RESUMO
Understanding the kinetics of the crystallization process for organometal halide perovskite formation is critical in determining the crystalline, nanoscale morphology and therefore the electronic properties of the films produced during thin film formation from solution. In this work, in situ grazing incidence small-angle X-ray scattering (GISAXS) and optical microscopy measurements are used to investigate the processes of nucleation and growth of pristine mixed halide perovskite (MAPbI3-x Cl x ) crystalline films deposited by bar coating at 60 °C, with and without additives in the solution. A small amount of 1,8-diiodooctane (DIO) and hydriodic acid (HI) added to MAPbI3-x Cl x is shown to increase the numbers of nucleation centers promoting heterogeneous nucleation and accelerate and modify the size of nuclei during nucleation and growth. A generalized formation mechanism is derived from the overlapping parameters obtained from real-time GISAXS and optical microscopy, which revealed that during nucleation, perovskite precursors cluster before becoming the nuclei that function as elemental units for subsequent formation of perovskite crystals. Additive-free MAPbI3-x Cl x follows reaction-controlled growth, in contrast with when DIO and HI are present, and it is highly possible that the growth then follows a hindered diffusion-controlled mechanism. These results provide important details of the crystallization mechanisms occurring and will help to develop greater control over perovskite films produced.
RESUMO
In situ wide-angle X-ray scattering (WAXS) has been measured during the spin coating process used to make the precursor films required for the formation of thin films of perovskite. A customized hollow axis spin coater was developed to permit the scattered X-rays to be collected in transmission geometry during the deposition process. Spin coating is the technique most commonly used in laboratories to make thin perovskite films. The dynamics of spin-casting MAPbI3-x Cl x and FAPbI3-x Cl x films have been investigated and compared to investigate the differences between the dynamics of MAPbI3-x Cl x and FAPbI3-x Cl x film formation. In particular, we focus on the crystallization dynamics of the precursor film formation. When casting MAPbI3-x Cl x , we observed relatively fast 1D crystallization of the intermediate product MA2PbI3Cl. There was an absence of the desired perovskite phase formed directly; it only appeared after an annealing step that converted the MA2PbI3Cl to MAPbI3. In contrast, slower crystallization via a 3D precursor was observed for FAPbI3-x Cl x film formation compared to MAPbI3-x Cl x . Another important finding was that some FAPbI3-x Cl x perovskite was generated directly during spin-casting before annealing. These findings indicate that there are significant differences between the crystallization pathways for these two perovskite materials. These are likely to explain the differences in the lifetimes of the resulting perovskite solar cell devices produced using FA and MA cations.