TADF polymer enables over 20% EQE in solution-processed green fluorescent OLEDs

Solution-processed fluorescent organic light-emitting diodes (OLEDs) are believed to be favorable for low-cost, large-area, and flexible displays but still suffer from the limited external quantum efficiency (EQE) below 5%. Herein, we demonstrate the EQE breakthrough by introducing a donor-acceptor type thermally activated delayed fluorescence (TADF) polymer as the sensitizer for the typical green-emitting fluorescent dopants. Benefitting from their matched energy alignment, the unwanted trap-assisted recombination directly on fluorescent dopant is prevented to avoid the additional loss of triplet excitons. Indeed, triplet excitons are mainly formed on the polymeric TADF sensitizer via a Langevin recombination and then spin-flipped to singlet excitons due to the good upconversion capability. Followed by an efficient Forster energy transfer, both singlet and triplet excitons can be harvested by fluorescent dopants, leading to a promising solution-processed green hyperfluorescence with a record-high EQE of 21.2% (72.2 cd/A, 59.7 lm/W) and Commission Internationale de L'Eclairage coordinates of (0.32, 0.59). The results clearly highlight the great potential of solution-processed fluorescent OLEDs based on TADF polymers as the sensitizer. Solution-processed green hyperfluorescence has been demonstrated based on thermally activated delayed fluorescence polymer as the sensitizer for the traditional fluorescent dopant. In addition, a breakthrough to over 20% external quantum efficiency is achieved successfully due to the good solution processibility, suitable energy level alignment, and efficient upconversion, followed by a subsequent Forster energy transfer. image