f = the light’s frequency (in cycles per second = c/λ)
c = the velocity of light (300,000,000 meters per second)
λ = the wavelength (in meters)
Using the fact that an angstrom unit is 10⁻¹⁰ meter, the energy of a 5500 A photon could be calculated as
E = hf = hc/λ = (6.62 × 10⁻³⁴ × 3.00 × 10⁸)/(5.50 × 10⁻⁷)
= 3.61 × 10⁻¹⁹ joule = 2.2 electron volts
Comparing this result, 2.2 electron volts, with the energies required to cause atomic ionization or molecular dissociation (an electron volt or so), we see that it is in the right range to actuate direct conversion devices based on such phenomena.
Harnessing the Sun’s Energy
Historically, the sun’s energy has most often been used by concentrating it with a lens or mirror and then converting it to heat. We could do this and run a heat engine, but a more direct avenue is open.
About a decade ago it was found that the junction between p and n semiconductors would generate electricity if illuminated. This discovery led to the development of the solar cell, a thin, lopsided sandwich of silicon semiconductors. As shown in [Figure 12], the top semiconductor layer exposed to the sun is extremely thin, only 2.5 microns. Solar photons can readily penetrate this layer and reach the junction separating it from the thick main body of the solar cell.