Researchers at the University of North Carolina at Chapel Hill and CubicPV have developed mini solar modules based on perovskite cells treated with zinc trifluoromethane sulfonate [Zn(OOSCF3)2]. The scientists found that using a small amount of this zinc salt in the perovskite solution can address the issue of interstitial iodides, which are the most critical type of defects in perovskite solar cells that limits efficiency and stability. The zinc salt helps control the iodide defects in resultant perovskites ink and films.
The scientists explained that this is a low-cost material that is used as an additive at a very small percentage in perovskite inks and that its use makes perovskite module fabrication more reproducible, which helps to also make it cheaper.
The researchers added five different zinc salts into five samples of perovskite ink, and fabricated small solar cells via a blade coating process. The different zinc formations were zinc formate [Zn(OOCH)2], zinc acetate [Zn(OOCCH3)2], zinc trifluoroacetate [Zn(OOCCF3)2], zinc trifluoromethane sulfinate [Zn(OOSCF3)2], and zinc trifluoromethane sulfonate [Zn(OO2SCF3)2]. They also fabricated a reference cell without the addition of zinc.
In addition, each cell was tested with different additive concentrations of 0.14%, 0.28%, 0.42%, and 0.55%. On the cell level, the scientists found the Zn(OOSCF3)2 with a concentration of 0.28% to deliver the best results, with an average open circuit voltage of 1.18 V and a fill factor of 82%. This compares with an open-circuit voltage of 1.16 V and a fill factor of 80% in the control cell.
The team stated that the results suggest that Zn(OOSCF3)2 improves the device efficiency through defect passivation as both open-circuit voltage and fill factor are significantly enhanced.
They then used the Zn(OOSCF3)2 at the optimized concentration to blade coat 78 cm2 of perovskite films, from which they fabricated mini modules with an area of 78 cm2, 84 cm2, and 108 cm2, respectively. They all had 20 sub-cells, each with a width of 6.5 mm.
The fabricated minimodules reportedly showed power conversion efficiencies of 19.60% and 19.21% with aperture areas of 84 cm2 and 108 cm2, respectively, as certified by National Renewable Energy Laboratory (NREL). According to the team, this is the highest efficiency certified for minimodules of these sizes.
Conducting further investigation into the minimodules' optoelectronic and morphologic properties, the scientists emphasized that Zn(OOSCF3)2 passivated the defects of the perovskites despite its induction of negligible grain size changes. They said that the CF3SOO‒ anions can reduce the generated iodine during perovskite solution or device aging. At the same time, the zinc cations can precipitate the excess iodide so that the iodide interstitial concentration is diminished throughout the films, resulting in improved device efficiency and stability.
In order to be used in a commercial setting, this method will need to be applied to equipment that can handle bigger size modules, such as slot-die coater, or roll-to-roll coaters, the team mentioned.
