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Efficiency Battle For CIGS Solar Cells Hots Up

Incorporating miniscule amounts of sodium and potassium into the copper indium gallium diselenide layer can alter its electronic properties and improve efficiency
Empa scientists have developed a new technique for manufacturing high-efficiency, flexible, thin film solar cells from CIGS semiconductors.

This has enabled them to achieve an efficiency of 20.4 percent for the conversion of sunlight into electrical energy. As the solar cells are deposited onto plastic foils, they could be produced on an industrial scale using cost-effective roll-to-roll manufacturing.

New CIGS thin film solar cell courtesy of Empa

For many years scientists and engineers have been trying to provide low-cost solar energy by developing a cheap solar cell that is both highly efficient and at the same time simple to build, enabling it to be mass produced.

Now, the team led by Empa researcher Ayodhya N. Tiwari has made a major leap forward in this respect.

The researchers have developed a new manufacturing technique for CIGS solar cells, in which tiny quantities of sodium and potassium are incorporated into the CIGS layer.

The post-deposition treatment of the CIGS layer with sodium and potassium fluoride enables fabrication of flexible photovoltaic devices with a remarkable conversion efficiency due to modified interface properties and mitigation of optical losses in the CdS buffer layer.

This treatment leads to a significant depletion of copper and gallium concentrations in the CIGS near-surface region and enables a significant thickness reduction of the CdS buffer layer without the commonly observed losses in photovoltaic parameters.

The researchers say the exchange processes, well known in other research areas, are proposed as underlying mechanisms responsible for the changes in chemical composition of the deposited CIGS layer and interface properties of the heterojunction.

The special treatment alters the chemical composition of the complex sandwich structure. This alters its electronic properties, as confirmed by various methods including detailed electron microscopy.

The new method has been published in the journal Nature Materials

With this technique, the Empa team has been able to significantly increase the energy conversion efficiency from sunlight into electricity using CIGS thin film solar cells on flexible plastic foils - to a new record level of 20.4 percent, representing a marked improvement over the previous record of 18.7 percent established by the same team in May 2011.

This finally enables CIGS cells to compete with the best polycrystalline silicon cells.

Until recently, the Empa CIGS cells were the most efficient in the world.

At the end of October, though, a German research team at the Zentrum für Sonnenenergie-und Wasserstoff-Forschung (ZSW) in Stuttgart presented CIGS cells with an efficiency of 20.8 percent, although they use far higher processing temperatures and (rigid) glass as the substrate.

The slightly improved record shows that CIGS thin film technologies are a hot topic and that Empa is right at the cutting edge.

This work has been described in detail in the paper, "Potassium-induced surface modification of Cu(In,Ga)Se2 thin films for high-efficiency solar cells," by Adrian Chirilă et al in Nature Materials (2013). DOI:10.1038/nmat3789

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