Cesium trifluoroacetate induced synergistic effects of grain growth and defect passivation on high-performance perovskite solar cells

The prejudicial defects accumulated at the grain boundaries and surface are one of the most important reasons that cause perovskite solar cells (PSCs) to deviate from the theoretical Shockley-Queisser efficiency limit and poor stability. Consequently, it is highly desirable to reduce defect densities of the polycrystalline perovskite film through manipulating perovskite crystallization and introducing passivation strategy. Herein, the synergistic effects of cesium trifluoroacetate (Cs-TFA) as a novel additive are demonstrated, where it not only provides cesium cations (stabilizer) for perovskite lattice, but also can simultaneously modulate crystal growth and passivate grain boundaries defects. The resultant perovskite films possess more regular morphology, larger grain sizes, and lower defect density. The enlarged crystal grains derive from the incorporation of Cs-TFA that slows down nucleation process, which is demonstrated by in-situ laser confocal photoluminescence imaging. As a result, the encouraging efficiency up to 22.82% is realized for the resulting optimized device. Additionally, the unencapsulated devices sustain around 91% of its initial efficiency after storing 850 h in the air with 40-45% relatively humidity (R.H.) under dark.