Theoretical Studies of Plasmonic Effects in Organic Solar Cells - Organic Solar Cells: Materials and Device Physics - page 192

absorption enhancements by the SPRs will be of a great help for high-efficiency
photovoltaic cells.
In conclusion, we have formulated the equation of the generalized Lambert's
cosine law and comprehensively studied the Au nanostrip OSC structure for
enhancing the TE- and TM-polarized light with the detailed explanations of the
enhancements by the Wood's anomalies and SPRs.
7.4.2 Multiple Nanospheres
We studied in detail on the near-field multiple scattering effects of plasmonic
nanospheres (NSPs) embedded into thin-film OSCs. The fundamental physics of
the optical absorption shows remarkable differences between the NSPs embedded
into a spacer and those embedded into an active layer. The direction-dependent
features of near-field scattering from NSPs significantly affect the absorption
enhancement when NSPs are embedded into the spacer. The interaction between
longitudinal and transverse modes supported in the NSP chain plays a key role in
the absorption enhancement when NSPs are embedded into the active layer.
Through properly engineering the position and spacing of NSPs, our theoretical
results show that the absorption enhancement can be improved by about 100 %.
Moreover, we demonstrate the breakdown of the electrostatic scaling law that
should be considered in the design of OSCs. The work provides the physical
guidelines for plasmonic OSCs.
Figure 7.6 a, b illustrates a schematic pattern of a bulk heterojunction OSC
nanostructure [ 2 , 3 ] to be investigated. The active layer is a typical blend polymer of
P3HT (poly(3-hexylthiophene)) and PCBM (methanofullerene). A hole conduction
layer is PEDOT:PSS chosen as a spacer between an electrode and the active layer.
Figure 7.6 c shows the absorption coefficient of the active material. With tunable size
and spacing, a spherical chain comprising multiple silver (Ag) NSPs is embedded
into the spacer or active layer as near-field concentrators. An incident light is
propagated from the spacer to the active layer with a TM polarization ð H z ¼ 0 Þ at a
vertical ð 0 Þ and an oblique ð 60 Þ incidences. In comparison with the TE polari-
zation, the TM polarization supports much stronger dipole-dipole couplings
between NSPs. Moreover, the absorption enhancement by a silver NSP array, which
has a polarization-independent feature, can be regarded as a superposition of those
by silver NSP chains with both the TM and TE polarizations. To unveil device-
related multiple scattering mechanism of NSPs, we develop a rigorous VIE method
presented in Sect. 7.3.3 to characterize the optical properties of the OSC. The
coupling between multiple NSPs, as well as the interaction between NSPs and
multilayer device structure, is fully taken into account in our model. It is worth
mentioning that we only calculate the optical absorption of the active material
excluding the metallic absorption of NSPs especially when NSPs are embedded into
the active layer. Figures 7.7 and 7.8 show the spectral enhancement factors (defined