624
22 Light-to-Electricity Conversion
Fig. 22.53 Solar spectra (power per area and wavelength interval) for AM0 (black line, extraterrestrial irradiance) and
AM1.5 (sun at 41.8 ◦ elevation above horizon) for direct normal irradiance (blue line) and global total irradiance (red
line) on a sun facing surface (tilted 37 ◦ towards the equator). Left (right) graph in log-log (linear) scales
Fig. 22.54 Schematic path
of sunlight through the
atmosphere and definition
of the air mass AMx
A M 3 .9 1
A M
1 . 5
A M 2
A M 1 . 1 3
AM1
=14.8°
horizon
earth
This value and the according spectrum of the sun’s emission, which is similar to a blackbody with
temperature 5800 K (Fig. 22.53), is termed air mass zero (AM0). The total energy that reaches the earth
from the sun is 1.8 × 10
17 W per year. This value is 10
4 times the world’s primary energy need.
Air mass zero (AM0) is important for solar cells in satellites. When the solar spectrum passes
the earth’s atmosphere, it is changed with regard to its shape and the total energy density due to
gas absorption (ozone, water, CO 2 , . . .). Depending on the meridian of the sun γ (Fig. 22.54), the
spectrum on the surface of the earth is termed AMx with x = 1/ sin γ . In spring and fall (March
21st and September 21st), Leipzig (51
◦ 42
N latitude) has about AM1.61. At the summer (June 21st)
and winter (December 21st) solstices the air mass in Leipzig is AM1.13 (γ = 61.8
◦ ) and AM3.91
(γ = 14.8
◦ ), respectively. Additionally, the duration of sunshine and thus the light power density is
regionally different across the earth due to climate and weather (Fig. 22.55). For AM1.5, the incident
power density is 844 W/m
2 .
The global radiation reaching a photovoltaic cell has three components: (i) the direct radiation, (ii)
diffuse radiation and (iii) reflected radiation. The relative amounts and their spectra depend on details
such as the climate (e.g. humidity) or the environment (e.g. outdoors vs. urban).
22.4.2 Ideal Solar Cells
When a solar cell made from a semiconductor with a band gap E g is irradiated by the sun, only photons
with hν > E g contribute to the photocurrent and the output power. The I –V characteristic under
22 Light-to-Electricity Conversion
Fig. 22.53 Solar spectra (power per area and wavelength interval) for AM0 (black line, extraterrestrial irradiance) and
AM1.5 (sun at 41.8 ◦ elevation above horizon) for direct normal irradiance (blue line) and global total irradiance (red
line) on a sun facing surface (tilted 37 ◦ towards the equator). Left (right) graph in log-log (linear) scales
Fig. 22.54 Schematic path
of sunlight through the
atmosphere and definition
of the air mass AMx
A M 3 .9 1
A M
1 . 5
A M 2
A M 1 . 1 3
AM1
=14.8°
horizon
earth
This value and the according spectrum of the sun’s emission, which is similar to a blackbody with
temperature 5800 K (Fig. 22.53), is termed air mass zero (AM0). The total energy that reaches the earth
from the sun is 1.8 × 10
17 W per year. This value is 10
4 times the world’s primary energy need.
Air mass zero (AM0) is important for solar cells in satellites. When the solar spectrum passes
the earth’s atmosphere, it is changed with regard to its shape and the total energy density due to
gas absorption (ozone, water, CO 2 , . . .). Depending on the meridian of the sun γ (Fig. 22.54), the
spectrum on the surface of the earth is termed AMx with x = 1/ sin γ . In spring and fall (March
21st and September 21st), Leipzig (51
◦ 42
N latitude) has about AM1.61. At the summer (June 21st)
and winter (December 21st) solstices the air mass in Leipzig is AM1.13 (γ = 61.8
◦ ) and AM3.91
(γ = 14.8
◦ ), respectively. Additionally, the duration of sunshine and thus the light power density is
regionally different across the earth due to climate and weather (Fig. 22.55). For AM1.5, the incident
power density is 844 W/m
2 .
The global radiation reaching a photovoltaic cell has three components: (i) the direct radiation, (ii)
diffuse radiation and (iii) reflected radiation. The relative amounts and their spectra depend on details
such as the climate (e.g. humidity) or the environment (e.g. outdoors vs. urban).
22.4.2 Ideal Solar Cells
When a solar cell made from a semiconductor with a band gap E g is irradiated by the sun, only photons
with hν > E g contribute to the photocurrent and the output power. The I –V characteristic under