Gold quantum dots and metallic gratings work together to boost efficiency of inverted organic solar cells

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Recent microscopic advances could make solar panels more efficient. (AP Photo/Steve Helber)

Adding two microscopic features to inverted organic solar cells enhances their ability to absorb light and convert it into electricity by up to 16%, according to a recent study demonstrating how quantum dots can be used to harvest in-demand solar energy.

The researchers tested designs that included gold particles less than 2 nanometers in diameter called quantum dots and a silver grating nanostructure, and saw that these features were most effective when paired. The team reported the findings March 24 in Royal Society Open Science.

The Biden administration is working toward a goal of transitioning the United States away from fossil fuels by 2035. Lowering the cost of solar energy from 4.6 to 2 cents per kilowatt-hour by 2030 is a crucial part of this plan, the Department of Energy has said. 

Most photovoltaic panels on the market today are made of silicon and convert around 18% to 22% of the sunlight that hits them into electricity. Organic photovoltaic cells made of carbon-containing materials aren't as efficient as traditional silicon solar cells, but are cheaper to produce, lighter and more flexible, and can be made transparent. 

Researchers have recently become intrigued by the possibility of using gold quantum dots — which are just five to 25 atoms in size — to improve organic solar cells. Solar cells mostly convert visible light into electrical energy. Gold quantum dots, however, can absorb ultraviolet light and convert it into visible light. 

This means that the solar cell can harvest additional light from the fluorescence of the gold quantum dots, says Akira Baba, a professor of engineering at Niigata University in Japan and last author of the new study. 

Another way to improve the efficiency of organic solar cells is to add a metallic grating to their surface, he said. This nanostructure scatters incoming sunlight and intensifies its electric field, which allows the solar cell to absorb more energy.

Baba and his team have previously reported that adding both gold quantum dots and a metallic grating can make conventional organic solar cells about 19% more efficient. For the new study, the team examined inverted organic solar cells, which are designed such that the electric current flows through the bottom of the cell.

"This is [the] opposite way to the conventional organic solar cells," Baba said.

According to the study, inverted organic solar cells "have received significant attention because of their superior device performance and better air stability" compared with conventional ones. The inverted design shields the easily corroded metals of the bottom electrode from the open air, making the device more stable and improving its performance. 

Baba and his team tested out three types of gold quantum dots on inverted organic solar cells. The differing sizes of the dots caused them to emit green, red or blue fluorescence, respectively. Compared with unaltered cells, the solar cells with dots that emitted green light were 10.77% more efficient, while the cells with red and blue fluorescence improved by 5.23% and 3.41%, respectively. The first group of dots was most effective because the solar cells absorb green light most efficiently, Baba said.

Then, when the researchers added both green fluorescent quantum dots and the grating to the solar cells, the efficiency improved by 16% relative to solar cells that didn't have either feature. This meant that the overall efficiency of the cells at converting sunlight into electricity increased from 3.25% to 3.77%.

This overall improvement was slightly weaker than the efficiency boost the team saw in its previous work on conventional organic solar cells. That's because the inverted setup required the researchers to place the grating farther from the layer of material that converted light into electrical energy, making it less effective. 

On the other hand, the inverted design also allowed the researchers to place the quantum dots closer to the metallic grating than was possible in their earlier experiments with conventional organic solar cells. As a result of its interaction with the grating, the light that reached the gold quantum dots was more intense, which in turn boosted the fluorescence from the dots. 

"Our strategy was thus identified as a promising approach for enhancing the performance" of inverted organic solar cells, Baba and his colleagues wrote in the study.

The technique could also be used to improve other kinds of electronic devices that detect or use light, they added.

"We are aiming to use this system not only for organic solar cells, but also in the application of sensors and photodetectors," Baba said. 

Wearable sensors, which collect physiological and movement data on the people who wear them, would be one potential application for the research, he said.

The study, "The effect of gold quantum dots/grating-coupled surface plasmons in inverted organic solar cells," published March 24 in Royal Society Open Science, was authored by Chutiparn Lertvachirapaiboon, Kazunari Shinbo, Keizo Kato and Akira Baba, Niigata University; Ryousuke Ishikawa, Niigata University and Tokyo City University; Kulrisa Kuntamung and Patrawadee Yaiwong, Niigata University and Chiang Mai University; and Kontad Ounnunkad, Chiang Mai University.

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