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