Endowed by a thermally activated
delayed fluorescence (TADF) sensitizer
with a high constant rate of reverse intersystem crossing, the singlet
excitons could be accumulated and then delivered to emitting states
through favorable Förster resonance energy transfer, bypassing
the inefficient intersystem transition processes of emitters. However,
the conventional intermolecular sensitization strategies suffer from
inherent aggregation-induced quenching and inevitable phase segregation
of TADF sensitizers and emitters. In this context, we proposed a novel
intramolecular sensitization strategy by covalently incorporating
the TADF sensitizer into conjugated polymeric emitters. After rationally
regulating the proportions of sensitizer and emitter units in polymers,
the intramolecular sensitized conjugated TADF polymers with anticipated
photophysical properties and stable device performance were obtained.
A superior k
RISC value over 106 s–1 accompanied by a suppressed nonradiative transition
of the triplet exciton could be gained; therefore, the photoluminescence
quantum yield (PLQY) could reach nearly 90%. In accord with the superior
PLQY values enhanced by our intramolecular sensitization strategy,
the solution-processed organic light-emitting diodes (OLEDs) can achieve
a maximum external quantum efficiency (EQE) value of 17.8% while still
maintaining 16.0% at 1000 cd/m2 with extremely low efficiency
roll-off. These results convincingly manifest the significance of
an intramolecular sensitization strategy for designing high-efficiency
polymeric TADF emitters.
Conjugated polymers featuring thermally activated delayed fluorescence (TADF) attract tremendous attention in both academic and industry communities due to their easy solution processing for fabricating large-area and low-cost high-performance polymer light-emitting diodes (PLEDs). However, current nondoped solution-processed PLEDs frequently encounter significant efficiency roll-offs and unreasonably high operating voltages at high brightness, especially for red-emitting polymers. Herein, we design hyperbranched conjugated polymers (HCPs) with D−A−D type TADF characteristics for high-performance red-emitting PLEDs. Multiple intramolecular charge transfer (ICT) channels induced by quasi-equivalent donors of the TADF core strongly boost the reverse intersystem crossing (RISC) process and singlet excitons radiative transition. Coupling with the efficient energy transfer process generated by structure advantages of HCPs, the strongly electron-withdrawing oxygen atoms located on the TADF cores further accelerate hole transportation from the host chains to the TADF cores. Under a rational regulation of the TADF core ratio, the related nondoped red-emitting device performs an outstanding performance with an EQE max of 8.39% and exhibits no roll-off while the luminance is less than 100 cd/m 2 and only 3.3% decrease at 500 cd/m 2 . Simultaneously, the EQE can maintain 7.4% under 1000 cd/m 2 . Furthermore, the corresponding nondoped device exhibits a low turn-on voltage of around 2.5 V and achieves a luminance of 500 cd/m 2 at 3.5 V and even 1000 cd/m 2 at 3.9 V. To our knowledge, this is the best performance among all nondoped red PLEDs with high brightness obtained at low operating voltage.
Blue conjugated polymers‐based OLEDs with both high efficiency and low efficiency roll‐off are under big challenge. Herein, a strategy of local conjugation is proposed to construct high‐efficiency blue‐emitting conjugated polymers, in which the conjugation degree of polymeric backbones is adjusted by inserting different spacers. In this way, the energy level of triplet state and the energy transfer direction of the polymeric main‐chains can be effectively regulated. Benefiting from such fine regulation, the prepared alternative copolymers Alt‐PB36 with local conjugated main‐chains can better suppress the accumulation of long‐lived triplet excitons comparing with the complete conjugated polymers. The higher PLQY of Alt‐PB36 also verifies the effective energy transfer from the polymeric main‐chains to the TADF units. Accordingly, Alt‐PB36 based solution‐processed OLEDs achieve an EQEmax of 11.6% and a very low efficiency roll‐off of 2.8% at 100 cd m−2 and 15.2% at 500 cd m−2. This result represents the best efficiency among blue light‐emitting conjugated polymer‐based OLEDs so far under high luminance.
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