Gone too soon: Solid state upconversion is a race against time

Achieving photochemical upconversion could dramatically boost solar energy efficiency, but practical implementation of the technique will require a molecule that remains excited for long enough to get the job done.

Photochemical upconversion involves gluing two low-energy photons of light together to create more energetic, visible light, which can be captured by solar cells or harnessed for other practical purposes. The technical term for the gluing process is ‘triplet-triplet annihilation’ and produces a ‘singlet exciton’. In theory, controlled and reliable triplet-triplet annihilation and the photochemical upconversion it enables could raise the efficiency limit of solar energy devices from 33.7% to 40% or beyond.

In 2020, researchers at the ARC Centre of Excellence in Exciton Science demonstrated that a certain type of dye, a molecule called violanthrone-79, can perform upconversion from below the band gap of silicon. The band gap is the minimum energy that is required to excite an electron in silicon up to a state where it can participate in energy conduction. However, those tests were performed in liquid. For the mechanism to be useful in real-world applications, it must be effectively demonstrated in a solid state.

Corresponding author Professor Timothy Schmidt of UNSW Sydney said: “We want to make it work in the solid state. There’s all sorts of advantages because it’s more compatible with real devices and technology. That’s where we want to go. And it’s always a criticism of the solution phase that this is not device relevant.”

Tim and his collaborators have now explored how violanthrone performs when fixed in a lattice, mimicking the type of solid-state behavior likely to occur in a solar cell. They found that the useful behavior and characteristics violanthrone demonstrated in solution do not effectively transfer to a solid-state context.

A major issue is that while low energy photons of light are able to be glued together, in the solid state they are very likely to undergo ‘singlet fission’, degrading and falling apart too quickly for the higher energy state to be useful. This is because, within the fixed lattice, many other violanthrone molecules, which have not undergone triplet-triplet annihilation, remain in close proximity to the singlet exciton, and are ready to interact with and downgrade it back to separate low-energy excitons.

“Singlet fission occurs in less than a nanosecond,” Tim said.

“So it’s very hard to get that excited state to emit light. If the excited state is too short lived, you can’t really use it.”

Overcoming this obstacle will not be easy. For a molecule to be capable of undergoing triple-triplet annihilation, it will also be inherently able to experience singlet fission.

Tim added: “We’ve really got to tune the interaction so that triplet-triplet annihilation can still occur fast enough, while slowing down the reverse process so that it won’t compete with singlet fission.”

Like Tim, first author Dr Elham Gholizadeh knows significant challenges remain to build on the existing work, but she is confident further progress can made.

“The final goal (of upconversion) would be to apply it in real-world solar cells,” Elham said.

“So, it must be done in a solid phase. It seems a bit tough at this point, but I’m very optimistic regarding finding solutions for these obstacles. I think an alternative to violanthrone-79 can assist us in reaching our final goal.”

The results of the research have been published in The Journal of Physical Chemistry C and are available here: https://chemrxiv.org/engage/chemrxiv/article-details/60c756d6bdbb892ab6a3aafc