Quasi-2D perovskite semiconductors can be created by introducing organic interlayer cations into 3D perovskites and possess large binding energy, superior stability, high luminance efficiency, and tunable bandgap, holding promising applications in blue-light emitting devices. Compared with mixed halide perovskites, quasi-2D bromide perovskites emit blue light with high color stability. However, multiple-phases usually co-exist in quasi-2D bromide perovskites, resulting in low color purity. In this work, three ammonium bromides, namely, 3-(trifluoromethyl)phenyltrimethylammonium bromide (F-PTABr), benzyltributylammonium bromide (BTBABr), and phenethylammonium bromide (PEABr), are employed to fabricate quasi-2D perovskite films with narrow phase distribution. PEA serves as the scaffold to stabilize the quasi-2D PEA2Csn-1PbnBr3n+1 perovskite structure and provides enough space to allow the F-PTA cation entering the interlayer, which further triggers the entrance of a larger BTBA cation into the interlayer. The large F-PTA and BTBA cations reduce the crystal size of the perovskite and narrow the phase distribution to n = 2. As a result, the photoluminescence spectrum of the PEA0.9F-PTA0.05BTBA0.05-based perovskite film becomes unimodal and blue-shifts from 498 to 484 nm compared with the film using the PEA1.0 cation. A sky-blue light-emitting diode with an external quantum efficiency of 0.6% is achieved using the PEA0.9F-PTA0.05BTBA0.05-based perovskite as the emitter. We, therefore, demonstrate a strategy to prepare phase narrow quasi-2D perovskites with improved color purity by introducing ternary organic cations into the quasi-2D perovskites and envisage that promising device performance can be achieved with a further dedicated structure design of the ammonium cations.

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