Efficient and Spectra‐Stable Pure‐Red Mixed‐Halide Quasi‐2D Perovskite Light‐Emitting Diodes Enabled by Cesium Triiodide Additives

Mixed‐halide quasi‐2D pure‐red perovskite light‐emitting diodes (PeLEDs) have shown significant progress in recent years. However, the unsatisfied emission efficiency and spectral stability caused by unfavorable distribution of low‐dimensional phase, non‐radiative recombination losses, and halide phase segregation remain the primary unsolved challenges for pure‐red quasi‐2D PeLEDs. Here, it is shown that the addition of CsI 3 into both perovskite and interface layer precursors can significantly enhance efficiency and spectral stability of pure‐red quasi‐2D PeLEDs. The introduced CsI 3 can improve perovskite film quality by rearranging low‐dimensional phase, decreasing defect state density and reducing crystallite size. In addition, the addition of CsI 3 in the interface layer leads to better band alignment between hole transport layer and the perovskite layer. Consequently, efficient and stable quasi‐2D pure‐red PeLEDs with an EQE max of 20.05% at 635 nm are achieved, which exhibit negligible wavelength shift under different operating voltages and indicate excellent spectral and operational stability. This work not only addresses the critical challenges in iodine‐bromine mixed pure‐red PeLEDs, but also provides a promising strategy for their further development in optoelectronic applications.