Fig. 1. Characteristics of perovskite single crystals and films.

a Experimental PLQYs of quasi-2D perovskite and 3D perovskite films as a function of carrier density; the curves indicates that serious Auger recombination takes place in quasi-2D case compared to 3D one, leading to rapid PLQY declining at a relatively lower threshold of carrier density; the black curve illustrates an ideal PLQY evolution trend for the optimal perovskite emitter which we want to achieve. Lattice structure and corresponding simulated total electron charge density for b PEA₂PbBr₄ and c p-FPEA₂PbBr₄, respectively. d, e Temperature-dependent PL spectra and f absorption spectra of PEA₂PbBr₄ and p-FPEA₂PbBr₄ single crystals. g Extracted E_b and corresponding PLQYs for PEA and p-FPEA based quasi-2D films with different <n > -values; the PLQYs of the films were obtained under low carrier densities of around 1.2 × 10¹⁵ cm⁻³.