Boosting the efficiency of CIGS thin-film solar cells

Researchers from Germany’s Centre for Solar Energy and Hydrogen Research Baden-Württemberg (ZSW), Martin Luther University Halle-Wittenberg (MLU) and Helmholtz-Zentrum Berlin (HZB) have identified a key point where the performance of thin-film solar cells can be improved for the cell to convert more sunlight into electricity. Published in the journal Nature Communications, their results reveal how manufacturers of copper, indium, gallium and selenium (CIGS) thin-film solar cells can achieve even higher efficiencies.

Great strides have been made in recent years towards CIGS thin-film solar cells’ maximum theoretical efficiency of about 33%, but around 10 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 among experts; now scientists at the ZSW, MLU and HZB have learned more about their origins.

“Some of the losses occur at the boundaries between the individual CIGS crystals in the solar cell,” said project manager Dr Wolfram Witte from the ZSW. “Positive and negative electrical charges can neutralise each other at these grain boundaries, some of which are electrically active. This reduces the cell’s performance.”

The researchers were able to identify this type of loss mechanism by combining experimental measurement methods with computer simulations. The HZB analysed 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 the MLU.

The 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 antireflective 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,” Dr Witte said. This could be achieved by augmenting the CIGS layer with additives, adapting the substrate material or optimising the temperature balance during coating. These would be promising points of departure for the photovoltaic industry’s efforts to elevate the efficiency of CIGS modules.

Image caption: Lab plant for depositing the CIGS layer in a co-evaporation process. Image credit: ZSW/Alexander Fischer.

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