Singlet Fission for Better Solar Cells

Spin of electrons is related to the dynamics of electrons excited as a result of singlet fission. This process could be used to extract energy in future solar cell technologies. (Source: L. Weiss, Cambridge)

Spin of electrons is related to the dynamics of electrons excited as a result of singlet fission. This process could be used to extract energy in future solar cell technologies. (Source: L. Weiss, Cambridge)

Physicists from HZB, Freie Uni­versität Berlin and Univer­sity of Cambrigde 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 employed lasers, microwave radiation and magnetic fields to analyze the spin of excitons, which are energe­tically excited particles formed in molecular systems. Their expe­riments have been performed at the Joint EPR Lab, which is jointly funded by HZB and FU Berlin.

Using materials exhi­biting singlet fission in solar cells could make energy production much more efficient in the future, but the process needs to be fully under­stood in order to optimize the relevant materials and design appro­priate techno­logies to exploit it. 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 then poten­tially be extracted as electrical current. In some materials, however, the absorption of a single photon initially creates one higher-energy spin singlet exciton. This singlet can also share its energy with another molecule, forming two lower-energy excitons, rather than just one. These spin triplet excitons can move through the molecular structure of the material and be used to produce charge.

The splitting process 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, relative to the amount of light put in. If materials capable of singlet fission can be integrated into solar cells, it will become possible to generate energy more effi­ciently from sunlight. But achieving this is far from straight­forward. 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. Studying and measuring spin through time, from the initial formation of the pairs to their decay, is essential if they are to be harnessed.

The researchers utilised a method that allows the spin pro­perties of materials to be measured through time. The 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.

Because the 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. The approach allowed the researchers to effec­tively watch and mani­pulate the spin state of triplet pairs through time, following formation by singlet fission. Leah Weiss, a Gates-Cambridge Scholar at Trinity College, Cambridge, says: “This research has opened up many new questions. What makes these excited states either separate and become inde­pendent, or stay together as a pair, are questions that we need to answer before we can make use of them.”

The researchers were able to look at the spin states of the triplet excitons in consi­derable detail. 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 spin­tronic devices, which similarly rely on being able to measure and control the spin properties of electrons. (Source: HZB)

Reference: L. R. Weiss et al.: Strongly exchange-coupled triplet pairs in an organic semiconductor, Nat. Phys., online 17. October 2016; DOI: 10.1038/nphys3908

Link: Institute Silicon Photovoltaics, Helmholtz-Zentrum Berlin

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