Photovoltaic Power Generation - Renewable Energy: Technology, Economics and Environment - page 243

spectrum. Nevertheless, highest solar module efficiencies are achieved at clear cold

days during wintertime.

Table 6.1 Efficiencies of solar cells (also refer to /6-16/; only cells with a cell surface
larger than 1 cm 2 have been considered)
Material

Type Efficiency State

of tech-
nology a

Lab. Manufac.

in %

Silicon
Polysilicon, simple
MIS inversion layer (silicon)
Concentrator solar cell (silicon)
Silicon on glass substrate
Amorphous silicon, simple
Tandem 2 layers, amorphous silicon
Tandem 3 layers, amorphous silicon
Gallium indium phosphate /
Gallium arsenide b
Cadmium-telluride c
Copper indium di-selenium d

1
1
2
2
3
1
2
1
2
2
2
Lab. Laboratory; Manufac. Industrial manufacturing; technol. technology; a 1 large scale production, 2
small scale production, 3 pilot production, 4 development on a laboratory scale; b GaInP/GaAs; c
CdTe; c CuInSe 2

monocrystalline
polycrystalline
monocrystalline
monocrystalline
transfer technol.
thin film
thin film
thin film
tandem cell
thin film
thin film

24.7
19.8
17.9
26.8
16.6
13.0
13.0
14.6
30.3
16.5
18.4

14.0 - 18.0
13.0 - 15.5
16.0
25.0
8.0
8.8
10.4
21.0
10.7
12.0

To assess the power output of a photovoltaic module under the site-specific
meteorological conditions the so-called annual efficiency concept has been devel-
oped. Actual module temperatures, solar irradiation, and solar spectra are assessed
according to the frequency of their occurrence and according to the product-
specific parameters of the efficiency dependence on temperature, radiation, and
spectrum. Based on this approach evaluation of the power output of various solar
modules may thus differ from the efficiencies determined under STC conditions.
However, for the plant operator in the end only the annual efficiency is of impor-
tance since it determines the energy yield /6-13/.

Cell types. Due to the energy gap, shown in Fig. 6.8, crystalline silicon is not
regarded as an ideal semiconductor material for photovoltaic cells. Furthermore,
silicon is a so-called indirect semiconductor whose absorption coefficient for solar
radiation shows relatively low values. Solar cells made of such semiconductor
material must thus be relatively thick; a conventional crystalline silicon cell of
simple planar structure, as shown in Fig. 6.6, must have a layer thickness of at
least 50 µm to nearly completely absorb the incident sunlight. High layer thick-
ness implies high material consumption and thus high costs. Nevertheless, crystal-
line silicon is commonly used for photovoltaic cells. The main reason is that sili-
con is the semiconductor material that shows the widest market penetration, that
has been theoretically best understood, and that is most easily controlled.