Integrating the Emitter and Host Characteristics of Donor–Acceptor Systems through Edge‐Spiro Effect Toward 100% Exciton Harvesting in Blue and White Fluorescence Diodes

Abstract

EQE) and power efficiencies (PE) beyond 20% and 50 lm W −1 , respectively, they are indeed inferior to their fluorescence (FL) counterparts in all other aspects. [2] However, the drawback of FL emitters in triplet exciton-harvesting forces the employment of combined emissive layers (EMLs) containing blue FL emitters and low-energy phosphors, viz., so-called hybrid WOLEDs, which can also realize the high efficiencies, but actually sacrifice the other advantages of FL devices. [3] Recently, pure-organic molecule-based thermally activated delayed fluorescent (TADF) emitters and their diodes were emerged with the feature of 100% exciton harvesting through converting irradiative triplet excitons to radiative singlet excitons in virtue of the reverse intersystem crossing (RISC) process, [4] which provide a promising way to develop pure FL WOLEDs with the state-of-the-art efficiencies. Nevertheless, the key issue for constructing high-performance white FL diodes is still remained about TADF-involved WOLEDs, namely, how to achieve the rational and efficient exciton allocation. Figure 1 shows four types of the potential emitter combinations for EMLs in FL WOLEDs. Superior to type-I devices, the other types can theoretically achieve 100% internal quantum efficiency (IQE), but still accompanied by the respective challenges. It is known that FL emitters can utilize all of the electrogenerated excitons through Förster resonance energy transfer from high-energy TADF sensitizers, which can be further used to construct type-II WOLEDs. [5] However, for both type-II and III devices, their white emissions are based on rational exciton allocations, viz., either triplet exciton confinement on TADF sensitizers for the former or singlet exciton confinement on FL emitters for the latter through doping concentration and position management, which is believed to be crucial for device efficiencies and therefore renders the contradiction between white emission and electroluminescence (EL) efficiencies. [6] In this sense, type-IV devices, namely, TADF WOLEDs, would be more flexible and superior in realizing highly efficient WOLEDs since the singlet and triplet excitons can be utilized by both high-energy and low-energy TADF emitters, giving rise to high EL efficiencies Herein, by utilizing edge-spiro effect to modulate intramolecular charge transfer and intermolecular interactions, emitter and host characteristic are integrated in simple donor-acceptor systems named SFInxTz. As emitter, SFI34pTz realizes true-blue thermally activated delayed fluorescent (TADF) diodes with an external quantum efficiency (EQE) of 25.3% and Commission Internationale de L'Eclairage (CIE) coordinates of (0.15, 0.20). As host, it can endow the yellow devices with the maximum EQE about 18%. When SFI34pTz serves as both blue emitter and yellow host material in pure fluorescence white device, the maximum efficiencies are as high as 22.9% for EQE and 52.4 lm W −1 for power efficiency, and stable electroluminescence spectra are successfully realized with CIE coordi...

Interface Exciplex Anchoring the Color Stability of Solution‐Processed Thermally Activated Delayed Fluorescent White Organic Light‐Emitting Diodes

“…The WOLEDs with x = 3.5 nm show the best efficiency and high CRI: a CE of 21.10 cd A −1 , a PE of 20.71 lm W −1 , and an EQE of 10.02% are achieved. Notably, evident white emission can be observed even at low luminance, and the stability of EL spectra is enhanced remarkably, which is comparable to the reported WOLEDs by evaporation method . The device exhibits a CIE coordination of (0.32, 0.34) at 500 cd m −2 and affords a CRI of 82, and the CIE coordination of the device retains to (0.32, 0.33) with a CRI of 85 at 1000 cd m −2 .…”

Theoretical investigation of the effects of various substituents on the large energy gap between triplet excited-states of anthracene

“…[1][2][3][4][5][6][7][8] In this process, the use of triplet excitons is the most effective way to improve the luminous efficiency of organic electroluminescent materials. [9][10][11][12][13][14] Recently, the laboratory of Professor Ma found a new way to increase the utilization rate of triplet excitons, with this new way involving reverse intersystem crossing (RISC) through a specic exciton channel to the singlet state. However, to ensure that this channel is clearly open, there should be a large energy gap in the triplet state (DE T 1 -T 2 ), which blocks the relaxation of triplet excitons.…”

White light emitting diode based on purely organic fluorescent to modern thermally activated delayed fluorescence (TADF) and perovskite materials

“…Wang and co-workers established an edge-spiro strategy to develop an efficient blue TADF host, SFI34mTz for all-TADF WOLEDs (type IV), by considering the issue of interaction-induced quenching between adjacent TADF molecules and fluorescence molecules, and manifested previously explored F-WOLEDs (Type 1, II, and III) (shown in Fig. 20) [51]. A series of spiro[fluorene-indenocarbazole] (SFI)-based SFInxTz (n = 23 and 34; x = m and p), have been synthesized, SFI23mTz (5′-(3-(4,6-diphenyl-1,3 5-triazin-2-yl)phenyl)-5′ H -spiro[fluorene-9,7′-indeno[2,1-b] carbazole]), SFI23pTz (5′-(4-(4,6-diphenyl-1,3,5-triazin-2-yl) phenyl)-5′ H -spiro[fluorene-9,7′-indeno[2,1-b]carbazole]), SFI34mTz (5′-(3-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-5′ H spiro[fluorene-9,12′-indeno[1,2-c]carbazole]), and SFI34pTz (5′-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-5′ H -spiro[fluorene-9,12′-indeno[1,2-c]carbazole]), these blue TADF dyes are ornamented with different spiro-fluorene groups to achieve a low ∆E ST and high PLQY, Φ PL due to a strong steric hindrance and good electronic distribution within the molecule.…”

“…b DTPATXO a yellow TADF structure, c luminance and J–V and EL spectra (inset) of the devices, d luminance efficiency curves of the devices(Reproduced with permission from Ref. [51] Copyright 2018, ©WILEY–VCH Verlag GmbH & Co. KGaA, Weinheim)…”

“…(Reproduced with permission from Ref. [51] Copyright 2018, ©WILEY–VCH Verlag GmbH & Co. KGaA, Weinheim)…”