PECVD equipment for fabrication of hydrogenated amorphous silicon thin film solar cells.

Crystec Technology Trading GmbH
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Hydrogenated amorphous silicon 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,, +49 8671 882173, FAX 882177

Configuration of an hydrogenated amorphous silicon thin film solar cell

a-Si:H solar cell

Amorphous silicon or a-Si is not suited to be used for solar cells because it contains many dangling bonds. Charge carriers have a short lifetime in such a material. If the dangling bonds are saturated by hydrogen, then you get hydrogenated amorphous silicon a-Si:H with good properties for thin film solar cells. This material absorbs enough light and the carrier lifetime is sufficient. Also this material can be doped with Boron or Phosphorous.
The solar cell consists of a transparent, conductive front layer, followed by a p-doped a-SiC:H-layer, the absorber layer of intrinsic a-Si:H or a-SiGe:H and an n-doped a-Si:H-layer, forming a p-i-n-structure. The two p- and n-doped layers generate an electric field used to separate the charge carriers. The upper, p-doped layer should absorb only little light, because the carriers generated in this layer recombine fast. This can be achieved by alloying the silicon with carbon. For the intrinsic absorber layer, a compromise has to be found. A thicker layer absorbs more light, but less carriers reach the electrodes, because of the increased recombination rate. This reduces the efficiency of the thin film solar cell. One solution is the preparation of stacked multijunction solar cells, so called tandem or dual or triple junction cells. They consist of 2 or 3 p-i-n-structures and can therefore increase the total light absorption. Voltage is also doubled or tripled by the serial connection. In case of such multiple layer solar cells, the lower layers are quite often alloyed with germanium to a-SiGe:H in order to increase the absorption of light of lower wavelength.
Same as for solar cells made from mono-crystalline or poly-crystalline silicon, the conductive front side layer is textured in order to scatter the light and increase its path length. Light absorption and efficiency of the solar cell is increased.
If the glass is used as a basis for the deposition of the layers, then the glass is above the solar cell and this structure is called superstrate configuration, while the deposition of the various layers on a non-transparent carrier on the backside is called substrate configuration.

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

Manufacturing of thin film solar cells from amorphous silicon by PECVD technology

The main components of a a-Si:H solar cell are the three layers of p-doped, intrinsic and n-doped material (p-i-n). Usually, they are deposited by plasma enhanced chemical vapour deposition (PECVD). This process is operated at rather low temperature. The main gas is silane SiH4. When using silane diluted by hydrogen, the properties of the deposited layers can be improved. By adding phosphine PH3 or diborane B2H6 doping of the two outer amorphous silicon layers can be achieved. Addition of methane CH4 will generate an alloyed front layer a-SiC:H, while the addition of germane GeH4 will alloy the absorber layer of stacked solar cells and form a-SiGe:H.
For the deposition in a laboratory, a simple PECVD parallel plate reactor with a 13,56 MHz rf generator is sufficient. The radicals formed in this plasma consist mainly of SiH3 radicals. They are adsorbed at the silicon surface and diffuse on it to the next dangling silicon bonding. For production scale, the deposition rate of a parallel plate reactor is too low. It can be increase by using the microwave technology. Plasma is generated externally by a microwave generator and conducted into the reaction chamber. This way a higher deposition rate can be achieved without increasing the formation of SiH2 radicals, which would influence the material properties negatively.

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