Theoretical Studies of Plasmonic Effects in Organic Solar Cells - Organic Solar Cells: Materials and Device Physics - page 193

Fig. 7.6 a, b A schematic OSC nanostructure. To enhance the optical absorption of the OSC, a
spherical chain comprising multiple NSPs is embedded into a spacer layer (PEDOT:PSS) or into
an active layer (P3HT:PCBM). The geometric size is t 1 ¼ 1 : 25D and t 2 ¼ 2 : 5D. The spacing
between adjacent NSPs is d ¼ D or d ¼ 0 : 2D for the separated or close-packed ones,
respectively. The diameter is D ¼ 20 or D ¼ 40 nm for the small or large NSPs, respectively.
The yellow arrows represent the propagation direction of an incident light with a TM polarization
ð H z ¼ 0 Þ and the red dashed lines denote corresponding near-field profiles of NSPs. c Absorption
coefficient of the active material. d, e Near-field polarization current distributions of the OSC
nanostructure at a vertical and an oblique incidences, respectively. 2011 AIP; Ref. [ 30 ]
in Sect. 7.3.4 ). Table 7.1 lists the total enhancement factors (defined in Sect. 7.3.4 )
corresponding to Figs. 7.7 and 7.8 .
When Ag NSPs are embedded into the spacer, as the incident angle increases,
both the spectral and total enhancement factors increase independent of the NSP's
size and spacing. The fundamental physics is that the near-field energy of a metal
nanoparticle is mainly distributed along the polarization direction of incident E-
field, which is critically different from the far-field scattering, where the energy
scatters to the propagation direction of the incident light. As shown in Fig. 7.6 a, d,
the concentrated electric near field is distributed along the lateral direction at the
vertical incidence, which deters the plasmonic resonance from enhancing the light
absorption of the active material even though very strong near field is obtained.
Hence, a careful design in introducing NSPs into a multilayer device structure is
critical for enhancing the performances of plasmonic OSCs. Interestingly,
absorption enhancement improves at the oblique incidence, because more energy
transfers to the active layer having a significant directivity as depicted in Fig. 7.6 e.