Chapter 1

Introduction

Throughout Part 2, we followed a photon on an extraordinary journey from the core of the Sun to the surface of a solar panel.

Now our attention shifts to the material it is about to strike.

To understand how sunlight becomes electricity, we must first understand what a solar cell is made of. 

Like everything else in the Universe, solar panels are built from atoms. Although atoms are unimaginably small, the way they are structured determines whether a material can conduct electricity, block it completely or convert sunlight into electrical energy.

Our journey into photovoltaic physics begins with one of the smallest building blocks of nature: the atom.

What Is Matter Made Of?

Everything we can see, touch and measure is made from matter. A grain of sand, a drop of water, a leaf and a solar panel may look completely different, but they are all built from the same fundamental building blocks: atoms.

More than one hundred different types of atoms, known as elements, exist in nature. By combining in different ways, they form every material in the Universe.

Atoms are incredibly small. A single grain of sand contains trillions upon trillions of them, far too small to be seen with an ordinary microscope.

For many years, scientists believed atoms were the smallest possible particles. They were wrong.

Inside an Atom

At the centre of every atom lies a tiny, dense nucleus. The nucleus contains protons, which carry a positive electrical charge, and neutrons, which carry no charge.

Surrounding the nucleus are electrons, tiny negatively charged particles that occupy the space around it. Despite containing almost all of an atom's mass, the nucleus occupies only a tiny fraction of its volume. Most of an atom is actually empty space. If the nucleus were enlarged to the size of a marble, the outermost electrons would be hundreds of metres away.

Although this simple model explains the basic structure of an atom, protons and neutrons are not fundamental particles themselves.

They are built from something even smaller.

Quarks: Building the Nucleus

Protons and neutrons belong to a family of particles called hadrons, which are themselves made from even smaller particles known as quarks.

A proton contains two up quarks and one down quark, while a neutron contains two down quarks and one up quark.

These quarks are held together by particles called gluons, which carry the strong nuclear force.

As its name suggests, the strong nuclear force is the strongest of the four fundamental forces of nature. Without it, protons and neutrons could not exist, and neither could the atoms that make up everything around us.

Fortunately for us, this force acts only over incredibly small distances inside the nucleus.

Holding the Nucleus Together

The nucleus presents an interesting puzzle. Every proton carries a positive electrical charge, and like charges repel one another.

If this were the only force acting inside the nucleus, the protons would push each other apart and every atom would fly apart almost instantly. The reason this doesn't happen is because the strong nuclear force is enormously more powerful than electrical repulsion at very short distances.

It acts like an incredibly powerful glue, binding protons and neutrons together into a stable nucleus.

Neutrons play an important role in this process. Although they carry no electrical charge, they still experience the strong nuclear force, helping to hold the nucleus together without adding any extra electrical repulsion.

Without this delicate balance of forces, atoms could never exist.

Why Are Electrons Different?

Electrons are different to protons and neutrons. As far as current scientific evidence shows, they are fundamental particles, meaning they are not made from anything smaller. They also do not participate in the strong nuclear force, which is why they are not bound inside the atomic nucleus.

Instead, they are held around the nucleus by the electromagnetic force. The positively charged nucleus attracts the negatively charged electrons, keeping them bound to the atom.

Electrons do not orbit the nucleus like planets orbiting the Sun. Instead, quantum mechanics tells us they exist in regions of space called orbitals, where there is a high probability of finding them.

For the purposes of understanding solar cells, however, the important point is much simpler.

Electrons occupy different energy levels around the nucleus, and those furthest from the centre are held less tightly than those closer in. These outer electrons are the ones that determine how a material behaves electrically.

The Electrons That Matter

Although every electron is identical, not every electron plays the same role. The electrons closest to the nucleus are held so tightly that they rarely participate in chemical or electrical processes.

The outermost electrons are different.

Because they are less tightly bound, they can sometimes move between atoms when given enough energy.

This simple difference explains why some materials conduct electricity easily, others block it almost completely, and a special group of materials, known as semiconductors, lie somewhere in between.

One of those semiconductors is silicon.

Its unique atomic structure makes it perfectly suited for converting sunlight into electricity.

Looking Ahead

We now understand that every material is built from atoms, and that the behaviour of their outermost electrons determines many of their electrical properties.

But why do electrons move freely through copper, remain trapped inside rubber, yet behave somewhere in between inside silicon?

The answer lies in the unique structure of semiconductor materials.

In the next chapter, we'll discover why silicon has become the foundation of almost every solar panel ever built.