Stress-induced stabilization of the photoactive FAPbI₃ phase under ambient conditions without using an additive approach

Formamidinium lead iodide (FAPbI(3)) as the absorber material has garnered significant interest due to its suitable bandgap for perovskite solar cell applications. Herein, a novel fabrication process is developed to stabilize the photoactive alpha phase of FAPbI(3). This approach involves the formation of an intermediate phase and the introduction of strain within the perovskite film. The strain in the perovskite film is generated because of the interfacial lattice mismatch between the perovskite and SnO2 (electron transport layer) and the temperature difference between the substrate and the perovskite precursor solution. The microstructural stress analysis using X-ray diffraction (XRD) and photoluminescence spectroscopy (PL) analysis confirm the strain within the film. The fabricated perovskite film remained stable and without transforming into delta-FAPbI(3) even after being exposed to air for three months. The fabricated solar cells demonstrated a power conversion efficiency (PCE) of 20.46% with excellent reproducibility. Moreover, the non-encapsulated device retained normalized PCE above 85% when stored under relative humidity levels below 40% for 1200 hours. Therefore, this study provides a method of stabilizing alpha-phase FAPbI(3)-based perovskite solar cells without using additive approaches.