Abb. 2: Die aus den Blöcken geschnittenen Siliziumscheiben werden zu Zellen weiterverarbeitet.
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From quartz sand to crystalline solar modules

The path taken from quartz sand via the silicon preparation and the wafer manufacture to a standard solar cell and the further processing to form a laminated solar module with single-pane safety glass and a Tedlar rear side is described below:

Silicon production

After oxygen, silicon is the second most common element in the earth’s crust, where it occurs in the form of oxides and silicates. Millions of tonnes of metallurgic silicon are produced from quartz stone each year. It is manufactured at a process temperature of more than 2,000 °C in electric arc furnaces for the metal and plastics industry at a price of around EUR 1/kg.

Silicon purification

For semiconductor applications, silicon is only allowed to contain a few foreign atoms. Whereas a particularly high degree of purity is required for microelectronics (so-called electronic grade eg-Si has atomic impurities of less than 1 ppb), the requirements for solar grade sog-Si are considerably less for photovoltaics (99.99% or 1 ppm). Around 100,000 tonnes of solar grade silicon are currently produced each year. The Siemens process is usually deployed in which metallic silicon reacts with hydrogen chloride at 650 °C to form liquid trichlorsilane, which is purified in an energy-intensive distillation process. It is then evaporated and, with the help of hydrogen, deposited as polycrystalline silicon on hot silicon ingots. The increasingly thicker ingots have to be replaced on a weekly basis. The entire process requires between 100 and 160 kWh of electrical energy to purify one kilogram of silicon.

A new fluidised bed process avoids these interruptions and the use of energy when restarting the process. With the granulate reactor developed by Wacker Chemie in Burghausen, Germany, hot granular silicon provides the start material for depositing the silicon. They grow to a diameter of up to a millimetre and can be removed as granules during the ongoing operation. The pilot plant is already in operation and a production reactor with an annual capacity of 500 tonnes is being planned.

Another approach is being taken with the joint SUNSIL 2010 project, which is being carried out by the company Joint Solar Silicon. This is concerned with the cost- and energy-efficient manufacture of solar silicon from monosilane. The purified monosilane decomposes in an 800 °C tube reactor to form silicon powder, which can be continually removed. All process stages are being investigated, including the pyrolysis of the silane, the deposition of the silicon powder, the mechanical or thermal post-treatment of the product as well as its universal applicability in the downstream processing stages. A preliminary pilot plant with an annual capacity of 850 tonnes was already realised in 2008.

Crystal growth

Purified polysilicon is then melted, doped with boron and hardens to form rectangular, multicrystalline ingots weighing up to several 100 kilograms or cylindrical monocrystalline ingots weighing approximately 100 kilograms. These crystal ingots are mechanically processed to form blocks with a length of 156 mm. A new experimental crystallisation plant enables multicrystalline silicon ingots to be more quickly manufactured up to a weight of 1,000 kg. In a plant for crystallising multicrystalline Si ingots, Fraunhofer ISE is developing new crystallisation processes, whereby it is using innovative silicon sources: the institute is testing upgraded metallurgical-grade (umg) silicon with the aim among other things of achieving highly doped silicon wafers as a substrate for crystalline silicon thin-film solar cells, the so-called wafer equivalent concept. 24 companies and research partners have combined together as part of the joint ‘SolarFocus – SolarSilizium- Forschungs-Cluster’ project. They are investigating the currently used silicon, in particular in terms of the influence of impurities and structural defects on the efficiency of solar cells.

Wafer production

The wafers, roughly 200 μm-thick silicon discs, are cut with wire saws from the crystal ingots. Although the resulting silicon swarf (2010: 50,000 tonnes) is still not recycled, the considerably more valuable SiC is reused. Process innovations are aimed at considerably reducing the sawdust. The intention is to achieve saw widths of 100 μm and cost benefits of up to 20% in the sawing process. Schott Solar is drawing on its experience gained from its previous use of Edge-defined Film-fed Growth (EFG) wafer production, which is saw-free, in order to improve the material quality of ingot-crystallised, multicrystalline silicon wafers. Together with the University of Konstanz Solar World Innovations is developing a film-drawing process in which the wafer is drawn very quickly out of liquid silicon.

Cell design and production

For manufacturing the solar cells, the wafer is cleaned and generally etched to produce a textured surface. This reduces the reflection of the sunlight and considerably increases the light absorption, particularly in the longwave part of the spectrum. A highly concentrated phosphorus dopant is diffused into the wafer to a depth of 0.3 μm at a temperature of around 900 °C. This creates a so-called pn-junction on the surface, which consists of a p-base doped with boron and an n-emitter doped with phosphorus that turns the Si wafer into a large-scale semiconductor diode. In order to improve this solar cell, a hydrogen-containing, 70 nm-thick silicon nitride layer is deposited on the front side using a plasma enhanced chemical vapour deposition (PECVD) process. This layer acts as an anti-reflection layer and gives the solar cell its blue colour. Positive space charges and the hydrogen content in the nitride enable the n-type emitter to be particularly well passivated, i.e., only a few of the photo-generated charge carriers are lost on the front side of the solar cell. With the current standard metallization used, metal-containing pastes are applied using screen-printing processes to the front and rear sides and fired in a joint stage to form stable contacts. A finger and busbar grid made of silver is applied to the front side and full-surface metallization made of aluminium is applied to the rear side that creates an aluminium-doped back surface field (BSF). In addition, solderable silver pads are printed on the rear side.

Intensive research work is being conducted on all subprocesses in cell production. For example, together with industry partners Fraunhofer ISE’s Labor- und Servicecenter Gelsenkirchen is developing a new process for achieving high efficiencies with thin silicon wafers (d < 180 μm). The core task is to achieve rear passivation by means of one-sided etching and cleaning processes, and by using PECVD-produced dielectric layers and layer systems that are not damaged during the metallization.

The University of Konstanz is working on transferring new back surface processes for solar cells into production lines. These include dielectric passivation, boron-doped back surface fields and the definition of local contact areas. The front side is executed in accordance with industrial standards using screen-printing metallization in order to facilitate the introduction into production lines.

Optimisation of solar cells

HToday’s industrially produced standard solar cells achieve efficiencies between 14 and 17%. Depending on the performance, researchers accordingly distinguish between high efficiency (17 to 20%) and very high efficiency (> 20%) solar cells. Laboratory cells are now achieving efficiencies of 25%. Fraunhofer ISE is working on very high efficiency cells where materials and processes from microelectronics are deployed. All German cell manufacturers are now working on high efficiency concepts for solar cells with efficiencies of 20% and more. For example, Fraunhofer ISE is developing processes for the industrialmanufacture of solar cells based on PERC and MWT-PERC structures (Metal Wrap Through – Passivated Emitter and Rear Contact) with the aim of achieving very high quality passivation and very fine metallization and doping structures. In addition, it is also planned to manufacture modules that achieve 17.5% efficiencies using PERC structures and 18% efficiencies using MWT-PERC structures. Q-Cells is developing a highly efficient Emitter-Wrap-Through (EWT) solar cell for mass production based on industrial polycrystalline 6-inch silicon material with an envisaged cell efficiency of 18.5%, whereby it is also investigating the usability of upgraded metallurgical grade (umg-) Si for the EWT high-efficiency process. Q-Cells is also working on a second generation of rear side-collecting monocrystalline silicon solar cells for large-scale industrial production and the accordingly required electrical connections in the module. They are looking to achieve a module efficiency of 18%.

With its new bifacial rear-contact solar cells, Institut für Solarenergieforschung (ISFH) in Hameln is demonstrating the potential efficiency of newly developed technologies for manufacturing industrial solar cells. It is deploying local etching and deposition of functional layers, lowtemperature surface passivation, metallization of both contacts in a vapour deposition process and metallization firing at low temperatures of less than 300 °C. This noncontact technology reduces the mechanical wear, less material is used and the metallization is reduced from three stages to one.

Solar World has managed to further increase the laboratory efficiency of its monocrystalline solar cells, which was already more than 18%. The selective emitter structures, BSF production, passivation and metallization have been further developed so that they are ready for production, enabling solar cells with an efficiency of 18.8% to be produced. The same cell concept achieves consistently high efficiencies of up to 18.3% with n-type silicon. With the aim of achieving thinner rear-contact solar cells, a cost-effective manufacturing process with pure screen-printing metallization has also been developed and the first 125 x 125 mm BSC solar cells produced with efficiencies of up to 12%. The International Solar Energy Research Centre (ISC) in Constance is developing a 156 x 156 mm double-sided contact n-type solar cell with a selective emitter, selective or local BSF, and a stable efficiency of more than 20% for industrial pilot production. Together with four industrial partners, the ISFH and the Helmholtz Centre Berlin (HZB) want to develop concepts for heterojunction solar cells where laboratory cells will achieve efficiencies of at least 21%. Their transfer to industrially relevant sizes is intended to ensure that the technology is commercially utilised at a later stage.

Module design, manufacture and recycling

In standard designs, solar cells are electrically connected in series by soldering copper bands between them, whereby the front and rear sides are connected to consecutive cells. These so-called strings are encapsulated in a weatherresistant sandwich made of single-pane safety glass /ethyl vinyl acetate (EVA) film/cell string/EVA/rear side (Tedlar) film. A conventional module is completed with a junction box and the frame. An alternative type of module is now being developed by Fraunhofer ISE. It utilises a process from the construction industry for automatically sealing the edges of double-glazed units. This enables considerable material savings and provides a simple concept for recycling the cells.

A new method enables thin rear-side contact cells to be simultaneously connected from a single side. This increases the packing density and provides a more economic and less invasive, shade-free cell connection.

State-of-the-art technology now enables recycling of production waste, completely worn-out modules and broken modules, achieving recycling rates of more than 95% (see BINE-Projektinfo brochure 2/2010 “Recycling photovoltaic modules”). The solar power industry has founded the PV CYCLE association as a joint initiative to develop an EU-wide blanket recycling system.


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