Spectrally Stable Deep‐Blue Perovskite Light‐Emitting Diodes Enabled by a Deep Eutectic Solvent

Abstract Full‐color LED display requires Commission Internationale de l'Eclairage (CIE) 1931 coordinates of (0.14, 0.08), based on the National Television Standards Committee (NTSC) blue standard. However, such deep‐blue perovskite light‐emitting diodes (PeLEDs) still lag far behind in terms of device efficiency. Moreover, spectral stability remains a big challenge. To settle these issues, a kind of deep eutectic solvent (DES) is introduced as the perovskite precursor solvent additive, which is formed from a hydrogen bond acceptor (HBA) choline chloride and a hydrogen bond donor (HBD) urea via the hydrogen bond interaction. The incorporation of DES has multiple advantages, shown as follows: 1) improves the solubility of CsCl to yield dense perovskite morphology, 2) inhibits the growth of the perovskite crystal to achieve low‐ n phases with narrow distribution, and 3) stabilizes the halide anions in the lattice to ensure stable electroluminescence (EL) spectra. As a result, the as‐prepared PeLED shows the maximum external quantum efficiency (EQE max ) of 2.71% with a CIE coordinate of (0.134, 0.055). With the increase in the DES concentration, the spectra stability is also obviously enhanced.