Swedish researchers have used a silver alloy to improve grain growth and crystal quality in a chalcopyrite absorber, which is said to partially compensate for its high band-gap energy. The scientists produced a master cell based on a ZTO buffer layer.
“The goal of our current EU-funded Stable Inorganic TAndem (SITA) solar cell project is to make a wide-gap chalcopyrite-topped cell for tandem PV devices with a silicon-based cell,” said researcher Jan Keller. pv magazine. “So the next step is to move the process to transparent back contacts. In our previous work, we have shown the potential to fabricate wide-gap chalcopyrite solar cells with slightly smaller bandgaps, around 1.45 eV, on highly transparent back contacts.
The use of a silver alloy improves grain growth and crystal quality, while reducing adverse defects and partially compensating for the low bandgap energy of CuGaSe.2 absorber. CuGaSe2 it has a bandgap energy of 1.7 eV and has so far been used in solar cells with limited duty factor and open circuit voltage.
“In addition to losses directly related to CuGaSe2 layer itself, front-interface recombination is supposed to be clear when using a standard cadmium sulfide (CdS) buffer, the researchers said, noting that they tested devices with CdS buffer layers and layers made of atomically grown zinc tin oxide (ZTO). layered deposition (ALD).
The research team used a three-step co-evaporation process to deposit a silver-doped absorber (ACGS) onto several solar cell samples.
“The carrier collection improves with Ag addition, higher deposition temperature and towards a stoichiometric absorber composition,” they said. “Light soaking increases the fill factor of most samples, possibly by reducing internal transport barriers.”
The master cell produced by the researchers was based on a ZTO buffer layer, and the power conversion efficiency of the device was 11.2%.
“This is the highest efficiency ever measured for an ACGS solar cell without an anti-reflective coating,” they said. “Possible explanations for the beneficial effects of light soaking could be photodoping of the ZTO layer, which would reduce the effective barrier height of electrons, or redistribution of sodium in the heterojunction.”
The researchers said they plan to use a transparent back contact (TBC) in the cell, rather than one made of molybdenum (Mo), as is used in tandem devices.
They described the solar cell technology in the paper “Silver doping in highly efficient CuGaSe2 Solar Cells with Different Buffer Layers”, which was published recently RRL sun.