Theoretical Studies of Plasmonic Effects in Organic Solar Cells - Organic Solar Cells: Materials and Device Physics - page 201

Fig. 7.12 The near-field distributions in the active polymer layer at the wavelengths denoted
with the arrows of Fig. 7.11 a: a 500 nm; b 580 nm; c 800 nm. 2011 OSA; Ref. [ 33 ]

find two eigenvalues of the TM wave closest to the momentum of surface plasmon
polariton (SPP). The SPP propagates at the interface between semi-infinite poly-
mer and Ag half spaces and its dispersion relation is of the form

r

e Ag e pol
e Ag þ e pol

K z ¼ K 0 ð 7 : 48 Þ

Particularly, the eigenvalue pair essentially corresponds to the symmetric and
asymmetric surface wave modes supported by the Ag/polymer/Ag system [ 40 , 86 ].
These modes are also associated with the guided-wave poles of the generalized
reflection coefficient for the planar layered media [ 59 ]. Figure 7.11 b shows the
dispersion relations. At long wavelengths, the dispersion relation of the SPP is
located between those of symmetric and asymmetric modes. Then we calculate the
decay lengths of the surface plasmon waves penetrated into the active material as
plotted in Fig. 7.11 c. The peaks of the decay length of the symmetric mode
strongly coincide with those of the spectral enhancement factor as shown in
Fig. 7.11 a. The incident light excites symmetric surface plasmon waves at the
bottom corners of the cavity and the waves propagate in the active layer bounded
between the metal claddings. A short decay length away from the metal claddings
makes E-fields concentrate at the surfaces of the metals. Contrarily, a long decay
length induces a concentrated E-field in the center of the active material.
Figure 7.12 demonstrates the near-field distributions in the active layer at the
wavelengths denoted with the arrows of Fig. 7.11 a. At 800 nm, the slowly
decaying near field away from the metal claddings leads to the most significant
enhancement.
Third, we study the hybrid plasmonic system, which comprises a plasmonic
cavity coupled with a nanosphere. The optical enhancement shows little
improvements when the cavity is coupled with the dielectric or MC-DS sphere as
illustrated in Fig. 7.11 a. However, a significant enhancement can be achieved if
the cavity is coupled with metal or DC-MS sphere as shown in Fig. 7.11 d.
Employing the cavity coupled with the DC-MS sphere, the optical absorption of
the active polymer material has a 4-fold increase according to the obtained total
enhancement factor (defined in Sect. 7.3.4 ). We calculate the algebraic summation
of the spectral enhancement factor by the uncoupled single cavity and that by the
uncoupled single DC-MS sphere. As shown in Fig. 7.13 , the summation is