Photovoltaic Power Generation - Renewable Energy: Technology, Economics and Environment - page 242

− Due to the different refractive indices, reflection losses occur when radiation is
transmitted from air to the semiconductor material. Anti-reflecting coatings and
structured cell surfaces considerably reduce these losses.
− Short-wavelength light usually does not penetrate as deep into the semiconduc-
tor material as long-wavelength light. To utilise short-wavelength light the
structuring of the upper semiconductor layer properties are of major impor-
tance. The higher the layer doping, the thinner the layer should be, since charge
carriers tend to re-combine very quickly in such layers. The adsorbed light thus
contributes only very little to the photocurrent of the solar cell.
− High short-circuit currents, open-circuit voltages and fill factors imply maxi-
mum diffusion lengths. However, charge carriers tend to re-combine at imper-
fections and impurities of the crystal lattice. Thus the bulk material must be of
good crystallographic quality and must meet maximum purity requirements.
− Also the surface of the semiconductor material (i.e. the photovoltaic cell) is a
large-surface imperfection of the crystal lattice. There are various techniques
available to passivate such surface imperfections and to reduce the resulting ef-
ficiency losses.
− Further losses occur when transferring energy from the solar cell. Resistance
losses occur as charge carriers move towards the contacts and as they are trans-
ferred through the connecting cables. Manufacturing imperfections may cause
local short-circuit between the front and rear side of the solar cell.
For highly efficient laboratory silicon solar cells these losses amount to
approximately 10 %. Under otherwise optimum conditions, the theoretical maxi-
mum efficiency of a solar cell of 28 % (Fig. 6.8) is thus reduced to an actual effi-
ciency of 25 % (maximum laboratory values achieved, see Table 6.1).
The efficiencies indicated for photovoltaic cells usually only apply for deter-
mined standardised measuring conditions, since the power output of a solar cell
depends on spectral light composition, temperature and irradiation intensity. The
standardised conditions mentioned above generally refer to so-called "Standard
Test Conditions" (STC): radiation 1,000 W/m 2 , solar cell temperature 25 °C, spec-
tral distribution of the irradiation according to AM (air mass) = 1.5 (AM = 1.5
implies an effective atmosphere thickness of 1.5 times the vertical light penetra-
tion; spectral distribution of solar irradiation is thus changed in a characteristic
manner mainly due to the absorption of photons at certain frequencies within the
atmosphere; the AM 1.5 spectrum has been standardised and the light used for
solar cell or module calibration has to comply with this spectrum). The power
generated by solar cells under these conditions is referred to as peak power.
However, standard test conditions (STC) occur only very rarely in practice,
precisely spoken, almost never. In Europe, for example, at a radiation of
1,000 W/m 2 modules heat up by 20 to 50 K above ambient temperature, depend-
ing on the concept of module mounting or integration into the building environ-
ment. STC temperature and radiation thus only occur under ideal conditions in
winter when ambient temperature amounts to 0 °C or below. But due to the low-
angle of the sun AM values increase in winter and lead to a shift of the solar