Silver-alloyed CGIS solar cell with 18.7% efficiency

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Scientists from Switzerland’s Federal Laboratories for Materials Science and Technology (EMPA) have built a thin-film solar cell based on copper, gallium, indium and selenium (CIGS) by using a small amount of silver (Ag) to alloy the CGIS absorber. They claim the device can be used for applications in tandem solar cells.

“The amount of Ag in CIGS is very tiny; only about 20nm or thinner Ag precursor layer for a total CIGS thickness of about 3000 nm,” researcher Maximilian Krause told pv magazine. “But the benefits in terms of structural and electronic properties to enhance the photovoltaic conversion efficiency are high.”

The researchers said the use of silver improves grain growth and crystal quality, while reducing detrimental defects and compensating for the low bandgap energy of 1.0 eV of the CIGS absorber. The researchers used a precursor layer method to deposit Ag on the absorber, in a three-stage co-evaporation process on soda-lime glass.

“The absorbers were completed by a 35nm cadmium sulfide (CdS) buffer layer deposited in a chemical bath,” they explained. “The solar cells did not undergo a heat-light soaking.”

They built a cell with a surface of 0.57 cm2 and an antireflective coating based on magnesium fluoride (MgF2). Tested under standard illumination conditions, the best-performing device showed a power conversion efficiency of 18.7%, an open-circuit voltage of 602 mV, a short-circuit current of 42.1 mAcm-2, and a fill factor of 73.9%.

“While Ag leads to higher open-circuit voltage and fill factor values, the short-circuit current does not reveal similar trends,” they said.

The academics said the increase in performance may be due to the fact that Ag-alloying modifies the cell interface as it consists of a bi-layer structure with a rubidium, indium, selenium (Rb-In-Se) layer and a Cu-depleted ordered vacancy compound OVC layer, which reduce the density of interfacial defects and act like passivation layers.

They described the cell technology, which they claim can be used for applications in tandem solar cells, in “Silver-alloyed low bandgap CuInSe2 solar cells for tandem applications,” which was recently published in RRL Solar.

“Up to now, we have not yet calculated the extra costs, but benefits could be significant when one considers the implication on industrial-level productions,” Krause said, when asked about the cell's production costs. “Our lab is very interested in further developments of technology and towards industrialization.”

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