High-performance and good-stability hollow Sn-based perovskite solar cells using ethylenediammonium and formamidinium cations.

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) AMX 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, H NMR, TGA, SEM/EDX), we have assigned the general formula (A)( en) (M)(X) to the hollow perovskites. The incorporation of en in the 3D perovskite structure leads to massive M and X vacancies in the 3D [ MX] 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 MASnI and FASnI. The hollow perovskite compounds pose as a new platform of promising light absorbers that can be utilized in single junction or tandem solar cells.

Alternative all-inorganic halide perovskites are sought to replace the hybrid lead halide perovskites because of their increased stability. Here, the (111)-oriented defect perovskite family A 3 M 2 X 9 based on trivalent M 3+ is expanded through the use of mixed halides, resulting in Cs 3 Bi 2 I 6 Cl 3 . This compound shares the (111)-oriented 2D bilayer structure of α-Cs 3 Sb 2 I 9 (space group P3̅ m1), with Cl occupying the bridging positions of the bilayers and I in the terminal sites, in contrast to the parent compound Cs 3 Bi 2 I 9 , which consists of 0D molecular [Bi 2 I 9 ] 3− dimers. The increased dimensionality induces a direct band gap as calculated by density functional theory but has an absorption edge of 2.07 eV, nearly identical to the indirect band gap compound Cs 3 Bi 2 I 9 . Intriguingly, there is a remarkable lack of Cl orbital contribution to the band edge states of Cs 3 Bi 2 I 6 Cl 3 , despite Bi−Cl bonds binding all octahedra together. This highlights the importance of interlayer interactions in the defect perovskite family, which enhances the effective dimensionality of these 2D and 0D materials and may improve their optoelectronic performance. However, these changes in the excitonic absorption do not reflect free excitons, as Cs 3 Bi 2 I 6 Cl 3 exhibits broad photoluminescence as a result of self-trapped excitons, which appear to be universal in the (111)-oriented defect perovskites.

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The high Z chalcohalides HgQI (Q = S, Se, and Te) can be regarded as of antiperovskite structure with ordered vacancies and are demonstrated to be very promising candidates for X- and γ-ray semiconductor detectors. Depending on Q, the ordering of the Hg vacancies in these defect antiperovskites varies and yields a rich family of distinct crystal structures ranging from zero-dimensional to three-dimensional, with a dramatic effect on the properties of each compound. All three HgQI compounds show very suitable optical, electrical, and good mechanical properties required for radiation detection at room temperature. These compounds possess a high density (>7 g/cm) and wide bandgaps (>1.9 eV), showing great stopping power for hard radiation and high intrinsic electrical resistivity, over 10 Ω cm. Large single crystals are grown using the vapor transport method, and each material shows excellent photo sensitivity under energetic photons. Detectors made from thin HgQI crystals show reasonable response under a series of radiation sources, including Am andCo radiation. The dimensionality of Hg-Q motifs (in terms of ordering patterns of Hg vacancies) has a strong influence on the conduction band structure, which gives the quasi one-dimensional HgSeI a more prominently dispersive conduction band structure and leads to a low electron effective mass (0.20 m). For HgSeI detectors, spectroscopic resolution is achieved for both Am α particles (5.49 MeV) andAm γ-rays (59.5 keV), with full widths at half-maximum (FWHM, in percentage) of 19% and 50%, respectively. The carrier mobility-lifetime μτ product for HgQI detectors is achieved as 10-10 cm/V. The electron mobility for HgSeI is estimated as 104 ± 12 cm/(V·s). On the basis of these results, HgSeI is the most promising for room-temperature radiation detection.