From Optical Transitions Through Morphology Changes to Charge Transport: A P3HT Benchmark Study Using the Critical Point Model

Organic and polymer semiconductors are central to modern electronic and optoelectronic technologies, including organic thin-film transistors (OTFTs), solar cells, and bioelectronic devices. Controlling thin-film morphology is critical for optimizing the performance of organic electronic devices. However, the spin-coating process provides limited morphological control, and conventional characterization tools such as X-ray diffraction (XRD) and atomic force microscopy (AFM) are often insensitive to subtle structural changes that strongly influence charge transport. In this work, we demonstrate that spectroscopic ellipsometry (SE) combined with critical point (CP) analysis enables rapid, non-destructive assessment of microstructural evolution in regioregular poly(3-hexylthiophene) (rr-P3HT) thin films during thermal annealing. OTFTs based on P3HT were selected as a benchmark system due to their simple device architecture and well-defined structure–property relationships. The CP model provides physically meaningful optical parameters (transition energy, amplitude, broadening, and dimensionality) that correlate with electrical performance. Our results reveal a distinct shift in dimensionality of the primary optical transition upon annealing, indicating increased molecular ordering and reduced energetic disorder. These optically derived descriptors complement conventional structural probes and correlate with the observed improvements in charge transport. These findings offer a practical, non-destructive materials-focused approach for early-stage evaluation and process monitoring in organic electronics.