White Light‐Emitting Electrochemical Cells Employing Phosphor‐Sensitized Thermally Activated Delayed Fluorescence to Approach All‐Phosphorescent Device Efficiencies

Abstract

Solid-state light-emitting electrochemical cells (LECs) show promising advantages of simple device architecture, low operation voltage, and insensitivity to the electrode work functions such that they have high potential in low-cost display and lighting applications. In this work, novel white LECs based on phosphor-sensitized thermally activated delayed fluorescence (TADF) are proposed. The emissive layer of these white LECs is composed of a blue-green phosphorescent host doped with a deep-red TADF guest. Efficient singlet-to-triplet intersystem crossing (ISC) on the phosphorescent host and the subsequent Förster energy transfer from the host triplet excitons to guest singlet excitons can make use of both singlet and triplet excitons on the host. With the good spectral overlap between the host emission and the guest absorption, 0.075 wt.% guest doping is sufficient to cause substantial energy transfer efficiency (ca. 40 %). In addition, such a low guest concentration also reduces the self-quenching effect and a high photoluminescence quantum yield of up to 84 % ensures high device efficiency. The phosphor-sensitized TADF white LECs indeed show a high external quantum efficiency of 9.6 %, which is comparable with all-phosphorescent white LECs. By employing diffusive substrates to extract the light trapped in the substrate, the device efficiency can be further improved by ca. 50 %. In the meantime, the intrinsic EL spectrum and device lifetime of the white LECs recover since the microcavity effect is destroyed. This work successfully demonstrates that the phosphor-sensitized TADF white LECs are potential candidates for efficient white light-emitting devices.

Cationic Ir(III) Complexes with 4-Fluoro-4′-pyrazolyl-(1,1′-biphenyl)-2-carbonitrile as the Cyclometalating Ligand: Synthesis, Characterizations, and Application to Ultrahigh-Efficiency Light-Emitting Electrochemical Cells

“…By carefully selecting appropriate ligands, it becomes possible to achieve high luminance efficiency and create multicolor LECs. ,,,− To obtain emission at 450–650 nm, one approach consists of using either electron-withdrawing group-substituted ligands (such as F) or electron donating group-substituted ligands (such as CH 3 or OCH 3 ) showing a high-lying lowest-unoccupied molecular orbital (LUMO) while azole-containing moieties as the highest-occupied molecular orbital (HOMO). − It was recently shown that the replacement of the pyridine ring of C∧N ligand by the five-membered nitrogen-rich heterocyclic moiety, e.g., pyrazole, triazole, or tetrazole, gives in most cases a hypsochromically shifted PL emission due to the HOMO stabilization . According our previous work, tethering electron-withdrawing fluorine atoms onto phenylpyrazole and introducing electron-donating CH 3 moieties onto bipyridine (bpy) lead to an enhanced band gap ( E g ) and excellent performance of a maximal external quantum efficiency (EQE max ) = 4.6% in blue-green LECs. ,− In 2020, Lu et al successfully developed a yellow complex known as YIr, denoted as [Ir­(bppz) 2 (Bphen)]­PF 6 , using the host–guest strategy and embedding a diffusive layer, achieving a promising EQE max up to 23.7% at 565 nm, marking it as the best performance observed to date for yellow LECs . In the subsequent year, a green complex, [Ir­(CF 3 -dPhTAZ) 2 (bpy)]­PF 6 , which introduced by He et al achieved an EQ E max of 10.4% at 525 nm while effectively mitigating phosphorescence concentration-quenching using the C∧N ligand …”

“… 33 According our previous work, tethering electron-withdrawing fluorine atoms onto phenylpyrazole and introducing electron-donating CH 3 moieties onto bipyridine (bpy) lead to an enhanced band gap ( E g ) and excellent performance of a maximal external quantum efficiency (EQE max ) = 4.6% in blue-green LECs. 30 , 34 − 36 In 2020, Lu et al successfully developed a yellow complex known as YIr, denoted as [Ir(bppz) 2 (Bphen)]PF 6 , using the host–guest strategy and embedding a diffusive layer, achieving a promising EQE max up to 23.7% at 565 nm, marking it as the best performance observed to date for yellow LECs. 20 In the subsequent year, a green complex, [Ir(CF 3 -dPhTAZ) 2 (bpy)]PF 6 , which introduced by He et al achieved an EQ E max of 10.4% at 525 nm while effectively mitigating phosphorescence concentration-quenching using the C∧N ligand.…”