Ambient-air-stable inorganic Cs2SnI6 double perovskite thin films via aerosol-assisted chemical vapour deposition

Abstract

Air-stable caesium tin iodide double perovskite (Cs₂SnI₆) thin films have been fabricated via aerosol-assisted chemical vapour deposition (AACVD). We compare the properties of the double perovskite films made using AACVD with those made by the widely used spin-coating method. Films with purer crystalline phase (less CsI impurity) and far better stability in ambient air can be obtained by AACVD compared with spin coating. The AACVD-grown Cs₂SnI₆ films retain high phase purity for at least ∼100 days aging in air with negligible CsI impurities detected over this time, as determined by X-ray diffraction. The films exhibit an optical band gap energy (E_g) of ca. 1.3 eV and a homogeneous morphology with the expected nominal stoichiometry within error, as probed by energy-dispersive X-ray spectroscopy. Overall, the characteristics of the Cs₂SnI₆ films are highly process-dependent, e.g. they are influenced by the presence of hydroiodic acid (HI) in the precursor solution. Without HI addition, an iodine-deficient film with more CsI is produced, which also exhibits a larger E_g of ca. 1.6 eV. In addition to bulk properties, we utilise X-ray photoelectron spectroscopy (XPS) to scrutinise the surface characteristics in detail. We find excess Sn and I located at the surfaces. This can be attributed to the presence of SnI₄ from the deposition precursor vapour. Furthermore, following aging in air, an increase in CsI impurity for the AACVD (+HI)-grown film is observed, along with a reduction in SnI₄ at the surfaces. Near-ambient pressure XPS (NAP-XPS) is used to examine the surface stability of AACVD (+HI)-grown films on exposure to O₂ and H₂O. No enhancement in the amount of CsI impurity is observed after both H₂O vapour (9 mbar) and O₂ (5 mbar) exposure. Nevertheless, the concentrations of tin and iodine change after exposure, suggesting that SnI₄ protects Cs₂SnI₆ from degradation. This passivation effect of SnI₄ on Cs₂SnI₆ surfaces is proposed to explain the additional stability of Cs₂SnI₆ fabricated via AACVD.