High Fluorescence Rate of Thermally Activated Delayed Fluorescence Emitters for Efficient and Stable Blue OLEDs

Abstract

A lack of an efficient and stable blue device is a critical factor restricting the development of organic light-emitting diode (OLED) technology that is currently expected to be overcome by employing thermally activated delayed fluorescence (TADF). Here, we investigate the TADF and electroluminescence (EL) performance of six carbazole/triphenyltriazine derivatives in different hosts. A good linearity between lg(LT50/k F 2 ) and the EL emission wavelength is found, where LT50 is the half-life of the devices and k F is the fluorescence rate of the emitters, suggesting the dominance of the singlet exciton energy and lifetime in device stability. An indolylcarbazole/triphenyltriazine derivative (ICz-TRZ) with the capability to suppress solid-state solvation exhibits blue-shifted emission and an increased k F (1.5 × 10 8 s −1 ) in comparison to the control emitters in doped films. ICz-TRZ-based devices achieve a maximum external quantum efficiency (EQE) of 18% and an EQE of 5.5% at a very high luminance of 7 × 10 4 cd/m 2 . Ignoring the poor electrochemical stability of ICz-TRZ, the device offers an LT50 approaching 100 h under an initial luminance of 1000 cd/m 2 and CIE coordinates of (0.14, 0.19). The findings in this work suggest that computer-aided design of high k F TADF emitters can be an approach to realize efficient and stable blue OLEDs.

Degradation Mechanisms in Blue Organic Light-Emitting Diodes

“…Recently, using an indolocarbazole/triphenyltriazine derivative (ICz-TRZ) as a luminescent material, we fabricated a stable blue TADF device successfully, which has the color coordinate CIE of 0.14, 0.19 and an LT 50 of up to 95 h with an L 0 of 1000 cd/m 2 . 23 Note that bluish-green TADF devices could be much more stable than the blue TADF OLEDs as mentioned above. 44,45 Some preliminary studies based on controlled experiments indicated that blue TADF OLEDs could be much more stable than blue PHOLEDs.…”

“…A small amount of the narrow bandgap coupling products could quench the emission from new molecules effectively. 62,85 As found in some TADF systems mentioned above, 15,[20][21][22][23] efficient and stable blue TADF OLEDs could be achieved by employing a TADF emitter with relatively large ΔE ST and long TADF lifetime. A possible explanation is that the S 1 state of most TADF molecules is a large-polarity CT state, that is, a diradical state.…”

“…Very recently, with the assistance of quantum computational chemistry, Su et al 23 designed and synthesized a series of blue TADF molecules with a high fluorescence rate based on BCz-TRZ (Figure 8c, blue-emitting molecules). Among them, the compound ) and the EL emission wavelength, as shown in Figure 8e.…”

“…where μ is the dipole moment, f is the polarizability of the solvent, and a is the solvent cavity radius. 84 Accordingly, the specific methods for suppressing SSS are to reduce the excited-state dipole moment of the emitter (Figure 8f) 15,62 by utilizing low-polar host materials, 23 and to use sterically shielded design for the emitter. 16 In addition to the bond homolysis and free radicals formation caused by exciton-exciton and excitonpolaron annihilations, the coupling reaction between two CT state molecules might also be responsible for the degradation of TADF OLEDs.…”

Isotope Effect of Host Material on Device Stability of Thermally Activated Delayed Fluorescence Organic Light‐Emitting Diodes

“…[ 7,8 ] Furthermore, the fluorescent radiative decay rate constant ( k r ) of TADF emitters is also an important factor for the device stability as well. [ 11,12 ] In addition to the rational molecular design of TADF emitters for having a longer device operational lifetime, the choice of host materials should have a pronounced effect on device stability. In fact, it has been reported that the device operational stability of TADF or phosphorescent (Ph) OLEDs are significantly increasing when chemically stable materials are utilized as the host.…”