Chapter 4

Introduction

We've now followed two separate journeys.

In Part 2, we discovered that sunlight is made up of tiny packets of energy called photons, each carrying a precise amount of energy. So far in Part 3, we have learned about the atomic structure of silicon and discovered that its electrons normally occupy the valence band, separated from the conduction band by a small band gap. Until now, these two stories have remained completely separate.

This is where they finally come together.

Every second, trillions upon trillions of photons strike the surface of a solar panel. Most are reflected away, pass straight through the material or lose their energy as heat. But occasionally, a photon carrying just the right amount of energy collides with an electron inside the silicon. That single interaction marks the beginning of an extraordinary chain of events. For the first time, light begins its transformation into electricity.

A Photon Is Absorbed

A photon is far more than just a tiny particle of light; it is also a packet of energy. As we discovered in Part 2, not all photons carry the same amount of energy. Blue visible light photons carry more energy than red photons, while infrared photons carry less. Whether a photon can generate electricity depends entirely on how much energy it carries.

If a photon doesn't have enough energy to overcome silicon's band gap, nothing significant happens. It may pass through the material, be reflected away or simply become heat. However, if the photon carries enough energy, the photon is absorbed by one of the electrons sitting in the valence band. The photon disappears completely, transferring all of its energy to that single electron.

Crossing the Band Gap

Receiving this energy doesn't simply make the electron move faster. Instead, it gives the electron exactly what it needs to overcome the band gap. Almost instantaneously, the electron jumps from the valence band into the conduction band.

This is a crucial moment. While the electron remained in the valence band, it was tightly associated with its atom and helped hold the silicon crystal together through chemical bonds. Once it reaches the conduction band, it is no longer confined to a single atom. Instead, it becomes a free electron, capable of moving throughout the crystal. This single jump is the fundamental process that allows a solar cell to generate electricity. Without enough photon energy it can never happen, but with enough energy it occurs almost instantly.

The Hole Left Behind

As the electron leaves the valence band, it doesn't simply disappear from its original position. It leaves behind an empty space where it once existed. This empty space is known as a hole.

Although a hole is not a real particle, it behaves as though it carries a positive electric charge. Nearby electrons can move into this empty position, leaving another hole behind them. As this process repeats, the hole appears to move through the crystal in the opposite direction to the electrons. For this reason, physicists and engineers often describe holes as if they were positively charged particles, even though they are simply the absence of an electron. Thinking of holes in this way makes it much easier to understand how electricity flows inside semiconductor devices.

An Electron-Hole Pair

The moment a photon is absorbed, two equally important things are created: a free electron in the conduction band and a hole in the valence band. Together they are known as an electron-hole pair.

Every electron-hole pair represents a tiny opportunity to convert sunlight into useful electrical energy. On a bright sunny day, billions upon billions of these pairs are created inside every square centimetre of a solar cell every second. Without them, electricity could never be generated.

However, there is still one major problem. The negatively charged electron is naturally attracted back towards the positively charged hole it left behind. If nothing stops them, they simply recombine. The electron falls back into the valence band, the hole disappears, and all of the energy originally carried by the photon is released as heat instead of becoming useful electricity.

Simply creating free electrons isn't enough. They must be separated before they have a chance to recombine.

Looking Ahead

We've finally seen how sunlight creates free electrons inside silicon. But creating an electron-hole pair is only the beginning. 

Left alone, the electron and hole will quickly find each other again, releasing the photon's energy as heat instead of useful electricity.

If a solar panel is going to generate electrical power, it needs a way of separating these charges before recombination can occur. Remarkably, it does this without using a battery or any external source of energy.

The answer lies in one of the most ingenious features inside every solar cell: the PN junction.