CaI2: a more effective passivator of perovskite films than PbI2 for high efficiency and long-term stability of perovskite solar cells

Abstract

A large number of previous studies demonstrate that excess PbI₂ in perovskite films is essential for solar cells to achieve high efficiency due to defect passivation, but few studies consider its negative impact on the devices' long-term stability. Herein, we introduced CaI₂ instead of PbI₂ with much less additive content into the CH₃NH₃PbI₃ (MAPbI₃) film and obtained a dense, uniform and surface-smooth morphology with much enlarged crystal grains. It has been demonstrated that addition of only 0.5% CaI₂ additive to MAPbI₃ is enough to enlarge the average grain size from 260 nm to 550 nm; accordingly, the defect density is decreased from 5.76 × 10¹⁶ cm⁻³ to 3.35 × 10¹⁶ cm⁻³ and the photoluminescence (PL) lifetime is increased from 65 ns to 133 ns; as the reference, addition of 5% PbI₂ additive to MAPbI₃ results in an average grain size of 410 nm, a defect density of 4.06 × 10¹⁶ cm⁻³ and a PL lifetime of 98 ns. In all of these key parameters, CaI₂ as a new passivator of the MAPbI₃ film, is found to be more effective than conventional PbI₂. As a consequence, the champion cell based on MAPbI₃(CaI₂)_0.005 achieved a high efficiency of up to 19.3%, superior to those based on pristine MAPbI₃ (16.1%) and MAPbI₃(PbI₂)_0.05 (18.0%). In addition, 10 times less additive of CaI₂ than PbI₂ made the perovskite film much less deviated from the ideal stoichiometry of MAPbI₃, which is found to be essential for the device to achieve long-term stability.