Molecular single crystals induce chain alignment in a semiconducting polymer

The blending of π-conjugated molecules with polymeric semiconductors into a composite system is an effective strategy to promote charge carrier mobility because of the transmission path by the conductive polymers through electrical bridge connection of the small organic molecule crystalline domain. In this work, pentacene single crystal was prepared to induce the conjugated polymer chain alignment of polymeric semiconductor PDPP2T-TT-OD to form the semiconducting composite, which led to enhanced field-effect mobility of the organic thin-film transistor (OTFT) by improving the crystallinity due to nucleation and growth phase separation. Besides, with the addition of anti-solvents, the crystallization of the blend film was further improved, 27 times higher than that of a pure polymer semiconductor–based OTFT. That was because the pentacene nuclei induced polymer crystallization through π-π interactions and the addition of anti-solvent promoted the aggregation of polymer chains in solution, enabling the molecular chains packed more closely in solid films. Therefore, the chain arrangement of polymers induced via small molecular single crystals provides a new idea to improve mobility in composite semiconductor thin films for the construction of novel organic optoelectronic devices.


Introduction
In recent years, organic thin-film transistors (OTFTs) in composite systems with high performance based on solution processing have attracted extensive attention in academia and industry. Impressively, the method of blending soluble small molecules with conjugated polymers into a mixture does not require complicated orientation design, which is an efficient and feasible route to adjust semiconductor processability, regulate solid-film microstructure, enhance charge transport, and improve device stability [1][2][3][4][5][6]. Polymeric semiconductors with lightweight, low-cost, mechanical flexibility and easy processing merits demonstrate great potential in various flexible electronic and optoelectronic applications [7,8]. The prosperity of these industries depends on gradually deepening insights into the interaction between microstructure and electrical properties in conjugated polymers. For conjugated polymer semiconductors with rigid backbones and flexible side chains, the charge transport routes mainly include intrachain transport and interchain transport via π − π interactions [9,10]. In addition, the nucleator has the potential to induce heterogeneous nucleation of polymers, so then increasing the rate of crystallization and improving the crystal structure to enhance charge carrier mobility. Thus, the crystallization process of conjugated polymers has profound implications for charge transport. Conjugated polymer interchain interaction is driven by weak van der Waals forces, so the resulting polymer chains have numerous degrees of conformational freedom and irregular interchain entanglement [11,12]. In such systems, the charge transport process is significantly retarded because the charge carriers may be decelerated or trapped caused by a large amount of lattice disorder as they hop between the chains [13,14]. Accordingly, how to construct high crystallinity structures with close intermolecular packing and high degree of lattice perfection for more efficient interchain hopping is the key to achieve high charge carrier mobility (μ) [15][16][17].
To address the above challenges, extensive efforts have been devoted to constructing oriented conjugated polymer films with strengthening intermolecular interactions through a variety of alignment processes, as follows: capillary action [18], nanogroove-guided [19], magnetic force-induced [20], directed self-assembly [21][22][23], and others [24,25]. Despite the high charge mobility of polymer-based transistors being obtained through the above processing methods, they all need external directional force guidance to achieve aligned crystalline thin films, rendering the process more complicated. Owing to the limited understanding of the dynamic process of carrier generation and evolution, the known methods thus far for film texture control still confront considerable restrictions such as large scale, convenience, and universality [26,27]. Currently, a great deal of endeavors has been conducted to develop a thin film with aligned structure preparation in molecular orientation, packing, and crystallinity. Besides, tuning the blending thin-film microstructure composed of both small molecule and polymeric semiconductors is of significance to optimize the performance of the device [28,29]. Blending realizes the compatibility of crystallinity in small molecules and the control of film uniformity in polymers, which is a feasible pathway to meet the needs of processability, stability, and reproducibility. All in all, since the molecular dynamics of the interaction and phase transition behavior in small molecules and polymers is still ambiguous, the intrinsic relationship between microstructure and charge transport ability of organic thin film requires continued discussion in depth.
In this study, the thin films with high crystallinity were prepared by inducing the crystallization of conjugated polymers through the growth of pentacene single-crystal island during spin coating. Meanwhile, the effects of pentacene on the phase structure, surface morphology, and carrier transport property of polymers were explored. Pentacene, formed by annealing a soluble pentacene precursor (Pre-PENT) under a special conversion temperature range similar to the annealing temperature of polymers, behaves as crystal nuclei inducing polymer crystallization [30]. Taking the typical conjugated polymer with an empirical formula (C 60 H 90 N 2 O 2 S 4 ) n , poly (2,5-bis (2-octyldodecyl)-3-(5-(thieno [3,2-b] thiophen-2,5-yl) thiophen-2-yl)-6-(thiophen-2,5-yl) pyrrolo [3,4-c] pyrrole-1,4 (2H,5H)-dione) (PDPP2T-TT-OD) as an example [31,32], shows an excellent charge carrier mobility due to the high planarity, robust aggregating performance, and ambient stability of the groups with thieno-thiophene parts, which was majorly utilized for various organic electronics such as photovoltaic cells, light emitting diodes, and field-effect transistors [33][34][35]. The organic thin-film transistor (OTFT) constructed by the optimized blending film of PDPP2T-TT-OD/pentacene with a mass ratio of 15:1 exhibits an obvious mobility increase (16 times higher) compared with the device based upon a pure PDPP2T-TT-OD film due to the formation of local order structure in the blending film. Furthermore, the addition of a suitable anti-solvent can enhance the phase separation of polymer and pentacene, allowing the complete separation of the pentacene from the overlapping region (π-π interaction region) between the conjugated polymer chains. The separated pentacene molecules gather in the flexible side chain region, reducing the charge transport barrier and improving the order of the molecular chains. The field-effect mobility increased about 27 times higher in contrast to the pure PDPP2T-TT-OD film.

General information
The polymeric semiconductor PDPP2T-TT-OD and smallmolecular Pre-PENT were purchased from Sigma-Aldrich, and used as received without any fractional distillation. Fourier transform infrared (FTIR) spectra were recorded on a Tensor-27 spectrometer (Bruker, Germany) in the region of 4000-400 cm −1 on a silicon wafer coated with the Pre-PENT thin film in reflectance mode at room temperature. UV-visible (UV-vis) absorption spectra were obtained on an Agilent Cary 5000 spectrophotometer (Agilent, USA) on quartz glass coated with the Pre-PENT thin film under the same condition as above. The crystallization behavior was determined by X-ray diffraction (XRD) analysis via utilizing an Ultima IV X-ray diffractometer (Rigaku, Japan) equipped with Cu Ka radiation (λ = 0.15 nm). The operating current and voltage were 30 mA and 35 kV, respectively. The scanning angle range and rate were set at 2θ = 3 ~ 60° and 5°/min separately. To justify the crystallinity of PDPP2T-TT-OD/pentacene composites, the following Eq. (1) is often used and expressed by the ratio of the area of the crystalline region (A c ) to the sum of the area of the crystalline region and the amorphous region (A c + A a ).
The acquired XRD profiles of PDPP2T-TT-OD/pentacene composite thin films were analyzed by peak differentiating and imitating. The area of the diffraction peak of each crystalline region and amorphous region could be obtained. The surface morphology of the thin films was evaluated by using an atomic force microscope (AFM) Icon (Bruker, Germany) in peak force tapping mode. The scanning area was set at 1 × 1 μm 2 ; the integral gain value and the proportional gain value were 0.3 and 0.05, individually.

Fabrication and characterization of organic thin-film transistor devices
OTFTs were fabricated on highly n-doped silicon which was the gate electrode, and a 50 nm thick SiO 2 formed by thermal deposition on the silicon surface was the gate dielectric layer. Next, the Si/SiO 2 substrates were ultrasonicated sequentially in acetone, isopropanol, and deionized water for 20 min each and then treated with UV ozone for 20 min. Then, PDPP2T-TT-OD/ Pre-PENT (mass ratio of 1:0, 20:1, 15:1, 10:1) was dissolved respectively in chloroform at 0.3 wt% and spin-coated on the Si/SiO 2 substrate at 1500 rpm for 60 s, afterward heating and annealing at 155 °C under vacuum for 2 h. Later, source and drain Au electrodes (50 nm thick) were thermally evaporated through the shadow mask at an evaporation rate of 0.2 Å s −1 under a pressure of 3 × 10 −4 Pa. Finally, the devices were baked in a vacuum drying oven at 120 °C for 20 min to remove the residual solvents. The channel length and width of OTFTs were 50 and 1200 μm severally. Again, the anti-solvent treatment is that the selected PDPP2T-TT-OD/Pre-PENT (mass ratio of 15:1) was dissolved separately at 0.3 wt% in chloroform/ethanol (volume ratio of 50/1), chloroform/methanol (volume ratio of 20/1), and chloroform/cyclohexane (volume ratio of 20/1).
Other steps are the same as mentioned above. The hole charge transport mobilities were extracted in the saturation regime using the accordingly following formulas (2) and (3) as below: where L and W are the channel length and width, respectively. I DS and V GS are the saturation source-drain current and gate-source voltage separately, C is the areal capacitance of the gate dielectric, and V th is the threshold voltage determined by extrapolating the corresponding I DS 1/2 vs. V GS plots. ε 0 is the vacuum permittivity with a value of 8.85 × 10 −12 F/m, ε r and d are the relative permittivity with the value of 3.9 and the thickness (~ 50 nm) of SiO 2 dielectric layer. All the electrical performance tests of OTFT devices were carried out with a Keithley 4200A-SCS semiconductor parameter analyzer under atmosphere conditions at room temperature.

Solution-processed conjugated polymer/ small-molecule organic semiconductor composite thin film
Pentacene is one of the most widely used materials in small-molecule organic semiconductors with exceptional (3) C = 0 r ∕d thermal stability, whereas it is essentially insoluble in conventional organic solvents at room temperature. Therefore, vacuum deposition methods are frequently used to fabricate pentacene films. Additionally, solution processing is a desirable choice for organic semiconductors because of the low cost and simple operation by solution deposition of thin films. Hence, soluble Pre-PENT was selected as the nucleating agent, which was able to be dissolved in conventional organic solvents and reverted to pentacene in one step at a certain temperature (Fig. 1a).
The chemical structures of the Pre-PENT after and before heating were characterized by UV-vis absorption spectra (Fig. S1) and FTIR (Fig. 1b). For the sake of the noticeable comparison of characteristic peaks in Fig. 1b, the peak intensity of the pentacene precursor before heating was subjected to an overall weakening treatment (that is, the peak intensity × 0.4). The characteristic absorption peak at 1706 cm −1 of the Pre-PENT before thermal decomposition attributed to the stretching vibration of the carbonyl group (C = O), the multiple absorption peaks in the range of 1000 and 1300 cm −1 assigned to the stretching vibration of N-sulfonyl carbamate group, and the stretching vibration peak at 2978 cm −1 corresponding to the unsaturated -CH bond was able to be apparently observed. Nevertheless, the above absorption peaks were evidently weakened after the precursor was heated at 150 °C for 1 h, and new absorption peaks appeared at 905 cm −1 and 728 cm −1 , confirming the precursor was decomposed to form pentacene after heat treatment as expected [36].
The Pre-PENT and polymeric semiconductor (PDPP2T-TT-OD) thin film was spin-coated from chloroform solution on the silicon substrate. Later, the Pre-PENT was thermally decomposed into pentacene and then crystallized; the polymer crystal structure was going to be further adjusted during annealing at the same time (Fig. 1c). In fact, the interactions between solute and substrate resulted in phase separation due to diverse adhesion of polymer/ small-molecule semiconductor to the substrate [37]. As PDPP2T-TT-OD and pentacene existed in a thin-film state, an electrical potential developed during phase separation bringing about remarkable changes in the microstructures of the thin film [38,39]. Physicochemical characterizations of the composite semiconductor films were carried out via manifold test methods, and the influence of smallmolecule crystal nuclei on the crystallization behavior of polymers was investigated in the following sections.

Characterization of structure and surface morphology
In order to more intuitively observe the accumulation of polymer chain segments near the pentacene crystals, a specific area was selected for AFM microscopic morphology. There were two-phase substances presented in Fig. 2a; one (A area) was the small-molecular phase tending to nucleate uniformly, and the other (B area) was a large-area polymer phase with a flat surface. As illustrated in Fig. 2b, the height of the monolayer molecular chain was about 3 nm (red line area) demonstrated by the cross-sectional line profile analysis, which was similar to the monolayer height of PDPP2T-TT-OD [40]. Analogously, the average height of the steps was 1.4 nm (blue line area) in the island-like phase as shown in Fig. 2d, which was consistent with the van der Waals size of pentacene in the long axis direction [41]. The above corresponding molecular dimensions are revealed by a ball-and-stick model in Fig. 2c. Since pentacene dispersed in the polymer matrix in the form of particles, the phase separation phenomenon of the blend was able to be explained by the nucleation growth mechanism [42]. In this way, an island-like structure where one phase was Fig. 1 Characterization of pentacene and preparation of PDPP2T-TT-OD/pentacene composite thin film: a diagrammatic presentation of thermal decomposition reaction process of the pentacene precursor. b FTIR spectra of the pentacene precursor after heating at 150 °C for 1 h (red line) and before heating (black line). Schematic representations of c the fabrication process for PDPP2T-TT-OD/pentacene precursor thin film spin-coated from chloroform solution with the corresponding chemical structures and the annealing process for PDPP2T-TT-OD/ pentacene composite thin film distributed in another phase was formed, namely, "seaisland" structure. The influence of the introduction of pentacene on the crystallization of PDPP2T-TT-OD for hybrid thin films was examined by XRD profiles as shown in Fig. 2d. The pure pentacene (0:1) film displayed three plane diffraction peaks at 2θ = 6.10°, 12.24°, and 19.26°, which separately belonged to (001), (002), and (003) lattice planes [43]. Moreover, the pure PDPP2T-TT-OD (1:0) film had a sharp (100) lattice plane diffraction peak at 2θ = 4.44° [44]. With the increase of the pentacene in the composite thin film (5:1, 2:1, 1:1), the peak intensity of the polymer moiety (100) lattice plane was clearly faded, and the characteristic peak of pentacene appeared. The weakening of the absorption band in polymer moiety was owing to the large amount of pentacene, blocking the arrangement in polymer molecular chains, so then suppressing the crystallization. However, when the pentacene was lessening in the composite thin film (10:1, 15:1, 20:1), the diffraction peak intensity of the polymer moiety (100) lattice plane became stronger, which was higher than that of the pure polymer indicating a better crystallinity [45]. In addition, a new weak (200) lattice plane diffraction peak arose in the composite thin film, suggesting the crystallinity had improved through π-π stacking interaction between polymer chains and small-molecule moieties due to the addition of pentacene as well [46]. The crystallinity of the PDPP2T-TT-OD/pentacene composite thin films calculated by the above-mentioned Eq. (1) is listed in Table 1.
The effect of introducing pentacene on the optical absorption properties of PDPP2T-TT-OD for hybrid thin films was studied via UV-vis absorption spectra in Fig. 2e. There were two absorption bands named band I and band II. Band I was the absorption peak subdivided into (0-0), (0-1), and (0-2) absorption peaks for charge transfer within the molecule, c Ball-and-stick model for the molecular dimensions of PDPP2T-TT-OD (left) and pentacene (right). The relationship between atomic species and ball colors: carbon (grey), hydrogen (green), oxygen (red), nitrogen (blue), and sulfur (yellow). d XRD profiles and e normalized UV-vis spectra for PDPP2T-TT-OD/pentacene composite thin films prepared via various mass blending ratios and band II was the weak π-π transition peak with the summit at 400-500 nm. The PDPP2T-TT-OD film had a dominant peak at 740 nm (0-1) and another two shoulder peaks at 675 nm (0-2) and 820 nm (0-0), indicating the molecular aggregation resulting in the intermolecular interactions [47]. The PDPP2T-TT-OD/pentacene composite thin films also exhibited a dominant peak and two shoulder peaks. Accordingly, the intensity of the shoulder peak (0-2) weakened with the increase of pentacene, which was assigned to the enhancement of chain order and aggregation in polymer molecules.
Notably, the crystallinity of PDPP2T-TT-OD had a trend of change with the pentacene content, which was represented by XRD profiles (Fig. 2d). The inherent driving force for this change was that the pentacene crystal nuclei induced the ordered arrangement of the polymer backbones through π-π interactions [48], and this induction could also be manifested via the absorption peaks of electrons transitioning between two kinds of molecular energy levels [49], as shown in UV-vis spectra (Fig. 2e).
So as to explore the surface morphology and phase structure of PDPP2T-TT-OD/pentacene composite thin films, the AFM measurement was implemented in tapping mode as illustrated in Fig. 3a-d. The results show micropores on the surface of the polymer film (1:0) marked with the green arrow as the black areas in the image, which was potential to be the traps during the charge transfer process and thus decrease the device performance [50,51]. The composite thin films in the mass blending ratios of 20:1 and 10:1 had a few large holes without the orientation of polymer chain packing. But the composite thin film (15:1) possessed the distinctive fibrous structure assigned to the more uniform dispersion of pentacene in the polymer matrix. A similar micromorphology could also be observed by POM (Fig. 3e-h). And yet the chain packing existed the evident orientation induced via π-π interaction revealed in Fig. 3i, which was beneficial to the transportation of the carriers to achieve a better performance. Briefly, the oriented film with low surface roughness means high planarity and structural uniformity, contributing to the preparation of organic electronic devices [52,53].

Analysis of carrier transport properties
OTFTs with a bottom-gate/top-contact (BGTC) configuration (Fig. 4a) were subsequently fabricated to study the charge transport properties of the composite thin films. The optical image of the devices with the channel length and width of 50 and 1200 μm, respectively, is presented in Fig. 4b. The complete device fabrication and characterization methods were included in the experimental section. The transfer and output characteristics of OTFT devices based on PDPP2T-TT-OD/pentacene composite thin films are depicted separately in Fig. 4c, e and Fig. S4. The performance parameters of the devices are summarized in Table 2. The data demonstrate that the device based upon the composite thin film in a mass blending ratio of 15:1 exhibited peak value (6.56 × 10 −3 cm 2 V −1 s −1 ), which was 16 times higher than that of the pure polymer-based (1:0) OTFT (4.06 × 10 −4 cm 2 V −1 s −1 ), attributed to the improvement of crystallinity in the composite. It is suggested that the introduction of pentacene facilitated the orderly accumulation of polymer chains, so then enhancing the interaction between molecular chains and achieving the purpose of optimizing the charge transport path [54,55].
Furthermore, the effect of anti-solvent on the charge transport property in composite thin films was investigated with the same configuration of OTFTs. The transfer and output characteristics of OTFT devices based on PDPP2T-TT-OD/pentacene (15:1) composite thin films after antisolvent treatment are illustrated in Fig. 4d, f and Fig. S5, respectively. The results indicate that the mobility of composite thin film OTFTs after the mixed solution of chloroform/ethanol (volume mixing ratio of 50/1) treatment increased to 1.11 × 10 −2 cm 2 V −1 s −1 , which was 27 times than the pure polymer-based OTFT. It was ascribed that the addition of the appropriate amount of anti-solvent can effectively promote the further aggregation and orderly intermolecular packing of polymer chains in solution. In other words, the crystallinity, surface morphology, roughness, and grain size of the blending films were able to be optimized by regulating the type and ratio of the solvent and anti-solvent to acquire the optimum charge transport performance, as demonstrated by XRD profiles (Fig. S2) and AFM morphological images (Fig. S3). The detailed description and quantitative analysis of the relationship between grain size and the full width at half maxima (FWHM) via the Scherrer equation are shown in the Supplementary Information (SI).
Whereas, in contrast to the performance of PDPP2T-TT-OD-based transistors in the previous reports [56,57], our field-effect mobility value of OTFTs was lower. On the one hand, perhaps due to the fact that the SiO 2 surface of the substrate was not passivated with silane molecules, such as octadecyl trichlorosilane (OTS) and hexamethyl disilazane (HMDS) [58], resulting in high defect density and charge carrier trap states at the interface of the semiconductor/dielectric layer, hindering the effective transport of carriers. On the other hand, there was a nonlinear relationship at the low drain voltage (V D ) from the output characteristics, signifying a higher electron injection barrier and a larger interface contact resistance at the electrode/semiconductor layer interface, which affected the effective injection of electrons [59]. Thus, the interface of the dielectric/semiconductor layer and the electrode/semiconductor layer can be adjusted to optimize the effective

Conclusion
To sum up, the present research provided a novel consideration to boost the carrier transport properties via regulating and controlling the polymer chain arrangement. The orientation mechanism of introducing π-conjugated molecules (pentacene) into polymeric semiconductor (PDPP2T-TT-OD) was explored, which brought about an intensive charge carrier transport of the OTFTs through facilitating the crystallinity owing to nucleation and growth phase separation accordingly. Moreover, along with the addition of anti-solvents, the crystallization of the composite thin film was further ameliorated. Impressively, the results confirmed that in contrast to the mobility of pure polymer-based OTFTs, the maximal fieldeffect mobility arrived at 6.56 × 10 −3 cm 2 V −1 s −1 and the mobility of the transistor after ethanol anti-solvent treatment was further raised to 1.11 × 10 −2 cm 2 V −1 s −1 , which was 16 and 27 times higher than that of pure polymer semiconductor-based OTFTs separately. It was worth emphasizing that the crystallization induction of pentacene nuclei to the polymer by π-π interactions and the addition of anti-solvent enhanced the aggregation of polymer chains in the solution rendering the molecular chains more compactly packed in the solid-phase film. In view of these results, we believe this work, involving the strategy for molecular single crystals to induce the chain alignment of polymers to heightening fieldeffect mobility in composite semiconductors, paves a practicable pathway to manufacture an innovative type of organic optoelectronic devices.