Furnaces for activation of CdTe thin-film solar cells

Most photovoltaic solar cells today are still made of monocrystalline or polycrystalline silicon, sometimes also germanium. However, thin-film solar cells are becoming increasingly widespread. There are currently essentially three types of thin-film solar cells:

  • Thin-film solar cells made of amorphous silicon

  • Thin-film solar cells made of cadmium telluride (CdTe), cadmium selenide (CdSe), cadmium sulfide (CdS)

  • Thin-film solar cells based on chalcopyrites, e.g. copper indium selenide, copper indium gallium selenide or -sulfide, abbreviated CIGS.

Because these materials absorb light better than monocrystalline or polycrystalline silicon, they can be deposited in thin layers on a substrate, often glass. Using less material leads to lower costs if the production of the thin layers can be realized cost-effectively.

When sunlight shines, charge carriers are generated in the solar cell and then diffuse to the electrodes, building up a voltage and producing photovoltaic current. Charge carriers in the semiconductor have a limited lifetime or diffusion length. The thinner the layer, the easier the charge carriers reach the electrodes and the more effective the solar cell.

Structure of the cadmium telluride (CdTe) solar cell

A cadmium telluride thin-film solar cell consists of several layers. When a glass substrate is used, the cell is built from the front in the superstrate configuration, i.e. the substrate is located above the cell: on (actually under) the glass superstrate you find at first a transparent, conductive indium tin oxide layer (ITO).

On top of that, a very thin cadmium sulfide layer is deposited. Cadmium sulfide is highly transparent to sunlight but has relatively high electrical resistance. A thin cadmium telluride layer is then deposited. At the interface between the CdS and CdTe layers, charge carrier generation by sunlight illumination takes place.

Both semiconductor layers must be treated and tempered with cadmium chloride (CdCl2) to optimize the crystal structure of the layers and the quality of the CdS–CdTe interface and thus the electrical properties. Metallic contacts on the rear of the solar cell collect the charge carriers. The rear encapsulation of the cell can be opaque.

If non-transparent substrates such as metal foil are to be used, the cell is built in the substrate configuration, i.e. the substrate is below the cell and the layers must be applied in reverse order. This layer arrangement is more complicated and is therefore not yet used on a large scale.

CdTe solar cell
Thermal activation of the cadmium telluride (CdTe) solar cell

To obtain semiconductor layers with good electrical properties, the deposited layers must be activated, thermally treated or sintered for about 20 minutes at approximately 400°C, or at 550°C if the semiconductor layers were applied by screen printing. This process step is very important and influences the cell's properties more than the deposition technique used for the CdS and CdTe layers.

Because the glass substrates on which thin-film solar cells are produced can be relatively large, large, clean and energy-efficient roll conveyor furnaces are used for this thermal activation. The glass plates move continuously and very gently on ceramic rollers through the furnace. A special glass muff is used to keep the furnace clean. The air is cleaned via HEPA filters.

Targeted gas supply and exhaust extraction are as important for the process result as very good temperature uniformity in the furnace. The system itself must be protected against the corrosive chlorine components. A multi-story furnace design is possible to increase throughput and reduce the furnace footprint.

The company JTEKT Thermo Systems (formerly Koyo Thermo Systems) already has extensive experience in manufacturing such conveyor furnaces.

JTEKT Thermo Systems and Crystec look forward to building a cost-effective system for you that meets your most stringent requirements.

Roll conveyor furnace