A review on halide perovskite film formation by sequential solution processing for solar cell applications

Perovskite solar cell performance is closely related to the quality of the perovskite absorbing layer which is highly dependent on deposition processes. Sequential process is shown to be effective in fabricating both single-junction and tandem solar cells delivering comparable efficiencies compared with devices by a single-step process. Sequence processes exhibit the benefits of controlling crystallization speed, overcoming solvent incompatibility, and greater flexibility. Here, mechanisms of film formation in two common sequential solution processes, namely chemical bath deposition and sequential spin coating, showing that film formation is highly dependent on precursors or deposition conditions, are reviewed. Herein, further review is conducted on how three main strategies improve perovskite crystallization. The first is by PbI₂ complex formation or via intramolecular exchange which is shown to result in a better perovskite conversion and a more ordered perovskite growth. The second strategy is by changing the condition of perovskite precursors, e.g., by solvent, cation, halide, and additive engineering. The last strategy is by altering precursor dispensing conditions and adding vapor or solvent annealing, thereby affecting reaction conditions. Many of these strategies are demonstrated to improve perovskite film morphology with reduced defects.