Solar cells.

Step 1 of 7

Light in, current out

In the Unit on electricity we saw that a current is charge moving round a closed circuit, and that it needs a source to push it. In a solar panel the source is the light itself. Zoom to the junction is at Junction: a slice of one cell, with a thin layer of n-type silicon on top and a thick p-type base below. Where they meet, the junction, there is a built-in electric field.

Step 2 of 7

Half the light

Choose Panel under Zoom to the junction, and look at the meter beside the bulb: it shows the voltage, the current and the power of one cell. Angle to the Sun is locked, so the panel faces straight at the Sun.

Step 3 of 7

How much of the light becomes electricity

A cell’s efficiency is the share of the light power falling on it that becomes electrical power:

$\eta = \frac{P_{\text{out}}}{P_{\text{in}}}$$\eta =$$\frac{P_{\text{out}}}{P_{\text{in}}}$⁠

The light power falling on the cell is the strength of the light, in watts per square metre, times the cell’s area. The electrical power is $P = VI$⁠. Both controls are locked.

Step 4 of 7

Where the rest goes

Choose Junction under Zoom to the junction again, and watch the photons: not all of them free a pair, and not all the energy of a photon that does free one reaches the circuit.

Step 5 of 7

Tilted away from the Sun

A beam of sunlight has a fixed width. Tilt the panel, and the same beam spreads over a larger area, so each square metre gets less. If $\theta$⁠ is the angle between the way the panel faces and the direction of the Sun, the light on each square metre is

$I = I_0 \cos\theta$⁠

Both controls are locked, with Angle to the Sun at 60°.

Step 6 of 7

Half the power

Sunlight is locked at 1000 W/m², and Angle to the Sun is free. The power the cell gives depends on the light it catches, but not only on that: the voltage, as you saw, drops a little when the light weakens.

Step 7 of 7

The photocurrent

With the two terminals joined directly, every electron–hole pair the light makes flows round the circuit. That current, the short-circuit current, counts the photons.