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]PbBr4to form [H3N(CH2)6NH3]PbBr4·Br2, which contains molecular
bromine (Br2) intercalated between the layers of corner-sharing
PbBr6octahedra. Bromine intercalation in [H3N(CH2)6NH3]PbBr4·Br2results 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 Br2intercalation 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·Br2has 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 Br2and Br in the
[PbBr4]∞layers, which is likely to have
important effects in a range of organic–inorganic metal halides.