Ultrathin C8-BTBT Films for 2D Organic Transistors

The organic semiconducting small molecule, C8-BTBT (2,7-Dioctyl[1]benzothieno[3,2-b][1]benzothiophene), has shown good carrier mobilities in solution processed field effect transistors. This is partly enabled by exotic solution processing strategies that control film morphology and crystallinity [1]. We have used organic beam molecular beam deposition to grow ultrathin (0.5-10 layer) films of C8-BTBT to determine whether this high performance material can be incorporated into the intriguing “2D OFET” approach to organic electronics [2]. We investigate film nucleation and growth morphology at different substrate temperatures on a silane-treated SiO2 surface and find very large domains of ~ 10’s of micrometers at high substrate temperatures. In a bottom-contact device geometry, non-uniform growth morphology with discontinuities at the contacts is observed to disrupt a very high-quality growth in the first few layers that negatively impacts device performance. The development of techniques for the deposition of ultrathin C8-BTBT film will aid in the ability to produce inexpensive thin film transistors with reliable and reasonably high carrier mobility.
[1] Y. Yuan et al., Nature Commun. 5, 3005 (2014).
[2] Smits et al., Nature 455, 956 (2008).