Chemistry Reference
In-Depth Information
As a result of the research, several research groups reported attaining 5% efficiencies with the
combinations of poly(3-hexyl thiophene) with 1-(3-methoxycarbonyl)propyl-1phenyl-[6,6]-
methanofullere. Numerous other conducting polymers including copolymers containing fluorene, car-
bazole, cyclopentadithiophene were investigated.
The efficiency of organic solar cells is usually defined as:
¼ðJ
sc
V
oc
FF
Þ=P
in
V
oc
is the open circuit voltage. FF is the fill factor. Much of the research effort to date has been based on
attempts to increase
V
oc
. This is based on empirical correlation between the magnitude of the open
circuit voltage and the difference in energies between HMO of the donor and LUMO of the acceptor.
It is expected that by lowering the HMO of the donor,
P
in
is the input energy of solar radiation. The output short circuit current density is
J
sc
and
where
V
oc
can be increased [
266
].
To achieve this goal, two research teams headed by Yu and by Yang tested a series of copolymers
prepared by reacting a benzodithiophene derivative with various thienothiophenes. The aim was to
lower the polymers' HMO by attaching successively stronger electron-withdrawing groups to the
polymer backbone. The result was that by replacing an alkoxy group that was adjacent to a carbonyl
group with an alkyl chain at the same position, the group lowered the HOMO level by roughly 0.1 eV.
They lowered the level by another 0.1 eV by adding a fluorine atom. Solar cells prepared with this
polymer were found to be 6.8% efficient [
267
]. Subsequently, Yu et al. reported slightly improved
conversion efficiency of over 7% [
268
].
It is interesting that at the time of the publication of the review, it was reported in
Chem. and Eng
News
[
269
] that Heeger and Gong developed a broad spectrum donor acceptor combination that can
detect photons throughout the whole light spectrum, from the ultra-violet to the infra-red. The
combination of the two materials can be illustrated as follows:
S
N
N
S
S
S
S
H
21
C
10
C
10
H
21
