Prof Henry Snaith FRS

Abstract:

Mixed-halide perovskites offer bandgap tunability essential for multijunction solar cells; however, a detrimental halide segregation under light is often observed. Here we combine simultaneous in situ photoluminescence and X-ray diffraction measurements to demonstrate clear differences in compositional and optoelectronic changes associated with halide segregation in MAPb(Br_0.5I_0.5)₃ and FA_0.83Cs_0.17Pb(Br_0.4I_0.6)₃ films. We report evidence for low-barrier ionic pathways in MAPb(Br_0.5I_0.5)₃, which allow for the rearrangement of halide ions in localized volumes of perovskite without significant compositional changes to the bulk material. In contrast, FA_0.83Cs_0.17Pb(Br_0.4I_0.6)₃ lacks such low-barrier ionic pathways and is, consequently, more stable against halide segregation. However, under prolonged illumination, it exhibits a considerable ionic rearrangement throughout the bulk material, which may be triggered by an initial demixing of A-site cations, altering the composition of the bulk perovskite and reducing its stability against halide segregation. Our work elucidates links between composition, ionic pathways, and halide segregation, and it facilitates the future engineering of phase-stable mixed-halide perovskites.

Abstract:

All‐inorganic perovskite materials are attractive alternatives to organic‐inorganic perovskites because of their potential for higher thermal stability. While CsPbI3 is compositionally stable under elevated temperatures, the cubic perovskite α‐phase is thermodynamically stable only at >330°C and the low‐temperature perovskite γ−phase is metastable and highly susceptible to non‐perovskite δ‐phase conversion in moisture. Many methods have been reported which show that incorporation of acid (aqueous HI) or “HPbI3” – recently shown to be dimethylammonium lead iodide (DMAPbI3) – lower the annealing temperature required to produce the black, perovskite phase of CsPbI3. The optical and crystallographic data presented here show that DMA can successfully incorporate as an A‐site cation to replace Cs in the CsPbI3 perovskite material. This describes the stabilization and lower phase transition temperature reported in the literature when HI or HPbI3 are used as precursors for CsPbI3. The Cs‐DMA alloy only forms a pure‐phase material up to ∽25% DMA; at higher concentrations the CsPbI3 and DMAPbI3 begin to phase segregate. These alloyed materials are more stable to moisture than neat CsPbI3, but do not represent a fully inorganic perovskite material.