posted on 2021-04-05, 11:34authored byBenny Febriansyah, Yulia Lekina, Jagjit Kaur, Thomas J. N. Hooper, Padinhare Cholakkal Harikesh, Teddy Salim, Ming Hui Lim, Teck Ming Koh, Sudip Chakraborty, Ze Xiang Shen, Nripan Mathews, Jason England
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 on 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 cations upon the
structure of n = 1 2D tin iodide perovskites (where n refers to the number of contiguous two-dimensional (2D)
inorganic layers, i.e., not separated by organic
cations) 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 the formation
of (110)-oriented materials, including 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 of the imidazolium rings with the Sn–I
framework, and the comparatively high tendency of Sn2+ ions
to stereochemically express their 5s2 lone pairs . 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 among the shortest ever
recorded for 2D tin iodide perovskites. These 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 measurement of excitonic activation energies
as low as 83(10) meV for ImEA[SnI4]. When combined with
superior light absorption capabilities relative to their lead congeners,
this allowed the 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 among the highest values reported for
pure n = 1 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.