Electrochemical Depositon of End-Capped Triarylamine and CBP Dendrimers: Alternate
Technique for the Fabrication of Organic Light-Emitting Devices
Ho-Jin Son
1
, Won-Sik Han
1
, Ji-Yun Chun
1
, Kyung Ryang Wee
1
, Dae Hyun Kim
1
, Kuk-Wha
Lee
2
, Ha Jin Jung
2
, Chongmok Lee
2
, Jaejung Ko
1
, and Sang Ook Kang
1
1
Advanced Material Chemistry, Korea University, 208 Seochang, Jochiwon, 339-700, Korea,
Republic of
2
Department of Chemistry, Ewha Womans University, 11-1 Daehyun-dong, Seoul, 120-750,
Korea, Republic of
ABSTRACT
End-capped triarylamine and carbazole dendrimers were prepared through the divergent
synthesis based on the reaction of diethenyl propagating carbosilane dendrimers with suitable
functional groups such as the naphthylphenylaminophenyl (NPB) or carbazolylphenyl (CBP)
unit. The electrochemical deposition behavior in higher generation dendrimer was observed in
cyclovoltammetric experiments, leading to the good film formation on ITO. The deposited films
remained intact in the depositing solvent and the film thickness was adjusted by varying the
number of CV cycles. To obtain the reason of this behavior, the electrochemical property of
compounds based on silicon (Me
4-n
Si(CBP)
n
) was systematically studied. Moreover, multi-layer
films could be fabricated without causing any damage to the previous layer. To this ends, the
fabrication of OLEDs using this electrochemical deposition process was also investigated.
INTRODUCTION
Dendritic molecules have been proposed as alternative materials for convenient solution
processing. The sphere-like rigid structures of these molecules enable the easy fabrication of
homogenous films by casting [1]. In addition, dendrimeric precursors exhibit properties that are
desirable for OLED applications, such as amorphous structure, high moldability, and high
thermal stability [2]. Recently, we found that carbosilane dendrimers adorned with either
triarylamine or carbazole units in their periphery exhibit novel electrochemical behavior in which
the electrochemical deposition is controlled by dendrite generation. In addition, the deposited
layers remained intact in the depositing solvent, methylene chloride, allowing a second layer to
be deposited on top of the first layer. In the present study, we sought to establish the suitability of
this electrochemical deposition technique for use in the construction of multi-layer OLEDs,
which cannot be fabricated via conventional spin-coating with a polymeric precursor. Thus, the
electrochemical deposition-based process could potentially offer an ideal combination of
deposition control on the one hand and multi-layer fabrication on the other. We report herein the
novel electrochemical deposition behavior of triarylamine or carbazole end-capped carbosilane
dendrimers of the type Gn-2
n+1
NPB or Gn-2
n+1
CBP and their use for the formation of multi-
layer structures of the type used in OLEDs’.
Mater. Res. Soc. Symp. Proc. Vol. 965 © 2007 Materials Research Society 0965-S03-19
Figure 1. Electrochemical deposition of G3-16NPB and G3-16CBP Dendrimers
DISCUSSION
The electrochemical deposition behavior of end-capped triarylamine/carbazole dendrimers has
not been previously reported, although arylamine
[3] and carbazole
[4] units are known to
undergo electrochemical polymerization upon oxidation. Due to effective end-group isolation
derived from branching ethenyl units, each peripheral unit cannot undergo a conventional
electrochemical polymerization. In the case of end-capped NPB dendrimers, effective deposition
is observed from the third generation G3-16NPB, while for more rigid end-capped CBP
dendrimers, deposition started even in the first generation, G1-4CBP. To maximize the film
growth, the deposition process for each NPB or CBP end-capped dendrimer was optimized by
using the third generation of the dendrimer.
For the third generation dendrimers, a thick film was deposited on the electrode surface
because higher generation dendrimers contain highly charged species and hence have an
increased tendency toward good film formation [5].
An additional advantage of electrochemical
deposition is that the film thickness can be controlled by varying the number of redox cycles in
the CV. With the aid of this novel electrochemical deposition method, NPB and CBP dendrimer
films were grown onto an indium tin oxide (ITO) surface as shown in Figure 2. The thicknesses
of the deposited films were measured using a profilometer and the surface coverages were
evaluated by CV [6]. This indicates that the proposed technique may be a viable alternative
method for constructing layers.
To demonstrate the successive formation of multiple layers, we electrochemically deposited a
second layer. The film of end-capped NPB dendrimers formed after electrodeposition of G3-
16NPB was insoluble in methylene chloride. Therefore, to test the feasibility of successive
deposition, a second electrochemical deposition with G3-16CBP onto the preformed NPB
dendrimer film was conducted. The results of this electrochemical deposition are presented in
Figure 2A. In the depositions of the NPB and CBP dendrimer films, the potential scan ranges
were varied to obtain films with similar thicknesses. A representative cross-sectional scanning
electron microscope (SEM) image of the double-layer structure is shown in Figure 2B. These
10 u A
2 uA
ITO
ITO
ITO
Film Formation
Electrodeposition
experimental data clearly establish that, by using suitable dendrimer precursors, multilayer
structures can be fabricated by applying successive electrochemical processes.
Figure 2. (a) CVs for the successive electrodeposition of NPB and CBP dendrimers and (b) a
typical cross-sectional SEM image of the NPB/CBP dendrimer double-layer film.
CONCLUSION
In summary, we have demonstrated that (1) rigid ethenyl linked carbosilane dendrimers
adorned with NPB or CBP units were well suited to electrochemical deposition. (2) Once
electrochemically deposited, the dendrimer films remained intact in the depositing solvent. (3)
The film thickness could be adjusted by varying the number of CV cycles. (4) Most importantly,
multi-layer films could be fabricated without causing any damage to the previous layer. The
fabrication of OLEDs using this electrochemical deposition process as well as the optimization
of such device structures is currently being investigated.
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