Stability Investigations of Triple Halide Perovskite: A Detailed Study Using Angle Resolved XPS

Organic-inorganic halide perovskite materials have garnered significant interest in the scientific community due to their exceptional photovoltaic properties and cost-effective manufacturing processes. These materials can be deposited using various near-room temperature solvent-based techniques, including spin-casting, blade-coating, slot-die printing, and inkjet printing. Triple halide perovskites are particularly noteworthy for their tunable wide bandgap, making them suitable for tandem solar cell applications when paired with silicon bottom cells. Despite the impressive advancements in efficiency and reduced production costs, perovskite solar cells face challenges in device stability that must be addressed before they can become commercially viable. To investigate these materials further, a study was conducted using angle-resolved X-ray photoelectron spectroscopy (XPS) on spin-coated (Cs_0.22FA_0.78)Pb(I_0.85Br_0.15)₃ + 3 mol % MAPbCl₃ absorber layers, also known as Cs₂₂Br₁₅. Five triple halide perovskite films were prepared using DMF-based inks in a controlled glovebox environment and spin-cast onto plasma-cleaned glass/ITO substrates. Four samples were then annealed at 100°C for 30 minutes. After that three of them were exposed to 100°C, 150°C and 50% humidity respectively for 1 hour. The XRD plots confirm that a lot of intermediate phase peaks are observed when the samples which are treated at higher temperature and exposed to high humidity. Angle-resolved XPS was employed to examine the chemical composition of the perovskite film surfaces. This technique is particularly useful for studying photochemical and thermal decomposition in perovskite solar cells. Prior to XPS measurements, samples were cleaned using a low-energy ion/cluster beam to avoid damaging the perovskite layer. The survey spectrum of a freshly prepared glass/perovskite sample revealed characteristic peaks associated with perovskite crystals, including Pb4f, Br3d, I3d, Cs3d, and C1s. A closer examination of the 283-287 eV range showed a primary peak at ~286 eV (C-N bond) and a secondary peak at ~284 eV (C-C bond) . The XPS spectra of Pb suggests that the samples which are exposed to higher temperature and more moisture exhibit a small peak towards lower binding energy, indicating that metallic lead was formed. The ARXPS data shows that at elevated temperature, the concentration of C is less compared to Pb at the surface of the sample. That suggests formamidinium (FA) was depleted from the surface.
"We acknowledge support from TxState University through a Research Enhancement grant"