Major strides have
been made in the development of materials and devices based around
low-dimensional hybrid group 14 metal halide perovskites. Thus far, this work
has mostly focused upon compounds containing highly toxic Pb, with the
analogous less toxic Sn materials being comparatively poorly evolved. In
response, the study herein aims to (i) provide insight into the impact of
templating cation upon the structure of 2D tin iodide perovskites, and (ii)
examine their potential as light absorbers for photovoltaic (PV) cells. It was
discovered through systematic tuning of organic dications, that imidazolium
rings are able to induce formation of (110)-oriented materials, including the
first examples of “3 × 3” corrugated Sn-I perovskites. This structural outcome
is a consequence of a combination of supramolecular interactions of the two
endocyclic N-atoms in the imidazolium functionalities with the Sn-I framework
and the higher tendency of Sn<sup>2+</sup> ions to stereochemically express their
5s<sup>2</sup> lone pairs relative to the 6s<sup>2</sup> electrons of Pb<sup>2+</sup>.
More importantly, the resulting materials feature very short separations
between their 2D inorganic layers with iodide–iodide (I···I) contacts as small
as 4.174 Å,
which is amongst the shortest ever recorded for 2D tin iodide perovskites. The
proximate inorganic distances, combined with the polarizable nature of the
imidazolium moiety, eases the separation of photogenerated charge within the
materials. This is evident from the excitonic activation energies as low as
83(10) meV, measured for ImEA[SnI<sub>4</sub>]. When combined with superior
light absorption capabilities relative to their lead congeners, this allowed
fabrication of lead-free solar cells with incident photon-to-current and power conversion
efficiencies of up to 70 % and 2.26 %, respectively, which are amongst the
highest values reported for pure 2D group 14 metal halide perovskites. In fact,
these values are superior to the corresponding lead iodide material, which
demonstrates that 2D Sn-based materials have significant potential as less
toxic alternatives to their Pb counterparts.
The Impact of Phase Retention on the Structural and Optoelectronic Properties of Metal Halide Perovskites
