Theoretical Studies of Plasmonic Effects in Organic Solar Cells - Organic Solar Cells: Materials and Device Physics - page 196

In contrast to the NSPs embedded into the spacer, our results show that the NSPs
embedded into the active layer offer stronger optical absorption, which can be
observed in Table 7.1 . As seen in Fig. 7.6 b, the scattering energy from the NSPs is
directly and sufficiently absorbed by the contiguous active material uncorrelated
with the directional property of the electric near field. Owing to the plasmon cou-
pling and hybridization, the close-packed NSPs have more concentrated near-field
distribution leading to larger enhancement (Table 7.1 ). Remarkably, the absorption
of the OSC has about 2-fold increase by the small close-packed NSPs. For the large
close-packed NSPs, the excessive red-shifted resonance reduces the spectral overlap
between the resonance and the absorption peak of the active material as illustrated in
Figs. 7.8 d and 7.6 c. At vertical incidence, the reduced spectral overlap gives a
reason why the total enhancement factor by the large close-packed NSPs is smaller
than that by the small ones. However, the total enhancement factor by the large
close-packed NSPs increases at oblique incidence, which distinguishes from the
close-packed small NSPs. The interplay between longitudinal and transverse modes
[ 40 , 84 ] supported by the NSP chain is a physical origin of the phenomenon. Having
larger geometric size and stronger retardation effect, the large close-packed NSPs
support more red-shifted longitudinal modes at the vertical incidence and more blue-
shifted transverse modes at the oblique incidence (see Fig. 7.8 b, d). In comparison
with the red-shifted longitudinal modes, the blue-shifted transverse modes have a
better spectral overlap with the absorption coefficient of the active material and can
be further exploited or engineered in a future design of OSCs.
Regarding the electrostatic limit described by the Laplace equation, the near-
field or far-field response of a subwavelength scatterer is independent of the
scatterer's size and depends only on its shape [ 85 ]. Therefore, it may cause a
misunderstanding that the same enhancement can be obtained if the scaling ratio of
a device structure to a concentrator remains constant. However, using the same
scaling ratio as shown in Fig. 7.6 a, b, we find that the large NSPs and small ones
have noticeable differences both in the spectral and total enhancement factors. The
breakdown of the scaling law can be explained by the retarded and multiscale
effects. The electromagnetic response of a single NSP is dominated by the elec-
trostatic (nanocircuit) physics, but that of multiple NSPs is governed by the
electrodynamic (wave) physics with nonnegligible retardation and long-range
interplay between each NSPs. Furthermore, large-scale OSC nanostructure and
small-scale NSPs strongly couple with each other, which makes the optical path
very complicated; and the trapping confinement, together with leaky loss, must be
considered quantitatively.
In conclusion, we study the near-field multiple scattering effects of plasmonic
NSPs embedded into the thin-film OSC. The absorption enhancement of the OSC
strongly depends on the directional property of near-field scattering from NSPs
and the interplay between longitudinal and transverse modes supported for the
NSPs embedded into the spacer and active layer, respectively. Moreover, the
complex coupling between NSPs and device makes the scaling law in electro-
statics inapplicable. The work provides the fundamental physical understanding
and design guidelines for a typical class of plasmonic photovoltaics.