Chapter 5

Introduction

In the previous chapter, we discovered how sunlight creates an electron-hole pair inside silicon. A photon transfers its energy to an electron, lifting it into the conduction band and leaving behind a hole in the valence band. However, there was one major problem.

The electron and hole are naturally attracted to one another. Left alone, they quickly recombine, releasing the photon's energy as heat and undoing everything that the photon had achieved.

If a solar cell is going to generate useful electricity, it needs a way of separating these charges before they can reunite. Remarkably, every solar cell solves this problem without using a battery or any external power source.

Instead, it contains its own invisible electric field, built directly into the silicon itself.

Pure Silicon Isn't Enough

Pure silicon is an excellent semiconductor, but by itself it still isn't capable of producing electricity efficiently. Although sunlight continually creates electron-hole pairs, there is nothing to stop them from finding each other again. Most simply recombine almost as quickly as they are created.

The solution is surprisingly simple.

Instead of using perfectly pure silicon, manufacturers deliberately introduce tiny quantities of other elements into the crystal. This process is known as doping.

Although only a handful of atoms are added for every million silicon atoms, these impurities completely transform the electrical behaviour of the material.

Creating P-Type and N-Type Silicon

Two different types of doped silicon are created.

If a small amount of phosphorus is added, each phosphorus atom brings an extra electron that isn't needed for bonding. These spare electrons are only loosely held and can move relatively easily through the crystal. This material is known as N-type silicon, where N stands for negative, because electrons are the majority charge carriers.

If boron is added instead, one electron is missing from each boron atom's bonds. This creates additional holes that can move through the crystal as neighbouring electrons fill the empty spaces. This material is known as P-type silicon, where P stands for positive, because holes become the majority charge carriers.

On their own, neither material can generate electricity from sunlight.

The real magic happens when they are joined together.

Forming the PN Junction

When P-type and N-type silicon are brought into contact, something fascinating happens almost immediately.

Some of the free electrons on the N-type side naturally wander across the boundary into the P-type side, where they recombine with nearby holes. At the same time, holes begin moving in the opposite direction.

As more electrons and holes disappear through recombination, the region around the boundary becomes almost completely depleted of mobile charge carriers.

This narrow region is known as the depletion region.

Although it contains very few free electrons or holes, it is the most important part of the entire solar cell.

The Built-In Electric Field

When electrons leave the N-type side, they expose positively charged phosphorus ions that cannot move. Likewise, when holes disappear from the P-type side, negatively charged boron ions are left behind.

These fixed charges create something extraordinary. An electric field forms naturally across the depletion region.

Unlike electrons and holes, these charged atoms are locked into the crystal lattice, so the electric field is always present. No battery is needed to create it, and no external power is required to maintain it.

This invisible electric field acts like a microscopic one-way slope. Whenever sunlight creates a new electron-hole pair near the junction, the field immediately pushes the negatively charged electron towards the N-type side while forcing the positively charged hole towards the P-type side.

Instead of recombining, the charges are pulled apart almost instantly.

This is the breakthrough that makes a solar cell possible.

Separating the Charges

Every photon absorbed near the PN junction now produces far more than just a free electron.

It creates an electron-hole pair that is immediately separated by the electric field before recombination can occur. The electron is swept towards the N-type side, while the hole is pushed towards the P-type side.

For the first time, the charges are no longer trying to reunite.

Instead, they accumulate on opposite sides of the junction, storing electrical potential energy that can be harnessed by an external circuit.

The solar cell now has everything it needs to begin generating an electric current.

When the solar cell is connected to an external circuit, the separated electrons can flow from the N-type side through the circuit towards the P-type side. As they travel through this external path, their movement forms an electric current that can deliver energy to an electrical load. We’ll explore exactly how this current, along with voltage and electrical power, behaves in much more detail in Part 4.

Looking Ahead

We’ve now built every essential component of a solar cell. Sunlight creates electron-hole pairs, the PN junction separates them, and an external circuit gives the electrons a path to flow.

But a single solar cell can only produce a small amount of electrical power. To turn the photovoltaic effect into a practical source of energy, we need to combine many cells together and protect them from the outside world.

In the next chapter, we’ll see how individual solar cells are connected and packaged together to create the modern solar panel, the familiar technology found on rooftops and solar farms around the world.