The Solar Energy Cell: what it is made of and how it works.

To understand the functioning of a solar energy cell, we need to know about the behaviour of several elements and the three states of matter (solids, liquids and gases).

PREAMBLE ON ELEMENTS: An element is a pure substance where all the atoms have an identical number of Protons (+ charges) and electrons (- charges) and all have the same chemical properties, meaning they react in the same way. All elements are stable when they have what is called a FULL OUTER SHELL, which is when they have EIGHT ELECTRONS in that outermost layer.

Elements come in three main categories. We'll assume for now that all the atoms in our examples start with the same number of protons (+) and electrons (-) and are therefore neutral. The operation of the solar energy cell depends on these atoms achieving EIGHT electrons in their outer shell.

NON-METALS: These GAIN electrons to obtain a full outer shell. When a non-metal atom does this it becomes a negatively charged ion. It effectively absorbs free electrons from the environment. Oxygen and Fluorine are good examples.

TRANSITION ELEMENTS: These elements SHARE electrons with others to gain a full outer shell. Silicon is one of these elements. These can make large, very stable chains or lattices. Each bond is a PAIR of electrons. By looking at the Silicon lattice diagram we can see that each Silicon atom has four of these bonds for a total of eight electrons in the outer shell.

Now...Let's have a look at SOLID MATERIALS.

There are three broad categories of solids.

1. Conductors. The atoms in these solids easily lose hold of their (outer shell) electrons. Those electrons can move easily and so the solid can conduct electricity. This is also the general definition of a metal. Examples from everyday use are copper in house wiring and gold in computer chips. Both these elements have excellent conductive properties.

2. Insulators, or non-conductors. The atoms in a non-conducting solid have a very strong grip on their electrons and will not readily give them up. As the electrons cannot move, the solid cannot conduct electricity. A good example of an insulator is plastic which is mostly made of carbon and Hydrogen atoms bonded together.

3. Semiconductors. These solids are like insulators. However, when heated or exposed to a magnetic field the atoms of the semiconductor material may lose their grip on their electrons and so are capable of conducting, by having those now loose electrons move. The amount of conductivity a semiconductor displays can be controlled by modifying these factors.

SILICON

Silicon is a very popular choice as a semiconductor material in solar energy cells. Silicon can form a 3 dimensional covalent network lattice structure where each silicon atom is bonded covalently to four other silicon atoms.

Semiconductors are used in solar energy cells and can be made from pure substances that have the right properties such as Silicon and Germanium (Ge). They can also be alloys such as a Si/Ge alloy where two materials of similar properties are mixed together. Another type of semiconductor is called a compound, where two or more substances with different properties are mixed together.

Either way, they all behave in a similar way to pure silicon.

HOW DOES THAT GENERATE ELECTRICITY FROM SUNLIGHT?

In short, it does not. All we have so far is a piece of material, a silicon lattice, that loses its grip on some of its electrons when heated or exposed to a magnetic field. That's no more likely to start generating electricity than an iron nail lying in the sunshine.

What we need to do is get electrons moving through this piece of silicon. The Silicon lattice is held together by electron pairs, the bonds. When the lattice is able to conduct, electrons can be pushed or pulled from one bond to another; the net number of electrons does not change. The lattice is like a piece of hosepipe filled with marbles. If you push one in, one comes out straight away, and ALL the marbles have shifted their positions.

In order to get the silicon lattice making electricity from sunshine, we need to add some different elements to the lattice. This is called DOPING. The added elements are bonded covalently into the lattice; they become part of the structure.
Let's take an element called Boron, a favourite with solar energy cell semiconductor makers. Boron has three outer shell electrons. So when we slot it into the lattice, there is an electron MISSING; this is called a hole. You can see it on the top of the Boron atom in the *diagram. That hole will be filled by any nearby electron so that both the B atom and the Si atom above it have a full outer shell. Since silicon doped with boron swallows up electrons, it is acting in a POSITIVE manner by pulling the electrons. Therefore it is called a P-TYPE semiconductor.

However the Boron atom, while having 8 electrons in its outer shell and therefore being stable to a degree, only has a very weak grip on the electron. A dose of energy will be enough to knock it free from the bond it is in, and then it is free and energized. It is the SUNLIGHT that provides that energy!

Now that our electron is free and charged up, it moves. But it can't go back to the N area because we have cleverly added a one way barrier between the layers. What can it do? It goes down a wire attached to the P area, carrying the energy with it. Thus for each sunlight-released electron in the P-silicon area an electron enters the wire from the P area to the circuit we wish to power.

That then means that there is a shortage of electrons in the P area . More electrons migrate from the N area to fill the gaps (the dark phase again), and so it goes on as long as the sun is shining on the solar energy cell. Of course there are a multitude of factors involved in the manufacture of a solar energy cell, their arrangement into grids, modifying the power output and so on.