Hybrid halide perovskite materials are thin-film semiconductors of interest to various applications in optoelectronics.^1–3  They are soft yet crystalline solution-processable mixed ionic–electronic conductors that display excellent optoelectronic characteristics, which are highly versatile and tuneable by using different compositional elements.^4–9  A hybrid organic–inorganic perovskite structure can be described by the AMX₃ formula (Fig. 1A). It is based on an inorganic {MX₆} octahedral corner-sharing framework, where M is a divalent metal (mostly Pb²⁺ or Sn²⁺), X is a halide anion (Cl⁻, Br⁻, I⁻), and A central cation (A-cation), which can be either inorganic (e.g. Cs⁺) or organic (e.g. methylammonium (MA), formamidinium (FA)).^9–13  Hybrid perovskites have demonstrated excellent light absorption and long-living photogenerated charges with long diffusion lengths and high defect tolerance, despite routine preparation by solution-processing.^9,14,15  Their optoelectronic properties are defined by the orbital (M–X) interactions within the inorganic framework, whereas organic components indirectly contribute by affecting the structural characteristics.^9,16  These properties can be tailored by using different AMX₃ perovskite compositions, changing their octahedral connectivity, as well as by applying other organic species as additives or molecular modulators (Fig. 1B and C).^16–18  The incorporation of larger organic moieties within the perovskite framework can lead to the formation of low-dimensional or layered (2D) perovskite structures (Fig. 1D and E).^19–23  They represent a class of materials typically described by the S₂A_n−1M_nX_3n+1 or S′A_n−1M_nX_3n+1 formulae, where A (MA⁺, FA⁺, Cs⁺) are central A-cations, M divalent metal cations, X halide anions (Cl⁻, Br⁻, I⁻), and S or S′ (typically mono- or bifunctional ammonium cations, respectively) organic spacers, which often form Ruddlesden–Popper (RP) or Dion Jacobson (DJ) phases (Fig. 1D and E).^24,25  Layered hybrid perovskites feature a layered structure of perovskite slabs separated by the organic spacers, where the value of n represents the number of {MX₆} octahedra in the perovskite stack (n = 1, 2, etc.).^22,23  The organic spacer affects the structure, orientation, as well as the morphology of the resulting material, and consequently its optoelectronic properties.²³  Most commonly used organic spacers are based on hydrophobic alkyl chains terminated by ammonium groups, such as n-buthylammonium (BA) or phenylethylammonium (PEA),²³  which are electronically insulating. These materials have nonetheless already shown promise in optoelectronic devices, such as solar cells,^26,27  light-emitting diodes,^28,29  piezoelectrics,³⁰  transistors,^30,31  and memory elements.^5,8,31,32