Figure 6.

a) Energy‐level diagram of the typical excitonic and band‐to‐band transitions of 2D lead halide perovskites (FC = free carrier; FE = free exciton; GS = ground state). Colored arrows indicate absorption or PL, and the black arrow represents nonradiative relaxation. Reproduced with permission.106 Copyright 2018, American Chemical Society. b) Solution‐phase absorption (dotted lines) and PL (solid lines) spectra for n = 1 and n = 2 nanoplatelets in toluene, highlighting the changes that occur when the halide (X) changes from Cl to Br to I and when the metal changes from Pb to Sn. Reproduced with permission.52 Copyright 2016, American Chemical Society. c) PL of different 2D hybrid perovskites and the corresponding optical PL images. Scale bars are 2 mm for (i–v) and 10 mm for (vi). Reproduced with permission.[qv: 17c] Copyright 2015, AAAS. d) PL spectra of (PEA)₂PbX₄ NSs (X = Cl, Br, I) with different compositions. e) Photograph of solutions of (PEA)₂PbX₄ NSs with different composition under the irradiation of a 365 nm UV lamp. Reproduced with permission.[qv: 15d] Copyright 2017, Wiley‐VCH. f) PL spectra under excitation of 330 nm for the quasi‐2D compound with equimolar BA:MA ratio, without and with solvent evaporation, and with solvent evaporation in the presence of SPPO1. Reproduced with permission.107 Copyright 2017, Royal Society of Chemistry. g,h) PLQY and time‐resolved photoluminescence of the (PEA)₂FA₂Pb₃Br₁₀ perovskite films with and without TOPO passivation. Reproduced with permission.108 Copyright 2018, Springer Nature.