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Annealing temperature is the temperature at which a material is held after deposition or forming in order to change its structure rather than its shape. Raising it supplies the activation energy for diffusion, so grains grow, point defects are removed and a metastable phase converts to the equilibrium one; in a film the same treatment relaxes growth stress and changes the preferred orientation, and both the microstructure and the mechanical response can therefore pass through an optimum rather than improving steadily ^[1]. Where the film is grown from solution, the anneal sets composition and crystallinity together, so a hotter treatment may improve crystal quality while a volatile constituent is lost ^[2]. Grain growth continues throughout the hold and coarsens the microstructure once densification is complete ^[3]. Optical properties follow the same trade-off, since the band structure and the film density change with the treatment until decomposition sets the upper bound ^[4], and in a hybrid perovskite the precursor concentration and the temperature act together on the final optical response ^[5]. Because kinetics and thermodynamics respond differently, the most favourable microstructure usually lies in a window rather than at the highest temperature available.