Photovoltaic
Like sand on a beach
Silicon is the second most frequently found element in the ground. Its simplest compound is silicon dioxide or silica (SiO2), also known as quartz sand.
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Silicon is the most important basic material used in the manufacture of semiconductors, which in turn are used to produce computer microchips, transistors and 99 % of all solar cells. In order to obtain semiconductor silicon, the oxygen must first be extracted from the silicon. The raw silicon thus obtained must then be cleaned by means of expensive processes in order to achieve the required extremely high level of cleanliness. The melt of this material is either cast into blocks or drawn into pillar-like crystals. A wire saw is then used to cut these silicon pillars into very thin slices, also called wafers, which form the basis for the manufacture of solar cells. |
The sand pillar on the right of the entrance gives an impression of the level of cleanliness required.
From Wafer to Cell
If n- and p-silicon are combined, the differently charged free charge carriers are attracted to each other, move into the respective neighbouring area and so charge it with electricity. A strong, inner electric field is created at the boundary layer, the pn-crossover.
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When illuminated, this field will separate the created charge carriers and a voltage of approx. 0.5 V is created at the outer contacts. The pn-crossover ensures that the charge carriers created by the light do not join up again, but can be used as current. Silicon solar cells are differentiated from untreated, extremely clean semiconductor material by two main stages: doping and pn-crossover. If small quantities of foreign atoms are introduced into the silicon ("doping"), free moving charge carriers are created according to the type of atoms. Phosphorus atoms lead to free electrons (n-silicon), boron to free holes (p-silicon). Doping therefore provides free charge carriers in otherwise isolating silicon. |
The introduction of light into an untreated silicon wafer creates free charge carriers, but these cannot be used and will therefore join up again after a short time.
With n-doping almost every thousandth silicon atom (Si, quadrivalent) is replaced by a phosphorus atom (P, pentavalent), creating - even without light incidence - free moving, negative charges (electrons). In the basic material almost every millionth silicon atom is replaced by a boron atom (B, trivalent).
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This p-doping leads to freely moving positive charges (holes). If n- and p-doped silicon are now combined, the electrons will move into the p-field and the holes into the n-field. The p-field charges itself negative and the n-field positive, thereby creating a strong inner field. The inner electric field separates the charge carriers released by the light incidence, and these are then collected by means of metallic contacts and flow back via a user element (e.g. a lamp), after having provided a service - thus completing the circuit. |
From Cell to System
Solar cells are fragile. In a solar module they must therefore be protected against ambient influences. In addition, most photovoltaic systems require further components such as batteries, charge controller(s) and power inverters to transform the solar current into a practical and usable form. How these components are used to create a sensible system must be tried out during practical tests with the modules. Here some of the components are explained in greater detail.
Solar current from the socket:
The power inverter and solar cells provide direct current. In order to feed solar current into the roof installation using the public mains network or to operate alternating current user units, a power inverter must first convert the direct current into the standard domestic 230 V alternating current. A good power inverter has an effectiveness in excess of 90 %.
Storage for solar current:
The battery, a system not connected to the mains network, is called a satellite system. Here a battery (charging process) stores the current produced by the solar cell. It then provides the electric user units with electricity during the dark hours or bad weather conditions (discharge process).
Everything under control:
In order to prevent the battery from being damaged by overcharging or excessive discharge in the case of satellite systems, a charge controller is fitted between the solar module, battery and electric user units.
Interactive factory: How to produce cells and systems? How do they work? Table of experiments: study properties of cells in an application show case: overview variety types.
Different types of Solar Cells
Monocrystalline Silicon Solar Cells (Pe-1)
Multicrystalline Silicon Solar Cells (Pe-2)
Gallium Arsenide Solar Cells (Pe-3)
Gallium Antimonide Cells (Pe-4)
Amorphous Silicon Solar Cells (Pe-5)
Copper Indium Gallium Diselenide Solar Cells (Pe-6)
Cadmium Telluride Solar Cells (Pe-7)
Crystalline Silicon Thin-film Solar Cells (Pe-8)
Dye-sensitized Solar Cells (Pe-9)
Mechanically Stacked Concentrator Tandem Solar Cells (Pe-10)
Monolithic Tandem Solar Cells (Pe-11)
Monolithic Tandem Solar Cells (Pe-12)
Efficiency
Only a part of the radiation reaching the solar cell is converted into electrical power. The efficiency indicates the proportion of the radiant power which is converted into electrical power by the solar cell. The other energy contained in the radiation is transformed into heat. That is why solar cells heat up more the stronger the radiation reaching them is. The solar cells are measured according to their efficiency and graded into the corresponding quality classes.
Quality control
The frontal contact deposited by means of screen pressure is inspected visually. The aim is to achieve lines which are as high and as narrow as possible, and without contractions, so that the electrical resistance of the lines and the shadowing of the cells by the metallization are as low as possible.
When the contacts have been sintered, the cells are turned round so that they can subsequently be measured. To this end the current-voltage characteristic of the solar cell is measured under defined conditions and the efficiency of each cell calculated.
Production line
From left to right: In the diffusion oven the phosphorous glass formed in diffusion are removed in the APCVD system (Atmospheric Pressure Chemical Vapor Deposition). This is followed by an operation in which an anti-reflective coat is deposited in order to improve the coupling of the sunlight into the cell. The previously bright metal wafer now appears blue, because blue light is being reflected. The reflection is the lowest for red light. The front and rear contacts are then deposited by means of the screen pressure process. The contacts are also sintered, i.e. the bars are compressed. In this way a good contact is established between the metallization and the silicon disk.
Solar cells are elegantly simple, made from special materials that are neither insulators (like plastics) nor conductors of electricity (like copper wire). When they are exposed to light and absorb photons (particles of light and other forms of electromagnetic radiation), these materials - called "semi-conductors" - allow an electrical current to be generated.
Photons contain various amounts of energy depending on the different wavelengths of the solar spectrum. This energy level determines what happens when photons strikes a photovoltaic cell, where they will either be absorbed, reflected or pass right through. Some of the absorbed photons generate electricity, others generate heat, and some never reach the external circuit.
Why do electrons move about?
The electrons in a semiconductor material live in a range of defined energy levels, each one known as a band.
The conduction band is partially filled with electrons, creating a negative charge.
The valence band has areas where electrons are missing - known as holes - equivalent to a positive charge.
In the absence of light, the positive and negative charges balance each other out. But when light energy in the form of protons strikes the semiconductor material, electrons are dislodged and the equilibrium is disturbed.
This causes electrons to move down an external circuit in the form of light-generated electricity: a phenomenon which is called the photovoltaic effect.
How much electricity is generated?
Around fifteen per cent of the energy of sunlight can be used to produce electricity using photovoltaic solar technology.
The individual solar cell's size determines the amount of current and power it is capable of producing - at most some 0.5 Volts (V). In order to generate a significant amount of electricity, several solar cells are assembled into modules.
Certain kinds of cells, such as those made from thin films, can be made directly into modules without needing to make separate cells first. Generally constructed to give an output somewhere between twenty W and 100 W, these modules can themselves be connected together to make arrays that could potentially supply several megawatts of power.