Chemistry Reference
In-Depth Information
10.4.4 Materials for Electron or Hole Transporting
Photochemical devices including OLEDs and solar cells require materials that can
transport electrons and holes. Recently, PEDOT-PSS is one of the most utilized
polymeric materials for hole transport [165]. Although there is no established
compound providing easy film fabrication and high performance, aryl amine deri-
vatives that contain the triphenylamine (TPA) or carbazole unit are some of the potent
candidates superior to PEDOT-PSS [166,167]. Dendrimers that contain these aryl
amine components are quite remarkable materials. One of the advantages of using this
material is its excellent film-forming property based on its low viscosity and low
crystallinity. Due to their spherical morphology, the dendrimers that have hole-
transporting units as the termini allows high carrier mobility, regardless of the core
structure and the functionality [35,168-170]. Thus, a flexible design is available based
on the luminescence and charge transport. Examples of the application in OLED
devices are presented in the following section.
Bulk charge transport through a semiconducting filmis an essential process in organic
electronics. However, a molecular-level charge transfer and its management is also an
important key for any improvement. Especially, for photovoltaic devices, the initial
electron transfer (charge separation) upon photoexcitation determines most of the
photoenergy conversion efficiency. If back-electron transfer against the charge separa-
tion occurs, the total conversion efficiency will be significantly reduced. Although
several strategies to suppress the back-electron transfer have been reported, nanos-
tructure construction based on the dendrimer is a powerful tool to optimize the distance
between the electron donor and acceptor. The photosynthetic reaction center contains a
well-established redox sequence in order to facilitate long-range charge separation for an
efficient photoenergy conversion. The dendrimer structure enables the fine construction
of the function based on a topological and geometrical molecular design.
10.5 APPLICATIONS FOR PHOTOCHEMICAL DEVICES
The function of devices is established as an integration of material properties such as
photoexcitation, luminescence, energy transfer, and electron transfer. In addition to
the molecular-level design, all chemical processes including the formation of
multilayer, heterojunction, fine-tuning of the electron- and hole injection should be
precisely designed for the device construction. Not only the electric property but also
the fabrication process of ultrathin films must be taken into account. In the following
sections, examples of using the dendrimer structure in actual devices are introduced.
10.5.1 Fabrication of an Ultrathin Organic Film for Molecular Devices
In general, molecules with the dendritic architecture show a very good solubility in
most organic solvents. In the case of a linear polymer, the solubility drastically
decreases if the backbone structure is composed of a rigid
-conjugating architecture.
In sharp contrast, the dendrimer is still dissolved even if the rigid structure is the main
p
