Chemistry Reference
In-Depth Information
Contents
1 Principles of Solar Energy Conversion in Organic Active Layers . . . . . . . . . . . . . . . . . . . . . . . . 118
2 Optimization of Photovoltaic Features of Langmuir-Blodgett Porphyrin Films . ........ 120
3 Fullerene Derivative-Based Solar Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121
4 Covalently Bonded Dyads and Triads . . . . . . . ............................................... 123
4.1 Role of the Interlayer in the Kinetics of Photoinduced Electron Transfer in LB
Films . . ................................................................................. 126
5 Systems Without Fullerene Derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135
1 Principles of Solar Energy Conversion in Organic Active
Layers
The processes that rule the mechanisms leading to the production of electrical
energy from light in organic photovoltaic cells start from the absorption of a photon
with appropriate energy, which promotes an electron transition toward an excited
state (Fig.
1
). This quasiparticle, made of a hole (h
+
) and an electron (e
) electro-
statically bounded, is called exciton [
1
], and it diffuses into the material where it is
generated. The couple h
+
-e
can recombine wasting the photon absorption, or if the
diffusion length is long enough to allow the exciton to meet an internal field,
represented, for example, by the interface between a donor element (
D
) and an
acceptor one (
A
)[
2
], it can separate in two free charges that move toward the
electrodes.
The following reactions can be used to summarize the above-described
phenomena:
hν
D
þ A
D þ A!
ð
1
Þ
D
þ A ! D
þ
þ A
ð
2
Þ
*-LUMO
band of the p-type [
3
] donor material. The electron transfer from the donor element
to the LUMO of the acceptor generates the excited, but still neutral exciton with an
e
-h
+
coupling energy smaller than the energy gap. Then, the exciton can diffuse up
to meet the internal field generated at the donor-acceptor interface, and exciton
separation occurs. The carriers are, then, transported toward the electrodes, and
during the pathway, they can be trapped by defects of the organic matrices. In light
of this, the transfer method used to obtain an adequate phase separation and an
ordered film appears crucial to optimize the efficiency of an organic photovoltaic
device [
4
].
Furthermore, the thickness of the organic layer should be thick enough to ensure
a high photon absorption and should be thin enough to allow a high interface where
excitons can dissociate.
The generation of electron-hole pairs takes place from
π
-HOMO to
π
