Solar cells are familiar to many of us, and we can see examples of their usefulness in many aspects of our everyday life. Solar-powered calculators are perhaps the most commonly-seen example and they, along with never needing a battery, often don’t come with an on/off button. As long as there is a light supply in constant view of the small panel on the calculator it will run indefinitely. Larger examples of solar panels in daily usage include those seen on emergency road signs, on buoys out at sea and even in parking lots, where they are used to supply power to lights.

Solar cells are useful in terms of providing electricity converted from the sun, as on a bright and sunny day the shining sun provides approximately 1,000 watts of energy per square metre of the surface of the Earth. This remarkable figure means that if only we were able to harness, convert and then utilise all of this solar energy it would be possible to provide power to every home and office at no charge. When solar cells are looked at in this way it is no surprise that many industry analysts expect solar energy to be the dominant alternative energy source in the coming decade or two.

The most common form of solar cells which are the ones commonly seen on our calculators and watches are called photovoltaic cells, also known as modules. A module, when you come across the term, refers to a group of cells connected and grouped into a single frame. PV cells fulfil their function by converting sunlight directly into an electrical supply, and were once entirely used in the domain of space satellites and in other devices with limited access to conventional energy. Technological advances have brought them more and more into everyday usage that we have seen, and in some cases they have even been used to power homes.

The PV cells themselves are comprised of materials called semiconductors like silicon. When the light from the sun contacts the cell part of the light is absorbed by the silicon or other semiconductor material, although silicon is the most common-used material. The light absorbed in this way is transported to the semiconductor and the energy produced loosens and frees trapped electrons, thereby enabling them to move around freely from layer to layer of the cell. The energy produced by this, as well as the fact that every cell has at least one electric field that make freed electrons flow in a certain direction, make a current. In order to draw off this electrical current in order to facilitate its external use metal contacts are positioned on the top and bottom of the photovoltaic cell. The current along with the voltage of the cell is what determine the wattage (or power) that the cell is capable of producing.

Silicon is the most often used material for solar cells as it has many very particular chemical properties, especially when it is in a crystalline state. Basically, silicon atoms have 14 electrons arranged into three separate shells, with the first two shells closest to the middle being totally full. The outer shell is only half full, however, and the atom will continually look to fill up this last shell, with the electrons constantly seeking out its neighbouring silicon atoms in order to do so. We therefore end up with effectively each atom ‘holding hands’ with its neighbour – and in the case of the structure of crystalline silicon every atom having four hands joined to four neighbours, a structure that is vital to a photovoltaic cell. In crystalline silicon the electrons are locked into the structure in the way previously described, which makes it an effectively conductor of electricity. Conversely, pure silicon would make a very poor conductor of electricity due to the fact that its electrons are not at liberty to move freely about in contrast to crystalline silicon or other excellent conductors like copper.

Our solar cells today contain silicon that also has impurities, which are other atoms that are mixed together with the silicon atoms. The cell would be unable to work correctly without these impurities, and such impurities are there for a good reason. This is because if we have silicon with a phosphorous atom liberally scattered around phosphorous has five instead of silicon’s four atoms in its outer shell. As a result it has no neighbour with whom to ‘hold hands’, and therefore does not form part of a bond. Despite this, however, the phosphorous nucleus does contain a positive proton that holds it in place. When the rays from the sun hit the panel adding heat this makes several electrons break their bonds and leave their atoms, which leaves a hole. The wandering electrons then travel, looking to find another hole in which to fall into. They are referred to as ‘free carriers’, and these are the electrons that are able to carry an electrical current. Pure silicon contains very few of these, and this is why impure silicon with its phosphorous atoms is so essential. It also needs significantly less energy to knock an ‘extra’ phosphorous electron loose due to the fact that they are not manacled in a bond as they have no ‘neighbours’ to hold them back. The result is that the majority of these electrons are able to free themselves, which results in many more free carriers than could ever be found in pure silicon.

The adding of an impurity such as phosphorous is known as ‘doping’, and the silicon that ensues as a result of this doping with phosphorous is known as N-type ( ‘N’ meaning negative) due to the sheer number of free electrons that ensue. This type of silicon is a far more efficient conductor than pure silicon. This accounts for one part of the solar cell. The remaining part of the solar cell is doped with boron, which becomes a P-type (positive) silicon as it has merely three electrons in its outer cell rather than four. These positive type silicones have free holes which are basically a lack of electrons. As a result, they carry a positive charge and can move around also. When the positive and negative silicon types are combined, things start to happen.

Every photovoltaic cell has a minimum of one electric field, without which the cell would be unable to function. The electrical field is able to form once the positive and negative silicon types combine. At this point all of the free electrons in the negative side, which have been seeking holes to fall into, rush to fill all of the free holes on the positive side. Eventually this balances and a barrier forms between the remaining N and P electrons, and an electric field forms. The field allows electrons to flow from side P to side N, but not vice versa. When the light from the sun hits the panel, this energy frees hole pairs. We then get a current as a result of electron flow, and voltage from the cells’ electric field. Our power, in the form of electricity produced by the cell, is the produce of these two things.

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