Broadband and massive Stokes shift luminescence in fully inorganic 2D-layered perovskite CsPb2Cl5: single crystal growth and self-trapped exciton emission

Fully inorganic perovskite materials of the Cs-Pb-Br system have been developing rapidly because of their overwhelming stabilities and splendid optoelectronic performance. However, there are few investigations on the parallel system of Cs-Pb-Cl, let alone the two-dimensional (2D) CsPb2Cl5. To fill in the gaps about CsPb2Cl5, we successfully grew large-scale CsPb2Cl5 single crystals (SCs) and partially substituted by Br. The quintessential 2D crystal structure determines the morphology character of slices, with a tetragonal system (I4/mcm). The ledge-connected [Cs-Cl](10) polyhedrons and face-connected [Pb-Cl](8) polyhedrons endow them with asymmetry and further bring crystal lattice distortions, which is beneficial for self-trapped exciton (STE) emission. Additionally, the band gap of the transparent CsPb2Cl5 SC is confirmed to be similar to 4.01 eV. The photoluminescence (PL) of CsPb2Cl5 SC is dependent on the excitation energies. Exciton emission centered at 2.32 eV is predominant under higher energy excitation (>3.85 eV), whereas STE emission centered at 1.8 eV with a broad band (FWHM > 120 nm) is the unique emission when the excitation energy is lower than 3.85 eV. In addition, appropriate replacement of Cl with Br can bring larger lattice distortion and facilitate STE emission.