Image Credit: U.S. Department of Agriculture, 2011, CC by 2.0.

The photocoltaic effect

In 1839 the photovoltaic effect was first observed by the French physicist Alexandre Edmond Becquerel (the father of Henri Becquerel – yes, they are related).  This phenomenon occurs when two different materials are in close contact with each other. When light hits one of the materials, energy is consumed and electrons are lifted to an excited state where they have a higher energy than in the ground state. As a result an electric field is formed along the contact to the second material. This field applies a force on the excited electrons and can force them into an external electrical load where their energy can be used to power an electronic device.

The photoconductivity of selenium

The English engineer Willoughby Smith experimented on selenium and found in 1873 that the normally insulating material becomes electronically conductive when exposed to light. This phenomenon is called photoconductivity. William Grylls Adams and Richard Evans Day discovered three years later that selenium can also produce electricity when light is shone on it. The first solar cell was finally created in 1883 by the American engineer Charles Fritts when he coated selenium with a thin layer of gold. His results were reproduced and confirmed later by the German engineer Werner von Siemens. Nevertheless, these prototype cells could only convert about 1 % of sunlight into electricity (in other words they had an efficiency of 1 %) and the phenomenon could not be well understood at the time. For these reasons solar cells were not developed further back then.

The photoelectric effect

The photoelectric effect was first observed by the German physicist Heinrich Hertz. This effect occurs when solid materials emit free electrons under exposure to light (or other electromagnetic radiation like X-rays). Modern silicon solar cells rely on this phenomenon to create electricity from sunlight. Albert Einstein later received the Nobel Prize in Physics for explaining the photoelectric effect in detail. (No, he did not get it for the relativity theory.)

The silicon solar cell

Daryl Chapin, Calvin Fuller and Gerald Pearson from the Bell Laboratories (New Jersey, US) discovered in the early 1950s that silicon is much better at converting sunlight into power than selenium. This lead to the first practical silicon solar cell being demonstrated in 1954 which showed an efficiency of 6 %. The first commercial silicon solar cells entered the market 1956, but they were still very expensive and not very succesful at first. The situation changed with the dawn of spaceflight where solar cells were used by NASA to power satellites like Vangguard 1 from 1958 onwards. This application enabled further research which resulted in lower prices for solar power. Nevertheless, it was not before 1982 that the first solar park was installed in California (US). Today silicon solar cells reach an efficiency of 15 – 20 %. Due to climate change, smog and pollution, solar cell technology has received a lot of interest during the past years as a clean alternative to burning of fossil fuels like coal. Now we are at a point where the cost for silicon solar cells if falling rapidly which could make them even more popular in the future.

The future of solar cell technology

Despite the historically low prices of silicon solar cells, they might soon encounter some serious competition as new materials and concepts are being studied. One contender could be perovskite solar cells (discovered 2009 in Japan) which promise a cheaper and simpler manufacturing process than silicon solar cells. In addition, dye-sensitized solar cells, which were discovered already in the 1960s and rely on the photovoltaic effect, could eventually prove another low-cost competitor. Up to now these new technologies are less efficient than silicon solar cells, but especially perovskite solar cells are on a good way to reach an efficiency of 15 – 20 % within just a few years. Another strategy could be the combination of perovskite solar cells and silicon solar cells in hybrid modules. It remains exciting to see which road solar cell technology will take next.