Effects of thermal annealing on structure, morphology and optoelectronic properties of TiOPc ultrathin films

Abstract

This article reports the effects of thermal annealing on structure, morphology and optoelectronic properties of titanyl phthalocyanine (TiOPc) ultrathin films grown at room temperature. When the thermal annealing temperature (TA) is over 160 °C, a peak at 2θ =7.5° with d010 = 1.194 nm was observed, which suggests that TiOPc molecules were restructured and crystalline α-form films were formed. By the detailed x-ray diffraction analysis, the optimum thermal annealing conditions are as follows: TA is 160 °C, and the thermal annealing time is 4 h. The root-mean-square roughness and grain size are also increased due to recrystallization with increasing TA. Similarly, these absorption spectra of TiOPc ultrathin films reveal the gradual formation of the crystalline α-phase, as evidenced by the development of a red-shifted absorption band, peaking 845 nm, and the corresponding reduction of the 720 nm amorphous phase peak when TA was increased from 120 °C to 180 °C. Furthermore, device performance of TiOPc thin film transistors could be improved and mobility was enhanced by thermal annealing, which originated from amorphous TiOPc films transformed into crystalline α-TiOPc films and increasing of grain size. At TA= 160 °C, the device achieved the highest performance with field-effect mobility of 6.34 × 10-3 cm2V-1s-1.