Charge carrier localised in zero-dimensional (CH₃NH₃)₃Bi₂I₉ clusters

A metal-organic hybrid perovskite with 3-D framework of metal halide octahedra has been reported as a low-cost, solution processable absorber for a thin film solar cell with a power conversion efficiency over 20 %. Low-dimensional layered perovskites with metal halide slabs separated by the insulating organic layers are reported to show higher stability, but the efficiencies of the solar cells are limited by the anisotropy of the crystals because of the confinement of excitons. In order to explore the confinement and transport of excitons in zero-dimensional metal-organic hybrid materials, a highly-orientated film of methylammonium bismuth halide, (CH₃NH₃)₃Bi₂I₉, with a nanometer sized core cluster of Bi₂I₉^3- surrounded by insulating CH₃NH₃⁺, was deposited on a quartz substrate via solution processing. The (CH₃NH₃)₃Bi₂I₉ film shows highly anisotropic photoluminescence emission and excitation due to the large proportion of localized excitons coupled with a small number of delocalised excitons from inter-cluster energy transfer. The abrupt increase in photoluminescence quantum yield at excitation energy above twice band gap could indicate a quantum cutting due to the low dimensionality.