Ammonium sulfate treatment at TiO2/perovskite interface boosts operational stability of perovskite solar cells

Titanium dioxide (TiO2) electron transport layers (ETLs) are still widely used in perovskite solar cells (PSCs) due to their compatibility with existing printing technologies and favorable energy level alignment for efficient electron extraction. However, TiO2 ETLs suffer from surface defects, e.g. oxygen vacancies, that are detrimental to perovskite/ETL interface stability, especially under operational conditions. Furthermore, hydroxyl groups present on the TiO2 surface also contribute to deprotonation of acidic organic cation in PSCs. We thus hypothesize that the metal oxide surface turns chemically reactive under 1-sun illumination whereby devices are highly populated with charge carriers and experience elevated temperature (ca. 60 oC). Here, we introduced a facile incorporation of sulfate species on the metal oxide surface to minimize chemical degradation at the perovskite/ETL interface. The sulfate treatment was found to minimally influence the perovskite film morphology grown on top of the ETLs so ruling out the morphological effects and allowing us to study the perovskite/ETL interface stability. We found that sulfate treated devices exhibited enhanced operational stability under initial maximum power point voltage (VMPP) over 1800 s measurement. Sulfate treated devices retained 95% of their initial efficiency while pristine devices already lost more than 40% of their initial efficiency. We also thermally aged encapsulated perovskite films coated on top of pristine and treated ETLs. We found that thermally aged perovskite films coated on pristine ETL contained perovskite hydrate species while the treated samples did not. We thus postulate that the water molecules contributing to the hydrate formation were generated solely from the ETL/perovskite interface. Lastly, a better energy alignment was also found between perovskite and sulfate-treated ETL which also contributes to the improved operational and thermal stability.