Published on February 5th, 2018 | by Tina Casey0
Scientists Drop Organic Solar Cell Fission Bomb On Coal
February 5th, 2018 by Tina Casey
It seems like only yesterday, but back in 2014 CleanTechnica took note of a mysterious solar cell something called singlet fission and now the whole thing has blown wide open. A team of scientists working through Lawrence Berkeley National Laboratory has reported that the mystery is solved, or at least part of it, leading to the potential for a new generation of super cheap, super efficient organic solar cells.
As if the US coal industry did not have enough to worry about! President* Trump campaigned on promises for coal miners, their families and their communities, but it seems that the presidential golf schedule has taken priority. After all, what’s more important?
Singlet Fission And Organic Solar Cell Performance
CleanTechnica’s 2014 piece provides this handy definition of singlet fission:
Scientists were pretty excited when they discovered you could convert light energy directly into electricity by capturing photons in semiconductors, exciting them into “excitons” (bound electron with negative charge and hole with positive), and capturing the resultant current through electrodes. Now a group of four chemists from the University of California, Riverside, has worked out a way for one photon to generate a pair of excited states rather than just one.
If you could harness the power of singlet fission, the difference in organic solar cell efficiency could be striking. Here’s a member of the Riverside team enthusing over the potential:
“If a triplet exciton has half the energy of a singlet, then it is possible for one singlet exciton, generated by one photon, to split into two triplet excitons. Thus, you could have a 200% yield of excitons—and hopefully, electrons—per absorbed photon.”
The new singlet fission breakthrough builds on this body of knowledge through a new research consortium called the Center for Computational Study of Excited-State Phenomena in Energy Materials. It was established at Berkeley Lab in 2016 and it seems the members have not let any grass grow under their feet.
The study was published last December in the journal Physical Review Letters under the title, “Origins of Singlet Fission in Solid Pentacene from an ab initio Green’s Function Approach.” Here’s a snippet from the abstract (pentacene is an organic semiconductor btw):
…For crystalline pentacene, symmetries dictate that the only purely Coulombic fission decay process from a bright singlet state requires a final state consisting of two inequivalent nearly degenerate triplets of nonzero, equal and opposite, center-of-mass momenta. For such a process, we predict a singlet lifetime of 30–70 fs, in very good agreement with experimental data, indicating that this process can dominate singlet fission in crystalline pentacene.
Got all that? Here’s C2SEPEM Director Steven G. Louie with another way to look at it:
“We actually discovered a new mechanism that allows us to try to design better materials.”
Okay then. Basically, the research team was able to describe why singlet fission takes place at such an incredibly fast rate, as in a few quadrillionths of a second. The speed is vital to organic solar cell efficiency.
Until this study, singlet fission research was focused on examining localized reactions consisting of just a few molecules. That approach has yielded some results, but the C2SEPEM was looking for a more holistic understanding. Here’s co-lead author Felipe H. da Jornada with the explainer:
“It’s like trying to explain the ocean by either looking at it molecule by molecule, or looking at a whole wave…Our approach directly captures the whole crystal.”
The team found that symmetry of structure is a key characteristic for a material to produce singlet fission efficiently. Another characteristic is the tight packing of molecules within each symmetrical element:
The efficiency of the singlet fission process appears to rely heavily on the number of molecules packed within each repeating pattern or “motif” in the crystal, and on a particular type of symmetry that in which there is a 180-degree rotation and mirroring of these motifs. This relationship between symmetry and efficiency, the researchers found, allows them to make powerful predictions on the efficiency of the overall fission.
Okay, so don’t hold your breath for that new singlet fission organic solar cell. The new study provides a pathway for improving organic solar cell efficiency, not a device ready to roll off the factory floor.
However, it’s a good start for C2SEPEM, which is after all only two years old.
Sorry, Coal: More And Better Organic Solar Cells On The Way
For those of you new to the topic, organic solar cells are generally not as efficient as their silicon cousins, but they are relatively cheap, lightweight and flexible.
Until recently, cheap natural gas has been Public Enemy #1 when it comes to coal power plant closings. Now that low cost solar and wind power are also starting to pile on, coal-dependent states like Montana are beginning to plan for a low carbon economy.
As for the golfer-in-chief, his own Energy Secretary is back on the clean energy bandwagon after making a half-hearted stab at protecting coal. For that matter, the Department of Energy’s website still features a full blown, no holds barred section on climate change.
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Image: “…an optically excited spin-singlet state (red), which features electron-hole pairs, splits into a pair of spin-triplet states (blue). The individual triplets have equal and opposite center-of-mass momenta – they behave like waves moving in opposite directions along a crystal. The gray and white spheres represent carbon and hydrogen atoms, respectively” by Florian Brown-Altvater/Berkeley Lab.
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