by Steve Lussing

Created on: October 24, 2008

The process whereby a solar cell converts light into energy is called photovoltaics. In 1839, the French physicist Antoine-Csar Becquerel first noticed that a voltage developed when light fell upon a solid electrode placed in an electrolyte solution.

Albert Einstein won his Nobel Prize for describing the photovoltaic effect, whereby ultraviolet light striking a sodium block placed in a vacuum jar and which completes an electric circuit causes the sodium block to act as a cathode by emitting electrons from its surface, deflecting the pointer in a current meter. He said that high frequency photons from the ultraviolet light had enough energy to knock electrons from the outer orbitals of the sodium atoms on the surface of the block.

Other researchers experimented with photovoltaics but they found that the efficiency of the conversion of light into energy was very low, less than one percent. Then, in 1941, Russel Ohl invented the solar cell. By 1954, research by others had led to the design of a silicon solar cell which had an energy conversion efficiency of six percent, and to the creation of the first solar panel.

There is continuing research ongoing in the effort to increase the efficiency of solar cell power conversion because more efficient solar cells mean lower cost per watt ratios. The latest efforts have seen the team at Boeing-Spectrolab develop a solar cell that converts 40.7 percent of the light it receives into electricity. That is remarkable progress.

The development of solar cell technology has made possible safe and reliable methods of powering satellites and space stations. Here on Earth we receive much less solar energy than in space because of reflection and absorption by our atmosphere. About ten times as much solar energy per unit area strikes the surface of space-based solar cells compared with those based on the surface of our Earth.

But there is a trade-off. Radiation damage to a space-based solar cell array limits its usable lifetime. This can be very expensive because this limits the lifetime of the satellite which it powers. Additionally, micrometeoroids can impact and damage the fragile solar cells. Also, bombardment by high-energy particles results in significant degradation of space-based solar cells over time.

Later research into methods to increase the lifetime of space-based solar arrays resulted in the development of a solar cell with an orbital lifetime ten years longer than earlier types of solar cells. Radiation damage was analyzed to be the result of secondary effects of the radiation acting on the primary defects of the solar cell which then react with themselves and with each other, causing rapid degradation. By introducing a "getter" layer into the cell, which drew away the secondary effects, the solar cell was made much more radiation resistant, thereby enhancing its longevity.

Solar cells are the green approach to powering space-based electrical systems. They have made obsolete the radioisotope thermoelectric generator, which uses dangerous Plutonium 238, and which can cause potential harm if damaged when used in applications in Earth orbit. Spin-offs from this research to improve the space-based solar array will have great repurcussions for the continued improvement of Earth-based alternative energy systems.

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