13.1 Nanostructured solar cells

Nanostructured solar cells are revolutionizing energy harvesting. These innovative designs use quantum dots, dyes, perovskites, and nanowires to boost light absorption and efficiency. They're pushing the limits of traditional solar technology, offering exciting possibilities for clean energy.

Advanced solar cell designs like multi-junction and tandem cells are taking things further. By stacking different materials and using clever light-trapping techniques, these cells are achieving record-breaking efficiencies. It's a game-changer for solar power's future.

Nanostructured Solar Cell Types

Quantum Dot and Dye-Sensitized Solar Cells

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• Quantum dot solar cells utilize semiconductor nanocrystals to absorb light and generate electron-hole pairs
  • Quantum dots offer tunable bandgaps allowing for optimization of light absorption across the solar spectrum
  • Fabrication involves depositing quantum dots onto a conductive substrate (titanium dioxide)
  • Advantages include low-cost production and potential for high efficiencies
• Dye-sensitized solar cells employ organic dye molecules to absorb light and inject electrons into a semiconductor
  • Structure consists of a photoanode (dye-sensitized titanium dioxide), electrolyte, and counter electrode
  • Ruthenium-based dyes commonly used for their broad absorption spectrum
  • Benefits include flexibility, transparency, and performance under low-light conditions

Perovskite and Plasmonic Solar Cells

• Perovskite solar cells use hybrid organic-inorganic materials with a specific crystal structure
  • Perovskite materials (methylammonium lead halides) exhibit excellent light absorption and charge transport properties
  • Rapid efficiency improvements achieved in recent years, surpassing 25% in laboratory settings
  • Challenges include stability issues and the presence of lead in most efficient formulations
• Plasmonic solar cells incorporate metallic nanoparticles to enhance light absorption
  • Surface plasmon resonance concentrates light near the metal-semiconductor interface
  • Gold and silver nanoparticles commonly used due to their strong plasmonic effects
  • Plasmonic enhancement can increase absorption in thin-film solar cells, improving efficiency

Nanowire Solar Cells

• Nanowire solar cells employ arrays of vertically aligned semiconductor nanowires
  • Nanowires provide direct pathways for charge transport, reducing recombination losses
  • Enhanced light trapping due to the nanowire geometry increases absorption
  • Materials used include silicon, gallium arsenide, and indium phosphide
  • Potential for high efficiency and material savings compared to traditional planar cells

Advanced Solar Cell Designs

Multi-Junction Solar Cells

• Multi-junction solar cells stack multiple p-n junctions with different bandgaps
  • Each junction absorbs a specific portion of the solar spectrum, maximizing overall efficiency
  • Typical structure includes top cell (gallium indium phosphide), middle cell (gallium arsenide), and bottom cell (germanium)
  • Achieves record efficiencies exceeding 40% under concentrated sunlight
  • Used in space applications and terrestrial concentrator photovoltaic systems

Tandem Solar Cells

• Tandem solar cells combine two or more subcells with complementary absorption spectra
  • Subcells connected in series or parallel depending on the design
  • Perovskite-silicon tandems show promise for high efficiency and cost-effective production
  • Four-terminal tandem designs allow for independent operation of each subcell
  • Challenges include current matching and interface engineering between subcells

Efficiency Enhancement Techniques

Light Trapping Strategies

• Light trapping increases the optical path length within the solar cell
  • Textured surfaces scatter light, increasing the chance of absorption
  • Antireflective coatings reduce reflection losses at the cell surface
  • Plasmonic nanostructures concentrate light in specific regions of the cell
  • Photonic crystals can be used to manipulate light propagation within the device
• Back reflectors redirect unabsorbed light for a second pass through the active layer
  • Metallic (silver) or dielectric (distributed Bragg reflector) back reflectors commonly used

Carrier Multiplication and Advanced Concepts

• Carrier multiplication generates multiple electron-hole pairs from a single high-energy photon
  • Occurs in quantum dots and other nanostructures due to quantum confinement effects
  • Potential to exceed the Shockley-Queisser limit for single-junction solar cells
  • Challenges include efficiently extracting the additional carriers before recombination
• Hot carrier extraction aims to collect carriers before thermalization
  • Requires ultrafast extraction and specialized contact materials
  • Potential to significantly increase cell efficiency by reducing thermalization losses
• Intermediate band solar cells introduce an additional energy level within the bandgap
  • Allows for absorption of sub-bandgap photons, increasing current generation
  • Realized through quantum dots or highly mismatched alloys