CRYSTALLINE SILICIUM (c-Si)
At the moment the crystalline silicium is the most used from the industrial point of view.
It is divided into POLYCRYSTALLINE (mc-Si) and MONOCRYSTALLINE SILICIUM (Cz-Si).
The reasons of this widespread use are mainly the costs compared to the efficiency per m² of the photovoltaic module and the easiness of finding the silicium on the Earth. However, the refining of the silicium itself is not so easy. It has to be pure at 99,9%, fused into ingots and then cut into slices (wafers). The cells in monocrystalline silicium can be also obtained through wafers rejected by the microprocessors industry. These wafers are pickled through simple acide attacks and then grown as photovoltaic cells. However, this production technology “suffers” from the optimization of the microchips production, which was able to decrease the rejects thanks to the research.
The improvement curve of the crystalline silicium technology is almost at the peak, as industrial efficiencies of 15-17% have been reached (the silicium efficiency in the laboratory is of 25% but it is difficult that it becomes a parameter applicable to industrial solutions too). Therefore, the research is now moving from the material to the production technologies of the cell, optimizing the rejects (losses for the ingots cuts, new materials for doping, new systems for the cell growing, testuring, recovery of active area with new systems of fingher deposition, ext.). A new and interesting technology is called “Spheral Solar”: it uses microspheres of crystalline silicium to obtain flexible cells, without realising and cutting slices from ingots, avoiding losses and expensive technologies for the ingot and cell growing.
At the moment, thanks to several financing programmes of the various European and Extraeuropean governments, the silicium ingots and wafers market has had a great boom with a consequent increase of costs and difficulty in the availability.

Wafers ready to become photovoltaic cells

Ingot of crystalline silicium

TANDEM & TRIPLE JUNCTION a-Si:H TECHNOLOGY This innovative technology derives from the traditional a-Si:H and allows a bigger functioning efficiency, which reaches, in some cases, 12% (triple junction). Further junctions are added, treated in a different way, which have different reactions to the light spectrum, increasing the functioning band in the solar spectrum. For example, a photovoltaic module realised with this type of technology is able to produce 20% more of energy per year compared to C-Si crystalline silicium because it has a better reaction to the diffused solar radiation. Unisolar Triple Junction technology (pictures on the right) allows to realise flexible photovoltaic modules for various uses and to stick the various cells on steel substrates (sheet roofs) or roofs in other materials (PVC, ext.). Unisolar is the only THIN FILM technology granting 20 years of warranty.

Composition of Triple Junction cell and Unisolar PV module

CADMIUM TELLURIUM/CADMIUM SULPHIDE TECHNOLOGY (CTS) The solar cell CTS is composed by a p layer (CdTe) and by a n layer (CdS), which create a p-n heterojunction. The creation process is similar to the one of the Amorphous Silicium. Unlike the a-Si:H technology, the cell CTS is able to reach bigger efficiencies: 8-10% for industrial products (15,8% reached in the laboratory). One of the problems for the production in large scale of the CTS technology is the cadmium contained in the cell, which can become an environmental problem if it is not correctly recycled or used.

Tests of PV cells with CTS technology

GALLIUM ARSENIDE (GaAs) AND HIGH EFFICIENCY DEVICES At the moment GaAs technology is the most interesting as regards efficiency, which is more than 25-30%, but the production of these cells is limited because of very high costs and shortage of raw material, which is mainly used in the industry of semiconductors at high speed of commutation and of optoelectronics (leds and phototransistors). In fact GaAs technology is mainly used for space applications, where small weight and dimensions are very important. The results obtained with GaAs cells show a conversion efficiency of more than 30%. In 1999 a project carried on by Spectrolab and National Renewable Energy Laboratory (NREL) reached an important record in the photovoltaic conversion, realising a solar cell with a conversion efficiency of 32,3%. This triple junction cell has been manufactured using three layers of semiconductor materials, indium gallium phosphide, gallium arsenide and germanium (GaInP2/GaAs/Ge). In the near future further developments are expected, reaching 40% of efficiency.

Cell with GaAs technology