Precursor polymorph determines the organic semiconductor structure formed upon annealing

Abstract

Films of the chemical precursor ^tBoc-quinacridone obtained by the spin-coating and bar-assisted meniscus shearing methods were subjected to thermal deprotection to recover the organic semiconductor quinacridone in its crystalline form. We found that the final crystal structure of the semiconductor on the Si/SiO₂ substrate is in fact determined by the chemical precursor starting structure, which is in turn induced by the deposition method. Indeed, the samples prepared by spin coating display the precursor structure known from the literature, which transforms into the β-quinacridone phase. The shearing technique instead yields highly homogeneous films composed of a novel ^tBoc-quinacridone polymorph, which acts as a trigger for the subsequent formation of a pure, well oriented α-quinacridone phase. Although this crystalline form is the least stable of the many quinacridone polymorphs, here it turns out to be selectively induced and stabilized. Finally, the organic field effect transistor charge mobility of the α-quinacridone films was measured.