Controlling the Microstructure of Solution-Processable Small Molecules in Thin-Film Transistors through Substrate Chemistry

Solution-processable small molecules have tremendous potential in macroelectronics applications by providing both high charge carrier mobility and low cost processing. Fluorinated 5,11-bis(triethylsilylethynl) anthradithiophene enables high performance thin film transistors due, in part, to a self-patterning process where crystals grow from chemically tailored contacts and bridge the transistor channel. This paper outlines a detailed microstructural study that identifies the crystallization mechanisms of the self-patterning. Two crystal habits are observed: we find that crystals on chemically modified electrodes predominantly form (001) oriented platelets while untreated surfaces form a fine mixture of (001) and (111) oriented crystals. For (001) oriented platelets, the (010) fast growth face lies in the plane of the film and allows extended growth from platelets nucleated on the electrode into the transistor channel. The in-plane charge carrier mobility of the (001) platelets is high; for short channel lengths, crystal growth fronts from adjacent electrodes bridge the channel gap, resulting in the excellent device performance. On untreated surfaces between devices, the low charge carrier mobility, finely mixed state provides self-isolation for stable device operation.