The incorporation of Cl anion and MA/Cs cations into FAPbI 3 perovskite has been shown to dramatically improve solar cell performance. However, the microscopic properties of hybrid metal halide perovskite materials are not well understood yet, and it is still unclear how ion incorporation stabilizes the cubic FAPbI 3 perovskite. In this work, we conduct a systematic study on the effect of the CsCl/MACl additives on the microstructure, crystal structure, and defects (nanotwins and stacking faults) of FA-based perovskite solar cells (PSCs). We find that the cubic α-phase in pure FAPbI 3 is unstable with evidence of additional phases in the experimental electron diffraction analyses, namely the hexagonal δ-phase, the cubic supercell structure (with double the lattice constant of the α-phase) and a rhombohedral phase. The addition of CsCl/MACl effectively stabilizes the cubic FAPbI 3 with a 2× 2× 2 supercell expansion and the Im 3 ̅ space group. X-ray diffraction and photoluminescence studies show that the addition of CsCl/MACl results in a change in both the lattice parameter and the optical bandgap, respectively. The lattice contraction is a result of the incorporation of Cs/MA cations and Cl anion in the FAPbI 3 perovskites. Moreover, the addition of CsCl is shown to minimize the density of defects and improve the photoluminescence yield as well as the minority carrier lifetime of the perovskite films. All of these factors contribute to the improved device performance with a maximum efficiency of 21.98% measured for the 10 mol% CsCl perovskite layer.