Theoretical Studies of Plasmonic Effects in Organic Solar Cells - Organic Solar Cells: Materials and Device Physics - page 178

Mode hybridization. The superradiant plasmonic mode oscillating in-phase can
be realized by the mode coupling and hybridization mechanisms [ 42 , 43 ], which
occur in the close-packed metallic nanostructures. For example, longitudinal modes
in the metallic nanosphere chain or symmetric modes in two coupled metallic plates
can increase the optical absorption of OSCs by several folds [ 30 , 33 ]. Likewise,
the LPR by a metallic nanosphere interacted with the SPR by a metallic plate leads
to coherent near-field enhancement [ 33 ]. Moreover, the plasmonic mode can couple
to the Fabry-Pérot mode through carefully optimizing both device and metallic
structures [ 31 , 32 ]. Mode coupling and hybridization open up a more effective and
hopeful way in the optical design of OSCs.
Intrinsic limit and beyond. The intrinsic limit of plasmonic effects is the bad
spectral overlap between the absorption of active materials and plasmonic reso-
nances. In other words, strong plasmonic resonances only appear in the weak
absorption region of active materials. For example, the absorption peak of the
active material P3HT:PCBM is around 500 nm; however, the embedded into
P3HT:PCBM are respectively at 600 and 650 nm. There are two potential schemes
to go beyond the limit. One is to employ large and lossless dielectric concentrators
[ 44 - 46 ]. Nevertheless, embedding too large dielectric scatterers seems not to be
practical in fabricating thin-film OSCs. Exploring the mode hybridization mech-
anism is an alternative way to overcome the difficulty. By tuning the thicknesses of
active and spacer layers, plasmon-coupled Fabry-Pérot mode can overlap with the
absorption peak of active materials [ 16 ]. Symmetry breaking and retardation
effects allow us to excite the dark modes or high-order modes in the vicinity of
metallic structures with broadband and strong resonances [ 47 ].
7.3 Theoretical Model
7.3.1 Comparisons of Various Models
Computational electromagnetics [ 48 ], which are used for modeling the interaction
of electromagnetic fields with physical objects and surrounding environment, play
an important role in characterizing and optimizing the optical design of OSCs.
A rigorous, fast, and efficient solution to Maxwell's equations facilitates under-
standing the underlying device physics, reducing the experimental cost, and
accelerating the research and development period. With the aid of state-of-the-art
methods, the critical physical quantities in the optical design of OSCs, such as
optical absorption of active material, can be illustrated for observation and ana-
lyzed for optimization. It is highly desirable to know the strengths and weaknesses
of various theoretical methods in modeling the optical properties of OSCs.
Time-domain methods versus frequency-domain methods. Most optical
materials are dispersive, therefore, a recursive convolution method [ 49 ] or a piece-
wise linear recursive convolution method [ 50 ] must be adopted for time-domain