The slow-moving organic solar cell industry finally achieved a breakthrough in its quest to optimize energy conversion, and thanks to an accidental discovery during experimentation, this breakthrough came by a process of coaxing electrons with layers of fullerene molecules—popularly known as “buckyballs.” University of Michigan scientists made the discovery when experimenting with organic solar cell architecture. The researchers added two layers of fullerene molecules on top of an organic cell's energy-producing layer where photons hit solar cells to dislodge electrons.
What they found was that electrons move more freely and go farther in the fullerene layer and that an “energy well” (technically known as a potential well) was also created where electrons could not escape. The result was that these electrons—when coaxed with the layers of fullerene molecules—could travel up to several centimeters (compared to nanometers), which enabled them to produce a greater amount of electrical current.
Organic cells are known to have a weak electronic conductivity because they have loose bonds between individual molecules. Instead of having an efficient conduit between molecules, electrons often get trapped and can only travel up to a couple hundred nanometers at most. In an organic solar cell, this trapping of the electrons is the main obstacle that limits the distance that these electrons can travel. If they are free to move through the structure without impedance, they can travel much further. This is the same for all solar cells, but the organic network presents a much harder challenge for these electrons to travel through. Because the electrons don’t travel far enough before their entrapment (where they cannot move), they are unable to participate in the electrical circuit. This block in the electrons’ participation lowers the conductivity of the cell, and in turn, the conversion efficiency decreases as fewer free-flowing electrons can circulate. As a result, organic solar cells, which consist of non-metallic semiconductors such as polymers, only offer efficiencies of up to 13.1 percent. This level of efficiency doesn’t compete well with silicon-based inorganic solar cells that offer a 26.6 percent power efficiency and are in wide use in solar panels today.
Several positive characteristics of organic solar cells, however, highlight the need for further research into extending their efficiency. For example, besides the potential of lower costs due to simpler polymer processing technologies, organic solar cells are also thinner, more flexible, and transparent. These characteristics are crucial for efficiently converting sunlight into electricity. Also, where projects aim to build net-zero energy buildings (NZEBs) or retrofit existing structures to become more energy efficient, companies can integrate organic solar cells into the structure itself, as on rooftops and walls, where the heavier, inflexible, silicon-based inorganic solar cells are not practical or feasible. These organic solar cells are also beneficial in that they come in a variety of colors and configurations, offering better aesthetics as well.
The need to discover ways to bring organic solar cells to their full potential is clear, and this recent breakthrough may do just that. According to the University of Michigan’s article entitled “Semiconductor Breakthrough May Be a Game-Changer for Organic Solar Cells,” its researchers started with the organic cell’s power-producing layer, where photons hit solar cells to dislodge electrons. “Using a common technique called vacuum thermal evaporation, they layered in a thin film of C60 fullerenes—each made of 60 carbon atoms.” What they found was that the electrons moved freely in the fullerene layer, rather than being trapped in the loose bonds between organic molecules.
Interestingly, fullerenes are known to be excellent acceptor molecules because of their variable hybridization states, ability to rehybridize, and curved topology. (However, it is significant to note that since this discovery of use for fullerenes in solar cells, a new highly-efficient class has emerged that is now known as the non-fullerene acceptor (NFA) organic solar cells, which has similar electron accepting properties as fullerenes—but is obviously non-fullerene molecules.) Fullerenes are also electronically confined materials, and they contain potential (i.e., quantum) wells. This means that it is hard to remove an electron once it has entered the potential well of a fullerene molecule. The use of an electron blocking layer that sandwiches the fullerene layer to prevent any electrons from leaving and recombining with the holes creates an additional barricade.
The only way that an electron can influence the realm outside of the potential well is through electron tunneling. However, when you place quantum wells side by side, that is, in a way that the fullerenes molecules can be placed next to each other in a layer, they can form what is known as a “superlattice.” If the distances between quantum wells are less than the reach of an electron’s tunneling wavefunction, the electronic wavelengths can overlap and create a connection between the potential wells, which enables the electrons (and a current) to flow. So, by trapping electrons within the fullerene layers, the close proximity of the potential wells from molecule to molecule enables the electrons to flow unimpeded with no risk of entanglement.
Again, because they are freely moving and cannot recombine with the holes in the energy-producing layer, electrons can travel farther—up to several centimeters, rather than mere nanometers—which enables them to produce a greater amount of electrical current. This is a result of, as mentioned, a greater current that is now possible, not because a single electron carries more energy but because there are more current (i.e., charge) carriers flowing around the electrical circuit. Ultimately, these increases in specific current (and efficiency) within organic solar cells is dependent on how many electrons were flowing around the system before the fullerenes were added compared to after.
The researchers at the University of Michigan acknowledge that this discovery is only a start and that there’s more work to be done to improve solar cell design, in particular in researching what else makes good electron conductors in organic materials. Stephen Forrest, professor of engineering at the University of Michigan, anticipates that it could take as many as 10 years for a major organic solar cell solution to emerge.
This fullerene discovery, though, paves the way for organic materials to be useful to create transparent solar cells that are very efficient over longer distances. Solar cell manufacturers can shrink a solar cell's conductive electrode into an invisible grid, for example, and combined with the organic solar cells’ other characteristics, solar cells can form laminations over any surface without obstruction. With lower polymer processing costs associated with organic solar cells, these solutions can be reasonably inexpensive for a wide range of applications. Perhaps the biggest breakthrough in this discovery involving organic solar cells is that more discoveries leading to more advances are on the horizon.
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