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Solar Cells and Photovoltaics
The Inner Workings of Photovoltaic Cells
The idea with this section is to give a fundamental understanding of how solar cells work. Where possible, we have omitted in-depth descriptions af the atomic structure and mathematics in order to present a clear picture for as many as possible.
There are a few different types of solar cells nowadays and they all work in slightly different ways. The main idea behind all of them is to make use of the energy the sun is providing us with by catching the electromagnetic radiation with solar cells. But the outputs are not always the same, sometimes you want heat, sometimes you need electricity. We begin with taking a look at the inner workings of the materials used in solar panels, then we will move on to describe the pn-junction, the photovoltaic effect and lastly how the cells are made and how they differ. There are more things to think about than just efficiency for example. For many years, developers of solar cells were fighting against the fact that it cost more money to make each cell than it’s ability to produce equivalent amount of energy during it’s lifetime. That is, the amount of energy produced had less financial value than the investment to make the cell. This barrier has since long been broken, but still, todays hi-efficient solar cells can be very expenesive and are not always of main focus. The key is to hit the peak, to find the highest efficiency for the least cost.
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Firstly, a short introduction of the atom and little update on our highschool physics. An atom is made up of the nuclei and it’s surrounding electrons. There are different energylevels within the atom that each electron can occupy and the closer to the nuclei the stronger the binding force. The outer electron is called the valence electron and it determines most of the chemical and physical characteristics for the different types of materials that atoms can form. It’s improtant to note that electrons are not little balls of negative charge, (in science, no one knows what anything is, we can only describe the behaviour of what we’re observing by the use of mathematics) rather, they’re thought of as a zero-dimensional points in space with infinite charge density. They, like all matter and light, have wave-particle duality in their nature. They travel like waves and exchange energy as particles. When bound to a nuclei, their ”paths” are called orbitals and for each energy level, there are a certain number of orbitals that the electrons can take. If an atom recieves just the right amount of energy, the electrons can jump to a higher energy-level, and are said to be excited.
There are three main types of material we talk about here. Conductors, insulators and semiconductors.
Conductors are made up of atoms that have very losely bound valence electrons. When forming a solid the electrons are largely ”free” to move within the solid boundaries and the atomic cores are set in place. The electrons hardly interact at all with the cores and just randomly collide with other electrons. To put it another way, the bulk of a conductor, the nucleis, are highly transparent to the ”electron-cloud”. When applying an electric field, the electrons will move with ease in the opposite direction of the applied field.
In Insulators the valence electrons are tightly bound to the atomic core and require a lot of energy to break free.
Semiconductors have properites that range in between the two previously described. These possess interesting characteristics and will be the main focus for us.
Semiconductors
Silicon, which is the dominant material used in semiconductors, has four valence electrons. Since all atoms strive to achieve the status of the noble gases they can bond covalently (share electrons) with four other silicon atoms to fill their outer most shell. This way they all share eight electrons rather than having four for them selves. Together they create a lattice that go to make up the solid. At low temperatures this arrangement tend to act like an insulating material, this because the valence electrons are tightly bound with other atoms and there is no thermal energy to free them from their bonds. When increasing the temperature of the semiconductor the lattice starts to vibrate more frequently, resulting in some of the bonds being broken. When a bond breakes, an electron is set free and leaves behind a hole. The free elctrons are now able to move in an externally applied electric field and are called the negative charge carriers. The holes are often filled by electrons in adjecent atoms, creating a new hole from the replacement-electrons former place. This way the holes move about too and are called the positive charge carriers.
Conduction Bands and Bandgaps
Before we move on with the semiconductors, we had better cover an important aspect of atoms and their valence electrons. There is more than one way to arrive at the bandtheory of solids and we have chosen to do it the intuitively easy way. Leaving quantum mechanics aside as much as possible.
For simplicitys sake, let us say we have an isolated sodium atom in the 3s state. This means it has only one electron in it’s outer shell, the third energy level (3), and will be in the ”first” orbit (s). If we were to let a second identical sodium atom move in from a far, the valence electrons of the atoms will start to interact more and more the closer they get. Because of their wavelike nature, the valence electrons of the two atoms will start to overlap when brought closer to eachother, and this will result in a splitting of energy levels, as shown in figure 1b.
As we bring more atoms together, there will be more splittings of the isolated-atom energylevels. The width of the energyband, designated ∆E, is always determined by atoms close enough to interact strongly. Because of the large number of atoms in any material, in the range of 1023 atoms/cm3, there will be many splittings in the close range of ∆E and it may be considered a continous band. Because of the complex inner workings of most atoms, there will be many different energy bands. The gaps in between these bands are called band gaps, or forbidden energy bands.
With a little imagination we can easily apply this example to silicon and other materials that go and make up the parts of most semiconductors.
Conduction
To understand conduction in metals we continue with our sodium sample. The outer shell is only occupied by one electron, which makes that particular band only half filled (fig. 2), since there can be two electrons in each orbital. This is called the Fermi Energy. The next band, 3p, actually overlaps with the 3s one, making it really close energywise. When an electric field is applied, the electrons that are close to the Fermi Energy, that is, the top most level, will easily be excited (jump up) to higher levels and are then free to move, forming a current.
Insulators on the other hand have filled energy levels and a large gap to the next energy band. The filled lower band is usually referred to as the valence band and the upper empty band as the conduction band. Since the difference in energy is large between the bands, it will require large amounts of energy to excite the electrons from the valence band all the way up to the conduction band, and is the reason for insulators high resistivity.
Semiconductors have a small yet distinct gap between the valence band and the conduction band. At lower temperatures they are still poor conductors because their electrons are all in the valence band. But in roomtemperature and above, the uppermost electrons are thermally excited (by the heat) and often located in the conduction band. Since there are many nearby energy states it requires little energy to form a current.
p-n junction
Probably the biggest application of semiconductors is the p-n junction. It consists of two parts, one positively charged (p) and one negatively charged (n). In order to get a neutral sample charged, either negative or positive, we have to do something called doping. Doping means that we take a neutrally charged sample, say silicon, and add impurities. As we described earlier, the silicon atom has four valence electrons and will bond to four other silicon atoms in a given sample. If we were to add a certain amount of atoms with five valence electrons, like arsenic, there would form new bonds with excess electrons. Because four of the arsenic electrons will bond to the nearby silicon atoms in the lattice, there will be one electron left over. This electron can almost be considered free and is easily influenced by externally applied electric fields. A sample with these characteristics would be negatively charged and called an n-type semiconductor.
To aquire a positively charged sample, a p-type, we would only have to replace the arsenic atom with say aluminum, that only has three valence electrons. With impurities like these there will form an excess of holes in the sample and a net positive charge. Materials that are doped are much better conductors than their neutral counterparts.
When the two samples are joined, it’s called a p-n junction. In every junction there are three areas of interest, one positively charged, one negatively charged, and one neutrally charged area in between, called the depletion region. This middle region arises naturally when the two pieces are brought together and some of the excess electrons of the n-type semiconductor move over to the p-type. The arsenic cores that are still impurities in the n-type are left in their place with and create a positive charge. Remember that arsenic has five protons in their core, unlike silicon with only four. This one extra proton will contribute to the net positive charge once the fifth electron has diffused over to the p-type semiconductor. The reverse goes for the aluminum atoms in the p-type part of the junction. This difference in charge creates an electric field, going from the positive part of the n-type semiconductor to the negative part of the p-type semiconductor. When strong enough, this electric field prevents further movements of electrons across the region.
If we were to apply an electric field to this junction and let the external field be positive on the p-type, the depletion region would shrink, letting more current through. If we on the other hand reversed the external field, the depletion region would widen, resulting only in a very mild current. The most important part of the characteristics of a p-n junction is that it can only pass current in one direction.
Light Emission and Light Absorption
If an electron is excited to a higher energy band it can easily jump back to a hole in the conduction band. When the electron was excited it was given enrgy from the electric field and when it jumps back down, this energy has to go somewhere. In the the transformation from the conduction band to the valence band, the energy is emitted in form of light of the equivalent energy of the band gap. Light Emitting Diodes (LEDs) and lasers are examples of applications of the fenomena.
An electron in the valence band might also absorb a light quanta (a small amount of energy called a photon) and be excited to the conduction band. Once in the conduction band it comes under the influence of an external field and contribute to a total current. The description of this phenomena, the Photo-Electric Effect, is what gave Albert Einstein the Nobel Prize in Physics in 1921 and is the fundamental mechanics of solar cells.
When creating solar cells there are a few aspects that we have to take into consideration. Only light with energy equal or higher than the band gap contributes to the excitation of electrons and the current we want. With a small band gap, much of the sunlights energy spectrum is absobed by the p-n junction, but it only creates a small voltage. If we want a high voltage output we have to use materials for our semiconductors that have a large band gap. The problem of course is that only a small part of the energyspectrum is absorbed. Finding the optimal solution can sometimes be a tedious process and often priorties have to be taken into account on what the most desired outcome is.
Lately, scientists have made use of multiple systems of cells. Instead of trying to absorb the whole spectrum with one cell, they have a high energy absorber on top and one or several low energy absobers underneath.