Recently, Boston College researchers observed the so-called “hot electron” effect in a solar cell for the first time, and then harvested the charges with ultra-thin solar cells.
What does this mean for you and for me? The short answer is: improved solar panel efficiency.
Generally speaking, when UV light is captured in solar cells, it generates free electrons in a range of energy states. In order to capture the charges and convert them to renewable electricity, the electrons must reach the bottom of the conduction band. In the past, one of the biggest hurdles has been the fact that the highly energized “hot” electrons lose much of their energy to heat along the way.
You can see hot electrons in devices like semiconductors. Yet, the high kinetic energy can cause the electrons, also known as “hot carriers,” to degrade a device. For years, researchers have theorized about the benefits of harnessing hot electrons for solar power through so-called “3rd generation” devices.
Scientists are now using ultra-thing solar cells (less than 30 nanometers thick) in a mechanism that can extract hot electrons before they cool. This may effectively open an “escape hatch” through which they typically don’t travel.
Professor of Physics Krzysztof Kempa used the analogy of trying to heat a swimming pool with a pot of boiling water:
Drop the pot into the center of the pool and there would be no change in temperature at the edge because the heat would dissipate en route. But drop the pot into a sink filled with cold water and the heat would likely raise the temperature in the smaller area.
“We have shrunk the size of the solar cell by making it thin. In doing so, we are bringing these hot electrons closer to the surface, so they can be collected more readily. These electrons have to be captured in less than a picosecond, which is less than one trillionth of a second.”
With ultra-thin solar cells, an overall increase in efficiency of 3% can be expected with absorbers 1/50th as thick as conventional cells! The Boston College research team determined that the gains were the result of capturing hot electrons and an accompanying reduction in voltage-sapping heat. Currently, the film’s efficiency is limited by the negligible light collection of ultra-thin junctions. However, by combining the film with new light-trapping technology — such as nanowire structures –we could significantly increase efficiency in an ultra-thin hot electron solar cell technology.