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How Do ‘Plastic’ Solar Cells Work? New Research Provides Insight

The exact mechanics of how ‘plastic’ solar cells actually work has remained somewhat of a mystery since their creation, despite their use to date. That all appears to finally be changing though, as new multi-university research has begun to shed light on the processes involved — the means by which light beams excite the chemicals in solar panels and lead to the production of a charge.

With the improved understanding, it should become possible to improve the cost efficiency of the technology, or that’s the aim behind the work anyways. The new work was headed by researchers at the University of Montreal, the Science and Technology Facilities Council, Imperial College London, and the University of Cyprus.

Three laser beams are needed to record the excited vibrational modes of PCDTBT with the method called femtosecond stimulated Raman spectroscopy. First, the green pulse is absorbed by the polymer, just as sunlight would be in a solar cell, which creates the excited state. Then, a pair of infra-red and white pulses probe this excited vibrational mode. Very short pulses of light and precise timing enable an impressive time resolution of less than 300 femtoseconds. Image Credit: University of Montreal

Three laser beams are needed to record the excited vibrational modes of PCDTBT with the method called femtosecond stimulated Raman spectroscopy. First, the green pulse is absorbed by the polymer, just as sunlight would be in a solar cell, which creates the excited state. Then, a pair of infra-red and white pulses probe this excited vibrational mode. Very short pulses of light and precise timing enable an impressive time resolution of less than 300 femtoseconds. Image Credit: University of Montreal

“Our findings are of key importance for a fundamental mechanistic understanding, with molecular detail, of all solar conversion systems — we have made great progress towards reaching a ‘holy grail’ that has been actively sought for several decades,” stated the study’s first author, Françoise Provencher of the University of Montreal.

The press release from the University of Montreal provide a bit of background:

The researchers 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. Polymers are large molecules made up of many smaller molecules of the same kind — consisting of so-called ‘organic’ building blocks because they are composed of atoms that also compose molecules for life (carbon, nitrogen, sulphur). A fullerene is a molecule in the shape of a football, made of carbon.

“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 study author Sophia Hayes of the University of Cyprus.

“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 allowed the researchers “to ascertain how the molecules were evolving.”

There are two major findings of the new work.

  • 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 (0.0000000000003 s). A femtosecond is a quadrillionth of a second — a femtosecond is to a second as a second is to 3.7 million years. 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,” stated the University of Montreal’s Carlos Silva, who was senior author of the study.

The new findings were just published in the journal Nature Communications.

Speaking of cutting-edge solar power, India recently announced its plans to build the largest floating solar power plant in the world. The 50 MW solar PV project will be built over bodies of water in the southern state of Kerala by India’s leading hydro power generator National Hydro Power Corporation.

The project is expected to cost around $64-72 million. The floating solar power plant technology concept being used was developed last year by a team led by SP Gon Choudhury, Chairman of the Renewable Energy College.

 

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Written By

James Ayre's background is predominantly in geopolitics and history, but he has an obsessive interest in pretty much everything. After an early life spent in the Imperial Free City of Dortmund, James followed the river Ruhr to Cofbuokheim, where he attended the University of Astnide. And where he also briefly considered entering the coal mining business. He currently writes for a living, on a broad variety of subjects, ranging from science, to politics, to military history, to renewable energy.

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