One of the main challenges for the scientific community and the PV industry to bring perovskite solar cells (PSCs) to commercial production is the need to improve moisture stability. Protecting the perovskite layer from moisture is key to preventing excess water from forming on the layer itself, as it can damage the crystallinity of the cell structure, which affects overall performance.
An international group led by the Basque Center for Materials, Applications, and Nanostructures in Spain has developed a moisture-resistant perovskite solar cell using sulfur-based additives. “By tuning the iodide and bromide tails of additives, the influence of sulfur heteroatom containing ammonium salts on the photo-physical and device properties of a formamidinium-rich perovskite absorber is uncovered,” the research's corresponding author, Shahzada Ahmad, told pv magazine. “By adopting this additivization strategy, we investigated the obtained stability with proton migration analysis and kelvin probe microscopy under humidity.”
The scientists used, in particular, a type of widely utilized ammonium additives, which underwent deprotonation reaction to form volatile amines and halogens, which they said nullify the passivation effects during the long-term cell's operations. “Employing resonance stabilized amidinium counterparts was our strategy,” said Ahmad. We proposed for the first time a rational additivization strategy to augment the long-term water and humidity stability of the pure formamidinium lead triiodide (FAPbI3) layer by multifunctional organic salt containing ammonium and amidinium ends, and oxygen-assisted water uptake on the perovskite surface.”
According to the researchers, induced lead sulfide (Pb-S) interaction and manipulated crystallization growth were responsible for the cell's enhanced stability. “Moisture-assisted perovskite degradation mechanisms were investigated including surface adsorption of water molecules, proton migration with perovskite ammonium ends, and oxygen-assisted water uptake on the perovskite surface,” Ahmad went on to say.
The research team built the cell with a fluorine-doped tin oxide (FTO) front electrode, compact and mesoporous titanium dioxide (c-TiO2 and m-TiO2, respectively), the perovskite absorber, a spiro-OMeTAD hole-blocking layer, and a gold (Au) metal contact.
The champion solar cell developed with this structure achieved a power conversion efficiency of 25.1%, an open-circuit voltage of 1,165 mV, a short-circuit current of 25.88 mA cm–2, and a fill factor of 83%. A reference cell built without sulfur-based additives achieved an efficiency of 22.49%, an open-circuit voltage of 1,115 mV, a short-circuit current density of 25.16 mA/cm2, and a fill factor of 80%.
“The additives arguably contribute to strengthening the crystal lattice of the perovskite grains, which reduces their propensity toward undergoing irreversible conversion to lead(II) iodide (PbI2),” the research group explained, noting that the cell was also able to retain around 99.6% of its initial efficiency after 1000 h.
“Our developed methodology with heteroatomic additivization strategy not only achieves high intrinsic stability for formamidinium-enriched thin films but also pushes the performance of the devices from 22.49% to 25.14%,” the academics concluded.
The device was described in “Uncovering the Nanoscopic Humidity Ingression in Multifunctional Addivated Halide Perovskites,” published in Advanced Energy Materials. The team also included scientists from Huazhong University of Science and Technology in China, the University of Bordeaux in France, and the Max Planck Institute for Polymer Research in Germany.
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