Photovoltaic Effect

A silicon solar cell has no moving parts, no fuel, and no combustion. Sunlight hits it, and electricity comes out. The mechanism is remarkably simple: a photon knocks an electron loose, and a built-in electric field sweeps the electron into a circuit.

The photovoltaic effect converts light directly into electricity in three steps. First, a photon with energy above the semiconductor's band gap strikes the material and is absorbed, transferring its energy to an electron. Second, the energized electron breaks free from its atomic bond, creating an electron-hole pair (a free negative charge and a "missing electron" that acts as a positive charge). Third, a built-in electric field at the cell's p-n junction (where two differently treated layers of silicon meet) separates the electron and hole, pushing them in opposite directions. Connect a wire between the two sides, and current flows.

The p-n junction is the key. It is created by doping one layer of silicon with an element that has extra electrons (n-type, like phosphorus) and another with fewer electrons (p-type, like boron). Where they meet, a permanent electric field forms. This field does the work of separating charges; no external power source is needed.

Worked Example

A silicon solar cell has a surface area of 0.016 m². Sunlight delivers 1,000 W/m² (standard test conditions).

• Calculate power hitting the cell. 1,000 x 0.016 = 16 watts.
• Apply cell efficiency. A typical commercial cell is about 22% efficient. 16 x 0.22 = 3.5 watts per cell.

A standard 60-cell residential panel contains 60 of these cells. What is the panel's output?

60 x 3.5 = 210 watts. This matches the typical 200-220 watt rating of a standard residential panel.

The 22% efficiency in this example is a manufacturing choice, not a hard limit. Every panel on a roof or a solar farm runs the same three-step process. The expensive ones just capture more photons and lose fewer of them to heat.

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