Electron spin. Spin, an intrinsic property of electrons, is related to the dynamics of electrons excited as a result of singlet fission – a process which could be used to extract energy in future solar cell technologies. Credit: Image provided by Leah Weiss
Physicists have successfully employed a powerful technique for studying electrons generated through singlet fission, a process which it is believed will be key to more efficient solar energy production in years to come. Their approach, reported in the journal Nature Physics, employed lasers, microwave radiation and magnetic fields to analyse the spin of excitons, which are energetically excited particles formed in molecular systems.
These are generated as a result of singlet fission, a process that researchers around the world are trying to understand fully in order to use it to better harness energy from the sun. In most existing solar cells, photons are absorbed by a semiconducting material, such as silicon. Each photon stimulates an electron in the material’s atomic structure, giving a single electron enough energy to move. This can be extracted as electrical current.
In some materials, however, the absorption of a single photon initially creates one higher-energy, excited particle, called a spin singlet exciton. This singlet can also share its energy with another molecule, forming 2 lower-energy excitons, rather than just one. These lower-energy particles are called spin “triplet” excitons. Each triplet can move through the molecular structure of the material and be used to produce charge.
The splitting process – from 1 absorbed photon to 2 energetic triplet excitons – is singlet fission. For scientists studying how to generate more solar power, it represents a potential bargain – a two-for-one offer on the amount of electrical current generated. But one challenge is that the pairs of triplet excitons only last for a tiny fraction of a second, and must be separated and used before they decay. Their lifespan is connected to their relative “spin,” which is a unique property of elementary particles and is an intrinsic angular momentum. Studying and measuring spin through time, from the initial formation of the pairs to their decay, is essential if they are to be harnessed. Electron spin resonance (ESR) spectroscopy, has been used and improved since its discovery over 50 years ago to better understand how spin impacts on many different natural phenomena.
It involves placing the material being studied within a large electromagnet, and then using laser light to excite molecules within the sample, and microwave radiation to measure how the spin changes over time. This is especially useful when studying triplet states formed by singlet fission as these are difficult to study using most other techniques. As excitons’ spin interacts with microwave radiation and magnetic fields, these interactions can be used as an additional way to understand what happens to the triplet pairs after they are formed. In short, the approach allowed the researchers to effectively watch and manipulate the spin state of triplet pairs through time, following formation by singlet fission.
They observed pairs had formed which variously had both weakly and strongly-linked spin states, reflecting the co-existence of pairs that were spatially close and further apart. Intriguingly, the group found that some pairs which they would have expected to decay very quickly, due to their close proximity, actually survived for several microseconds. “Finding those pairs in particular was completely unexpected,” Weiss added. We think that they could be protected by their overall spin state, making it harder for them to decay. Continued research will focus on making devices and examining how these states can be harnessed for use in solar cells.”
Beyond trying to improve photovoltaic technologies, the research also has implications for wider efforts to create fast and efficient electronics using spin, so-called “spintronic” devices, which similarly rely on being able to measure and control the spin properties of electrons.
https://www.cam.ac.uk/research/news/researchers-road-test-powerful-method-for-studying-singlet-fission
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