Effect of Substituents on the Electronic Structure and Degradation Process in Carbazole Derivatives for Blue OLED Host Materials

Abstract

We investigate the dissociation mechanism of the C−N bond between carbazole and dibenzothiophene in carbazole-dibenzothiophene (Cz-DBT) positional isomers, selected as representative systems for blue host materials in organic lightemitting diodes (OLEDs). The C−N bond dissociation energies, calculated at the density functional theory level, are found to depend strongly on the charge states of the parental molecules. In particular, the anionic C−N bond dissociations resulting in a carbazole anion can have low dissociation energies (∼1.6 eV) with respect to blue emission energy. These low values are attributed to the large electron affinity of the carbazole radical, a feature that importantly can be modulated via substitution. Substitution also impacts the energies of the first excited electronic states of the Cz-DBT molecules since these states have an intramolecular charge-transfer nature due to the spatially localized character of the frontier molecular orbitals within the carbazole moiety (for the HOMO) and the dibenzothiophene moiety (for the LUMO). The implications of these results must be considered when designing blue OLED hosts since these materials must combine chemical stability and high triplet energy.

Enhancing the Kinetic Stability and Lifetime of Organic Light‐Emitting Diodes based on Bipolar Hosts by using Spiroconjugation

“…This observation was also reported recently by Hong et al [21] The BDE À of the CN s-bond in the ET À -HT 0 anion is small largely because the N-sp 2 (HT) level of HT lies much lower than the p* LU(ET) level of ET. This observation was also reported recently by Hong et al [21] The BDE À of the CN s-bond in the ET À -HT 0 anion is small largely because the N-sp 2 (HT) level of HT lies much lower than the p* LU(ET) level of ET.…”

Degradation Mechanism and Lifetime Improvement Strategy for Blue Phosphorescent Organic Light‐Emitting Diodes

“…Continuous oxidation and reduction processes happening inside the device by carrier injection generate positive and negative polarons, which cause electrochemical degradation. Many works have shown that organic materials become unstable under positive or negative polarons even though they are stable without any polarons …”

Rigid Oxygen‐Bridged Boron‐Based Blue Thermally Activated Delayed Fluorescence Emitter for Organic Light‐Emitting Diode: Approach towards Satisfying High Efficiency and Long Lifetime Together

“…[ 1,10,11 ] Generally, the C–N bond between donor and acceptor moieties is weaker than the C–C bond and simultaneously causes the low bond dissociation energy (BDE). [ 12,13 ] On the other hand, blue TADF emitters have high T 1 , resulting in the utilization of phosphine oxide‐based host materials such as bis[2‐(diphenylphosphino)phenyl] ether oxide (DPEPO) and dibenzo[b,d]furan‐2,8‐diylbis(diphenylphosphine oxide) (DBFPO) due to their high T 1 values. [ 14,15 ] However, such phosphine oxide‐based host materials show low operational stability because of their very low BDE.…”