Chemistry Reference
In-Depth Information
Fig. 29 Typical sandwich-
like architecture for a (OPV)
solar cell. Polymers and
fullerene derivatives are
bended on the active layer
F
3
C
NO
2
N
N
F
3
C
C
12
H
25
O
H
3
C
N
C
12
H
25
O
C
8
H
17
OC
6
H
13
N
S
S
OC
6
H
13
S
S
Fig. 30 Some examples of modified fullerenes bearing different organic addends used to prepare
photovoltaic devices
The cyclopropanation reaction to superior fullerenes to form PCBM analogues is
more complex than for C
60
. Indeed, the less symmetry and the presence of more
than one reactive double bond are often responsible for the formation of
regioisomeric mixtures. Nevertheless, the loss of symmetry of C
70
induces a
stronger absorption, even in the visible region. As a result, PC
71
BM [
233
]is
considered a suitable candidate for more efficient polymer solar devices. Moreover,
such devices displayed the highest verified efficiency determined so far in a BHJ
solar cell, with an internal quantum efficiency approaching 100% [
234
]. Analo-
gously, PC
84
BM [
235
] has been obtained as a mixture of three major isomers. The
stronger electron affinity and the diminished solubility gave rise, however, to poor
power conversion efficiencies.
Although the PCBMs are the acceptors that guarantee best performances, it does
not mean that they are necessarily the optimum fullerene derivatives. Therefore, a
variety of other fullerene derivatives [
236
] have been synthesized in order to
improve the device efficiency or to achieve a better understanding of the depen-
dence of the cell parameters on the structure of the acceptor.
One of the most promising modified fullerene prepared so far is the diphenyl-
methanofullerene (DPM12, 56) prepared by Mart
ยด
n et al. [
237
] endowed with two
