Air-stable and high-performance organic field-effect transistors based on ordered, large-domain phthalocyanine copper thin film

• The influence of domain size and boundary on the electrical properties and the stability of CuPc thin films fabricated by the weak epitaxial growth method were studied.
• The CuPc/p-6P thin film with large and ordered domain is very stable even stored in air.
• The air stability mechanism of the CuPc/p-6p with large and ordered domain is the presence of lower density of voids at the boundaries and lower degree of misorientation.

The development of air-stable and high-performance organic field-effect transistors (FETs) is highly important for practical applications. In present research we studied how the domain size and boundary influence the electrical properties and the stability of CuPc thin films fabricated by the weak epitaxial growth (WEG) method. The electrical properties of fresh CuPc devices have a strong dependence on the domain size, which has been demonstrated based on the analysis of film morphology, electrical properties, and kelvin probe force microscopy (KPFM) measurements. The field-effect mobility of fresh CuPc devices increased with the domain sizes, and the mobility as high as of 0.18 cm2/Vs was obtained for a large crystalline domain size of about 60 μm2. Furthermore, the CuPc/p-6P FETs with large domains of ordered crystallites show excellent stability after being exposed to ambient conditions for 20 days. In contrast, it was found that the device with randomly aligned crystallites stored in atmosphere with the same temperature and humidity for 20 days exhibited large changes in the electric characteristics including positive-shifted threshold voltage, much lower Ion/Ioff and mobility. X-ray photoelectron spectroscopy (XPS) results proved that the oxygen content in ordered, large-domain CuPc thin film is much less than that in randomly aligned CuPc thin film with small-size crystallites as stored in the same conditions. It results from the diffusion of more oxygen and water into the randomly aligned CuPc thin film with small-size crystallites, which has more boundaries and higher degree of misorientation than the one with ordered, large-domain crystallites. The investigation reveals the importance of domain boundaries in the device stability, and provides a guide for rational optimization of film morphology for air-stable, high-performance organic FETs.