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

Abstract

A metal-organic hybrid perovskite (CH₃NH₃PbI₃) with three-dimensional 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 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 (CH₃NH₃)₃Bi₂I₉ with nanometre-sized core clusters of Bi₂I₉^3- surrounded by insulating CH₃NH₃⁺ was prepared via solution processing. The (CH₃NH₃)₃Bi₂I₉ film shows highly anisotropic photoluminescence emission and excitation due to the large proportion of localised excitons coupled with delocalised excitons from intercluster 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.Understanding the confinement and transport of excitons in low dimensional systems will aid the development of next generation photovoltaics. Via photophysical studies Ni et al. observe 'quantum cutting' in 0D metal-organic hybrid materials based on methylammonium bismuth halide (CH₃NH₃)3Bi₂I₉.

Fig. 1

Crystal structure of (CH₃NH₃)₃Bi₂I₉ from single-crystal diffraction The hydrogen atoms are omitted. The thick light-blue line (0.43 nm) shows the shortest distance between two Bi₂I₉ bioctohedra and the thick red line (0.85 nm) shows the distance between two furthest iodine atoms. ½ C and ½ N represent the half-occupied carbon and nitrogen sites, respectively

Fig. 2

Structure characterisation of the orientated (CH₃NH₃)₃Bi₂I₉ coating on quartz. a X-ray diffraction results from the coating and powdered samples. b Optical microscope and c scanning electron micrograph of the (CH₃NH₃)₃Bi₂I₉ coating processed by depositing and drying of the solution of BiI₃ and CH₃NH₃I in N,N-Dimethylmethanamide. The scale bars represent 25 μm

Fig. 3

Photoluminescence (PL) anisotropy of (CH₃NH₃)₃Bi₂I₉ coating. a Experimental set-up and coating orientation. The detector and incident angle are positioned at right angles and the substrate is rotated to control the angle. The sample is positioned vertically and the wavelength of incident beam is 350 nm. b–d show the respective PL spectra at 25°, 45° and 65°. The black lines at the bottom of b–d are the PL measurement on the bare fused silica substrate at the respective angles

Fig. 4

Excitation property of the charge carriers. a Ultraviolet–visible absorption derived from diffuse reflectance spectra of (CH₃NH₃)₃Bi₂I₉ and the angular dependence of photoluminescence excitation (PLE) of 650 nm emission (λ _det = 650 nm) at 25°, 45° and 65°. b PLE spectra for emission at λ _det = 650, 610 and 600 nm while keeping incident angle, θ, at 45°. The open circles show the major PLE peaks. c Time-resolved photoluminescence of the coating placed at θ = 0° under a excitation beam of 375 nm (λ _ex = 375 nm) and 450 nm (λ _ex = 450 nm). d Schematics of quantum-well structure and multiphonon relaxation. The mid-gap energy level is from the organic termination. E _hh represents the energy level of heavy hole state, while E ₁, E ₂ and E ₃ show the discrete energy levels in the conduction band

Fig. 5

Photoluminescence (PL) at difference excitation energies. a PL spectra of the powdered (CH₃NH₃)₃Bi₂I₉ under different excitation energies. The blue and red arrows show the Stokes shift of 0.13 eV. The dashed line at the bottom is the PL measured on the substrate excited at 400 nm. b Energy diagram for the band gap of orientated film and the peak energy of PL at different angles. c Integration of PL intensities at different excitation energies. d Relative PL quantum yield (PLQY) for the (CH₃NH₃)₃Bi₂I₉ powder and coating on quartz, respectively