Solar cells based on APbX₃ halide perovskites with (A = CH₃NH₃, HC(NH₂)₂, Cs and X = Br, Cl, I) have shown record efficiencies of up to 21.1%.¹ However, there are concerns related to these absorbers, such as the toxicity of Pb and solar cell instability from UV induced damage to the organic component. One route to reduce the toxicity of the absorber material is replacing Pb by Sn. Moreover, completely substituting the organic cation(s) with (inorganic) Cs can improve device longevity.² Incorporating these modifications leads to a class of inorganic Pb-free perovskites – CsSnX₃ – as promising alternative absorbers. Devices based on CsSnX₃ have thus far exhibited only relatively low efficiencies, possibly due to the oxidization from Sn⁺² to Sn⁺⁴ producing deep defects in the absorber.³ Adding SnF₂ during preparation inhibits this oxidation,⁴ however, the exact mechanism behind the SnF₂ treatment, in particular its impact on the electronic structure, is not yet fully understood. Another approach to preventing Sn oxidation is changing the deposition environment/technique. Sn is more likely to oxidize when CsSnX₃ is prepared wet-chemically (e.g., via spin-coating) than if it is prepared by thermal evaporation under vacuum conditions. For the latter, depositing the absorber in ultra-high vacuum (UHV) allows preparation of films without Sn oxidation.

To identify the roles of the SnF₂ treatment, halide composition, and deposition route, thin films of (spin-coated) CsSnBr₃/compact-TiO₂/FTO/glass and (UHV deposited) CsSnCl₃/Mo/glass were characterized by hard x-ray photoelectron spectroscopy (HAXPES). We find improved substrate coverage when SnF₂ is added to the precursor solution or when the absorber is deposited via thermal evaporation in UHV. Furthermore, we can identify different chemical species depending on the preparation process. Our measurements also reveal an impact of the SnF₂ treatments on the perovskite electronic structure, i.e., it enhances the density of states close to the valence band maximum (VBM). Also, we find a VBM shift of approximately 1 eV away from the Fermi level if CsSnCl₃ is compared to CsSnBr₃.

In our contribution, we will discuss in detail the chemical and electronic properties of inorganic Pb-free perovskites. These findings can lead to insights required to identify pathways for efficiency improvements of solar devices based on solution- and thermal-deposition-processed CsSnX₃ perovskite absorber layers.

¹Saliba et al., Energy Environ. Sci.9 (2016)1989.

²Kulbak et al., J. Phys. Chem. Lett. 7 (2015) 167.

³Chung et al., J. Am. Chem. Soc. 134 (2012) 8579.

⁴Kumar et al., Adv. Mater. 26 (2014) 7122.