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

Schematic of a dual thin film layered solar cell. The sun enters the top and reaches the CIGS and CZTSSe layers that absorb light and form positive and negative particles that travel to the top and bottom contact layers, producing electricity. Sincerely: Akhilesh Lakhtakia, Penn State

Solar cells have come a long way, but inexpensive, thin film solar cells still lag far behind more expensive, crystalline solar cells in efficiency. Now, a team of researchers has suggested that using two thin films of different materials may be the way to make inexpensive, thin film cells with about 34% efficiency.

“Ten years ago I knew little about solar cells, but it became clear to me that they were very important,” Akhilesh Lakhtakia, Ivan Pugh University professor and Charles Godfrey Binder Professor of Engineering Science and Mechanics, Penn State he said.

Investigating the field, they found that the researchers approached the solar cells from two sides, the optical side – how sunlight is collected – and the electrical side – seeing how converting collected sunlight into electricity goes. Optical researchers try to optimize light capture, while electrical researchers try to optimize conversion to electricity, with both sides simplifying the other.

“I have decided to build a model in which both electrical and optical aspects will be treated equally,” Lakhtakia said. “We need to increase the actual efficiency, because if the efficiency of a cell is less than 30% it is going to make no difference.” Researchers report their results in a recent issue Applied Physics Letters.

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

Solar cells appear to be simple devices, he explained. A clear top layer allows sunlight to fall on the energy conversion layer. The material chosen to convert energy absorbs light and creates currents of negatively charged electrons and positively charged pores move in opposite directions. The differentially charged particles migrate to a top contact layer and a bottom contact layer that eject electricity from the cell for use. The amount of energy a cell can produce depends on the amount of sunlight and the capacity of the conversion layer. Different materials react and convert to different wavelengths of light.

“I realized that to increase efficiency we had to absorb more light,” Lakhtakia said. “To do this we have to make the absorbent layer nonhomogeneous in a particular way.”

That particular method was to use two different absorbent materials in two different thin films. The researchers chose commercially available CIGS-copper indium gallium deslenide- and CZTSSe-copper zinc tin sulfur selenide- for the layers. By itself, the efficiency of CIGS is around 20% and that of CZTSSe is around 11%.

These two materials work in the solar cell because the structure of both materials is the same. They have almost the same lattice structure, so they can be grown one above the other, and they absorb different frequencies of the spectrum, so they should increase efficiency, according to Lakhtakiya.

“It was amazing,” Lakhtakia said. “Together they produced a solar cell with 34% efficiency. It creates a new solar cell architecture – layer by layer. Others that can actually make solar cells are discover other aggregates of layers and perhaps better. can do.”

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

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more information:
Faiz Ahmed et al, Double-absorber thin film solar cell with 34% efficiency, Applied Physics Letters (2020). DOI: 10.1063 / 5.0017916

Provided by Pennsylvania State University

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