Extended Data Fig. 1: Photoluminescence (PL) properties of the perovskite films.

a-c, PL maps of the perovskite films modified with MABr (a), PEABr (b) and t-BABr (c) as a function of the molar ratio of the organic cation and photon energy. The emission features (b) of n = 2 and n = 3 indicate the quasi-2D phases. The blue shifts of the PL emission and absorption onsets can be observed as the molar ratio of PEABr is increased. The PL peaks slightly shift with the increase of t-BABr but no quasi-2D features are appeared, even at a high concentration (x = 1.0). Logarithmic-PL spectra are used to plot maps in the linear scale.