Pure PbI₂ crystal particles with a flaky shape were prepared by a pH-constant double-jet precipitation process, which has the significant advantages of easy operation and scaling-up. It was found that a pH below 5.0 of the mixing solution is the appropriate range for the formation of pure PbI₂ phase, while at a pH above 5.0, the PbIOH phase would appear immediately and decrease its solubility in DMF (dimethylformamide) for the preparation of a high-quality film of perovskite solar cells.

Various instruments, including XRD, FTIR, SEM/EDS were used to characterize the precipitated particles obtained under different experimental conditions, and the effect of various parameters, including pH, concentration of the lead ions, feeding rate, and the characteristics of the surfactants on the particle was investigated systematically. Thermodynamic calculation of species distribution in the solution systems of Pb²⁺-I^-- H₂O, Pb²⁺-I^--Cit-H₂O and Pb²⁺-I^--EDTA-H₂O were carried out to identify the indispensable role of pH on the formation of highly pure lead iodide crystals. The crystallization of PbI₂ was regarded as the basis of the formation of flake-like products, which was also strongly dependent on the pH value of the solution.

It was found that at a low concentration of the PbI₂ precursor, such as with very dilute lead ions or with a very slow feeding rate, the XRD reflection peaks at 12.67°, 38.67° and 52.39° will dominate, while the peaks at 25.91°, 34.27°, and 39.51° will become dominant in the case of high concentration. The lead iodide particles were tested by mixing them in DMF, and it was found that the samples precipitated at a pH of 2 and 4 could dissolve and form a homogeneous solution easily, while the sample produced at a pH of 6 would form a turbid suspension, and could not dissolve completely to obtain a clear solution.

The results presented in this work provide detailed and significant information about the synthesis of highly pure PbI₂ which may be applied in the fabrication of perovskite solar cells.