Chemistry Reference
In-Depth Information
(21, Fig.
8
)[
110
]. Intravenous injection of 21 into tumor bearing mice followed by
irradiation showed a significant anti-tumor PCT effect which depended on the
timing of light exposure that correlated with tumor accumulation as detected by
the enhanced intensity of MRI signals. Finally, the photoactivity of star-shaped
poly(
ʵ
-caprolactone)-C
60
22 has been successfully proven in the benchmark trans-
formation of 9,10-anthracene dipropionic acid into its endoperoxide [
111
]. In fact,
large amounts of
1
O
2
have been obtained upon irradiation of 22 with visible light.
These kinds of polymers are of interest especially because they are biodegradable,
biocompatible, and non-toxic to living organisms.
Among the possible practical applications of fullerene-polymers, their use as
electron acceptors in the active layer of organic photovoltaic devices seems to be
one of the most realistic. Despite the disappointing results, in terms of efficiency,
associated with the “double-cable” polymers, investigations of which were time
consuming, the very latest few years are witnessing a new momentum in the use of
polyfullerenes. This is probably due to improved technologies and methodologies
that are allowing scientists to achieve efficiencies comparable to those obtained in
bulk heterojunction cells based on molecular C
60
-derivatives. The first improve-
ment in organic solar cells performances has been obtained by using a new
approach in which the glycidol ester of [6,6]-phenyl C
61
butyric acid has been first
prepolymerized in the presence of a Lewis acid as the initiator (23, Fig.
9
)[
112
].
Second, after spin coating the prepolymer in a blend with P3HT (poly-3-hexyl
thiophene), the ring-opening polymerization has been completed by heating the
photovoltaic device which showed 2.0% conversion energy efficiency, probably
due to the morphological stabilization of the active layers architecture. Soon after, a
new approach was reported in which the amphiphilic diblock-polymer 24 carries
both the C
60
units and P3HT fragments acting as a compatibilizer between
PCBM and P3HT in the active layer [
113
]. When added to a blend of PCBM:
P3HT at 17 wt% the photovoltaic device prepared displayed an efficiency of 2.8%,
along with enhanced stability of the devices against destructive thermal phase
segregation. This improvement has been accounted by for the higher control in
the blend morphology of the active layer due to the presence of fragments of P3HT
in the polymer backbone which act as compatibilizer between PCBM and P3HT.
Analogously, rod-coil block copolymer 25 has been used at various concentrations
as surfactant/compatibilizer for the active layer of bulk-heterojunction solar cells
in blends with PCBM [
114
]. This approach resulted in 35% increase of the
photocurrent efficiency, increasing from 2.6% to 3.5% when the copolymer was
used at 5 wt%. Such enhancement has been ascribed to the improvement in the
bicontinuous interpenetrating network due to the compatibilizing action of the
copolymer, as also evidenced by AFM studies.
Finally, a revolutionary approach has recently been described in which the cross-
linked C
60
-polymer 26 is generated in situ allowing the subsequent deposition of
the active layer to avoid interfacial erosion [
115
]. The inverted solar cell ITO/ZnO/
26/P3HT:PCBM/PEDOT:PSS/Ag showed an outstanding device performance with
a PCE of 4.4% and an improved cell lifetime with no need for encapsulation. The
strength of this new approach is its wide and general application. In fact, changing
