I look at the construction of silicon based PV panels and other types of solar PV panels. This article comes from discussions about what solar panels present value for money. Compare output for the entire installed system (in Watts) against ‘dollars per watt’.
This article is about how monocrystalline silicon fits into solar photovoltaic (PV) modules. I discuss solar efficiency and the cost of production of the types of solar modules. Commercial monocrystalline modules have an advertised efficiency of around 14.9%. Polycrystalline modules have an advertised efficiency of 14.1%.
Those efficiency figures come from trade catalogues. They show that these generic PV types perform with similar efficiency. The modules look different in appearance because the manufacturing process is different. We will get into the similarity and differences later through this writing.
Solar Panels Generally
Buildings need ample roof space facing in the correct orientation for erected solar panels to work near their peak efficiency. Efficiency is not the single most important consideration in solar system design.
The roof orientation with its slope plane usually fixes the tilt angle of solar panels. This aspect impacts your system performance. Your system’s performance depends on there being no overshadowing. Also size and quality of individual components affect the performance results you achieve. The total system cost accrues from panels, brackets, solar inverter, wiring and switches, installer’s costs, and any ancillary solar equipment.
Some people worry about selecting the right component or a particular one of these technologies. If you are wondering whether all that is important, I hope this article will assist with your answer.
Have you wondered about what is in that solar rooftop installation? New modules look interesting. Are there advantages of one type of solar panel array, inverter, battery type, or whatever, over some other type?
Solar PV Panels Made With Silicon
There are several types silicon used in the manufacture of PV cells in solar panels. The structures of silicon in amorphous, polycrystalline and monocrystalline silicon based systems. I will explain these types of silicon-based solar cells as we progress through this article. These names reflect silicon categories of structure from not crystalline to very crystalline frameworks.
Wikipedia has a good schematic of silicon atomic arrangements.
All of the solar PV technologies have continued to improve over time. Rooftop solar PV modules are usually made using monocrystalline or polycrystalline silicon cells and they develop from proven PV technologies. The typical monocrystalline solar cell is blue-black colour with octagon corners. Polycrystalline solar cells are a lighter dark blue colour with some lighter variation shades within.
Semiconductors Are Made From Silicon
Solar Energy Cells are made out of silicon or wafers coated in silicon semiconductor material. A semiconductor is a special type of material that conducts electricity at certain conditions. For a solar energy cell it depends on the different amounts of light of particular wavelengths that it accepts. The PV semiconductor material accepts that particular light energy and converts it into electricity.
There are several types of materials that can be classified as semiconductors. Silicon is the most common one used in the photovoltaics and solar panel industry. Silicon is the second most abundant element in the earth’s crust.
Metals, Non-Metals and Metalloids
I will briefly explain a few terms that will be used later in the article. When you stay with this story you will understand why PV solar works. For greater detail for semiconductor sources and some definitions try looking here.
First let me explain metals, non-metals and metalloids so you understand where silicon fits into the PV solar story.
In the context of the elementary periodic table a metal is a chemical substance that readily conducts electricity and heat. Metals are good conductors and are used for electrical wires and contacts. An alloy is a mix of metals and it’s also a conductor. A metal’s conductivity decreases while increasing the temperature of the metal substance.
Metals exist in crystalline atomic structures that provide hardness yet are malleable, ductile and have (metallic) lustre. A larger number of elements on the periodic table are metals.
Usually non-metals are located on the right side of elementary periodic table. Non-metals tend to be brittle, easily vaporized, and poor conductors of electricity and heat. They are good as insulators. The non-metal chemical elements include carbon, nitrogen, fluorine, phosphorus and sulphur. Non-metals oxides form acids and those that have stable compounds with hydrogen form negative ions when in solution.
Seventeen elements are classified as non-metals. Hydrogen, helium, nitrogen, oxygen, fluorine, neon, chlorine, argon, krypton, xenon and radon are gases. The only liquid non-metal is bromine with the others of carbon, phosphorus, sulphur, selenium, and iodine being solids.
A metalloid on the elementary periodic table is an element in the table positioned between metals and non-metals. Metalloids can hold properties from both metals and non-metals or a mixture of them. Boron, silicon, germanium, arsenic, antimony, tellurium, and polonium are metalloid substances. Metalloid substances have an electrical conductivity between that of a metal and a non-metal.
Semiconductors: Using Silicon & Germanium
Some metalloid substances, such as silicon and germanium, have semiconductor properties. A semiconductor’s conductivity increases with temperature. A conductor with the addition of particular impurities can develop some semiconducting properties.
Silicon occurs naturally within clay, feldspar, granite, quartz and sand. Silicon (Si14) is a tetravalent metalloid element. The silicon crystal structure is an ordered tetrahedron, with each atom in a pyramid position with electrons exhibiting predictable behaviours.
Silicon is an atom with four valence electrons in its outermost electron shell. That means the atom has a chemical valence of four. Silicon could combine with two O2 forming silicate (SiO4). The mineral composition of silica (SiO2) is quartz. Next to Oxygen, Silicon is the second most abundant element in the earth's crust.
Germanium (Ge32) is a brittle grey crystalline element that is a used in making semiconductor transistors. Germanite is the source of this semiconductor element. It is quite a rare mineral that consists as copper iron germanium sulfide.
Argyrodite is another source of the semiconductor element Germanium. Argyrodite is a rare steel-grey mineral that whose chemical composition is Silver Germanium Sulfide (Ag8GeS6).
Semiconductors And Electron Flows
In electronics conductors are devices designed to transmit electricity in a circuit. Semiconductors are conductors made with a semiconducting material like silicon. A silicon chip is a semiconductor device in electronic equipment consisting of a small crystal of a silicon semiconductor material. The chip is usually fabricated to carry out certain functions in an electronic circuit.
A semiconductor device may contain of a p-n junction, which consists of:
- n-type semiconductor –a semiconductor in which electrical conduction is due to the movement of negative charged electrons.
- p-type semiconductor – a semiconductor in which electrical conduction is due to the movement of positive holes (places in the atom’s structure from which electrons have been displaced).
A solar cell contains semiconductor material in layers that have p-n junctions.
Production of Silicon Semiconductor Cells
The purity of silicon in polysilicon semiconductors produced for the electronics industry is disproportionately better than solar grade polycrystalline silicon. China, Germany, Japan, Korea and the United States together produce most of the world’s solar grade polycrystalline silicon.
There are several types of solar energy cells that take their name from the general categories of silicon semiconductor used.
Monocrystalline silicon (Mono-Si) means single crystal of silicon. That mono crystalline framework is homogenous because the entire section is from one single, continuous and unbroken crystal. Such mono crystals are not found in nature but can be produced with a crystallization process.
Monocrystalline silicon is the material that absorbs light in the solar photovoltaic cells to produce electricity. Each solar cell monocrystalline silicon wafer is sliced from a long sausage shaped ingot extruded from molten silicon liquid. Mono-Si wafers are pure silicon crystal structures without grain boundaries. That means the wafer of pure silicon crystal is solid and unbroken to its edges.
Monocrystalline silicon is developed from expensive refining techniques using pure polysilicon feedstock. The Czochralski process involves filtration, separation and recrystallization to develop large single monocrystalline ingots.
Monocrystalline silicon is an expensive but more efficient semiconductor material base than polycrystalline silicon. These purer silicon ingots are sliced into thin silicon wafers that will be used for the production of semiconductors.
Polycrystalline silicon is also called polysilicon (Poly-Si). These polysilicon semiconductors consist of many small fused crystals (crystallites). Poly-Si semiconductors are used in the photovoltaic devices like solar cells.
High quality raw silicon is cast into bricks that are manufactured into Polysilicon semiconductor slices. These semiconductor materials are used in the solar photovoltaic and electronics industries.
Polysilicon completes fewer energy intensive filtration and separation processing cycles than are required of monocrystalline so it’s cheaper to produce. The Poly-Si material can be cast into square blocks of silicon out of which the solar cells are cut.
Polysilicon is produced from metallurgical grade silicon within the Siemens purification process. The Siemens process involves breaking down volatile silicon compounds at high temperatures to form pure silicon. New refinement processes are evolving to lower the costs of silicon for meeting increasing demands from the photovoltaic industry.
Polysilicon when referenced as multicrystalline silicon usually refers to semiconductor cells with crystals larger than one micrometer. Each grain is crystalline over the width of the grain. The typical grain size should be as large as possible for maximum solar cell efficiency. Multicrystalline solar cells are the most common polysilicon product type in the solar consumer market. The grain boundary separates the grains where the adjoining grain is at a different orientation from its neighbor.
The multiple crystal structures of a multicrystalline silicon blocks have contiguous crystals that touch at defined ‘grain boundaries.’ Grain size affects the efficiency of polycrystalline solar cells. Solar cell efficiencies increase with grain size. This effect is due to reduced recombination of electrons and holes in the solar cell.
Amorphous silicon (a-Si) differs from single crystal monocrystalline silicon. It is also different to polycrystalline silicon, which consists of small grain crystals. Amorphous silicon is the non-crystalline form of silicon.
The a-Si material uses lower purity silicon for making solar cell manufacture. Solid amorphous silicon is not crystalline so the atoms and molecules are not organized in the Silicon crystal lattice pattern. Moreover any crystal grain boundaries that exist do so with gaps and spare bonds at boundary edges.
Amorphous silicon was the first type of semiconductor material used for solar energy cells. They are used in consumer products such as watches, solar calculator and non-critical outdoor applications due to their low cost.
Amorphous silicon is used as semiconductor material for flexible substrate solar cells. It is the type of semiconductor material being adapted for thin-film silicon solar cell technology. Also a-Si can be layered in thin films onto a flexible substance, such as glass, metal or plastic.
A downside of Amorphous silicon cells is they have a much lower PV efficiency than polysilicon and monocrystalline silicon. The upside is the a-Si semiconductors use less silicon material and cheaper technology to produce. That means the lack of PV efficiency can be made up in other ways, such as greater solar PV area.
The starting point of this article topic was about explaining something of the Monocrystalline silicon solar PV cell. Commercially manufactured solar PV panels use silicon for semiconductor device technology. Silicon is the second most common element in the earth’s crust.
I have generally covered the make-up of Solar panel silicon noting that it’s the source feedstock of most semiconductor material. The installed and wholesale prices of solar panels have been dropping over the last ten years.
In 2008 there were only twelve factories in the world producing solar-grade polysilicon. At the end of 2013 over 100 factories were manufacturing solar-grade polysilicon. The semiconductor's growth of demand and supply has been fantastic.
The various types of silicon produce different semiconductor technologies. It’s the processed quality of silicon that creates good solar panels that we rely on to effectively produce renewable electricity.