In situ spectroscopic ellipsometry of buried organic layers under thermal treatment - part I: model discrimination and confinement-induced optical anisotropy in PEDOT:PSS/TAPC stacks.

In situ spectroscopic ellipsometry of buried organic thin films is inherently model-dependent, and parameter correlations in multilayer stacks can yield non-unique optical solutions. We investigate the thermal evolution of buried layers in ITO/PEDOT:PSS/TAPC/Ag structures using in situ spectroscopic ellipsometry complemented by ex situ Mueller-matrix ellipsometry. Although several optical models reproduce the dynamic Ψ-Δ spectra comparably, identifying a physically consistent solution requires additional constraints. The thermo-optic coefficient of TAPC provides a robust discriminator, while Mueller-matrix data reveal confinement-induced uniaxial anisotropy in PEDOT:PSS after prolonged annealing at 50°C. This behavior indicates thermally activated structural relaxation under constrained boundaries, producing refractive-index changes beyond intrinsic thermo-optic effects, whereas TAPC remains optically stable over 72 h.