Manipulation of the Structure and Optoelectronic Properties through Bromine Inclusion in a Layered Lead Bromide Perovskite

One of the great advantages of organic-inorganic metal halides is that their structures and properties are highly tuneable and this is important when optimizing materials for photovoltaics or other optoelectronic devices. One of the most common and effective ways of tuning the electronic structure is through anion substitution. Here, we report the inclusion of bromine into the layered perovskite [H₃N(CH₂)₆NH₃]PbBr₄ to form [H₃N(CH₂)₆NH₃]PbBr₄·Br₂, which contains molecular bromine (Br₂) intercalated between the layers of corner-sharing PbBr₆ octahedra. Bromine intercalation in [H₃N(CH₂)₆NH₃]PbBr₄·Br₂ results in a decrease in the band gap of 0.85 eV and induces a structural transition from a Ruddlesden-Popper-like to Dion-Jacobson-like phase, while also changing the conformation of the amine. Electronic structure calculations show that Br₂ intercalation is accompanied by the formation of a new band in the electronic structure and a significant decrease in the effective masses of around two orders of magnitude. This is backed up by our resistivity measurements that show that [H₃N(CH₂)₆NH₃]PbBr₄·Br₂ has a resistivity value of one order of magnitude lower than [H₃N(CH₂)₆NH₃]PbBr₄, suggesting that bromine inclusion significantly increases the mobility and/or carrier concentration in the material. This work highlights the possibility of using molecular inclusion as an alternative tool to tune the electronic properties of layered organic-inorganic perovskites, while also being the first example of molecular bromine inclusion in a layered lead halide perovskite. By using a combination of crystallography and computation, we show that the key to this manipulation of the electronic structure is the formation of halogen bonds between the Br₂ and Br in the [PbBr₄]_∞ layers, which is likely to have important effects in a range of organic-inorganic metal halides.