Question E6F01:
What absorbs the energy from light falling on a photovoltaic cell?  

Choices:  
A: Protons  
B: Photons  
C: Electrons  
D: Holes    

Correct Answer is  C: Electrons  

To understand photovoltaic (PV) action: incident light photons transfer their energy to electrons in the semiconductor. If a photon has enough energy to overcome the bandgap, it excites a valence electron into the conduction band, creating an electron-hole pair. The built-in field separates them and current flows.

Why the correct answer is correct:
- Electrons: Electrons in the semiconductor absorb the photon’s energy, gaining enough energy to jump from the valence band to the conduction band. This is the core of the photovoltaic effect.

Why the other choices are incorrect:
- Protons: These are in atomic nuclei and do not participate in the photovoltaic absorption process.
- Photons: Photons are the energy carriers that are absorbed; they don’t “absorb” energy in the cell—rather, they are absorbed by electrons.
- Holes: A hole is the absence of an electron. Holes are created as a consequence of electron excitation, but they do not absorb the photon’s energy.

Useful formulas and rules of thumb:
- Photon energy:
  $$E_{\text{photon}} = hf = \frac{hc}{\lambda}$$
  where $h$ is Planck’s constant, $f$ is frequency, $c$ is the speed of light, and $\lambda$ is wavelength.
- Absorption condition for PV generation:
  $$E_{\text{photon}} \ge E_g$$
  where $E_g$ is the semiconductor bandgap energy.
- Maximum open-circuit voltage per junction (ideal upper bound):
  $$V_{OC} \lesssim \frac{E_g}{q}$$
  where $q$ is the electron charge.
- Rule of thumb: Each absorbed photon with $E_{\text{photon}} \ge E_g$ ideally creates one electron-hole pair; photocurrent is roughly proportional to light intensity.