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.