emitters exhibit good color purity with small full width at half maximum (FWHM) of electroluminescent spectra, which promises next-generation ultrahighdefinition display. Up to now, most of the high-performance MR-TADF OLEDs were fabricated by vacuum deposition, [7] which requires expensive equipment and excessive energy consumption. Solutionprocessing methods have special advantages, such as large-area and low-cost manufacturing, which is a more economical approach for the mass production of OLED displays. [24][25][26][27][28][29][30][31][32][33] However, the device performance of solution-processed MR-TADF OLEDs remains far away from that of their vacuum-deposited counterparts. Especially, the efficiency roll-off at high luminance is very large, [6,10,20,[34][35][36][37] which hinders its practical application.The MR-TADF emitters usually have a moderate singlet-triplet energy splitting (ΔE ST ), [1,11,[19][20][21][22] which results in slow reverse intersystem crossing (RISC) process. So, the conversion of triplet excitons to singlet ones on MR-TADF emitters is inefficient. And the large number of unconverted triplet excitons would accumulate at the light-emitting layer (EML) to induce triplet-triplet annihilation (TTA) and triplet-polaron annihilation (TPA). The triplet exciton annihilation becomes even more serious at practical luminance with high exciton density, which leads to large efficiency roll-off. [38,39] This problem could be solved by codoping a thermally activated delayed fluorescent (TADF) material with high RISC rate in the EML as the sensitizer for the MR-TADF emitters. [15] The triplet excitons could be converted to singlet ones rapidly on the sensitizer, and then transfer to the MR-TADF emitter through long-range Förster resonance energy transfer (FRET) for narrowband emission. [17] With this approach, a lot of high-performance vacuumdeposited MR-TADF OLEDs have been reported by developing efficient TADF sensitizers. [40][41][42][43][44][45][46][47][48] However, this approach has not successfully achieved promising device performance in solution-processed MR-TADF OLEDs, due to the lack of solutionprocessible TADF sensitizers with high RISC rate. [37] Solution-processed organic light-emitting diodes (OLEDs) based on multipleresonance thermally activated delayed fluorescence (MR-TADF) emitters exhibit high color purity for next-generation ultrahigh-definition display. However, they suffer from low efficiency and large efficiency roll-off due to slow triplet exciton upconversion of MR-TADF emitters, resulting in serious triplet exciton quenching. Here, efficient solution-processed blue MR-TADF OLEDs featured with small efficiency roll-off are developed by using a new bulky TADF sensitizer consisting of five di-tert-butylcarbazoles and one triazine with high reverse intersystem crossing rate of 2.0 × 10 7 s −1 , which can rapidly convert triplet excitons to singlet ones to avoid exciton quenching. The Dexter energy transfer from the sensitizer to the MR-TADF emitter is blocked by using tertiary b...

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