Cadmium-free kesterite solar cell with 9.7% efficiency

Scientists from the Barcelona Institute of Science and Technology in Spain have developed a cadmium-free superstrate kesterite solar cell with a new approach for silver (Ag) refining.

“Kesterite is a PV material consisting of earth-abundant, non-toxic elements,” the research's corresponding author, Gerasimos Konstantatos, told pv magazine. “However, a technological bottleneck originated from the material defects, unstable interface, and toxic cadmium sulfide (CdS) buffer layer has to be overcome. In our work, we used a 200 nm ultrathin cadmium-free buffer layer made of titanium oxide (TiO₂), inverted device stacks and, more importantly, silver refining. This refining step is beneficial in controllably promoting grain growth and improving carrier transport.”

The researchers built the cell with a kesterite substrate known as CAZTS, a tin oxide (FTO) substrate, an electron transport layer (ETL) based on titanium oxide (TiO₂), and a precursor film made of PTB7 polymer, molybdenum trioxide (MoO3) and gold (Au). “Refining of Ag in the absorber was realized during the precursor film construction where an additional layer of copper-free solution was coated on top of the pristine precursor,” they explained.

The research team tested the cell under standard illumination conditions and found it achieved a power conversion efficiency of 9.7%, which it described as a new record for this type of device. The cell was also able to retain around 90% of its initial efficiency after 100 days without encapsulation. The academics said silver refining resulted  in averagely doubled grain sizes, expanded depleted region width, and improved
transport kinetics, which in turn was responsible for increasing the cell's open-circuit voltage and fill factor.

“Estonian company Crystalsol Inc. recently fabricated flexible, light-weight, transparent, and low-cost solar modules,” Konstantatos said. “According to its report, the production cost of a kesterite module is around $0.35/W. Since kesterite only consists of earth-abundant elements, it does not have a fundamental limit to scale up to terawatt, rather than the scarcity of indium and gallium in copper, indium, gallium and selenium (CIGS) thin-film solar cells. In addition, given the current kesterite module efficiency is around 10%, which compares to about 20% for CIGS, further reduction of cost can be achieved with improved efficiency and mass production.”

They presented the device in the paper “Ag-Refined Kesterite in Superstrate Solar Cell Configuration with 9.7% Power Conversion Efficiency,” published in Advanced Functional Materials. “In this study, we introduced silver which is a rather expensive element, but with ultrathin absorber that is around 10 times thinner than conventional kesterite film, hence the resulting cost for material remained the same, which perseveres the low-cost advantage of pristine kesterite.”