Rapid Self‐Assembly Process at Air/Water Confined Interface for Highly Aligned Crystalline Polymeric Semiconductor Films

Highly aligned aggregation state structures have great significance for effective charge‐carrier transport in organic semiconductors. Several methods have been developed to provide organic semiconducting molecules with highly oriented aggregation state structure; among these, using a water surface to form organic semiconductor films is a widely implemented strategy, wherein solutions are spread on the surface of water. However, common techniques of film formation on water surfaces generally result in a nonuniform orientation of the film owing to the isotropic spread process of droplets on the water surface. In this study, a spatially confined air/water interfacial assembly method is proposed to obtain uniformly aligned monolayer and multilayer poly(diketopyrrolopyrrole‐thieno[3,2‐b]thiophene) thin films with controlled thickness. The structural and morphological characterizations obtained using atomic force microscopy, high‐resolution transmission electron microscopy, and grazing incidence wide‐angle X‐ray scattering indicates the crystalline structure of the thin films and high alignment of the molecular chains. The maximum mobility of the thin films reaches up to 2.06 and 0.5 cm2 V−1 s−1 in the parallel and perpendicular direction, respectively, indicating apparent anisotropic electrical properties. Furthermore, an inverter based on these thin films exhibits a voltage gain of up to 70, demonstrating the potential of applying the proposed technique to logic circuits.


Introduction
The vigorous development of organic electronics has benefited from continuous improvement of the performance of organic www.advelectronicmat.de polymeric semiconductor is considerably lower than the theoretical value. In addition to the impact of defects in the chemical structure of organic semiconductors, such as oxidized sites and poor planarity, certain drawbacks of polymeric semiconductor films fabricated using traditional methods, such as the large bends of chains, charge-carrier traps in the films, packing model of chains and boundary between crystalline/amorphous regions, hinder the effective charge carrier transport. [5] Therefore, a simple and effective strategy is required for controlling the orientation of polymer molecular chains to achieve efficient charge-carrier transport and improve device performance.
The existing common technologies such as dip coating and solution shearing can effectively help polymeric semiconductors molecular chains to orientate and obtain excellent chargecarrier transport capabilities. [6] However, these techniques also have their limitations such as the alignment of molecule backbones in films are ununiform, which will cause considerable contact resistance that limiting the requirements of highfrequency operation of organic electric devices. [7] In order to obtain a thin organic semiconductor layer, the technology that use water surface to form thin organic semiconductor films or other aggregate state was proposed and achieved a number of advances. Guan et al. proposed an air/water interfacial assembly method to achieve high-performance stretchable semiconducting nanofilms. [8] During this process, the poly(3-hexylthiophene) semiconductor solution was dropped on the water surface and then spread and slowly evaporated to form a thin film with a high mobility of 8.57 cm 2 V −1 s −1 . Similarly, Zhang and coworkers utilized the same method and achieved ordered conjugated polymer thin films with charge-carrier mobility remarkably boosted. [9] Jung et al. utilized water surface to form monolayer nanomaterials and embedded them in elastomer to realize stretchability and thin layer at same time. [10] Apart from polymers, small molecules were also carried out an air/water interface method grow crystals. [11] Jie and coworkers utilized the water-surface drag coating techniques to form high-quality conjugated small molecule thin films with a 4.7 times enhancement of charge-carrier mobility. [12] The films achieved from the air/ water interface are usually much thinner than that from other solution processing methods such as spin-coating. Usually, the few semiconductor layers next to the semiconductor/dielectric interface play a dominant role in charge-carrier transport. The thick films are spatially inhomogeneous with the inconsistence of a stacking model and looseness of molecular layers, which hinders device performance. [13] For films achieved from the air/ water interface, the air/water interface provides an ideal condition allowing the molecules to reorganize to form better molecular packing, leading to high device performance. Moreover, the monolayer obtained at the water surface are provided with a two-dimensional nature, which is beneficial for the charge-carriers transport pathways in two dimensions and leads to optimized conducting pathways. [14] However, during general air/ water interfacial assembly process, it is easy to notice that the direction of the semiconductor solution spreading on the water surface is 360° from the drop point as the center, which means that in this process, the ordering direction of polymer chains is axially scattered anisotropic distribution in the whole system. In other words, the thin films obtained by nonconfined air/ water interface assembly were polycrystalline and ununiform in alignment. In that case, the orientation of organic semiconductor layer of each device in multiple devices and basic circuits is different so the performance of the whole system may not be balanced, which will limit the application of these technologies. Therefore, it is necessary to propose improvements on general air/water interface assembly technology to obtain more uniformly oriented polymer chains in organic semiconductor layer for performance enhancement and excellent processability in organic circuits.
In this study, a spatially confined air/water interfacial assembly method is proposed. Herein, the space of the air/ water interface is confined, which effectively regulated the spread of organic semiconductor solutions, resulting in more uniformly oriented organic semiconductor thin films. In this method, two slim rods were introduced to construct a narrowconfined area at the air/water interface. Highly aligned organic semiconductor films were formed along the sides of the rectangular confined area. Thin films with different thicknesses and monolayer films were obtained using the film-transfer process. The mobility of poly(diketopyrrolopyrrole-thieno[3,2-b] thiophene) (PDPP-TT) films based on this method reached up to 2.06 cm 2 V −1 s −1 in the direction parallel to solution spread. However, the mobility along the direction perpendicular to the solution spread was only 0.5 cm 2 V −1 s −1 . The difference in performance in the two orthogonal directions exhibited a typical anisotropic property, reflecting the control of the polymer chain orientation. This method serves as a simple and efficient strategy for fabricating highly oriented polymer films, which can be beneficial to both fundamental and applied researches in organic polymeric semiconductors.

Results and Discussion
Two slim rods were used to form the spatially confined air/ water interface, and a narrow-confined area on the water surface was obtained. As shown in Figure 1, ultrapure water was poured into a clean Petri dish until the water level was slightly above the edge. Due to the surface tension of water, the upper surface of the water exhibited slightly upward convexity without overflow. Two slim rods were placed parallel on the Petri dish and embedded at the air/water interface to form a pair of long sides of a confined rectangular area. PDPP-TT was used as the semiconductor, which was dissolved into chlorobenzene to form a solution with a concentration of 0.5 mg mL −1 . During the assembly process at the air/water interface, 2.5 µL of 0.5 mg mL −1 PDPP-TT chlorobenzene solution was dropped at the center of the confined area. The semiconductor solution spread rapidly on the water surface. A high-speed video (Video S1, Supporting Information) was recorded to observe this rapid spreading process. Figures S1-S3, Supporting Information, illustrate a set of comparison photos and schematics of the air/water interfacial assembly method and the confined interface coating process. Owing to the existence of slim rods, the solution was hindered along the short-side direction of the rectangular confined area and rapidly spread only along the long-side direction to generate thin films.
The spreading of semiconductor solution at air/water interface can be explained by the Marangoni effect, which is caused www.advelectronicmat.de by surface tension gradient at the two-phase interface. [15] As the surface tension of water and chlorobenzene at 25 °C are 71.99 mN m −1 and 32.42 mN m −1 , respectively, [16] the PDPP-TT solution can spread rapidly on water surface during the air/ water interfacial assembly process. Marangoni flows are known to spread only for a finite radius on nonconfined water surface because of critical micelle concentration. [15b,c,17] Following the fast Marangoni flows, there are secondary flows that are slow and unstable and the spreading radius are comparable with primary Marangoni flows, [17a] which is the crucial stage that confined process works. During the assembly process at the confined interface, the existence of slim rods limited the plumbing instability of the secondary flows. Consequently, the distance of spreading get much larger than Marangoni flows radius because the inertia of boundary layer and its slow diffusion makes the spreading of Marangoni flow get into an inertial jet. [17a] In other words, the lateral confinement of the flow can change the extent of these inertial secondary flows and form an inertial surface jet to spread to larger distances in the longaxis direction. [17a] In this case, the films obtained by the spatially confined air/water interfacial assembly method had a larger diffusion distance than Marangoni radius and the continuous inertial action made polymer chains highly aligned during the jet quickly spreading process.
After the film was completely formed, the slim rods were carefully removed, and the obtained films can be transferred onto any substrate. Figure 2a,b show the optical microscope (OM) and polarize OM (POM) images of PDPP-TT monolayer films obtained using the spatially confined air/water interfacial assembly method. As the monolayer film was too thin to be observed on the Si/SiO 2 substrate background, a copper mesh was covering as a reference. As shown in Figure 2a, the  www.advelectronicmat.de film was nearly transparent in the OM imaging mode because of its low thickness and only the Si/SiO 2 substrate could be intuitively observed. When using the POM imaging mode and rotating from 0° to 45°, the film exhibited an obvious color change from dark to pale-yellow. The obtained image color returned to the dark shade when the film rotated from 45° to 90° (Figure 2b). This process verified the existence of the thin film and validated its molecular alignment and crystalline characteristics. Interestingly, certain highly aligned textures were observed in the POM image of the 45° rotation, exhibiting a wide range of consistent alignment of films obtained based on the spatially confined air/water interfacial assembly method.
The molecular and/or crystalline alignment of the films obtained based on the spatially confined air/water interfacial assembly method was analyzed using microscopes. The thickness of the obtained film measured using atomic force microscopy (AFM) was only 2.5 nm (Figure 2c), indicating that the film possessed a monolayer structure. Furthermore, two-and three-layer film were obtained using the same method and different volumes of the same concentrate PDPP-TT solution.
Here, the thickness of the two-and three-layers films measured using AFM were ≈5 and 7.5 nm, respectively ( Figure S4). This implied that the thickness of the monolayer film obtained by the spatially confined air/water interfacial assembly method is controllable and repeatable. As shown in Figure S5 To further investigate the structure and morphology of the thin films, polarized ultraviolet-visible-near-infrared (UV-Vis-NIR) absorption spectra were separately obtained for the films produced using spatially confined and nonconfined processes. As shown in Figure 3a,b, both films exhibited broad absorption from 600 to 900 nm, and the maximum absorption peak appeared at ≈800 nm. The film fabricated by the confined interface in the parallel direction exhibited two strong absorption peaks at 750 and 830 nm which were much more obvious compared to the vertical direction (Figure 3a), and the dichroic ratio is 2.1. Conversely, the films obtained using the nonconfined air/water interfacial assembly method exhibited no apparent difference in the two orthogonal directions (Figure 3b), and the dichroic ratio was close to that of spin-coated films. [18] These results clarified that the highly aligned polymer chains were formed in the film when the spatially confined air/water interfacial assembly method was used. Furthermore, grazing incidence wide angle X-ray scattering (GIWAXS) measurements were characterized to analyze the molecular packing in thin films and monolayer films obtained using the spatially confined air/water interfacial assembly method. As illustrated in Figure 3c-e, three-layer, eight-layer, and spin-coated films exhibited similar GIWAXS signals. The four clear signals of out-of-plane (h00) signals were attributed to lamellar stacking of PDPP-TT chains, and the in-plane peak (010) at 1.79 Å −1 was attributed to π−π stacking of PDPP-TT. It can be obtained that PDPP-TT backbones in the thin films obtained using the spatially confined air/water interfacial assembly method take an edge-on orientation, which facilitates the transport of chargecarriers, particularly in 2D conducting networks. The different layers of films exhibited similar GIWAXS signals, indicating the uniformity of films and polymer chains stacking. However, the monolayer film was extremely thin to obtain identifiable GIWAXS signals. As an alternative, the monolayer sample . Polarized ultraviolet-visible-near-infrared absorption spectra in parallel and perpendicular directions of thin films obtained using the a) spatially confined air/water interfacial assembly method and b) general air/water interfacial assembled method. c) Grazing incidence wide-angle X-ray scattering (GIWAXS) images of three-layers, eight-layer films, and spin-coating films. d) In-plane and e) out-of-plane diffraction intensities of GIWAXS.
www.advelectronicmat.de was characterized by HRTEM. Figure 2e shows highly clear HRTEM lattice fringes, and 3.689 Å spacing concurred with the GIWAXS results. The Q-space signal of π−π stacking at 1.79 Å −1 (Figure 3d and Figure S6, Supporting Information) was close to the data of PDPP-TT π−π stacking distance. [19] The HRTEM results provided direct evidence of highly aligned crystalline chains in the monolayer films of PDPP-TT obtained using the spatially confined air/water interfacial assembly method.
The electrical properties of the thin films were investigated with OFETs. Bottom-gate top-contact (BGTC) OFETs were fabricated using a heavily doped Si/SiO 2 substrate, which was cleaned and modified with octadecyltrichlorosilane (OTS) via a vapor evaporation process. After the thin films were fabricated using the spatially confined air/water interfacial assembly method, the OTS-modified Si/SiO 2 substrate was immersed in water and transferred the film to the Si/SiO 2 substrate. Annealing was performed at 105 °C for 30 min to remove moisture from the film, and the source and drain electrodes were formed with gold stripes.  Figure S9, Supporting Information, depicts an OM image of the OFETs-based monolayer films that were obtained using spatially confined air/water interfacial assembly method. The mobility of PDPP-TT in parallel direction of solution spread reached up to 2.06 cm 2 V −1 s −1 whereas the same in perpendicular direction was only 0.5 cm 2 V −1 s −1 . This difference of approximately four times indicated that the polymer chains in the film were oriented in parallel directions of solution spread. To demonstrate the generality of this method, a semiconducting polymer poly((N,N′-bis(2octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl)alt-5,5′-(2,2′-bithiophene)) (P(NDI2OD-T2)) was also used for monolayer formation and mobility measurement with the developed method ( Figure S10). The mobility of the obtained P(NDI2OD-T2) film exhibited ambipolar characteristics with a hole mobility of 0.16 cm 2 V −1 s −1 and an electron mobility of 0.23 cm 2 V −1 s −1 . These values were also higher than those of value of P(NDI2OD-T2) monolayer formed on a nonconfined aqueous subphase. [20] Furthermore, the difference in mobilities of films obtained considering varying confined widths (W C ) with a certain solution concentration and droplet volume was compared to identify the optimal width for a specific concentration and volume. Confined areas that were unreasonably narrow would cause the solution to spread back and forth, resulting in nonuniform film thickness; conversely, confined areas that were excessively wide would cause the film to break down without covering the confined area. Thus, the optimal W C had to correspond one-to-one with solution concentration and the droplet volume. Table 1 and Figures 4j and S11, Supporting Information, indicated the mobilities of thin films fabricated with different W C values. The mobilities in parallel and perpendicular direction were also compared. When the W C increased from 3 to 10 mm, mobility in parallel and perpendicular directions decreased from 2.06 to 0.78 cm 2 V −1 s −1 and from 0.5 to 0.24 cm 2 V −1 s −1 , respectively. The large decay of mobility in parallel direction implies that the highly aligned aggregation state structure was destroyed when the confined width increased. Nevertheless, the mobility of the device was still higher than that of the nonconfined films (0.1 cm 2 V −1 s −1 ), where slim rods were not introduced. This result demonstrated the effectiveness of confined areas for polymer chains orientation and mobility improvement in thin films. When W C was less than 3 mm, a droplet of 2.5 µL was slightly large for droplets placed completely on the water surface, therefore, W C = 3 mm was determined to be the optimum width for experimental conditions in this study. Considering the excellent performance of OFETs, an inverter circuit was fabricated using the thin films obtained based on the spatially confined air/water interfacial assembly method. As shown in Figure 4k,l, the inverter was fabricated on a glass substrate with three electrodes of voltage-in (V in ), voltage-out (V out ), and constant voltage-supply bias (V d ). The inverter exhibited an apparent response when switched from −60 to 0 V, and the voltage gain was 70. The results unambiguously indicated that the uniformity of the thin films over a large area and the adequate switching capability of the devices exhibit potential for application in logic circuits.

Conclusion
In this study, a spatially confined air/water interfacial assembly method was proposed to optimize the general air/water interfacial assembly method, resulting in uniform and highly aligned PDPP-TT films. The monolayer of PDPP-TT film was 2.5 nm thick, and the layer can be controlled by changing volume of the dropped solution. This implied that the thin films can be fabricated from monolayer to any needed thickness. The polarized UV-Vis-NIR absorption spectra indicated an anisotropic aggregation state structure in the films. Additionally, the carrier mobility of OFETs devices based on PDPP-TT thin films shows obvious anisotropy, which was consistent with the polarized UV-Vis-NIR absorption spectra dichotomous results. The impact of confined width on mobility was also analyzed, and the highest mobility of 2.06 cm 2 V −1 s −1 was achieved when the confined width was 3 mm for 2.5 µL PDPP-TT solution with a concentration of 0.5 mg mL −1 . An inverter circuit with a voltage gain of 70 was fabricated using the obtained thin films. The results verify the effectiveness of confined area for improving orientation and performance of films with air/water interface coatings. This method can serve as an efficient strategy for fabricating highly aligned polymer films, providing a new reference for the preparation of high-performance organic electronic devices.

Experimental Section
Materials: The semiconducting polymer PDPP-TT was synthesized by using a previously reported method, [21] and P(NDI2OD-T2) was also synthesized according to the previously reported procedures. [22] Highly Aligned Monolayer Films Fabrication: A clean petri dish was filled with ultrapure water. Due to the surface tension of the water, the water surface was maintained slightly higher than the edge of the Petri dish without overflowing. Two clean slim rods were placed above the water surface and fixed to the edge of the Petri dish with tape at a set www.advelectronicmat.de distance. The PDPP-TT solution was prepared as a concentrate of 0.5 mg mL −1 with chlorobenzene as the solvent. During the fabrication process of the highly aligned monolayer films, a drop of 2.5 µL PDPP-TT solution was dropped at the center of the confined air/water interface. The droplet rapidly spread to both sides after making contact with the water surface, forming a thin film oriented along the direction of the spreading motion at the air/water surface. The thin films with thicknesses of twoand three-layers were prepared by controlling the volume of the same concentration PDPP-TT solution with 5 µL and 7.5 µL. The thickness cannot be controlled by volume of the solution for films thicker than three layers, and the thicker films were obtained by stacking one to three layers thin films. . Schematics of organic field-effect transistors (OFETs) based on thin films obtained using the a) spatially confined air/water interfacial assembly method in the parallel direction, b) spatially confined method in the perpendicular direction, and c) nonconfined assembly method. d-f) Transfer curves and g-i) output curves of corresponding OFETs. j) Statistics and comparison of electrical performance with different confined widths in both parallel and perpendicular directions. k) Optical microscope image of an inverter based on the thin films obtained using the spatially confined air/water interfacial assembly method. l) Static switching characteristic of the inverter, the inset depicts the corresponding circuit diagram.

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Device Fabrication: BGTC OFETs devices were fabricated with a "gold stripes" method. [23] A heavily doped Si/SiO 2 substrate was used as the gate electrode and dielectric layer. The substrate was firstly cleaned with a piranha solution for 30 min and ultrasonic cleaned in sequence with ethanol, acetone, and isopropanol for 5 min each. After drying with nitrogen, the OTS modification process was performed using the vapor deposition method at 120 °C for 3 h. Subsequently, the Si/SiO 2 substrate was ultrasonic rinsed with hexane, isopropanol, and chloroform for 5 min each. The Si/SiO 2 substrate was immersed into the water after drying with nitrogen guns. When transferring film, the substrate was slowly removed from the deeper part of the water to the air/water interface and the film was slowly transferred to the Si/SiO 2 substrate. The substrate with organic film was then placed in a vacuum oven at 105 °C for 30 min to remove moisture. Finally, the gold strips were transferred onto the films as a source and drain electrode with "gold stripes" method. An inverter with top gate bottom contact structure was fabricated on glass substrate. The cleaning process of glass substrate was conducted with ultrasonic rinsed with deionized water, acetone, and isopropanol for 5 min each. Gold was used as source and drain electrodes deposited with a thermal evaporation process. The transfer and annealing processes of polymer thin films was the same as the OFETs processing. Polymethyl methacrylate with a concentrate of 80 mg mL −1 in butyl acetate was used to fabricate a dielectrics layer (C i ≈ 3 nF cm −2 ) by spin-coating process, followed by an annealing process at 90 °C for 30 min. Subsequently, through-holes are drilled mechanically with a micro-operation. Finally, silver was used as the gate electrode and deposited on the surface of the dielectric layer also using a thermal evaporation process. For the mobilities extraction, the channel width and length of the OFET devices were measured with a microscope after electrode formed. The inverter was formed with masks and the channel length was fixed at 75 µm, while the values of channel width were 4200 µm and 1200 µm for the two series connected OFETs devices.
Morphological, Structural, and Electrical Property Characterization: A microscope (Nikon LV100ND) with both OM and POM imaging mode was used to observe the morphology and molecular alignment of PDPP-TT thin films. A high-speed camera (SONY FDR-AX700) was used to record the entire rapid self-assembly process at the air/water interface. AFM (Park XE7) was used with non-contact mode to measure the morphology and height of PDPP-TT thin films fabricated by spatially confinement at air/water interface. TEM (Talos F200S) and HRTEM were used to measure the morphology and crystallinity structure of PDPP-TT monolayer films with the help of a commercial measure company (Huasuan Technology, Shenzhen, PRC). GIWAXS measurements were conducted at BL14B1/15U beamlines of the Shanghai Synchrotron Radiation Facility (SSRF). Polarized UV-Vis-NIR absorption spectra (SHIMADZU 2600) was used to compare the aggregation state structure of films in parallel and perpendicular directions. The electrical properties of OFETs devices were measured with semiconductor parameter analyzer (Platform Design Automation FS380 Pro). The charge-carrier mobility was extracted from the saturation regime and calculated with formula: I ds = WµC i (V g − V th ) 2 (2L) −1 , where I ds denotes the current between source and drain electrode, W and L denote the channel width and length, respectively, C i denotes the capacitance per unit area of dielectrics, and V g and V th denote the gate voltage and threshold voltage, respectively.

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