Chemistry Reference
In-Depth Information
comparison with the PL spectra in solid state, whichwas explained by the formation of
ketonic defects during device operation, similar to polyfluorene-based devices.
Encouraged by these initial results, we continued our search for better truxene-
based DLED materials. We then explored an interesting molecular design, that is,
dendrimers constructed solely by truxene units. Figure 9.9 shows the chemical
structures of all-truxene dendrimers [51]. Compared with the previous series, their
absorption peaks red-shifted considerably to 350 nm, indicative of more effective
conjugation between truxene moieties. The emission spectra of all-truxene dendrimer
showed three peaks at 386, 405, and 430 nm. Again, the generation number does not
have a significant influence on their photophysical properties [52].
Smooth thin films of
13
and
14
showed one main peak around 406 nm with high
LQYs (60% for
13
and 62% for
14
). Subsequently, devices were fabricated using
13
and
with the configuration of ITO/PEDOT:PSS/PVK/dendrimer/TPBI/Ba/Al. The
turn-on voltages were 6.0 V and 5.8 V for
14
, respectively. Both devices
emitted pure-blue light with 1931 Commission Internationale de l'Eclairage (CIE)
coordinates of (0.16, 0.08). Notably, no ketonic defect or excimer emission was
observed in these devices, showing great advantage over devices using polyfluorene.
The maximum EQEs were as high as 1.93% at 7.0 V for
13
and
14
13
and 1.99% at 6.3 V for
14
.
Such a performance was remarkable compared to previous examples.
Our most successful light-emitting materials developed so far are E-stilbenyl-
truxene dendrimers (see Figure 9.10 for the chemical structure). Reported in
2006 [53], the synthesis of such dendrimers was based on repetitive Suzuki and
Horner-Wadsworth-Emmons reactions.
In the absorption spectra in solution
(Figure 9.11a), compounds
displayed two absorption peaks at 330 and
378 nm, corresponding to phenyl-functionalized truxene at the periphery and the
central E-stilbenyl-truxene unit, respectively. Two strong emission bands in the
visible region were observed for both
16
and
17
16
and
17
, one at 424 nm and the other at
448 nm. The absorption and emission maxima of
16
and
17
showed certain degree of
red-shift to those of model compound
18
.
in thin films (Figure 9.11b) were quite
similar to those in solution. This fact reflects that no obvious interchain and/or
intermolecular aggregation existed in the ground state, probably due to the presence of
bulky branches and a large number of alkyl groups. The emission spectra of
The absorption behaviors of
16
and
17
17
in the thin films showed slight red shift (about 10-20 nm) in comparison with those in
solution. Annealing the films at different temperatures in nitrogen atmosphere did not
significantly change its absorption and emission spectra, but did have significant
impact on device performances, as will be mentioned later.
Dendrimer
16
and
was soluble in many common organic solvents, which makes it
suitable for many solution processes such as spin-coating, inkjet-printing, and dip-
coating.
1
Devices with the configuration of ITO/PEDOT:PSS/PVK/
16
16
/Ba/Al were
fabricated [54]. Because
has a HOMO level at 5.8 eV, a thin layer of PVK was
introduced on top of PEDOT:PSS as the hole-injection layer. For an annealed device,
16
1
Recently, we have optimized the synthesis protocol and were able to obtain
16
on a large scale, setting the
basis for its commercialization.
