therefore, this study presents a design principle for the development of a high‐performance stretchable semiconducting polymer.

dicarboxamide-based spacers, and isophthalamide-based spacers, have been introduced into the backbone to improve the stretchability of copolymers. [33] Thermoplastic elastomer containing hard segments of high melting point and soft segments, which are easily decomposed at low temperatures, has excellent stretchability and strong mechanical properties. Similar to the structure of thermoplastic elastomer, the thermoplastic soft semiconductor is synthesized with hard segments of high crystallinity and soft segments, exhibiting an effective charge transport from the microstructure with semi-crystalline or amorphous domains. [19,22] And only the soft segments in the stretched thermoplastic soft semiconductor will be deformed, while the hard segment maintains the crystalline domain. In previous studies, diketopyrrolopyrrole-carbazole-diketopyrrolopyrrole (DPP-Cz-DPP)-based organic semiconductors exhibited high thermal stability and a highly crystalline microstructure. [51][52][53][54][55] Although the chemical structure of DPP-Cz-DPP has some torsion due to the steric hindrance between DPP and carbazole, it achieves high crystallinity and good aggregation properties, as shown from the differential scanning calorimetry (DSC)' curves of DPP-Cz-CPP. [52] Therefore, DPP-Cz-DPP moiety is suitable to be used as a hard segment of a thermoplastic soft semiconductor, owing to a larger crystalline domain than that of the previously reported moieties for stretchable polymers such as diketopyrrolopyrrole or thiophene-based small-sized conjugated cores.
Herein, we synthesized a set of thermoplastic soft semiconductors, denoted as PbDCT-based polymers, containing a DPP-Cz-DPP core and thiophene-based aliphatic soft segments via random copolymerization (Scheme 1). The mechanical and electrical properties of the polymers were investigated by observing the film morphology and fabricating organic field-effect transistors (OFETs). In the thermal phase transition analysis through DSC, the thermoplastic polymers exhibited decreased glassy temperatures owing to the increased content of the soft segments in the polymer backbone. The PbDCT-based copolymer films with semi-crystalline domains exhibited enhanced stretchability owing to the incorporation of the soft segments. OFETs with PbDCT-based polymers showed decreased mobilities with increasing content of the soft segments. However, the mobilities decreased less, even if the content of the soft segment is increased, whereas the stretchability is improved steeply. Therefore, our method demonstrates an effective approach for fabricating high-performance stretchable electronics by modulating soft segments in thermoplastic soft semiconductors, with minimal sacrifice of mobility.

Materials and Characterization
All starting materials and reagents were purchased from commercial sources and used without further purification unless otherwise specified. 1,12-Di(thiophen-2-yl)dodecane (T12) and Br-DPP-Cz-DPP-Br were prepared according to a previously reported procedure. [50,52] Proton nuclear magnetic resonance ( 1 H NMR) spectra were recorded on a Bruker AVANCE III 300 MHz spectrometer. The number-average molecular weight (M n ), weight-average molecular weight (M w ), and polydispersity index (PDI = M w /M n ) of the polymers were determined via gel permeation chromatography (GPC) at 40 °C using polystyrene as the standard and chloroform as the eluent. The solution and thin film absorption spectra were measured using a Thermo Scientific Evolution 600 UV-vis spectrophotometer. Thermogravimetric analysis (TGA; TA Instruments Auto TGA Q500) and differential scanning calorimetry (TA Instruments DSC 250) were performed under a nitrogen atmosphere at a heating rate of 10 °C min −1 . A conventional three-electrode cell was used for the cyclic voltammetric (CV) measurements. A platinum was used as the counter electrode, a thin indium tin oxide (ITO) film was used as the working electrode, and an Ag wire was used as the reference electrode in a computer-controlled VersaSTAT3 instrument (Ametek) at room temperature. The films were prepared by spin-coating a chlorobenzene (CB) solution of the polymers on the indium-tin-oxide working electrode. All measurements were performed in anhydrous acetonitrile with 0.1 M tetrabutylammonium perchlorate as the conducting electrolyte.

Synthesis of 1,12-Bis(5-bromothiophen-2-yl)dodecane (DiBr-T12)
T12 (0.50 g, 1.5 mmol) was added to a flask containing 20 mL chloroform. A solution of N-bromosuccinimide (0.56 g, 3.15 mmol) was added slowly in three folds to the reaction mixture and stirred for 6 h at room temperature. The reaction mixture was poured into deionized (DI) water and extracted with dichloromethane. The organic phase was dried over anhydrous magnesium sulfate and the solvent was removed under vacuum. The residue was purified via column chromatography on silica gel using hexane as the eluent to obtain pure DiBr-T12 (0.54 g, 73%) as an orange solid. 1 Figure S1, Supporting Information.

Film Characterization
To characterize the surface morphology of the PbDCT-based polymers, they were dissolved in anhydrous CB with a concentration of 5 mg mL −1 and spin-coated at 2000 rpm for 1 min on SiO 2 substrates. The coated films of PbDCT, PbDCT12_7:3, PbDCT12_5:5, PbDCT12_3:7, and PbDCT12_1:9 showed 4, 11, 16, 17, and 20 nm thickness, respectively. And the annealed films exhibited 4, 11, 13, 15, and 17 nm thickness, respectively. Height and electrical conductivity images of the organic semiconductor films were obtained via atomic force microscopy (AFM; Park Systems, NX-10) at the Cooperative Center for Research Facilities, Yonsei University. For the conductive AFM, a bias of 5 V was applied to the polymer films. To analyze the thin-film microstructure, grazing incidence wide-angle X-ray scattering (GIWAXS) was performed at the 9A beamline of the Pohang Accelerator Laboratory. The 2D scattering patterns of the samples were obtained via X-ray diffraction at a grazing angle of 0.12°. To measure the crack-forming strain of the polymer films, the PbDCT-based polymer films were transferred to PDMS-based stretchable substrates using polyvinyl alcohol (PVA)-modified substrates. PDMS substrates were prepared by blending a precursor (Sylgard 184) and a cross-linker with a ratio of 12:1. The thickness of the PDMS substrate is approximately 1.5 mm after cross-linking. PVA was dissolved in DI water to 5 wt% and spin-coated at 2000 rpm for 1 min on a SiO 2 substrate as the sacrificial layer. The PbDCT-based polymers were spin-coated at 2000 rpm for 1 min on PVA-modified substrates. The samples were annealed at the optimized temperature of 15 min and cooled at room temperature. The polymer-coated samples were attached to stretchable PDMS substrates. Water was then added to the gap between the two substrates. After the dissolution of PVA, the polymer films remained on the PDMS substrates.

Organic Field-Effect Transistor (OFET) Fabrication and Characterization
OFETs composed of PbDCT-based polymers were fabricated with a top-gate/bottom-contact (TG/BC) structure. The source and drain electrodes of Au/Ni (15/5 nm thickness) were patterned on Corning Eagle 2000 glass substrates using conventional photolithography and lift-off processes. The substrates were cleaned using bath ultrasonication in DI water, acetone, and isopropyl alcohol for 10 min each. After drying in an oven, the substrates were treated with oxygen plasma for 3 min at 120 W. For the active layers, dissolved PbDCT-based polymers in CB were spin-coated at 2000 rpm for 1 min on the substrates in a nitrogen-filled glovebox. The samples were annealed at various temperatures for 20 min. For the dielectric layers, poly(methyl methacrylate) was dissolved in n-butyl acetate, spin-coated at 2000 rpm for 1 min to a thickness of 500 nm, and annealed at 80 °C for 1 h. The Al gate electrode (50 nm thickness) was deposited with shadow masks using a thermal evaporator. Electrical characterization of the OFETs was performed using a semiconductor parameter analyzer (Keithley 4200-SCS) in a nitrogenfilled glovebox. The mobilities of the saturation regions were evaluated using the standard formula: µ 2

Polymer Synthesis
First, a thiophene-based aliphatic monomer (DiBr-T12) was synthesized to endow stretchability into the polymer backbone through bromination with N-bromosuccinimide from T12, as shown in Scheme 2. To determine the difference in properties according to the mixing ratio of the highly aggregated DPP-Cz-DPP and flexible aliphatic spacer (T12) in the polymer backbone, the polymers were synthesized with different ratios of DPP-Cz-DPP (100%, 70%, 50%, 30%, 10%, and 0%.) The synthesis procedures of all PbDCT-based polymers are summarized in Scheme 1, and the details are described in Experimental Section. Six PbDCT-based polymers were prepared via the Stille cross-coupling polymerization with different mixing ratios of the monomers under the same reaction conditions. After random copolymerization, the crude polymers were washed using Soxhlet extraction with methanol, acetone, and hexane to remove any by-products and oligomers. Finally, the chloroform fractions were recovered through precipitation in methanol, filtered, and dried under vacuum to obtain a dark purple solid. Except for the polymer without DPP-Cz-DPP, denoted as PT12T, the other five polymers exhibited sufficient solubility in organic solvents, such as chloroform, CB, and o-dichlorobenzene. The M n and PDIs of the five polymers were determined via gel GPC at 40 °C using chloroform as the eluent relative to the polystyrene standards.

Thermal Properties
The thermal stabilities of the polymers were investigated via thermogravimetric analysis; the results were summarized in Table 1 and Figure S7, Supporting Information. All polymers exhibited high thermal stability, and no decomposition was observed below 400 °C. As the amount of flexible aliphatic backbone spacer increased, the ash content decreased at temperatures above 500 °C owing to the low thermal stability, as shown in Figure S7, Supporting Information. The thermal phase transition behaviors of the random polymers were investigated via DSC, as shown in Figure 1. All polymers were scanned in the range of −50 to 330 °C under nitrogen at a heating rate of 10 °C min −1 . Unlike the previously reported DPP-Cz-DPP-based polymer, namely, PCbisDPP, [52] PbDCT exhibits broad melting (T m ) and recrystallization (T c ) temperatures during the heating and cooling scans. After incorporating the flexible aliphatic backbone spacers, T m and T c decreased, with more pronounced peaks. In addition, the T m of the flexible aliphatic backbone spacers was observed below 100 °C, which implies high ductility. [22] On the other hand, the PT12T composed of a flexible aliphatic backbone showed a higher T m than other random polymers, probably due to the alternated sequence in the chain of PT12T.

Optical and Electrochemical Properties
The normalized UV-vis-NIR absorption spectra of the five PbDCT-based polymers in CB solution and as thin films were presented in Figure 2. The corresponding optoelectronic properties, including absorption peak wavelengths, absorption edge wavelengths, and optical band gaps, were summarized in Table 1. All the polymers in the solution and film states exhibited broad and dual absorptions in the range of 300-800 nm, which are attributed to the localized π-π* and inter-and intramolecular charge transfer transitions. These absorption profiles   are typically observed for donor-acceptor-type conjugated polymers. As the amount of flexible aliphatic backbone spacers increased, a hypsochromic shift was observed in the solution and thin film states because of the decrease in the conjugation length. In addition, vibronic features were clearly observed for PbDCT and PbDCT12_7:3, whereas those of PbDCT12_5:5 to PbDCT12_1:9, which contained higher contents of flexible aliphatic backbone spacers, decreased. This phenomenon can be ascribed to an increase in solubility owing to the flexible aliphatic backbone.
To evaluate the electronic structures of the polymers, CV measurements were performed on the polymer films in acetonitrile with tetrabutylammonium hexafluorophosphate as the supporting electrolyte. The highest occupied molecular orbital (HOMO) of each polymer was calculated from the onset of the electrochemical oxidation. The HOMO levels of PbDCT, PbDCT12_7:3, PbDCT12_5:5, PbDCT12_3:7, and PbDCT12_1:9 were −5.08, −5.09, −5.14, −5.17, and −5.19 eV, respectively. As the proportion of flexible aliphatic backbone spacers increased, the HOMO levels of the polymers deepened owing to the increasing bandgap.

Film Morphology Characterization
To observe the surface morphology of the PbDCT-based polymer films via AFM, the thin films were spin-coated on SiO 2 substrates and heated at 100 °C or annealed at optimized temperatures, which are close to the transition temperatures of the polymers. The optimized temperatures were 250 °C for PbDCT, 200 °C for PbDCT12_7:3, PbDCT12_5:5, and PbDCT12_3:7, and 150 °C for PbDCT12_1:9. Nanograined morphologies were observed in the polymer films, as shown in Figure 3a and Figure S8, Supporting Information. After heating at 100 °C, PbDCT, PbDCT12_7:3, PbDCT12_5:5, PbDCT12_3:7, and PbDCT12_1:9 exhibited root-mean-square surface roughness values of 0.60, 0.63, 0.52, 0.69, and 2.68 nm, respectively, which decreased to 0.57, 0.62, 0.49, 0.60, and 1.34 nm in the annealed films, respectively. The smoothest surface was achieved with the annealed PbDCT12_5:5 film by incorporating flexible aliphatic moieties. However, the roughness of the surface increased again with the addition of more aliphatic moieties. In particular, the aliphatic composition formed large grains, leading to phase separation in PbDCT12_1:9. Therefore, the excessive aliphatic content of the backbone disrupts the stacking of DPP-Cz-DPP, thereby reducing the conjugation lengths.
From the conductive AFM measurements at a bias of 5 V, the brightness of the PbDCT film increased, as shown in Figure 3b, indicating its high electrical conductivity. In addition, several bright spots exhibiting high electrical conductivity were observed on the PbDCT12_7:3 film. However, low conductivities were measured for the other films. These results suggest that the decreased electrical conductivity is attributed to the broken conjugation owing to the increased content of the insulating non-conjugated spacer in the backbone, which is consistent with the results of the OFET performance evaluation, which are discussed in a subsequent section. To investigate the structural order and molecular distance of the semi-crystalline polymers, GIWAXS measurements were performed, and two-dimensional patterns were obtained from the organic semiconductor films, as shown in Figure 3c and Figure S9, Supporting Information. The thin films were prepared similarly to that of the AFM measurement samples on Si substrates. After annealing, the PbDCT-based polymers were reorganized and exhibited lamellar stacking in the (100), (200), and (300) plane directions, except for PbDCT and PbDCT_1:9. PbDCT and PbDCT12_1:9 exhibited nearly isotropic orientations. From the DSC curve, PbDCT had no glassy temperature, suggesting that the reorganization of the polymers by annealing occurred in random directions. In contrast, the crystallinity of the PbDCT12_1:9 film decreased because of the excessive aliphatic spacers, as seen in the AFM image. From the one-dimensional GIWAXS profiles of the PbDCT-based polymer films in the out-of-plane direction (Qz direction), π-π stacking peaks are observed for PbDCT12_7:3, PbDCT12_5:5, and PbDCT12_3:7 after annealing ( Figure S10 (200) planes did not differ significantly. However, the peak positions of the (300) plane varied depending on the content of the soft segments on the annealed films. For PbDCT12_3:7 in the in-plane, a prominent (010) π-π stacking peak is observed ( Figure S11, Supporting Information), which can be ascribed to the phase separation caused by excess aliphatic spacers.

Crack Formation Strain
The crack on-set strain of the polymers was measured as a representative stretchable property. And strain-induced morphological changes on polymer films were observed under single tensile loading, as a method of evaluating the fracture strain. To measure the fracture strain of the PbDCT-based polymers, the polymer films transferred to stretchable PDMS substrates were imaged using optical microscopy (OM) under various strains (Figure 4). The fracture strain of the polymer was determined by observing the crack propagation in the polymer films under strain. The film was stretched in one direction, and a strainrelated change was observed in the film morphology. PbDCT, which comprised hard segments only, was easily damaged at low strains. Similar to previously reported crystalline-domain polymers, the fracture strain was less than 10%. [21,22] In contrast, copolymers with soft segments were less damaged even under a higher strain. Therefore, thermoplastic soft semiconductors could withstand increased strain without cracks in the polymer film owing to the increased content of the soft segments. PbDCT12_1:9 with a soft segment proportion of 90% could withstand strains of less than 70%.

OFET Performance
To investigate the properties of the charge carrier transport in PbDCT-based OFETs, we fabricated devices using a solution process. The dissolved PbDCT polymers were spin-coated on rigid substrates with pre-patterned gold source-drain electrodes for the top-gate/bottom-contact structure. The deposited films were annealed at 100, 150, 200, and 250 °C (Figure 5, Figures S12 and S13, Supporting Information). The charge carrier mobility was evaluated using a gradual channel approximation equation, where the channel width and length were 1000 and 20 µm, respectively (see Experimental section). The threshold voltage (V th ) and the on/off ratio were calculated in the saturation regime. The performances of the OFETs are summarized in Table S1, Supporting Information.
The OFETs exhibited enhanced mobility upon annealing at a temperature close to the transition temperature. The improved mobility is ascribed to the effective charge transfer in the oriented microstructures by annealing treatment. The OFETs annealed at 200 and 250 °C using PbDCT12_7:3 exhibited comparable performances owing to the transition temperature of 223 °C. From the DSC data, these optimized temperatures for obtaining semi-crystalline domains decreased because of the increased aliphatic spacer content. The performance of the OFETs decreased due to conjugation breaking, resulting from the increased content of the insulating spacers (Figure 5a-e). The PbDCT-based OFETs exhibited the highest mobility of 0.125 cm 2 V −1 s −1 without conjugation breaking, whereas PbDCT12_1:9 demonstrated the lowest mobility of 0.005 cm 2 V −1 s −1 owing to the decreased electrical conductivity caused by the excessive insulating spacers, as shown in the conductive AFM image. The mobilities of the OFETs and crack onset strains were plotted depending on the content of the DPP-Cz-DPP unit, as shown in Figure 5f. The extended π-conjugation is broken by an increased content of soft segment possessing the long aliphatic chain, and the charge carrier transport is disturbed. Therefore, the stretchability is improved by the increased content of the soft segment, but the mobility of the charge carrier is decreased, resulting in a trade-off relationship between mobility and fracture strain. A trade-off relationship between mobility and fracture strain was noted. [18,20] However, the mobilities decreased less with the decreasing DPP-Cz-DPP proportion from 70% to 30%, whereas crack-formed strains rapidly increased. Therefore, devices with improved mechanical properties and suitable charge transport behavior were successfully fabricated by introducing soft segments at an optimized proportion.

Conclusion
This paper presents the synthesis of a series of stretchable polymer semiconductors employing a new design of thermoplastics with random copolymerization, using a largesized DPP-Cz-DPP hard segment and a thiophene-based aliphatic soft segment. The soft segment in the backbone of the copoly mer affected the thermal, electrical, and mechanical properties of the polymer semiconductors. The PbDCT-based thermoplastic soft semiconductors exhibited decreased glassy temperatures with increasing content of the soft segments in the polymer backbone. PbDCT-based copolymers with a proportion of soft segments of 90% exhibited enhanced stretchability under strains of up to 60%. A trade-off relationship was observed between the field-effect mobility and fracture strain of the PbDCT-based stretchable polymer. The OFET with the PbDCT-based polymer exhibited the highest mobility of 0.125 cm 2 V −1 s −1 without conjugation breaking, whereas the OFETs with an insulating spacer with a content of 90% in the polymer backbone demonstrated the lowest mobility of 0.005 cm 2 V −1 s −1 . However, the mobilities decreased less depending on the increasing ratio of the soft segment, whereas the rate of fracture strain abruptly increased. Therefore, the OFETs with improved mechanical properties and suitable charge-transport behavior can be fabricated by introducing soft segments at an optimized proportion. Therefore, this study suggests a design guideline that a conjugated core forming a large-sized crystalline domain should be introduced in the backbone of stretchable semiconducting polymer to reduce the conjugation breaking. Additionally, the study findings demonstrated that the fabrication of high-performance stretchable electronics can be achieved by modulating the content of soft segments in thermoplastic soft semiconductors.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.