Anti-solvent engineering to rapid purify PbI2 for efficient perovskite solar cells

The purity of PbI2 plays a critical role in determining the performance, stability, and reproducibility of perovskite solar cells (PSCs). However, the cost of high-purity PbI2 is much higher than that of low-purity, and even the expensive high-purity PbI2 suffers from batch variance issues. Although several purification methods have been developed, the straightforward and rapid purification of PbI2 remains a challenge. The solar cell efficiency achieved by previous direct purification of PbI2 is much lower than the state-of-the-art value. Therefore, people still doubt whether the low-purity PbI2 is eligible for the fabrication of efficient solar cells even after purification. Herein, we report an anti-solvent engineering strategy for the rapid and facile purification of low-purity PbI2, which allows us to efficiently eliminate both the insoluble and soluble impurities. Eventually, the formamidinium-cesium (FA0.95Cs0.05PbI3) PSCs made from directly purified PbI2 show much enhanced performance and reproducibility. A champion power conversion efficiency of 23.91% is realized, which is the highest value ever reported for PSCs made from purified low-purity PbI2. The in-depth PbI2 purification mechanism is further discussed from the perspective of solvent-coordination and solvent–solvent interactions by combining the donor number (DN) value of solvents and Hansen solution parameters. The results reveal that the weaker interaction between PbI2 and solvent, together with the stronger interaction between solvent and anti-solvent, is crucial for achieving successful purification. This study not only provides a simple and rapid method for the purification of PbI2 but also enhances our understanding of the selective crystallization of PbI2 and perovskite by elucidating the solvate and de-solvation processes of PbI2 within the perovskite precursor.