Selective stabilization and photophysical properties of metastable perovskite polymorphs of CsPbI₃ in Thin Films

All-inorganic cesium lead iodide (CsPbI₃) perovskite has improved thermal stability over the organic-inorganic hybrid perovskites and a suitable bandgap for optoelectronic and photovoltaic applications, but it is thermodynamically unstable at room temperature and has multiple structural polymorphs. Here, we show that the use of long-chain ammonium additives during thin film deposition as surface capping ligands results in the stabilization of metastable bulk CsPbI₃ perovskite phases without alloying mixed cations or anions into the perovskite lattice. Moreover, two different metastable CsPbI₃ perovskite polymorphs in the cubic (α-CsPbI₃) and the much less common orthorhombic (β-CsPbI₃) structures can be directly synthesized in a one-step spin coating film deposition by using oleylammonium or phenylethylammonium additives, respectively, and both phases are stable at room temperature for months. Time-resolved photoluminescence and photoluminescence quenching experiments show that the photoexcited species in the stabilized orthorhombic CsPbI₃ thin film are mainly free carriers under solar illumination with a carrier lifetime of -50 ns and carrier diffusion length on the order of 100 nm, which implies efficient carrier transport within the film despite the presence of surface ligands. Our results provide a new chemical strategy to synthesize metastable all-inorganic CsPbI₃ perovskites, which, together with the good photophysical properties, will open them up for applications in photovoltaic and other optoelectronic devices.