A solar cell is a solid state device which converts the energy from the sunlight into electrical energy by the photovoltaic effect. In order to generate useful power, it is required to connect a number of cells together to form a solar panel.Â The nominal output voltage of a solar panel is usually 12 Volts, and they may be used singly or wired together into an array. The number and size required is determined by the available light and the amount of energy required.
The amount of power generated by solar cells is determined by the amount of light falling on them, which is in turn determined by the weather and time of day. In the majority of cases some form of energy storage will be necessary.
Solar cells are made predominantly from pure silicon. Silicon naturally does not carry a positive or negative charge, but when it is combined with another charged element, it will take on that charge.
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For electricity to flow, a conductive link must be created between a positive and negative charge--therefore, solar cells are made up of two plates, one positive and one negative. The negatively charged plate in a solar panel is usually made from a silicon and phosphorus mixture, while the positively charged plate is often created from a combination of silicon and boron. The two plates of the solar cell are joined through a network of wires.
Solar cells work when natural sunlight, rich in photon particles, shines on the negative plate and excites the electrons that surround the bonded silicon and phosphorus atoms. The additional electron that makes the plate negative is freed from the atoms andÂ travelsÂ along the connective wiring to the electron-hungry and positively charged atoms on the silicon and boron plate.
This electron transfer creates energy, and the process will continue as long as photons shine on the negative plate. Energy that is not initially used can be transferred from the cell and stored in chemical batteries for future use. The amount of power a solar cell can produce is directly related to the size of the cell, as the number of electron transfers occurring at the same time determine the amount of energy harvested. Therefore, the greater the panel's surface area the more reactions can occur.
How does a solar cell work?
A solar cell operates by photogeneration of charge carriers in a light-absorbing material, and separation of the charge carriers to a conductive contact that will transmit electricity. When sunlight hits a solar cell, photons are absorbed by the semiconducting material, creating electron-hole pairs.
The PN junction prevents electron-hole recombination from taking place by separating the positive charges from negative charges. If the carrier recombines, then the light-generated electron-hole pair is lost and no current or power can be generated.
When the light-generated minority carrier reaches the PN junction, it is swept across the junction by the electric field at the junction, where it is now a majority carrier. For example, electrons from the p-side are swept across the junction to the n-side whereas holes from the n-side are swept across the junction to the p-side.
If the emitter and base of the solar cell are connected together, then the light-generated carriers flow through the external circuit and a short circuit current is produced.
If the light-generated carriers are prevented from leaving the solar cell, then the collection of light-generated carriers causes an increase in the number of electrons on the n-type side of the PN junction and a similar increase in holes in the p-type material. This produces a open circuit voltage.
Figure Typical Silicon Solar Cell
Figure Under Illumination
Figure Electron-hole Pair Generation
Figure Photocurrent Generation
Figure Photocurrent Generation
Energy Band Diagram
Figure Energy Band Diagram
The range of energies that an electron may possess in an atom is known as the energy band.
The three important energy bands are:
Valence Band is the range of energy possessed by valence electrons.
Conduction Band is the range of energy possessed by conduction electrons.
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Forbidden Band is the condition band between the Valence Band and Conduction Band.
The three solids classified into types are:
Insulators - The materials in which the conduction band and valence band are separated by a wide energy gap. A wide energy gap means that a large amount of energy is required to free the electrons by moving them from the valence band into the conduction band.
Since at room temperature, the valence electron of an insulator do not have enough energy to jump into the condition. Therefore insulators do not have the ability conduct current. Thus, insulators have high resistivity at room temperatures.
However, if the temperature is raised, some of the valence electrons may acquire energy and jump into the conduction band. It causes resistivity of insulator to decrease. Therefore, an insulator has negative temperature co-efficient of resistance.
Conductors - The materials in which conduction and valence bands overlap. The overlapping indicates a large number of electrons available for conduction.
Semiconductors - The materials in which conduction and valence bands are separated by a small energy gap. A small energy gap means that a small amount of energy is required to free the electrons by moving them from the valence band to the conduction band.
The semiconductors behave like insulators at 0K because there are no electrons in the conduction band. If the temperature is increased further, more valence electrons will acquire energy to jump into the conduction band. Thus, semiconductors are similar as of insulators as they have negative temperature co-efficient of resistance. This means that conductivity of semiconductors increases proportional to the temperature.
Doping of Boron
The purpose of P-type doping is to create an abundance of holes. Â In the case ofÂ silicon, a trivalent atom is substituted into theÂ crystal lattice. The result is that one electron is missing from one of the fourÂ covalent bondsÂ normal for the silicon lattice. Thus the dopant atom can accept an electron from a neighboring atom's covalent bond to complete the fourth bond.
This is why such dopants are called acceptors. The dopant atom accepts an electron, causing the loss of half of one bond from the neighboring atom and resulting in the formation of a "hole". Each hole is associated with a nearby negatively-charged dopant ion, and the semiconductor remainsÂ electrically neutralÂ as a whole. However, once each hole has wandered away into the lattice, one proton in the atom at the hole's location will be "exposed" and no longer cancelled by an electron.
For this reason a hole behaves as a quantity of positive charge. When a sufficiently large number ofÂ acceptorÂ atoms are added, the holes greatly outnumber the thermally-excitedÂ electrons. Thus, the holes are theÂ majority carriers while electrons are theÂ minority carriersÂ in P-type materials.
Possible causes for low current output
Firstly, it would be the sunlight's intensity. The higher the intensity, the higher current output will be produced. Thus, if the sunlight is less intense, it is unable to "kick loose" the electrons from their parent atoms and this reduces the flow of current.
Secondly, it would be the series resistance. When tying solar cells together, it is important to keep the resistance at a minimum. Resistance directly affects both voltage and current. Thus, an increasing resistance will cause the IV curve to move away from the MPP. Since the material in solar cell acts as a resistor to current flow, it is advisable to limit the amount of serially connected solar cells.
Thirdly, it would be the surface texturing. This is because the wafer surface had the solar cells in other shapes and thus, energy is lost through reflection. Surface texturing is used to increase the solar-cell surface area based on inverted pyramids which reduces reflection losses at the solar cell surface by promoting light trapping upon multiple reflections.
Fourthly, it would be shading. Even if one cell is shaded, it would affect the current output. This is because the solar cells are connected in a series string. The current output will decrease proportional to the percentage of the area of cells being shaded.
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Lastly, it would be the materials used in the wafer. The overall efficiency of a solar cell depends on the composition of the semiconductor, metallic contacts, presence and properties of anti-reflection coatings and type and architecture of the cell layers.
Ways to improve process to increase the efficiency of fabricated solar cells
Back surface field
Firstly, using the Etch-back of emitter, under the finger contacts, the structure has a deep junction and low sheet resistance. It also has a shallow junction and higher sheet resistance between fingers. In between the fingers, the shallow junction will increase the absorption of blue light into the base. This will increase the efficiency of the solar cell as blue light has the highest energy content.
Figure Types of surface
Secondly, having surface texturing, in a textured surface, light reflected can strike the surface of the silicon again. However, unlike in a flat surface whereby reflected light is being lost to the surroundings. Thus, trapping light and increasing the efficiency of the solar cell.
Thirdly, doing passivation, it is a process that lowers recombination rates in the bulk, grain boundaries or at the surface. With lower recombination rates, there will be many free electrons moving around.
Figure Anti-Reflection Coating
Fourthly, by applying anti-reflection coatings, it will cause the wave reflected from the anti-reflection coating top surface to be out of phrase with the waves reflected from the semiconductor surfaces. Thus, minimizing light reflections and increasing the transmission coefficient.
Figure High surface recombination velocity
Fifthly, by having the formation of back surface field, the back surface field refers to the built-in electric field at the rear surface which reduces the recombination rate and also acts as a blockage to minority carriers.
Lastly, by having gettering, there are two types of gettering, phosphorus diffusion from the front side and aluminum alloying at the back surface. It is a process of removing the device-degrading impurities from the active circuit regions of the wafer.
The wafer has to go through many stages in the process flow to be fabricated and when the sunlight hits the wafer, many things are occurring in the wafer which converts the sunlight energy to electrical energy which we cannot see through the naked eye. The output of the solar cell is usually associated with the sunlight intensity. However, there are many more factors which affect the solar cell output. With regard to this, there are also many ways to improve the efficiency of the solar cell so as to give the certain amount of output that is sufficiently needed.