Generally, the overall efficiency of any traditional silicon-based solar cell has an absolute limit. This is partly based on the fact that a single electron can be displaced or knocked loose by each photon of light, even if that photon carried double the energy required to do so.
However, scientists have now demonstrated a new method for acquiring high-energy photons that strike silicon to knock out two electrons rather than one. This paves the way for a new type of solar cell that has more efficiency than was thought before.
Traditional silicon cells possess an absolute theoretical maximum efficiency of approximately 29.1% conversion of solar energy, but over the last several years, investigators at MIT and elsewhere have developed a new method that could exceed that limit, possibly adding a number of percentage points to that maximum output.
The results of the study have been recently reported in the journal Nature, in a paper authored by professor of chemistry Moungi Bawendi, graduate student Markus Einzinger, professor of electrical engineering and computer science Marc Baldo, and eight others at Princeton University and at MIT.
The fundamental concept of this novel technology has been known for many years. In fact, six years ago, some of the team members carried out the initial demonstration that the principle may actually work. However, it took years of hard work to actually translate the technique into a complete, operational silicon solar cell, informed Baldo.
That first demonstration “was a good test platform” to show that the concept may indeed work, explained Daniel Congreve PhD ’15, currently an alumnus at the Rowland Institute at Harvard, and also the lead author of that previous report and a co-author of the latest paper. Now, armed with the latest results, “we’ve done what we set out to do” in that project, he stated.
It was the original study that showed the production of a pair of electrons from a single photon, but demonstrated this in an organic photovoltaic cell, which is less efficient when compared to a silicon solar cell. It was eventually discovered that transferring the pair of electrons from a top collecting layer composed of tetracene into the silicon cell, “was not straightforward,” added Baldo.
According to Troy Van Voorhis, a professor of chemistry at MIT who was part of that original group, the idea was initially suggested earlier in the 1970s, and he added wryly that converting that concept into a practical device, “only took 40 years.”
A group of materials possessing “excited states” known as excitons is integral for splitting the energy of a single photon into two electrons, stated Baldo: In these kinds of excitonic materials, “these packets of energy propagate around like the electrons in a circuit,” but with relatively different characteristics than electrons. “You can use them to change energy—you can cut them in half, you can combine them.”
In this situation, these packets of energy were undergoing a process known as singlet exciton fission, which is how the light’s energy gets divided into two separate and independently moving packets of energy. After absorbing a photon, the material forms an exciton that quickly goes through fission into a pair of excited states, each containing 50% of the energy of the original state.
However, the tricky part was to couple that energy over into the silicon — a material that is not excitonic. This is the first-ever coupling to be achieved so far.
As a transitional step, the researchers attempted to couple the energy from the excitonic layer into a material known as quantum dots. “They’re still excitonic, but they’re inorganic,” Baldo stated. “That worked; it worked like a charm,” he added. By interpreting the mechanism that occurs in that material, “we had no reason to think that silicon wouldn’t work,” he further stated.
According to Van Voorhis, that work demonstrated that the surface of the material itself, not in its bulk, was integral to these energy transfers.
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.
Van Voorhis, Professor, Department of Chemistry, MIT
According to Voorhis, that focus on the surface chemistry could have enabled this research team to succeed where others had failed.
The key lies 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 stated, by utilizing a layer of a material known as hafnium oxynitride.
Measuring just a few atoms thick, or just 8 angstroms (ten-billionths of a meter), this layer served as a “nice bridge” for the excited states, stated Baldo. That eventually made it viable for the single high-energy photons to activate the release of the pair of electrons present within the silicon cell.
That led to a two-fold increase in the amount of energy generated by a specified amount of sunlight in the green and blue portion of the spectrum. On the whole, that can possibly cause an increase in the power generated by the solar cell—from a theoretical maximum of 29.1% up to a maximum of approximately 35%.
However, actual silicon cells are not yet at their maximum efficiency and so does the novel material. Hence, this needs to be further developed, but the major step of combining both the materials as efficiently as possible has now been demonstrated.
“We still need to optimize the silicon cells for this process,” Baldo added. For one thing, thanks to the latest system, those cells can be thinner compared to existing versions. More research also needs to be done with regards to stabilizing the materials for durability. On the whole, commercial applications could still be a few years off, stated the researchers.
Other methods for enhancing the efficiency of solar cells may involve adding yet another type of cell, for instance, a perovskite layer, over the silicon.
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.
Marc Baldo, Professor, Department of Electrical Engineering and Computer Science, MIT
The team has determined a unique trait of hafnium oxynitride that allows it to 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,” stated Einzinger. To date, no other material tested by the researchers can actually match its characteristics.
The study was supported as part of the MIT Center for Excitonics, funded by the U.S. Department of Energy.