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.

新近报道的一类被称为“空心”钙钛矿的三维卤化物钙钛矿，近期已被证实可作为吸光半导体材料，用于制备效率更优、稳定性更佳的无铅钙钛矿太阳能电池。空心钙钛矿源自三维（3D）AMX₃型钙钛矿（其中A为甲胺（methylammonium, MA）、甲脒（formamidinium, FA）；M为锡（Sn）、铅（Pb）；X为氯（Cl）、溴（Br）、碘（I）），乙二胺阳离子（en）等小分子可作为双阳离子掺入其中，且不会改变结构的维度。本研究中，我们系统阐明了空心钙钛矿的固有结构特性，并将该类材料的范围拓展至含铅（Pb）的衍生物。通过结合多种物理与光谱表征手段（X射线衍射（X-ray Diffraction, XRD）、气体比重瓶法（gas pycnometry）、氢谱核磁共振（¹H Nuclear Magnetic Resonance, ¹H NMR）、热重分析（Thermogravimetric Analysis, TGA）、扫描电子显微镜/能谱仪（Scanning Electron Microscope/Energy Dispersive X-ray Spectroscopy, SEM/EDX）），我们确定了空心钙钛矿的通式为(A)₁–ₓ(en)ₓ(M)₁–0.7ₓ(X)₃–0.4ₓ。乙二胺阳离子掺入三维钙钛矿结构后，会在三维[MX₃]骨架中引入大量金属位点与卤素空位，这也是“空心”这一命名的由来。所得材料均为半导体，其直接带隙发生显著蓝移：锡基钙钛矿的带隙从1.25 eV拓宽至1.51 eV，含铅衍生物的带隙则从1.53 eV拓宽至2.1 eV。结构无序程度提升与空心结构特性，通过单晶X射线衍射分析以及对分布函数（pair distribution function, PDF）分析得到了验证。密度泛函理论（Density Functional Theory, DFT）计算结果与实验趋势相符，表明观测到的带隙拓宽现象源于大量金属位点与卤素空位的存在，这使得三维结构的连通性下降。此类材料具备优异的空气稳定性，其中锡基空心钙钛矿的空气稳定性相较于传统全结构钙钛矿MASnI₃与FASnI₃提升了两个数量级以上。空心钙钛矿化合物为一类极具应用前景的吸光材料平台，可应用于单结或叠层太阳能电池的制备。