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Article Cite This: ACS Omega 2018, 3, 9290−9295
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Modulating Surface Morphology and Thin-Film Transistor Performance of Bi-thieno[3,4‑c]pyrrole-4,6-dione-Based Polymer Semiconductor by Altering Preaggregation in Solution Guangcheng Ouyang,†,§,∥ Hongzhuo Wu,†,§,∥ Xiaolan Qiao,† Jidong Zhang,‡ and Hongxiang Li*,†
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†
Key Laboratory of Synthetic and Self-Assembly Chemistry for Organic Functional Molecules, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032, China ‡ State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China § The University of Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *
ABSTRACT: Due to their strong intermolecular interactions, polymer semiconductors aggregate in solution even at elevated temperature. With the aim to study the effect of this kind preaggregation on the order of thin films and further transistor performance, bi-thieno[3,4-c]pyrrole-4,6-dione and fluorinated oligothiophene copolymerized polymer semiconductor P1, which shows strong temperaturedependent aggregation behavior in solution, is synthesized. Its films are deposited through a temperature-controlled dip-coating technique. X-ray diffraction and atomic force microscopy results reveal that the aggregation behavior of P1 in solution affects the microstructures and order of P1 films. The charge transport properties of P1 films are investigated with bottom-gate top-contacted thin-film transistors. The variation of device performance (from 0.014 to 1.03 cm2 V−1 s−1) demonstrates the importance of optimizing preaggregation degree. The correlation between preaggregation degree and transistor performance of P1 films is explored.
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INTRODUCTION Polymer transistors have attracted great attention because they can be fabricated in large scale on flexible substrates at low cost.1−6 Besides the intrinsic charge carrier property, the order of polymer semiconductor in films determines the performance of polymer transistors.7−10 Although great efforts have been made, many defects still exist in the as-spun films, and the films usually display poor transistor performance. Post-treatments, such as thermal annealing11−13 and solvent annealing,14−17 are required to improve the order and lower the defects of films so as to facilitate the charge carrier transport. However, thermal annealing process is usually conducted at relatively high temperature (>150 °C, sometimes >200 °C),18,19 and solvent annealing is subject to a variety of solvents.16,17 Hence, it is highly required to optimize the order of polymer semiconductors during the film deposition process. Due to their strong intermolecular interactions, polymer semiconductors usually aggregate in solution (even at elevated temperatures). 20−23 It is believed that this kind of preaggregation is helpful to form ordered thin films.24,25 Unfortunately, the effects of preaggregation degree on the order of films and the performance of devices are rarely reported due to the difficulty in controlling the self-assembly of polymer semiconductors. It is well known that the selfassembly of polymer semiconductors is influenced by temperature in solution; therefore, we believe the preaggregation © 2018 American Chemical Society
degree of polymers can be controlled through regulating the solution temperature. To achieve this goal, the polymer used should have strong temperature-dependent preaggregation at relatively low temperature. Donor−acceptor (D−A)-type polymer semiconductors have attracted great attention due to their potential applications in organic transistors and organic solar cells.26−30 Currently, most of the reported D−A polymer semiconductors are diketopyrrolopyrrole-,31,32 naphthalenediimde-,33,34 and isoindigo35,36based ones. The development of polymer semiconductors with new acceptor building blocks is not only crucial to the exploration of high-performance polymer semiconductors, but also important to summarize the structure−property relationship of polymer semiconductors. Bi-thieno[3,4]pyrrole-4,6dione (bi-TPD) is an interesting acceptor unit and has the advantages of planar structure, extending π-conjugated length vs thieno[3,4]pyrrole-4,6-dione (TPD), and minimizing conformational disorder with intramolecular S···O interactions. Despite its merits, only few bi-TPD-based polymers have been reported, and the ones related to the organic transistors are even less.37−40 In this manuscript, bi-thieno[3,4-c]pyrrole-4,6dione (bi-TPD) and fluorinated oligothiophene copolymerized Received: July 17, 2018 Accepted: August 1, 2018 Published: August 16, 2018 9290
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Scheme 1. Chemical Structure and Synthetic Route of P1
polymer semiconductor P1 was synthesized. P1 shows strong temperature-dependent self-assembly behavior at relatively low solution temperature. With a temperature-controlled dipcoating technique,41 the influence of preaggregation degree on the order and charge carrier transport property of P1 films was systematically investigated. Atomic force microscopy (AFM) and X-ray diffraction (XRD) results reveal preaggregation degree has strong impact on the quality of films. The variation of device performance (from 0 to 1.03 cm2 V−1 s−1) demonstrates the importance in optimizing the preaggregation degree. The correlation between preaggregation degree and transistor performance of P1 films is explored.
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RESULTS AND DISCUSSION Synthesis and Physicochemical Property of P1. The synthetic route of polymer semiconductor P1 is shown in Scheme 1. P1 was prepared by coupling monomers 1 and 2 via Pd-catalyzed Stille reaction. The crude polymer was ultimately purified by precipitation into methanol, followed by successive Soxhlet extractions with organic solvents. Its number-average molecular weight (Mn) was measured with high-temperature (150 °C) gel-permeation chromatography in 1,2,4-trichlorobenzene by using polystyrene as the standard. The Mn/ polydispersity index (PDI) values were 54 kDa/1.82. The purity of P1 was characterized with elementary analysis. The thermal property of P1 was investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). P1 exhibited good thermal stability with a 5% weight loss over 400 °C. No peaks were observed in the DSC curve in the temperature range of 25−300 °C (Figure S1). Figure 1a illustrates the normalized UV−vis absorption spectra of P1 in 1,1,2,2-tetrachloroethane (TCE) solution at room temperature and thin film. P1 has strong absorptions in the range of 400−700 nm. The similarity of solution absorption and film absorption indicated P1 was strongly aggregated in solution. The corresponding optical energy band gap (Eg) of P1 is 1.82 eV, which was estimated from the onset of the film absorption spectrum. To investigate the effect of solution temperature on the preaggregation degree of P1, its absorption measurements were conducted at varied temperatures (from 30 to 120 °C) (Figure 1b). It is clearly seen, with solution temperature increasing, the intensity of absorption peak at 635 nm decreased. When the temperature was above 60 °C, this peak was nonidentified. At the same time, the absorption peak at 597 nm also changed gradually with the increase of temperature, that is, the width got narrowed and the intensity lowered instead of increased. This suggested the absorption peak at 635 nm did not disappear at higher
Figure 1. (a) UV−vis absorption spectra of P1 in 1,1,2,2tetrachloroethane (TCE) solution at room temperature and thin film. (b) UV−vis absorption spectra of P1 in TCE solution at different temperatures. (c) Cyclic voltammograms of the P1 films on a platinum electrode. Bu4NPF6 was used as electrolyte, CH3CN was used as solvent, the scan rate was 50 mV s−1, and ferrocene was used as external standard. (d) Illustration of temperature-controlled dipcoating technique.
temperature but was covered by the absorption peak at 597 nm. The absorption changes of P1 indicated its aggregation behavior is temperature-sensitive and can be controlled at relatively low temperature, which provides a platform for investigating the effect of preaggregation degree on the order of films and the performance of transistors.42,43 Morphology and Microstructure of P1 Films. With the aim to minimize the change of preaggregation degree during the film deposition process, P1 films were deposited with the temperature-controlled dip-coating technique (Figure 1d). In this technique, an oil bath and a heating source were used to control the temperatures of P1 solution and substrate. To eliminate the temperature difference, P1 solutions and substrates were stood at a given temperature for 1 h before each film deposition. The microstructures and orders of P1 films were investigated with AFM and X-ray diffraction (Figure 2). Nanowire-type structures were clearly observed for the films deposited at 40 and 50 °C. Notably, these films were very thin, and the thickness was 3−4 nm for the films deposited at 40 °C and 4−5 nm for those deposited at 50 °C (Figure S2), which provides a new way to prepare ultrathin films. When the 9291
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Figure 2. (a−f) AFM images of P1 films deposited at different solution temperatures. (g) Out-of-plane and (h) in-plane XRD patterns of P1 films deposited at different solution temperatures.
Figure 3. Typical transfer (a) and output curves (b) of P1 thin films deposited with temperature-controlled dip coating (at 70 °C).
ance, the charge transport properties of P1 films deposited through temperature-controlled dip coating were studied with field-effect transistors. The devices were bottom-gate topcontacted structures. Figures 3 and S5 show the representative output and transfer curves of the transistors, and the transistor performance is summarized in Table 1. The films dip coated at 40 °C did not show transistor performance, which was ascribed to their ultrathin films (only 3−4 nm).44,45 The thickness of films deposited at 50 °C was relatively thicker (5−6 nm), and a mobility of 0.014 cm2 V−1 s −1 was observed. The device performance increased dramatically when the temperature was 60 °C, and a mobility of 0.78 cm2 V−1 s−1 was observed. The highest device performance was achieved for the films dip coated at 70 °C, and a mobility of 1.03 cm2 V−1 s−1 was reached. The performance of the devices gradually decreased, when the solution temperature was further increased. The mobility was 0.94 cm2 V−1 s−1 for the films deposited at 80 °C and 0.50 cm2 V−1 s−1 for those deposited at 90 °C. The variation of device
temperature was further increased, the size of microstructures decreased, which was further proved by the scanning electron microscope results (Figure S3), and demonstrated the great effect of preaggregation degree on the microstructures of films. No continuous films were obtained at 100 °C due to the low viscosity of the solution and the high solvent evaporating rate at this temperature. Due to the low thickness, no diffraction peaks were observed for the films deposited at 40 and 50 °C (Figure 2g). Singlefamily Bragg reflections were observed for the films deposited at higher temperatures. The intensity of diffraction peaks was on the order of 70 °C > 80 °C ≈ 60 °C > 90 °C, indicating the different order of films. The distinct changes of the morphology and quality of P1 films demonstrate the importance of preaggregation degree. AFM and XRD results suggested the preaggregation degree at 70 °C was optimal for P1. Charge Carrier Transport Property of P1 Films. To study the effect of preaggregation degree on device perform9292
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chlorobenzene. The chlorobenzene solution was concentrated by evaporation to about 20 mL and was poured into 100 mL of methanol. The precipitate was collected by filtration and dried under vacuum to afford P1 (0.224 g, yield: 89.2%). Mn: 54.0 kDa, PDI: 1.82. Anal. Calcd for C100H154F2N2O4S6: C, 71.55; H, 9.25; N, 1.67%. Found: C, 71.08; H, 9.33; N, 1.35%. Device Fabrication and Characterization. Heavily ndoped silicon wafers with a 300 nm SiO2 layer as the dielectric layer were used as substrates for dip coating. The silicon wafers were modified by octadecyltrichlorosilane (OTS). The cleaning and modification of silicon wafers were performed according to the reported process.39 Films deposited with the temperature-controlled dip-coating technique: 2.0 mg mL−1 of P1 in TCE solution was first heated to 90 °C and stirred for 24 h. The solution was transported to the temperature-controlled dip-coating machine. The solution was stood at a given temperature for 1 h before film deposition. The parameters for dip coating are 30 s of immersed time and 25 mm min−1 of lifting speed. Films deposited with the varied-temperature drop-casting technique: P1 was dissolved in TCE (0.05 mg mL−1). Then, the solution and OTS-modified Si/SiO2 substrate were heated to a given temperature. P1 films were prepared by drop casting 40 μL solution onto the substrate. The transistors were bottom-gate top-contacted structures. Gold source and drain electrodes (50 nm in thickness) were deposited under vacuum with a shadow mask. The channel length and width were 31 and 273 μm, respectively. The transistor measurement was carried out at room temperature by using a Keithley 4200 semiconductor parameter analyzer in ambient condition. The mobilities were calculated in the saturation regime according to the following equation: IDS = (μWCi/2L)(VG − VT)2, where IDS is the drain−source current, W is the channel width, L is the channel length, Ci is the capacitance per unit area of the gate dielectric layer, and VT is the threshold voltage.
Table 1. Transistor Performances of P1 Films Deposited with Temperature-Controlled Dip Coating solution temperature (°C)
Vth (avg) (V)
Ion/Ioff
μmax (avg)a (cm2 V−1 S−1)
40 50 60 70 80 90
−29.2 −18.7 −16.4 −21.6 −15.6
103−4 104−5 104−5 104−5 104−5
0.014 (0.0078 ± 0.0042) 0.78 (0.44 ± 0.24) 1.03 (0.77 ± 0.20) 0.94 (0.59 ± 0.27) 0.50 (0.27 ± 0.17)
a More than 20 devices were measured for the films deposited at each temperature.
performance was well consistent with the XRD and AFM results and proved the great impact of preaggregation degree on device performance. Moreover, the performance of the dipcoated device performance could not be further optimized with thermal annealing, demonstrating the unique merit of temperature-controlled dip-coating technique for flexible polymer transistors. To demonstrate the generality of preaggregation degree on device performance, P1 films were further deposited through variable-temperature drop-casting technique. Due to the poor quality of these films, which is a common disadvantage of drop-casting technique, the mobility of drop-casting films was lower than that of dip-coating ones. The highest mobility for drop-casting films is 0.18 cm2 V−1 s−1. As we expected, these films showed the same trends in device performance as those deposited through temperature-controlled dip coating (Table S2), further proving the importance of preaggregation on device performance.
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CONCLUSIONS Bi-TPD and fluorinated oligothiophene copolymerized polymer semiconductor P1 was synthesized and characterized. P1 displays strong temperature-dependent aggregation behavior in solution. With a temperature-controlled dip-coating technique, the effect of aggregation behavior of P1 on the order and microstructures of films and further the thin-film transistor performance were studied. Experimental results showed the order and microstructures of P1 films were modulated by changing the preaggregation of P1, and the variation of transistor performance revealed the great impact of preaggregation of P1. The device performance could not be further optimized with post-treatment, demonstrating the unique merit of the temperature-controlled dip-coating technique for high-performance flexible polymer transistors. All these results show the importance of tuning preaggregation during the fabrication of polymer transistors.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b01690.
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TGA, DSC, and elemental analyses of P1; AFM image and XRD results of P1 films; transfer and out curves of P1 transistors (PDF)
AUTHOR INFORMATION
Corresponding Author
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*E-mail:
[email protected].
EXPERIMENTAL SECTION Material Synthesis. Monomer 1 was synthesized according to the reported literature,39 and monomer 2 was purchased from commercial source. Synthesis of P1. Compound 1 (0.245 g, 0.15 mmol), compound 2 (0.079 g, 0.15 mmol), Pd2(dba)3 (0.0054 g, 0.006 mmol), and P(o-tolyl)3 (0.0036 g, 0.012 mmol) were added in toluene (20 mL) solution and then were heated to reflux for 10 h under nitrogen. After being cooled to room temperature, the reaction mixture was slowly dropped into 50 mL of methanol. The precipitate was filtered and extracted by Soxhlet with acetone, petroleum ether, dichloromethane, chloroform, and
ORCID
Hongxiang Li: 0000-0002-1032-1583 Author Contributions ∥
G.O. and H.W. contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant nos. 21672252 and 21472116), the “Strategic Priority Research Program” of the Chinese 9293
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DOI: 10.1021/acsomega.8b01690 ACS Omega 2018, 3, 9290−9295
