A new type of solar cell using "quantum dots" may double the theoretical efficiency of current solar cells--allowing a panel to convert around 60 percent of the sun's energy that it laps up into electricity. The research on these new cells appeared Friday in Science. Current silicon-based solar cells lose about 80 percent of the sun's energy they take in. It's an inherent flaw: even working at their theoretical ideal, these cells would still lose 70 percent. We can blame the sun's diversely energized photons for this inefficiency. Silicon cells can only purposefully harvest photons with just the right amount energy. When they strike the cell, photons with just enough juice will prod an electron into motion (and create an electric current). An overly energized photon will excite the electrons to no purpose; the electrons will just quickly give off that photon's energy as heat. In two steps, this project, funded in part by the Department of Energy, salvages these "hot electrons."
"There are a few steps needed to create what I call this 'ultimate solar cell,'" says [Xiaoyang] Zhu, professor of chemistry and director of the Center for Materials Chemistry. "First, the cooling rate of hot electrons needs to be slowed down. Second, we need to be able to grab those hot electrons and use them quickly before they lose all of their energy." [University of Texas at Austin]
Step 1 -- Keep Hot Electrons Hot The researchers from the University of Texas at Austin kept the hot electrons from shedding their energy--by hindering them with quantum dots, nanoscale structures with quantum behaviors:
The group used nanoscale (less than 100 nanometers, or 10-9 meters) crystals of a compound called lead selenide. Like silicon, lead selenide is a semiconductor, meaning it absorbs light energy within a certain bandgap, or range of energies. But semiconducting nanocrystals, also known as quantum dots, exhibit very different properties than their larger counterparts. For one thing, they can hold on to a hot electron for a longer period of time, stretching out the amount of time it takes for the electron to cool. In fact, previous research has shown that quantum dots can increase the lifetime of hot electrons by as much as 1000 times. [Popular Mechanics]
Step 2 -- Forcing the Flow The team next spurred these energetic electrons by pushing them into a conducting material where they could more easily move.
Zhu's team has now figured out the next critical step: how to take those electrons out. They discovered that hot electrons can be transferred from photo-excited lead selenide nanocrystals to an electron conductor made of widely used titanium dioxide. "If we take the hot electrons out, we can do work with them," says Zhu. "The demonstration of this hot electron transfer establishes that a highly efficient hot carrier solar cell is not just a theoretical concept, but an experimental possibility." [Science Daily]
There's just one problem keeping these more efficient cells from competing with their silicon predecessors--hooking them up to a wire to use all that electric current. The hot electrons, it seems, are too hot to handle:
"If we take out electrons from the solar cell that are this fast, or hot, we also lose energy in the wire as heat," says Zhu. "Our next goal is to adjust the chemistry at the interface to the conducting wire so that we can minimize this additional energy loss." [University of Texas at Austin]
But quantum dots are not the only solar cell solution. DISCOVER reporter Andrew Moseman describes other front runners on page 14 of our July/August magazine issue, which is on newsstands now. Related content: 80beats: Tech Prize Goes to Inventor Who Wants to Turn Skyscrapers Into Generators 80beats: The Dream: Print-Out Solar Panels That Can Be Stapled to Your Roof 80beats: Glitter-Sized Solar Cells Could Be Woven into Your Power Tie 80beats: Self-Assembling Solar Panels Use the Vinaigrette Principle DISCOVER: Sun Catcher Promises Cheaper Solar PowerImage: The University of Texas at Austin