Controlling the solid-state microstructure of organic semiconducting materials by molecular compound formation

Abstract

Controlling the thin-film nano- and microstructure of active components in organic-based electronic devices such as organic field-effect transistors, organic light-emitting diodes and organic photovoltaic devices is still a challenging but crucial task [Sirringhaus H, Brown PJ, Friend RH, Nielsen MM, Bechgaard K, Langeveld-Voss BMW, Spiering AJH, Janssen RAJ, Meijer EW, Herwig P, de Leeuw DM. Two-dimensional charge transport in self-organized, high-mobility conjugated polymers. Nature. 1999;401:685–688; Yu G, Gao J, Hummelen JC, Wudl F, Heeger AJ. Polymer photovoltaic cells: enhanced efficiencies via a network of internal donor-acceptor heterojunctions. Science. 1995;270:1789–1791; Ray B, Alam MA. Optimum morphology and performance gains of organic solar cells. 2011 37th IEEE Photovoltaic Specialists Conference. IEEE;2011;2:003477–003481]. Over the past decade, numerous strategies have been explored for this purpose, including chemical modification of relevant materials and substrates, optimisation of existing processing procedures (through selection of solvents and processing conditions, annealing procedures, etc.), as well as the development of entirely new deposition techniques [Subramanian S, Park SK, Parkin SR, Podzorov V, Jackson TN, Anthony JE. Chromophore fluorination enhances crystallization and stability of soluble anthradithiophene semiconductors. J. Am. Chem. Soc. 2008;130:2706–2707; Reese C, Roberts M, Ling M, Bao Z. Organic thin film transistors. Mater. Today. 2004;7:20–27; Halik M, Klauk H, Zschieschang U, Maisch S, Effenberger F, Dehm C, Schu M, Brunnbauer M, Stellacci F. Low-voltage organic transistors with an amorphous molecular gate dielectric. Nature. 2004;431:963–966; Kim CS, Lee S, Gomez ED, Anthony JE, Loo YL. Solvent-dependent electrical characteristics and stability of organic thin-film transistors with drop cast bis(triisopropylsilylethynyl) pentacene. Appl. Phys. Lett. 2008;93:103302–103304; Dickey KC, Anthony JE, Loo YL. Improving organic thin-film transistor performance through solvent-vapor annealing of solution-processable triethylsilylethynyl anthradithiophene. Adv. Mater. 2006;18:1721–1726; Street RA. Thin-film transistors. Adv. Mater. 2009;21:2007–2022; de Gans BJ, Duineveld PC, Schubert US. Inkjet printing of polymers: state of the art and future developments. Adv. Mater. 2004;16:203–213; Krebs FC. Fabrication and processing of polymer solar cells: a review of printing and coating techniques. Sol. Energy Mater. Sol. Cells. 2009;93:394–412]. Additives have thereby been shown to provide extra means to significantly influence the molecular assembly and order in the solid state of the active species [Peet J, Kim JY, Coates NE, Ma WL, Moses D, Heeger AJ, Bazan GC. Efficiency enhancement in low-bandgap polymer solar cells by processing with alkane dithiols. Nat. Mater. 2007;6:497–500; Wei Q, Lin Y, Anderson ER, Briseno AL, Gido SP, Watkins JJ. Additive-driven assembly of block copolymer-nanoparticle hybrid materials for solution processable floating gate memory. ACS Nano. 2012;6:1188–1194]. Here, we will discuss the use of a class of additives that can form molecular compounds with the active materials, adding to the extensive tool box for microstructural control because crystalline solid-state structures can be realised where chromophores are spatially separated from each other by the additive molecules without compromising the overall long-range order of the system.