Decoding Perovskite Crystallization Dynamics Via Absorption‐Normalized In‐Situ Photoluminescence: A Case Study on the Role of Humidity

ABSTRACT Real‐time photoluminescence (PL) spectroscopy is widely used to probe crystallization kinetics and defect evolution during perovskite film formation. However, as both the amount of emissive material and its radiative ability evolve during crystallization and both influence the raw PL intensity, their contributions must be decoupled to properly interpret the nanostructure development of perovskite thin films. Here, we develop an in‐situ spectroscopy platform that enables tracking both of PL and UV–vis absorption during spin coating and thermal annealing, allowing the PL intensity to be normalized by the absorption at the excitation wavelength. This normalization decouples intrinsic radiative efficiency from emitter amount in the wet film, providing a rigorous metric for monitoring non radiative recombination in real time. Applying this method, we find that humidity does not simply alter crystallization kinetics but actively hijacks the early‐stage pathway by stabilizing intermediate species that persist after annealing, which directly correlates with suppressed absorption‐normalized PL intensity and degraded device performance. This work establishes absorption‑normalized in‑situ PL as a powerful and potentially widely applicable diagnostic tool for monitoring perovskite thin‑film development and beyond.