Chemistry Reference
In-Depth Information
6.1 Fullerenes for Organic Photovoltaics
Global dependence on fossil fuels is a key issue with important consequences in the
world today. A reasonable alternative to overcome this need is the use of renewable
energy sources, like solar energy, which could, in principle, fulfil our energy
requirements with environmentally clean procedures and low prices. Actually, the
energy received from the Sun, calculated to be 120,000 TW (5% ultraviolet, 43%
visible, and 52% infrared), surpasses by several thousandfold that consumed by the
planet. [
226
].
Photovoltaic (PV) solar cells are currently a hot topic in science and since the
original silicon-based device was prepared by Chapin in 1954 exhibiting an effi-
ciency around 6% [
227
], different semiconducting materials (inorganic, organic,
molecular, polymeric, hybrids, quantum dots, etc.) have been used for transforming
sunlight into chemical energy. Among them, photo- and electro-active organic
materials are promising due to key advantages such as the possibility of processing
directly from solution, thus affording lighter, cheaper, and flexible all-organic PV
devices. The most widely used configuration of polymer solar cells is based on the
use of a fullerene derivative as the acceptor component. Indeed, fullerenes have
been demonstrated to be the ideal acceptor because of their singular electronic and
geometrical properties and the ability of their chemically functionalized derivatives
to form a bicontinuous phase network with
ˀ
-conjugated polymers acting as
electron conducting (n type) material (Fig.
29
).
A great variety of chemically modified fullerenes were initially synthesized for
blending with semiconducting polymers and to prepare photovoltaic devices. These
fullerene derivatives were covalently linked to different chemical species such as
electron acceptors, electron donors,
-conjugated oligomers, etc. [
119
] (Fig.
30
).
However, in general, the obtained blends resulted in PV devices exhibiting low
energy conversion efficiencies [
228
].
The best known and most widely used fullerene derivative as acceptor for PV
devices is [6,6]-phenyl-C
61
butyric acid methyl ester (PCBM, 55)[
229
]. Since its
first reported application in solar cells [
230
], it has been by far the most widely used
fullerene, being considered as a benchmark material for testing new devices. This
initial report inspired the synthesis of many other PCBM analogues [
231
]inan
attempt to increase the efficiencies of the cells by improving the stability or PV
parameters such as the open circuit voltage (
V
oc
) by raising the LUMO energies of
the fullerene acceptor.
In this regard, only small shifts (
ˀ
100 meV) of the LUMO level have been
obtained by attaching a single substituent on the fullerene sphere, even by using
electron-donating groups. In contrast, significantly higher
V
oc
values have been
achieved through the polyaddition of organic addends to the fullerene cage
(~100 mV raising the LUMO per saturated double bond). Recently, an externally
verified power-conversion efficiency of 4.5% has been reported by Hummelen et al.
employing a regioisomeric mixture of PCBM bisadducts as a result of an enhanced
open-circuit voltage, while maintaining a high short-circuit current (
J
sc
) and fill
factor (FF) values [
232
].
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