Spectroscopic ellipsometry is a useful characterization technique to support the development of high-quality perovskite materials and PSCs. When performing spectroscopic ellipsometry, a known incident polarization state of light interacts with the perovskite sample surface (bulk material) and the reflected (transmitted) light is analyzed to determine the change in amplitude ratio (Ψ) and phase (Δ). From this characterization approach, the fundamental optical properties can be derived including the refractive index (n), extinction coefficient (k), dielectric functions (ε1 and ε2), and absorption coefficient (α). These interrelated optical properties play a crucial role toward the optimization and resulting efficiency of engineered PSCs. However, inadequate sample preparation, modeling assumptions, and optical dispersion data analysis can lead to an inaccurate derivation of perovskite optical properties (see Section 3.1 and Figure ). As a result, to ensure proper characterization of complex index materials using spectroscopic ellipsometry, it is often necessary to employ more rigorous optical dispersion data analysis methods (discussed here) and quantify associated process-dependent material structure-property relationships (see Section 3.2).