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S. Leu and D. Sontag
• Silicon is the second most abundant element of our Earth’s crust after oxygen.
Weighted by atomic per cent, the earth’s crust contains
1 :
– 60.4% oxygen
– 20.4% silicon
– 6.3% aluminium
– 2.9% hydrogen.
• The bandgap of silicon is 1.12 eV and is, thus, not too far from the optimal value
for converting—in solar cells—sunlight into electricity.
• Silicon melts easily, has good mechanical properties and is easy to machine
(sawing, polishing, etching).
• Silicon can be made into a highly pure form, as a single crystal. This means
that silicon has a homogeneous crystal lattice, which is very regular and has few
unwanted foreign atoms.
• With doping, the type and conductivity of silicon can be easily changed.
• Silicon is non-toxic.
• Silicon forms a native oxide layer, which serves as a high-quality insulator,
whereupon different layers can be deposited.
• Silicon can be thoroughly cleaned. Ultrapure silicon for the semiconductor industry is manufactured in purity levels of up to 11N
2 in mass production. For lowefficiency solar cells, a purity of 7N is sufficient but, as the demand for higher
efficiency solar cells increases, the specifications on silicon feedstock are also
requiring lower concentrations of impurities.
• The vast amount of knowledge gained during the last 50 years by semiconductor
industry in how to process silicon and how to make industrial tools for siliconbased devices. A lot of this knowledge has been used for Si PV as well.
Jöns Jacob Berzelius a Swedish chemist discovered silicon in 1823. Other semiconductors such as germanium and gallium arsenide have higher charge carrier
mobilities and, thus, allow for higher switching speeds in integrated circuits. But
Germanium has two disadvantages in connection with solar cells:
(a) Its bandgap is too small to form single-junction solar cells
(b) Germanium oxide is unstable and not suitable for surface passivation and
insulation layers.
Germanium and Gallium Arsenide (GaAs) are both too expensive to be suitable
for the production of solar cells for terrestrial applications
1 The earth’s crust forms the outer part of the earth and extends about 35–40 km into the earth’s
interior. Its composition is very diverse. If one arranges the elements of the earth’s crust according
to weight percent, a shift of the portions is noticeable in comparison to the distribution according to
atomic percent: Oxygen (46.6%), silicon (27%) and aluminum (8%) are still the three most frequent
elements.
2 11N means: 11 nines. 11N is 99.999999999% pure.
S. Leu and D. Sontag
• Silicon is the second most abundant element of our Earth’s crust after oxygen.
Weighted by atomic per cent, the earth’s crust contains
1 :
– 60.4% oxygen
– 20.4% silicon
– 6.3% aluminium
– 2.9% hydrogen.
• The bandgap of silicon is 1.12 eV and is, thus, not too far from the optimal value
for converting—in solar cells—sunlight into electricity.
• Silicon melts easily, has good mechanical properties and is easy to machine
(sawing, polishing, etching).
• Silicon can be made into a highly pure form, as a single crystal. This means
that silicon has a homogeneous crystal lattice, which is very regular and has few
unwanted foreign atoms.
• With doping, the type and conductivity of silicon can be easily changed.
• Silicon is non-toxic.
• Silicon forms a native oxide layer, which serves as a high-quality insulator,
whereupon different layers can be deposited.
• Silicon can be thoroughly cleaned. Ultrapure silicon for the semiconductor industry is manufactured in purity levels of up to 11N
2 in mass production. For lowefficiency solar cells, a purity of 7N is sufficient but, as the demand for higher
efficiency solar cells increases, the specifications on silicon feedstock are also
requiring lower concentrations of impurities.
• The vast amount of knowledge gained during the last 50 years by semiconductor
industry in how to process silicon and how to make industrial tools for siliconbased devices. A lot of this knowledge has been used for Si PV as well.
Jöns Jacob Berzelius a Swedish chemist discovered silicon in 1823. Other semiconductors such as germanium and gallium arsenide have higher charge carrier
mobilities and, thus, allow for higher switching speeds in integrated circuits. But
Germanium has two disadvantages in connection with solar cells:
(a) Its bandgap is too small to form single-junction solar cells
(b) Germanium oxide is unstable and not suitable for surface passivation and
insulation layers.
Germanium and Gallium Arsenide (GaAs) are both too expensive to be suitable
for the production of solar cells for terrestrial applications
1 The earth’s crust forms the outer part of the earth and extends about 35–40 km into the earth’s
interior. Its composition is very diverse. If one arranges the elements of the earth’s crust according
to weight percent, a shift of the portions is noticeable in comparison to the distribution according to
atomic percent: Oxygen (46.6%), silicon (27%) and aluminum (8%) are still the three most frequent
elements.
2 11N means: 11 nines. 11N is 99.999999999% pure.