Investigation of the Photocross-Linking Mechanism in Oxetane-Functionalized Semiconductors

Oxetane-functionalized organic semiconductors have been used for fabricating multilayer OLEDs from solution and for lithographic patterning. These materials are commonly cross-linked photochemically by cationic ring-opening polymerization (CROP) in the presence of a photoacid generator (PAG). Due to the sensitization of the reaction by the semiconductor itself, illumination leads to the photoinduced electron transfer (PET) from the PAG to the semiconductor. This in turn leads to – besides the intended cross-linking – redoxchemical doping of the layers, which may have an important impact on the device performance. Until now, the exact quantities of this unintentional side reaction were unknown. In this study, we use organic field-effect transistors to investigate and quantify the mechanism of photosensitized cross-linking of oxetane-functionalized organic semiconductors. By comparing the compound of interest, for example, a derivative of the commonly used hole conductor triphenylamine dimer (TPD), with an almost identical, noncross-linkable TPD-derivative and by using a quantitative oxidant in comparison with the PAG, we were able to quantitatively assign all possible reaction pathways. Furthermore, the experiments provide detailed information of the charge transport characteristics of doped cross-linked films. The charge-carrier mobility increases with the doping level by a factor of up to two. It turns out that the PAG induces a stoichiometric fraction of mobile charge-carriers of 2.5 × 10^–4. Finally, it is shown that thermal annealing of cross-linked films leads to controlled dedoping of the semiconductor.