Universal Host Materials for High-Efficiency Phosphorescent and Delayed-Fluorescence OLEDs

Abstract

A series of bipolar hosts, namely, 5-(2-(9H-carbazol-9-yl)-phenyl)-1,3-dipyrazolbenzene (o-CzDPz), 5-(3-(9H-carbazol-9-yl)-phenyl)-1,3-dipyrazolbenzene (m-CzDPz), 5-(9-phenyl-9H-carbazol-3-yl)-1,3-dipyrazolbenzene (3-CzDPz), and 5-(3,5-di(9H-carbazol-9-yl)-phenyl)-1,3-dipyrazolbenzene (mCPDPz), are developed for phosphorescent and thermally activated delayed fluorescence (TADF) organic light-emitting diodes (OLEDs). They are designed by selecting pyrazole as n-type unit and carbazole as p-type one. The triplet energy (E(T)), the frontier molecular orbital level, and charge transporting abilities, are adjusted by varying the molar ratio of pyrazole to carbazole and the linking mode between them. They have high E(T) values of 2.76-3.02 eV. Their electroluminescence performance is evaluated by fabricating both phosphorescent and TADF devices with blue or green emitters. The m-CzDPz hosted blue phosphorescent OLEDs achieves high efficiency of 48.3 cd A(-1) (26.8%), the 3-CzDPz hosted green phosphorescent device exhibits 91.2 cd A(-1) (29.0%). The blue and green TADF devices with 3-CzDPz host also reach high efficiencies of 26.2 cd A(-1) (15.8%) and 41.1 cd A(-1) (13.3%), respectively. The excellent performance of all these OLEDs verifies that these pyrazole-based bipolar compounds are capable of being universal host materials for OLED application. The influence of molar ratio of n-type unit to p-type one and the molecular conformation of these hosts on their device performance is discussed and interpreted.

Dual n-type units including pyridine and diphenylphosphine oxide: effective design strategy of host materials for high-performance organic light-emitting diodes

“…Apparently the direct linkage of the dual n-type units of the DPPO and pyridine groups successfully pull down the LUMO levels but keep the HOMOs unchanged. The HOMO levels of these compounds are close to that of the widely used hole transport material 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC, –5.50 eV), 21,23 while their LUMO levels are reduced but still higher than those of FIrpic (–3.0 eV) and Ir(ppy) 3 (–2.8 eV), indicating a small hole injection barrier from TAPC and also a decreased electron barrier to the emitting layer when these materials are used as hosts in OLEDs.…”

“…The similar HOMO levels indicate that the attachment of DPPO directly on the pyridine ring for m -PyPOmCP, p -PyPOmCP did not affect the HOMO alignment. In addition, the HOMO levels of these compounds are not lower than that of mCP (HOMO – 5.72 eV), 23 which does not contain any n-type groups, further revealing the possibility to adjust the HOMO or LUMO independently without affecting the other one if the p-type and n-type units are spatially separated from each other. After the reversible oxidation waves, one additional reduction peak at 0.80–0.85 V was detected for each compound, which has been frequently observed for carbazole derivatives containing non-protected carbazole at 3,6-sites in previous reports, and should be due to the instability of the radical cations of the carbazole moiety.…”

“…After the reversible oxidation waves, one additional reduction peak at 0.80–0.85 V was detected for each compound, which has been frequently observed for carbazole derivatives containing non-protected carbazole at 3,6-sites in previous reports, and should be due to the instability of the radical cations of the carbazole moiety. 5,21,23 During the cathodic scan, two irreversible reduction waves were detected for each compound, and the first reduction wave started at a less negative potential. LUMO energies were determined from the onset potential of the first reduction wave ( E onsetrex) according to the equation E LUMO = –e( E onsetrex + 4.4) (ref.…”

Molecular Engineering of Host Materials for High‐Performance Phosphorescent OLEDs: Zig‐Zag Conformation with 3D Gridding Packing Mode Facilitating Charge Balance and Quench Suppression

“…Upon photoexcitation, their thin films emit purple blue fluorescence with peaks at 402–433 nm. Also, their optical bandgaps were estimated from the threshold of absorption spectra of the neat films by absorption threshold method, as 3.42 eV ( m ‐BPyDCz), 3.27 eV ( p ‐BPyDCz), 3.48 eV ( m ‐BPySCz), and 3.36 eV ( p ‐BPySCz), and the lower bandgaps of two para ‐substituted analogs as verified by both longer‐wavelength absorption onsets and the red‐shifted fluorescence should be due to the more expanded π‐conjugation . Simultaneously, their absorption and fluorescence spectra in various solvents with different polarities were investigated to explore the excited states and solvatochromic behavior.…”

Purely Organic Thermally Activated Delayed Fluorescence Materials for Organic Light‐Emitting Diodes

“…Thus, the charge transport is more balanced in the 4CN34BCz : 4CzIPN device, which achieved a maximum EQE of 20.8% compared with only 13.8% using 3CN34BCz as the host. Li et al prepared four novel ambipolar hosts based on carbazole and pyrazole ( o ‐CzDPz , m ‐CzDPz , 3‐CzDPz , and m CPDPz ) (Figure ). The different interconnection patterns result in varying degrees of twisting of the conformations of the hosts, which thereby affect important properties such as triplet energies and charge mobility.…”