Question E6F09:
What is the efficiency of a photovoltaic cell?  

Choices:  
A: The output RF power divided by the input DC power  
B: The output in lumens divided by the input power in watts  
C: The open-circuit voltage divided by the short-circuit current under full illumination  
D: The relative fraction of light that is converted to current    

Correct Answer is  D: The relative fraction of light that is converted to current  

To understand “efficiency” here, the exam is referring to the optoelectronic (quantum) efficiency of a photovoltaic device.

Why the correct answer is correct (D)
- A photovoltaic cell converts incident light into electrical current. Its efficiency in this context is the fraction of incident light that actually produces charge carriers (current), i.e., the quantum efficiency.
- Quantum efficiency (QE) can be expressed as:
  $$\eta = \frac{\text{electrons per second}}{\text{photons per second}} = \frac{I/q}{P_{\text{opt}}/(h\nu)}$$
  where $I$ is photocurrent, $q$ is electron charge, $P_{\text{opt}}$ is optical power, $h$ is Planck’s constant, and $\nu$ is optical frequency.

Why the other choices are incorrect
- A: “Output RF power divided by input DC power” describes the efficiency of an RF power amplifier, not a photovoltaic cell.
- B: “Lumens per watt” is luminous efficacy for light sources (LEDs, lamps), not PV cells that absorb light.
- C: “Open-circuit voltage divided by short-circuit current” has units of resistance and is not an efficiency. For power-conversion efficiency, you need power out over power in, and PV max power is:
  $$P_{\text{max}} = V_{\text{OC}} \cdot I_{\text{SC}} \cdot FF$$
  where $FF$ is the fill factor (dimensionless, < 1).

Useful formulas and rules of thumb
- Quantum efficiency to responsivity (A/W):
  $$R = \frac{I}{P_{\text{opt}}} = \frac{\eta q}{h\nu} \approx \eta \cdot \frac{\lambda\ (\text{nm})}{1240} \quad \text{A/W}$$
- Power-conversion efficiency (often quoted for solar cells):
  $$\eta_P = \frac{P_{\text{out, electrical}}}{P_{\text{in, optical}}}$$
  Typical crystalline silicon solar panels have $\eta_P \approx 15\%\text{–}25\%$, but the exam’s “efficiency” for a PV cell here refers to the fraction of light converted into current (quantum efficiency).