# Lesson 4 - Solar cells

In this lesson a basic introduction to the workings of a photovoltaic cell is given. In order to do that, some new concepts must be introduced. It is not necessary to fully comprehend these concepts or to do complicated calculations. The exercises indicate the level of understanding that is required.

What is a photovoltaic/solar cell?

In everyday use the word ‘photovoltaic’ is often omitted and we speak of a ‘solar cell’. Strictly speaking the two terms are not interchangeable as there are other kinds of solar cells than photovoltaic cells. It has become so common practice however to just say ‘solar cell’ that we will not be strict about it.

A photovoltaic cell is a device that converts light into electrical energy.

As we have seen in lesson 3, light is a form of energy and consists of particles called ‘photons’. The concept that light can be converted into other forms of energy is not new. As, for example, we are all familiar with the fact that sunlight feels warm as light is begin converted into heat. The direct conversion from light into electrical energy is more complicated however.

One might wonder ‘What is electrical energy anyway?’. In this context it is the ability to let charged particles flow through a device.

Power plants generate electrical energy, and when you hook up, for example, a vacuum cleaner the current that will flow through that vacuum cleaner enables it to work.

How is a photovoltaic cell constructed?

A photovoltaic cell can be made of different kinds of materials, as long as these materials have certain properties. The material used most is crystalline silicon (c-Si) with the addition of certain elements to obtain the wanted properties. In figure 17 an example of a silicon based cell is shown. The most important property of a photovoltaic cell is that electrons that can move freely are drawn towards the N-type material. This is due to a natural electric field that is present on the border between the N-type and the P-type material.

How does a photovoltaic cell work?

To convert light into electrical energy there needs to be a link between light particles (photons) and charged particles that flow.

One might be tempted to think photons are transformed into charged particles. Such a thing is not possible!!!

Due to the materials used in a photovoltaic cell, there is always an electric field present inside the cell at the border between the N-type and the P-type material. If we connect the solar cell to a device, this electric field can cause a current to flow and thus provide electrical energy.

So why do we need light at all? Only charged particles that can move freely in the photovoltaic cell can be used for this process. However, most electrons are bound by atoms and can’t move freely. Without light there are simply not enough of electrons (and holes) that can move freely to be of any use. The role of light is then to free electrons from there atoms.

The freeing of electrons costs energy and light provides that energy.

So what happens in a photovoltaic cell:

1. Light penetrates into the solar cell.
2. Bound electrons in the p-type material absorb that light and become free.
3. These free electrons get drawn to the n-type material due to the natural electric field in the cell.
4. The free electrons can be used by an electrical device hooked up to the solar cell.

Is there enough energy in sunlight?

Before we even consider the use of direct solar energy we first have to estimate whether it will be profitable to do so. An estimation doesn’t need to be exact, as long as we get numbers that approximate reality. The worlds electrical energy consumption is about 20∙1012 kWh per year (20.000.000.000.000 kWh). Let’s say the solar energy at the earth’s surface is 1000 W/m2 and the average number of hours of sunlight is 6 hours per day. In these assumptions we already accounted for all kinds of effect, for example clouds, seasonal changes etc. Modern solar cells are capable of producing electrical power at about 30% efficiency. That means that 30% of the sunlight at the earth’s surface is converted into electrical power. At this moment 30% is only achieved under optimal conditions, in real world applications we have to consider effects that decrease this efficiency drastically. In the following exercises assume a photovoltaic cell has an efficiency of 15%.