Direct In Situ Conversion of Lead Iodide to a Highly Oriented and Crystallized Perovskite Thin Film via Sequential Deposition for 23.48% Efficient and Stable Photovoltaic Devices

In the sequential deposition method of perovskite films, the crystallinity and microstructure of PbI₂ are often sacrificed to solve the problem of an incomplete reaction between organic halide and lead halide. As a result, the crystal orientation of the perovskite film prepared by the sequential deposition method is generally worse than that of the perovskite film prepared by a one-step antisolvent method. Here, we preplaced formamidine formate (FAFa) on the buried interface to regulate the formation mechanism from PbI₂ to perovskite. As shown by the XPS measurement of the perovskite buried interface, the HCOO^– anion of FAFa first partially replaces I^– to coordinate with Pb²⁺. With the subsequent annealing process, some HCOO^– anions were released and migrated upward, which promoted the recrystallization of PbI₂, obtaining a PbI₂ film with enhanced crystallinity and orientation. Additionally, the lift-off process proves that the HCOO^– anions suppress the anion vacancy defects enriched at the buried interface and promote charge transport because the HCOO^– anions are small enough to adapt to the iodide vacancy. Grazing incidence wide-angle X-ray scattering and X-ray diffraction measurements show that the in situ conversion mechanism is responsible for the PbI₂-to-perovskite process, resulting in the highly oriented perovskite film without increasing the residual PbI₂ content in the perovskite film. As a result, our strategies enabled a champion power conversion efficiency of 23.48% with improved storage stability and photostability. This work provides a new strategy to improve the crystallinity of sequential deposition perovskites without destabilizing the device due to more PbI₂ residues.