Electron transport properties in fluorinated copper-phthalocyanine films: Importance of vibrational reorganization energy and molecular microstructure

Electron transport (ET) properties of a series of fluorinated copper-phthalocyanine (F₁₆CuPc) thin films, which were deposited at different substrate temperatures (T_sub) ranging from 30 to 150 °C, have been investigated by quantum mechanical calculations of the reorganization energy (λ_reorg), X-ray diffraction (XRD), atomic force microscopy (AFM), and microRaman spectroscopy. Density functional theory calculations were used to predict the vibrational frequencies, normal mode displacement vectors, and electron-vibrational λ_reorg for the F₁₆CuPc molecule. The electron mobilities (μ_e) of F₁₆CuPc thin films are strongly dependent on the T_sub, and the value of μ_e increases with increasing T_sub from 30 to 120 °C, at which point it reaches its maximum value. The importance of electron-vibrational coupling and molecular microstructures for ET properties in F₁₆CuPc thin films are discussed on the basis of theoretical vibrational λ_reorg calculations and experimental observations of resonance Raman spectra. We observed a good correlation between μ_e and the full-width-at-half-maximum of the vibrational bands, which greatly contributed to λ_reorg and/or which reflects the molecular microstructural quality of the active channel. In contrast, the crystal size analysis by XRD and surface grain morphology by AFM did not reveal a clear correlation with the ET behaviours for these different F ₁₆CuPc thin films. Therefore, we suggest that for organic films with weak intermolecular interactions, such as F₁₆CuPc, optimized microscopic molecular-scale parameters are highly important for efficient long-range charge transport in the macroscopic devices.