ABSTRACT
Photovoltaic (PV) cells directly convert light to electricity. The most common source of light on earth is
the sun. The photosphere, or the external region of the sun, emits radiation closely approaching that of a
thermodynamic “black body,” or perfect radiator, at a temperature of 6000 K, with a spectral distribution
governed by Planck’s radiation law, as shown in Figure 8.1.1 The radiant power per unit area
perpendicular to the direction of the sun just outside the earth’s atmosphere is essentially constant
and is known as the solar constant. This is also known as the air-mass zero (AM0) spectrum, which has
the value of 1.367 kW/m2. The AM0 spectrum, plotted in Figure 8.1, is used for determining the
performance of PV cells in space. Sunlight is attenuated by at least 30% while passing through the earth’s
atmosphere due to scattering of light by molecules, aerosols, and dust particles, as well as absorption by
gases such as oxygen, ozone, water vapor, and carbon dioxide. In addition, sunlight passes through more
or less of the atmosphere depending on the time of day, with the most direct path being when the sun is
directly overhead, defined as air-mass one (AM1). Therefore, terrestrial sunlight varies in both intensity
and spectral composition. To allow a meaningful comparison between the performance of solar cells at
different locations, a terrestrial standard has been defined that corresponds to the sun being 488 off
overhead, known as AM1.5. This spectrum is also plotted in Figure 8.1, and the total power density is
normalized to PinZ1 kW/m 2, which is close to the actual value received at the earth’s surface.
