3 Plasmonically Enhanced Dye-Sensitized Solar Cells
129
3.2.1 Mechanism of Enhanced Dye Absorption
In contrast to inorganic p-n junction type cells where plasmonically active metal
nanostructures are used to scatter light into the photoabsorbing layer [5], metal
nanoparticles directly increase photoabsorption of the dye in DSSCs by near-field
coupling [13]. The local electric field felt by a dye molecule absorbed on or near
a metal nanoparticle can be hundreds (or even thousands) of times the strength of
the incident field [14]. The strength of the near-field around a nanoparticle depends
upon the energy stored in the plasmon mode and the number and placement of
sharp features in the nanostructure. The stored energy is approximately related to
the absorption cross-section of the particle, which in the quasi-static limit (where the
particles are very small compared to the incident photon wavelength) is given by:
C abs = kIm(ω),
(3.1)
where k is the free space wavevector (2α/ε) and ω is the polarizability of the nanoparticle. The polarizability of a sphere is given by:
ω = 4αa
3 λ m − λ d
λ m + 2λ d
,
(3.2)
where a is the radius of the sphere and λ m and λ d are the dielectric constants of
the metal and surrounding dielectric, respectively. When the real part of the metal
dielectric constant is equal to −2λ d , the denominator of (3.2) approaches zero and
the system is resonant. Increasing the dielectric constant of the surrounding medium
causes the resonance to move to a higher wavelength, or ‘red-shift’. For metals such
as silver and gold, the red-shift moves the resonance away from interband transitions
in the metal. If a plasmonic structure is designed such that the LSPR is near the
interband transitions, the LSPR will be heavily damped due to decay into electronhole pairs. Thus, red-shifting the resonance from interband transitions increases the
polarizability and hence the absorption cross-section of the nanoparticle.
Although the introduction of bare Au nanoparticles into a DSSC has been shown
to increase the cell photocurrent [15], in most DSSCs the use of corrosive redox
mediators such as the iodide/triiodide redox couple requires that the nanoparticles be
protected with a capping layer. Both TiO 2 [16] and SiO 2 [17] capping layers have been
investigated on gold and silver nanoparticles incorporated into DSSCs. Depending
on the thickness of the capping layer, the particle experiences an effective dielectric
environment in between the value of λ c for the capping layer and the value of λ d for
the solution. As the thickness increases the proportion of effective medium dielectric
constant tends towards λ c , and the resonance red-shifts (if λ c is greater than λ d ) in
accordance with Eq. 3.2. Additionally, as the thickness of the capping layer increases,
the distance between the adsorbed dye and the metal particle increases. Because
the electric field associated with the plasmon resonance on the nanoparticle decays
exponentially away from the metal surface, the observed photocurrent decreases. For
129
3.2.1 Mechanism of Enhanced Dye Absorption
In contrast to inorganic p-n junction type cells where plasmonically active metal
nanostructures are used to scatter light into the photoabsorbing layer [5], metal
nanoparticles directly increase photoabsorption of the dye in DSSCs by near-field
coupling [13]. The local electric field felt by a dye molecule absorbed on or near
a metal nanoparticle can be hundreds (or even thousands) of times the strength of
the incident field [14]. The strength of the near-field around a nanoparticle depends
upon the energy stored in the plasmon mode and the number and placement of
sharp features in the nanostructure. The stored energy is approximately related to
the absorption cross-section of the particle, which in the quasi-static limit (where the
particles are very small compared to the incident photon wavelength) is given by:
C abs = kIm(ω),
(3.1)
where k is the free space wavevector (2α/ε) and ω is the polarizability of the nanoparticle. The polarizability of a sphere is given by:
ω = 4αa
3 λ m − λ d
λ m + 2λ d
,
(3.2)
where a is the radius of the sphere and λ m and λ d are the dielectric constants of
the metal and surrounding dielectric, respectively. When the real part of the metal
dielectric constant is equal to −2λ d , the denominator of (3.2) approaches zero and
the system is resonant. Increasing the dielectric constant of the surrounding medium
causes the resonance to move to a higher wavelength, or ‘red-shift’. For metals such
as silver and gold, the red-shift moves the resonance away from interband transitions
in the metal. If a plasmonic structure is designed such that the LSPR is near the
interband transitions, the LSPR will be heavily damped due to decay into electronhole pairs. Thus, red-shifting the resonance from interband transitions increases the
polarizability and hence the absorption cross-section of the nanoparticle.
Although the introduction of bare Au nanoparticles into a DSSC has been shown
to increase the cell photocurrent [15], in most DSSCs the use of corrosive redox
mediators such as the iodide/triiodide redox couple requires that the nanoparticles be
protected with a capping layer. Both TiO 2 [16] and SiO 2 [17] capping layers have been
investigated on gold and silver nanoparticles incorporated into DSSCs. Depending
on the thickness of the capping layer, the particle experiences an effective dielectric
environment in between the value of λ c for the capping layer and the value of λ d for
the solution. As the thickness increases the proportion of effective medium dielectric
constant tends towards λ c , and the resonance red-shifts (if λ c is greater than λ d ) in
accordance with Eq. 3.2. Additionally, as the thickness of the capping layer increases,
the distance between the adsorbed dye and the metal particle increases. Because
the electric field associated with the plasmon resonance on the nanoparticle decays
exponentially away from the metal surface, the observed photocurrent decreases. For