Blue perovskite light-emitting diodes (LEDs) have shown external quantum efficiencies (EQEs) of more than 10%; however, devices that emit in the true blue—those that accord with the emission wavelength required for Rec. 2100 primary blue —have so far been limited to EQEs of ~6%. We focused here on true blue emitting CsPbBr ₃ colloidal nanocrystals (c-NCs), finding in early studies that they suffer from a high charge injection barrier, a problem exacerbated in films containing multiple layers of nanocrystals. We introduce a self-assembled monolayer (SAM) active layer that improves charge injection. We identified a bifunctional capping ligand that simultaneously enables the self-assembly of CsPbBr ₃ c-NCs while passivating surface traps. We report, as a result, SAM-based LEDs exhibit a champion EQE of ~12% [CIE of (0.132, 0.069) at 4.0 V with a luminance of 11 cd/m ² ], and 10-fold–enhanced operating stability relative to the best previously reported Rec. 2100-blue perovskite LEDs.