Chemistry Reference
In-Depth Information
FIGURE 9.4
Chemical structures of E-stilbenyl dendrimers
3
-
5
(from the first to the third
generation) [33].
By changing one of the reaction substrates in the final step of the synthesis,
E-stilbenyl dendrimers with different emission colors could be facilely obtained [34].
For example, introducing a porphyrin core resulted in a red DLED. Likewise, a
distyrylanthracene core gave a yellow-green DLED. In other words, the processabil-
ity and luminescent property were independently provided by the dendron and the
core, respectively, demonstrating the advantage of the dendritic architecture.
Generally, dendrimers constructed from E-stilbenyl units are hole-dominated
materials. Therefore, it is reasonable to expect that introduction of electron-trans-
porting moieties into the dendrimer would balance the charge transport. Following
this line of reasoning, Kim et al. have designed and synthesized triazine-functiona-
lized dendrimers [36] such as
6
(Figure 9.5a). However, two-layer devices (ITO/PVK/
dendrimer/Al:Li) using
as the active material showed only modest EQEs
(0.03-0.5%). Another interesting effort was made by Jenekhe and coworkers
(Figure 9.5b) [37]. They incorporated diphenylquinoline units, which have been
extensively studied as promising electron acceptors [38], into the E-stilbenyl den-
drimer. Thereby, the dendrimer was converted to an n-type material. Unlike previous
examples, the chromophores responsible for the emission color are now at the
dendrimer surface. But such a design did not seem to work well for active material
in DLED. Intimate contact among diphenylquinoline units was extensive in the solid
state: the emission color changed from blue to yellow on going from solution to thin
film, and the LQY dropped significantly. Consequently, DLED devices using these n-
type dendrimers alone showed a low EQE (about 0.01%). Nevertheless, these
dendrimers can be used as excellent electron-transporting layers. To demonstrate
this point, Jenekhe's group fabricated two-layer diodes (ITO/PEDOT/MEH-PPV/
dendrimer/Al) by sequential spin coating of MEH-PPV layer and the dendrimer layer.
The best performance of thus-obtained devices had amaximumEQE of 5.0%, a power
efficiency of 1.3 lm/W, a device efficiency of 1.5 cd/A at 6.5 V, and a brightness level
of 240 cd/m
2
(for dendrimer
6
7
). All the parameters are much better than those from
