Question E6F04:
What is the photovoltaic effect?  

Choices:  
A: The conversion of voltage to current when exposed to light  
B: The conversion of light to electrical energy  
C: The effect that causes a photodiode to emit light when a voltage is applied  
D: The effect that causes a phototransistor’s beta to decrease when exposed to light    

Correct Answer is  B: The conversion of light to electrical energy  

Correct answer: B — The conversion of light to electrical energy

Why B is correct
- The photovoltaic effect is the generation of a voltage and current in a material when it is illuminated. Photons create electron–hole pairs that are separated by an internal electric field (e.g., a p–n junction), producing usable electrical energy. This is how solar cells work.

Why the other choices are incorrect
- A: “The conversion of voltage to current when exposed to light” describes the photoconductive effect (increased conductivity under illumination, often with a reverse bias), not the photovoltaic effect which produces an EMF with no external bias.
- C: A device emitting light when a voltage is applied is electroluminescence (e.g., an LED), the inverse process of photovoltaic.
- D: Light typically increases a phototransistor’s conduction by generating base current. A “beta decrease when exposed to light” is not the defining effect and is generally incorrect.

Useful formulas and rules of thumb
- Solar cell I–V relation:
  $$I = I_{ph} - I_0\left(e^{\frac{qV}{nkT}} - 1\right)$$
- Open-circuit voltage:
  $$V_{OC} \approx \frac{nkT}{q}\ln\left(\frac{I_{ph}}{I_0}+1\right)$$
- Short-circuit current:
  $$I_{SC} \approx I_{ph} \propto \text{illumination level}$$
- Photon energy threshold:
  $$E = hf = \frac{hc}{\lambda},\quad E \ge E_g$$
  For silicon, $E_g \approx 1.1\ \text{eV}$, so $\lambda \lesssim 1100\ \text{nm}$.
- Rule of thumb: A silicon solar cell produces about $0.5$–$0.7\ \text{V}$ open-circuit per cell in full sun.