Deep blue emission is crucial for achieving high-quality display and lighting, while high-efficiency is required to obtain energy-saving devices. A wet- and dry-process feasible efficient deep-blue emitter is highly desired to realize, respectively, low cost roll-to-roll fabrication of large area and high performance devices with precise deposition of organic layers. We demonstrate here high-efficiency and ultra-deep blue organic light-emitting diodes using a cyanofluorene-acetylene conjugate based emitter, 7,7′ ((9,9-dibutyl-9H-fluorene-2,7-diyl)bis(ethyne-2,1-diyl))bis(9,9-dipropyl-9H-fluorene-2 carbonitrile) (C3FLA-2). By spin coating, C3FLA-2 shows maximum external quantum efficiency (EQE) of 6.1%, and at 100 cd m−2 for example, its color coordinates are (0.156, 0.055) with an EQE of 5.8%, the highest among all reported wet-processed deep-blue devices of fluorescent, phosphorescent, and thermally activated delayed fluorescent types. By vapor deposition, the device shows maximum EQE of 8.0%, and at 100 cd m−2 color coordinates of (0.156, 0.048) with EQE of 6.5%, the highest among all the dry-processed counterparts. The record high efficiency may be attributed to the emitter having a high quantum yield of 86%, low doping concentration preventing concentration quenching, and a suitable host facilitating an effective host-to-guest energy transfer.