Blocking Energy‐Loss Pathways for Ideal Fluorescent Organic Light‐Emitting Diodes with Thermally Activated Delayed Fluorescent Sensitizers

Abstract

Organic light-emitting diodes (OLEDs) based on thermally activated delayed fluorescence-sensitized fluorescence (TSF) offer the possibility of attaining an ultimate high efficiency with low roll-off utilizing noble-metal free, easy-to-synthesize, pure organic fluorescent emitters. However, the performances of TSF-OLEDs are still unsatisfactory. Here, TSF-OLEDs with breakthrough efficiencies even at high brightnesses by suppressing the competitive deactivation processes, including direct charge recombination on conventional fluorescent dopants (CFDs) and Dexter energy transfer from the host to the CFDs, are demonstrated. On the one hand, electronically inert terminal-substituents are introduced to protect the electronically active core of the CFDs; on the other hand, delicate device structures are designed to provide multiple energy-funneling paths. As a result, unprecedentedly high maximum external quantum efficiency/power efficiency of 24%/71.4 lm W in a green TSF-OLED are demonstrated, which remain at 22.6%/52.3 lm W even at a high luminance of 5000 cd m . The work unlocks the potential of TSF-OLEDs, paving the way toward practical applications.

All‐Fluorescence White Organic Light‐Emitting Diodes Exceeding 20% EQEs by Rational Manipulation of Singlet and Triplet Excitons

“…[ 26 ] This technique has made promising progress in monochromatic emitting dyes with relatively low‐lying states, such as green, yellow, and red fluorophores. [ 27–29 ] Thus, it seems reasonable and accessible to achieve all‐fluorescence WOLEDs by incorporating sensitized and complementary fluorescence dyes for high efficiency. Nevertheless, very few efficient all‐fluorescence WOLEDs are successfully designed on the basis of this strategy, [ 30,31 ] which may be attributed to the following reasons: i) the energy level of deep blue fluorescence emitter is difficult to be matched with the appropriate TADF sensitizer; ii) meanwhile, the relatively low‐lying triplet energy level of blue fluorescent emitters () could induce triplet exciton quenching or detrimental Dexter energy transfer with non‐radiative process, thus reduce the exciton utilization ratio for radiative transition of other color‐complementary emitters; iii) furthermore, the trap effect and accumulation of triplet excitons would accelerate triplet–triplet annihilation (TTA), triplet–polaron annihilation (TPA) degradation; [ 32,33 ] iv) unlike a monochromatic OLED, it is also important but difficult to maintain rational and stable exciton allocation under different current injections, thereby induces a steady CIE coordinates for WOLED systems composed of multiple emissive units.…”

White OLEDs with an EQE of 21% at 5000 cd m−2 and Ultra High Color Stability Based on Exciplex Host

“…Furthermore, due to the spatially separated donor (D) and acceptor (A) molecules, most exciplex hosts exhibit minor singlet–triplet differences (E ST ). Through reverse intersystem crossing, triplet excitons can convert to singlet excitons which is beneficial to facilitate the long‐range Förster energy transfer, which may further improve device efficiency . In general, exciplex host which matches the above requirements simultaneously is ideal to realize highly efficient WOLEDs at practical working brightness.…”

Design of Thermally Activated Delayed Fluorescent Assistant Dopants to Suppress the Nonradiative Component in Red Fluorescent Organic Light‐Emitting Diodes

“…Therefore, singlet exciton harvesting of the fluorescent emitter assisted by the TADF material produces high EQE FOLEDs with good color purity. It has already been demonstrated that TADF‐assisted FOLEDs could reach high EQE greater than 20 % by chemical engineering of the fluorescent emitters . However, the effect of the TADF assistant dopant on the EQE of TADF assisted FOLEDs is not understood and the requirements of TADF materials need to be established.…”

“…[1,11,12] The TADF materials are used as emittersi nT ADF OLEDs and assistant dopantsi nF OLEDs. [13][14][15][16][17] They have as mall energy gap DE ST betweens inglet and triplet excited states, which facil-itates reverse intersystem crossing( RISC) from the triplet excited state (T 1 )t ot he singlet excited state (S 1 ). They show ah igh EQE similart ot hat of phosphorescent emitters by the RISC process, which makest hem useful as emitters.…”