Two layers are better than one for efficient solar cells – Affordable thin film solar cells with 34% efficiency.

Solar cells have a history, but inexpensive thin-film solar cells are still far behind crystalline solar cells and more expensive in terms of efficiency.

Now, a team of researchers suggests that using two thin layers of different materials may be the way to go to create affordable thin-film cells with around 34% efficiency.

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

Akhlesh Lakhtakia, professor at Evan Pugh University.

The researchers approached solar cells from two fronts, the optical side – looking at how sunlight is collected – and the electrical side – looking at how collected sunlight is converted into electricity.

Optics researchers strive to optimize light capture, while electrical researchers strive to optimize conversion to electricity, simplifying both sides of the other.

I decided to create a model in which both aspects, electrical and optical, are treated the same. We had to increase the actual efficiency, because if the efficiency of a cell is less than 30%, it will not be taken into account.

Akhlesh Lakhtakia.

Lakhtakia is a theorist. He doesn’t do thin films in the lab, but rather creates mathematical models to test the possibilities of configurations and materials so that others can test the results. The problem, he said, is that the mathematical structure of optical and electrical optimization is very different.

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

I realized that in order to increase efficiency, we had to absorb more light. For this, we had to make the absorbent layer non-homogeneous in a particular way.

Akhlesh Lakhtakia.

This special shape was use two different absorbent materials on two different thin films. The researchers chose CIGS – copper, indium, gallium, diselenide – and CZTSSe – copper, zinc, tin, sulfur, selenide – for the layers. The efficiency of CIGS alone is about 20% and that of CZTSSe about 11%.

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

It was unbelievable. Together, they formed a 34% efficient solar cell. This creates a new architecture of solar cells layer by layer. Others who can actually make solar cells can find other diaper formulations and maybe do better.

Akhlesh Lakhtakia.

According to the researchers, the next step is to create them experimentally and see what the options are for improving them.

More information: