In situ spectroscopic ellipsometry of buried organic layers under thermal treatment - part II: model for...

Spectroscopic ellipsometry is inherently model dependent, and in buried organic multilayer stacks, reduced optical sensitivity and strong parameter correlations can lead to non-unique solutions. The thermal evolution of an OLED-relevant ITO/PEDOT:PSS/TAPC/CBP/Ag stack is investigated using in situ spectroscopic ellipsometry complemented by ex situ Mueller-matrix measurements. Measurements from the glass-substrate side, including backside reflection, yield small Ψ-Δ amplitudes due to interplay between limited refractive-index contrast and organic layer thicknesses, allowing multiple optical models to fit the data with comparable statistical quality. Model ambiguity is resolved by applying physically constrained modeling based on independently determined room-temperature optical constants, substrate characterization, and the temperature-dependent optical response of PEDOT:PSS and TAPC established in Part I. Within this framework, the time- and temperature-dependent optical properties of the CBP layer and the CBP/Ag interphase are extracted during prolonged annealing at 50 °C, revealing relaxation-driven refractive-index changes and interfacial reorganization.