Organic thin film transistors using a liquid crystalline palladium phthalocyanine as active layer

Abstract

70 nm thick solution-processed films of a palladium phthalocyanine (PdPc_6) derivative bearing eight hexyl (–C_6H_13) chains at non-peripheral positions have been employed as active layers in the fabrication of bottom-gate bottom-contact organic thin film transistors (OTFTs) deposited on highly doped p-type Si (110) substrates with SiO_2 gate dielectric. The dependence of the transistor electrical performance upon the mesophase behavior of the PdPc_6 films has been investigated by measuring the output and transfer characteristics of the OTFT having its active layer ex situ vacuum annealed at temperatures between 500 ºC and 200 ºC. A clear correlation between the annealing temperature and the threshold voltage and carrier mobility of the transistors, and the transition temperatures extracted from the differential scanning calorimetric curves for bulk materials has been established. This direct relation has been obtained by means of a compact electrical model in which the contact effects are taken into account. The precise determination of the contact-voltage drain-current curves allows for obtaining such a relation.