Toward in-line monitoring of the microstructure of organic semiconductor materials for photovoltaic applications via spectroscopic ellipsometry

High-throughput (HT) and automated fabrication methodologies are transforming the development of thin film organic optoelectronic devices. Organic semiconductor materials typically exhibit morphology-dependent performance, and their integration into lab-scale automated systems and roll-to-roll workflows would benefit from non-destructive, rapid, and reliable characterization tools. Distinguishing vertical segregation and microstructural evolution during processing remains a major challenge for in-line quality control and materials screening. In this work, we evaluate the sensitivity of variable angle spectroscopic ellipsometry (VASE) as a fast and non-invasive technique to monitor vertical microstructure stratification in organic semiconductor thin films based on polymer donor/non-fullerene acceptor bilayers (BLs) using two common materials for photovoltaic applications. We deduce the optical properties of the reference materials using standard critical point and Tauc–Lorentz models and then analyze model BL samples with donor/acceptor interfaces before and after thermal annealing. The ellipsometric response suggests that the technique can distinguish with high accuracy sharp BL interfaces and vertically separated blends. The findings in this work demonstrate that VASE can serve as an effective HT diagnostic tool for tracking layer dynamics in automated fabrication environments, providing a valuable bridge between process control and device optimization.