Abstract
Organic field-effect transistors (OFET) are presently attracting significant research and development interests as they potentially offer unique advantages over their inorganic counterparts in terms of cost-reduction, compatibility with low-temperature and printing-based manufacturing1). However, carrier transport mechanism at the interface between organic semiconductor and gate insulator is still not clear because it is difficult to clarify the relationship between complex structures and transport at the channel interface which are composed of self-assembled molecules and/or polymers via week Van der Waals force. Recently, however, it has been revealed that dynamic and static disorders at the interface play important roles in both charge-transport and localization of these systems based on the quantum-mechanical principles2),3). Here, an experimental evidence of the disorder-induced carrier localization in molecular semiconductor will be shown, in which prolonged gate-bias stress were applied at the interface between pentacene semiconductor and polymer insulator (Fig.1(a)). Pentacene is the most popular molecule applied to OFET applications and has at least four different crystallographic morphologies (polymorphs) sensitively depending on the temperature, thickness, surface energy, and other parameters. Especially, two morphologies with d(001) spacing at 14.1A and 14.5A are energetically most stable and coexist in the wide temperature range, can be reversibly transformed by thermal annealing with a small inter molecular sliding4)(inset of Fig.1(b)). We observed a coincidence of electron trapping at the interface and formation of the structural-disorder by polymorphic mixture in pentacene film by using micro-Raman spectroscopy and X-ray diffraction after contentious gate-bias stress for 15 hours (Fig.1(b)). The trapped electrons and the disorders were reversibly recovered by thermal annealing and it was accelerated by direct photo-excitation of the pentacene films, which clearly indicated that electrons were trapped in the disordered molecular matrix. Possibility of Anderson localization model to explain these trapping states will be discussed5)-7). Fig.1. Effect of bias, anneal and light irradiation on transfer characteristics (a) and micro-Raman spectroscopy (b) of pentacene FETs.
Refs.: 1) Sirringhaus & Ando, MRS Bulletin, July (2008), 2) Gershenson, RMP78, 973(2006), 3) Troisi, PRL 96, 086601 (2006), 4) Batlogg, Adv. Mater. 19, 2079(2007), 5) McKenzie, PRB66, 214528 (2002), 6) Stafström, PRB74, 235403(2006), 7) Heeger, PRL96, 246403(2006).