Theoretical Studies of Plasmonic Effects in Organic Solar Cells - Organic Solar Cells: Materials and Device Physics - page 177

absorption of OSCs. Noting that the obscure and disparate glossaries describing
the same resonance mechanism, we will focus more on their physical under-
standings and unique features.
Fabry-Pérot mode. The planar multilayer device can support the Fabry-Pérot
mode whose spectrum has symmetric Lorentzian line shape. Considering the
plane-wave excitation in OSC problems, the eigenmode of the multilayer device
cannot be excited due to the momentum or phase mismatch ð b [ k 0 Þ: The Fabry-
Pérot mode can be understood by the mode coupling between the excitation
solution and the eigenmode. The fundamental limit in the optical design of OSCs
forbids the Fabry-Pérot mode bouncing in a single active layer but possibly in
several layers. Compared to thicker amorphous silicon SCs, OSCs support weaker
Fabry-Pérot mode. The nonplanar device structure can also support the Fabry-
Pérot mode if the nonplanar structure can be approximately decomposed as
multiple planar structures [ 15 ].
Quasi-guided mode. The space harmonics in periodic nanostructure provide
additional momentum, so that the eigenmode or guided mode can be excited with
the momentum matching condition of Re ð b Þ¼ k 0 sin h þ 2 P m ; m ¼ 0 ; 1 ;
2 ; ... : Here, we consider a 1-dimensional periodic structure with a periodicity
P and an incident angle h. The complex propagation constant b implies that the
excited guided mode cannot be perfectly trapped in the grating layer but giving
rise to a leaky wave. Arising from the constructive and destructive interference
of a narrow discrete guided mode with a broad continuum (incident light), the
quasi-guided mode [ 35 ] with an asymmetric and narrow Fano line shape has
extraordinary transmittance and reflectance called Wood's anomaly [ 36 - 38 ]. The
pronounced quasi-guided mode enhances the absorption of OSCs; however, its
performance is limited by the narrow bandwidth.
Plasmonic mode. The excitation of plasmons by light is denoted as a surface
plasmon resonance (SPR) for planar surfaces or localized plasmon resonance
(LPR) for nanometer-sized metallic structures [ 39 - 41 ]. SPRs are electromagnetic
excitations propagating at the interface between a dielectric and a metal, eva-
nescently confined in the perpendicular direction. Using light to excite the SPR,
the momentum matching condition can be satisfied by using a periodic structure
with space harmonics or a subscatterer producing evanescent waves. LPRs are
nonpropagating excitations of the conduction electrons of metallic nanostructures
coupled to the electromagnetic field. The curved surface of the particle exerts an
effective restoring force on the driven electrons, so that a resonance can arise at a
specific wavelength independent of wave vector. The half-wavelength limit in the
optical design of OSCs compels researchers to pay more attenuation on the
physical mechanism of near-field concentration (not far-field scattering). Hence,
plasmonic mode, which has unique features of near-field enhancement, is one of
the best candidates to boost the optical absorption of OSCs. The resonance peaks
of plasmonic mode strongly depend on the material, geometry, and surrounding
environment and can be highly tunable and manipulated.