Chemistry Reference
In-Depth Information
performance. This observation has two origins. One is that higher generation
thiophene dendrimers possess longer
-conjugated pathways and absorb light at
longer wavelengths where the solar photon flux is higher. The other reason is that
higher generation dendrimers have higher hole mobility. One of the disadvantages of
these thiophene dendrimers over P3HT is that their absorption spectra do not cover the
red and near-infrared regions, caused by shorter conjugation length and decreased
intermolecular interaction. In addition, the hole mobility of the dendrimer is also
lower than P3HT. Nevertheless, this work demonstrated that conjugated dendrimer
can be used as the third type materials in OSC. Also, such large dendrimer with
diameter approaching 10 nm brings up an interesting question. As has been proved in
previous work, a nanoscale phase separated morphology is the optimum one for
BHJSC [102]. In this respect, the 3-D persistency of the dendrimer sets a lower limit to
the domain size, which enables unique control over the degree of the phase separation.
Similar work using phenyl cored thiophene dendrimers is reported byKopidakis et al.,
and a maximum
a
auerle's group
attached the thiophene dendrons to a ruthenium(II) phthalocyanine core using
coordination chemistry between pyridine and metal-phthalocyanine. A solution
processed BHJSC with
Z
of 1.3% is achieved [110]. In a recent work, B
€
of 1.6% has been achieved [111].
Following our work on OTE-truxene dendrimers, we also investigated their
applicability as OSC materials. However, in most cases, their absorption spectra are
too narrow. To extend the absorption spectrum to longer wavelength, we have
replaced the OTE segments with oligo(thienylenevinylene) (OTV) segments [112],
which have lower energy bandgap due to better conjugation through the double bond.
Figure 9.28 illustrates the chemical structures of two zeroth-generation extended
dendrimer with OTV branches and truxene nodes, along with their model compounds.
From the absorption spectra of thin films, it was seen that the OTV-truxene dendrimers
covered a much broader spectrum than OTE-truxene dendrimers, even though the
generation is lower. Typically, poly(thienylenevinylene)-based OSC devices do not
show good performances due to the nonluminescent nature of thienylenevinylene
unit [113]. In our case, the introduction of truxene unit effectively improved the
luminescent ability of the OTV segments. As a result, OSC devices using
Z
showed
better performance than the poly(3-hexylthienylenevinylene), despite narrower
absorption spectrum. The highest
37
.In
subsequent works, we have further introduced an electron-accepting unit such as
benzothiadiazole into the dendrimer scaffold to further extend the absorption
spectrum into the red region, and preliminary results showed that
Z
is 0.40% for OSC devices based on
37
Z
could be improved
to 0.54% (unpublished results) (Figure 9.29) [112].
Shape-persistent conjugated dendrimers also showed application potential in
DSSCs. For example, Yamamoto and coworkers used their phenylazomethine
dendrimers as a charge separator in DSSC [114]. DSSCs prepared by casting
phenylazomethine dendrimers onto dye-sensitized TiO
2
film exhibits higher open-
circuit voltage than the bare film, due to the suppression of back electron transfer by
the dendrimer.
A general problem for dendrimer-based OSC is their narrower light absorption
range and lower absorption coefficient in the red and near-infrared regions, compared
