Problem:

Solar cells have come a long way, but inexpensive, thin film solar cells are still far behind more expensive, crystalline solar cells in efficiency. Theoretically, two layers would be better than one for solar-cell efficiency.

Solution:

A team led by Akhlesh Lakhtakia, Evan Pugh University Professor and Charles Godfrey Binder Professor of engineering science and mechanics at the Pennsylvania State University, has suggested that using two thin films of different materials may be the way to go to create affordable, thin film cells with about 34% efficiency.

To do that the Penn State team had to make the absorbent layer nonhomogeneous in a special way. That special way was to use two different absorbent materials in two different thin films. They chose commercially available CIGS — copper indium gallium diselenide — and CZTSSe — copper zinc tin sulfur selenide— for the layers. By itself, CIGS’s efficiency is about 20% and CZTSSe’s is about 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 one on top of the other, and they absorb different frequencies of the spectrum so they should increase efficiency
“It was amazing,” said Lakhtakia. “Together they produced a solar cell with 34% efficiency. This creates a new solar cell architecture — layer upon layer. Others who can actually make 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 options are to get the final, best answers.The National Science Foundation supported this research.

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