What is an easy way to understand solar energy?

Answer 1

Please see the related link about the energy band theory. It is easier to understand semiconductor operations with the concept.

At room temperature (20 degrees Centigrade) or higher (the sun's radiation will warm up the earth and everything in the sun), some electrons, but not enough, will be roaming in the conduction band of the solar cell, made of silicon mostly, or other III-V or II-V semiconducting compounds. Each material has a distinct band gap (Si: 1.1eV), the forbidden energy zone, that an electron in the valence band has to cross/overcome in order to be in the conduction band. What can excite an valence electron (what can supply sufficient energy) to the conduction band?

The answer is photons. When a photon is absorbed in a semiconductor, the photon releases its energy to the lattice as heat or to the electrons. Photons also have a second personality, a behavior that is best described as an electromagnetic wave (see the related link about EM spectrum), which encompasses a wave with frequency ranging from 1x104 to 1x1021 [Hz]. The sun is a good source of EM waves (see the related link about the sunlight), the visible part of which spans from 400 nm to 700 nm in wavelength.

We can convert frequency to wavelength to energy for photons/light. They are interchangeable here.

wavelength = c / f, where c = the speed of light = 3x108 [m/s]; f = frequency [Hz]

energy = h * frequency, where h = the Planck constant = 6.626 068 96(33) × 10−34[J s].

Fortunately, we also have a visual aid (see the table in the EM spectrum link). All we need to know is do we have photons of enough energy to overcome at least 1.1eV of the energy gap? According to the table, we need a wave of wavelength less than about 1x10-6 [m], which is, viola, within the range of visible light.

Conclusion: the sun provides the photons needed to excite valence electrons to the conduction band of a semiconductor. Once the electron is in the conduction band, it is free to roam. With the aid of an electric field (by applying a voltage across two terminals), you have electron flow in the closed circuit (electricity).

P.S. The business of solar cell remains to be how to increase the conversion efficiency (different materials, reflective coating, sun-tracking, etc) without incurring too much cost. (See the related link about solar cells).