Origin of Contrasting Emission Spectrum of Bromide versus Iodide Layered Perovskite Semiconductors.

The origin of broadband emission is studied using temperature-dependent time-resolved photoluminescence (PL) spectra for two-dimensional (2D) layered halide perovskites (i.e., (PEA) 2 PbBr 4 = phenylethylammonium lead bromide and (PEA) 2 PbI 4 = phenylethylammonium lead iodide) semiconductors. Both perovskite systems show only a single peak exciton emission at room temperature, which becomes multipeak exciton emissions at low temperatures. For temperatures below 100 K, the (PEA) 2 PbBr 4 film gives broad PL emission, Stokes shifted by 750 meV from narrow exciton emission peaks, whereas the (PEA) 2 PbI 4 film does not show any broad emission. Kinetics of various peaks could provide useful insight to propose a consistent energy level scheme associated with a barrier (PEA) and well (PbX 6 4- ) material system's electronic states. This broad emission in (PEA) 2 PbBr 4 perovskite is observed due to coupling of triplet states in the inorganic well (PbBr 6 4- ) and organic barrier (PEA) layer, which is in contrast to a proposed model based on self-trapped exciton.