Environmental Engineering Reference
In-Depth Information
Scheme 2.5 Several
polythiophene derivatives
the HT-HT isomer proportion in polymers is named as regioregularity. Regio-
regular poly(3-substituted thiophene) can easily access a low energy planar con-
formation, leading to highly conjugated polymers. An increase of the torsion
angles between thiophene rings leads to greater band gaps, with consequent
destruction of high conductivity and other desirable properties, hence the regio-
regularity is an important factor in characterization of poly(3-substituted thio-
phene), and P3HT with high regioregularity ([98 %) is first required for high
photovoltaic performance. Currently, the influence of regioregularity on photo-
voltaic properties of the other polymer systems has not been well studied, but this
parameter would be one of the keys to do further improvement of molecular
structures.
Although, P3HT exhibited promising photovoltaic properties, the absorption
band (from 500 to 650 nm) of this polymer is still not broad enough to get good
harvest of the sunlight. Two-dimensional conjugated polythiophenes (2D-PTs)
provided a feasible way to broaden absorption band. A representative structure of
2D-PT, absorption band of 2D-PT is formed by two parts [ 18 ]. One part located at
short wavelength direction is from the conjugated side chain; and another located
at long wavelength direction is from the conjugated main chain. By adjusting the
conjugated length of the side chain, the absorption peak position at short wave-
length direction can be tuned, and by adjusting the m:n value, a broad and strong
absorption band can be obtained. 2D-PTs also exhibited better hole mobilities than
P3ATs, and 2 and 3 orders improvement of hole mobilities have been observed in
some kinds of 2D-PTs [ 19 ].
To replace the alkyl side groups of P3ATs by alkoxy groups is an effective way
to reduce the band gap of polythiophenes. Band gaps of poly(3-alkyloxythioph-
ene)s (P3AOTs in Scheme 2.4 ) are *1.55 eV, which is much lower than that of
P3ATs. Although, P3AOTs have much better absorption band compared to that of
P3ATs, P3AOTs are not suitable as electron donors due to their high-lying HOMO
levels. Since, the alkoxy has much stronger electron donating effect than alkyl,
HOMO levels of P3AOTs were ca. 0.4 eV higher than those of P3ATs. Since, the
electron donating effect of side groups of polythiophenes is one of the keys to tune
their HOMO levels and alkyls can be seen as weak electron donating groups, to
use less alkyl substituents should be helpful to get deep HOMO level. P3HDTTT
(see Scheme 2.5 ) was designed based on this strategy. It can be seen that for
P3HT, each thiophene unit has one alkyl; for P3HDTTT, three thiophene units
possess one alkyl. As a result, P3HDTTT exhibits deeper HOMO level than that of
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