Fluorinated 2-Phenylethylammonium Spacer Cations for Improved Stability of Ruddlesden–Popper Pb/Sn Halide Perovskites: Insight from First-Principles Calculations

Abstract

Improving the stability of hybrid halide perovskite solar cells is essential for their commercialization in emerging cutting-edge technology. Here, we report the results of state-of-the-art first-principles calculations to achieve and understand enhancement in the stability of Ruddlesden−Popper (RP) lead/tin halide perovskites with fluorinated 2-phenylethylammonium (PEA) spacer cations. It is found that the substitution of up to five F atoms in the phenyl ring makes C slightly positively charged due to ∼0.5e charge transfer to electronegative F. This leads to the formation of electric dipoles in the spacer cation region and polarization of the charge density. This improves the formation energy and enhances the stability of the layers. The charge transfer makes some of the C 2p orbitals partly empty so that they hybridize with F 2p orbitals near the conduction band minimum. This is interesting for the charge transport perpendicular to the layers. Our study further shows that changing the metal cations Pb with Sn does not affect the orientation of the cation molecules in the pure as well as chemically tuned RP layers whereas changing the halide ions I with Br has an impact on the orientation of the organic molecules. The calculated structures of the layers agree well with the available experimental results and suggest structures of some unexplored layers that would be interesting to fabricate. The calculated electronic structure and optical properties show that the absorption coefficient is high (∼10 5 cm −1 ) for the layers having heavily fluorinated spacer cations. Furthermore, we find higher stability and a desirable lower direct band gap for Sn-based green perovskite layers compared to the Pb counterparts. Our results provide computational insight into the surface engineering of RP perovskites for further developments in this fast-growing area.

Synthesis and Optical Properties of Potassium‐Based Wide‐Bandgap Mixed–Halide Perovskite KPbF2Cl Nanorods

“…To overcome this in organic–inorganic HPs, a wide variety of materials in this class have been studied by changing the organic cation and/or halide anion to improve the chemical stability. [ 10,11 ] In this respect, all‐inorganic HPs are expected to have much better chemical stability. [ 7,12 ] Moreover, fluorine has shown its ability to form a large variety of fluorides with high level of chemical stability by participating with low electronegative alkali metals (e.g., Na, K, etc.)…”

Reductive 2D Capping Layers through Dopamine Salt Incorporation for Pb–Sn Mixed Perovskite Solar Cells

“…The 2D capping layer not only effectively suppresses nonradiative charge recombination but also obtains significant enhancement of light, thermal, and humidity stability of Pb based perovskite. Furthermore, the 2D passivation approach in Pb–Sn perovskite has also been widely explored to enhance both the performance and stability. − Notably, Loi and co-workers reported on quasi-2D lead–tin perovskite solar cells with a gradient structure by in situ formation of tin selenide. These devices retained 91% of the original PCE after 10 days in ambient air without encapsulation.…”