Environmental Engineering Reference
In-Depth Information
7.1 Introduction
Organic solar cells (OSCs) [ 1 - 3 ] have drawn much attentions in recent years, due
to their interesting properties in terms of light incoupling and photocurrent gen-
eration, as well as the prospect of large-area production and low-cost processing.
Many organic semiconductors exhibit very high absorption coefficients, making
them promising for photovoltaic devices. However, short lifetime and diffusion
length of exciton result in ultrathin-active-layer configuration in OSCs with a
typical thickness of a few hundreds nanometers. The configuration limits the light
absorption efficiency, and thereby the power conversion efficiency of OSCs.
Having unique features of tunable resonance and unprecedented near-field
concentration, plasmon is an enabling technique for light manipulation and
management [ 4 - 6 ]. By altering the metallic nanostructure, the properties of
plasmons, in particular their interactions with light, can be tailored, which offers
the potential for developing new types of optoelectronic devices. Meanwhile, the
use of metallic materials with negative permittivity is one of the most feasible
ways of circumventing the fundamental (half-wavelength) limit and achieving
localization of electromagnetic energy (at optical frequencies) into nanoscale.
Breaking the half-wavelength limit has a fundamental significance for the optical
design of thin-film OSCs.
Plasmonic effects allow us to significantly improve the optical absorption of
thin-film OSCs [ 7 ] and promote emerging solar cell technology meeting clean
energy demands. So far, plasmonic nanostructures can offer three principles to
enhance the optical absorption of OSCs. The first one is surface plasmon resonance
by metallic gratings fabricated on the top or bottom of the active layer [ 8 - 17 ]. The
second one is local plasmon resonance by metallic nanoparticles incorporated into
or near the active layer [ 18 - 30 ]. The third one is plasmon coupling and hybrid-
ization, such as surface plasmon resonance coupled with local plasmon resonance
or plasmon resonance coupled with photonic resonance [ 31 - 33 ].
7.2 Resonance Mechanisms
Critically different from the thin-film polycrystalline or amorphous silicon SCs
with active layer thickness of a few microns [ 34 ], the active polymer layer of thin-
film OSCs only has a few hundreds nanometers or even thinner thickness due to an
extreme short exciton diffusion length [ 1 ]. Such thin active layer with low
refractive index induces not only the weak optical absorption of OSCs but also
fundamental (half-wavelength) limit of the optical design. On the one hand, the
strong Fabry-PĂ©rot mode (or waveguide mode) cannot be expected in the ultrathin
active layer. On the other hand, the physical mechanism of near-field concentra-
tion (not far-field scattering) should be taken into account in the design. Taking
full advantage of versatile resonance mechanics is essential to enhance the optical
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