Question E6F11:
What is the approximate open-circuit voltage produced by a fully illuminated silicon photovoltaic cell?  

Choices:  
A: 0.5 volts  
B: 0.7 volts  
C: 1.1 volts  
D: 1.5 volts    

Correct Answer is  A: 0.5 volts  

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Correct answer: 0.5 volts

Why it’s correct:
- A single silicon photovoltaic (PV) cell typically produces an open-circuit voltage around $0.5$–$0.6\ \text{V}$ under full sunlight at room temperature. The exam standard answer is $0.5\ \text{V}$.

Key idea:
- Open-circuit voltage is limited by the diode physics of the cell, not by the silicon bandgap directly.

Useful formula:
$$V_{OC} \approx \frac{n k T}{q}\,\ln\!\left(\frac{I_{ph}}{I_s}+1\right)$$
where:
- $n$ = diode ideality factor (≈ 1–2)
- $k$ = Boltzmann’s constant
- $T$ = absolute temperature (K)
- $q$ = electron charge
- $I_{ph}$ = photocurrent
- $I_s$ = diode saturation current

At $25^\circ\text{C}$, $\frac{kT}{q} \approx 25.9\ \text{mV}$, giving typical $V_{OC} \approx 0.5$–$0.6\ \text{V}$.

Why the other choices are incorrect:
- 0.7 volts: This is a common silicon diode forward drop, not the typical PV cell open-circuit voltage. PV $V_{OC}$ is usually lower, about $0.5$–$0.6\ \text{V}$.
- 1.1 volts: Roughly the silicon bandgap energy in electron-volts (≈ 1.12 eV), often confused with voltage. It does not equal the cell’s $V_{OC}$.
- 1.5 volts: Typical for an alkaline battery cell, not a PV cell. You need about three silicon PV cells in series to approach this voltage open-circuit.

Rules of thumb:
- Single Si PV cell: $V_{OC} \approx 0.5$–$0.6\ \text{V}$ at 25°C in full sun.
- Temperature coefficient: $V_{OC}$ drops about $-2\ \text{mV}/^\circ\text{C}$ per cell.
- For panels: series strings of ~36 cells yield $V_{OC} \approx 18\ \text{V}$; $V_{MP} \approx 0.8\,V_{OC}$ (roughly).