Pb contamination in aquatic environments causes severe pollution; therefore, harmless absorbents are required. In this study, we report a novel synthesis of whitlockite (WH, Ca₁₈Mg₂(HPO₄)₂(PO₄)₁₂), which is the second most abundant biomineral in human bone, and its application as a high-performing Pb²⁺ absorbent. Hydroxyapatite (HAP) and WH are prepared via a simple precipitation method. The Pb²⁺ absorption performance and mechanism of the synthesized biominerals are investigated in aqueous solutions at neutral pH. The results demonstrate that WH exhibits an excellent Pb²⁺ absorption capacity of 2339 mg g⁻¹, which is 1.68 times higher than the recorded value for HAP. Furthermore, the absorbed Pb²⁺ ions are recycled into high-purity PbI₂. This is employed as a precursor for the fabrication of perovskite solar cells (PSCs), resulting in a conversion efficiency of 19.00% comparable to that of commercial PbI₂ powder (99.99% purity). Our approach provides an efficient way to remove Pb²⁺ ions from water and reuse them in the recycling of PSCs.

The effect of the residual thermal stress of NiO films on the performance of an inverted type perovskite solar cell was studied. In this study, NiO films were grown on fluorine doped tin oxide (FTO) substrates of different surface roughness by thermally oxidizing Ni film and were tested as a hole transport layer for large-scale perovskite solar cells. Experimental and simulation results show that it is very important to suppress the appearance of the residual stress at the NiO-FTO interface during the oxidation of the Ni film for effective hole extraction. The Ni oxidation on the flat FTO film produced in-plane compressive stress in the NiO film due to the Ni film volume expansion. This led to the formation of defects including small blisters. These residual stress and defects increased leakage current through the NiO film, preventing holes from being selectively collected at the NiO-perovskite interface. However, when Ni was deposited and oxidized on the rough surface, the residual stress of the NiO film was negligible and its inherent high resistance was maintained. Stress-free NiO film is an excellent hole transport layer that stops the photogenerated electrons of the perovskite layer from moving to FTO. The improvements in the structural and electrical qualities of the NiO film by engineering the residual stress reduce the carrier recombination and increase the power conversion efficiency of the perovskite solar cells to 16.37%.