Quantum Confinement via Formation-Energy-Controlled Phase-Distribution Engineering in Quasi-2D CsPbI ₃ for Spectrally Stable Pure-Red Light-Emitting Diodes

Spectrally stable pure-red light-emitting diodes (LEDs) are essential for wide-gamut displays, yet halide-mixing approaches commonly suffer from spectral drift under electrical bias due to phase segregation. Here, we report a phase-distribution engineering strategy based on formation energy to achieve quasi-two-dimensional CsPbI₃ perovskites possessing spectrally stable electroluminescence (EL) properties. By introducing two organic spacer cations whose corresponding n = 1 phases exhibit distinct formation energies , phenylethylammonium (PEA⁺) and 1-naphthyl-methylammonium (NMA⁺), the phase distribution of quasi-2D CsPbI₃ is successfully converged into moderate-n phases, enabling efficient carrier transport and suppressed emission from other extreme-n-phases. It is further discovered that the strength of quantum confinement plays a critical role in modulating the excitonic absorption peak positions, and there exists a composition-dependent crystal growth mode, where PEA-rich films favor a more horizontal alignment of the [PbI₆]^4- octahedral layers, while NMA-rich films promote a more vertical alignment. Through regulating thermodynamic phase preference, crystal growth kinetics, and quantum confinement effects, the developed perovskites deliver a film photoluminescence quantum yield >60% and a pure-red PeLED with a peak EL of 645 nm, a spectral shift <1 nm, a CIE (0.693, 0.306) aligning with the Rec.2020 standards, a peak external quantum efficiency of 6.21%, and a maximum brightness of 800 cd·m^-2.