Enabling the next generation of organic solar cells

Most of today's solar cells are made from Silicon and are heavy, rigid, and expensive to produce. By contrast, organic solar cells -- which are made from materials and elements found in plants and animals -- hold the promise of being lightweight, flexible, and cheap to make. However, organic solar cells have not yet reached the sunlight-to-electricity efficiencies of their Silicon-based counterparts, preventing their commercialisation

Now, researchers from the University of Cambridge, in a global collaboration with experts from Canada, Belgium, New Zealand, and China, have discovered a new fundamental way for energy to move in organic materials at a speed up to 1000's of times faster than normal, getting steps closer to fully realise the promise of organic photovoltaics. Their findings are reported in the journal Science Advances.

This new movement mechanism, coined "transient exciton delocalization" allows energy to move and transfer to the surrounding electrical wires incredibly much faster than normal.

"This improvement is made possible by the quantum-mechanical nature of reality, where energy can exist in many places at once, simultaneously," said first author Alexander Sneyd, a PhD student at Cambridge's Cavendish Laboratory. "By taking advantage of these quantum-mechanical elements which allow for highly-efficient energy movement, we can make better, more efficient solar cells."

The research team began by using a highly advanced nanotechnology technique called 'living crystallization driven self-assembly' to create nanofibers made from a Sulphur and Carbon-based polymer. This allowed them to precisely control the position of each of the atoms in the organic nanofiber to create a 'perfect' model material. "This was really the secret to the success," said Dr. Akshay Rao of the Cavendish Laboratory who led the research. "We were able to attain an unprecedented level of structural control, which one could only dream of until very recently."

The team then shone a laser at the nanofibers to mimic sunlight, and watched the energy move over time using a technique called transient-absorption microscopy to create 'films' of the energy transport. This allowed them to observe energy movement at extremely short timescales, with a resolution of almost a single femtosecond, or 0.000000000000001 of a second, which is equivalent to a film with a frame rate of 1 million billion frames per second. "When we performed the experiments, we were very surprised," explained Sneyd. "The energy was moving at speeds of 100's or even 1000's of times faster than what was typically observed in the scientific literature."

Finally, they used a supercomputer to simulate at the quantum level what was occurring physically in the nanofibers. By comparing the results of simulation with the experiment they concluded that it was indeed the ability for the energy to 'delocalize', or be in many places at the same time, that was primarily responsible for the unexpected behaviour.

"This new mechanism provides many opportunities to significantly improve the performance of traditional organic solar cells," said Professor Sir Richard Friend of the Cavendish Laboratory, who co-led the study. "But even more excitingly, it's also opening up prospects of whole new types of devices based on inexpensive and adaptable organic materials."