286
J. B. Liyanage et al.
Keywords Titanium dioxide · Photoanode · Dye-sensitized solar cells ·
Nanostructures
15.1 Introduction
The increasing demand for energy in conjunction with our dependence on fossil fuel
has fast-tracked the reduction of the oil reserves of the earth. Moreover, the use of
fossil fuel has resulted in severe ecological contamination and change. Renewable
energy sources promise to alleviate both these problems and among these sources,
solar energy has garnered much interest.
Photovoltaic cells, which convert solar radiation into electric current, are in the
forefront of plans for sustainable energy. Presently, first generation solar cells based
on crystalline or multi-crystalline Si hold a majority (>90%) of the market-share of
solar cells. However, the adoption of Si photovoltaic cells has not reached desired
levels, and research into more commercially viable technologies continues. Dye sensitized solar cells (DSCs) are third-generation, semiconductor photovoltaic devices
that incorporate dyes to mediate efficient conversion of radiation. DSCs are considered a promising alternative (and complement) to single crystal silicon solar cells,
due to their lower manufacture cost, easy fabrication on flexible substrates, and performance at ambient temperatures.
Although research into several materials has been reported, TiO 2 remains the
most widely utilized photo-anode material for DSCs. In this chapter, we discuss
modifications to the TiO 2 component which have been reported, and their effect on
DSC performance. We begin with a general treatise on DSCs, and then move on to
reviewing literature on modifications to TiO 2 . Finally, we briefly discuss alternative
semiconductor materials to TiO 2 , in terms of DSC function.
A contemporary DSC utilizes five components for operation, namely;
1. Transparent conductive oxide, the anode of the device. This is the side of the
device receiving solar radiation.
2. Semiconductor, deposited on the anode. This layer activates electronic conduction.
3. Sensitizer, a charge transfer dye bonded to the semiconductor.
4. Electrolyte, a redox mediator which restores the dye with each cycle.
5. Catalyst, coated on the cathode, which accelerates the redox reaction.
An illustration of the electron flow in a working DSC is given in Fig. 15.1. When
the DSC is exposed to sunlight, electrons in the sensitizer can absorb a photon with
energy hν, and excite to the LUMO. Dye sensitizers are chosen such that the LUMO
has high energy, and excited electrons can inject into the conduction band (CB) of
the working electrode (semiconductor) material. These electrons diffuse through the
anode and can be used to drive an external circuit. Electrons returning from the
counter electrode can reduce oxidized molecules of the redox-couple, which in turn
undergo a redox reaction with dye, regenerating it and completing the cycle.
J. B. Liyanage et al.
Keywords Titanium dioxide · Photoanode · Dye-sensitized solar cells ·
Nanostructures
15.1 Introduction
The increasing demand for energy in conjunction with our dependence on fossil fuel
has fast-tracked the reduction of the oil reserves of the earth. Moreover, the use of
fossil fuel has resulted in severe ecological contamination and change. Renewable
energy sources promise to alleviate both these problems and among these sources,
solar energy has garnered much interest.
Photovoltaic cells, which convert solar radiation into electric current, are in the
forefront of plans for sustainable energy. Presently, first generation solar cells based
on crystalline or multi-crystalline Si hold a majority (>90%) of the market-share of
solar cells. However, the adoption of Si photovoltaic cells has not reached desired
levels, and research into more commercially viable technologies continues. Dye sensitized solar cells (DSCs) are third-generation, semiconductor photovoltaic devices
that incorporate dyes to mediate efficient conversion of radiation. DSCs are considered a promising alternative (and complement) to single crystal silicon solar cells,
due to their lower manufacture cost, easy fabrication on flexible substrates, and performance at ambient temperatures.
Although research into several materials has been reported, TiO 2 remains the
most widely utilized photo-anode material for DSCs. In this chapter, we discuss
modifications to the TiO 2 component which have been reported, and their effect on
DSC performance. We begin with a general treatise on DSCs, and then move on to
reviewing literature on modifications to TiO 2 . Finally, we briefly discuss alternative
semiconductor materials to TiO 2 , in terms of DSC function.
A contemporary DSC utilizes five components for operation, namely;
1. Transparent conductive oxide, the anode of the device. This is the side of the
device receiving solar radiation.
2. Semiconductor, deposited on the anode. This layer activates electronic conduction.
3. Sensitizer, a charge transfer dye bonded to the semiconductor.
4. Electrolyte, a redox mediator which restores the dye with each cycle.
5. Catalyst, coated on the cathode, which accelerates the redox reaction.
An illustration of the electron flow in a working DSC is given in Fig. 15.1. When
the DSC is exposed to sunlight, electrons in the sensitizer can absorb a photon with
energy hν, and excite to the LUMO. Dye sensitizers are chosen such that the LUMO
has high energy, and excited electrons can inject into the conduction band (CB) of
the working electrode (semiconductor) material. These electrons diffuse through the
anode and can be used to drive an external circuit. Electrons returning from the
counter electrode can reduce oxidized molecules of the redox-couple, which in turn
undergo a redox reaction with dye, regenerating it and completing the cycle.