The short answer? Photons knock electrons loose, and a built-in electric field pushes them around an external circuit. The long answer involves quantum mechanics, band theory, and a few numbers that explain why silicon dominates the market and why single-junction efficiency caps at 33.7%. We covered the basics in our how solar panels work guide. Here we go deeper.
TL;DR: Photons above silicon's 1.12 eV bandgap excite electrons from the valence band to the conduction band, creating electron-hole pairs. The P-N junction's built-in electric field (0.6-0.7V across a 0.5-2 micrometer depletion zone) separates these pairs before they recombine, generating DC current. A typical 60-cell panel produces ~36V Vmp at 11A Imp, around 400W under STC. The inverter's MPPT continuously adjusts operating voltage to the V-I curve's power peak as conditions change. Shockley-Queisser analysis caps single-junction silicon at 33.7% theoretical efficiency; production panels hit 22-23% as of 2026.
When I started learning photovoltaics, I expected the physics to be the hard part. It's the easy part. The hard part is grasping how dozens of secondary effects (surface recombination, contact resistance, spectral mismatch, temperature coefficients) drag the theoretical 33.7% efficiency down to 22-23% in production, and the industry has spent 70 years chiseling them away one at a time.
What Energy Does a Photon Need to Generate Electricity?
For silicon, a photon needs energy above 1.12 eV (the bandgap) to lift an electron from the valence band into the conduction band. Photons below that threshold pass straight through, useless for power. Photons above it are absorbed, but any excess energy beyond 1.12 eV converts to heat instead of electricity, a loss called thermalization. That 1.12 eV threshold corresponds to about 1,100 nanometres: UV and visible light absorb well but lose more to thermalization, near-infrared absorbs efficiently with minimal loss, and anything above 1,100 nm passes through unabsorbed.
Why silicon specifically? The solar spectrum at Earth's surface (AM 1.5G reference) peaks in the visible band around 500 nm. The Shockley-Queisser detailed balance analysis (1961) puts the optimal single-junction bandgap around 1.34 eV, gallium arsenide (1.42 eV) sits closer, reaching a 27% theoretical limit versus silicon's 33.7%. Silicon won anyway on economics: abundant, well-understood, and endlessly scalable, while GaAs is rarer and harder to grow defect-free.
How Does the P-N Junction Separate Electron-Hole Pairs?
The P-N junction is the boundary between two silicon regions doped differently: phosphorus for N-type (extra electrons) and boron for P-type (holes). Electrons from the N-side diffuse into the P-side and holes diffuse the other way, leaving behind fixed ionized dopant atoms that create a permanent electric field, the "built-in field" of about 0.6-0.7V across roughly 0.5-2 micrometers. That region is the depletion zone. When a photon is absorbed inside or near it, the field splits the electron-hole pair: the electron accelerates toward the N-side, the hole toward the P-side. Further out, pairs can recombine before reaching the field.
For mono-crystalline silicon, minority carrier diffusion length runs 100-300 micrometers, much longer than typical cell thickness (180 micrometers), so most absorbed photons produce useful current. Polycrystalline silicon has grain boundaries that act as recombination sites, cutting effective diffusion length to 50-150 micrometers and trimming efficiency.
What Does the V-I Curve Tell You About a Cell?
The V-I curve plots voltage versus current under a set illumination. It's the fundamental characterization of a cell and the basis for everything an inverter does. A typical 400W panel under STC (1,000 W/m2, 25 deg C cell temperature) gives these key points:
| Parameter | Symbol | Typical value | What it means |
|---|---|---|---|
| Short-circuit current | Isc | 10-11A | Current when output is shorted, max possible current |
| Open-circuit voltage | Voc | 41-45V (60-cell panel) | Voltage with no load, max possible voltage |
| Maximum power voltage | Vmp | 33-38V | Voltage at maximum power point |
| Maximum power current | Imp | 9-10A | Current at maximum power point |
| Fill factor | FF | 0.78-0.84 | Imp x Vmp / (Isc x Voc), curve "squareness" |
The curve starts at Isc (zero voltage), stays roughly flat as voltage rises, then drops sharply near Voc. The maximum power point sits at the "knee" where the V and I product is largest, a "400W" nameplate means Vmp x Imp = Pmp at that point under STC. Fill factor for a good panel runs around 0.80, capturing 80% of the Isc x Voc rectangle. Temperature and irradiance both shift the curve: higher temperature mainly cuts Voc, why panels lose ~0.30%/deg C in TOPCon cells and ~0.35%/deg C in PERC, while lower irradiance cuts Isc proportionally without touching Voc much. See our TOPCon vs HJT vs PERC comparison for how cell technologies shift this curve.
How Does MPPT Find and Hold the Maximum Power Point?
Maximum Power Point Tracking is the inverter's algorithm for landing the string on the V-I curve's power peak. That peak drifts constantly with irradiance and temperature, so a fixed operating point bleeds energy. The most common algorithm is Perturb and Observe (P&O): measure V and I, compute power P = V x I, nudge the voltage 1-3%, then check whether power rose or fell, reversing if it fell. It oscillates around the peak, holding within 1-2% under stable light, though accuracy can drop to 95-97% under passing clouds. Fancier algorithms (Incremental Conductance, particle swarm) trade complexity for precision; SolarEdge HD-Wave and Enphase IQ8 claim 99%+ accuracy under fast-changing conditions.
Multiple MPPT channels matter for mixed orientations or shading, a residential string inverter usually has 2-3 channels, each tracking one string. Module-level electronics push this further: the SolarEdge P370 optimizer and Tigo TS4-A-O retrofit put a tiny MPPT controller on each panel, and the Enphase IQ8A microinverter converts DC to AC at each panel with its own MPPT. Is that worth it on every install? Honestly, no, for an unshaded south-facing roof, string-level MPPT captures nearly all available power. Module-level electronics pay back where shading or mismatch exist.
What Limits Real-World Cell Efficiency?
The Shockley-Queisser limit caps single-junction silicon at 33.7% theoretical efficiency under STC, yet production panels hit only 22-23%. That ~10-point gap breaks down into specific loss mechanisms: below-bandgap photons (15-19% loss, photons below 1.12 eV pass through unabsorbed, no cell-level fix), thermalization (33-40% loss, excess photon energy above 1.12 eV dumps as heat), recombination (1-3% loss, mostly at grain boundaries and unpassivated surfaces, TOPCon's tunnel oxide and HJT's passivation attack this), optical losses (3-5%, reflection and busbar shading, addressed with anti-reflective coatings and texturing), resistive losses (1-2%, more busbars and thinner cells reduce it), and temperature derating (5-15% in operation, summer roofs run 50-70 deg C, not the 25 deg C of STC).
The commercial cell record sits around 27.3% (Longi 2024 HBC structure), module efficiency runs 22-23% for volume TOPCon and HJT. The gap to 33.7% narrows each year, but the easy gains are gone, the next 3-4 points will be harder than the last 8.
Summary
Solar panels make electricity by absorbing photons above silicon's 1.12 eV bandgap, separating the resulting electron-hole pairs across a P-N junction's built-in 0.6-0.7V field, and collecting the DC current through metallization on the front and back of the cell (NREL Photovoltaic Cell Physics). A typical 60-cell panel produces ~36V Vmp at 11A Imp, around 400W under STC. The inverter's MPPT continuously adjusts operating voltage to the V-I curve's power peak as conditions change, at 96-98% efficiency. Real production efficiency runs 22-23% versus the 33.7% Shockley-Queisser theoretical limit, the gap split among thermalization, below-bandgap photons, recombination, optical losses, resistive losses, and temperature derating. For the materials and construction behind those cells, our piece on what solar panels are made of covers the build details.