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How a surface treatment improves the inside of a solar cell

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Published on Monday, 03 February 2020

In the frame of the European project Sharc25, physicists from the University of Luxembourg with European experts have succeeded in explaining the recent efficiency improvements in thin film solar cells. The work of the whole consortium has been published in the prestigious journal Advanced Energy Materials.

Thin film solar cells are the electricity source with the lowest CO2 emissions. Thin film solar cells based on chalcopyrites, a semiconductor material, are among the technologies showing the highest efficiencies and they are stable in the field. 

Comprehensive model

After 3.5 years of collaborative effort between European partners, researchers have demonstrated that the recent improvements are due to changes inside the active layer, not at the surface as originally thought. 

The effort led to several steps forward in efficiency - and to deeper understanding of the effects leading to these efficiency improvements. “In this new paper we bring all the evidence together to come up with a comprehensive model of the effects of alkali treatments. Originally it was thought that the surface is responsible for the efficiency improvement, because it’s a surface treatment and the chemistry at the surface is changed considerably”, explains Prof. Susanne Siebentritt who leads the Laboratory for Photovoltaics at the University of Luxembourg. 

With the combined effort of several groups across Europe, researchers could show that the main change is inside the active layer, in the atomic and electronic structure of grain boundaries. “It was the photoluminescence work at the University of Luxembourg that gave the decisive hint that the electronic structure inside the absorber is improved by the treatment”, says Prof. Siebentritt. 

European collaboration

In order to better understand thin film solar cells and increase their efficiency, the European project Sharc25: “Super high efficiency Cu(In, Ga)Se2 thin-film solar cells approaching 25%” was launched in 2015 for 3.5 years and and a budget of 4.6 million euros from the European Union under the research framework programme Horizon 2020. 

It gathered eleven research partners from eight countries: Swiss Federal Laboratories for Materials Science and Technology (EMPA); Universities of Luxembourg, RouenParma and Aalto; Interuniversity Microelectronics Centre (IMEC); Helmholtz-Zentrum Berlin für Materialien und Energie (HZB), International Iberian Nanotechnology Laboratory (INL); Flisom ; Manz CIGS Technology and Zentrum für Sonnenenergie- und Wasserstoff-Forschung Baden-Württemberg (ZSW). 

Prior to the project, researchers at EMPA discovered that these solar cells can be made better by a surface treatment with alkali atoms, like potassium while during the project researchers at ZSW improved those treatments and obtained several world record efficiencies. In 2015, the two groups decided to join forces with several experts in various characterisation techniques, among them the Laboratory for Photovoltaics at the University of Luxembourg. 

Publication "Heavy alkali treatment of Cu(In,Ga)Se2 solar cells: surface versus bulk effects", Advanced Energy Materials, January 2020