| Thermally evaporated pentacene, as one of the organic molecular crystals with adequate electronic properties, has been extensively studied in various thin-film applications such as field-effect transistors and flexible displays. However, there are two major concerns with this material: it is incompatible with solution process, and is known to adopt a herringbone packing in solid state, which is not facilitating pi-pi overlapping. Bis-(triisopropylsilylethynyl) (TIPSP) pentacene was synthesized to address those two issues. The bulky side groups offer TIPS pentacene good solubility in common solvents, and at the same time help to establish a two-dimensional columnar stacking between acene planes.; In this work the solubility of TIPS pentacene in different solvents (such as THF, and toluene) was examined, and structural morphologies of TIPS pentacene films with those solvents were compared using a range of process conditions. According to diffraction results, TIPS pentacene molecules tend to lie on the solid substrate with their side groups touching down the substrate and the pi-pi stacking directions along the film long axis. A thermally induced solid-to-solid phase transition of TIPS pentacene at 124°C was identified by using differential scanning calorimetry, hot-stage microscopy and diffractions. The unit cell parameters at room temperature (a=0.755 nm, b=0.773 nm, c=1.676 nm, alpha=89.5, beta=78.7, gamma=84.0 degrees) were found to be distinctly different from the ones beyond the phase transformation (a=0.780 nm, b=0.720 nm, c=1.720 nm, alpha=90.0, beta=85.0, gamma=88.0 degrees). Nanoindentation experiments demonstrated that, because of the bulky side-group, the stiffness of TIPS pentacene films was about an order of magnitude lower than that of pentacene films at loading rates of 0.05∼0.7 mN/s. In addition, a solution deposition chamber with controllable gas flushing rate and substrate temperature was designed and built for fabricating thin film TIPS pentacene transistor active layers. Based on the structural analysis of TIPS pentacene films, the effect of thermal cracking, grain boundary density, film thickness, and crystal orientation on charge carrier mobility were examined in solution-processed thin film transistors. |
