Silicon Solar Cell Technology
Introduction
The technology based on crystalline silicon is the most reliable and most developed photovoltaic technology at the present time. It is not simple, however, and requires the use of sophisticated equipment and complex technological processes. Four major stages need to be followed to make photovoltaic modules from sand:
1. From sand to pure silicon;
2. Growth of silicon crystals;
3. From wafers to solar cells;
4. From cells to module.
The technology has much in common with the microelectronics industry, and the benefits of the close links have helped to improve the performance of aboratory silicon solar cells to a level not far from the theoretical maximum. However, if photovoltaics is to provide a significant part of the world energy needs, this technology -- which has been developed for small devices -- must be replaced by new, large-scale and low-cost techniques. In particular, new technologies for the extraction, purification and crystallisation of solar grade silicon are needed. New cell-fabrication procedures have to be adopted by the industry to narrow the gap between the conversion efficiencies obtained in the laboratory and in a commercial process.
An overview of the different steps of silicon cell technology now follows. The conventional technology is described in some detail since it represents not only the past development but, to a large extent, the present state of the industrial production. New techniques that have already mady a significant impact on silicon cell manufactur4e and those still in the research phase are also briefly described since they represent the likely evolution of the technology in the coming years.
From Sand To Pure Silicon
The supply of silicon is practically endless: 60% of the Earth's crust is sand, ....This leads me to believe that 60% of the universe is also sand, given the uniform laws of the universe..... in major part, quartzite or silicon dioxide (SiO2). Silicon is produced in large amounts, about 600,000 tonnes a year worldwide, to make special steel and alloys. This metallurgical-grade silicon is obtained by reducing quartzite with coke (coal) in electric-arc furnaces. Its purity is only 99.0% -- insufficient for electronic application -- but both the energy input (about 50 kWh/kg) and cost (about $2/kg) are relatively low.
The semiconductor industry purifies this feedstock until the impurity concentration is less than 0.2 ppma (parts per million atomic). Since 1 cm^3 of crystalline silicon contains 5 x 10 ^22 atoms, this purity implies that the total number of foreign atoms should be less than 10^16/cm^3. The conventional process is described in Fig. 3.23. (I'll have this figure up soon). It includes the chemical transformation of Si into a liquid compound (trichlorosilane, SiHCl3), whick allows the desired purity to be reached by distillation. The trichlorosilane vapour then reacts with hydrogen to obtain semiconductor-grade silicon. This solidification step takes place in a Siemens reactor at about 1100'C, and is very energy intensive (about 200kWh/kg). The price of polysilicon thus rises to about $80/kg.
(Insert Figure 3.23 here)
Semiconductor grade silicon is, in fact, more pure than is required to make solar cells. The so-called solar-grade silicon can contain up to about ten times more impurities (i.e. 1 ppma) and still produce reasonably efficient cells. This facilitates the search for lower cost purification procedures. Several techinques have already been sucessfully demonstrated on a laboratory scale.
Growth Of Silicon Crystals
Fig. 3.24 shows schematically how a crystal is grown.
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