Why Solar Panels Can’t Get Much More Efficient (And That’s Okay!)

Shockley-Queisser and the limits to converting sunlight into electricity

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Ground-mount solar panels in Jurupa Valley, CA. Image by US Department of Energy (US government work).
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LEDs and solar cells are just the inverses of each other! Left side of figure US government work; overall figure by the author.
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Periodic table of elements and the first few rows highlighted: As we move across columns from left to right, we add electrons to the element’s outermost shell, until a stable, full valence is reached on the right. By virtue of their position in a common column, carbon and silicon have similar chemistries. Periodic table adapted from https://pubchem.ncbi.nlm.nih.gov/periodic-table/ (US government); overall figure by the author. All remaining figures by the author.
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  1. p-type (e.g. boron), which creates holes in the lattice.
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  1. Radiative recombination: Some electrons randomly fill holes before they can generate a useful current, and about 25% of light energy is lost to this.
  2. Current impedance: About 14% of energy is lost via resistance to current flow.
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The left-hand figure gives the solar spectrum in terms of wavelengths hitting the surface of the Earth (as well as the “top of the atmosphere”). On the right side, the energy of a photon with any given wavelength is indicated by the blue curve: Any electron with a wavelength above about 1,150 nm carries less than 1.1 eV, and so its energy is lost. For electrons with shorter wavelengths, only 1.1 eV is used for photoexcitation, and so the energy gap between 1.1 eV and whatever the photon carries is wasted. The inscribed spectrum is simply for reference (not to scale).
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Energy losses and “spectral efficiency” based on a standard solar spectrum and a band gap energy of 1.1 eV.
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Key Points

  • Solar cells are based on p-n semiconductor junctions and the photoelectric effect
  • Sunlight creates “electron-hole pairs” in a silicon matrix via the photoelectric effect
  • Dopants in this matrix create a permanent charge separation in the matrix
  • This allows free electrons generated by photoelectric effect to flow across an external load and generate useful current
  • Band-gap = photon energy required to create electron-hole pair (measured in eV). If an incoming photon has…
    Less energy = no effect
    Any excess energy = wasted
  • 1.1 eV for silicon is close to optimum for the Sun’s light spectrum
  • Spectral losses + radiative recombination + circuit resistance limit theoretical PV efficiencies
  • Silicon PV is limited to 32% efficiency
  • Typical commercial tech (15–20%) is already amazing

References

[1] Shockley, W., & Queisser, H. J. (1961). Detailed balance limit of efficiency of p‐n junction solar cells. Journal of applied physics, 32(3), 510–519.

Academic with a background in medicine, mathematics, and engineering (MD,PhD). Interested in agriculture and how consumption drives global environmental change.

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