Highly efficient electroluminescence from purely organic donor–acceptor systems

Abstract

Thermally activated delayed fluorescence (TADF) emitters are third-generation electroluminescent materials that realize highly efficient organic light-emitting diodes (OLEDs) without using rare metals. Here, after briefly reviewing the principles of TADF and its use in OLEDs, we report a sky-blue TADF emitter, 9-(4-(benzo[d]thiazol-2-yl)phenyl)-N 3 ,N 3 ,N 6 ,N 6 -tetraphenyl-9H-carbazole-3,6-diamine (DAC-BTZ). DAC-BTZ is a purely organic donor-acceptor-type molecule with a small energy difference between its lowest excited singlet state and lowest triplet state of 0.18-0.22 eV according to fluorescence and phosphorescence spectra of a DAC-BTZ-doped film. In addition, the doped film exhibits a high photoluminescence quantum yield of 0.82. Time-resolved photoluminescence measurements of the doped film confirm that DAC-BTZ emits TADF. An OLED containing DAC-BTZ as an emitter exhibits a maximum external quantum efficiency (EQE) of 10.3 %, which exceeds those obtained with conventional fluorescent emitters (5-7.5 %). TADF from DAC-BTZ makes a large contribution to the high EQE of its OLED.

Electronic and optical properties of novel carbazole-based donor-acceptor compounds for applications in blue-emitting organic light-emitting diodes

“…Recent work has explored the use of intramolecular donor-acceptor (D-A) systems, which tether electron-donating and electron-accepting moieties on the same structure. 2,[7][8][9] This spatially separates the HOMO and LUMO, creating an intramolecular charge-transfer (CT) excited state, allowing a single material to stabilize either a radical cation or anion. Use of one material eliminates the complexities associated with forming multi-layer or blended systems, which is highly advantageous for production.…”

“…2 Recent studies by Adachi et al show that many carbazole compounds undergo a process known as thermally-activated delayed fluorescence (TADF). [9][10][11][12][13][14] In TADF, excited compounds in their triplet state undergo reverse intersystem crossing, mediated by the available thermal energy. 14 This process allows for non-emissive triplets to be recovered as emissive singlets through delayed fluorescence, which generally occurs on the sub-microsecond to the sub-millisecond timescale in deoxygenated solution 15,16 and in thin films.…”

Quantitative analyses of high electroluminescence efficiency of thermally activated delayed fluorescence emitters based on acridine–triazine hybrids

“…[1][2][3][4][5][6][7][8] Since the pioneering introduction of thermally activated delayed fluorescence (TADF) emitters in OLEDs by Uoyama et al, 2 TADF materials have attracted intensive research in the past few years. This is mainly due to the capability of TADF materials in harvesting triplet excitons by the thermally activated upconversion and in giving possible 100% internal quantum efficiency in electroluminescence (EL), through purely organic molecular frameworks and without the need to incorporate transition metals like in phosphorescent emitters.…”

“…This is mainly due to the capability of TADF materials in harvesting triplet excitons by the thermally activated upconversion and in giving possible 100% internal quantum efficiency in electroluminescence (EL), through purely organic molecular frameworks and without the need to incorporate transition metals like in phosphorescent emitters. [1][2][3][4][5][6][7][8] In TADF molecules, through reducing the spatial overlap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital, the energy gap (ΔE ST ) between the lowest excited singlet (S 1 ) and triplet (T 1 ) states can be effectively reduced to facilitate efficient upconversion of T 1 excitons to S 1 . 2,[5][6][7][8] On the other hand, the optical outcoupling is also a critical issue for enhancing the overall external quantum efficiency (EQE) of OLEDs for practical use.…”

“…[1][2][3][4][5][6][7][8] In TADF molecules, through reducing the spatial overlap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital, the energy gap (ΔE ST ) between the lowest excited singlet (S 1 ) and triplet (T 1 ) states can be effectively reduced to facilitate efficient upconversion of T 1 excitons to S 1 . 2,[5][6][7][8] On the other hand, the optical outcoupling is also a critical issue for enhancing the overall external quantum efficiency (EQE) of OLEDs for practical use. 9,10 In addition to many other optical approaches and structures, [11][12][13][14][15] reports in recent years have revealed the importance of the emitting dipole orientations of OLED emitters relative to the device plane.…”