In 1961, William Shockley and Hans-Joachim Queisser calculated that the maximum theoretical efficiency of a silicon-based solar panel is 30%. In other words, less than a third of the sunlight that strikes a solar panel can be turned into electricity.
Today, only high-end solar panels intended for use in spacecraft get near that maximum efficiency limit. Those panels are far too expensive for normal commercial use. The average panels used on rooftops and in solar farms are much less expensive, but have an efficiency of around 22%.
The problem is that silicon only responds to certain wavelengths, particularly those in the red and yellow portion of the electromagnetic spectrum. Longer light waves in the infrared part of the spectrum are too weak to create an electrical current. Shorter light waves in the blue and green part of the spectrum don’t create any electrical current when they strike the silicon in a solar cell — at most, they bounce off. At worst, they generate heat, which degrades the efficiency of panels.
A Bright Idea Becomes A New Business
In 2014, Akshay Rao and a team of researchers at the University of Cambridge had a bright idea. What if there was a way to convert those blue and green light waves into red light waves? That would boost the efficiency of a solar panel to around 35% — roughly 50% more than the conventional solar panels in use today. Can you image what that would mean to the world of renewable energy?
“Rao developed a photon multiplier film made up of a layer of an organic polymer called pentacene studded with lead selenide quantum dots — small, light emitting clumps of inorganic material. The polymer absorbs blue and green photons and converts them into pairs of excitons. These excitons flow to the quantum dots, which absorb them and emit lower energy red or infrared photons.
“When the film is placed on top of a silicon solar cell, the light from the quantum dots shines onto the silicon. Meanwhile, the red and infrared wavelengths directly from the sun pass through the polymer film and hit the silicon as they normally would. The result is that more usable photons strike the silicon, increasing production of electrical current.”
“You’re preserving the total energy that comes in and out, but you’re making the silicon receive a higher photon flux in the portion of the spectrum that it’s good at converting into electricity,” Wilson says.