Furnace, Furnaces for activation of CdTe thin film solar cells
Koyo Thermo Systems

Koyo Thermo Systems Co., Ltd. is represented in Europe by
Crystec Technology Trading GmbH

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solar   doping   metallization   ARC   a-Si:H   CdTe   CIGS   modules

Furnaces for activation of CdTe thin film solar cells

Today most photovoltaic solar cells consist still of mono-crystalline or poly-crystalline silicon material, sometimes also germanium. However thin film solar cells find more and more interest. Three main types of thin film solar cells are existing:

Because of the better light absorption of these materials in comparison to monocrystalline or polycristalline silicon, these materials can be deposited in thin layers on a substrate, which is in many cases a glass substrate. Less material consumption can reduce the price in case the deposition can be achieved in a cost effective manner.
By the irradiation with sun light, charge carriers are generated in the solar cell, which diffuse then to the the electrodes and generate voltage and photovoltaic current. The electric carriers have a limited life time and diffusion length in the semiconductor material. The thinner the layer can be made, the more carriers can reach the electrodes and the more effective will the photovoltaic cell be then.

Crystec Technology Trading GmbH, Germany, www.crystec.com, +49 8671 882173, FAX 882177

Configuration of a Cadmiumtelluride CdTe solar cell

CdTe solar cell

A cadmiumtelluride thin film solar cell consists of several layers. When glass support is used, then the solar cell is built from the front side in a superstrate configuration with the support above the cell: On (or better below) the glass support, you find at first a transparent, conductive indium tin oxide layer (ITO). On top of this ITO layer a very thin cadmiumsulfide layer is deposited. The cadmium sulfide is very transparent for sun light but has a rather high electrical resistivity. Then a thin cadmium telluride layer is applied. At the interface between the CdS- and the CdTe-layer the generation of the electrical carriers takes place by sun light irradiation. Both semiconductor layers have to be treated, annealed and activated with cadmiumchloride CdCl2, in order to optimize the crystalline structure, the quality of the CdS-CdTe-interface and therefore the electrical properties of the solar cell. On the backside the carriers are drained by a metallic contact layer. The backside encapsulation can be done with non-transparent material.
In case that a non-transparent support like metal foil should be used, then this has to be done in the substrate configuration, meaning that the support is below the cell and the cell is built from the backside and in reverse order. This setup is more complicated and is therefore right now not yet used in mass production.

Crystec Technology Trading GmbH, Germany, www.crystec.com, +49 8671 882173, FAX 882177

Thermal activation treatment of cadmium telluride solar cells

roller hearth transportation furnace

In order generate layers with good electrical properties, the deposited semiconductor layers have to be activated for around 20 Min. at around 400C or at 550C in case that the semiconductor layers have been deposited by screen printing technology. This process step is very important and influences the cell properties more than the deposition technology for the CdS and CdTe layer. As the glass substrates on which thin film solar cells are produced are rather large and same size as the finished solar module, large, clean and energy saving roller hearth furnaces are used for thermal activation treatment. The glass plates move continuously and in a very gentle manner on the ceramic rollers through the furnace. A special glass muffle is used in order to keep the furnace clean. Air is cleaned by a HEPA filter. A dedicated gas flow and exhaust is very important for achieving good process results as well as an excellent temperature uniformity inside the furnace. The equipment itself must be protected against corrosive chlorine contamination. The furnace can be constructed in a multilevel configuration in order to increase the throughput and keep the footprint low. Koyo Thermo Systems has already a lot of experience with the construction of such conveyor furnaces.

Koyo Thermo Systems and Crystec will be pleased to engineer a cost effective system to satisfy your most demanding and exacting requirements.

Crystec Technology Trading GmbH will be pleased to further discuss details with you.
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