Photovoltaic technologies

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Technologies

"Nature uses only the longest threads to weave her patterns, so each small piece of her fabric reveals the organization of the entire tapestry."
(Richard Feynman)

Silicon - atomic structure Clean room - solar cell production Solar cell manufacturing Solar cell testing

From silicon to the solar cell
(source/copyright: Hahn-Meitner-Institut Berlin)

According to the crystalline structure amorphous, poly-crystalline and mono-crystalline solar cells are known. According to technological procedures used by production solar cells can be divided into silicon solar cells, produced from Si wafers, and thin-film solar cells produced with vacuum technologies. Basic features from different solar cell types can be found below. Solar cells are connected together and many solar cells represent a solar module with typical power range of up to 100 W or even more. For large PV system special PV modules are produced with typical power range of up to several 100 W. The solar module properties depend mainly on the solar cell type used. The most important tasks in the future are utilisation of less pure silicon and increasing efficiency (monocrystalline solar cells) and increasing efficiency and life-time (amorphous solar cells).


Solar cell materials - production and features
Silicon The most important material for solar cells production is silicon. At the time being it is almost the only material used for solar cell mass production. As the most often used semiconductor material it has some important advantages: In nature it can be easily found in large quantities. Silicon oxide forms 1/3 of the Earth's crust. It is not poisonous, and it is environment friendly, its waste does not represent any problems. It can be easily melted, handled, and it is fairly easy formed into mono-crystalline form. Its electrical properties with endurance of 125°C allow the use of silicon semiconductor devices even in the most harsh environment and applications. In technics, pure silicon is the only widely used chemical element produced so pure. The percentage of pure silicon in material is at least 99.9999999 %. According to density of silicon, which is 5x10²² atoms/cm³, it means 5x10¹³ impure atoms/cm³. Impure atoms values are investigated due to numerous specific physical methods like mass spectrometry and similar sophisticated measurements. Pure silicon is produced from sand (SiO₂). In production the following procedure steps are used: Pure silicon is produced from silicon by reduction in specially designed furnaces at 1800°C. The produced material contains 98-99% of pure silicon. As a reducer carbon electrodes are used. The complete reaction is as follows: SiO₂ + C -> Si + CO₂ Silicon - atomic structure (source/copyright: Hahn-Meitner-Institut Berlin) Such silicon is used as raw material in production of pure silicon. It is also used in steel and aluminium production procedures as a supplement material. The most important producers of raw silicon are Canada, Norway and Brazil. 15 to 25 kWh of electrical energy is needed to produce a kg of silicon. We get silicon tetra-chloride (gas) by chlorination of fine ground metallurgic silicon in special reactor. Additions or impurities are eliminated in the form of chlorine salt. Si + 2Cl -> SiCl₄ The following reactions result in tri-chlorine-silan gas: SiCl₂ + HCl -> SiHCl₃ The gas is then additionally purified, removing any remaining tetra-chlorine-silan and other silans. The purifying is followed by reduction in hydrogen atmosphere at 950°C: 4SiHCl₃ + H₂ -> 2Si + SiCl₄+ SiCl₂+6HCl Besides pure silicon the procedure results in a number of side products. Their origin is gaseous and they condense outside of the reactor. Tetra-chlorine-silan is one of the side products. At 1200°C, it can be converted into tri-chlorine-silan using the following reaction: SiCl₄ + H₂ -> SiHCl₃ + HCl The presented example depicts one possible way of producing pure silicon. There are other production procedures with different chemical reactions used, yet the end product is the same - pure silicon.	Polycrystalline silicon production The procedure of extracting pure poly-crystalline silicon from tri-chlorine-silan can be (among others) performed in special furnaces developed by Siemens. Furnaces are heated by electric current, which flows through (in most cases) silicon electrodes. 2 m long electrodes measure 8 mm in diameter. The current flowing through electrodes can reach up to 6000 A. The furnace walls are additionally cooled preventing formation of any unwanted reactions due to gas side products. The procedure results in pure poly-crystalline silicon used as a raw material for solar cell production. Poly-crystalline silicon can be extracted from silicon by heating it up to 1500°C and then cooling it down to 1412°C, which is just above solidification of the material. The cooling is accompanied by origination of an ingot of fibrous-structured poly-crystalline silicon of dimensions 40x40x30 cm. The structure of poly-crystalline silicon in part of the material is settled, yet it is not adjusted to the structure of the other part. Mono-crystalline silicon production Two different technological procedures are used to produced mono-crystalline silicon from pure silicon: Czochralski method: By utilization of the Czochralski method, silicon is extracted from melt in induction oven with graphite lining at the temperature of 1415°C. Silicon crystal of defined orientation is placed on a rod. In the melt, spinning the rod makes the crystal grow. The rod spinning speed comes to 10 to 40 turns per minute, whilst the movement at length comes between 1 micro-metre and 1 millimetre per second. It allows production of rods, which measure 30 cm in diameter and several metres in length. It all takes place in inert atmosphere. Possible impurities burn or eliminate in the melt. More about Czochralski method (german language)... (1600 kB) Float zone: Using float zone monocrystalline silicon is produced from polycrystalline silicon. The main advantage of this procedure is higher pure silicon production. The silicon rods produced Silicon rod measure 1 m in length and 10 cm in diameter. The procedure, where induction heater travels along the rod melting silicon, also takes place in inert atmosphere. Mono crystal silicon originates from the cooling. Monocrystalline or polycrystalline silicon ingots are then sawn and the wafers are worked upon until they can serve as foundation for solar cell production. By sawing approximately 50% of material is wasted. Production of Amorphous Silicon Amorphous silicon is produced in high frequency furnaces in partial vacuum atmosphere. At presence of high frequency electrical field, gases like silan, B₂H₆ or PH₃ are blown through the furnaces supplying silicon with boron and phosphorus. Solar cell materials and solar radiation spectrum (source/copyright: Hahn-Meitner-Institut Berlin)
Gallium arsenide (GaAs) GaAs is used for production of high efficiency solar cells. It is often utilized in concentrated PV systems and space applications. Their efficiency is up to 25%, and up to 28% at concentrated solar radiation. Special types have efficiency over 30%. Cadmium telluride (CdTe) Thin-film material produced by deposition or by sputtering is a promising low cost foundation for photovoltaic applications in the future. The procedure disadvantage is poisonous material used in production. Lab solar cells efficiency is up to 16%, whilst the commercial types efficiency is up to 8%. Copper-indium-diselenide (CuInSe₂, or CIS) Thin-film material with efficiency of up to 17%. The material is promising, yet not widely used due to production specific procedures.	CIS solar cell (source/copyright: Hahn-Meitner-Institut Berlin)

Technologies

Crystalline solar cells

Polycrystalline as well as monocrystalline solar cells belong into this group. The basic form for crystalline solar cells production is silicon ingot (please see production procedure description above). The ingot (block of silicon), sawn with diamond saw into thin silicon wafers, is a foundation for solar cell production. Wafers of 1 mm in thickness sawn with 1/10 mm precision are placed between two plan-parallel metal plates, which rotate into opposite directions. The procedure enables wafer thickness adjustment to 1/1000 mm precisely. The subsequent solar cell production procedure consists of the following steps:

Doped wafers are first etched some micro-metres deep. The procedure removes crystal-structure irregularities caused by sawing and provides wafer cleaning. The material is doped as melt at polycrystal silicon or adequate gas is added whilst extracting pure silicon.
The above procedure is followed by diffusion. Phosphorus, which is supplied inside the material in gaseous form, diffuses at the temperature of 800°C. N doped layer and oxide layer rich with phosphorus form on top of wafers due to oxygen reaction.
Wafers are then folded to form a cube and etched in oxygen plasma, removing N layer from the edges.
The following phase removes oxide layers from top of wafer by wet chemical etching.
In the back, contact surface is produced from silver containing 1% aluminium. Special procedures enable silver print over mask on cell surface.
Pressed cells are then sintered at high temperatures.
Similar procedure is used to print contacts in the front cell surface.
Anti-reflex layer is applied in a similar manner. We have titanium paste at choice, which at sintering form titanium dioxide TiO₂ or silicon nitride Si₃N₄.

Amorphous solar cells

Amorphous solar cells are produced with similar technological procedures than integrated circuits. Due to the procedure these modules are also known as thin-film solar cells (thin-film modules). Herein, amorphous solar cells production is described briefly:

Glass substrate is thoroughly cleaned.
Lower contact layer is applied
The surface is then structured - divided into bands.
In vacuum, under high frequency electric field amorphous silicon layer is applied.
The surface is re-banded.
Upper metal electrodes are fixated.

Other solar cells

Among less frequently used solar cell types we find solar cells produced by EFG (Edge Defined Film fed Growth) method and Apex solar cells from silicon, cadmium telluride solar cells and copper-indium selenide (CIS) solar cells. EFG monocrystalline solar cells are produced directly from silicon melt eliminating sawing to wafers, which results in lower production costs and material saving for there is no waste due to sawing. Using EFG procedure, a silicon ribbon shaped in proper tube with eight flat sides is drawn from silicon melt. The tube length amounts to several metres. Flat sides are sawn by laser into separate solar cells. Most solar cells are proper square shaped in dimension of 100x100 mm. Consequently, the module power is greater with lesser surface compared to crystal modules of square shaped cells with truncated sides. Contacts are made in shape of copper bands. Separate cells are then combined in a similar manner than with other cell types. EFG cells are produced by Schott Solar. In contrast to EFG cells, Apex cells are poly-crystalline. Their production procedure is protected. Production procedure was developed by Astropower Inc. Cadmium telluride and copper-indium selenide (CIS) cells are thus far scarcely used, mostly in lab research. Commercial modules from above mentioned materials are still hard to find. In the table below you will find comparison between different solar cell types with their advantages and disadvantages.

Material	Thickness	Efficiency	Colour	Features
Monocrystalline Si solar cells	0,3 mm	15 - 18 %	Dark blue, black with AR coating, grey without AR coating	Lengthy production procedure, wafer sawing necessary. Best researched solar cell material - highest power/area ratio.
Polycrystalline Si solar cells	0,3 mm	13 - 15 %	Blue with AR coating, silver-grey without AR coating	Wafer sawing necessary. Most important production procedure at least for the next ten years.
Polycrystalline transparent Si solar cells	0,3 mm	10 %	Blue with AR coating, silver-grey without AR coating	Lower efficency than monocrystalline solar cells. Attractive solar cells for different BIPV applications.
EFG	0,28 mm	14 %	Blue, with AR coating	Limited use of this production procedure Very fast crystal growth, no wafer sawing neccesary
Polycrystalline ribbon Si solar cells	0,3 mm	12 %	Blue, with AR coating, silver-grey without AR coating	Limited use of this production procedure, no wafer sawing neccesary. Decrease in production costs expected in the future.
Apex (polycrystaline Si) solar cells	0,03 to 0,1 mm + ceramic substrate	9,5 %	Blue, with AR coating, silver-grey without AR coating	Production procedure used only by one producer, no wafer sawing, production in form of band possible. Significant decrease in production costs expected in the future.
Monocrystaline dendritic web Si solar cells	0,13 mm incl contacts	13 %	Blue, with AR coating	Limited use of this production procedure, no wafer sawing, production in form of band possible.
Amorphous silicon	0,0001 mm + 1 to 3 mm substrate	5 - 8 %	Red-blue, Black	Lower efficiency, shorter life span. No sawing necessary, possible production in the form of band.
Cadmium Telluride (CdTe)	0,008 mm + 3 mm glass substrate	6 - 9 % (module)	Dark green, Black	Poisonous raw materials, significant decrease in production costs expected in the future.
Copper-Indium- Diselenide (CIS)	0,003 mm + 3 mm glass substrate	7,5 - 9,5 % (module)	Black	Limited Indium supply in nature. Significant decrease in production costs possible in the future.
Hybrid silicon (HIT) solar cell	0,02 mm	18 %	Dark blue, black	Limited use of this production procedure, higher efficiency, better temperature coefficient and lower thickness.

Overwiev of solar cell materials

Photovoltaic manufacturing equipment
	Applied Materials - is a leading provider of advanced thin film deposition equipment and service. The Applied SunFab Solar Module Production Line enables customers to manufacture world class, 5.7m² thin film silicon photovoltaic modules. The Applied SunFab Line delivers state-of-the-art manufacturing capability needed to produce the lowest cost per watt modules. Languages: Webmaster's choice - interesting on-line presentations available.
	GT Solar - Company offers wide range of photovoltaic (PV) fabrication lines and PV manufacturing equipment. Languages:
	NPC group - The NPC group, whose headquarters is located in Tokyo, Japan, has been providing its clients with the most advanced vacuum-related and automated machines over 40 years. Languages:
	Oerlikon - Oerlikon Solar offers field proven production solutions for Thin Film Silicon Solar Modules based on its extensive experience in thin film mass production technology. Languages:
	Spire - Spire Corporation is the world's leading supplier of the manufacturing equipment and technology needed to manufacture solar photovoltaic power. Languages:
	ULVAC - ULVAC is an international corporation that designs, manufacturers and markets equipment and materials for industrial applications of vacuum technology. Languages:
Photovoltaic technologies related web sites
	PV-Tech.org - is a wholly-owned subsidiary of Semiconductor Media Ltd. Our aim is to provide the most up-to-date independent news coverage of developments within the photovoltaics industry in an easily accessible and navigable format. Languages:
	Wafernet, is an innovative silicon resource on the web. Many interesting information about silicon and wafer manufacturing. Languages: Webmaster's choice - interesting presentations about silicon wafer manufacturing available on-line.

Perspective or especially interesting technologies
	Evergreen Solar - brief description of perspective string ribbon production procedure. Languages:
New technologies
	Kyosemi Corporation - provides high reliability products of opto-electronics to be used for optical communications, ATM, auto-vending machines, OA/FA equipments, precision optical instruments and hazard prevention equipments. Languages:
	Global Photonic Energy Corporation - Incorporated in 1994 Global Photonic Energy Corporation, Inc. is a renewable energy technology development company. GPEC is harnessing photonic energy (Sunlight) using small-molecule organic materials to produce electricity and hydrogen - or - "Photo Fuel™". Languages:

Technology specific information

Jäger-Waldau, A.: Environmentally Research, Solar Cell Production and Market Implementation in Japan, USA and the European Union, EUR 20850 EN - European Commission, DG JRC, Institute for Environment and Sustainability, Renewable Energies Unit, Ispra, Italia (655 kB).
Jäger-Waldau, A.: Environmentally Status of PV Research, Solar Cell Production and Market Implementation in Japan, USA and the European Union, EUR 20245 EN - European Commission, DG JRC, Institute for Environment and Sustainability, Renewable Energies Unit, Ispra, Italia (383 kB).
Lauinger, T., Schmidt, W., Wösten, B.: EFG-Silicium: Material, Technologie und zukünftige Entwicklung, ForschungsVerbund Sonnenenergie, 2000 - Germany (183 kB).
Schönecker, A., Laas, L., Gutjahr, A., Goris, M., Wyers, P., Hahn, G., Sontag, D.: Ribbon-Growth-on-Substrate: Status, Challenges and Promises of High Speed Silicon Wafer Manufacturing, 12th Workshop on Crystalline Silicon Solar Cells, Materials and Processes (102 kB).
Madou, M.: Si crystal growth, Si crystal orientation, oxidation and interface defects-3 (3496 kB).