Crystallite size effect on the hole mobility of uniaxially aligned copper phthalocyanine thin-film field-effect transistors

We fabricated copper phthalocyanine (CuPc) thin-film field-effect transistors (FETs) with three different orientations of CuPc with respect to channels. The substrate rubbing treatment induced uniaxial orientation in CuPc layers, yielding higher hole mobility $(\sim 0.02{\mathrm{cm}}^2∕\mathrm{V}\mathrm{s})$ than that of untreated FETs. Although the rubbing treatment bore high-aspect-ratio $(>10)$ CuPc domains oriented to rubbing direction, the mobility anisotropy of the film was only 1.4. This discrepancy was explained by analyzing grazing-incidence x-ray diffraction profiles, i.e.: (1) In-plane mean size of crystals was smaller than their appearance in atomic force microscopy, and (2) the crystallites were much shorter in the apparent long-hand direction $\left(11\mathrm{nm}\right)$ than the direction perpendicular to it $\left(44\mathrm{nm}\right)$ .