The Effect of Buried Interface on the Orientational Crystallization and Thermal Stability of Halide Perovskite Thin Films

The need for sustainable and renewable energy sources has led to the development of new photovoltaic technologies, with perovskite solar cells (PSCs) showing great potential as the next generation of solar cells. However, to fully realize the potential of PSCs, improving device stability is a challenge that still needs to be addressed [1]. In this work, we demonstrate how the orientational crystallization of perovskites plays a crucial role in film stability, and how the perovskite underlayer dictates the oriented growth. We investigated the crystallographic, morphological, optical, and electrical properties of Cs_0.10FA_0.90PbI₃ films grown on FTO/SnO₂ and FTO/NiO_x/MeO-2PACz – the typical underlayers in devices with n.i.p. and p.i.n. architecture, respectively. Films of the same composition grown on FTO were also prepared for comparison. In all cases, X-ray diffraction (XRD) patterns showed preferential growth along the (100) plane of the cubic perovskite structure. However, an increase in (111) plane orientation from SnO₂ to FTO to MeO-2PACz was observed, which was further confirmed by selected area electron diffraction (SAED). When subjected to thermal stress at 85 °C for 500 hours under N₂ atmosphere, the more oriented films showed less segregation of PbI₂. PbI₂ is known to be detrimental to PSCs, as it increases series resistance, decreases fill factor values, and creates recombination pathways at interfaces, which affect open-circuit voltage [2]. Exposure of the films to ambient conditions for several days resulted in complete degradation of the perovskite layer, except for the perovskite film grown on FTO/NiO_x/MeO-2PACz, indicating better stability. It has been reported that water molecules adhere more favorably to the (100) facet than the (111) facet, suggesting that (111)-oriented films should exhibit greater moisture stability [3]. Cathodoluminescence (CL) mapping along with energy-dispersive X-ray spectroscopy (EDS) measurements indicated that the less oriented films also showed CsPbI₃ segregation after thermal stress. Topographical maps obtained by atomic force microscopy (AFM) revealed that the more oriented films had flatter grains with smaller grain size distribution, which could also impact the stability of the perovskite layer. Conductive AFM indicated that Cs-rich grains formed after thermal stress exhibited significantly lower conductivity than the FA-rich phase, contributing to increased resistance in the films. These results suggest that promoting oriented crystallization in perovskite thin films is crucial for improving stability against temperature and moisture, minimizing phase transitions, and enhancing carrier mobility.