Photovoltaic cells and dye-sensitized solar cells (DSSC)

Nature of science:

Transdisciplinary—a dye-sensitized solar cell, whose operation mimics photosynthesis and makes use of TiO2 nanoparticles, illustrates the transdisciplinary nature of science and the link between chemistry and biology. (4.1)

Funding—the level of funding and the source of the funding is crucial in decisions regarding the type of research to be conducted. The first voltaic cells were produced by NASA for space probes and were only later used on Earth. (4.7)

• Molecules with longer conjugated systems absorb light of longer wavelength.
• The electrical conductivity of a semiconductor increases with an increase in temperature whereas the conductivity of metals decreases.
• The conductivity of silicon can be increased by doping to produce n-type and p- type semiconductors.
• Solar energy can be converted to electricity in a photovoltaic cell.
• DSSCs imitate the way in which plants harness solar energy. Electrons are "injected" from an excited molecule directly into the TiO₂ semiconductor.
• The use of nanoparticles coated with light-absorbing dye increases the effective surface area and allows more light over a wider range of the visible spectrum to be absorbed.

Applications and skills:

• Relation between the degree of conjugation in the molecular structure and the wavelength of the light absorbed.
• Explanation of the operation of the photovoltaic and dye-sensitized solar cell.
• Explanation of how nanoparticles increase the efficiency of DSSCs.
• Discussion of the advantages of the DSSC compared to the silicon-based photovoltaic cell.

• The harnessing of solar energy could change the economic fortunes of countries with good supplies of sunlight and unused land.

Theory of knowledge:

• A conjugated system has some similarities with a violin string. How useful is this metaphor? What are the underlying reasons for these similarities? What role do models and metaphors play in the acquisition of knowledge?

Utilization:

• Syllabus and cross-curricular links:
• Topic 3.2—patterns in ionization energy
• Topic 9.1—redox reactions
• Biology topic 2.9—photosynthesis

Aims:

• Aim 6: Students could build an inexpensive dye-sensitized solar cell and investigate their photovoltaic properties.
• Aim 7: The properties of DSSCs can be best investigated using data loggers.

• The relative conductivity of metals and semiconductors should be related to ionization energies.
• Only a simple treatment of the operation of the cells is needed. In p-type semiconductors, electron holes in the crystal are created by introducing a small percentage of a group 3 element. In n-type semiconductors inclusion of a group 5 element provides extra electrons.
• In a photovoltaic cell the light is absorbed and the charges separated in the silicon semiconductor. The processes of absorption and charge separation are separated in a dye-sensitized solar cell.
• Specific redox and electrode reactions in the newer Grätzel DSSC should be covered. An example is the reduction of l₂/l₃ ^- ions to l^-