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Theoretically, two layers are better than one for solar cell efficiency



Theoretically, two layers are better than one for solar cell efficiency

Schematic overview of a double thin film layered solar cell. The sun enters the top and reaches the CIGS and CZTSSe layers, which absorb the light and create positive and negative particles that move to the top and bottom contact layers and produce electricity. Credit: Akhlesh Lakhtakia, Penn State

Solar cells have come a long way, but cheap, thin film solar cells are still far behind more expensive, crystalline solar cells in efficiency. Now a team of researchers suggests that the use of two thin films of different materials may be the way to go to create affordable, thin film cells with approx. 34% efficiency.


“Ten years ago, I knew very little about solar cells, but it became clear to me that they were very important,”

; said Akhlesh Lakhtakia, a professor at Evan Pugh University, and Charles Godfrey Binder, a professor of engineering and mechanics, Penn State.

He studied the field and found that scientists were approaching solar cells from two sides, the optical side – looking at how the sun’s light is collected – and the electric side – looking at how the collected sunlight is converted into electricity. Optical scientists strive to optimize light capture, while electrical scientists strive to optimize conversion to electricity, both sides simplify the other.

“I decided to create a model where both electrical and optical aspects would be treated equally,” Lakhtakia said. “We needed to increase the actual efficiency, because if the efficiency of a cell is less than 30%, it will not make a difference.” The researchers report their findings in a recent issue of Physics letters used.

Lakhtakia is a theorist. He does not make thin films in a laboratory, but creates mathematical models to test the possibilities of configurations and materials so that others can test the results. The problem, he said, was that the mathematical structure for optimizing the optical and the electrical is very different.

Solar cells appear to be simple entities, he explained. A clear top layer allows sunlight to fall on an energy conversion layer. The material chosen to convert the energy, absorbs the light and produces currents of negatively charged electrons and positively charged holes that move in opposite directions. The differently charged particles are transferred to a top contact layer and a bottom contact layer that channels the electricity out of the cell for use. The amount of energy that a cell can produce depends on the amount of sunlight collected and the ability of the conversion layer. Different materials respond to and convert different light wavelengths.

“I realized that in order to increase efficiency, we needed to absorb more light,” Lakhtakia said. “To do that, we had to make the absorbent layer non-homogeneous in a special way.”

The special way was to use two different absorbent materials in two different thin films. The researchers selected commercially available CIGS – copper indium gallium dieselene – and CZTSSe – copper zinc zinc sulfuride – for the layers. In itself, CIGS’s efficiency is approx. 20%, and CZTSSe is approx. 11%.

These two materials work in a solar cell because the structure of both materials is the same. They have roughly the same lattice structure, so they can be grown on top of each other, and they absorb different frequencies in the spectrum, so they should increase efficiency according to Lakhtakia.

“It was amazing,” Lakhtakia said. “Together, they produced a solar cell with 34% efficiency. This creates a new solar cell architecture – layer upon layer. Others who can actually manufacture solar cells can find other formulations of layers and perhaps do better.”

According to the researchers, the next step is to create these experimentally and see what the possibilities are for getting the final, best answers.


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More information:
Faiz Ahmad et al, double-absorbing thin-film solar cell with 34% efficiency, Physics letters used (2020). DOI: 10.1063 / 5.0017916

Provided by Pennsylvania State University

Citation: Theoretically, two layers are better than one for solar cell efficiency (2020, 15 September) retrieved 15 September 2020 from https://phys.org/news/2020-09-theoretically-layers-solar-cell-efficiency.html

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