Abstract
Introduction: 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. Materials and Methods: 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. Results: 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. Conclusion: 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.
Highlights
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 can be seen from Fig. (2) that the diffraction peaks of lead iodide powder prepared at pH=2 and 4 are in good agreement with the diffraction peak of standard PbI2, which indicates that the purity of the sample was quite high
On the basis of the above study, it could be concluded that the acidic pH condition determines the formation of pure PbI2 in the synthesis process that should be carefully controlled in the complete precipitation process
Summary
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. Considering that lead iodide is a important basic raw material in the fabrication of perovskite solar cells, its purity, composition and degree of crystallization will influence its solubility in DMF, which is used as the solvent, as well as the quality of the fabricated perovskite solar cell film. Its preparation method is quite important as it may directly influence the photoelectronic transferring performance of the perovskite solar cell film. According to the literature [36, 37], the chemical composition and purity of lead iodide should be carefully controlled in its synthetic processes
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