Fast Optical Reflectance Measurements during Spin Coating and Annealing of Organic–Inorganic Perovskite Precursor Solutions

The spin‐coating and subsequent low‐temperature annealing processes of organic/inorganic halide perovskites are investigated using in situ high‐speed spectral reflectance measurements. Methyl ammonium lead triiodide (CH 3 NH 3 PbI 3 , MAPI), methyl ammonium lead tribromide (CH 3 NH 3 PbBr 3 , MAPBr), formamidinium lead triiodide (HC(NH 2 ) 2 PbI 3 , FAPI), and triple cation‐based (TripleCat) perovskite Cs 0.05 (FA 0.83 MA 0.17 ) 0.95 Pb(I 0.83 Br 0.17 ) 3 are used as precursor solutions. After ≈1 s of spin coating, interference fringes develop that are used to determine the thickness of the layers. The spontaneous formation of MAPBr and TripleCat perovskite polycrystalline layers is observed during spin coating at room temperature. The phase transition from the other precursor solutions to the polycrystalline perovskite layers is monitored with diffuse reflectance measurements during the low‐temperature annealing step. These measurements permit the monitoring of the crystallization process that affects the bandgap of perovskites. Furthermore, it is possible to investigate the phase separation that occurs in perovskite mixtures during annealing by a second relative minimum in the reflectance spectra. The obtained results show that fast reflectance measurements are very useful to characterize kinetics and phase transition. Moreover, this fast experimental method provides insight into the formation of perovskite thin films and allows monitoring these processes on a timescale of some tens of milliseconds.