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Projektinfo – Detailed information on energy research

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Wafer production at SolarWorld in Freiberg: control of the produced wafer.
© SolarWorld
Wafer production for crystalline solar cells
Projektinfo 02/2017

Schematic of the wire saw process with a multi-wire saw.
© Fraunhofer IWM

After sawing and cleaning, the silicon wafers are tested and sorted.
© SolarWorld

SEM image of a used wire. The total number of diamond grains that can be counted (white dots, below) indicates the overall diamond density; the number of exposed diamond grains (black dots, centre) shows the proportion of cutting diamonds. This is an important parameter for the cutting efficiency of a wire.
© SolarWorld
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Cheaper production of solar cells

There is intense competition in the market for photovoltaic systems. With constantly new innovations, the manufacturers are reducing their production costs and are increasing the efficiency of the cells and modules. For this purpose they are improving the production processes along the chain from silicon to the module. Companies and research facilities are working together to produce high-quality silicon crystals and wafers that save as much material as possible. They are also improving the material quality using an innovative solidification process for quasi-monocrystalline silicon. And with a new separation process they are producing more wafers from the same amount of silicon.

Wafers made of monocrystalline or multicrystalline silicon are usually used for silicon-based solar cells. Multicrystalline silicon is produced cost-effectively by means of ingot casting, but does not attain the efficiency of standard monocrystalline silicon, which is grown in a complex process using the Czochralski method. The standard cell efficiencies achieved by this method of more than 21% can also be attained, however, using the newly developed quasi-mono silicon, which researchers from SolarWorld can produce more cost-effectively using a new, crucible-free crystal growing process. This replaces the ingot casting method customarily used for microcrystalline silicon cells, in which the crucible and its coating provide a source for impurities and disruptive foreign nucleating agents. The new process enables them to produce a monocrystalline, dislocation-free and low-oxygen silicon.

In the next step, the crystal is cut into fine slices known as wafers. In order to improve this production step, the researchers are replacing the previously used lapping abrasive-based sawing technology with diamond wire cutting and specially adapted cooling liquid. Diamond saws can cut the crystals more quickly into wafers with less material loss. These are currently about 180 μm thick. Substantial material savings are still possible here, and within the next ten years the researchers want to attain 100 μm. By way of comparison, a sheet of paper is about 80 μm thick.

Before the wafers can be further processed into solar cells, they have to be cleaned. With newly developed processes, residues from the coolant and lubricant are removed along with organic adhesions and particles.

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Crystallisation: Directional solidification

Directional solidification is the established process used for the large-scale production of multicrystalline silicon ingots for solar cells. New solidification concepts are aimed at producing quasi-monocrystalline silicon ingots that have higher efficiencies and fewer defects that reduce the life of solar modules. Here magnetic flux fields are also used. They allow the melt currents, which are mainly influenced by magnetic fields generated by the resistance heaters, to be adjusted specifically at the solid-liquid growth front, thereby improving the solidification process. A measurement method for such flow structures developed with the TU Freiberg confirms the results of the numerical simulations carried out in the research project.

The researchers developed the current process based on a previous project in which they melted polycrystalline silicon in the crucible to produce quasi-mono material. For this purpose, they laid monocrystalline silicon on the bottom of the crucible, allowed the crystal to grow from there, and then gradually lowered the temperature from below upwards. The process requires very accurate temperature control. It is also more complicated than the production of polycrystalline material as the monocrystalline crystallisation seeds are only allowed to melt slightly.

Since disturbances in the crystal structure and impurities from the crucible still limit the efficiency that can be achieved with this method, the researchers developed the crucible-free quasi-mono process. In addition to improving the structural quality of the crystals with fewer dislocations and smaller recombination-active grain boundaries, this process also enables them to achieve a considerably lower concentration of impurities such as oxygen, carbon and metals. This in turn improves the efficiency of the processed solar cell.

Within the scope of the experimental work, they used various raw material deposits and seed templates, and selectively varied process parameters such as the heat outputs or the growth rate. In order to further improve the manufacturing process and the furnace, the researchers used a newly developed simulation software for the crucible-free technology as well as a new measuring system. This model enables them to predict dislocations and dislocation clusters caused by thermo-mechanical stresses, as well as the distribution of residual stresses in the cooled crystal.

Projektinfo 02/2017:
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Addresses

Project management, development of quasimono technology and wafer production
SolarWorld Innovations GmbH

Evaluation of auxiliary materials and cleaning method
Fraunhofer ISE

Development of quasimono technology
Leibniz-Institut für Kristallzüchtung

Production of quasimono silicon crystals
Fraunhofer IISB

Basic saw and separation process for quasimono silicon
Fraunhofer IWM

Development of cooling lubricants and cleaning chemicals
PETROFER CHEMIE

Model experiments for the directional crystallization of Si bricks
TU Freiberg, INEMET

Optimisation of separation and cleaning processes
Fraunhofer CSP

Simulation of tension dispersion in quasimono-silicon bricks
ACCESS e.V.

Ultrasonic measurement technology for directional solidification model experiments
TU Dresden, IEE

Links

Publication
Richter, T.: Entwicklung hoch- und kosteneffizienter PV-Si-Wafer. Teilprojekt: Entwicklung einer quasimono-Kristallisationstechnologie in Verbindung mit einem spezifischen hocheffizienent Vielspalten-Sägeprozess. FKZ 0325646A. SolarWorld Innovations GmbH, Freiberg (Hrsg.). [2015] (only in German)

Video
Frieder Braun, Universität Konstanz (2011): Production of a silicon-based solar cell (only in German)

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