Sublimation point depression of small-molecule semiconductors by sonocrystallization

Small-molecule semiconductor solids such as pentacene and tris(8-hydroxyquinoline)aluminum(III) (Alq3) were dispersed in a poor solvent, like water, and insonated in a 10 mL scintillation vial with an output frequency of 20 kHz, a voltage of 1500 V, and an optimal induction time for 10 min at -13 °C. Sonocrystallization, at a low bulk solution temperature, gave pentacene powders of a high lattice energy difference value, ΔE_latt, of 2.258 J/g (i.e., 0.6285 kcal/mol) caused by the poor crystallinity of 69% and produced Alq3 powders with only 37 wt % of the stable α-form and 63 wt % of the metastable ε-form mixed with an amorphous phase. Therefore, insonated pentacene and Alq3 powders had depressed sublimation points of 210 and 180 °C, respectively. However, surface energy and impurities had nothing to do with the sublimation point depression. The sublimation point depression of target materials could reduce the heating and cooling duty of the vapor-phase deposition method for the manufacturing of organic light-emitting diodes (OLEDs), organic thin-film transistors (OTFTs), and photovoltaic (PV) cells drastically, because the total radiant-heat-transfer rate between heated surfaces is proportional to the fourth power of the absolute temperature according to the Stefan-Boltzmann law.