Direct Energy, 2018a

6 PHOTOVOLTAICS 127
Figure 6.14: Diagram of atmospheric windows wavelengths at which electromagnetic
radiation will penetrate the Earth's atmosphere. Chemical
Notation (CO 2 ,O 3 ) indicates the gas responsible for blocking sunlight at
a particular wavelength. This gure is used with permission [72].
energy cannot.
If a temperature gradient is applied across a pn junction, charges ow.
When one side of the device is heated, charges move more rapidly and these
energetic charges diuse to the cooler side. This eect, called the Seebeck
thermoelectric eect, is discussed in Chapter 8.
6.6 Solar Cells
6.6.1 Solar Cell Eciency
Energy conversion devices are never 100% ecient. Eciency is dened as
the output power over the input power. Eciency of a solar cell is often
dened as the ratio of electrical power out to optical power in to the device.
η eff = P electrical out
P optical in
(6.23)
Not all sunlight reaches a solar cell because some of it is absorbed by
the earth's atmosphere. This atmospheric absorption is strongly dependent
on wavelength. Figure 6.14 is a plot of the transmissivity of the
atmosphere as a function of wavelength. It plots the percent of light which
passes through the atmosphere without getting absorbed. Some gases in
the atmosphere, such as water vapor and CO 2 , absorb a signicant amount
of energy at particular wavelengths. The gure indicates which gas is responsible
for atmospheric absorption at some particular wavelengths. For
example, ozone O 3 absorbs ultraviolet light. Ozone in the atmosphere oers
benets because ultraviolet light can damage eyes and skin. The intensity
of the optical power from the sun that is hits a solar cell varies from day