Photovoltaic cells of different chemistries and morphologies have very different tolerance of light at a variety of angles. Various geometrical factors, such as the tilt of the earth rotational axis, its orbital motion around the sun or the sunlight incident angle, affect the light intensity on the earth surface. Moreover, the atmosphere alters the sun radiation by absorbing and reflecting selected wavelengths. For terrestrial measurements (at the Earth's surface) the number following the AM designation is the inverse of the cosine of the zenith angle subtended by the sun and the surface normal (Equation 7). The solar spectrum that strikes the earth surface at the equator at noon at the equinox is called AM1.0 (air mass 1.0), but most of the time and for most locations on the globe, sunlight actually passes through more than one air mass before reaching the earth surface. Therefore solar cells are generally characterized with a normalized spectrum, called AM 1.5 radiation, of 1000 W/m2 or 1 sun irradiance at 25 oC (IEC 904-3). The light passes the atmosphere at an angle of 48.2 degrees its path is 1.5 times the minimum distance (sun at zenith). The figure compares the actual solar spectra for AM0 and AM1.5. The difference between these two spectra is due to the spectral absorption from components of the Earth's atmosphere, such as H2O and O2.
Figure - Schematic diagram of the origin of the different AM light spectra
Source: IDTechEx