Catching some rays: Organic solar cells make a leap forward

Jun 14, 2012 By Jared Sagoff
Catching some rays: Organic solar cells make a leap forward
Improvements in the efficiencies of organic solar cells will eventually make them competitive with traditional silicon-based solar cells and, hopefully, ultimately with fossil fuels.

(Phys.org) -- Drawn together by the force of nature, but pulled apart by the force of man – it sounds like the setting for a love story, but it is also a basic description of how scientists have begun to make more efficient organic solar cells.

At the atomic level, function like the feuding families in Romeo and Juliet. There’s a strong natural attraction between the positive and negative charges that a photon generates after it strikes the cell, but in order to capture the energy, these charges need to be kept separate.

When these charges are still bound together, they are known to scientists as an exciton. “The real question that this work tries to answer is how to design a material that will make splitting the exciton require less energy,” said senior chemist Lin Chen of the U.S. Department of Energy’s (DOE) Argonne National Laboratory.

Excitons can be thought of as a sort of “quasiparticle,” Chen said, because they exhibit certain unique behaviors. When the two charged regions of the exciton – the electron and a region known as a “hole” – are close together, they are difficult to pry apart.

“The closer the hole and the electron regions are inside an exciton, the more likely they are to recombine without generating electricity,” Chen said.

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When energy is added to the system, however, the charges begin to separate, rendering the electrons and holes completely free and eventually allowing for the possibility of generating current and extracting electricity

“The closer the hole and the electron regions are inside an exciton, the more likely they are to recombine without generating electricity,” Chen said. “But if they are already ‘pre-separated,’ or polarized, the more likely they are to escape from this potential trap and become effective charge carriers.”

In the new experiment, Chen and her colleagues examined how four different molecules in the polymer layer in the middle of a solar cell generated different exciton dynamics. They discovered that more heavily polarized excitons yielded more efficient polymer-based

“If the conventional exciton, right after it is generated, contains the hole and electron in almost the same location, these new materials are generating an exciton that is much more polarized at the beginning,” Chen said. Currently, the collaborative team is exploring new materials for high-efficiency organic solar cells based on these findings.

Organic solar cells still have a ways to go to get close to the efficiency of their inorganic, silicon-based competitors, but they remain much more attractive from a cost perspective. Further research into the electronic dynamics of organic photovoltaics is essential to improving their efficiency and thus making solar power cost-competitive with conventional energy sources, Chen said.

The work has been recently published in the Journal of the American Chemical Society.

Chen’s work on organic solar cells represents one of several avenues of solar energy research currently underway as part of the Argonne-Northwestern Solar Energy Research Center (ANSER), a collaborative enterprise between Argonne and Northwestern University that seeks to investigate a number of possible improvements to the current generation of photovoltaic devices. ANSER is one of 46 Energy Frontier Research Centers established in 2009 by DOE’s Office of Science at universities, national laboratories, and other institutions across the nation to advance basic research on energy.

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User comments : 8

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Birger
not rated yet Jun 14, 2012
While there is still a long way to go to competitive photovoltaic solar cells, this is certainly exciting news.
We need a plethora of good technological solutions to carbon-free power generation/storage, so they can complement each other -each technology has its own set of shortcomings.
dschlink
not rated yet Jun 14, 2012
Within broad limits, the cost per installed watt is much more important than the efficiency. In small systems, the installation cost far outweigh the cost of the cells. More focus is needed on reducing the man-hours required to design, install, and connect the systems.
Eikka
not rated yet Jun 15, 2012
More focus is needed on reducing the man-hours required to design, install, and connect the systems.


A simple solution would be to mass-manufacture, but because the whole field is still moving you will find that a large factory to output cheap pre-fab panel kits will be obsolete in a year.

Furthermore, while putting small solar panels on private roofs may seem like a good idea, it has no economic sustainability because all the solar panels tend to produce power at the same time, so as you scale up your overall installed power, the system price for selling the power falls down due to oversupply.

Unless you buy batteries to go along with the panels, you're ultimately limited to how much power you can use by yourself, because selling the power will eventually be impossible. That translates to very small benefits from solar panels, because they don't produce power all the time.

For private roofs, heat collectors would be more economical and would save more energy.
SatanLover
5 / 5 (1) Jun 15, 2012
Furthermore, while putting small solar panels on private roofs may seem like a good idea, it has no economic sustainability because all the solar panels tend to produce power at the same time, so as you scale up your overall installed power, the system price for selling the power falls down due to oversupply.


Patently false, in many western countries or states the sun doesnt shine everywhere. Also oversupply is no problem if ppl charge their cars during that time.
Eikka
not rated yet Jun 15, 2012
Of course, if you live in the south, much of your power will be consumed in air conditioning, which opens the possibility for cold storage using the solar power peak to freeze water. That way you can install more panels and use the excess later.

A cubic meter of ice in the basement will store on the order of 100 kWh of energy for cooling purposes. For comparison, a well insulated fridge would keep cool for three months with that amount of energy.

Eikka
not rated yet Jun 15, 2012
Patently false, in many western countries or states the sun doesnt shine everywhere. Also oversupply is no problem if ppl charge their cars during that time.


A timezone is sufficiently wide that it creates an oversupply problem if everyone has solar panels. The US for example is about three hours wide, which means that when the center part is at 100% the east and west coast will be at 70-80% production. Save for random cloud cover, this is effectively everyone producing at the same time.

The peak to average solar power ratio during the year is roughly 10:1 over much of the earth, which means that in order to produce more than 10% of the total energy with solar power, you have to have massive oversupply that is stored somewhere and used later.

Electric car batteries are one option, but they are ultimately not enough. Out of the total energy used, transportation on petrol represents about 27% of all energy consumed, and with electricity that figure would be three times less.
TkClick
not rated yet Jun 15, 2012
Organic solar cells still have a ways to go to get close to the efficiency
The low efficiency in not the only problem. The instability of organic compounds at sunlight makes all organic solar cells disadvantageous, where the price of installation is the main component of the overall TCO.
Eikka
not rated yet Jun 17, 2012
transportation on petrol represents about 27% of all energy consumed


That should read petroleum, since it's counting diesel and light oil as well.