An international group of collaborators have used the femtosecond Raman spectroscopy lasers available on the CLF's Ultra system to reveal chemistry at work in plastic solar panels. The team, led by Prof. Sophia Hayes of the University of Cyprus, was a collaboration between University of Cyprus, the University of Montreal (Francoise Provencher, Carlos Silva, Nicola Berube and Michael Cote), Imperial College, London (Christoph Hellman and Natalie Stingelin) and the CLF (Tony Parker, Greg Greetham and Mike Towrie).
The research, published in Nature Communications (link opens in a new window)
, focused on the fundamental beginnings of the reactions that underpin solar energy conversion devices, studying the new brand of photovoltaic diodes that are based on blends of polymeric semiconductors and fullerene derivatives. A better understanding of how 'plastic' solar panels work is required to improve and make more cost efficient devices that can be more widely used for our ever increasing needs for energy.
In these devices, the absorption of light fuels the formation of an electron and a positive charged species. To ultimately provide electricity, these two attractive species must separate and the electron must move away. If the electron is not able to move away fast enough then the positive and negative charges simply recombine and effectively nothing changes. The overall efficiency of solar devices compares how much recombines and how much separates.
The team used the femtosecond stimulated raman spectroscopy (FSRS) technique available on the Ultra system to reveal what happens within the solar panels under photo-stimulated chemistry. The technique gives details on how chemical bonds change during extremely fast chemical reactions.
FSRS requires 3 laser beams to record the structural changes occurring when the plastic is excited. First, a green pulse activates the polymer to create an electronic excited state. Then, a pair of near infra-red and white light continuum pulses are used to generate the Raman spectrum that records vibrational modes of the excited molecule. The ultrashort lasers pulses enable a time resolution of less than 300 femtoseconds. Credit: University of Montreal.
Firstly, they found that after the electron moves away from the positive centre, the rapid molecular rearrangement must be prompt and resemble the final products within around 300 femtoseconds. This promptness and speed enhances and helps maintain charge separation.
Secondly, the researchers noted that any ongoing relaxation and molecular reorganisation processes following this initial charge separation, as visualised using the FSRS method, should be extremely small.
These findings open avenues for future research into understanding the differences between material systems that actually produce efficient solar cells and systems that should be as efficient but in fact do not perform as well. A greater understanding of what works and what doesn't will obviously enable better solar panels to be designed in the future.