Question

The material used for solar cell should have band gap ______.

• A. equal to zero.
• B. less than 1.0 eV (non-zero).
• C. more than 1.8 eV.
• D. between 1.0 eV and 1.8 eV.

Hint:

Band Gap Cheat Sheet:
Conductors = 0 eV
Solar Cells = 1.0 to 1.8 eV (Matches visible light photons!)
LEDs (Visible) = 1.8 to 3.0 eV
Insulators = > 3.0 eV

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
The question asks for the optimal energy band gap ($E_g$) range for a semiconductor material to be efficiently used in manufacturing a solar cell.

Step 2: Detailed Explanation:
Solar cells generate electricity by absorbing photons from sunlight. For a photon to successfully excite an electron from the valence band to the conduction band (creating an electron-hole pair), its energy ($h\nu$) must be strictly greater than the material's band gap ($E_g$).
The sun's emission spectrum peaks heavily in the visible light range ($\sim 1.5$ eV) and extends into the near-infrared.
- If the band gap is too high (e.g., $> 1.8$ eV), the material will be completely transparent to most of the solar spectrum. Most photons simply won't have enough energy to excite electrons, resulting in very low current.
- If the band gap is too low (e.g., $< 1.0$ eV), while many photons are absorbed, the excess energy of the high-energy visible photons is wasted purely as heat (thermalization). Additionally, the output voltage of the solar cell drops drastically.
Theoretical calculations (the Shockley-Queisser limit) show that the absolute maximum theoretical efficiency for a single-junction solar cell peaks when the band gap is roughly 1.3 to 1.4 eV.
Therefore, materials with band gaps ideally between 1.0 eV and 1.8 eV are chosen.
Classic examples include Silicon ($E_g = 1.1$ eV) and Gallium Arsenide (GaAs, $E_g = 1.43$ eV).

Step 3: Final Answer:
The band gap should be between 1.0 eV and 1.8 eV, matching option (d).