Direct Energy, 2018a

6 PHOTOVOLTAICS 101
6 Photovoltaics
6.1 Introduction
This chapter discusses solar cells and optical detectors, both of which are
devices that convert optical electromagnetic energy to electricity. The next
chapter discusses lamps, LEDs, and lasers which convert energy in the opposite
direction. The photovoltaic eect is the idea that if a light shines on
a pure piece of semiconductor, electron-hole pairs form. In the presence of
an external electric eld, these charges are swept apart, and a voltage develops
across the terminals of the semiconductor. It was rst demonstrated
in 1839 by Edmond Becquerel. In a photovoltaic device, also called a solar
cell, this eect typically occurs at a semiconductor pn junction. This same
eect occurs on a smaller scale in photodiodes used to detect light and
in optical sensors in digital cameras. To understand the physics behind
these devices, we need to further study crystallography in semiconductors.
Energy level diagrams, which illustrate the energy needed to remove an
electron from a material, are another topic studied in this chapter.
Unlike fossil fuel based power plants, photovoltaic cells produce energy
without contributing to pollution. The solar power industry is growing at
a fast pace. Worldwide as of April 2017, photovoltaic cells were capable of
generating over 303 GW of power, and 75 GW of this total was installed
within the past year [67]. This generating capacity was sucient to satisfy
1.8% of the worldwide demand for electricity [67]. In the United States as
of April 2017, photovoltaic cells installed were capable of generating 14.7
GW [67].
6.2 The Wave and Particle Natures of Light
The physics of electromagnetic radiation is described by Maxwell's equations,
Eqs. 1.5 - 1.8, and discussed in Sections 1.6.1 and 4.4.1. Optical
energy is electromagnetic energy with wavelengths roughly in the range
400 nm λ 650 nm.
This wavelength range corresponds to the frequency range
4.6 · 10 14 Hz f 7.5 · 10 14 Hz.
We often think of electromagnetic radiation as behaving like a wave. However,
it has both wave-like and particle-like behavior.
One way to understand light is to think of it as composed of particles
called photons. A quantum is a small chunk, and a photon is a quantum,