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Renewable Energy The photoelectric effect and its role in solar photovoltaics

From Luke James

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Photovoltaic energy allows us to generate renewable energy using the sun. It works by converting solar radiation into electricity using the photoelectric effect, but what is it and how did it become so prevalent?

Photovoltaic energy is based on the photoelectric effect—the emission of electrons when electromagnetic radiation (i.e. light) hits a material.
Photovoltaic energy is based on the photoelectric effect—the emission of electrons when electromagnetic radiation (i.e. light) hits a material.
(Source: gemeinfrei / Pixabay)

Photovoltaic solar energy is generated by converting sunlight into energy, a type of clean, renewable, and inexhaustible energy that can be produced in installations ranging from small panels on the top of houses to large photovoltaic plants. This is achieved using a technology based on the photoelectric effect.

What exactly is photovoltaic energy?

Photovoltaic energy is a clean, renewable source of energy that uses solar radiation to produce electricity. It is based on the photoelectric effect—the emission of electrons when electromagnetic radiation (i.e. light) hits a material. Electrons that are emitted in this manner are known as photoelectrons and they generate an electric current.

To harvest electromagnetic radiation and turn it into useable electricity, a semiconductor device known as a photovoltaic cell is used. These are commonly made from monocrystalline, polycrystalline, or amorphous silicon, however, other thin-film semiconductor materials can also be used.

The type of material used has an impact on how efficient the solar cell is. Those made from monocrystalline, for example, are obtained from a single crystal of pure silicon and can achieve a maximum efficiency of between 18 % and 20 % on average. In contract, those made from amorphous silicon have a disordered crystalline network and this leads to a lower performance efficiency of between 8 % and 9 % on average (but they are also much cheaper).

A very brief history of light

Research on the nature of light is known to have started way back in the times of the Ancient Greeks. During this time, philosophers such as Plato, Socrates, and Pythagoras all offered their opinions and theories on the matter, with scientists from the medieval times coming along later to work on theories of light and vision.

However, it wasn’t until 1678, when Christian Huygens developed a technique for proving how light moves in waves and where light waves propagate, until we truly began understanding the nature of light. At the time, it was not considered as good enough evidence but further works in the 19th century by Thomas Young (in 1801) and James Clerk Maxwell (in 1865) culminated in what we understand a beam of light to be today—a traveling wave of electric and magnetic fields.

Discovering the photoelectric effect

In 1887, Heinrich Hertz devised some experiments with a spark gap generator to test James Clerk Maxwell’s hypothesis. These experiments are thought to have produced the first transmission and reception of electromagnetic waves.

In the experiment, sparks generated between two small metal spheres in a transmitter created sparks that jumped between two brass knobs in a copper wire loop that acted as a receiver. A tiny spark then jumped between these two electrodes. Hertz discovered that by illuminating the electrodes with UV light, he could make the receiver spark more vigorously. This was the first observation of the photoelectric effect.

Although Hertz did not explain the observed phenomenon (the photoelectric effect), his results were confirmed by Wilhelm Hallwachs around one year later. He showed that UV light shining on an evacuated quartz bulb with two zinc plates acting as electrodes and connected to a battery generated a current due to the photoelectric current (electron emission).

Stoletov’s law (the “first law of photoeffect”) also confirms this: That there is a proportionality between the intensity of electromagnetic radiation acting on a metallic surface and the induced photoelectric current.

Electrons and Einstein’s theory

In 1897, electrons (or “corpuscles” as they were then called) were discovered by JJ Thompson. He then proposed a structure for the atom and in 1899, he showed that the increased sensitivity in Hertz’s experiments was because of light pushing on corpuscles. Also, in 1899, Philipp Lenard showed that irradiating metals with UV light may produce photoelectrons. He found that the kinetic energy of these photoelectrons was independent from the intensity of light of the same frequency. Despite this, he did agree that more photoelectrons would be emitted by a bright source than a dim one.

13 years later, Thompson’s student, Rutherford, proposed a model for the atom as a positively charged core (nucleus) with electrons (negative charges) circulating at varying distances around it. This nuclear structure of the atom is still accepted today. Rutherford’s work has since earned him the title of “the father of nuclear physics.”

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Einstein attempted to explain the photelectric effect by bringing back the idea of light corpuscles that were advocated by Isaac Newton centuries prior. He also considered the works of Max Planck that proposed that light comes in bundles of energy, and that in a light beam, there are hundreds of “quanta”. He recognized that Planck’s model was real: Light isn’t a continuous wave of electromagnetic radiation but is rather a stream of discrete quanta. Einstein’s work resulted in an essential formula for quantum physics known as the Planck-Einstein relation:

= ½ · m · v2 = Ey - W = h · fW

Where is the energy of an electron, v is the speed of an electron, m is the mass of an electron, Eᵧ is the energy of the light quantum, and W is the work function, which is a constant dependent on the metal.

W is the energy that is required to release an electron from a metal to produce photoelectrons. The number of photoelectrons released depends on the metal, its crystalline structure, and how polished the surface is. Einstein theorized that when light hits a metal, some of the energy goes towards W and some goes to electrons as kinetic energy. Where energy supplied matches W, electrons are released at zero velocity. This explains Hertz’s observation that with higher intensities of UV light, more sparks were released.

For his work and “services to Theoretical Physics, and especially for his discovery of the law of the photoelectric effect,” Einstein was awarded the Nobel Prize in 1921.

And although Einstein’s work is still good science even today, it’s customary to use the term “photons” due to Gilbert Lewis’ work in 1926 which proposed that instead of light quantum, we should consider a new kind of atom, the photon, as the carrier of light. Scientists consider “photon” to be a suitable synonym for Einstein’s light quantum works.