Emission Wavelength Tuning Via Competing Lattice Expansion and Octahedral Tilting for Efficient Red Perovskite Light‐Emitting Diodes

The band‐edge electronic structure of lead halide perovskites (ABX3) is composed of the orbitals of B and X components and can be tuned through the composition and structure of the BX6 octahedron. Although A‐site cations do not directly contribute to near‐edge states, the bandgap of 3D metal halide perovskites can be affected by A‐cations through BX6 octahedron tilting or lattice size variation. Here, as confirmed by the Rietveld refinement results of X‐ray diffraction characterization, the competition between lattice expansion and octahedral tilting is identified for the first time in emission wavelength tuning when introducing a large A‐site cation (C2H5NH3+, EA+) into 1‐naphthylmethylammonium iodide‐passivated CsPbI3 system. The former dominates spectral redshift, while the latter leads to a blueshift of emission peak, which broadens the way to tune the emission wavelength. In addition, excess cations can also passivate the perovskites, leading to a photoluminescence (PL) quantum yield as high as 61%, increased average PL lifetime of 74.7 ns, and a high radiative and non‐radiative recombination ratio of 15.7. Eventually, spectral‐stable deep‐red perovskite light‐emitting diode with a maximum external quantum efficiency of 17.5% is realized, which is one of the highest efficiencies without using any light outcoupling and anti‐solvent techniques.