Theoretical Studies of Plasmonic Effects in Organic Solar Cells - Organic Solar Cells: Materials and Device Physics - page 190

The rapid and extraordinary variations in the absorption over narrow frequency
bands imply the resonant Wood's anomalies with asymmetric Fano line shape [ 36 ,
37 ]. Figure 7.4 a shows the E-field of the absorption peak pointed by the arrow in
Fig. 7.3 a for the Wood's anomalies. The intense E-field in the active layer is
bounded between the periodic nanostrip pattern and the BCP layer. The E-field is
related to the quasi-guided modes supported by the periodic strip structure with the
phase-matching condition of k 0 sin h þ 2pm = P ¼ Re ðÞ; m ¼ 0 ; 1 ; 2 ; ... ;
where P is the periodicity, k 0 is the incident wave number, and b is the complex
propagation constant of the quasi-guided modes. The condition is easily satisfied
in standard OSCs in view of the broadband and wide angle Sun illumination. For
confirming the forced-resonance behavior of the Wood's anomalies, we calculate
the averaged power density flowing in the x direction for the Au-PEDOT:PSS-Au
pattern (strip structure) and PEDOT:PSS layer (nonstrip structure). For the strip
structure, as shown in the inset of Fig. 7.3 a, the negative (opposite direction)
power density peak induced by the n ¼ 1 space harmonics coincides with the
absorption peak at h ¼ 40 and 460 nm. In addition, the spectral overlap between
the Wood's anomalies and the material absorption of C 60 will be of a good help for
short wavelength photon harvesting. Particularly, we can observe the sharp
oscillations of the enhancement factor at h ¼ 80 from 510 to 550 nm. The sharp
oscillations result from the overlap between the Fabry-Pérot mode in the nonstrip
structure and the quasi-guided mode in the strip structure. It should be noted that
the quasi-guided mode is an eigenmode of Maxwell's equations for arbitrary
periodic structure and cannot be excited in the planar nonstrip structure by the
plane wave due to the momentum mismatch ð b [ k 0 Þ: However, the Fabry-Pérot
mode can be found in the planar structure, and can be understood by the mode
coupling between the excitation solution and the eigenmode.
For the TM polarization, a broadband absorption enhancement is obtained from
650 to 800 nm, especially at the oblique angles. Figure 7.4 b shows the H-field of
the absorption peak pointed by the arrow in Fig. 7.3 b. The concentrated H-field at
the interface between Au and CuPc layers is due to the SPRs excited by the
evanescent waves produced by the subwavelength nanostrips. Regarding the
absorption peaks around 750-800 nm in Fig. 7.3 b, they are blue shifted as the
incident angle increases due to the blue shift of the plasmon-coupled Fabry-Pérot
mode. By studying the SPR spectrum (Fig. 7.3 b; black straight-dotted line) and the
absorption spectrum of CuPc (Fig. 7.2 b; red dashed line) together, it is observed
that the SPR peak at 675 nm is weaker than that at 755 nm because of the stronger
absorption of CuPc at 675 nm.
Figure 7.5 a, b shows the total absorptivity defined in Eq. ( 7.31 ) as a function of
the incident angle, respectively, for the TE and TM polarizations. The ideal total
absorptivity governed by the generalized Lambert's cosine law is calculated by
A 0 cos h, where A 0 is the total absorptivity under the vertical incidence condition.
The total absorptivity of the strip structure is noticeably better than that of the
nonstrip structure for both polarizations. The improvements are caused by the
Wood's anomalies and the SPRs that have been explained previously. When the
incident angle increases, the total absorptivity for the TM polarization decays