The Diode Equation in Reverse
A solar cell is a large-area p-n junction diode operated in reverse: instead of consuming current, it generates it. When photons with sufficient energy strike the semiconductor, they create electron-hole pairs that are separated by the built-in electric field at the junction. The resulting photocurrent Iph is nearly proportional to irradiance. The I-V curve emerges from the competition between this photocurrent and the diode's recombination current, which increases exponentially with voltage.
Anatomy of the I-V Curve
The I-V curve has two characteristic endpoints. Short-circuit current (Isc) is the current when the cell is shorted (V = 0) — it equals the photocurrent minus a tiny diode current. Open-circuit voltage (Voc) is the voltage when no current flows — it is set by the balance between photogeneration and recombination. The maximum power point (MPP) sits on the curve's knee, where the rectangle V × I has maximum area. The fill factor measures how square this knee is.
Parasitic Resistances
Real solar cells have series resistance Rs from the metal grid fingers, bus bars, and wiring, and shunt resistance Rsh from manufacturing defects that create leakage paths. Series resistance steepens the curve's drop near Voc and reduces fill factor. Shunt resistance tilts the flat portion near Isc downward. Both degrade the maximum power point. High-quality cells minimize Rs (below 0.5 Ohm) and maximize Rsh (above 500 Ohm).
Interactive Exploration
This simulation lets you adjust irradiance, temperature, diode ideality, and series resistance to see their effects on the I-V curve in real time. Watch how the MPP shifts as conditions change. See fill factor collapse under high series resistance. Observe the temperature penalty on Voc. The power curve (P vs V) overlays the I-V plot to clearly show where maximum extraction occurs.