Diffusion furnaces for doping crystelline silicon solar cells c-Si.
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Most solar cells which are used today are based on crystalline silicon. The silicon can be mono-crystalline or poly-crystalline. Monocrystalline material is produced by the Czrochalski-process, while polycrystalline material is usually prepared by molding. In both cases the generated material is cut to wafers by wire saws. The wafers serve then as substrate material for the solar cell.
The solar cells consists mainly of silicon and is called therefore thick film solar cell, in contrary to thin film solar cells where the semiconductor layers are deposited on substrate of a different material. The bulk silicon is usually lightly p-doped, and conductive for positive charge carriers or holes. On the front side a thin heavily n-doped layer has to be formed by doping, which is conductive for negative charge carriers or electrodes. This way a p/n-junction is formed, which can separate the charge carrier pairs, generated by the absorption of sunlight. On the front side and on the back side metallic contacts have to be formed in order to drain the solar current. At the backside a holohedral aluminium layer is deposited, while at the front side silver contact fingers are generated, which allow most of the sunlight to pass into the cell. Finally a silicon nitride antireflection coating ARC is attached to the front side in order to increase absorption of the sunlight. The last production step is the assembly of the solar cells to solar modules.
Crystec Technology Trading GmbH, Germany, www.crystec.com, +49 8671 882173, FAX 882177
The doping of the upper, heavily n-doped layer is done with phosphorous as doping material. Two main procedures are used:
Horizontal furnaces or diffusion furnaces from JTEKT Thermo Systems (previously Koyo Thermo Systems) ensure cost effective doping with high throughput. The LGO heating elements, used in this furnace have a very low thermal mass and can reduce therefore the process time. They can also save energy and costs for the doping process. All normal sizes of solar wafers can be processed in this type of furnace. Liquid POCl3 is supplied in a bubbler. Nitrogen passes at a well defined temperature through the liquid and is transporting the dopant. Typical doping temperature is 800 - 900°C.
The latest development of JTEKT allows now also the continous doping of silicon wafers with POCl3 in a tube furnace. The wafers are carried through the tube on quartz cars with a load of 100 wafers each. The design of the cars is critical in order to obtain a good temperature uniformity and the process results of the POCl3 doping is therefore also affected seriously. Gas curtains at both ends of the tube separate the process room from the environment.
For higher demands to the homogeneity of the doping profile or to the automation level,
vertical furnaces are available. The smallest
version of a JTEKT Thermo Systems (previously Koyo Thermo Systems) vertical furnace can be very well used especially for the use in research and
development of solar cells. The furnace model VF1000 is designed as a mini-batch furnace,
has a manual loading and is therefore very flexible regarding sample sizes.
This vertical furnace is equipped with a cost saving LGO heating element.
The process performance equals the big production versions of vertical furnaces
for IC production. The price of this unit is similar to the price of a horizontal
tube in a horizontal furnace.
For mass production, JTEKT Thermo Systems developed a special vertical furnace, which gives better process results compared to a horizontal one, but still does not increase much the production costs for solar cells. This twin furnace loads 3-4 boats in one vertical tube and has therefore a capacity of 600-800 solar cells. This is the same capacity that you get on a horizontal furnace. Automation is easier to do on a vertical furnace.
Crystec Technology Trading GmbH, Germany, www.crystec.com, +49 8671 882173, FAX 882177
Doping with doping paste works with rather harmless materials and allows the usage of a simple conveyor furnace, which is well suited for mass production and can be intergrated easily in in-line production systems. The dopand is diffusing in the furnace from the doping paste into the silicon material. Temperature and anneal time determine the thickness of the generated n-doped top layer. The JTEKT conveyor furnace model 47-MT with mesh belt transportation system can be used for this application.
For research and development, JTEKT can offer a small type conveyor furnace model 810, which is constructed similar to the large furnaces and can simulate a production environment.
JTEKT Thermo Systems and Crystec will be pleased to engineer a cost effective system to satisfy your most demanding and exacting requirements.
