  The triplet harvesting is a main challenge in organic light-emitting devices
(OLEDs), due to the radiative decay of triplet is spin-forbidden. Here, we
designed and synthesized two D-A type molecules, TPA-TAZ and TCP. The OLEDs
based on them exhibit deep-blue emission and the singlet formation ratios are
higher than the simple spin-statistics of 25 %. Specially, a TPA-TAZ-based OLED
achieves a maximum EQE of 6.8 %, which is the largest value of the undoped
OLEDs with CIE(y)< 0.06 (the EBU blue standard) up to date. Comprehensive
experiments eliminate the triplet-harvesting processes of thermally activated
delayed fluorescence and triplet-triplet annihilation. Instead, the
triplet-polaron interaction induced upconversion from triplet to singlet
through one-electron transfer mechanism is proposed, and proven by the
magneto-current measurement and quantum chemistry computation. Our results may
offer a new route to break through the 25 % upper limit of IQE of fluorescent
OLEDs, especially, the deep-blue fluorescent OLEDs.
