Researchers at the National Renewable Energy Laboratory (NREL) discovered new materials for super-efficient solar cells of the future.

The research focuses on a carefully designed molecule that can efficiently split the energy given away by one photon into two excited states. These states are separated for several microseconds, which is quite a long time on the molecular scale.

The research was published in Nature Chemistry by Nadia Korovina, Chris Chang, and Justin Johnson. The team was not only able to develop a promising energy-absorbing molecule but also to explain its function in detail.

Singlet Fission

‘Exciton’- an excited energy state- is created when a photon strikes a suitable superconductor. The exciton can even split, forming two triplet excitons. This process is called singlet fission, which loses some energy to heat.

Singlet fission can help draw more energy from each photon than a conventional solar cell. But when the two triplets come across each other, they merge and cease to exist. In an ideal scenario, a perfect organic photovoltaic molecule can ensure that singlet excitons are converted into triplets with no heat loss while keeping the triplets away from each other.

Instead of hunting for such a molecule, the researchers at NREL designed their own, as they knew which organic molecules show promise based on previous research. However, they still needed to figure out the length and the complexity of these molecules to prevent the triplets from merging.

Nadia Korvina synthesized a series of molecules of varying length, built from chains of light-absorbing molecular building blocks called chromophores.

Nadia said that after a year of trial and error, they finally had the right molecules from which they can learn the intricacies of the singlet fission process. However, the most challenging part was designing molecules with a delicate balance to achieve singlet and triplet energies.

On sorting the molecules by size, the research team realized that a chain of at least three chromophores is needed to isolate two triplet excitons successfully. A chromophore is an atom or group whose presence is responsible for the color of a compound.

To understand how the chromophores help isolate the triplets, the team created and refined a computer model. Through the model, the team discovered that a twisting motion gives the molecules the features needed to isolate the triplets.

The molecular chain is floppy and flexible when it’s not illuminated; however, as soon as it absorbs a photon, the chain twists around its central axis and stiffens initially. The twist results in a shape that facilitates the formation of two triplets.

Subsequent twisting after the completion of the initial process spatially separates the two triplets, augmenting their lifespans. According to Justin Johnson, discoveries like these are not possible without crossing disciplines, and combining expertise can yield a bigger impact.

By combining experimental and modeling approaches, the team has developed a promising energy-absorbing molecule for use in high-efficiency solar cells or other photoelectrochemical systems.

Mercom had earlier reported that researchers at the NREL recently conducted the first global assessment of various approaches used to manage solar PV modules at the end of their life spans.

It was also reported that the scientists at the NREL had developed a solar cell that has an efficiency exceeding 47%. The latest offering, which is a six-junction solar cell, now holds the highest solar conversion efficiency of 47.1% under concentrated illumination. A variant of the same cell has also set the record under one sun illumination at 39.2%.

 

Image credit: NREL