Tracking transformative transitions: From CsPbBr3 nanocrystals to bulk perovskite films.

The control of grain size and surface properties is an important parameter in controlling the optoelectronic and photovoltaic properties of metal halide perovskites. When CsPbBr3 nanocrystal (∼10 nm in diameter) films were annealed at 100–125 °C, they grow in size to produce ∼400 nm diameter crystallites while transforming into bulk perovskite films. Characteristic changes in the optical properties were noted when such transformation occurred from nanocrystals into bulk. By tracking absorbance and emission spectra and morphological changes of CsPbBr3 films at different annealing times and temperature, we were able to establish the mechanism of particle growth. The presence of nanocrystals and larger crystals during the intermediate annealing steps and narrowing size distribution confirmed the Ostwald ripening mechanism for the crystal growth. The energy of activation of crystal growth as determined from the temperature dependent optical properties was estimated to be 75 kcal/mol.