Solar Cell Interview Questions

Q1. Explain the photovoltaic effect and how a solar cell generates electricity.

The photovoltaic effect is the generation of a voltage and current in a semiconductor p-n junction when photons with energy greater than the bandgap are absorbed, creating electron-hole pairs that are separated by the built-in electric field at the junction. In a silicon solar cell, photons with wavelength below 1100 nm (above 1.12 eV bandgap) generate electron-hole pairs; the built-in field sweeps electrons to the n-side and holes to the p-side, creating a forward bias voltage. The open-circuit voltage of a typical silicon cell is around 0.6 V because it is limited by recombination, not by the full bandgap.

Follow-up: Why is the open-circuit voltage of a silicon solar cell much less than the silicon bandgap of 1.12 eV?

Q2. Describe the I-V characteristic of a solar cell and identify its key parameters.

The I-V curve of a solar cell is a shifted diode curve: I = I_L - I_0 (e^(qV/nkT) - 1), where I_L is the photocurrent, I_0 is the dark saturation current, and n is the ideality factor. Key parameters are short-circuit current Isc (x-axis intercept), open-circuit voltage Voc (y-axis intercept), maximum power point (Pmp = Vmp × Imp), and fill factor. A standard 60-cell silicon module has Voc ≈ 37.8 V and Isc ≈ 9 A under standard test conditions (1000 W/m², 25°C).

Follow-up: How does the fill factor relate to the shape of the I-V curve and cell quality?

Q3. What is fill factor and what causes it to decrease in a real solar cell?

Fill factor (FF) is the ratio of the maximum power (Vmp × Imp) to the product of Voc and Isc, and a high FF (typically 0.7–0.85 for silicon) indicates a squared-off I-V curve close to ideal. FF is degraded by series resistance (from metal contacts, bulk silicon, and soldering) and shunt resistance (from leakage paths and junction defects). A series resistance of just 0.5 Ω in a cell producing 9 A causes a 4.5 V drop, significantly reducing the maximum power point and FF.

Follow-up: How would you measure series resistance and shunt resistance of a solar cell experimentally?

Q4. What is the solar cell efficiency, and what limits it for silicon cells?

Efficiency is the ratio of maximum electrical power output to incident solar power, expressed as η = Pmp / (G × A), where G is irradiance and A is cell area. The theoretical Shockley-Queisser limit for a single-junction silicon solar cell is about 29.4%, and practical record efficiencies for silicon are around 26–27% (PERC/HJT cells), with commercial modules reaching 20–22%. Losses include thermalization of above-bandgap photons, sub-bandgap photon transmission, recombination, and optical reflection.

Follow-up: What is the Shockley-Queisser limit and what fundamental losses does it account for?

Q5. What is MPPT (Maximum Power Point Tracking) and why is it needed in solar systems?

MPPT is a control technique implemented in solar inverters and charge controllers that continuously adjusts the operating point of the PV array to extract the maximum available power as irradiance and temperature change. The SMA Sunny Boy inverter uses Perturb and Observe (P&O) MPPT, shifting the operating voltage in small steps and observing the resulting power change. Without MPPT, a fixed-voltage load would operate the array off its maximum power point, wasting potentially 10–40% of available energy under varying conditions.

Follow-up: Compare the Perturb and Observe and Incremental Conductance MPPT algorithms.

Q6. How does temperature affect solar cell performance?

Increasing cell temperature decreases Voc at approximately -2.3 mV/°C per cell for silicon, while Isc increases slightly, resulting in a net decrease in power of about -0.4% to -0.5% per °C. A 60-cell silicon module rated 300 W at 25°C produces approximately 270 W when the cell temperature reaches 50°C (a 10 V reduction in Voc alone). This temperature coefficient means modules in hot climates like Rajasthan must be derated, and NOCT (Nominal Operating Cell Temperature) must be factored into energy yield calculations.

Follow-up: Why does Isc increase slightly with temperature in a silicon solar cell?

Q7. What is the difference between monocrystalline, polycrystalline, and thin-film solar cells?

Monocrystalline silicon cells are cut from a single crystal ingot and achieve efficiencies of 20–22%, polycrystalline cells are cast from multi-grain silicon and reach 16–18%, while thin-film cells (CdTe, CIGS, a-Si) are deposited on substrates and reach 10–16% but use less material. First Solar's CdTe modules achieve 18–19% efficiency at the module level and have a lower temperature coefficient than silicon, making them better suited for hot climates. Monocrystalline PERC cells dominate the market today due to falling wafer costs.

Follow-up: What is the PERC cell structure and why does it improve efficiency over standard monocrystalline cells?

Q8. Explain the effect of partial shading on a solar panel and how bypass diodes help.

Partial shading causes the shaded cell to become reverse-biased and act as a resistive load, forcing the module current through it and causing a large power loss and potential hot spot damage. A bypass diode connected anti-parallel across a group of 20 cells (a sub-string) allows current to flow around the shaded sub-string, limiting power loss to that group rather than the entire string. Without bypass diodes, shading a single cell in a string of 60 cells can reduce module power from 300 W to under 50 W.

Follow-up: What is a hot spot in a solar panel and what damage can it cause?

Q9. What is a heterojunction solar cell (HJT) and what is its efficiency advantage?

An HJT cell uses thin amorphous silicon (a-Si:H) layers deposited on both sides of a crystalline silicon wafer to passivate the surface defects and form the p-n junction, achieving record efficiencies of 26–27% at the research level. The a-Si passivation layers reduce surface recombination velocity by orders of magnitude compared to diffused p-n junctions in standard PERC cells. Panasonic HIT (now Heterojunction with Intrinsic Thin-layer) cells also have a very low temperature coefficient of around -0.26%/°C versus -0.40%/°C for standard cells.

Follow-up: What is surface recombination and why does it limit solar cell Voc?

Q10. What is a tandem or multi-junction solar cell, and why does it have higher theoretical efficiency?

A tandem solar cell stacks two or more p-n junctions with different bandgaps to absorb different parts of the solar spectrum, with wider bandgap materials on top absorbing high-energy photons and narrower bandgap materials below absorbing lower-energy photons. A GaInP/GaAs/Ge triple-junction cell used in space satellites achieves over 40% efficiency because thermalization losses — which dominate in single-junction cells — are reduced by matching each sub-cell bandgap to a portion of the solar spectrum. The Shockley-Queisser limit for an infinite stack of junctions approaches 68% under direct sunlight.

Follow-up: Why are multi-junction cells primarily used in concentrated photovoltaic (CPV) systems rather than standard rooftop panels?

Q11. What is the equivalent circuit model of a solar cell?

The one-diode equivalent circuit models a solar cell as a photocurrent source I_L in parallel with an ideal diode, a shunt resistance R_sh representing leakage, and a series resistance R_s representing contact and bulk losses. A two-diode model adds a second diode with ideality factor n=2 to model recombination in the depletion region more accurately, important for low-irradiance performance simulation. Simulation tools like PVsyst use the one-diode model with extracted parameters for energy yield prediction of commercial PV systems.

Follow-up: How are the parameters I_L, I_0, Rs, and Rsh extracted from measured I-V curves?

Q12. How does irradiance level affect the I-V curve of a solar cell?

Isc scales nearly linearly with irradiance (halving irradiance roughly halves Isc), while Voc decreases logarithmically (Voc = (nkT/q) ln(I_L/I_0 + 1)), meaning a 50% reduction in irradiance drops Voc by only about 30–40 mV in silicon. A 300 W panel at 1000 W/m² produces approximately 150 W at 500 W/m², mainly due to the Isc reduction, with a modest Voc drop. This is why MPPT must track the changing operating point under cloud transients.

Follow-up: How does the maximum power point voltage (Vmp) shift with irradiance?

Q13. What is anti-reflection coating (ARC) in solar cells and how does it work?

An anti-reflection coating is a thin dielectric layer (typically silicon nitride, SiNx, at 75 nm thickness) deposited on the cell surface to minimize reflection losses by destructive interference of reflected waves. Silicon has a refractive index of 3.5, so without ARC, about 30% of incident light is reflected; SiNx with n ≈ 1.9 reduces reflection to under 3% at the design wavelength. The SiNx layer also passivates dangling bonds at the silicon surface, reducing recombination and improving Voc.

Follow-up: What is the optimal refractive index for a single-layer ARC on silicon?

Q14. What is quantum efficiency (EQE and IQE) in a solar cell?

External Quantum Efficiency (EQE) is the ratio of collected electrons to incident photons at each wavelength, while Internal Quantum Efficiency (IQE) is the ratio of collected electrons to absorbed photons, isolating optical losses from recombination losses. A silicon solar cell with good front surface passivation has IQE > 95% at 600–900 nm but lower EQE due to reflection. EQE measurement with a monochromator and lock-in amplifier is a standard characterization tool at research labs and production lines to diagnose ARC and passivation quality.

Follow-up: How would you use EQE data to identify whether a cell suffers more from optical or recombination losses?

Q15. What is a string inverter versus a microinverter in a PV system?

A string inverter connects to multiple series-connected panels (a string) and performs MPPT for the entire string, while a microinverter is attached to each individual panel and performs independent MPPT at the panel level. Enphase IQ microinverters allow each panel to operate at its own maximum power point, eliminating mismatch losses due to partial shading or panel-to-panel variation that limit string inverters. Microinverters cost more per watt but deliver 5–25% more energy in shaded or mixed-orientation installations.

Follow-up: What is a power optimizer and how does it compare to a microinverter?