What is band gap and why does it matter?

The band gap is the energy difference (in electron volts, eV) between the highest occupied electronic state and the lowest unoccupied state in a solid material.

Why It Matters

The band gap determines a material's fundamental electronic behavior:

• Zero band gap (metals): Electrons flow freely, making the material a conductor (e.g., copper, iron)
• Small band gap (0.1-1.5 eV): Semiconductor behavior; used in transistors, solar cells, infrared detectors
• Medium band gap (1.5-3.5 eV): Wider semiconductors for LEDs, power electronics, visible-light photodetectors
• Large band gap (> 3.5 eV): Insulators and ultrawide-gap semiconductors for high-voltage devices, UV optics

For Solar Cells

The optimal band gap for a single-junction solar cell is approximately 1.34 eV (the Shockley-Queisser limit). Materials with band gaps of 1.0-1.8 eV are prime candidates for photovoltaic absorbers.

DFT Caveat

Band gaps computed with standard DFT (GGA/PBE) are systematically underestimated by 30-50%. Use them for relative comparisons and screening, not as absolute values.