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Blue electroluminescence is highly desired for emerging light‐emitting devices for display applications and optoelectronics in general. However, saturated, efficient, and stable blue emission has been challenging to achieve, particularly in mixed‐halide perovskites, where intrinsic ion motion and halide segregation compromises spectral purity. Here, CsPbBr_3−xCl_xperovskites, polyelectrolytes, and a salt additive are leveraged to demonstrate pure blue emission from single‐layer light‐emitting electrochemical cells (LECs). The electrolytes transport the ions from salt additives, enhancing charge injection and stabilizing the inherent perovskite emissive lattice for highly pure and sustained blue emission. Substituting Cl into CsPbBr₃tunes the perovskite luminescence from green through blue. Sky blue and saturated blue devices produce International Commission on Illumination coordinates of (0.105, 0.129) and (0.136, 0.068), respectively, with the latter meeting the US National Television Committee standard for the blue primary. Likewise, maximum luminances of 2900 and 1000 cd m⁻², external quantum efficiencies (EQEs) of 4.3% and 3.9%, and luminance half‐lives of 5.7 and 4.9 h are obtained for sky blue and saturated blue devices, respectively. Polymer and LiPF₆inclusion increases photoluminescence efficiency, suppresses halide segregation, induces thin‐film smoothness and uniformity, and reduces crystallite size. Overall, these devices show superior performance among blue perovskite light‐emitting diodes (PeLEDs) and general LECs.

To realize ubiquitously used photonic integrated circuits, on-chip nanoscale sources are essential components. Subwavelength nanolasers, especially those based on a metal-clad design, already possess many desirable attributes for an on-chip source such as low thresholds, room-temperature operation and ultra-small footprints accompanied by electromagnetic isolation at pitch sizes down to ∼50 nm. Another valuable characteristic for a source would be control over its emission wavelength and intensity in real-time. Most efforts on tuning/modulation thus far report static changes based on irreversible techniques not suited for high-speed operation. In this study, we demonstratein-situdynamical tuning of the emission wavelength of a metallo-dielectric nanolaser at room temperature by applying an external DC electric field. Using an AC electric field, we show that it is also possible to modulate the output intensity of the nanolaser at high speeds. The nanolaser’s emission wavelength in the telecom band can be altered by as much as 8.35 nm with a tuning sensitivity of ∼1.01 nm/V. Additionally, the output intensity can be attenuated by up to 89%, a contrast sufficient for digital data communication purposes. Finally, we achieve an intensity modulation speed up to 400 MHz, limited only by the photodetector bandwidth used in this study, which underlines the capability of high-speed operation via this method. This is the first demonstration of a telecom band nanolaser source with dynamic spectral tuning and intensity modulation based on an external E-field to the best of our knowledge.