Substituted Indolo[3,2-b]Carbazoles: A New Class of Stable, High Mobility Organic Semiconductors for Thin Film Transisto
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Substituted indolo[3,2-b]carbazoles: A new class of stable, high mobility organic semiconductors for thin film transistors Yuning Li, Yiliang Wu, Beng Ong* Materials Design & Integration Laboratory Xerox Research Centre of Canada Mississauga, Ontario, Canada L5K 2L1 e-mail: [email protected] ABSTRACT Properly functionalized indolo[3,2-b]carbazoles represent a new class of stable, highmobility organic semiconductors for organic thin-film transistor applications. Both 5,11disubstituted and peripherally substituted indolo[3,2-b]carbazoles with proper substituents selforganized into highly crystalline terrace-layered structures under suitable processing conditions. Organic TFTs using channel semiconductors of this nature exhibited excellent field-effect transistor properties, with mobility to 0.14 cm2 V-1 s-1 and current on/off ratio to 107. By virtue of their relatively low HOMO levels and large band gaps, this class of semiconductors also displayed excellent environmental stability under ambient conditions, an appealing characteristic for organic TFT applications. INTRODUCTION Organic thin-film transistors (OTFTs) have been widely studied for use in large-area (e.g., flat-panel display), flexible (e.g., e-paper), and low-end (e.g., radio frequency identification tags) electronics, where high switching speeds are not essential. OTFTs are economically appealing as they can be manufactured at low cost using simple solution-based deposition/patterning technologies. Furthermore, OTFT circuits can also be fabricated on plastic substrates at plasticcompatible temperatures, thus offering the potential of building structurally compact, lightweight, flexible and mechanical robust electronic devices. For these applications, field effect transistor (FET) mobility on the order of that of amorphous silicon may be sufficient [1]. Accordingly, organic compounds such as pentacenes, tetracenes [2], thiophene oligomers and polymers [3], which have mobility similar to or better than that of amorphous silicon may be suitable semiconductor materials. However, most of high-mobility organic semiconductors suffer from their relatively poor environmental stability. They are generally prone to photooxidation in ambient atmosphere due to their narrow band gaps and, in the case of p-type semiconductors, high-lying HOMO energy levels. Poor environmental stability of semiconductors would lead to higher manufacturing cost due to the necessity to manufacture under well-controlled environments as well as use of costly encapsulation technology. Recently, several organic semiconductors such as pentacene precursors [4], oligofluorenes [5], and regioregular poly(3,3’’’-dialkyl-quaterthiophene)s (PQTs) [3e] have been reported to yield high mobility in OTFTs when fabricated under ambient conditions. These latest advances in material development may help bring the concept of low-cost OTFTs closer to commercial reality. We present here a new class of indolo[3,2-b]carbazole-based organic semiconductors (Figure 1) for OTFTs. Through subtle str
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