Selenisation and sulfurisation of chalkopyrite CIGS 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:

• Thin film solar cells made of amorphous silicon
• Thin film solar cells made of cadmiumtelluride CdTe, cadmiumselenide CdSe, cadmiumsulfide CdS
• Thin film solar cells based on chalkopyrite e.g. copperindiumselenide, copperindiumgalliumselenide or sulfid, abbreviated CIGS.

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.

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Configuration of a chalkopyrite CIGS or CIGSe solar cell

Chalkopyrite or yellow copper ore is a commonly occuring mineral and I-III-VI2-compound, containing copper (greek chalko), further metals like indium and gallium as well as sulfur and selenium. Many chalkopyrits absorb light very well and have semiconducting properties, which makes them well suited for the construction of thin film solar cells. It is possible to adjust properties of the semiconductor material like lattice spacing and band gap by the variation of the chemical composition and the mixture of the various elements. This is a good condition for engineering optimum solar cells.
A chalkopyrit or CIGS thin film solar cell consists of several layers. In most cases a glass plate is used as a substrate. A non-transparent molybdenium layer is deposited on it as a back side contact. For the fabrication of the chalkopyrite layer, first copper, indium and gallium have to be evaporated. Then selenidation and sulfurisation of this deposit has to be done in a furnace or RTP-unit at a temperature of around 550°C. As a source for selenium and sulfur, the gases hydrogen selenide H2Se and hydrogen sulfide H2S are used. On this CIGS-layer, also named absorber layer, a thin film of Cadmiumsulfid CdS is deposited, followed by another layer of a transparent conductive oxide TCO, serving as a front side contact and consisting e.g. of Zincoxide ZnO. At the interface between the CdS- and the CIGS-layer the generation of the electrical carriers takes place by sun light irradiation.
In general it is also possible to setup the CIGS solar cell in a reverse order, using the glass support at the front side. This version is called superstrate configuration, because the glass carrier is above the solar cell, compared to the substrate configuration described above, with the glass support at the backside.

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

Furnaces and RTP systems for selenisation and sulfurisation

Selendisation and Sulfurisation can be achieved in various types of furnaces and RTP systems. On the picture, you can see a large, vertical furnace type VFS-4000 manufactured by Koyo Thermo Systems. This furnace had been developed originally for the deposition of poly-silicon layers on glass plates for the manufacturing of TFT-LCDs. In this system, 25 solar size glass plates can be processed at a time in an upright position. This furnace can achieve an excellent temperature uniformity as most vertical furnaces, designed for semiconductor applications can. It can handle toxic gases like H2Se and H2S safely. Tube and boat of the furnace consist of quartz glass and a clean environment of the process can be ensured, although the price for these parts is high. It is possible to combine several furnaces to a process cluster and use a set of common cassette I/O stations together with one loading robot. Then operation is quasi-continuous.

Production can also be done in an continuous in-line process. After sputtering of the molybdenum contact, metallic precursors Cu, Ga, In are deposited. After pre-heating, the selenisation and sulfurisation process follows, either using hydrogenselenide and hydrogensulfide or thin layers of solid selenium and sulphur. In our example a continuous conveyor furnace from Koyo is used for this thermic process step. The process chambers are separated by locking doors. The thermal process chambers can be evacuated and purged. The transportation of the glass plates is achieved by a tacted roller hearth transportation system.

Recent developments allow the introduction of the selenium by diffusion from solid layers or phases of selenium. This allows now the usage or more simple roller hearth furnace or other conveyor furnaces as they are used also for manufacturing of CdTe-cells. It is no longer necessary to use high toxic, gaseous compounds and this helps to reduce production costs significantly.

Alternatively an RTP system can be used. In this case, lamp heating replaces the continuous heating by Moldatherm^® heaters.

RTP-systems can also be used for single wafer processing of smaller glass plates in the laboratory or for pilot lines.