High-efficiency and thermal/moisture stable CsPbI2.84Br0.16 inorganic perovskite solar cells enabled by a multifunctional cesium trimethylacetate organic additive

Abstract

All-inorganic cesium lead iodine perovskite materials (CsPbI₃) have been widely studied and applied to PSCs because of their excellent thermo-chemical stability and suitable bandgap. However, CsPbI₃ still faces the issue of phase transitions under humid conditions. In this study, cesium trimethylacetate (CsTa) organic cesium salt is employed as an additive in the CsPbI_2.84Br_0.16 film. The results show that CsTa can have multifunctional effects on CsPbI_2.84Br_0.16 perovskite stability. The steric hindrance blocking effect of Ta⁻ anions can efficiently prevent the tilt of the [PbI₆]⁴⁻ octahedrons to inhibit the phase transition process from the corner-shared perovskite to the edge-shared non-perovskite structure. The Ta⁻ groups are firmly located at the X site of the perovskite structure, leading to an increase in the energy barrier for X-site vacancy generation (from 0.816 eV to 1.217 eV), which slows the phase transition process induced by lattice disorder. Meanwhile, the decrease in crystal grain size after adding CsTa is beneficial for perovskite phase stability due to the decreased surface Gibbs free energy of CsPbI_2.84Br_0.16 crystals. In addition, the reduced defect density brought about by the improved crystal quality is also favorable for increasing the energy barrier for the phase transition from the perovskite phase to the δ phase. Finally, a champion PCE of 16.59% can be achieved from the HPbI₃-prepared PSCs with adding CsTa, which maintain 80.88% of the initial performance after more than 1200 hours in air (RH: ∼20%). Meanwhile, both 0.1-CsTa and 0.2-CsTa PbI₂-prepared PSCs deliver excellent stability and maintain above 80% of the initial PCE after nearly 2000 hours. Impressively, under a relatively high thermal/moisture (temperature: 80 °C/RH: ∼80%) environment, the 0.2-CsTa devices can still retain 87.41% of the initial performance after 500 hours of storage.