Alternative determination of thermal transitions of conjugated polymers through GIWAXS

Full text: Thermal transitions in conjugated polymers are fundamental for obtaining processing conditions of polymer solar cells. It has been observed in literature that thermal transitions such as glass transition temperature are hard to determine through conventional techniques such as DSC or DMA, therefore, alternative characterization techniques have been employed for this purpose. Since charge carriers mobility need to be improved in the orthogonal direction to the plane of deposition, it is important to evaluate how morphology changes; for this purpose, the diffraction peaks of semi-crystalline polymers can be analyzed, however, due to the fact that conventional X-ray diffraction is not able to detect any peak for thin films, Grazing incidence X-ray scattering was employed. The biggest experimental challenge to perform these analyses was the thermal expansion of thin films, which constantly modifies the critical angle in which the grazing beam must strike the sample and this makes the experiment more complex, challenging and time consuming, reason by which, the experiments were conducted using synchrotron radiation over several glass plates containing thin films of various conjugated polymers which have the benzodithiophene structure in their backbones. The study aimed to evaluate the energy needed for every different polymer chain and its chain organization in the solid state. Preliminary results showed that the complexity of polymer backbone has an important influence on thermal transitions, which can be useful for tuning proper annealing treatments in polymer solar cells. (author)