News Article
Scientists discover how plastic solar panels work
Findings are of key importance for all solar conversion systems
Scientists have determined exactly how light beams excite the chemicals in solar panels to produce charge. The findings, published today in Nature Communications, are of key importance for a fundamental mechanistic understanding, with molecular detail, of all solar conversion systems, according to lead author Francoise Provencher of the University of Montreal.
The researchers at University of Montreal, the Science and Technology Facilities Council, Imperial College London and the University of Cyprus have been investigating the fundamental beginnings of the reactions that take place that underpin solar energy conversion devices, studying the new brand of photovoltaic diodes that are based on blends of polymeric semiconductors and fullerene derivatives. "In these and other 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 simple recombine and effectively nothing changes. The overall efficiency of solar devices compares how much recombines and how much separates," explained Sophia Hayes of the University of Cyprus, last author of the study.
Two major findings resulted from the team's work. "We used femtosecond stimulated Raman spectroscopy," explained Tony Parker of the Science and Technology Facilities Council's Central Laser Facility. "Femtosecond stimulated Raman spectroscopy is an advanced ultrafast laser technique that provides details on how chemical bonds change during extremely fast chemical reactions. The laser provides information on the vibration of the molecules as they interact with the pulses of laser light." Calculations on these vibrations enabled the scientists to ascertain how the molecules were evolving. 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.
"Our findings open avenues for future research into understanding the differences between material systems that actually produce efficient solar cells and systems that should 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," said the University of Montreal's Carlos Silva.
The article 'Direct observation of ultrafast long-range charge separation at polymer-fullerene heterojunctions' was published in Nature Communications on July 1, 2014.
