Bandgap Energy

Bandgap energy is the amount of energy required to dislodge an electron from its covalent bond and allow it to become part of an electrical circuit.

When light shines on crystalline silicon, electrons within its crystal lattice may be freed. But not all photons—as packets of light energy are called—are created equal.

To free an electron, the energy of a photon must be at least as great as the bandgap energy. However, photons with more energy than the bandgap energy will expend the extra as heat when freeing electrons. So it is important for a photovoltaic (PV) cell to be "tuned"—through slight modifications to the silicon's molecular structure—to optimize the photon energy.

Crystalline silicon has a bandgap energy of 1.1 electron-volts (eV). (An electron-volt is equal to the energy gained by an electron when it passes through a potential of 1 volt in a vacuum.) The bandgap energies of other effective PV semiconductors range from 1.0 to 1.6 eV. In this range, electrons can be freed without creating extra heat.

The photon energy of light varies according to the wavelengths of the light. The entire spectrum of sunlight, from infrared to ultraviolet, covers a range of about 0.5 eV to about 2.9 eV. For example, red light has an energy of about 1.7 eV, and blue light has an energy of about 2.7 eV. Most PV cells cannot use about 55% of the energy of sunlight because this energy is either below the bandgap of the material used or carries excess energy.

Different PV materials have different energy band gaps. Photons with energy equal to the band gap energy are absorbed to create free electrons. Photons with less energy than the band gap energy pass through the material.