Colloids of semiconducting polymers for high-performance, environment-friendly polymer field effect transistors

doi:10.1016/j.orgel.2015.05.039

Organic Electronics

Volume 24, September 2015, Pages 160-164

https://doi.org/10.1016/j.orgel.2015.05.039 Get rights and content

Highlights

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We demonstrate polymer colloids technique for organic electronics.
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Polymer colloids techniques enable green technology.
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High mobility of 2.7 cm²/Vs is demonstrated from arrays of polymer particle.
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X-ray study supports the well aligned ordering of semiconductor in particles.
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Stable polymer colloids are fabricated by simple dispersion method.

Abstract

Considering the recent rapid growth of polymer field effect transistor (PFET) techniques and environmental pressure to avoid the use of toxic organic solvents in industrial processes, it is very important to develop colloids of semiconducting polymers in non-toxic solvents. Herein, we demonstrate the fabrication of surfactant-free, emulsion-dispersed nanoparticles of donor–acceptor copolymers. By optimizing the deposition methods of such semiconducting polymer colloids, high mobility (2.7 cm²/Vs) PFETs were fabricated via environmentally benign processes. Systematic analyses on the crystalline nature of the colloid-processed films revealed that the well-developed edge-on lamella structure comparable to that of the bulk polymer, was preserved during the colloid-fabrication processes.

Graphical abstract

Introduction

In the polymer industry, the use of toxic organic solvents can be a serious issue due to the attendant additional environmental costs at the mass-production stage. Consequently, tremendous effort has been directed toward switching from solvent-based to water-borne polymer systems, especially in the fields of adhesives, thermoplastics, textiles, paints, and biomedical applications [1], [2]. Such systems may be denoted as polymer colloids, where polymer microparticles or submicroparticles are dispersed in environmentally benign solvents such as water or alcohols. For example, in the field of conducting polymer colloids, conducting polymers such as polypyrrole or polyacetylene are sterically stabilized by water-soluble polymeric stabilizers that are physically adsorbed on the outer surface of the conducting polymer microparticles [3]. The conductivity of films prepared with such colloids is usually lower than that of films prepared using bulk polymer powders due to the presence of the electrically insulating polymeric stabilizer and the increased number of resistive interparticle contacts [3]. Nonetheless, the demand for these conducting polymer colloids is increasing because of the environmental pressure to avoid the use of toxic organic solvents [4].

In the field of semiconducting polymers, however, efforts to fabricate polymer colloids to meet the trend toward greener polymer technologies have been limited [5], [6], [7]. This can be attributed in part to the fact that the colloids of semiconducting polymers usually exhibit very low charge carrier mobility. For example, Zhou and coworkers demonstrated that colloids of poly-3(-hexylthiophene) could be dispersed in water either with or without the assistance of a surfactant, and relatively higher mobility was obtained from the colloids without surfactant, presumably due to better interparticle charge transport [7]. Although this result is very interesting, the mobility was still remarkably low compared to that of bulk poly-3(-hexylthiophene). This may be attributed to the generally poor charge transport within the microparticles of semiconducting polymers that assemble very quickly in hydrophilic environments. Note that in the field of traditional polythiophene-based polymer field effect transistors (PFETs), it is well known that slow formation of the polymer domain is a critical factor for achieving better intermolecular interactions [8].

Based on these considerations, we attempted to fabricate colloids of novel semiconducting polymers, represented by donor–acceptor conjugated polymers. Unprecedentedly high performance, with charge carrier mobilities higher than 10 cm²/Vs, has been achieved with these polymers, mostly having diketopyrrolopyrrole (DPP) or isoindigo (IID) as an electron acceptor [9], [10], [11], [12], [13]. More importantly, these polymers have exhibited high performance even in the case of thin films with low crystallinity [14]. Such interesting characteristics were observed especially for the donor–acceptor copolymers with strongly planar backbone structures [15]. Therefore, it is anticipated that these novel polymers should be able to maintain high mobility even in the form of (sub)microparticles constructed without sufficient time for self-assembly.

On this basis, we demonstrate herein a simple method for the fabrication of polymer colloids to achieve high mobility (>2 cm²/Vs) PFETs. Polymer colloids were prepared in butyl acetate (BA) or ethyl acetate (EA) by a simple emulsion dispersion method based on a donor–acceptor copolymer that has a torsion-free, planar backbone structure, i.e., diketopyrrolopyrrole–thienothiophene (DPP–TT) [16]. According to previous reports, BA and EA can be categorized as relatively greener solvents comparable to ethanol [17], [18], and therefore, the prepared colloids of the semiconducting polymer adequately satisfy the requirements of the colloid technique (also see Table S1). The submicroparticles of DPP–TT were fully characterized by employing scanning tunneling microscopy (FE-SEM) and grazing incident X-ray diffraction (GIXD). By optimizing the methodology for deposition of the submicroparticle film, high mobility (2.7 cm²/Vs) PFETs are achieved from polymer colloids for the first time.

Section snippets

Materials

The procedure for synthesis of DPP–TT is documented in a previous report. 16 BA, EA, and chloroform were purchased from Sigma–Aldrich and used without further purification. Molecular weight of DPP–TT used in this work was Mn = 34,200, Mw = 57,800.

Characterization

Optical images were obtained with a digital camera (Lumix DMC-LX5, Panasonic). SEM images were obtained via FE-SEM (Sigma/Carl Zeiss) with a Schottky Field Emitter. The GIXD measurements were performed at the PLS-II 9A U-SAXS beamline of Pohang Accelerator

Results and discussion

Fig. 1(a) shows the chemical structure of the semiconducting polymer, DPP–TT, used as a polymeric semiconductor in this work. A scrutiny of the chemical structure shows that the lactam moiety in the DPP segment should exhibit a strong electron-withdrawing effect and high electron affinity. Furthermore, because the thiophene unit is attached to the DPP unit, it is expected that close (2.1 Å) intramolecular hydrogen bonding between the carbonyl oxygen of DPP and the nearest thiophene hydrogen

Conclusions

In conclusion, high performance (mobility ∼ 2.7 cm²/Vs) PFETs based on colloids of semiconducting polymers were successfully developed for the purpose of realizing green organic electronics. To this end, a donor–acceptor copolymer with strong backbone planarity with the ability to maintain crystalline ordering even in the form of submicroparticles dispersed in colloidal solutions was employed. Based on various morphological and structural analyses, the optimized submicroparticles were proved to

Acknowledgements

This Research was supported by Space Core Technology Development Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education – South Korea (Grant Number: NRF-2014M1A3A3A02034707).

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Colloids of semiconducting polymers for high-performance, environment-friendly polymer field effect transistors

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Materials

Characterization

Results and discussion

Conclusions

Acknowledgements

J. Colloid Interface Sci.

Overview of Polymer Colloids: Preparation, Characterization, and Applications

Paint and Surface Coatings – Theory and Practice

Handbook of Conducting Polymers

Chem. Mater.

Macromolecules

Macromolecules

Nat. Mater.

Mater. Chem.

Chem. Mater.