Connecting Electrical and Molecular Properties of Semiconducting Polymers for Thin-Film Transistors
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Michael L. Chabinyc, Leslie H. Jimison, Jonathan Rivnay, and Alberto Salleo Abstract An overview of recent work on the connection between electrical and molecular properties of semiconducting polymers for thin-film transistors (TFTs) is presented. A description of the molecular packing and microstructure of amorphous to semicrystalline semiconducting polymers is presented. The features of basic models for electrical transport in TFTs are discussed. These studies indicate that defect states and traps are as important as ordered domains for understanding transport in semiconducting polymers. Advanced methods, such as electric force microscopy, useful for measuring the characteristics of defect states and charge traps, are briefly reviewed.
Introduction Integrated circuits based on singlecrystal silicon provide enormous capabilities for processing information, but there are many applications that do not require high-performance electronic components. In large-area electronic devices such as flat-panel displays, the switching speeds of the individual transistors are fairly low (10–100 kHz), but these elements must be fabricated over very large areas relative to integrated chips.1 The figure of merit for such devices is not speed, but utility of physical design. (See the article in this issue by So et al.) In other applications such as radio-frequency identification (RFID) tags or smart cards, the absolute requirements for data storage might be relatively small, but the cost of integrating Si-based memory can be prohibitive.2 Additionally, to decrease the cost of these
low-performance devices, there is great interest in fabricating them using processing methods developed in the printing industry for lightweight plastic substrates.3 Thus, there is value in acquiring the ability to manufacture reliable circuitry at low cost on both rigid and flexible substrates. (See the articles in this issue by Brabec et al., Someya et al., and Sirringhaus et al.) Semiconducting organic polymers represent an interesting solution for low-cost electronic devices. They readily form films from solution and can be deposited using a variety of coating methods.1 Thin-film transistors (TFTs) have been fabricated with semiconducting polymers, and circuits in prototype and commercial products are currently being developed (Figure 1).4,5 These initial applications do not require
MRS BULLETIN • VOLUME 33 • JULY 2008 • www.mrs.org/bulletin
exceptionally high-performance TFTs in terms of switching speed, but they do require stable electrical performance over their operating range. (See the article in this issue by Sirringhaus et al.) We review here some recent work toward understanding how molecular structure determines microstructure and electronic structure, thereby affecting the electrical performance of the semiconducting film. Recent improvements in the performance of polymeric TFTs have come from the synthesis of new materials and improvements in the synthesis of known materials. Comparison of the differences and similarities of these mater
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