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(a)
Top electrode
Active layer
Al grating
(b)
(c)
Fig. 13 a Periodic grating structure for propagating SSPs; grating period and height: 277 and
50 nm, respectively. b, c Absorption spectra of the polymer blends b APFO3/PCBM and c APFO
Green5/PCBM on the grating structures, recorded while illuminating at different polarization
directions [
53
]
contact of metal NPs with the semiconducting layer should be the most effective
way to improve the absorption process in OPVs. Nevertheless, this method still
encounters many problems in reality. For most of the studies reported so far, no
apparent plasmonic effect has been observed, presumably because of serious phase
separation between the nanostructures and the organic materials.
The third scenario is the adoption of an SPP structure (Fig.
9
c). Tvingstedt et al.
reported one of the earliest examples in 2007. They fabricated periodic nano-
structures to induce SPPs through an Al grating; Fig.
13
a presents the device
structure [
53
]. The plasmonic grating also served as the bottom cathode; a PEDOT:
PSS layer functioned as the top anode. The authors employed two different polymer
blends (APFO3:PCBM and APFO Green5:PCBM) as photoactive materials.
Figure
13
b, c display the corresponding measured absorption spectra. When com-
pared with the performance of the planar sample, the absorption profiles for the
systems incorporating both materials were altered by the SPP nanostructures. More
importantly, different spectra could be obtained when the samples were illuminated
with differently directed polarized light. The authors found that the transverse
electric (TE) polarized electromagnetic wave could not excite the SPPs. IPCE
measurements of the device properties revealed an apparent influence of the
polarization direction on the spectra, suggesting that the SPPs could indeed enhance
the photocurrent.
More recently, Li et al. combined two different plasmonic nanostructures to
improve
the
efficiencies
of
inverted
OPVs
[
54
].
Figure
14
displays
the
device
structures
and
the
chemical
structures
of
the
photoactive
materials.
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