NABCEP Solar PV Fundamentals 4 — Questions and Answers
Question 1: What is the theoretical maximum efficiency limit for a single-junction silicon PV cell under unconcentrated sunlight, known as the Shockley-Queisser limit?
- Around 15%
- Around 29–33% (Correct answer)
- Around 50%
- Around 65%
Correct answer: Around 29–33%
The Shockley-Queisser limit for a single p-n junction under AM1.5 illumination is approximately 29–33%, due to thermalization and transmission losses.
Question 2: What does NOCT stand for and how is it used in PV module performance estimation?
- Normal Operating Cell Temperature — used to estimate cell temperature under realistic outdoor conditions (Correct answer)
- Nominal Output Current Test — used to validate rated current at standard conditions
- Night-time Operating Charge Temperature — used for battery sizing calculations
- Net Operating Conversion Threshold — the minimum irradiance for module operation
Correct answer: Normal Operating Cell Temperature — used to estimate cell temperature under realistic outdoor conditions
NOCT is the cell temperature at 800 W/m², 20°C ambient, and 1 m/s wind; it is used to estimate real-world operating temperatures.
Question 3: What happens to minority carrier lifetime when a silicon solar cell has higher impurity or defect concentrations?
- Minority carrier lifetime increases, improving efficiency
- Minority carrier lifetime decreases, increasing recombination and reducing efficiency (Correct answer)
- Minority carrier lifetime is unaffected by impurities
- Minority carriers become majority carriers, improving conductivity
Correct answer: Minority carrier lifetime decreases, increasing recombination and reducing efficiency
Defects and impurities create recombination centers that reduce minority carrier lifetime, lowering open-circuit voltage and efficiency.
Question 4: Which type of charge carrier moves from the p-type to n-type layer across the depletion region in an illuminated p-n junction solar cell?
- Electrons drift from p-type to n-type
- Holes drift from n-type to p-type
- Photo-generated electrons are swept from p-type to n-type by the built-in electric field (Correct answer)
- Photo-generated holes are swept from p-type to n-type by the built-in electric field
Correct answer: Photo-generated electrons are swept from p-type to n-type by the built-in electric field
The built-in electric field at the depletion region sweeps photo-generated minority electrons from the p-side to the n-side, generating current.
Question 5: When comparing bifacial PV modules to monofacial modules, what is the primary performance advantage of bifacial modules?
- Lower degradation rate under UV exposure
- Additional energy generation from reflected or diffuse light hitting the rear surface (Correct answer)
- Better shading tolerance due to dual electrical connections
- Lower temperature coefficient enabling better hot-weather performance
Correct answer: Additional energy generation from reflected or diffuse light hitting the rear surface
Bifacial modules generate additional power from albedo (reflected) light incident on the rear cell surface, boosting total energy yield.
Question 6: What is potential-induced degradation (PID) in PV modules and how is it typically mitigated?
- Mechanical stress fracturing cells — mitigated by flexible mounting systems
- Electrochemical leakage current degrading cells — mitigated by grounding, module selection, or anti-PID inverter settings (Correct answer)
- Corrosion of bus bars from moisture — mitigated by encapsulant upgrades
- Delamination from UV exposure — mitigated by anti-reflective coatings
Correct answer: Electrochemical leakage current degrading cells — mitigated by grounding, module selection, or anti-PID inverter settings
PID is caused by high-voltage leakage currents through the module's glass and frame, which can be mitigated by grounding strategy, PID-resistant modules, or inverter voltage offset settings.
Question 7: What is the significance of the bandgap energy of a PV semiconductor material?
- It determines the maximum current the cell can deliver
- It sets the minimum photon energy needed to generate an electron-hole pair, affecting what portion of the solar spectrum is used (Correct answer)
- It determines how quickly the module degrades over time
- It controls the internal resistance of the cell
Correct answer: It sets the minimum photon energy needed to generate an electron-hole pair, affecting what portion of the solar spectrum is used
The bandgap energy determines the threshold photon wavelength; photons with less energy pass through unused, while excess energy is lost as heat.
NABCEP Solar PV Fundamentals 4
What is the theoretical maximum efficiency limit for a single-junction silicon PV cell under unconcentrated sunlight, known as the Shockley-Queisser limit?