1.3 Photovoltaics

Introduction to Solar Energy

  • The discussion begins with the history and development of solar energy technology.

Historical Foundations of Solar Energy

  • Edmund Becquerel's Discovery

    • Light can be converted into electrical energy.

    • First experiment involved:

    • Silver chloride or silver bromide solution.

    • Platinum electrodes.

    • Current generation upon exposure to light.

  • Charles Fritz and the First Solar Cell (50 years post-Becquerel)

    • Invented the first solar cell.

    • Utilized metal plates.

    • Serendipitous discovery of voltage generation upon light exposure.

    • Noted low efficiency of early solar cells.

  • Development by Bell Labs (1954)

    • Silicon solar cell created with approximately 6% efficiency.

    • Marked the beginning of photovoltaics (PV) technology.

Current Landscape of Solar Energy

  • Top PV Countries in 2024

    • China leads with nearly 1 terawatt of installed photovoltaics.

    • The USA is lower on the list but increasing installations yearly.

  • Global Energy Production

    • In 2024, solar energy accounted for 70% of global energy production, approximately 2 terawatts.

    • The industry reflects exponential growth and substantial financial investments.

Types of Solar Cells

  • First Generation Solar Cells

    • Mostly silicon-based.

    • Common in residential applications and solar parks.

    • Characteristics:

    • Abundant and cost-effective.

    • Longevity of over 30-40 years.

  • Second Generation Solar Cells

    • Include technologies like:

    • Copper Indium Gallium Selenium (CIGS).

    • Multi-junction solar cells.

    • Usage:

    • Primary applications in solar parks rather than residential settings.

    • Greater efficiency for specific environments (high sun concentration).

  • Third Generation Solar Cells

    • Comprise organic photovoltaics and perovskites.

    • Pros and cons:

    • Cheap and abundant materials.

    • Shorter lifespan (around 5 years).

    • Intended for portable or temporary uses.

Applications of Solar Technology

  • Home Integration

    • Solar panels can be retrofitted on roofs if oriented properly (ideally south-facing).

    • Financial impact:

    • Installation around £6,000; payback typically within 10 years.

  • Building Integration

    • Solar cells used as roof tiles and wall panels, generating electricity while serving structural roles.

    • Semi-transparent solar cells allow natural light penetration while generating electricity.

  • Special Applications

    • Utilization in off-grid situations (e.g., powering road signs).

    • Solar parks can match outputs of traditional gas or coal power plants.

    • Mobile applications such as charging devices, portable radios, and solar-powered drones.

Solar Energy Source

  • The primary energy source for solar cells:

    • The sun, a fusion reactor located 150 million km from Earth.

    • Light takes about 8 minutes to reach the Earth.

Mechanism of Solar Cells

  • Single Junction Solar Cells

    • Mechanism involves:

    • Absorption of a photon.

    • Generation of an electron-hole pair.

    • Charge separation facilitated by semiconductor materials.

      • N-type semiconductor: has excess electrons.

      • P-type semiconductor: has holes.

    • Formation of a built-in electric field at the junction.

  • Band Gap and Absorption

    • Example materials: Gallium Arsenide with a band gap of about 1.4 eV (850 nm wavelength).

    • Solar cells can only use photons exceeding the band gap energy; anything shorter is absorbed, while wavelengths longer don't generate energy.

Efficiency and Theoretical Limits

  • Performance Metrics

    • Power output measured via current-voltage (I-V) curves.

  • Shokley-Queisser Limit

    • The maximum theoretical efficiency of solar cells based on their bandgaps:

    • Single-junction cells have limits around 33%, and multi-junction cells up to 45%.

  • Factors Affecting Efficiency

    • Only a portion of sunlight can be absorbed, leading to conditions where efficiency cannot reach 100%.

Solar Cell Construction Techniques

  • Multi-Junction Solar Cells

    • Stack of different semiconductor materials with varied band gaps to absorb broader sunlight spectral range.

  • Manufacturing Process

    • Involves growing crystals, slicing wafers, and layering materials to create the solar cell.

  • Innovation in Solar Cell Production

    • Companies like MicroLink Devices focus on thin, flexible solar cells, utilizing Epitaxial Lift-Off technology to create high-efficiency materials.