Peripherally Fluorinated Silicon Phthalocyanines: How Many Fluorine Groups Are Necessary for Air-Stable Electron Transport in Organic Thin-Film Transistors?
Benjamin King, Mário C. Vebber, Raluchukwu B. Ewenike, M. Dupuy, Callum French, Jaclyn L. Brusso, Benoît H. Lessard
Abstract
Silicon phthalocyanines (R 2 –SiPcs) are an emerging class of high-performance n-type semiconductors used in organic electronics, which historically have suffered from operational instability in air. We report the synthesis of R 2 –SiPcs with varying degrees of peripheral fluorination, R 2 –F x SiPcs ( x = 0, 4, or 16), and different axial phenoxy groups leading to 12 derivatives with 0 to 26 fluorine atoms in the molecular structure and going from non-operational devices in ambient conditions to fully air-stable operation in organic thin-film transistors (OTFTs). Ultraviolet photoelectron spectroscopy revealed that the peripheral fluorination of R 2 –F x SiPcs caused a decrease of up to 0.9 eV in the energy level of the lowest unoccupied molecular orbital (LUMO). This reduction in the LUMO energy level coincides with a decrease in the threshold voltage ( V T ) of the resulting OTFT. All compounds containing 16 peripheral fluorine atoms and one of four compounds containing four peripheral fluorine atoms in OTFTs yielded air-stable electron transport in ambient conditions at a humidity of 40%. Power spectral density functions (PSDF) from atomic force microscopy imaging also demonstrated that for similar morphologies across a series of compounds with the same axial group, a larger domain size and a smaller density of grain boundaries in the film led to a higher overall electron mobility (μ e ).