There are some things we think will never change. 1 Plus 1 equals 2. The sun rises in the east. One photon can only create one electron in a silicon solar cell. The maximum theoretical efficiency of a solar cell is 29.1%. And then one day we wake up and find everything has changed and we have to rethink everything we thought we knew.
A team of researchers at MIT and Princeton has demonstrated a way to get every high energy photon striking silicon to kick out two electrons instead of one, opening the door for a new kind of solar cell with greater efficiency than ever thought possible. That doesn’t mean the overall efficiency is double but it could raise the efficiency of future solar cells to as much as 35%. That is nearly double what many conventional solar cells are capable of today.
The result of the research by graduate student Markus Einzinger, professor of chemistry Moungi Bawendi, professor of electrical engineering and computer science Marc Baldo, and eight others at MIT and Princeton was published recently in the journal Nature. According to Science Daily, the basic concept behind this new technology has been known for decades, but actually translating it into a full, operational silicon solar cell took years of hard work, Baldo says.
The secret to the improved efficiency is a class of materials known as excitons in which “packets of energy propagate around like the electrons in a circuit,” according to Baldo. These packets have quite different properties than electrons. “You can use them to change energy — you can cut them in half, you can combine them.”
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 hard part is making it work with silicon, a material that is not excitonic. Such a 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.”
Troy Van Voorhis, a professor of chemistry at MIT who was involved in basic research in this area 6 years ago, says the key to the process is 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” by using a layer of a material called hafnium oxynitride.
“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.
The layer is only a few atoms thick but it acts 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, which produces a doubling of the amount of energy produced by a given amount of sunlight in the blue and green part of the spectrum.
“We still need to optimize the silicon cells for this process,” Baldo adds. 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.”
More efficient solar cells. What could they mean for the renewable energy revolution? Perhaps the difference between a sustainable world and one where life as we know it is extinguished by humanity’s foolish dependence on fossil fuels.