Self-assembly and charge carrier transport of sublimated dialkyl substituted quinacridones

Quinacridone, an industrial pigment, has recently shown a high charge carriers mobility in field-effect transistors. In search for new cheap organic semiconductors of improved vacuum processability we have synthesized three dialkyl derivatives of quinacridone, namely N,N′-dialkylquinacridones (alkyl = butyl, octyl, dodecyl), abbreviated as QA-C4, QA-C8 and QA-C12. The alkylation of quinacridone results in a significant decrease of its melting temperature which drops from 390 °C for quinacridone to 261 °C, 177 °C and 134 °C for QA-C4, QA-C8 and QA-C12, respectively, while retaining the onset of thermal decomposition above 390 °C. The elimination of the hydrogen bonding network between the carbonyl groups and amine hydrogens through alkylation not only lowers the melting temperature, but also induces supramolecular ordering in contrast to unsubstituted quinacridone. Detailed morphological and structural investigations of the vacuum deposited thin films have revealed that the length of the alkyl substituent is crucial for the molecular self-organization. Compound QA-C4 forms poorly ordered films, whereas QA-C8 and QA-C12 grow into a spherulitic dense morphology with increasing domain size at higher deposition temperatures. The more pronounced morphology is related to the lower melting point of the compounds and strong molecular diffusion during deposition. The poorly ordered films of QA-C4 do not show any field-effect response, what is consistent with previous reports. In contrast, transistors with QA-C8 or QA-C12 as active layers exhibit hole transport. Optimization of the deposition temperature, in which nucleation and crystal growth are properly balanced, resulted in OA-C8-based transistors with a hole mobility of 0.3 cm2/V, i.e. higher than in devices with unsubstituted quinacridone.