Low-Band-Gap Polymer-Based Ambipolar Transistors and Inverters Fabricated Using a Flow-Coating Method

Abstract

The performances of organic thin film transistors (OTFTs) produced by polymer solution casting are tightly correlated with the morphology and chain-ordering of semiconducting polymer layers, which depends on the processing conditions applied. The slow evaporation of a high boiling point (bp) solvent permits sufficient time for the assembly of polymer chains during the process, resulting in improving the film crystallinity and inducing favorable polymer chain orientations for charge transport. The use of high bp solvents, however, often results in de-wetting of thin films formed on hydrophobic surfaces, such as the commonly used octadecyltrichlorosilane (ODTS)-treated SiO 2 gate dielectric. De-wetting hampers the formation of uniform and highly crystalline semiconducting active channel layers. In this manuscript, we demonstrated the formation of highly crystalline dithienothienyl diketopyrrolopyrrole (TT-DPP)-based polymer films using a flow-coating method to enable the fabrication of ambipolar transistors and inverters.Importantly, unlike conventional spin-coating methods, the flow-coating method allowed us to use high bp solvents, even on a hydrophobic surface, and minimized the polymer solution waste. The crystalline orientations of the TT-DPP-based polymers were tuned depending on the solvent used (four different bp solvents were tested) and the employment of a thermal annealing step. The use of high bp solvents and thermal annealing of the polymer films significantly enhanced the crystalline microstructures in the flowcoated films, resulting in considerable carrier mobility increase in the OTFTs compared to the spin-coated films. Our simple, inexpensive, and scalable flow-coating method, for the first time employed in printing semiconducting polymers, presents a significant step toward optimizing the electrical performances of organic ambipolar transistors through organic semiconducting layer film crystallinity engineering.

A Nonchlorinated Solvent-Processable Fluorinated Planar Conjugated Polymer for Flexible Field-Effect Transistors

“…This observation indicates that the interchain interactions between PDPP2DT-F2T2 polymer chains increased in this order and that the nonchlorinated solvents TOL, XY, and TMB can improve interchain interactions. 4,25,40,41 This argument is also supported by comparing these spectra with the optical properties of the PDPP2DT-F2T2 polymer in solution. The maximum absorption peaks of polymer solutions in TMB and CF were 794 and 788 nm, respectively (see Figure 1b and Figure S9a).…”

“…According to previous studies that have employed chlorinated solvent systems such as CB, DCB, or binary solvents (e.g., CF:CB or CB:DCB), the use of high bp solvent systems (rather than using CF) assisted the growth of crystalline domains and thus enhanced device performance. 4,33 Note that CB and DCB, which have been used in previous studies, have different dipole moments and surface energies (in addition to the bp difference), 33 while those for TOL, XY, and 1,2,4-trimethylbenzene (TMB) 34 are similar to one another (see Figure S1). This fact suggests that we can examine the pure dependence of solvent drying speed on the electrical properties of polymer FETs by using nonhalogenated solvents of TOL, XY, and TMB whose bp values are 110, 144, and 168 °C, respectively.…”

“…Semiconducting conjugated polymers have been developed for various applications of polymer field effect transistors (FETs), including radio frequency-identification, chemical sensors, switching elements, and logic circuits in electronics. − Carrier mobilities of conjugated polymer-based FETs have exceeded 1 cm 2 V –1 s –1 , which is even higher than the carrier mobility of amorphous silicon (a-Si) FETs. − Most of the reported high-performance semiconducting polymers are based on alternating linkages of electron-rich donor (D) units − and electron-deficient acceptor (A) units. − Combinations of the chemical structures of D and A units can be used to modulate the optical and electrical properties of conjugated polymers and can affect the morphology in film state. Side alkyl chains in these polymers also play a significant role in determining the solubility and the molecular interactions in the film state. ,− Among the D–A type polymers, diketopyrrolopyrrole (DPP) has been widely employed as an acceptor unit because DPP-containing polymers can provide high-quality crystalline films based on the planar backbone structure, which results in high carrier mobilities.…”

“…To date, only a few studies have attempted to replace halogenated solvents with nonhalogenated solvents such as toluene (TOL), xylene (XY), and tetralin. ,, Moreover, the drying speed of semiconducting polymer films influences the film morphology and FET performance, and this aspect needs to be investigated using a series of different boiling point (bp) nonchlorinated solvents. According to previous studies that have employed chlorinated solvent systems such as CB, DCB, or binary solvents (e.g., CF:CB or CB:DCB), the use of high bp solvent systems (rather than using CF) assisted the growth of crystalline domains and thus enhanced device performance. , Note that CB and DCB, which have been used in previous studies, have different dipole moments and surface energies (in addition to the bp difference), while those for TOL, XY, and 1,2,4-trimethylbenzene (TMB) are similar to one another (see Figure S1). This fact suggests that we can examine the pure dependence of solvent drying speed on the electrical properties of polymer FETs by using nonhalogenated solvents of TOL, XY, and TMB whose bp values are 110, 144, and 168 °C, respectively.…”

Ambipolar polymers for transistor applications

“…The incorporation of F atoms in the β‐positions of thiophene rings not only improved the efficiency of direct arylation but also lowered the HOMO and LUMO energy levels of the polymer, forming ordered films with an edge‐on alignment of polymer backbones 11 . Besides, a simple flow‐coating method was used to coat organic semiconductors onto a variety of hydrophobic surfaces to enhance the film crystallinity and optimize the electrical performances of organic transistors 12 . Due to the high insolubility of polymers based on DPP units, the organic semiconductor could be directly patterned by conventional photolithography to achieve well‐balanced hole and electron mobilities 13 .…”