Researchers at China's Hangzhou Dianzi University have modified the absorber of a conventional perovskite solar cell with potassium trifluoromethanesulfonate (KTFS) and found that the additive improved the device's performance and stability. The cell’s perovskite film reportedly showed less lead defects and lower J-V hysteresis.
“The KTFS molecule is a typical kind of potassium salt including the cationic potassium (K+) and anionic trifluoromethanesulfonate (SO3CF3−), indicating a bifunctional interaction between KTFS and perovskite,” the team explained. “The sulfonyl group can passivate the undercoordinated lead of the deep-level defect and thus inhibit the non-radiative recombination.”
The research group described the results of the proposed potassium-based salt additive as providing a better surface morphology and higher crystallinity in a film built with a lead iodide (PbI2) layer treated with KTFS via thermal annealing.
The performance of the KTFS-modified film was compared to that of a control perovskite film without the additive, and the former was found to have less lead impurities than the control film. “The presence of Pb impurities in perovskite films is the result of the decomposition of residual PbI2 when exposed to light or X-ray irradiation, indicating an iodine deficiency in the films,” the scientists said. “These metal lead clusters act as impurities, which can accelerate the degradation of perovskite films and affect the stability of perovskite solar cells.”
Also, KTFS was found not to affect the energy bandgap of 1.52 eV of the formamidinium lead iodide (FAPbI3) perovskite material selected for the film.
The group built the solar cell with a substrate made of indium tin oxide (ITO), an electron transport layer (ETL) based on either tin oxide (SnO2), the KTFS-modified perovskite absorber, a hole transport layer (HTL) relying on Spiro-OMeTAD, and a silver (Ag) metal contact.
Tested under standard illumination conditions, the device achieved a power conversion efficiency of 23.96%, an open-circuit voltage of 1.165 V, a short-circuit current density of 25.29 mA/cm2, and a fill factor of 81.33%. For comparison, a reference device without KTFS achieved an efficiency of 22.23%, an open-circuit voltage of 1.15 V, a short-circuit current density of 25.03 mA/cm2, and a fill factor of 77.24%.
“The device modified by KTFS shows a relatively low hysteresis index with a value of 4% compared to 6.8% for the control device,” the team said. “It is speculated that the existence of K+ in KTFS incorporated with perovskite can inhibit ion migration, thereby reducing J-V hysteresis.”
The group also found that the KTFS-modified cell was able to maintain 90% of its original efficiency after 1,200 h of storage, while the control device achieved only 80%.
