Growth Kinetics of Single Crystalline C8-BTBT Rods via Solvent Vapor Annealing

Abstract

The successful device fabrication of organic semiconducting single crystals relies on controlling their growth properties, which can only be achieved by understanding the growth kinetics of the crystals under various growth conditions. Here, we measure the growth kinetics of single crystalline 2,7-dioctyl[1] benzothieno[3,2-b][1]benzothiophene (C8-BTBT) rods on poly(methyl methacrylate) (PMMA) films at various substrate temperatures (T-sub) and chloroform gas pressures (P-chl) during solvent vapor annealing using in situ optical microscopy. Initially, the total volume of the rods quickly increases, but afterward, the volume splits into three different modes depending on the parameters: (mode 1) remaining constant at high T-sub and low P-chl, (mode 2) slowly decreasing at medium T-sub and P-chl (i.e., 28 degrees C and 220 Torr), and (mode 3) rapidly decreasing at low T-sub and high P-chl. These behaviors are explained by the competition between the adsorption and desorption of chloroform molecules on the PMMA film. With decreasing T-sub and increasing P-chl, the number of molecules adsorbed on the film increases with time, allowing the rods to be dissolved into chloroform. For the opposite conditions, the molecules are evaporated, thereby preventing the coarsening of the rods. Interestingly, during growth, each rod is encapsulated in a liquid chloroform drop, and the rods only interact when they are trapped in a liquid chloroform drop via the coarsening of multiple liquid drops. Additionally, we demonstrate the successful application of the rods grown at the mode 2, the best SVA condition, to field effect transistors (FETs), obtaining the hole mobility of 7.67 cm(2)/(V s), comparable to that of the best single crystal organic FET.