Bricks of multicristalline silicon

... produced via Vertical Gradient Freeze method at SIMTEC.

Silicon Material TEChnology research center (SIMTEC)

SIMTEC is our Silicon Material TEChnology research center at Fraunhofer ISE. It focuses on the front end processes of the solar value chain. Starting with various sources of silicon feedstock, it involves multicrystalline block crystallization, block shaping, wafering and – as a new technology in PV – silicon epitaxy for crystalline silicon thin-film wafer equivalents.

The need for cost reduction in silicon photovoltaics is also present at this early stage of the value chain. It has already brought new, varying qualities of silicon feedstock to the market, and it is calling for fast and inexpensive crystallization of high-quality crystals as well as lower cost, high-yield shaping and wafering processes. Supported by the German Ministry for the Environment, Nature Conservation and Nuclear Safety, we are setting up SIMTEC as a technology platform for research on all aspects of this part of the value chain. Additionally, the laboratory enables us to offer services related to existing technologies.

CVD-Lab

In addition to the standard technologies, we are establishing a “CVD Lab“ at SIMTEC, dedicated to silicon epitaxy. With the use of two in-house constructed, high-throughput silicon chemical vapor deposition reactors, we will address the main issues of silicon deposition relevant to nearly all crystalline silicon thin-film solar cell approaches.

With these reactors, process development will be done at a productionrelevant throughput, especially for the epitaxial wafer equivalent. New concepts like emitter epitaxy for wafer solar cells will also be pursued.

Research competence

The combination of wafer and wafer equivalent technologies at SIMTEC necessitates a very open philosophy concerning material purity. Therefore, the SIMTEC equipment has a large flexibility in use of silicon material steming from either high-purity polysilicon or from upgraded metallurgical feedstock. Particularly for the latter, the co-operation inside Fraunhofer ISE between SIMTEC and our PV-TEC and Module Lab facilities uniquely provides a continuous lab-line for material characterization from feedstock up to cell and module level.

Fraunhofer ISE has a long tradition in processing, characterization and optimization of solar cell materials, solar cells, solar cell processes and manufacturing equipment. SIMTEC completes our research competence along the whole value chain. With more than 150 people in all kinds of laboratory environments we can offer services across the entire range from single-sample to highthroughput processing, in-house.

  • Block Shaping and Wafering

    Drahtfeld einer Vieldrahtsäge
    © Foto Fraunhofer ISE

    Wire array of the multi-wire saw (foreground) and already sawn wafers (background). Multi-wire technology is the key technology today for producing thin silicon wafers for solar cell production.

    Wafer eines multikristallinen Siliciumblocks
    © Foto Fraunhofer ISE

    Wafers from a multicrystalline silicon block, prepared at SIMTEC in industrial quality.

    For shaping the ingots, a band saw is used at SIMTEC. This saw provides a high level of flexibility due to the manual operation such that we are able to prepare nearly any brick shape and sample size needed for characterization. Shaped bricks can be surface finished using a grinding and chamfering tool, adapted to 125 mm and 156 mm bricks.

    We cut wafers from blocks of up to 250 mm height with our industrial multiwire saw DS265 by Meyer&Burger. Its flexibility in cutting parameters allows us to research and improve the cutting process. In recent projects we were able to reproducibly cut wafers down to 70 μm thickness. A wet bench for cleaning the cut wafers completes the wafering section of SIMTEC.

  • High-Throughput Silicon Epitaxy

    ProCon CVD Anlage
    © Foto Fraunhofer ISE

    ProConCVD reactor for silicon deposition for epitaxial wafers.

    Solar Cell on Epitaxial Wafer Equivalent
    © Foto Fraunhofer ISE

    Solar Cell on Epitaxial Wafer Equivalent grown on a highly doped Cz substrate with an efficiency of 15.1%. éFraunhofer ISE

     

    Epitaxy of silicon has applications in photovoltaics both for the conventional wafer technology (e.g. for emitter growth) and for crystalline silicon thin-film solar cells. At SIMTEC, we will set up two in-house-constructed silicon epitaxy reactors designed specially for the requirements of photovoltaics. Our in-line test reactor, the ConCVD, features a throughput of approx. 1 m 2 /h at 10 μm layer thickness, and will be used for feasibility tests and process development.
    The ProConCVD, the “big sister“ of the ConCVD, is designed for a throughput of 15 MW/a of epitaxial wafer equivalents. Both reactors, together with supply infrastructure and a wet chemistry section, will be located in the ~600 m 2 “CVD Lab“ at SIMTEC.

  • Silicon Directional Solidification

    Kristallisationsanlage
    © Foto Fraunhofer ISE

    Crystallisation facility to produce multicrysatalline silcon ingots.

    Kristallisierter Silicumblock
    © Foto Fraunhofer ISE

    Crystallised silicon ingot of highly doped tops and tails, waste material from the microelectronics industry. The ingot has a base area of app. 68 x 68 qcm, and with a height of app. 25 cm, weighs about 270 kg.

    Our primary tool for directional solidification of silicon is a PVA Tepla Multicrystallizer, realizing the so-called “Vertical Gradient Freeze“ method. With this tool we are able to crystallize ingots of various sizes equivalent to 16, 4 or one brick of 156 x 156 mm 2 , corresponding to batch weights ranging from 20 kg to 250 kg.

    For preparation of the crucibles before crystallization an internally-developed coating unit and a sintering furnace complete the equipment. Within this coating facility the research focuses on optimizing crucible coatings with respect to purity, cost and residual impurities and special requirements for the use of UMG silicon.