Researchers have demonstrated a way to use quantum dots to significantly reduce the amount of energy that solar cells lose to heat, paving the way for future cells that are twice as efficient as today’s technology.
When it comes to turning sunlight into electricity, today’s technology leaves lots of room for improvement. The most efficient solar cells on the market, which are made of silicon, convert less than 20 percent of the light that hits them into electricity, and the theoretical maximum efficiency of these cells is around 31 percent.
One reason for this low efficiency is that much of the incoming light contains energy that is too high for solar cells to capture, so it’s lost as heat. Now researchers have shown that it’s possible to harvest that energy before it escapes, meaning that engineers could one day develop next-generation solar cells with efficiencies of up to 66 percent. The research, funded by the Department of Energy, is described in the June 18 edition of the journal Science.
When light hits a solar cell, a fraction of its energy is absorbed, exciting electrons in the cell’s material and knocking them free. An electric field then forces the free electrons to flow in a specific direction, producing electric current. The energy that is absorbed is determined by what scientists call the bandgap—a limited range of energies the cell’s material can capture.
But sunlight is composed of particles, called photons, representing a very broad range of energies. The energy from photons too high to be absorbed takes the form of high-energy electrons—or, as scientists call them, “hot electrons”–and is lost as heat. However, if one could remove the hot electrons before they cool, says study author Xiaoyang Zhu, a chemistry professor at University of Texas at Austin, “then you essentially shut down this heat-loss pathway, and you increase efficiency by more than a factor of two.”
To accomplish this, 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.
Once a hot electron is confined within a quantum dot, then comes the hard part: removing it so its energy can be harvested. The electron likes to stay inside the quantum dot, Zhu says, “so we needed to find something that would attract it out.” For this role, the researchers chose titanium oxide, a well-studied compound known for its ability to accept new electrons. Then came the really hard part: arranging the lead selenide quantum dots and titanium dioxide in such a way that their chemical interactions would induce electron transfer.
Not only was the transfer successful, it was also very fast. If verified, this result makes highly efficient quantum dot solar cells more realistic, according to Tianquan Lian, a chemistry professor at Emory University who was not part of this study, and whose research revolves around the use of nanomaterials for solar energy conversion. This is the first demonstration that, in principle, the vital electron transfer step is possible, he says.
The ultimate goal, Zhu says, is called a “hot-carrier solar cell,” which could convert up to 66 percent of incoming light into electricity. But many scientific and engineering steps remain before such a cell can be commercially developed. One challenge is to figure out how to transfer the hot electrons to a conducting wire. “This is science that has really striking implications, but implication is not application yet,” Zhu says, adding, “I’ll be extremely happy if, in my lifetime, I see [hot-carrier cells] on roofs.”
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Source: Popular Mechanics