Enhanced electroluminescence based on thermally activated delayed fluorescence from a carbazole–triazine derivative

Abstract

Thermally activated delayed fluorescence (TADF) properties of a dicarbazole-triazine compound, 9-(4,6-diphenyl-1,3,5-triazin-2-yl)-9'-phenyl-3,3'-bicarbazole (CzT), and its OLED characteristics were investigated. An estimated small energy gap of about 90 meV between the singlet and triplet energy states of CzT made the up-conversion of triplet excitons back to a singlet state possible. The origin of the observed delayed fluorescence has been shown to be thermally activated delayed fluorescence. An organic light emitting diode (OLED) with CzT as an emitter showed the maximum external quantum efficiency (EQE) of 6%. For comparison, another carbazole-triazine derivative of 3-(2'-(4,6-diphenyl-1,3,5-triazin-2-yl)-[1,1'-biphenyl]-2-yl)-9-phenyl-9H-carbazole (PhCzTAZ) with a similar structure was also studied. PhCzTAZ showed a low fluorescence quantum yield with no TADF.

Theoretical Rationalization of the Singlet–Triplet Gap in OLEDs Materials: Impact of Charge-Transfer Character

“…11,26 In the case of TTA, the triplet up-conversion could achieve efficiencies up to 37.5 %, because only one photon is emitted by every pair of annihilated triplet states, while all of triplet excitons can ideally be upconverted to singlet states in the case of TADF. 27 Note that the rate of the latter mechanism becomes high only when the energy gap between the lowest triplet and singlet states, ∆E ST , is small (see also Figure 1). Recent studies show how this requirement can be met by molecules containing a sufficiently spatially separated Highest Occupied (HO) and Lowest Unoccupied (LU) Molecular Orbitals (MO).…”

Dynamic nature of excited states of donor–acceptor TADF materials for OLEDs: how theory can reveal structure–property relationships

“…As an alternative to the use of costly and/or possibly toxic phosphorescent metal complexes, new fully organic materials relying on the Thermally Activated Delayed Fluorescence (TADF) mechanism have been recently developed [8][9][10][11] to ease the conversion of the non-emissive triplet excitons to emissive singlet states by a (Reverse) InterSystem Crossing ((R)ISC) mechanism. In TADF materials, the usual prompt fluorescence occurring on the sub-nanosecond timescale is followed by a long-lived component typically on the micro-to the millisecond scale, the so-called delayed fluorescence resulting from the interconversion of triplet excitons into singlets.…”

Carbazole Dendrimers as Solution‐Processable Thermally Activated Delayed‐Fluorescence Materials

“…This result indicates that the spin-coated GnTAZ neat films harvest the electrically generated triplet excitons through TADF processes. Even though the observed values are still lower than the calculated theoretical maximum EQE values with TADF, [24,25] further structural optimization of the devices could lead to improvements. The dependence of the EQE on the current density was determined by a roll-off analysis employing the TTA model ( Figure S18).…”

“…[a] According to Ref. [24]. [b] Assuming a hole-electron recombination efficiency of 1 and a light-extraction efficiency of 0.22.…”