Modelling of Organic thin film transistors incorporating liquid crystalline metal phthalocyanines

Abstract

The aim of this project has been to characterise spin-coated “as-deposited” and annealed films of liquid crystalline mono- and bis metal phthalocyanines acting as p-type semiconductors in OTFT devices. Structural variations such as long-chain alkyl substituents at peripheral and non-peripheral positions, and aza nitrogen replacement, have been considered. The main quantity for this characterisation is the field-independent mobility of the organic semiconductor material. The method by which this has been achieved involves extracting fundamental device parameters of the OTFT from experimental current-voltage (I-V data). An existing analytical model has been modified to describe charge transport of OTFT devices. In addition, source and drain contact resistances exhibiting possible Schottky behaviour, and bulk bypass resistance effects have been accounted for. A statistical modelling procedure, which simulated the experimental I-V data according to the defined model, has then been used to extract all the device parameters simultaneously, The field-independent mobility is the key characteristic that has been measured. In substituted copper phthalocyanines this mobility increases from 3.91×10-3cm2V-1s-1 to 4.89×10-3 cm2V-1s-1 when an aza-N atom is replaced by CH. Also, the mobility of an as-deposited film of copper phthalocyanine with a short-chain alkyl substituent is found to be higher by two orders of magnitude compared with that having a long-chain alkyl substituent. Similarly, the mobility is lower for an as-deposited film of zinc phthalocyanine having substituents at non-peripheral positions compared with substituents at peripheral positions. In lead phthalocyanine the mobility for an as-deposited film is found to be 7.56×10-4cm2V-1s-1. This value increases significantly to 4.19×10-2cm2V-1s-1 as the film is annealed at 150 °C, where an improved film morphology with a lower number of grain boundaries is believed to occur. In general, the mobility of a bis-gadolinium phthalocyanine film is found to be smaller compared with all the mono-metal phthalocyanines examined so far.