Frontiers in Chemistry - Solar Cells

Frontiers in Chemistry - CHEM1008

• Lecture 4: Solar Cells
• Lecturer: Darren Walsh
• Location: Room A09, GSK Carbon Neutral Laboratory for Sustainable Chemistry

Lecture 4 Learning Outcomes

• Students will understand the massive challenge of generating enough energy for future generations
• Students will appreciate that conventional methods cannot meet future energy demand and that solar power could be a solution
• Students will understand why semiconductors can conduct electricity using band theory
• Students will comprehend how P-N junction photovoltaics function and their limitations
• Students will be aware of other emerging technologies with the potential to improve solar cell efficiencies

Energy Demand

• Global energy consumption is approximately 500 x 10¹⁸ joules, equivalent to an average of 16 terawatts (TW)
• This energy consumption is expected to double by 2050 and triple by 2100
• Current alternative energy sources (Biomass, Nuclear, Wind, Hydroelectric) are insufficient to meet the projected energy demand.
  • Calculated potential from these sources combined is roughly 21 TW.

Solar Power

• Solar power provides ~1360 W m⁻² outside Earth's atmosphere.
• Only ~175 W m⁻² reaches Earth's surface after passing through and reflecting off the atmosphere. This varies with the angle of the sun.
• This represents 89,000 TW of potentially usable solar power
• This is sufficient to meet current global energy needs in a relatively short period (1.5 hours).

Land Requirements for Solar Power

• To generate the current global energy needs, 860,000 km² of land area would need to be covered with 10% efficient solar energy conversion systems
• This is roughly equivalent to the land area of Venezuela.

Harvesting Solar Energy - An Old Example

• Ancient Greeks, specifically Archimedes, are credited with demonstrating the use of mirrors to focus solar energy to damage enemy ships.

Harvesting Solar Energy - Modern Methods

Different methods for harvesting solar energy: direct (PV and solar thermal) and indirect (Ocean thermal, wave, wind, biomass, and hydroelectric)

The Solar Spectrum

• The maximum intensity of solar energy occurs at around 1eV photon energy.
• An "air-mass" 1.5 spectrum is used as a standard spectrum.

Photovoltaics - Band Theory

• Molecular Orbital (MO) theory explains the overlapping of atomic orbitals to produce bonding and antibonding orbitals.
• In semiconductors, the bonding and antibonding orbitals combine to form energy bands.
• Understanding band theory is crucial for designing efficient solar cells.
  • The Fermi level represents the highest occupied molecular orbital at 0K.

Band Gap Width

• The band gap width in semiconductors influences their conductivity.
• Metals have overlapping valence and conduction bands.
• Semiconductors have a narrow band gap, allowing for conductivity through thermal excitation.
• Insulators have wide band gaps, exhibiting no electrical conductivity under normal conditions except at very high temperatures.

Intrinsic and Extrinsic Semiconductors

• Intrinsic Semiconductors (like silicon and germanium): Naturally have low band gaps and few charge carriers.
• Extrinsic Semiconductors: Created by doping, adding impurities, so they exhibit enhanced conductivity.
  • N-type doping introduces impurities with more valence electrons than the host material.
  • P-type doping introduces impurities with fewer valence electrons than the host material.

P-N Junction Photovoltaics

• Contacting P-type and N-type semiconductors creates a depletion region with a built-in electric field.
• When sunlight strikes the junction with photons, electron-hole pairs are generated.
• The field separates the electrons and holes, creating a potential difference that can be harnessed via a circuit.

What Should We Make Our Cells From?

• Si (band gap = 1.1 eV) and GaAs (band gap = 1.4 eV) are suitable for solar cells because their band gaps have high solar intensity.
• Single-crystal silicon solar cells are effective but costly.
• Polycrystalline silicon cells are somewhat less efficient but cheaper to make.
• Amorphous silicon is the cheapest per watt, with a lower efficiency.

Newer Technologies for Increasing Efficiency

• Recent advancements in solar cell technology are continuously improving efficiency.

What is Limiting Maximum Efficiency?

• Approximately 23% of sunlight is reflected or passes through, while 54% is lost as heat.
• Sunlight with a spectrum exceeding 1.1 μm is not converted to electricity.

Using PVs to Store Energy

• PV cells act as small batteries with voltages as low as 0.58 V.
• Combining multiple solar cells in series creates higher voltages suitable for charging batteries (such as 12 V)

This quiz covers key concepts from Lecture 4 of CHEM1008, focusing on solar cells and their role in meeting future energy demands. Students will explore the limitations of conventional methods and understand how solar power and semiconductor technology, specifically P-N junction photovoltaics, could provide sustainable energy solutions.