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M. B. Ross et al.
3.1 Introduction
Increasing population and economic growth continue to drive global energy
consumption. Though fossil fuels, including natural gas, coal, tar sands, and oil,
could meet these demands; increasing concern about the sustainability of these energy sources has led to the invigoration of support for renewable energy sources.
Solar energy provides an alternative and abundant source of energy. More solar energy is transferred to the surface of the earth each hour than is consumed globally
each year [1]. However, module costs for solar cells must decrease to approximately
US$ 0.50 per peak Watt without subsidies for solar to be competitive with traditional
fossil fuels [2].
The efficiency of single-junction photovoltaic (PV) cells is intrinsically limited
by the band gap of the absorbing medium. In the case of silicon, with a band gap of
1.1 eV, much of the energy of ultraviolet (UV) and visible wavelength photons is lost
to the mismatch between photon energy and the band gap, limiting the maximum
efficiency to about 34 % [3]. Multi-junction cells can be used to improve the match
between photon energy and the band gap of the absorber. The efficiency record as of
2012 for a multi-junction p-n photovoltaic is 43.5 % for a 3-junction InGa cell with
a 306× solar concentrator [4].
An alternative to increasing efficiency is a dramatic reduction in manufacturing
cost for low efficiency devices; this has led to a steady increase in thinfilm PV
research, with recent devices utilizing hydrogen ion implantation and annealing to
produce 20 µm thick single crystal silicon substrates. The optical density of thin
film PV systems is low and efficiency drops with reduced thickness. Both localized
surface plasmons and surface plasmon polaritons have been utilized in thin film
PV devices to scatter light into the absorbing semiconductor and confine it there,
thereby increasing the path length of light in the cell, increasing the optical density
and photoelectron generation [5].
Dye-sensitized solar cells (DSSCs) present an attractive alternative to traditional
inorganic devices because the components can be of low purity and the manufacturing
process is conducive to high throughput printing. The basic components of a DSSC
are presented in Fig. 3.1. A sensitizing dye is chemisorbed to a mesoporous film
of wide band gap semiconductor, usually TiO 2 , which is attached to a transparent
conducting oxide film. Light is absorbed by the dye and a photoexcited electron is
transferred to the semiconductor and then to the electrode. A redox couple (usually
iodide/tri-iodide) then reduces the dye to the ground state and carries the hole to the
counter electrode.
Since the first DSSC by O’Regan and Grätzel in 1991, the field has expanded
greatly; however, until recently, efficiencies had stalled at around 9 % [6]. This is
in part due to the complexity of having separate materials responsible for the light
absorption and the electron/hole transport. However, the diversity of components
is attractive because novel materials can easily be incorporated into a variety of
photo-collection systems to understand and maximize their effect. For a review of
the diversity of dyes and electron/hole transport materials, see Ref. [4]. Recent ad-
M. B. Ross et al.
3.1 Introduction
Increasing population and economic growth continue to drive global energy
consumption. Though fossil fuels, including natural gas, coal, tar sands, and oil,
could meet these demands; increasing concern about the sustainability of these energy sources has led to the invigoration of support for renewable energy sources.
Solar energy provides an alternative and abundant source of energy. More solar energy is transferred to the surface of the earth each hour than is consumed globally
each year [1]. However, module costs for solar cells must decrease to approximately
US$ 0.50 per peak Watt without subsidies for solar to be competitive with traditional
fossil fuels [2].
The efficiency of single-junction photovoltaic (PV) cells is intrinsically limited
by the band gap of the absorbing medium. In the case of silicon, with a band gap of
1.1 eV, much of the energy of ultraviolet (UV) and visible wavelength photons is lost
to the mismatch between photon energy and the band gap, limiting the maximum
efficiency to about 34 % [3]. Multi-junction cells can be used to improve the match
between photon energy and the band gap of the absorber. The efficiency record as of
2012 for a multi-junction p-n photovoltaic is 43.5 % for a 3-junction InGa cell with
a 306× solar concentrator [4].
An alternative to increasing efficiency is a dramatic reduction in manufacturing
cost for low efficiency devices; this has led to a steady increase in thinfilm PV
research, with recent devices utilizing hydrogen ion implantation and annealing to
produce 20 µm thick single crystal silicon substrates. The optical density of thin
film PV systems is low and efficiency drops with reduced thickness. Both localized
surface plasmons and surface plasmon polaritons have been utilized in thin film
PV devices to scatter light into the absorbing semiconductor and confine it there,
thereby increasing the path length of light in the cell, increasing the optical density
and photoelectron generation [5].
Dye-sensitized solar cells (DSSCs) present an attractive alternative to traditional
inorganic devices because the components can be of low purity and the manufacturing
process is conducive to high throughput printing. The basic components of a DSSC
are presented in Fig. 3.1. A sensitizing dye is chemisorbed to a mesoporous film
of wide band gap semiconductor, usually TiO 2 , which is attached to a transparent
conducting oxide film. Light is absorbed by the dye and a photoexcited electron is
transferred to the semiconductor and then to the electrode. A redox couple (usually
iodide/tri-iodide) then reduces the dye to the ground state and carries the hole to the
counter electrode.
Since the first DSSC by O’Regan and Grätzel in 1991, the field has expanded
greatly; however, until recently, efficiencies had stalled at around 9 % [6]. This is
in part due to the complexity of having separate materials responsible for the light
absorption and the electron/hole transport. However, the diversity of components
is attractive because novel materials can easily be incorporated into a variety of
photo-collection systems to understand and maximize their effect. For a review of
the diversity of dyes and electron/hole transport materials, see Ref. [4]. Recent ad-