Styrenic polymer-additive engineering of crystalline defects and charge transport in TIPS-pentacene organic thin-film transistors

Solution-processed organic thin-film transistors (OTFTs) are promising emerging platforms for flexible micro- and nano-electronics; however, crystalline defects, orientational disorder, and incomplete semiconductor coverage can severely limit charge transport. Here, styrenic polymer additives are investigated as a solution-processable strategy for engineering the crystallization and defect morphology of 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS pentacene, TP). Polystyrene (PS) and poly (α-methylstyrene) (PαMS) were evaluated as styrenic polymer additives. Their effects on crystalline morphology and electrical performance were investigated using polarized optical microscopy, scanning electron microscopy, and transistor electrical characterization. While pristine TP films exhibited substantial orientational disorder and incomplete substrate coverage, PS reduced the misorientation angle but produced relatively narrow domains with pronounced dendritic boundaries. In contrast, TP/PαMS films exhibited nearly parallel crystalline domains, with an average misorientation angle of 1.07 ± 1.29°, a domain width of 75.13 ± 24.63 μm, and a crystal coverage of 83.17%. The simultaneous improvement in alignment, domain width, and coverage minimized morphological interruptions associated with grain-boundary-limited transport and created more continuous source-to-drain pathways. Consequently, TP/PαMS devices achieved a maximum extracted saturation mobility of approximately 0.5 cm 2 /Vs, more than three times that of pristine TP devices. Because the PS and PαMS used in this study differ in molecular weight and exhibit different optimized additive loadings, the observed differences are interpreted as reflecting the combined effects of polymer structure, molecular weight, and concentration rather than being assigned exclusively to α-methyl substitution. More broadly, this study demonstrates that polymer-additive engineering provides an effective strategy for controlling crystalline defects and charge transport in next-generation solution-processed flexible electronics.

As conventional silicon transistors approach fundamental and manufacturing constraints, increasing attention has been directed toward emerging materials and processing strategies for next-generation micro- and nano-electronics (Oh et al., 2026; Lim, 2025; Buot, 1993; Lu and Lieber, 2007). While two-dimensional materials (Bhimanapati et al., 2015; Butler et al., 2013), carbon nanotubes (De et al., 2013; Tasis et al., 2006), and oxide semiconductors (Shi et al., 2021; Wan et al., 2026; Pal et al., 2026) are being explored primarily for aggressive device architectures and advanced integration, solution-processable organic semiconductors provide a complementary platform for low-temperature, lightweight, flexible, and large-area microelectronics and nanoelectronics (Hussain, 2018; Eze et al., 2026). Organic thin-film transistors (OTFTs) are particularly attractive for wearable sensors (Zhang et al., 2025; Song et al., 2024), flexible displays (Park et al., 2024; Hao et al., 2024), radio-frequency identification devices (Panagiotidis et al., 2026; Anam et al., 2025; Saetia et al., 2025), and low-cost integrated circuits (Wang X.-Y. et al., 2025; Li and Chen, 2025; Cool et al., 2026). In contrast to conventional inorganic semiconductor devices, OTFTs can be fabricated at relatively low temperatures using solution-based deposition methods, providing compatibility with mechanically flexible substrates and scalable manufacturing processes (Kwon et al., 2024; Xu et al., 2026; Fraleoni-Morgera et al., 2026). Among solution-processable p -type organic semiconductors, 6,13-bis(triisopropylsilylethynyl) pentacene, commonly known as TIPS pentacene and abbreviated here as TP, has been widely studied because of its high solubility in common organic solvents, favorable environmental stability, and excellent hole transport capability (Diao et al., 2013; Giri et al., 2011; Li et al., 2009; Jung et al., 2010). TP has been extensively demonstrated in flexible electronic applications, including conventional OTFTs (Zhang et al., 2026; Wang H. et al., 2026), flexible and wearable devices (Nie et al., 2026; Guo et al., 2026; Wang Y. et al., 2025), phototransistors (Zhang et al., 2023; Park et al., 2013), photodetectors (Benavides et al., 2018; Zhao et al., 2018; Wang J. et al., 2026; Mumtaz et al., 2026), and chemical or gas sensors (Ramachandran and Nageswaran, 2026; Tene et al., 2026; Lee et al., 2022; Mehrotra et al., 2026). These attributes make TP a useful model material for understanding how solution processing and crystalline defects influence the performance of emerging organic transistors.

During solvent evaporation, TP molecules self-assemble into anisotropic crystalline domains whose orientation, width, spatial coverage, and connectivity strongly affect lateral charge transport (Shao et al., 2013; Lee et al., 2007). Efficient transistor operation requires the semiconductor domains to form continuous pathways between the source and drain electrodes. Grain boundaries, dendritic branches, exposed dielectric regions, and abrupt changes in crystal orientation can interrupt these pathways, introduce localized trap states, and increase interdomain transport barriers (Randle, 2010; Kim et al., 2011; Homer et al., 2015). Charge transport in TP is also strongly anisotropic, with mobility depending on the orientation of the crystalline domains relative to the source–drain direction (Chen et al., 2009). Consequently, controlling crystalline alignment and reducing boundary-limited transport are essential for improving field-effect mobility and device reproducibility. In this context, morphology control functions as a form of defect engineering in which the density and spatial distribution of electrically resistive regions are regulated during semiconductor deposition. Therefore, two major approaches, including external alignment techniques (He et al., 2022a; Kim et al., 2024; Tamayo et al., 2021) and polymer mixing (Ozorio et al., 2018; Intaniwet et al., 2011; Chung et al., 2010), have both been demonstrated as established strategies for controlling solution rheology, nucleation, solvent evaporation, phase separation, and crystal growth in solution-processed organic semiconductors. Although external alignment approaches, such as inkjet printing (Ryu et al., 2013; Chung et al., 2011; Lee et al., 2012), solution shearing (Kim et al., 2019; Kim et al., 2018), and zone casting (James et al., 2013; Kim et al., 2015; Su et al., 2013), can produce highly aligned crystalline films, they may require specialized equipment, additional processing steps, and precise control of externally applied forces. Polymer-additive engineering, on the other hand, provides an alternative strategy in which semiconductor crystallization is regulated intrinsically through changes in solution rheology, molecular interactions, phase separation, and crystal growth (He et al., 2024). Because polymers can be introduced directly into the semiconductor solution, this approach is compatible with flexible and large-area transistor applications.

The effects of polymer additives depend on their molecular structure, molecular weight, concentration, chain flexibility, glass-transition temperature, solubility, and miscibility with the semiconductor and solvent. Amorphous polymers can provide good solution compatibility and uniform film formation (He et al., 2022b), whereas conjugated polymers may contribute additional electronic interactions but introduce more complex phase behavior (Ding et al., 2023; Nguyen et al., 2023). Semicrystalline polymers can promote structural organization and mechanical stability but may also compete with semiconductor crystallization or induce excessive phase separation (Song et al., 2025). Polymer molecular weight further regulates chain mobility, solution viscosity, and entanglement. Using polyethylene oxide (PEO), our group previously demonstrated that low-molecular-weight polymer additives more effectively guided TP crystallization, whereas high-molecular-weight PEO restricted chain rearrangement, promoted excessive phase segregation, and reduced crystallinity and mobility (Bi et al., 2024). We also showed that the amorphous polyacrylates poly (ethyl acrylate), poly (butyl acrylate), and poly (2-ethylhexyl acrylate) regulate lateral and vertical phase segregation by altering miscibility and intermolecular interactions among the semiconductor, polymer, and solvent (He et al., 2013). These studies demonstrate that polymer additives can control semiconductor morphology through several interconnected solution-state and solid-state mechanisms.

Despite this progress, the influence of subtle variations in amorphous-polymer structure on organic-semiconductor crystallization and transistor performance remains insufficiently understood. Molecular structure is particularly important because the chemical composition and side-group configuration of a polymer govern its chain packing, local free volume, solution organization, and interactions with semiconductor molecules and advancing crystal interfaces (Cornil et al., 2001; Coropceanu et al., 2007; Shirota and Kageyama, 2007). Polystyrene (PS) and poly (α-methylstyrene) (PαMS) provide a useful pair of styrenic additives for investigating these effects. Both polymers contain pendant phenyl groups attached to a hydrocarbon backbone, while PαMS possesses an additional methyl substituent at the α-position of each repeating unit. This α-methyl group increases steric crowding near the polymer backbone and may modify chain organization and the local crystallization environment during solvent evaporation. Such changes could influence TP nucleation, lateral crystal growth, dendritic branching, and the formation of grain-boundary defects. In this study, we investigate how PS and PαMS regulate the crystallization, morphology, and charge-transport properties of TP-based OTFTs. Polarized optical microscopy, scanning electron microscopy, and quantitative image analysis were used to evaluate crystal orientation, domain width, substrate coverage, dendritic branching, and morphological discontinuities. Bottom-gate, top-contact OTFTs are subsequently fabricated to correlate these structural characteristics with electrical performance. This work sheds light on the intricate relationship between polymer-additive design and crystalline defect control, providing a practical materials-processing strategy for solution-processed organic transistors intended for flexible and large-area electronic applications.

Materials and Film Deposition: TIPS pentacene was used as the p -type small-molecule semiconductor. PS with a weight-average molecular weight of approximately 25 kg/mol and PαMS with a weight-average molecular weight of approximately 10 kg/mol were used as insulating polymer additives. Toluene was used as the solvent for preparing the semiconductor and polymer solutions. All materials and solvents were purchased from Sigma-Aldrich. Heavily doped silicon wafers with thermally grown SiO 2 were used as substrates. The doped silicon acted as the common bottom-gate electrode, while the SiO 2 layer served as the gate dielectric. The wafers were cut into individual substrates and cleaned by sequential sonication in acetone, isopropyl alcohol, and deionized water. The substrates were subsequently dried using nitrogen and used shortly after cleaning to minimize surface contamination.

Pristine TP solutions were prepared in toluene at a concentration of 5 mg/mL and stirred at room temperature for at least 30 min to ensure complete dissolution. Both PαMS and PS polymer stock solutions at the same concentration were separately prepared in toluene and combined with the TP solution. Both polymers were investigated at loadings of 5, 10, 33, and 50 wt% relative to the total TP/polymer solids. All blend solutions were stirred for an additional 30 min before deposition to promote uniform mixing. A fixed volume of each solution (∼200 µL) was drop-cast onto a cleaned Si/SiO 2 substrate. During drying, the substrate was positioned at a slight angle (∼5°) to guide solvent flow and promote directional crystallization. The films were allowed to dry under ambient conditions without additional thermal treatment. The same deposition volume, substrate geometry, and drying configuration were used for samples intended for direct comparison.

Morphological Characterization: The crystal morphology of the deposited films was examined using polarized optical microscopy. Representative images were collected from multiple regions of each sample. Top-view scanning electron microscopy (FEI Quanta 650 ESEM) was additionally used to examine crystal edges, dendritic branches, exposed substrate regions, and interdomain boundaries. Crystal orientation was quantified by measuring the angle between the long axis [210] of each selected TP domain and a defined reference growth direction. The absolute misorientation angle was calculated as ∣Δθ∣ = ∣θ i ​−θ ref ​∣, where θ i ​ is the orientation of an individual crystal and θ ref ​ is the reference direction. Thirty representative crystals were analyzed for each film condition. The reported values represent the mean ± standard deviation. Then, the grain widths of 30 representative TP domains were measured along the short axis [1 2 ¯ 0] of the crystals. Crystal coverage was determined from optical microscopy images by segmenting TP-rich crystalline regions from exposed SiO 2 regions using identical image-processing criteria for all samples.

Electrical Characterization: Bottom-gate, top-contact OTFTs were fabricated using pristine TP or TP/polymer films as the active semiconductor layer. The heavily doped Si substrate served as the gate electrode, and the thermally grown SiO 2 layer (250 nm) served as the gate dielectric. Following semiconductor deposition and crystallization, gold source and drain electrodes (50 nm) were thermally evaporated through a shadow mask. The channel length (L) and channel width (W) were defined by the shadow-mask geometry. The channel length ranges from 5 µm to 200 μm, while the channel width ranges from 20 μm to 2000 µm. The electrical characteristics of the fabricated OTFTs were measured using a semiconductor parameter analyzer under ambient conditions. Output characteristics were measured by sweeping the drain voltage (V DS) at different gate voltages (V G). Transfer characteristics were measured by sweeping V G at a fixed drain voltage, typically V DS = −60 V. Since TP is a p -type semiconductor, negative gate bias was used to accumulate holes at the organic semiconductor/SiO 2 interface. The field-effect mobility was extracted from the saturation regime using Equation 1: where I D S is the drain current, W and L are the channel width and length, C i is the capacitance per unit area of the SiO 2 dielectric (C i ​ = 13.8 nF/cm 2), μ is the field-effect mobility, and V t h is the threshold voltage. The mobility was obtained from the slope of the (− I D S 1/2 versus V G plot in the saturation region.

Threshold voltage (V th) was obtained from the x-intercept of the linear fit to (-I DS) 1/2 versus V G in the saturation regime. The subthreshold swing (SS) was extracted from the inverse slope of log (-I DS) versus V G in the subthreshold region. The on/off current ratio (I on /I off) was determined from the maximum and minimum drain currents measured during the transfer sweep. An SS-derived effective trap density (D it,eff) was estimated according to where C i is the gate-dielectric capacitance per unit area, q is the elementary charge, k B is the Boltzmann constant, and T is the measurement temperature. Because this SS-derived quantity can include contributions from dielectric/semiconductor interface states, localized semiconductor states, and grain-boundary-related trapping, it is referred to here as an effective trap density. Statistical electrical parameters are reported as mean ± standard deviation from 10 devices for each formulation. The resulting statistical electrical parameters for pristine TP and the optimized TP/polymer formulations are summarized in Table 1.

Figures 1a-c presents the molecular structures of TP, PS, and PαMS together with the bottom-gate, top-contact transistor architecture used in this study. Since polymer molecular weight influences semiconductor crystallization through chain mobility, solution viscosity, and entanglement (Ballantyne et al., 2008; Himmelberger et al., 2014), low-to-intermediate-molecular-weight PS and PαMS were selected in this work to maintain good solubility and sufficient chain mobility while minimizing viscosity- and entanglement-induced restrictions on TP crystallization. While PS and PαMS share a styrenic structure with pendant phenyl groups, their principal structural distinction is the α-methyl substituent in PαMS. This additional group increases steric crowding near the polymer backbone and may modify polymer-chain organization during solution drying. In the transistor configuration shown in Figure 1d, negative gate bias induces hole accumulation near the TP/SiO 2 interface. Charge transport therefore occurs laterally through the semiconductor domains between the source and drain electrodes. The orientation, width, continuity, and coverage of the TP-rich domains are consequently expected to influence the measured drain current and field-effect mobility.

Figure 2 presents representative polarized optical microscopy images of pristine TP films prepared without a polymer additive. Although TP forms elongated crystalline domains, the films exhibit pronounced dendritic growth, with substantial variations in crystal orientation, width, and spatial distribution. Large exposed SiO 2 regions remain between neighboring crystals, and the crystal long axes are poorly aligned. Akkerman et al. showed that dendritic TIPS-pentacene growth occurs during the early stage of solvent evaporation, when rapid solvent loss produces high supersaturation and fast crystal growth (Akkerman et al., 2012). Under these conditions, crystals nucleate and branch in multiple directions before the molecules can reorganize into uniformly aligned domains. As evaporation proceeds and supersaturation decreases, the growth rate slows, allowing the formation of longer and more ordered plate-like crystals. The initially formed dendritic regions therefore contain smaller, differently oriented branches, disordered molecular terraces, and numerous grain boundaries and physical discontinuities. These structural features disrupt continuous π–π-stacking pathways, increase charge trapping and interdomain transport barriers, and thereby hinder efficient hole transport. The transistor image in Figure 2d further shows that the source–drain channel is bridged by discrete crystals rather than a laterally continuous semiconductor film. Charge transport is therefore restricted to a limited number of crystalline domains that span the channel, making device performance highly sensitive to their orientation, connectivity, and spatial coverage. Chen et al. demonstrated that charge transport in TIPS-pentacene is strongly anisotropic (Chen et al., 2009). The long axis of the needle-shaped crystals, approximately corresponding to the ([210]) direction, provides an efficient transport pathway because it combines favorable π–π stacking with a relatively low density of grain and twin boundaries. When this direction is aligned parallel to the source–drain channel, holes can move along more continuous intermolecular pathways, whereas transport perpendicular to the crystal long axis requires carriers to cross more boundaries and structural discontinuities. As a result, the mobility decreased by as much as one order of magnitude as the ([210]) direction rotated from parallel to perpendicular to the channel. Besides, this strong anisotropy is governed not only by the intrinsic molecular packing but also by the crystal geometry and boundary distribution. Thus, the misalignment, incomplete surface coverage, and discontinuous connectivity observed in pristine TP films collectively increase transport barriers and device-to-device variability, motivating the use of polymer additives to regulate crystallization and produce more aligned and continuous transport pathways.

Both polymer additives were initially screened at 5, 10, 33, and 50 wt% relative to the total TP/polymer solids. The polymer loading produced a clear concentration-dependent effect on both crystalline morphology and transistor response. Based on this combined screening, 33 wt% PS and 10 wt% PαMS were identified as the respective optimized formulations and were therefore selected for the detailed quantitative morphology comparison presented below. When PS was mixed with TP, the crystal morphology was improved in general in terms of crystal alignment and domain regularities. The best morphology in terms of crystal alignment, grain width, and film coverage was obtained at 33 wt% PS weight ratios, as shown in the optical images of Figure 3. Images collected from different regions of the film reveal elongated TP crystalline domains with a preferential growth direction, indicating that PS reduces the orientational disorder observed in pristine TP films. However, the domain widths and spatial distribution remain nonuniform, and exposed SiO 2 regions continue to interrupt the crystalline pathways, particularly in Figure 3b. These observations demonstrate that the PS polymer additive improves the overall directional organization of TP but does not completely eliminate incomplete coverage, irregular domain boundaries, or morphological discontinuities.

On the other hand, the optimal morphology of TP/PαMS mixed film was achieved at 10 wt% weight ratio. As shown in the optical images of Figure 4, the film consists of elongated crystalline domains that remain nearly parallel over relatively large areas. Compared with pristine TP and the PS-containing films, the TP/PαMS domains display substantially less angular variation and more consistent lateral widths. At higher magnification, the TP/PαMS domains show comparatively smooth and continuous boundaries. Some exposed SiO 2 regions remain; however, the overall crystalline coverage is higher, and the spacing between neighboring TP-rich domains is reduced. This morphology should increase the probability that continuous semiconductor pathways extend across the source–drain channel. The comparison between Figures 3, 4 indicates that, under their respective optimized conditions, PS improves general orientation but frequently produces narrow domains and dendritic boundaries, whereas PαMS provides stronger directional alignment while maintaining relatively wide TP domains. Because the two additives differ in both molecular structure and molecular weight and are compared at different optimized loadings, the present data do not independently isolate the effect of α-methyl substitution. The observed morphological differences are therefore interpreted as reflecting the combined influence of polymer structure, molecular weight, and additive concentration under the investigated processing conditions.

The qualitative trends observed in the optical images were quantified using the absolute misorientation angle, crystalline-domain width, and crystal coverage, as summarized in Figure 5. Because the two polymers exhibited their most favorable morphologies at different concentrations, the subsequent comparison was conducted using the optimized formulation of each additive: 33 wt% PS and 10 wt% PαMS. Based on the measurement of 30 representative crystals, pristine TP exhibits an average misorientation angle of 36.46 ± 17.88 ∘, as shown in Figure 5a, indicating substantial orientational disorder and considerable variation among domains. Incorporating PS reduces the average misorientation angle to 9.97 ± 7.29 ∘, confirming that PS promotes a more consistent crystal-growth direction. The strongest alignment is obtained in the TP/PαMS film, which has an average misorientation angle of only 1.07 ± 1.29 ∘. This small mean value and narrow distribution indicate that nearly all measured TP domains are oriented parallel to the reference direction, thereby reducing angular deviations between neighboring crystalline transport pathways.

The crystalline-domain width also differs substantially among the three films. As shown in Figure 5b, pristine TP has an average domain width of 73.60 ± 59.03 μ m, based on the measurement of 30 representative crystals. Although the mean width is relatively large, the exceptionally high standard deviation reflects pronounced morphological nonuniformity, including both wide crystals and much narrower branches. TP/PS exhibits a smaller average width of 38.77 ± 24.10 μ m, suggesting that PS improves directional growth but restricts lateral domain expansion. By comparison, TP/PαMS maintains a large average domain width of 75.13 ± 24.63 μ m while substantially reducing the width variation relative to pristine TP. PαMS therefore promotes domains that are both wide and comparatively uniform across the film. The influence of domain width and orientation on charge transport can be explained using a grain-boundary-limited transport model (He et al., 2025). In this model, the highly conductive crystalline domains and the less conductive grain-boundary regions are treated as resistive elements connected in series along the transistor channel. For elongated domains, the number of boundaries encountered by a carrier is related to the domain width and orientation by n ≈ L ⁡ sin ⁡ θ / W G. Substituting this geometric relationship into the series-resistance expression gives Equation 2: where μ eff is the effective channel mobility, μ G and μ GB are the mobilities within a crystalline domain and across a grain boundary, respectively, L GB is the effective boundary width, W G is the domain width, and θ is the angle between the crystal long axis and the transport direction. Equation 2 may also be written in the simplified form given by Equation 3:

This shows that the inverse effective mobility increases approximately with the inverse domain width. This relationship predicts that wider domains reduce the number of low-mobility boundaries encountered across the channel, while better alignment reduces the orientation-dependent boundary contribution. Because crystalline defect and trap centers are mostly located at the grain boundaries, grain-boundary mobility in TIPS-pentacene has been estimated to be several orders of magnitude lower than the mobility within a crystalline domain (Chen et al., 2008), even narrow boundary regions can strongly limit the overall channel mobility.

Large domain width alone, however, is insufficient to ensure efficient charge transport. Pristine TP combines a relatively large average width with severe orientational disorder, a broad width distribution, and substantial uncovered substrate area. Its crystal coverage is only 64.34%, as shown in Figure 5c, indicating that more than one-third of the substrate remains exposed and that many potential transport pathways are physically interrupted. Coverage increases to 70.14% for TP/PS, representing a moderate improvement, but reaches 83.17% for TP/PαMS. The higher TP-rich coverage produced by PαMS reduces exposed SiO 2 regions and increases the probability of forming continuous semiconductor pathways between the source and drain electrodes. Although uncovered regions are not explicitly included in the simplified grain-boundary model, they introduce additional physical discontinuities and interdomain hopping barriers that can further suppress the measured mobility..

Together, these quantitative results show that the improvement produced by PαMS cannot be attributed to a single morphological parameter. Instead, PαMS simultaneously reduces the misorientation angle, preserves large and relatively uniform crystalline domains, and improves film coverage. According to the grain-boundary-limited transport model, the larger domain width decreases the number of resistive boundaries encountered by carriers, while the nearly parallel domain orientation provides more direct source-to-drain transport pathways. The higher coverage further prevents these pathways from being interrupted by exposed dielectric regions. In contrast, PS improves alignment but produces narrower domains and only a moderate increase in coverage, while pristine TP retains relatively large domains but suffers from substantial misorientation, width nonuniformity, and incomplete film coverage. The TP/PαMS morphology therefore provides the most continuous and least boundary-limited network for lateral charge transport.

Figure 6 shows representative output and transfer characteristics of TP/PαMS and TP/PS OTFTs. Both devices exhibit typical p -type transistor behavior. The maximum extracted mobility from all devices is approximately 0.5 cm 2 /Vs for TP/PαMS, compared with approximately 0.35 cm 2 /Vs for TP/PS, implying approximately a 1.4 fold improvement in the maximum mobility with the incorporation of PαMS. In contrast, the OTFTs based on pristine TP exhibited mobilities ranging between 3.1 × 10 –6 cm 2 /Vs to 0.16 cm 2 /Vs (He et al., 2011), indicating both polymers significantly improved the mobilities of TP organic semiconductor. The improved transport in TP/PαMS devices is consistent with the quantitative morphology analysis. First, the increase in crystal coverage from 70.14% for TP/PS to 83.17% for TP/PαMS reduces the fraction of uncovered SiO 2 within the transistor channel. Higher coverage increases the probability of forming continuous TP-rich pathways between the source and drain electrodes. Second, the average TP/PαMS domain width is nearly twice that of TP/PS. Wider domains reduce the number of domain boundaries that carriers must cross during lateral transport. Because these boundaries can contain structural disorder and localized trap states, reducing their density may decrease carrier trapping and scattering. Third, TP/PαMS exhibits an average misorientation angle of only 1.07°, compared with 9.97° for TP/PS. The nearly parallel domains provide a more directionally consistent transport network and reduce abrupt changes in crystal orientation between adjacent regions. The simultaneous improvement in coverage, domain width, and orientation is therefore consistent with the higher mobility observed for TP/PαMS, including a maximum value more than three times that observed for pristine TP.

The statistical electrical parameters further support the morphology-dependent transport trend. The average saturation mobility increases from 0.08 ± 0.06 cm 2 /Vs for pristine TP to 0.18 ± 0.08 cm 2 /Vs for TP/PS and 0.32 ± 0.09 cm 2 /Vs for TP/PαMS. At the same time, SS decreases from 6.2 ± 1.2 to 5.3 ± 0.9 and 4.3 ± 0.7 V/dec, respectively, while the SS-derived effective trap density decreases from (8.9 ± 1.7) × 10 12 to (7.6 ± 1.3) × 10 12 and (6.1 ± 1.0) × 10 12 cm -2 eV -1. TP/PαMS also exhibits the highest average I on /I off ratio of (3.8 ± 1.5) × 10 4. These trends are consistent with reduced electrical trapping and improved switching behavior as the TP morphology becomes more aligned, continuous, and uniformly covered. V th also shifts from 5.5 ± 2.6 V for pristine TP to 2.3 ± 1.4 V for TP/PS and 1.0 ± 1.2 V for TP/PαMS, consistent with a reduction in the gate bias required to establish channel conduction following polymer incorporation. Because V th can be affected by semiconductor morphology, charge trapping, injection, and semiconductor/dielectric interactions, the observed shift is not assigned exclusively to a single mechanism. For the representative transfer curves shown in Figure 6, the I on /I off ratios are approximately 4.71 × 10 4 for TP/PαMS and 1.18 × 10 4 for TP/PS. For the same representative devices, the maximum magnitude of the gate leakage current over the measured gate-voltage range is approximately 4.95 × 10 −9 A for TP/PαMS and 4.67 × 10 −8 A for TP/PS. In both cases, the maximum gate leakage current remains several orders of magnitude lower than the on-state drain current, indicating that gate leakage makes a negligible contribution to the measured on-state current.

Extracted OFET mobility can also depend on channel length because the relative contribution of contact resistance and the probability of crystalline defects vary with device geometry. Wang et al. reported that the apparent mobility of TIPS-pentacene OFETs initially increased with channel length as the relative effect of parasitic contact resistance decreased, followed by a reduction at longer channel lengths associated with an increased probability of crystalline defects (Wang et al., 2017). In the present comparative analysis, however, the representative TP/PS and TP/PαMS devices shown in Figure 6 use the same geometry (W = 1,000 μm, L = 15 μm); therefore, their electrical differences are not attributable to different channel lengths. A systematic channel-length-dependent analysis and separation of contact and channel resistance are beyond the present comparison and will be investigated in future work.

To further understand how the TP/PS crystalline morphology may contribute to its lower mobility, top-view SEM was conducted to provide additional information about the PS-containing films. As shown in Figure 7, at low magnification, the TP-rich domains exhibit an overall common orientation. However, individual domains retain a pronounced dendritic morphology characterized by secondary branches extending from the primary growth direction. At higher magnification, the domain edges appear feather-like or tooth-like rather than smooth and continuous. These branches increase boundary complexity and create narrow connections between different portions of the semiconductor film. The irregular geometry may introduce local constrictions in the transport pathway and produce nonuniform electrical fields within the transistor channel. Top-view SEM characterization was also performed for the optimized TP/PαMS films. Because these films consist of relatively large micrometer-scale crystalline domains, SEM did not reveal additional morphological features beyond those already resolved by polarized optical microscopy. The SEM observations were consistent with the comparatively wide and well-aligned morphology identified by optical microscopy; therefore, POM was retained as the primary technique for quantitative analysis of TP/PαMS domain orientation, width, and coverage. The SEM observations explain why improved optical alignment in TP/PS does not produce the same transport enhancement as the morphology obtained with PαMS. Although PS reduces large-scale orientational disorder, it does not fully suppress dendritic growth or the formation of narrow, irregular domains. PαMS, by comparison, produces wider and more uniform domains with substantially lower angular variation.

The results demonstrate that PS and PαMS regulate TP crystallization differently under the investigated conditions. However, the PS and PαMS used in this study differ in molecular weight, with weight-average molecular weights of approximately 25 and 10 kg/mol, respectively, and their optimized formulations also differ in additive concentration. The present experiments therefore cannot fully decouple the effects of polymer molecular structure, molecular weight, and concentration. Polymer molecular weight can influence chain mobility, solution viscosity, entanglement, phase separation, and semiconductor crystallization, whereas polymer molecular structure can independently affect the local crystallization environment. Accordingly, the differences observed between TP/PS and TP/PαMS are interpreted here as reflecting the combined effects of these factors rather than being assigned exclusively to the α-methyl substituent in PαMS. Previous reports likewise indicate that polymer molecular weight can influence TP morphology and device performance (Ohe et al., 2009; Jo et al., 2023). A systematic comparison of PS and PαMS at matched molecular weights and identical additive concentrations will be required to more directly isolate the contribution of α-methyl substitution.

The present polarized optical microscopy and top-view SEM measurements primarily characterize lateral crystalline morphology and do not directly determine the vertical composition of the TP/polymer blend films. Vertical phase separation can nevertheless be important in small-molecule/polymer OTFTs because charge accumulation occurs within a thin region adjacent to the SiO 2 /semiconductor interface. Previous studies have used depth-profile XPS to examine vertical segregation in TIPS-pentacene/PαMS blend films (Hwang et al., 2012), and have also reported vertical phase separation in TIPS-pentacene/PS systems (Bharti and Tiwari, 2016). Semiconductor enrichment near the dielectric interface may promote a more continuous charge-transport pathway, whereas excessive enrichment of an insulating polymer at the interface could interrupt TP connectivity and increase transport barriers. Conversely, an appropriately segregated polymer phase may modify the dielectric surface and reduce interfacial trapping. Therefore, the electrical consequences of vertical stratification depend on the local composition and molecular organization near the SiO 2 interface. Because no depth-resolved compositional measurements were performed in the present study, we do not assign a specific vertical distribution of TP, PS, or PαMS. Future work using depth-profile XPS or ToF-SIMS will be required to distinguish vertical-stratification effects from the lateral morphology effects quantified here.

The present study focuses on the initial morphology and electrical characteristics of the TP/polymer OTFTs and does not establish whether either polymer additive improves environmental or operational stability. Polymer incorporation may influence oxygen and moisture transport, semiconductor/polymer distribution, dielectric-interface trapping, and photoinduced degradation; consequently, improved initial crystalline morphology or field-effect mobility does not necessarily imply enhanced ambient stability. A rigorous stability comparison among pristine TP, TP/PS, and TP/PαMS will require controlled ambient aging, continuous electrical-bias stress, and illumination measurements using replicate devices while monitoring changes in mobility, threshold voltage, subthreshold swing, I on /I off ratio, and effective trap density as a function of exposure time.

Styrenic polymer additives were systematically investigated as a solution-processable strategy for controlling TP crystallization, crystalline defects, and charge transport in OTFTs. Pristine TP films exhibited substantial orientational disorder, highly nonuniform domain widths, and incomplete substrate coverage. PS improved directional alignment, reducing the average absolute misorientation angle from 36.46 ± 17.88° to 9.97 ± 7.29°. However, TP/PS films retained pronounced dendritic boundaries and relatively narrow domains with an average width of 38.77 ± 24.10 μm. In comparison, PαMS produced nearly parallel TP domains with an average misorientation angle of 1.07 ± 1.29°, a comparatively uniform domain width of 75.13 ± 24.63 μm, and a crystal coverage of 83.17%. These simultaneous improvements in alignment, domain width, and coverage reduced morphological interruptions associated with grain-boundary-limited transport and created more continuous source-to-drain pathways. Consequently, TP/PαMS devices achieved a maximum extracted saturation mobility of approximately 0.5 cm 2 /Vs, more than three times that of pristine TP devices. Because the PS and PαMS used in this study differ in molecular weight and are evaluated at different optimized additive loadings, the respective contributions of polymer structure, molecular weight, and concentration cannot be fully separated from the present data. The observed differences are therefore interpreted as the combined outcome of these factors under the investigated processing conditions rather than being attributed exclusively to α-methyl substitution. The statistical electrical analysis further supports the morphology-dependent transport trend: TP/PαMS exhibits the highest average mobility and I on /I off ratio together with the lowest subthreshold swing and SS-derived effective trap density among the three investigated conditions. However, the present measurements do not independently resolve the contribution of vertical semiconductor/polymer stratification, nor do they establish an environmental or operational stability advantage for PαMS. Overall, this study demonstrates that styrenic polymer-additive engineering provides a practical strategy for regulating crystalline defects and improving charge transport in solution-processed organic transistors. Future work will systematically compare PS and PαMS at matched molecular weights and identical additive concentrations, use depth-resolved characterization to determine vertical phase segregation, and evaluate environmental and operational stability under controlled ambient aging, electrical bias, and illumination.