Unraveling the Chemical Nature of the 3D “Hollow” Hybrid Halide Perovskites

The newly introduced class of 3D
halide perovskites, termed “hollow”
perovskites, has been recently demonstrated as light absorbing semiconductor
materials for fabricating lead-free perovskite solar cells with enhanced
efficiency and superior stability. Hollow perovskites derive from
three-dimensional (3D) AMX3 perovskites
(A = methylammonium (MA), formamidinium (FA); M =
Sn, Pb; X = Cl, Br, I), where small molecules such as ethylenediammonium
cations (en) can be incorporated as the dication
without altering the structure dimensionality. We present in this
work the inherent structural properties of the hollow perovskites
and expand this class of materials to the Pb-based analogues. Through
a combination of physical and spectroscopic methods (XRD, gas pycnometry, 1H NMR, TGA, SEM/EDX), we have assigned the general formula
(A)1–x­(en)x­(M)1–0.7x(X)3–0.4x to the hollow
perovskites. The incorporation of en in the 3D perovskite
structure leads to massive M and X vacancies in the 3D [MX3] framework,
thus the term hollow. The resulting materials are semiconductors with
significantly blue-shifted direct band gaps from 1.25 to 1.51 eV for
Sn-based perovskites and from 1.53 to 2.1 eV for the Pb-based analogues.
The increased structural disorder and hollow nature were validated
by single crystal X-ray diffraction analysis as well as pair distribution
function (PDF) analysis. Density functional theory (DFT) calculations
support the experimental trends and suggest that the observed widening
of the band gap is attributed to the massive M and X vacancies, which create a less connected 3D hollow structure.
The resulting materials have superior air stability, where in the
case of Sn-based hollow perovskites it exceeds two orders of temporal
magnitude compared to the conventional full perovskites of MASnI3 and FASnI3. The hollow perovskite compounds pose
as a new platform of promising light absorbers that can be utilized
in single junction or tandem solar cells.