Environmental Engineering Reference
In-Depth Information
Table 2.1
Properties and devices characteristics for the BDT-based low band gap polymers
Polymer
Eg(opt)
(eV)
HOMO/LUMO (eV/
eV)
J
sc
(mA/
cm
2
)
V
oc
(V)
FF
PCE
(%)
Refs
PBDTTT-E
1.77
-5.01/- 3.24
13.2
0.62
0.630
5.15
[
13
]
PBDTTT-C
1.67
-5.12/- 3.35
14.7
0.70
0.641
6.58
[
14
]
PBDTTT-
CF
1.77
-5.22/- 3.45
15.2
0.76
0.669
7.73
[
15
]
PBDTTT-S
1.63
-5.12/- 3.49
14.1
0.76
0.580
6.22
[
16
]
molecular structures. Since, it is hard to get a clear and exhaustive conclusion to
guide molecular design of photovoltaic materials with conjugated structures, to
have a comprehensive understanding of the properties of more organic photo-
voltaic
materials
would
be
the
best
way
to
understand
the
mechanisms
of
molecular design.
2.2 Active Layer Materials
2.2.1 Electron Donors
Numerous donor materials, including polymers and small molecular compounds,
have been developed in the past decades. It is hard to categorize these materials. In
this section, polythiophenes were used as an example for polymer materials with
homoconjugated backbones; polymers with 2,1,3-benzothiadiazole, pyrrolo[3,4-
c]-pyrrole-1,4-dione,
and
benzo[1,2-b;4,5-b
0
]dithiophene
were
used
as
three
examples
for
molecular
structure
design
of
conjugated
materials
with
D/A
structures.
2.2.1.1 Polythiophene and Its Derivatives
Poly(3-alkylthiophene)s (P3AT) have been used as active material for optoelec-
tronic devices, especially for photovoltaic cells. As mentioned above, P3HT is the
best one in P3ATs as photovoltaic material. Typically, P3ATs can be synthesized
easily through different methods. The repeating units of P3ATs were asymmetric,
so three relative orientations are available when two thiophene rings are coupled
between the 2- and 5-positions (Usually, the 2-position is called as head, and the 5-
position is called as tail). So, the first of these is head-to-tail (HT) coupling, the
second is head-to-head (HH) coupling, and the third is tail-to-tail (TT) coupling.
As shown in Scheme
2.5
, this leads to a mixture of four chemically distinct triad
regioisomers when 3-substituted (or asymmetric) thiophene monomers are
employed [
17
]. The HT-HT structure of polythiophenes are denoted as regio-
regular, the other three structures are denoted as regioregular or regiorandom, and
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