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Battery and energy materials

For a single-junction solar cell, there is an optimum band gap of the absorber semiconductor that maximizes the theoretical efficiency (about 1.1–1.4 eV under terrestrial sunlight). Which is the correct reason?

  1. AA small band gap gives a low voltage and loses excess energy as heat, while a large one leaves more photons unabsorbed
  2. BThe larger the band gap, the more photons are absorbed, but mobility falls, so the current decreases
  3. CThe smaller the band gap, the less recombination, but because sunlight has the same intensity at all wavelengths, the gain levels off somewhere
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Answer: A. A small band gap gives a low voltage and loses excess energy as heat, while a large one leaves more photons unabsorbed

A photon is absorbed only when its energy is at least the band gap Eg, and the energy in excess of Eg is handed to the lattice as heat by the carriers (thermalization loss). In addition, the open-circuit voltage cannot exceed Eg/q. With a small Eg more photons are absorbed but less energy can be extracted per photon; with a large Eg the voltage is higher, but photons with energy below Eg are not absorbed and pass through. Balancing these, the Shockley–Queisser theoretical efficiency under standard terrestrial sunlight (AM1.5) is high, about 32–33%, over the range of about 1.1–1.4 eV, with a maximum of about 33% at about 1.34 eV. The statement that a larger Eg increases absorption is backward. The intensity of sunlight varies greatly with wavelength and is not constant.

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