Question E6F10:
What is the most common material used in power-generating photovoltaic cells?  

Choices:  
A: Selenium  
B: Silicon  
C: Cadmium sulfide  
D: Indium arsenide    

Correct Answer is  B: Silicon  

Correct Answer: Silicon

Why silicon is correct:
- Silicon is by far the most common photovoltaic (PV) material due to its abundance, mature manufacturing, and good efficiency.
- Its bandgap is about $E_g \approx 1.12\,\text{eV}$, close to the single-junction optimum (roughly $1.1$–$1.4\,\text{eV}$) for the solar spectrum, giving a good balance of voltage and current.
- It’s used in crystalline (mono/poly) and amorphous forms for power-generating solar panels.

Why the other choices are incorrect:
- Selenium: Used in early photo cells and photoconductors; low efficiency and largely obsolete for power generation.
- Cadmium sulfide (CdS): Commonly used as a “window” or buffer layer in thin-film cells (e.g., CdTe), not as the main power-generating absorber.
- Indium arsenide (InAs): Very narrow bandgap suited for infrared detectors; too low bandgap for efficient single-junction solar power (low voltage, high thermal losses).

Useful formulas / rules of thumb:
- Bandgap–wavelength relation:
  $$E_g\text{ (eV)} \approx \frac{1240}{\lambda_g\text{ (nm)}}$$
  For silicon: $E_g \approx 1.12\,\text{eV} \Rightarrow \lambda_g \approx 1100\,\text{nm}$.
- Open-circuit voltage (order-of-magnitude guide):
  $$V_{OC} \approx \frac{kT}{q}\,\ln\!\left(\frac{I_{ph}}{I_s}+1\right)$$
  Silicon’s bandgap supports practical $V_{OC}$ values for efficient power conversion in single-junction cells.

Bottom line: Silicon dominates mainstream power-generating PV technology.