In-Stability of 2D Capping Layers for Halide Perovskite Optoelectronics

Surface treatments with bulky organic cations – too big to fit into a 3D perovskite lattice – have been instrumental for reducing non-radiative recombination, and therefore for increasing efficiencies, in metal halide perovskite films and devices. Bulky cation surface treatments are now included in most solar cell architectures with performances above 23%. In this contribution we study how changes in the surface heterogeneity, structure, chemistry, photoluminescence, and chemical composition of perovskite films treated with 2D capping layers affect performances and stability of complete devices, using techniques as grazing-incidence wide angle X-ray scattering, hyperspectral microscopy, and X-ray fluorescence, both in-situ and ex-situ. The role of bulky cation size, perovskite composition, halide counterion choice, and deposition route (solution vs. vapor) is discussed in correlation to the films’ responses to stressors as light, heat, moisture, and oxygen. Our results reveal that bulky cations help prevent degradation of the perovskite films under exposure to moisture and oxygen, but that the treated surfaces undergo continuous reconstruction under thermal stress and illumination, with impacts on device stability. We show that the structure of the bulky cation, as well as the choice of counterion, impact the dynamics of the interface reconstruction, enabling improved thermal stability of these interfaces. These results provide valuable insights to the community to help drive the design of capping layers to reduce surface reconstruction and increase the operational stability of perovskite optoelectronic devices.