In-Situ Ellipsometry for the Determination of Thermal Transitions and Relaxations in Organic Photovoltaic Materials

Characterization of thermodynamic transitions and kinetic processes in optoelectronic materials is critical for understanding the optimized processing conditions and final device structural stability. Differential scanning calorimetry (DSC) is traditionally used to determine melting, crystallization, and glass transition temperatures as well as additional transitions, such as polymorphic solid-solid transitions. These transition temperatures are utilized to understand the general structure-property relations of materials and can be used to inform processing protocols in device fabrication in order to facilitate the formation of preferable microstructures for optimized electronic properties. However, traditional DSC experiments are limited to bulk-like samples and cannot characterize device relevant, controlled thin film geometries. Here we demonstrate that ellipsometry is at least as capable as traditional DSC experiments to determine relevant thermal transitions through a direct comparison using a range of optoelectronic and benchmarking materials. In addition, ellipsometry measurement protocols can uncover kinetic characteristics and possible additional transitions that are not observed in traditional DSC. Furthermore, ellipsometry observes density changes associated with free volume and molecular packing and associated hysteresis during temperature sweeps directly, an avenue of inquiry underutilized to date. We anticipate that ellipsometry protocols will allow for a more widely used, powerful complement to DSC characterization.