If a photon hits something, something happens. In a solar cell, the photon knocks an electron out of silicon, as as you know, that is how electricity works. One photon gets to make one electron worth of electricity, maximum.
Photons come in a range, one would even say a spectrum, of energy levels. And yes, the higher energy photons have — more energy! But since one photon can only knock out one electron in a solar cell, no matter how much energy the photon has, it can only knock out that one electron. If only there was a way to harvest the additional energy that comes from those higher energy photons!
There is now possibly a way to free up more than one electron, by increasing the external quantum efficiency of the cell to go over 100%. The technique was discovered and reported in 2013, and since then has remained one of those “breakthrough” technologies that we don’t believe in until they happen. Technically, this may not be a true breakthrough level technology, since it would only increase solar cell energy production by single or low double digit percentage. But that is still a good thing.
The same team that reported this in 2013 has now demonstrated on the lab bench that they can do it. The earlier work also produced actual results, but using a form of solar cell that was very inefficient to begin with. So that’s like saying, “I can run faster than you” but only from a mile back from where you start.
From the press release from MIT, you can glean the details:
The key to splitting the energy of one photon into two electrons lies in a class of materials that possess “excited states” called excitons, Baldo says: In these excitonic materials, “these packets of energy propagate around like the electrons in a circuit,” but with quite different properties than electrons. “You can use them to change energy — you can cut them in half, you can combine them.” In this case, they were going through a process called singlet exciton fission, which is how the light’s energy gets split into two separate, independently moving packets of energy. The material first absorbs a photon, forming an exciton that rapidly undergoes fission into two excited states, each with half the energy of the original state.
But the tricky part was then coupling that energy over into the silicon, a material that is not excitonic. This coupling had never been accomplished before.
As an intermediate step, the team tried coupling the energy from the excitonic layer into a material called quantum dots. “They’re still excitonic, but they’re inorganic,” Baldo says. “That worked; it worked like a charm,” he says. By understanding the mechanism taking place in that material, he says, “we had no reason to think that silicon wouldn’t work.”
What that work showed, Van Voorhis says, is that the key to these energy transfers lies in the very surface of the material, not in its bulk. “So it was clear that the surface chemistry on silicon was going to be important. That was what was going to determine what kinds of surface states there were.” That focus on the surface chemistry may have been what allowed this team to succeed where others had not, he suggests.
The key was in a thin intermediate layer. “It turns out this tiny, tiny strip of material at the interface between these two systems [the silicon solar cell and the tetracene layer with its excitonic properties] ended up defining everything. It’s why other researchers couldn’t get this process to work, and why we finally did.” It was Einzinger “who finally cracked that nut,” he says, by using a layer of a material called hafnium oxynitride.
The layer is only a few atoms thick, or just 8 angstroms (ten-billionths of a meter), but it acted as a “nice bridge” for the excited states, Baldo says. That finally made it possible for the single high-energy photons to trigger the release of two electrons inside the silicon cell. That produces a doubling of the amount of energy produced by a given amount of sunlight in the blue and green part of the spectrum. Overall, that could produce an increase in the power produced by the solar cell — from a theoretical maximum of 29.1 percent, up to a maximum of about 35 percent.
Actual silicon cells are not yet at their maximum, and neither is the new material, so more development needs to be done, but the crucial step of coupling the two materials efficiently has now been proven. “We still need to optimize the silicon cells for this process,” Baldo says. For one thing, with the new system those cells can be thinner than current versions. Work also needs to be done on stabilizing the materials for durability. Overall, commercial applications are probably still a few years off, the team says.
Other approaches to improving the efficiency of solar cells tend to involve adding another kind of cell, such as a perovskite layer, over the silicon. Baldo says “they’re building one cell on top of another. Fundamentally, we’re making one cell — we’re kind of turbocharging the silicon cell. We’re adding more current into the silicon, as opposed to making two cells.”
The researchers have measured one special property of hafnium oxynitride that helps it transfer the excitonic energy. “We know that hafnium oxynitride generates additional charge at the interface, which reduces losses by a process called electric field passivation. If we can establish better control over this phenomenon, efficiencies may climb even higher.” Einzinger says. So far, no other material they’ve tested can match its properties.