3TPA-DiKTa shows an emission maximum at λPL = 561 nm (FWHM = 65 nm) with FPL = 93% while the emission of 3DPA-DiKTa is red-shifted (λPL = 613 nm, FWHM = 54 nm, FPL = 60%) in the 2 wt% mCP doped films. Benefitting from both the high ΦPL and the enhanced light- outcoupling associated with a preferential horizontal orientation of its transition dipole moment, the vacuum-deposited device with 3TPA-DiKTa exhibits an EQEmax = 30.8% at an electroluminescence maximum, lEL, of 553 nm (FWHM of 62 nm). The device with 3DPA-DiKTa shows an EQEmax of 16.7% with a lEL of 613 nm (FWHM of 60 nm). This work demonstrates how molecular engineering can lead to the most efficient ketone-containing MR-TADF emitters discovered so far.