Tailoring Crystallization Growth of Small‐Molecule Organic Semiconductors by Modification with Conjugated Polymers for Organic Field‐Effect Transistors

By investigating a typical small molecule, 6,13‐bis(triisopropylsilylethynyl) pentacene (TIPS), the crucial influence of conjugated polymers as additives on modulating the crystallization processes, crystalline structures, and carrier transport is unraveled. The conjugated polymer additives including poly[2,5‐bis(3‐alkylthio‐phen‐2‐yl)thieno(3,2‐b)thiophene (PBTTT) and poly(9,9‐di‐n‐octylfluorene‐alt‐benzothiadiazole) (F8BT) via a solution crystallization method can bring in crystalline structures that are not accessible by nonconjugated polymers, demonstrating superior order and enhanced carrier transport without external treatments. In such cases, polymorphism of the small molecules is manipulated by the features of conjugated polymers in the blend. According to optical microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X‐ray diffraction (XRD), different crystalline morphologies such as fibrous crystals from TIPS/PBTTT and rod‐like crystals from TIPS/F8BT are observed, which is determined by the crystalline habitats and solubility of conjugated polymers in small‐molecule/polymer blends. Meanwhile, organic field‐effect transistors (OFETs) based on TIPS/PBTTT and TIPS/F8BT blends are prepared for the purpose of exploring the electrical characteristics, yielding the mobility of 0.3 and 3.53 cm2 V−1 s−1, respectively. The conjugated‐polymer‐mediated polymorphism of small molecules can provide an attractive platform to explore the fundamental relationship between crystal stacking and electrical behaviors without altering the chemical structure.

structures. For example, Giri's group inflicted lattice strain on 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS) crystals via solution shearing, which can decrease the intermolecular distance and enhance the electrical properties of TIPS. [12] Other external alignment approaches of manipulating the crystallization process of small molecules include capillary forcebased [13][14][15][16] and patterning-based methods. [17][18][19] However, many external alignment approaches can just be utilized to small regional positions of the substrate, which is not compatible with large-area and flexible optoelectronic devices. Moreover, complicated device setup and precise modulation of experimental parameters are needed for exerting the external alignment on organic semiconductors. By stark contrast, the additive method is easier to scale-up and less technologically difficult to realize comparable performance, demonstrating attractive advantages over these external alignment ones owing to the facile manufacturing and low cost. [20][21][22][23][24][25][26][27][28][29][30][31][32][33] As for the additive method, nonconjugated polymers including poly(α-methylstyrene) and poly(2ethylhexyl acrylate), were added into TIPS, which can induce a vertical phase segregation in the TIPS-based blends. [23][24][25][26] It is worthwhile to mention that no TIPS lattice parameter variations were observed for the addition of nonconjugated polymers and most external alignments except for the solution shearing. In contrast, utilizing conjugated polymers has opened a new avenue. For instance, Chen et al. reported that the crystal morphologies and structures of TIPS can be altered by the incorporation of region-random pentacene-bithiophene polymer (PnBT-RRa) and poly(3-hexylthiophene) (P3HT), generating previously unreported crystal polymorphism with improved hole mobility of 0.14 and 0.39 cm 2 V −1 s −1 , respectively. [27] He et al. specifically explored the increasing contents of P3HT to modulate the morphologies of small-molecular organic semiconductors, including randomly-oriented crystal ribbons, wellaligned needles with improved long-range order, and grasslike curved microwires with interlinkages. [28] In another case, diketopyrrolopyrrole (DPP)-based material with P3HT as the additive was carried out to prepare self-assembly crystals, demonstrating the mobility of 10 −3 cm 2 V −1 s −1 . [29] Suzuki's group employed a tetracene derivative to fabricate nanowires with the addition of P3HT, leading to variations in crystalline morphologies, together with enhanced photoconductivity. [30] Despite the advancement, the investigation about the addition of conjugated polymers during the fabrication of small-molecular crystals still maintain unexplored and challenging. Besides, the influence of conjugated polymers on the crystalline mechanism and the resulting crystal is obscure, which are largely unexplored. Consequently, revealing the insight into the relation-ship between structures and functional characteristics is crucial to shed light on the crystallization mechanism of the blend system. The search for the small-molecule/polymer blends with superior physical and electrical characteristics as active materials for OFETs is highly desirable.
In this work, the addition of conjugated polymers into small molecules was demonstrated, which resulted in great longrange order and enhanced carrier transport without applying any external alignment methods. TIPS was used as a benchmark due to the good stability and effective carrier transport. [34] Two typical conjugated polymers, i.e., poly[2,5-bis(3-alkylthiophen-2-yl)thieno(3,2-b)thiophene (PBTTT) and poly(9,9-din-octylfluorene-alt-benzothiadiazole) (F8BT) with distinctively different crystalline characteristics as the agents, were added into TIPS to grow various crystalline crystals. As a result, conjugated polymer additives into TIPS during the solution crystallization lead to crystalline structures which are distinctly different from those obtained via nonconjugated polymer additives or external-field alignments. A combination of optical microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD) led to the observation of different crystalline features, which was determined by the crystalline habitats and solubility of conjugated polymers in small-molecule/polymer blends. With the aim to explore the electrical behaviors, OFETs based on TIPS/PBTTT and TIPS/ F8BT blends were prepared, demonstrating the mobility of 0.3 and 3.53 cm 2 V −1 s −1 , respectively. It is believed that our studies will enlighten and promote the development of well-aligned OFETs.

Results and Discussion
Scheme 1 depicts the chemical structures of TIPS, PBTTT, and F8BT. As shown in Figure 1a-c, TIPS, TIPS/PBTTT, and TIPS/F8BT blends (1:1 by weight) demonstrate diverse morphologies, which are slowly crystallized from dilute toluene solution via drop-casting. The pristine TIPS crystals are poorly covered with typical dimensions of 20-60 µm in width and 50-300 µm in length. TIPS/PBTTT blends show fiber bundles with enhanced long-range order while TIPS/F8BT blends form rod-like crystals. Figure 1d,e shows SEM images of TIPS/ PBTTT fibrous crystals and TIPS/F8BT rod-like crystals, respectively, demonstrating microstructures that are not observed from solution crystallization of pristine TIPS films or TIPS/ nonconjugated polymer blends. Additional optical images of TIPS/F8BT rod-like crystals are shown in Figure S1   . It is well acknowledged that the addition of nonconjugated polymers into TIPS facilitates the phase separation in a perpendicular direction with an optimized blend concentration, without varying the crystalline characteristics of the small molecule. In our cases, distinctively different crystalline morphologies are observed, which implies the interaction between the conjugated polymers and the small molecules, thus determining the crystalline behaviors. According to X-ray diffraction (XRD) curves in Figure 2, a strong series of (00l) peaks up to the third order that are associated with TIPS are observed for the three samples, indicating that preferred orientations along the c-axis with (001) planes are parallel to substrates.
For the purpose of exploring the spatial distribution of individual components in these blends, energy-filtered transmission electron microscopy (TEM) characterizations were performed for samples on copper grids. High-angle annular dark field (HAADF) images of TIPS/PBTTT and TIPS/F8BT blends are shown in Figure 3a,e. The energy-dispersive spectrometry (EDS) maps in Figure 3b,f reveals the sulfur portion of the blends, which are false-colored in yellow to demonstrate the sulfur-rich locations. Since both conjugated polymers such as PBTTT and F8BT consist of sulfur, the yellow locations indicate the presence of conjugated polymers in the blend. The coexistence of TIPS and conjugated polymers is confirmed in both blends, which can be corroborated by the element map intensity scans on the blends ( Figures S3 and S4, Supporting Information). Figure 3c,d,g,h show TEM images and selected-area electron diffraction (SAED) patterns of TIPS/PBTTT and TIPS/ F8BT blends. As shown in Figure 3d, there are two principal reflections that have been indexed as (020) and (200) in the TIPS/PBTTT blend, the corresponding spacing being 1.57 and 1.55 nm, respectively. These values match well with the dimensions of the ab projection reported previously, which has proposed a triclinic unit cell with a = 0.788 nm, b = 0.767 nm, c = 1.665 nm and γ = 81 o . [27] This TIPS polymorph in TIPS/ PBTTT blends is identical to that in TIPS/PnBT-RRa blends previously reported, which can be named as Form I. In contrast, as shown in Figure 3h, the SAED pattern of TIPS/F8BT blends conforms to the reported lattice parameter of the pristine TIPS crystals (a = 0.755 nm, b = 0.773 nm, c = 1.702 nm and γ = 81 o ). [27] The cell volume (V) of Form I and II are determined as 974.3 and 961.7 Å 3 , and the cell volume (V) of Form I are 1.3% larger than that of Form II. In a word, the addition of PBTTT can alter the crystalline packing of TIPS while F8BT has a negligible influence on that. The detailed reason will be elaborated later from the perspective of solution crystallization. According to the literature, TIPS polymorphs possess large V of 991.1-1013.5 Å 3 via a solution-sheared method. By stark contrast, the simple addition of conjugated polymers into TIPS can increase V in our cases and no external forces are needed during the crystallization process. Figure 4 summarizes our observations about the addition of conjugated polymers into TIPS, highlighting proposed variations in TIPS lattice parameters and long-range order due to the changes of intermolecular interactions between individual components of the blends. When there is only TIPS, the crystals are observed in random directions in the substrate without any limitation. The scenario is totally different if some conjugated polymers are introduced into TIPS. PBTTT and F8BT are long-chain conjugated polymers while TIPS is a small-molecule material.

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At the initial stage, the solvent evaporation facilitates homogeneous nucleation of TIPS crystals, rapidly generating a large quantity of crystal nuclei. As is well known, PBTTT is a crystalline semiconductor polymer. After the addition of PBTTT, the intermolecular interactions in TIPS/PBTTT blends guide the aggregated clusters into a metastable crystal structure with small lattice parameter variations. During the slow solution crystallization process, both TIPS and PBTTT can crystallize almost at the same time. These TIPS crystals subsequently act as efficient nucleation locations to induce crystallization of PBTTT chains. In this case, TIPS molecules are intertwined with PBTTT chains. The probability about which material takes part in the crystalline process at any particular time strongly relies on the local solubility of that material. The selection process occurs on the surface previously generated, and only the material of lower saturation solubility contributes to the crystal growth. Since the intermolecular interactions usually control the process, we propose that the capability of modulating the crystallization growth of one molecule with another is a general phenomenon.
In contrast, distinctively different crystalline morphologies are observed with the addition of F8BT into TIPS. Unlike PBTTT, a pristine F8BT film is amorphous, providing a different situation during the crystallization. Owing to the high aspect ratios of the rod-like crystals in Figure 1b, it seems polymer chains of F8BT preferentially adsorb to lateral faces, thus resulting in faster TIPS crystalline growth along the crystal axis in comparison to the lateral directions, which can achieve rod-like crystals. As TIPS crystals are highly terraced, F8BT probably adsorbs at step edges along crystal faces. That is to say, polymer chains of F8BT are inclined to deposit on the side of self-assembled TIPS crystals and serve as tube walls, limiting the random motion of TIPS. A similar phenomenon can also be observed for the blend of (2,5-di-(2-ethylhexyl)-3,6-bis(5″-nhexyl-2,2′,5′,2′′]terthiophen-5-yl)-pyrrolo [3,4-c]pyrrole-1,4-dione) (SMDPPEH) and P3HT. [29] In that case, P3HT tends to grow on the side of self-assembled SMDPPEH crystal and act as a tube wall, matching well with our work. Consequently, TIPS has  www.advelectronicmat.de to grow along this tube wall and eventually generated rod-like crystals with smooth surfaces. In a word, the crystallization is complex and hard to describe, but our model is beneficial for revealing some fundamental issues in the crystalline behaviors of small-molecule/polymer blends.
According to the study, the crystal morphologies in smallmolecule/polymer blends are greatly dependent on the crystalline habitats and the solubility of conjugated polymers. First, the crystalline properties of PBTTT can possess more intimate mixing due to enhanced intermolecular interactions compared with the amorphous characteristics of F8BT in terms of crystalline habitats. Second, the saturation solubility of different components has a remarkable influence on the crystallization process. PBTTT may crystallize into the TIPS unit cell because of relatively low saturation solubility, while long chains of F8BT still maintain randomly dispersed without participating in the crystalline behaviors of TIPS. In a word, TIPS/conjugated polymer blends result in various TIPS crystal morphologies and structures that can hardly be obtained by nonconjugated polymers and external treatments. Generally speaking, external forces, i.e., solution shearing, can improve the orientation of TIPS crystals, while nonconjugated polymers usually facilitate vertical phase separation. In our cases, the new morphologies can be ascribed to the different interactions between small molecules and conjugated polymers in which the crystalline habitats and the solubility of conjugated polymers likely represent a substantial contribution.
Top-contact, bottom-gate OFETs based on TIPS/PBTTT and TIPS/F8BT blends from toluene solution have been prepared. The crystals are across the channel, which is beneficial for carrier transport from one electrode to another. The typical transfer and output properties of OFETs are demonstrated in Figure 5. In the fabrication of devices, the copper grid based on a single crystal is used as the mask to evaporate Au electrode. The channel length (L) corresponds to the distance of the gridding and the channel width (W) corresponds to the width of this individual crystal. As for the device based on the TIPS/F8BT blend, W = 3.7 µm and L = 33.0 µm. In the device based on the TIPS/PBTTT blend, W = 29.7 µm and L = 33.0 µm. In total, 10 devices were examined. TIPS/PBTTT blends by employing toluene possessed the average mobility of 0.012 ± 0.008 cm 2 V −1 s −1 with the maximum mobility up to 0.02 cm 2 V −1 s −1 . In contrast, TIPS/F8BT blends possessed the average mobility of 2.30 ± 1.23 cm 2 V −1 s −1 with the maximum mobility up to 3.53 cm 2 V −1 s −1 , originating from the high crystallinity of rod-like crystals. The parameters of the device performance for these blends were summarized in Table S1 (Supporting Information). Both transistors based on crystals only have low on/off ratios, which may result from the low air stability of PBTTT and F8BT. It should be noted that the pristine PBTTT-based devices show the mobility below 10 −4 cm 2 V −1 s −1 ( Figure S6, Supporting Information), and neat F8BT-based devices exhibit the mobility of 10 −7 cm 2 V −1 s −1 ( Figure S7, Supporting Information). The recorded maximum mobilities from TIPS/PBTTT and TIPS/F8BT blends (0.02 and 3.53 cm 2 V −1 s −1 , respectively) are much higher compared with those values, indicating that TIPS makes a great contribution to carrier transport of the blends. Under this circumstance, the carrier transport is greatly associated with the lattice packing and π-orbital overlap between adjacent molecules. Because of weak non-covalent interactions, the small-molecular system TIPS can yield various crystalline morphologies, leading to great variations in their carrier mobilities.
Our work has demonstrated continuous efforts toward fundamental understanding of the small/polymer blends and broadened the range of previous studies. First, the selective www.advelectronicmat.de type of polymers is different from the work from Chen's group [27] since we specifically select a semicrystalline polymer, PBTTT, and an amorphous polymer, F8BT, as the model polymer and the variation of the crystalline morphologies and electrical properties in these blends mainly resulted from the crystalline habitats and solubility of conjugated polymers rather than the structural similarity between individual components of the blends from the work from Chen's group. [27] Second, the crystalline mechanism of TIPS/F8BT blends is different from those previously reported. Third, the mobility values of OFETs based on TIPS/PBTTT and TIPS/F8BT blends are higher than those based on TIPS/PnBT-RRa and TIPS/P3HT, achieving enhanced electrical characteristics. In a word, our work has significantly stepped forward toward the development of the small/polymer blends. Compared with previous studies [28] based on the increasing contents of P3HT for modulating the morphologies of small-molecular organic semiconductors, we have incorporated a new kind of polymers-F8BT into the smallmolecule/polymer blend and this amorphous polymer can yield a different morphology pattern which is newly reported. Meanwhile, the crystalline process associated with TIPS/F8BT blends is greatly different from those mainly based on the semi-crystalline polymers such as P3HT. Moreover, it is of great importance to further investigate the impact of chemical function-ality, molecular structure, or polymer architecture on different interactions. Further studies in this aspect are still ongoing in our lab.
With the aim to improve the crystalline morphology and device performance, we selected different solvents in the case of TIPS/PBTTT since the addition of PBTTT can alter the crystalline stacking of TIPS. Figure 6a shows the optical microscopy image by employing chloroform as the solvent, demonstrating the crystalline film. As shown in Figure S8 (Supporting Information), the corresponding polarized microscopy image is provided, demonstrating obvious optical anisotropy. As shown in Figure 6b, continuous dense filaments were observed with a large coverage and consistent orientation. To the best of our knowledge, the growth speed of nuclei was enhanced since chloroform was opted to evaporate because of the low boiling point. Consequently, the crystalline process was accelerated and the crystalline filaments were densely arranged. According to SEM images in Figure 6c, the crystals were piled upon each other with coarse surfaces. Despite the large coverage, the stacking behavior of the filaments had a negative influence on the carrier transport, which was not beneficial for achieving OFETs with high mobility. For modulating the crystalline morphology of TIPS/PBTTT, another solvent toluene was used as a second solvent in the mixed solvents. Under this Figure 6. a) Optical images and b,c) SEM images of TIPS/PBTTT by using chloroform as the solvent. d) Optical images and e,f) SEM images of TIPS/ PBTTT by using the mixed solvents with the volume ratio of chloroform and toluene as 9:1. g,h) XRD patterns of TIPS/PBTTT by using chloroform as the solvent (g) and using the mixed solvents with the volume ratio of chloroform and toluene as 9:1 (h).

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circumstance, chloroform was the main solvent and toluene acted as a second solvent to adjust the solvent environment for optimal morphologies. After a series of experiments, the mixed ratio of chloroform and solvent was optimized as 9:1, achieving improved fibrous crystals with smooth surfaces (Figure 6d). Compared with other fibrous crystals, they demonstrated woven structures with the absence of piling up (Figure 6e,f). Particularly, these fibrous crystals showed the length of 100 µm and the width of around 10 µm, demonstrating improved fibrous morphologies compared with those by using a single solvent such as toluene or chloroform, which can also be confirmed by XRD patterns in Figure 6g,h. In this case, the diffraction intensity of TIPS/PBTTT blends by using the mixed solvents was greatly enhanced in comparison to those by employing chloroform. Finally, OFETs were constructed based on TIPS/PBTTT by using the mixed solvents with the volume ratio of chloroform and toluene as 9:1. As shown in Figure 7, TIPS/F8BT blends by using the mixed solvents with the volume ratio of chloroform and toluene as 9:1 possessed the average mobility of 0.18 ± 0.12 cm 2 V −1 s −1 with the maximum mobility up to 0.3 cm 2 V −1 s −1 , which was superior to those by using toluene as the solvent. The enhanced electrical properties resulted from the improved fibrous crystals made from the mixed solvents. However, the leakage current in Figure 7 is high, which may result from the rougher surface and larger probability of oxygen vacancies existing in the crystalline structures. Therefore, further device optimization can be expected. In a word, even in the same blend system, the simple alteration of solvents during the crystallization can have an important impact on the resulting crystalline morphologies, thus affecting the mobility of OFETs. As far as we are concerned, the role of solvent effect on the crystalline morphologies and the device performance of OFETs has been widely and extensively studied during the past decades. [35][36][37] It is noted that different solvents have various characteristic features, such as the boiling points, molecular sizes and functional groups. The solvents have a strong interaction with organic molecules due to the formation of inter-molecular interactions. According to previous studies, the mixture of good solvent and poor solvent was employed to modulate the local solubility of organic molecules for the purpose of changing the crystalline morphologies. [38] By contrast, both chloroform and toluene are good solvents for these organic molecules in this work, which is used to adjust the selfassembly crystallization time via the selection of the combina-tion of solvents with high boiling points and low boiling points. Chloroform with a low boiling point is inclined to evaporate quickly and the addition of a solvent with a high boiling point can prolong the self-assembly crystallization time, facilitating the thermodynamically favored structures and thus achieving improved crystallinity. Consequently, chloroform was the main solvent and toluene acted as a second solvent to adjust the solvent environment and the self-assembly crystallization time for optimal morphologies.
Blending semiconducting small molecules with conjugated polymers can serve as a key route to tackle the problem of solution processing, solid-state microstructures, electrical characteristics, stability, and reproducibility, which is a useful method of satisfying the requirements for specific OFET applications. The advantages of blended strategies originate from the combination of the effective carrier transport in inherently crystalline small molecules and solution-based film-forming capability of conjugated polymers. Although great advancement has been achieved from the blend system, further study will be needed to attract extensive attention from various academic groups. If the small-molecule/polymer blends is endowed with some new functions such as stretching, biocompatibility, strong emission and enhanced electrical characteristics, OFETs based on the blend systems with multifunctional properties will be widely applied in healthcare and internet-of-things. In a word, the blend systems hold great promise for the development of advanced organic electronics.

Conclusion
Tailoring the crystallization growth of small-molecule semiconductors by conjugated polymers has been investigated in detail. In this case, conjugated polymers, i.e., PBTTT and F8BT, were intentionally employed to modulate the crystalline growth of TIPS, which just depended on the solvent evaporation to facilitate the crystalline behavior without any external alignment methods, providing the control on the kinetics of crystal nucleation and growth. TIPS/conjugated polymer blends result in various TIPS crystal morphologies and structures which can hardly be achieved by nonconjugated polymers and external treatments. Under this circumstance, the new morphologies result from the interactions between small molecules and conjugated polymers in which the crystalline habitats and solubility of www.advelectronicmat.de conjugated polymers likely represent a substantial contribution. For the purpose of exploring the electrical properties, OFETs made from TIPS/PBTTT and TIPS/F8BT blends exhibited 0.3 and 3.53 cm 2 V −1 s −1 , respectively. Our study can be simply utilized to manipulate the crystalline structures and improve the electrical performance of small molecules, and will provide a deep insight into the crucial application in organic electronics.

Experimental Section
Materials: All the reagents employed were purchased from Sigma-Aldrich, J&K or Xiya Reagent (China). If necessary, solvents and reagents were purified using standard procedures.
Sample Preparation: TIPS and the polymer with a weight ratio of 1:1 were first mixed together and then the mixture of solids were dissolved in toluene to produce a concentration of 0.5 mg mL −1 solution. As for the double solvent method, the mixed solvent was first obtained with the volume ratio of chloroform and toluene as 9:1 by adding a small amount of toluene into chloroform. Subsequently, the mixture of TIPS and the polymer was dissolved in the mixed solvent with the volume ratio of chloroform and toluene as 9:1. Meanwhile, the growth apparatus for the growth of crystals and the device structure of OFETs based on a single crystal are shown in Figure 8.
The sample preparation for the TEM imaging was as follows: A small amount of as-obtained solution (5-10 uL) was dropped onto the copper grid coated with a supporting film. Subsequently, the copper grid with the as-obtained solution was placed on a platform inside a cylinder container with a radius and height of 2.5 and 3.0 cm, respectively. The container was then carefully sealed for ensuring slow crystalline growth.
Device Fabrication: Transistors were fabricated on the wafer with a 300 nm-thick thermally grown SiO 2 as the gate dielectric. Octadecylsilane (OTS) was used to modify the substrate via vapor deposition. Subsequently, the container was put in a vacuum oven at 120 °C for 3 h to realize the substrate modification. The solutions were drop-coated onto the substrate. With the evaporation of the solvent, different crystalline morphologies were obtained. At last, the electrodes were obtained by evaporating Au through a shadow mask on the crystal with the thickness of around 50 nm. All electrical measurements were performed on Keithley 4200 source/measure units in air.

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