(Invited) Solution-Processable Liquid Crystalline Phthalocyanine Derivatives for Plastic Eectronic Devices

The investigation of small molecule semiconductors is playing a major role in the current rapid development of organic thin film transistors (OTFTs), a technology that ultimately may enable the low cost production of large area light-weight, low-cost printable plastic electronics. Particular issues involve compound design, the development of commercially adaptable solution processing methods for material deposition on flexible substrates, control of molecular alignment within the films for optimisation of the electrical properties, and gaining understanding of the fundamental conduction mechanisms. Semiconductivity exhibited by metallated phthalocyanine macrocycles is well documented and this, coupled with their well-established stability, renders them interesting compounds for applications in the area of organic electronics. Early p-type semiconductivity data were largely accrued for unsubstituted derivatives, compounds that have limited solubility in most solvents. These materials are formulated as thin films typically by vapour deposition techniques which are not suitable for large area deposition. The introduction of substituents onto the phthalocyanine nucleus can confer new and useful properties upon the ring system. Thus, derivatives bearing eight long alkyl chains at the non-peripheral positions typically exhibit columnar mesophase behaviour. They are also readily soluble in common organic solvents including tetrahydrofuran, hydrocarbons such as petrol and toluene, and also chlorohydrocarbons such as dichloromethane and chloroform. These materials have been proven from low angle X-ray reflectivity measurements to be ideal for deposition of ordered thin films by spin-coating. A high drift mobility up to 1.4 cm²V⁻¹s⁻¹has been achieved for holes in the crystal phase of the metal-free non-peripherally octahexyl substituted phthalocyanine. For all these reasons, this class of solution processable, low-molecular-weight liquid crystalline phthalocyanine has become a candidate for the semiconducting layer in the design of organic thin film transistors.

Bottom-gate, bottom-contact OTFTs have been  fabricated  using  spin coated films of non-peripherally octahexyl substituted phthalocyanineas an active semiconducting layer and the effect of surface passivation of the gate silicon dioxide (SiO₂) on the device characteristics has been recently reported. When the gate insulator was treated with a self-assembled monolayer of octadecyltrichlorosilane (OTS),  an increase in the saturation field effect  mobility by a factor of 20 and simultaneous   rise in the on/off current ratio by   were  achieved over  those obtained without surface passivation. These OTFTs show satisfactory stability to storage in the open laboratory environment for 30 days; lack of degradation is an important factor for applications in practical electronics as conventional organic semiconductors such as pentacene and rubrene are prone to instability arising from oxidation.  These phthalocyanine compounds are found to exhibit thermotropic columnar liquid crystalline behaviour. The annealing temperature of the active layer of these phthalocyanine derivatives is found to have produced a significant effect upon the transistor performance of the OTFT. Molecules are well aligned in the spincoated film with their columnar axis parallel to the substrate. The drain-source current is believed to be one dimensional hole transport via the overlap of p-p molecular orbitals through the accumulation layer. Although no noticeable changes in the molecular packing due to thermal annealing have been observed in the optical absorption spectroscopy, the disordered film structure is evident from AFM images, following the annealing temperature dependent growth of crystallites in different shapes, sizes and orientations.  This gives rise to the variation in the characteristic Meyer-Neldel energy. When the Meyer-Neldel energy is close to room temperature thermal energy, the structure is believed to nearly crystalline. Therefore, the OTFTs using the annealed  layer at this temperature produces optimum transistor parameters in terms of on-off ratio, threshold voltage and field-effect mobility. 

Acknowledgements

Thanks are due to Professor M J Cook and Professor A N Cammidge of the University of East Anglia for the provision of specially designed phthalocyanines for measurements. This research was financially supported by the UK Technology Strategy Board (Project No: TP/6/EPH/6/S/K2536J). The pre-patterned transistor substrates were prepared by QUDOS Technology, Rutherford Appleton Laboratory, Didcot, UK.