The 0D cesium lead halide perovskite Cs4PbBr6 has drawn remarkable interest due to its highly efficient robust green emission compared to its 3D CsPbBr3 counterpart. However, seizing the advantages of the superior photoluminescence properties for practical light‐emitting devices remains elusive. To date, Cs4PbBr6 has been employed only as a higher‐bandgap nonluminescent matrix to passivate or provide quantum/dielectric confinement to CsPbBr3 in light‐emitting devices and to enhance its photo‐/thermal/environmental stability. To resolve this disparity, a novel solvent engineering method to incorporate highly luminescent 0D Cs4PbBr6 nanocrystals (perovskite nanocrystals (PNCs)) into a 3D CsPbBr3 film, forming the active emissive layer in single‐layer perovskite light‐emitting electrochemical cells (PeLECs) is designed. A dramatic increase of the maximum external quantum efficiency and luminance from 2.7% and 6050 cd m−2 for a 3D‐only PeLEC to 8.3% and 11 200 cd m−2 for a 3D–0D PNC device with only 7% by weight of 0D PNCs is observed. The majority of this increase is driven by the efficient inherent emission of the 0D PNCs, while the concomitant morphology improvement also contributes to reduced leakage current, reduced hysteresis, and enhanced operational lifetime (half‐life of 129 h), making this one of the best‐performing LECs reported to date.

We utilized a modified reverse-microemulsion method to develop highly emissive and photostable zero-dimensional (0D) Cs 4 Pb(Br 1−x I x ) 6 perovskite nanocrystals (PNCs). We employed single-particle photoluminescence (PL) spectroscopy to explore blinking statistics and demonstrate single-photon emission from individual PNCs. Lowtemperature blinking and photon correlation studies revealed a transition from single-to multiphoton emission with progressively longer "delayed" PL components, reaching ∼70 ns at room temperature and representing a distinctive behavior to previously known iodide PNCs. Such thermally activated PL emission is explained by the existence of defect-related "reservoir" states, feeding back into the PNC's emissive state and providing multiple photons within a single excitation cycle. This work establishes a new member in the 0D class of perovskite materials, studies its photophysical properties, and reveals its potential for future optoelectronic applications.

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