Intensity on the power output of a solar cell.

Chris Critchley

Investigating the Effects of Distance and Light

Intensity on the Power Output of a Solar Cell.

Introduction:

As pollution becomes an increasingly important factor in energy production, the need arises to harness other ‘clean’ energy sources. Many of these energy sources, such as wind, tidal and solar, are used now but only in small amounts. In the near future it will become even more important that we use these as the non-renewable supplies, such as oil, finally run out.

This investigation focuses on solar energy and discusses its principles and how to use its potential. The actual experiments will look at how the power output of solar cells changes as the distance between the light source and the cell is altered, as well as looking for any specific relationship between the two.

Theory and Background Information:

Year after year, pollution and energy consumption worldwide reaches higher levels. As a result the need for new energy sources is increased. One possible solution is solar energy. However, as the diagram to the right shows, this only represents a very small percentage of energy consumption. This is mainly due to its cost and the fact that solar panels themselves are not very efficient. There are several disadvantages to solar power, one being that the amount of sunlight reaching the panels in variable, depending on weather and placement. In order to power fairly large devices, such as hot water tanks in homes, a significantly large solar panel is needed, sometimes even spanning the entire roof of a building. These large solar panels can be very expensive due to the fact that they are made up of hundreds of smaller solar cells. This brings me onto the main focus of this investigation. What is it that affects these solar cells?

Solar cells, also known as photovoltaic cells, are primarily made of a semiconductor, in most cases silicon. When light hits the cell a certain amount of it is absorbed within this semiconductor. This means that the energy of the absorbed light is also transferred to the semiconductor. The light energy, in the form of photons, knocks electrons loose, allowing them to flow freely. The process is known as the photovoltaic effect. This flow of electrons is a current (I = dQ/dt = rate of flow of charge). By placing metal contacts on the cell, this current can be drawn off in order to power various devices, for example a calculator.

An atom of silicon has fourteen electrons, arranged in three shells. The two inner shells are filled with two and eight electrons respectively. This leaves the third shell with only four out of a possible eight. The atom will always look to fill this gap by sharing electrons with four other silicon atoms. All of these atoms are sharing with four other atoms and as a result, a crystalline structure is formed. A downside to this however is that none of the electrons are free to move thus making silicon a poor conductor of electricity.

In order to get round this problem, the silicon in photovoltaic cells is mixed with impurities such as phosphorus and boron. This process is known as doping. Phosphorus has five outer electrons and when it bonds with silicon atoms, there is one electron left over. The part of the silicon containing phosphorus atoms is known as an N-type material (‘n’ for negative) as a result of the free electron. The other part of the silicon structure is doped with boron atoms. These only have three outer electrons meaning the boron-doped silicon becomes P-type silicon. This is the opposite of N-type in that there is an extra ‘hole’ for an electron.

When these two parts of the silicon are put together, the extra negative electrons rush to fill up the positive holes. However instead of filling the holes, they mix and form a barrier at the junction between the two types of silicon. This makes it harder for electrons to cross over to the positive side, before eventually reaching equilibrium. This has now produced an electrical field between the two sides. This electric field now allows electrons to move from the positive side to the negative but not the other way round.

Whenever light, in the form of photons, hits the photovoltaic cell its energy will normally be enough to release one electron, resulting in a free hole as well. If this happens close enough to the electric field, at the junction between the two types of silicon, the field will send the electron to the negative side and the hole to the positive side. This is extremely useful, as when we provide an external path for the current, the electron and hole that has just been moved will follow the path back to their original sides.

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