CsCl interface-driven structural resilience of FAPbI ₃ to humidity

Despite their power conversion efficiencies reaching 28.0%, formamidinium lead iodide (FAPbI3)-based perovskite solar cells (PSCs) suffer from moisture-induced phase degradation that initiates preferentially at the SnO2/FAPbI3 buried interface, yet the coupling between humidity exposure and thermal stress at the interface remains unclear. Here, we introduce a CsCl interlayer at the buried interface of n-i-p-structured PSCs and examine its role in reinforcing the structural resilience of FAPbI3 against moisture at elevated temperature. This interfacial role is distinguished from the improved crystallinity of the pristine FAPbI3 film that CsCl is already known to provide. The CsCl interlayer blocked the irreversible formation of Pb-O-related species and enabled partial reconversion of the moisture-driven δ-phase and PbI2 back to the α-phase during post-exposure annealing. Sequential changes in the lattice bonding environment across the pristine, humidity-exposed, and post-exposure annealed films were analyzed by complementary X-ray diffraction, grazing-incidence X-ray diffraction, Fourier-transform infrared spectroscopy, and X-ray photoelectron spectroscopy to elucidate the CsCl interlayer-induced stabilization mechanism in FAPbI3.