Near 14 cm2 V-1 s-1 Charge Carrier Mobility Enabled by Amorphous Indacenodithiophene-based Polymers with Controlled Regioregularity

Herein, we report the synthesis and characterization of four regioregular, well-dened donor–acceptor polymers (P1, P2, P3, and P4), comprising different compositions of axisymmetric cyclopentadithiophene (CDT) and centrosymmetric indacenodithiophene (IDT) donors in conjugation with the asymmetric 5-uoro-2,1,3-benzothiadiazole acceptor that is precisely oriented in the regular pattern along the backbone. Morphological analyses of the above polymer series show that exclusive CDT donor-containing P1 is semicrystalline, whereas the others (IDT donor-containing ones) are near-amorphous in nature. Comparatively, IDT donor-containing polymers have superior hole mobilities; in particular, exclusive IDT donor-containing polymer P4 offers an ultra-high mobility of 13.82 cm 2 V − 1 s − 1 at a channel length of 200 µm, which is comparable to the recently reported values for state-of-the-art semicrystalline semiconductors. In addition, the near-amorphous characteristics render the IDT donor-containing polymer lms highly ductile and stretchable. Such superior features, which are associated with excellent charge transport and ductility, demonstrate a promising possibility for application in viable stretchable electronics.

Introduction π-conjugated polymers have been extensively studied for their application in printing organic eld-effect transistor (FET) arrays and circuits to realize low-cost, large-area, exible, and even stretchable nextgeneration electronics that cannot be realized using traditional silicon technologies. [1][2][3][4][5][6][7] On the basis of the principle that charge carrier mobility is unambiguously linked to the degree of order in the packing, more than a decade of research has focused on increasing the crystallinity of π-conjugated polymers as a strategy for improving charge transport; this has reliably boosted mobility to exceed 10 cm 2 V − 1 s − 1 . 8−13 Three major design strategies exist for enforcing polymer crystallinity: (i) donor-acceptor architecture in the backbone induces a strong intramolecular charge transfer (ICT) effect as an attractive force between the donor and acceptor units, resulting in a more planar con guration that facilitates πelectron delocalization along the backbone; [14][15][16][17] (ii) the incorporation of fused aromatic building segments into the backbone generates a large π-orbital overlapping area, formulating large crystalline domains with only few disordered domain boundaries; 12, 18−21 and (iii) a precise regioregularity in the polymer repeating units realizes enhanced structural arrangement of the chains, thus promoting selforganization. 16, 22−24 As a successful example that integrates all of the abovementioned concepts for FET applications, Bazan et al. have reported a semicrystalline polymer with the donor-acceptor structure comprising cyclopentadithiophene (CDT) as the fused aromatic donor unit and 5-uoro-2,1,3benzothiadiazole (FBT) acceptor with the regioregular orientation of F atoms. 23 Compared to the axisymmetric CDT unit, indacenodithiophene (IDT), which contains peripheral thiophene rings held in a rigid arrangement through a central phenyl ring, is recognized as part of the extended CDT analogue centrosymmetric structure. Interestingly, polymers based on the extended fused-ring IDT core display high FET mobilities with high backbone coplanarity despite the lack of long-range crystalline order in the solid state. 2,3,[25][26][27][28] This intriguing feature considerably differs from the abovementioned theory and provides a new molecular-design guideline for achieving high mobilities in near-amorphous polymers, thus facilitating the development of intrinsically stretchable semiconducting polymers toward next-generation electronics. Driven by their unique, yet different properties, many polymers based on CDT and IDT units have been incorporated into high-performance FETs and continue to be studied (see Fig. 1 and Table S1). 2,14,16,23,26,[28][29][30][31][32][33][34][35][36] However, there is no systematic comparison study between CDT and IDT polymer systems with backbone regioregularity in terms of optoelectronic and morphological properties.
Herein, we aim to synthesize and characterize four donor-acceptor regioregular polymers (P1, P2, P3, and P4) with varied compositions of CDT and IDT donor subunits in conjugation with the regularly arranged FBT acceptor along the backbone; then, we investigate their intrinsic properties including their optical property, energetics, morphology, molecular packing, and charge transport. Notably, we discovered that in addition to their higher ductility, as evidenced by the crack onset strain measurement, FETs prepared using IDT donor-containing near-amorphous polymers show higher mobilities than those of exclusive CDT donor-containing semicrystalline P1. In particular, P4 exhibited an unprecedentedly high hole mobility of up to 13.82 cm 2 V − 1 s − 1 at a channel length of 200 µm. To the best of our knowledge, this is the highest mobility value among near-amorphous polymers reported to date.
As shown in Fig. 3 and Figures S14-17, in addition to the accurate peak area integrations, high-   Table 1. All polymers have π-π* transition bands that are centered at approximately 410 nm and broad ICT transition bands ranging from 450 to 1000 nm. Notably, upon transitioning from solution to thin lms, slight red-shifts occur for IDT donor-containing polymers (P2, P3, and P4), whereas no distinct absorption difference is observed for exclusive CDT donor-based P1. This suggests a breakup of the solid-state structural order within polymer chains to some extent with the incorporation of centrosymmetric IDT into the backbone. 30,39 In addition, one can observe that absorption spectra shift toward longer wavelengths with increasing CDT content in the repeating polymer unit. Therefore, optical bandgaps (E g opt ) determined by the onsets of absorptions in the lms are in the order of P1 < P2 ≅ P3 < P4, indicating that CDT is a stronger electron donor relative to IDT; this result is consistent with the abovementioned CV-derived oxidation potentials. The CV measurement of all polymers in thin lms was used to evaluate their electronic energy levels (highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO)). Figure 4c shows that all polymers displayed strong quasi-reversible oxidation peaks accompanied by irreversible reduction ones, indicating their p-type dominant character (vide infra). 40 (Table 1). As schematically visualized in their energy level diagram in Fig. 4d, the IDT donor-containing polymers (P2, P3, and P4) showed similar, deeper-lying LUMO levels compared to exclusive CDT donorbased P1 as well as gradual push-down HOMO levels with higher IDT content within the polymer repeating unit. Notably, the considerably higher-lying HOMO of P1 over the other polymers is attributed to the stronger electron-donating ability of CDT over IDT, which is in good agreement with the optical and electrochemical properties of CDT vs. IDT described above.
The DFT calculations of dimers of each polymer were modeled using the B3LYP/6-31G* basis set, with the long side chains replaced with methyl groups to reduce computation time (Figs. 5, S1, S2, Table S2). In addition to a similar trend of HOMO/LUMO levels with CV-derived data, for all cases, the LUMOs are centralized on electron de cient FBT units; however, the HOMOs are well-delocalized on molecular backbones. In the side view, it is observed that all dimers adopt almost coplanar backbones in the minimum-energy con guration most likely owing to noncovalent F···S and N···H intramolecular interactions within regular, alternating patterns between donor (CDT and/or IDT) and acceptor (FBT) units. 36,42 Further, the different conformation (syn (CDT) vs. anti (IDT)) of the S-atoms of thiophenes within the donor building blocks favors different optimized ground-state structures. For example, the P1 dimer can be described as a bent, banana-type structure; both P2 and P3 dimers adopt a sigmoid-like shape, whereas P4 forms a rather linear geometry. This observation suggests that P4 has a lower degree of energetic disorder compared to others. 27,43 Such varied conformations can signi cantly impact the magnitude and direction of dipole moments of dimer models, as summarized in Figure S2 and Table S2.
Note that relative to the models simulated for IDT-containing polymers, the P1 dimer has an overall higher dipole moment, which may increase the dipole-dipole interaction for better polymer chain assembly in the solid state. 44,45 Thin-Film Microstructure Analysis Tapping-mode atomic force microscopy (AFM) was used to investigate the surface morphology of polymer lms (Figs. 6a-d). To accurately compare the morphology with transistor characteristics, we passivated silicon dioxide (SiO 2 ) surfaces with n-decyltrichlorosilane (n-DTS) as a self-assembled monolayer before casting polymer solutions, which is described in detail in the experimental procedure. In addition, thin lms were spin cast from chlorobenzene (CB) solution and annealed at each optimal temperature determined from the following FET tests. All lms annealed at each optimal temperature reveal overall uniform and smooth surfaces with root mean square (RMS) values below 1.0 nm. Notably, the P1 lm forms a more pronounced brillar microstructural feature with a relatively larger RMS (0.99 nm) compared to other lms (0.31−0.39 nm), evidencing its high-order bulk organization structure, at least on the surface. 46 We further characterized the molecular orientation and crystallinity in the optimally annealed lms using grazing-incidence wide angle X-ray scattering (GIWAXS) analysis.  Table S3.
The P1 polymer shows distinct long-range ordered (n00) lamellar diffraction peaks in the out-of-plane which is indicative of the formation of larger crystallites in P1. Thus, we anticipate that P1 is typically favorable for e cient charge transport through the lateral charge-hopping mechanism. [47][48][49] However, IDT donor-containing polymers tend to adopt a face-on/edge-on coexisting bimodal texture, as con rmed by the strong (010) π-π peak in the OOP direction together with multiple (n00) lamellar peaks, which correspond to d (010) = 4.06-4.08 Å and d (100) = 24.0-24.2 Å, respectively. In particular, note that the ring-like halos surrounding the (010) π-π peak, as previously observed in other reported polymers employing an IDT unit, clearly appeared in IDT donor-containing polymers, which indicates their nearamorphous microstructure. 2,25,29,33 In addition, changes occur in the diffraction patterns among IDT donor-containing polymers.

Electrical Characterization
To evaluate the electrical properties of the polymers, two types of FET geometries, i.e., bottom-gate/topcontact (BGTC) and bottom-gate/bottom-contact (BGBC), were fabricated and characterized, wherein Au was used as source and drain contact electrodes. Both device architectures are schematically shown in Fig. 7a. The devices were subjected to the thermal annealing process ranging from 180 °C to 240 °C, which showed the optimal device performance at different annealing temperatures for each polymer, e.g., P1 (240 °C ), P2 (180 °C ), P3 (180 °C ), and P4 (200 °C ). Figure 7b and Figures S3-S4 show a representative transfer and output characteristics of the best-performing FETs fabricated with the polymers, and detailed FET parameters are summarized in Table 2 Table 2).
The observed highest mobility of P4 is attributed to reduced trap densities of P4 thin lms. To compare trap densities of the polymers, we calculated deep trap densities (N tr ) from subthreshold swings (S) of BGTC FETs with MoO 3 /Au using the following equation, 6 where C i is the capacitance of the gate dielectric per unit area, e is the elementary charge, k is the Boltzmann constant, and T is the measured temperature.  Figure S7). Performance dependence on device geometry is most likely due to the morphological change between bulk and surface regions of polymer lms. A similar phenomenon has also been observed in our previous study on thienoisoindigo-naphthalene polymer. 12 However, we cannot rule out the in uence of Ni/Au source/drain contact resistance on the electrical property of the BGBC structure.
Driven by the signi cantly improved performance of the BGBC structure, we further characterized the FET properties of polymers by varying the channel length from 20 to 200 μm. ( Figure S8) We determined that the mobilities of IDT donor-containing polymers (P2, P3, and P4) increased with longer channel length, whereas P1 showed nearly constant mobility values regardless of the channel length ( Table 2 and Table   S5). Because IDT donor-containing polymers have deeper-lying HOMOs than P1, their mobility enhancement is partially due to the decreased portion of contact resistance in the total resistance of longchannel FETs. In addition, we determined that P4-based devices with varied channel lengths outperformed other devices, as has been observed in the abovementioned studies. Notably, an unprecedented mobility of up to 13.82 cm 2 V −1 s −1 is achieved for P4-based BGBC FET at the channel length of 200 μm ( Figure 7e and Table 2). Thus far, this mobility is the highest value among amorphous polymers reported in the literature and is approximately comparable to the values obtained from the recently reported highly ordered crystalline polymers. Assuming that the highest mobility can be obtained with minimized contact resistance, this result implies that P4 can produce hole mobility that is higher than the current highest value reported herein (i.e., 13.82 cm 2 V −1 s −1 ) when ohmic contacts are favorably formed at semiconductor/contact interfaces. We anticipate that interfacial doping of the semiconducting polymer area adjacent to contact electrodes is a promising approach to obtain high mobility of shortchannel IDT-based FETs. 51 These results open up possibilities for achieving high mobility of IDT-based polymers by forming optimal contact junctions in FET con gurations.

Stretchability
To demonstrate a certain unique feature, which originates from amorphous IDT-containing polymers over semicrystalline P1, we performed crack onset strain measurements for all polymer thin lms. First, the polymer thin lms were spin-cast onto SiO 2 substrates and then transferred to PDMS substrates (approximately 500-μm thickness). The polymer/PDMS lms were stretched at discrete strain values (ε = 30%, 60%, and 100%) and then monitored under strain via optical microscopy. Figure 8 shows optical microscope images of annealed polymer thin lms at each strain value. Pristine (ε = 0%) P1 shows a rough, nonuniform surface with some gross aggregation, most likely due to its high crystalline nature. Numerous microcracks begin to form even at ε = 30%. However, pristine IDT-containing polymers appear to be featureless with a uniform surface and exhibit highly ductile behavior during stretching tests. Both P3 and P4 did not exhibit diamond-shaped cracks at ε = 60% strain, particularly for P3 with mechanical robustness up to ε = 100%. Although further investigation is required to understand reasons for the highest ductility of P3, we attributed such high stretchability to the amorphous nature of IDT-based polymers and more entangled IDT polymer chains. Notably, upon relaxation after being stretched at ε = 30% and 60%, amorphous IDT donor-containing polymers exhibit a wrinkled surface; in contrast, the P1 lm does not exhibit wrinkles but contains chunked lumps to some extent ( Figure S9).
In summary, we designed and synthesized four regioregular donor-acceptor polymers (P1, P2, P3, and P4) based on axisymmetric CDT and centrosymmetric IDT donors with the precise orientation of the asymmetric FBT counterpart acceptor relative to the backbone vector. The different backbone composition with the same regioregularity modi es photophysical and electrochemical properties and electronic structure of the polymers. Relative to the exclusive CDT donor-containing semicrystalline P1, IDT donor-containing amorphous polymers (P2, P3, and P4) exhibit higher hole mobilities in different FET con gurations tested herein, despite their lower degree of structural order within the lms, as indicated by the GIWAXS characterization. Speci cally, P4-based BGBC FET reached an unprecedented value up to 13.82 cm 2 V −1 s −1 at the channel length of 200 μm; this value was approximately comparable to the values obtained from recently reported highly ordered top-performing crystalline polymers. Moreover, amorphous IDT donor-containing polymer lms exhibit superior deformability and ductility, particularly for P3 that has a stretchabilty up to ε = 100%. The systematic study described herein reinforces the current understanding of the structure-property relationship in terms of backbone symmetricity, composition, and regioregularity; in addition, this study upgrades the FET performance level of sporadically reported amorphous polymers, which can facilitate the rational designing of top-performing polymers for stretchable electronics applications.