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 CsPbBr 3 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 CsPbBr 3 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. M etal halide perovskites have emerged as a promising material to be used in next-generation light-harvesting and-emitting devices. 1 Their attractive qualities, such as variable device architectures, 2−4 tunable band gaps, 5−8 and long charge carrier diffusion lengths, 9−12 have made this material an epicenter of energy research. Of particular interest is their ability to deliver high-efficiency solar cell devices and light-emitting diodes. 13−20 In our previous study, we employed a layer-by-layer deposition method using CsPbX 3 nanocrystals (NCs) to create bulk perovskite films through an annealing process. 21 This approach of annealing nanocrystals at high temperature is similar to the one employed to design other semiconductor films from nanocrystals. 22−24 This technique allows for facile, controllable film formation in which NCs sinter to become bulk crystallites, which has enabled the creation of solar cells with efficiencies of over 5%. 25 Many research laboratories have pointed out the importance of grain size and grain boundaries in dictating optical and photovoltaic properties of methylammonium lead halide films. 26−29 A thorough understanding of the growth mechanism of nanocrystals is essential to engineer metal halide perovskite films with fewer defects. The CsPbBr 3 system offers the convenience to track the changes in the optical properties as it can be prepared as nanocrystals (diameter of 5−10 nm) and grown into bulk films. By reducing the annealing temperature from the original study, we were able to control the growth process and probe the optical and physical changes associated with the transformation. Here, we discuss how the growth process was elucidated from absorption and emission measurements coupled with SEM and discuss the identified growth mechanism. CsPbBr 3 NCs were synthesized using a previously described method. 5 A high concentration solution of CsPbBr 3 NCs (∼53.6 μM) in hexane was spun cast onto microscope (2.5 cm × 2.5 cm) slides. Each glass slide with spun cast film was cut into thirds and then annealed at a constant temperature. The slides were periodically removed (each slide at different time interval) and were analyzed with optical and microscopy measurements (all measurements were carried out at room temperature (∼25°C) after the samples were cooled to room temperature). These sets of experiments allowed us to monitor the morphological changes that occur when annealed at a constant temperature. The SEM micrographs in Figure 1 show the morphological changes observed during the annealing temperature of 125 °C at predetermined time intervals between 0 and 60 min. With increasing time, we see the growth of crystal size and the disappearance of NCs. For example, the SEM image presented in Figure 1A shows a uniform layer of small (∼10 nm diameter) nanocrystals of pristine spun cast film (t = 0 min). Higher magnification of this samples could not be obtained