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.