The development and scale-up of perovskite photovoltaics require a detailed understanding of thin-film formation and suitable methods for monitoring critical material properties directly during processing. Optical metrology provides non-destructive and contactless access to film properties and can be integrated into a wide range of deposition and post-deposition processes. Here, we review recent results from in-situ and in-line approaches based on spectral reflectance and photoluminescence (PL) for monitoring the formation of perovskite absorber layers during solution- and vacuum-based processing.

Spectral reflectance provides information on the optical and structural evolution of the growing film. Fabry–Pérot interference features can be analyzed to determine film thickness and growth rate. Furthermore, changes in the spectral response provide information on absorption-edge formation, roughness and reaction kinetics. PL complements these measurements by directly probing the emissive semiconductor phases and their band gaps. Changes in PL peak position and intensity can therefore reveal compositional changes and phase formation during processing.

The applicability of these techniques is demonstrated for different perovskite fabrication routes, including spin coating, annealing, thermal co-evaporation and slot-die coating. During slot-die processing followed by air-knife treatment, time-resolved reflectance reveals rapid changes in wet-film thickness and optical properties, while PL emerges delayed after air-knife application and subsequently shifts towards the characteristic emission wavelength of the formed perovskite phase.

Optical modelling further indicates that a simple single-layer description is insufficient for the transient wet-film state. A multilayer model provides additional insight into the evolution of individual layers, refractive indices and film thickness during quenching. For vacuum-deposited FaCsPb(I,Br)₃, simultaneous reflectance and PL measurements demonstrate the complementary nature of both techniques. While PL monitors the evolution of the emissive perovskite phase, reflectance can reveal additional non-emissive components. In particular, an absorption-related reflectance signature around 500 nm indicates residual PbI₂ that may not be visible in PL, whereas the PL emission near 740 nm is attributed to the converted perovskite phase. The combination of both signals therefore provides a more complete picture of composition and conversion during film formation. Beyond laboratory-scale in-situ studies, reflectance measurements can be transferred to in-line configurations for direct monitoring of perovskite thickness during evaporation. Approaches adapted to textured silicon substrates further enable thickness extraction under technologically relevant conditions, with calculated values showing agreement with SEM measurements.

These results demonstrate how the combination of reflectance and photoluminescence can connect fundamental process understanding with scalable process control and provide complementary information on thickness, optical properties, composition and phase evolution during perovskite thin-film fabrication.