The Pathway To Today’s Solar Revolution: Discovering The Photosensitivity Of Selenium

This article is the launch of a CleanTechnica miniseries with physicist John Perlin, focusing on the discovery of photosensitivity. Our dialogue is based on Perlin’s book, Let It Shine: The 6000-Year Story of Solar Energy.

For any person wishing to keep abreast of today’s solar revolution, Perlin’s expertise on the subject provides an excellent archive. And as we soon discovered, not everyone has their facts about solar energy in the correct order… in fact, there’s a lot of misinformation about the history of solar that is widely shared.

Here are some questions we posed to Perlin and his answers:

CleanTechnica: When Let It Shine first came out, several experts criticized you for not recognizing Edmund Becquerel as the direct link to the solar cells that are in use today.

Perlin: Even though all the encyclopedias, wikipedias, and solar experts share similar views, they all have erred. Becquerel even states that he worked with an electrochemical cell and observed sunlight recharging it. But this was clearly not a solid-state device directly related to the current state of the art in photovoltaics.

CleanTechnica: Please elaborate on this.

Perlin: The discovery of the direct conversion of sunlight into electricity by a solid-state device happened due to the failure in the late 1860s of bars of selenium to act as expected for quality control of cabling before being submerged undersea as part of the first transatlantic communication network linking North America with Europe. Though the bars of selenium worked as expected at night, they performed dismally when the sun came out. Suspecting that the change in behavior had something to do with light, Willoughby Smith, the chief engineer of the cabling project, placed the bars in a box with a sliding cover. During the daytime, when he closed the cover, excluding any light from entering the interior, the bars’ high electrical resistance – the degree to which they hindered electricity flowing through them – acted as expected and did not fluctuate at all. But when the cover was opened, their conductivity – the enhancement of electrical flow – “immediately increased according to the intensity of light,” Smith observed.

CleanTechnica: How did Smith follow up?

Perlin: To determine whether it was the Sun’s heat or its light that affected the selenium’s increased conductivity, Smith placed one of the bars in a shallow trough of water. The water kept the selenium cool but had no effect on light penetrating it. When he covered and uncovered the trough as he had previously done, his results didn’t differ from these observations. He concluded the “bars’ resistance was altered according to the intensity of light.”

CleanTechnica: What reaction did the scientific community have to this observation?

Perlin: Selenium’s sensitivity to light intrigued the world’s scientific community. Never before had scientists seen light create such an alteration of an element’s electrical behavior. For this reason, “The attention of physicists is at present very much directed to the material,” commented Werner von Siemens, whose reputation in the field of electricity ranked alongside Edison’s. An intrigued James Clerk Maxwell, the greatest nineteenth-century physicist, wrote to a friend, “I saw conductivity of Selenium as affected by light. It is most sudden. Effect of a copper heater insensible. That of the sun great.”

Probably no scientific team subjected selenium to more experimentation than two British scientists, William Grylls Adams and Richard Evans Day. What they discovered would begin the long march to today’s photovoltaic revolution.