How Solar Panels Turn Sunlight into Electricity

A solar panel turns light directly into electricity. It has no moving parts and burns no fuel. Inside, it is made of many photovoltaic (PV) cells, usually thin slices of silicon, a semiconductor.

Three things happen in every cell. Light frees electrons inside the silicon. An electric field built into the cell pushes those electrons in one direction. Metal contacts then collect them, so they flow round a circuit as an electric current.

A cell is a sandwich of layers, each with a job:

• Anti-reflective coating: a very thin layer on top that cuts down how much light bounces off the shiny silicon.
• Front metal contacts: thin metal lines, called fingers, printed across the top. They collect electrons but are narrow so they block as little light as possible.
• n-type layer: a thin top layer of silicon mixed with a tiny amount of phosphorus. This gives it spare electrons that can move about, so it is called n-type (n for negative).
• p-type layer: a much thicker layer below with a shortage of electrons. The gaps where electrons are missing are called holes and act like positive charges, so it is called p-type.
• Rear metal contact: a metal layer across the back that completes the circuit.

The boundary where the n-type and p-type layers meet is the p-n junction. It is the most important part of the cell.

Light travels as tiny packets of energy called photons. When a photon hits the cell, it may be reflected, pass straight through, or be absorbed. Only absorbed photons can make electricity.

If an absorbed photon has enough energy, it knocks an electron free from its place in the silicon. The electron can now move through the material, and it leaves a hole behind. Scientists call this an electron-hole pair.

On their own, these pairs do not last long. A free electron soon drops back into a hole, and the energy it carried is wasted. To get useful electricity, the cell must pull the electrons and holes apart before that happens.

Joining the two kinds of silicon creates a permanent electric field across the junction. It is built into the cell when it is made, so no battery or power supply is needed to create it.

That field acts like a one-way gate. When light frees an electron near the junction, the field sweeps it across to the n-type side, and the matching hole is pushed towards the p-type side. Electrons pile up on the front of the cell and holes at the back. The front becomes a negative terminal and the back a positive one, just like the two ends of a battery.

Connect a wire from the front contacts to the rear contact, with a lamp in between, and the electrons have a path. They leave the n-type side through the front contacts, flow through the lamp and give it energy, then re-enter at the rear contact, where they fill holes in the p-type layer. As long as light keeps arriving, the flow continues.

The electrons always move the same way round the circuit, so a solar cell makes direct current (DC). Textbook circuit diagrams draw current the other way, from positive to negative: this is called conventional current, and it describes the same flow.

Homes and power grids use alternating current (AC), which keeps reversing direction. So a solar power system needs an inverter, a box of electronics that converts the DC from the panels into AC.

One cell produces only a little power. Cells are therefore wired together and sealed into a weatherproof panel (also called a module), and panels are joined into an array. More panels means more power. A typical home rooftop system in the United States has about 30 modules.

Panels make the most electricity when they face the Sun directly. Heat, dirt and shade all reduce how well they convert light. Even the best commercial panels convert only about 18–22% of the light energy that reaches them, and there is a theoretical limit of about 32% for a silicon cell. Efficiency has risen a lot: commercial panels averaged under 10% in the mid-1980s, and the most advanced modules are now approaching 25%.