We demonstrate two efficient blue-green thermally activated delayed fluorescence (TADF) compounds comprising a dimeric phenylcarbazole and four cyano substituents on the phenyl rings. A comparison of the 2,6-dicyano-substituted derivative (26IPNDCz) with the 3,5-dicyano-substituted derivative (35IPNDCz) shows that 26IPNDCz provides a larger dihedral angle and a lower decrease in the energy difference between the first singlet and triplet excited states (Delta E-ST) and the TADF lifetime. An organic light-emitting diode based on 26IPNDCz afforded an external quantum efficiency of 10% with reduced efficiency roll-off.