12 October 2020
Germany’s ZSW, MLU and HZB pinpoint losses in CIGS solar cells
Copper indium gallium diselenide (CIGS) thin-film solar cell efficiency has already exceeded 23%, but now a further increase looks to be within reach. A team at Germany’s ZSW (Zentrum für Sonnenenergie- und Wasserstoff-Forschung — or Center for Solar Energy and Hydrogen Research — Baden-Württemberg), Martin Luther University Halle-Wittenberg (MLU) and the Helmholtz-Zentrum Berlin (HZB) recently identified a key point where the performance of thin-film solar cells can be improved for the cell to convert more sunlight into electricity (Krause et al, ‘Microscopic origins of performance losses in highly efficient Cu(In,Ga)Se2 thin-film solar cells’, Nature Communications 11 (2020) 4189). The results of this investigation reveal how manufacturers of CIGS thin-film solar cells can achieve even higher efficiencies, it is reckoned.
Strides have been made in recent years towards CIGS thin-film solar cells’ maximum theoretical efficiency of about 33%, but around ten percentage points of potential remains untapped. This shortfall is attributable to loss mechanisms in the CIGS solar cell in the functional layers and at diverse interfaces. Where exactly and why these losses occur has been a point of conjecture and the subject of much debate.
Reducing the density of electrically active grain boundaries boosts performance
ZSW, MLU and HZB have now learned more about their origins. “Some of the losses occur at the boundaries between the individual CIGS crystals in the solar cell,” says project manager Dr Wolfram Witte at ZSW. “Positive and negative electrical charges can neutralize each other at these grain boundaries, some of which are electrically active,” he adds. “This reduces the cell’s performance.”
Researchers were able to identify this type of loss mechanism by combining experimental measurement methods with computer simulations. HZB analyzed a highly efficient CIGS solar cell with various electron microscopy techniques and optoelectronic measuring methods such as photoluminescence to provide realistic values to the two-dimensional device simulation developed at MLU.
Picture: Top image depicts measured grain structure of the CIGS solar cell produced at ZSW, with colors indicating the grains’ different crystallographic orientations. Bottom image shows the two-dimensional simulation based on these measurements. Artwork: ZSW, based on illustrations in Nature Communications.
ZSW manufactured the CIGS cell in a co-evaporation process that deposits copper, indium, gallium and selenium simultaneously in a vacuum. The cell’s efficiency was 21% without an additional anti-reflective layer. The physical microstructure of this cell and the values obtained in experiments with various analytical methods served as the input parameters for two-dimensional simulations.
Computer simulations showed that increased recombination at electrically active grain boundaries within the CIGS layer constitutes a significant loss mechanism. Above all, this decreases the open-circuit voltage and fill factor, which reduces the cell’s efficiency.
“What needs to be done to further improve the efficiency of CIGS thin-film solar cells and modules is to reduce the density of the electrically active grain boundaries and produce CIGS layers with larger grains,” says Witte. This could be achieved with technical means, for example, by augmenting the CIGS layer with additives, adapting the substrate material or optimizing the temperature balance during coating. These would be promising points of departure for the photovoltaic industry’s efforts to raise the efficiency of CIGS modules, it is reckoned.
The findings described in Nature Communications comprised one of several partial results obtained in the joint project EFFCIS. Funded by Germany’s BMWi (Bundesministerium für Wirtschaft und Energie, the Federal Ministry for Economic Affairs and Energy), this research venture ended in 2020 after three and a half years. Nine partners teamed up in a consortium that had experts from research institutes, universities and industry working together under the leadership of the ZSW. Their efforts focused on localizing and learning more about the dominant loss mechanisms in CIGS thin-film solar cells and modules to then reduce or eliminate these losses with innovative measures. The partners used analytical tools with high temporal and spatial resolutions to determine the chemical and physical properties of the functional layers and interfaces in CIGS solar cells.