Introducing MR‐TADF Emitters into Light‐Emitting Electrochemical Cells for Narrowband and Efficient Emission

Abstract

Organic semiconductors that emit by the process of multi‐resonance thermally activated delayed fluorescence (MR‐TADF) can deliver narrowband and efficient electroluminescence while being processable from solvents and metal‐free. This renders them attractive for use as the emitter in sustainable light‐emitting electrochemical cells (LECs), but so far reports of narrowband and efficient MR‐TADF emission from LEC devices are absent. Here, this issue is addressed through careful and systematic material selection and device development. Specifically, the authors show that the detrimental aggregation tendency of an archetypal rigid and planar carbazole‐based MR‐TADF emitter can be inhibited by its dispersion into a compatible carbazole‐based blend host and an ionic‐liquid electrolyte, and it is further demonstrated that the tuning of this active material results in a desired balanced p‐ and n‐type electrochemical doping, a high solid‐state photoluminescence quantum yield of 91%, and singlet and triplet trapping on the MR‐TADF guest emitter. The introduction of this designed metal‐free active MR‐TADF material into a LEC, employing air‐stabile electrodes, results in bright blue electroluminescence of 500 cd m−2, which is delivered at a high external quantum efficiency of 3.8% and shows a narrow emission profile with a full‐width‐at‐half‐maximum of 31 nm.

Highly efficient organic light-emitting diodes and light-emitting electrochemical cells employing multi-resonant thermally activated delayed fluorescent emitters with bulky donor or acceptor peripheral groups

“…The significantly improved operational stability of the TPA-tBu-DiKTa LECs can be attributed to the shorter value of τ d (Table 1), which will mitigate both exciton-exciton and exciton-polaron quenching and related deleterious side reactions. In this context, we mention that the measured operational lifetime of the optimized TPA-tBu-DiKTa LEC represents an improvement by a factor of 5 compared to the state-of-the-art [33] in the MR-TADF LEC field.…”

“…[53,54] This opportunity for material-and energy-efficient fabrication is desirable for cost-sensitive and sustainable electroluminescent devices, but it should be noted that the LEC performance is strongly dependent on the merits and combined function of the luminescent semiconductor and the mobile ions. For instance, it has recently been demonstrated that MR-TADF compounds can function as the luminescent organic semiconductor in LEC devices, [4,22,32,33] but also that the propensity for ACQ can be highly detrimental to both the LEC-critical ion redistribution process and the emission efficiency. [45] It is thus interesting and timely to investigate and compare the performance of LECs employing TPA-tBu-DiKTa and TPPO-tBu-DiKTa.…”

“…The high concentration of excitons formed in host materials at high voltage is more likely to undergo bimolecular quenching processes such as triplet-triplet or triplet-polaron annihilation, which results in severe efficiency roll-off at high current density and poor device stability. [31] In addition, there have been only a few reports of MR-TADF LECs [4,22,32,33] (Figure S25 and Table S11) and their efficiencies are much worse than those of D-A TADF LECs. [5] Thus, there remains outstanding materials and device efforts to demonstrate the potential of MR-TADF LECs to deliver color-saturated high efficiency devices.…”