2 research outputs found
Simple Bipolar Host Materials for High-Efficiency Blue, Green, and White Phosphorescence OLEDs
3-(1<i>H</i>-Pyrazol-1-yl)Âpyridine is used as electron-transporting
unit to construct bipolar host materials <i>o</i>-CzPyPz, <i>m</i>-CzPyPz, and <i>p</i>-CzPyPz for application
in phosphorescent organic light-emitting diodes (PhOLEDs). By varying
the ortho-, meta-, or para-linking mode between the n-type 3-(1<i>H</i>-pyrazol-1-yl)Âpyridine and the p-type carbazole on phenylene
bridge, the optoelectronic parameters are tuned to large extent. The
highly twisted <i>o</i>-CzPyPz has high triplet energy of
2.95 eV, while the isomer <i>p</i>-CzPyPz with more coplanar
conformation has smaller triplet energy of 2.67 eV. The <i>m</i>-CzPyPz-hosted blue PhOLED exhibits a peak current efficiency of
49.1 cd A<sup>–1</sup> (corresponding to an external quantum
efficiency of 24.5%) and low-efficiency roll-off, while the <i>p</i>-CzPyPz-hosted green PhOLEDs turns on at 2.8 V and exhibits
high efficiencies of 91.8 cd A<sup>–1</sup> (96.1 lm W<sup>–1</sup> and 27.3%). Furthermore, two-emitting-layer white
OLEDs are fabricated with <i>m</i>-CzPyPz or <i>p</i>-CzPyPz as common hosts for both blue and orange phosphors, which
realize high efficiencies of 57.8 cd A<sup>–1</sup> (45.4 lm
W<sup>–1</sup> and 23.6%) and 60.7 cd A<sup>–1</sup> (38.1 lm W<sup>–1</sup> and 23.1%). The optimization of host
structure for good matching of host and dopant and finally for the
ideal performance is discussed
Cyanopyridine Based Bipolar Host Materials for Green Electrophosphorescence with Extremely Low Turn-On Voltages and High Power Efficiencies
Low driving voltage and high power
efficiency are basic requirements when practical applications of organic
light emitting diodes (OLEDs) in displays and lighting are considered.
Two novel host materials <i>m</i>-PyCNmCP and 3-PyCNmCP
incorporating cyanopyridine moiety as electron-transporting unit are
developed for use in <i>fac</i>-trisÂ(2-phenylpyridine)ÂiridiumÂ(III)
(IrÂ(ppy)<sub>3</sub>) based green phosphorescent OLEDs (PhOLEDs).
Extremely low turn-on voltages of 2.01 and 2.27 V are realized, which
are even lower than the theoretical limit of the emitted photon energy
(<i>hv</i>)/electron charge (<i>e</i>) (2.37 V)
of IrÂ(ppy)<sub>3</sub>. High power efficiency of 101.4 lm/W (corresponding
to a maximum external quantum efficiency of 18.4%) and 119.3 lm/W
(24.7%) are achieved for <i>m</i>-PyCNmCP and 3-PyCNmCP
based green PhOLEDs. The excellent EL performance benefits from the
ideal parameters of host materials by combining cyano and pyridine
to enhance the n-type feature. The energetic favorable alignment of
HOMO/LUMO levels of hosts with adjacent layers and the dopant for
easy charge injections and direct charge trapping by dopant, their
bipolar feature to balance charge transportations, sufficiently high
triplet energy and small singlet/triplet energy difference (0.38 and
0.43 eV) combine to be responsible for the extremely low driving voltages
and high power efficiencies of the green PhOLEDs