Chemistry Reference
In-Depth Information
A similar effect was also reported for DPAs having a TPA core [74]. This material
showed a high hole-transport property in its film based on a low crystallinity. The
OLED cell assembled with the dendrimer and tris(8-hydroxyquinoline)aluminum
(Alq) showed ca. 1200 cd/m
2
of luminescence with a 10 Vapplied voltage. Interest-
ingly, the luminescent performance increased over 20 times only by the complexation
with 1 equiv. of SnCl
2
. This means that the effective doping of metal ions by finely
controlled hybridization using the DPA system enhanced the hole-transport property.
The role of the precise assembly in hole-transport materials is to repress aggregation
or crystallization. This synergetic effect was also available in other topological
systems (DPA-modified polymer, asymmetric DPA-CzD) based on the DPA
architecture.
An advantage of using these dendrimer-based metal-organic semiconducting
materials is their easy fabrication as a thin solid film only by a spin-coating method.
An amorphous film of ca. 50-100 nm thickness could be prepared without any partial
crystallization. This could be extended to the fabrication of filmswith a larger area that
cannot be produced by a vapor deposition method using monomeric molecules.
10.5.3 Solar Cells
Although the common components for OLED devices can be used, specially
developed materials are usually employed for use in solar cell devices. Materials
based on a macromolecular complex are also useful for photovoltaics and solar
energy conversion. There are several types of photovoltaic devices employing
organicmaterials; that is, an organic thin-layer solar cell [165] and a dye-sensitized
solar cell (DSSC) [179,180] that are currently in the advanced stage of development
for practical use. In both cases, efficient long-range carrier (hole and electron)
transport toward each electrode (anode and cathode) is important [181], as well as
the initial charge separation upon photoexcitation [182]. The charge separation
efficiency and the stability contribute to the internal conversion efficiency that
corresponds to a number of opposite charge pairs per one photon absorbed by the
dye. This efficiency can be significantly improved by the molecular-level nanos-
tructure as noted in the previous section.
In addition to the molecular-level design, the molecular assembled structure
on a submicrometer scale is also essential for enhancement of the photovoltaic
efficiency. First of all, the maximum absorption of photons irradiated from the
outside is necessary to maximize the output (photoelectrons). In order to
increase the absorption, a large amount of dyes as photon antennas and the
reaction center of the photo-induced electron transfer should be implemented in
the photovoltaic cell. Recently, a multichromophore system that allows absorp-
tion over a wide range of wavelengths was developed. Of course, an increase in
the film thickness is demanded for assembling a large number of chromophore
molecules onto a substrate. This strategy inevitably leads to an increase in the
cell resistance, thus electron- and hole-transporting materials are required to
have a very high carrier mobility.
