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Swedish researchers have used the silver alloy to improve grain growth and crystal quality in the chalcopyrite absorber, presumably partly offsetting its high bandgap energy. Researchers at Uppsala University (Sweden) have fabricated a chalcopyrite (CuGaSe2 or CGSe) solar cell using silver (Ag) to alloy the devices absorber. “The goal of our EU-funded Stable Inorganic Tandem Solar Cells (SITA) project is to fabricate a large-gap chalcopyrite top cell for photovoltaics in tandem with a silicon bottom cell,” researcher Jan Keller explains to pv magazine . . “The next step is to transfer the process to clear back contacts. In our previous work we have shown that it is possible to fabricate large-gap chalcopyrite solar cells with slightly lower bandgaps, around 1.45 eV, in highly transparent back contacts.” The use of silver alloy improves grain growth and crystal quality, while reducing deleterious defects and partly compensating for the low band energy of the CuGaSe2 absorber. CuGaSe2 has a bandgap energy of 1.7 eV and has so far been used in solar cells with limited fill factor and open circuit voltage. In addition to the losses directly related to the CuGaSe2 layer itself, front-end interface recombination is assumed to be pronounced when using a standard cadmium sulfide (CdS) interlayer, explain the scientists, noting that they tested devices with CdS interlayers and zinc tin oxide (ZTO) layers grown by atomic layer deposition (ALD). The research team used a three-stage coevaporation process to deposit the silver-alloyed absorber (ACGS) on various solar cell samples. Carrier uptake improves with the addition of Ag, higher deposition temperature, and toward a stoichiometric absorber composition, they said. Light immersion increases the fill factor for most samples, potentially mitigating internal transport barriers. The champion cell the scientists made was based on a ZTO buffer layer, and the device achieved a power conversion efficiency of 11.2%. “This is the highest efficiency ever measured for an ACGS solar cell without an anti-reflective coating,” they say. Possible explanations for the beneficial effect of light immersion may be photodoping of the ZTO layer, which would reduce the effective height of the electron barrier, or a redistribution of sodium at the heterojunction. The scientists said they plan to use a transparent back contact (TBC) instead of a molybdenum (Mo) one in the cell, due to its intended use in tandem devices. They described the solar cell technology in “ Silver Alloying in Highly Efficient CuGaSe2 Solar Cells with Different Buffer Layers,” recently published in RRL Solar. |
