632
22 Light-to-Electricity Conversion
Fig. 22.63 Schematic drawing of a solar cell (with contact grid), a module (36 cells) and an array of ten modules
22.4.6 Solar-Cell Types
First Generation Photovoltaics
Silicon is the most frequently used material for solar cells. Cells based on single-crystalline silicon
(wafers) have the highest efficiency but are the most expensive (Fig. 22.65a). The state-of-the-art industrial crystalline silicon solar cells technology and achievements (in 2014) has been reported in [1894].
The efficiencies of various solar cells are compiled in Table 22.1. Polycrystalline (for large grains also
called multi-crystalline) silicon (Fig. 22.65b) is cheaper but offers less performance. Material design
is oriented towards increasing the grain size and/or reducing their electrical activity. Grain boundaries
act as recombination centers with a surface (i.e. interface) recombination velocity of 10
2 cm/s [1895]
for particular, electrically fairly inactive grain boundaries, several 10
3 cm/s [1896], several 10
4 cm/s
[1897] or even 10
5 –10
7 cm/s [1898]. The grain boundaries reduce the effective diffusion length and
thus carriers recombine before they can reach the contacts. A detailed theory of solar cell performance
for polycrystalline material has been worked out in [1897] and explains the reduction of efficiency
with decreasing grain size as shown in Fig. 22.64.
These solar cells are also called ‘first-generation’ photovoltaics. Thin sheets of crystalline silicon
drawn from a melt between two seed crystals in a modified CZ growth (sheet silicon or ribbon silicon)
allow cheaper production compared to cells based on ‘traditional’ polished wafers cut from a large
silicon rod. Silicon made particularly for solar cell use is called ‘solar-grade’ silicon.
Second Generation Photovoltaics
Even cheaper are solar cells from amorphous silicon (Fig. 22.65c). Since silicon is an indirect semiconductor, a fairly thick layer is needed for light absorption. If direct band gap semiconductors are used, a
thin layer (d ≈ 1 µ m) is sufficient for complete light absorption. Such cells are called thin-film solar
cells [1901]. A typical material class used in this type of cell are chalcopyrites, such as CuInSe 2 (CIS).
The band gap is around 1 eV, which is not optimal. An improvement can be achieved by adding Ga
and/or S which increases the band gap, Cu(In,Ga)(Se,S) 2 (CIGS), to 1.2–1.6 eV. Using CIGS, an efficiency of over 19% has been reported in laboratory samples; 12–13% seeming realistic for production
22 Light-to-Electricity Conversion
Fig. 22.63 Schematic drawing of a solar cell (with contact grid), a module (36 cells) and an array of ten modules
22.4.6 Solar-Cell Types
First Generation Photovoltaics
Silicon is the most frequently used material for solar cells. Cells based on single-crystalline silicon
(wafers) have the highest efficiency but are the most expensive (Fig. 22.65a). The state-of-the-art industrial crystalline silicon solar cells technology and achievements (in 2014) has been reported in [1894].
The efficiencies of various solar cells are compiled in Table 22.1. Polycrystalline (for large grains also
called multi-crystalline) silicon (Fig. 22.65b) is cheaper but offers less performance. Material design
is oriented towards increasing the grain size and/or reducing their electrical activity. Grain boundaries
act as recombination centers with a surface (i.e. interface) recombination velocity of 10
2 cm/s [1895]
for particular, electrically fairly inactive grain boundaries, several 10
3 cm/s [1896], several 10
4 cm/s
[1897] or even 10
5 –10
7 cm/s [1898]. The grain boundaries reduce the effective diffusion length and
thus carriers recombine before they can reach the contacts. A detailed theory of solar cell performance
for polycrystalline material has been worked out in [1897] and explains the reduction of efficiency
with decreasing grain size as shown in Fig. 22.64.
These solar cells are also called ‘first-generation’ photovoltaics. Thin sheets of crystalline silicon
drawn from a melt between two seed crystals in a modified CZ growth (sheet silicon or ribbon silicon)
allow cheaper production compared to cells based on ‘traditional’ polished wafers cut from a large
silicon rod. Silicon made particularly for solar cell use is called ‘solar-grade’ silicon.
Second Generation Photovoltaics
Even cheaper are solar cells from amorphous silicon (Fig. 22.65c). Since silicon is an indirect semiconductor, a fairly thick layer is needed for light absorption. If direct band gap semiconductors are used, a
thin layer (d ≈ 1 µ m) is sufficient for complete light absorption. Such cells are called thin-film solar
cells [1901]. A typical material class used in this type of cell are chalcopyrites, such as CuInSe 2 (CIS).
The band gap is around 1 eV, which is not optimal. An improvement can be achieved by adding Ga
and/or S which increases the band gap, Cu(In,Ga)(Se,S) 2 (CIGS), to 1.2–1.6 eV. Using CIGS, an efficiency of over 19% has been reported in laboratory samples; 12–13% seeming realistic for production