Subchalcogenides are uncommon, and their chemical bonding results from an interplay between metal–metal and metal–chalcogenide interactions. Herein, we present Ir₆In₃₂S₂₁, a novel semiconducting subchalcogenide compound that crystallizes in a new structure type in the polar P31m space group, with unit cell parameters a = 13.9378(12) Å, c = 8.2316(8) Å, α = β = 90°, γ = 120°. The compound has a large band gap of 1.48(2) eV, and photoemission and Kelvin probe measurements corroborate this semiconducting behavior with a valence band maximum (VBM) of −4.95(5) eV, conduction band minimum of −3.47(5) eV, and a photoresponse shift of the Fermi level by ∼0.2 eV in the presence of white light. X-ray absorption spectroscopy shows absorption edges for In and Ir do not indicate clear oxidation states, suggesting that the numerous coordination environments of Ir₆In₃₂S₂₁ make such assignments ambiguous. Electronic structure calculations confirm the semiconducting character with a nearly direct band gap, and electron localization function (ELF) analysis suggests that the origin of the gap is the result of electron transfer from the In atoms to the S 3p and Ir 5d orbitals. DFT calculations indicate that the average hole effective masses near the VBM (1.19m_e) are substantially smaller than the average electron masses near the CBM (2.51m_e), an unusual feature for most semiconductors. The crystal and electronic structure of Ir₆In₃₂S₂₁, along with spectroscopic data, suggest that it is neither a true intermetallic nor a classical semiconductor, but somewhere in between those two extremes.

Organic–inorganic hybrid halide perovskites are promising semiconductors with tailorable optical and electronic properties. The choice of A-site cation to support a three-dimensional (3D) perovskite structure AMX₃ (where M is a metal and X is a halide) is limited by the geometric Goldschmidt tolerance factor. However, this geometric constraint can be relaxed in two-dimensional (2D) perovskites, providing us an opportunity to understand how various A-site cations modulate the structural properties and thereby the optoelectronic properties. Here, we report the synthesis and structures of single-crystal (BA)₂(A)Pb₂I₇ where BA = butylammonium and A = methylammonium (MA), formamidinium (FA), dimethylammonium (DMA), or guanidinium (GA), with a series of A-site cations varying in size. Single-crystal X-ray diffraction reveals that the MA, FA, and GA structures crystallize in the same Cmcm space group, while the DMA imposes the Ccmb space group. We observe that as the A-site cation becomes larger, the Pb–I bond continuously elongates, expanding the volume of the perovskite cage, equivalent to exerting “negative pressure” on the perovskite structures. Optical studies and DFT calculations show that the Pb–I bond length elongation reduces the overlap of the Pb s- and I p-orbitals and increases the optical bandgap, while Pb–I–Pb tilting angles play a secondary role. Raman spectra show lattice softening with increasing size of the A-site cation. These structural changes with enlarged A cations result in significant decreases in photoluminescence intensity and lifetime, consistent with a more pronounced nonradiative decay. Transient absorption microscopy results suggest that the PL drop may derive from a higher concentration of traps or phonon-assisted nonradiative recombination. The results highlight that extending the range of Goldschmidt tolerance factors for 2D perovskites is achievable, enabling further tuning of the structure–property relationships in 2D perovskites.

Two-dimensional (2D) halide perovskites have great promise in optoelectronic devices because of their stability and optical tunability, but the subtle effects on the inorganic layer when modifying the organic spacer remain unclear. Here, we introduce two homologous series of Ruddlesden-Popper (RP) structures using the branched isobutylammonium (IBA) and isoamylammonium (IAA) cations with the general formula (RA)(2)(MA)(n-1)PbnI3n+1 (RA = IBA, IAA; MA = methylammonium n = 1-4). Surprisingly, the IAA n = 2 member results in the first modulated 2D perovskite structure with a ripple with a periodicity of 50.6 angstrom occurring in the inorganic slab diagonally to the [101] direction of the basic unit cell. This leads to an increase of Pb-I-Pb angles along the direction of the wave. Generally, both series show larger in-plane bond angles resulting from the additional bulkiness of the spacers compensating for the MA's small size. Larger bond angles have been shown to decrease the bandgap which is seen here with the bulkier IBA leading to both larger in-plane angles and lower bandgaps except for n = 2, in which the modulated structure has a lower bandgap because of its larger Pb-I-Pb angles. Photo-response was tested for the n = 4 compounds and confirmed, signaling their potential use in solar cell devices. We made films using an MACl additive which showed good crystallinity and preferred orientation according to grazing-incidence wide-angle scattering (GIWAXS). As exemplar, the two n = 4 samples were employed in devices with champion efficiencies of 8.22% and 7.32% for IBA and IAA, respectively.

The advent of the two-dimensional (2D) family of halide perovskites and their demonstration in 2D/three-dimensional (3D) hierarchical film structures broke new ground toward high device performance and good stability. The 2D Dion–Jacobson (DJ) phase halide perovskites are especially attractive in solar cells because of their superior charge transport properties. Here, we report on 2D DJ phase perovskites using a 3-(aminomethyl)piperidinium (3AMP) organic spacer for the fabrication of mixed Pb/Sn-based perovskites, exhibiting a narrow bandgap of 1.27 eV and a long carrier lifetime of 657.7 ns. Consequently, solar cells employing mixed 2D DJ 3AMP-based and 3D MA_0.5FA_0.5Pb_0.5Sn_0.5I₃ (MA = methylammonium, FA = formamidinium) perovskite composites as light absorbers achieve enhanced efficiency and stability, giving a power conversion efficiency of 20.09% with a high open-circuit voltage of 0.88 V, a fill factor of 79.74%, and a short-circuit current density of 28.63 mA cm^–2. The results provide an effective strategy to improve the performance of single-junction narrow-bandgap solar cells and, potentially, to give a highly efficient alternative to bottom solar cells in tandem devices.

Structural transformations in molecules and solids have generally been studied in isolation, whereas intermediate systems have eluded characterization. We show that a pair of cadmium sulfide (CdS) cluster isomers provides an advantageous experimental platform to study isomerization in well-defined, atomically precise systems. The clusters coherently interconvert over an ~1–electron volt energy barrier with a 140–milli–electron volt shift in their excitonic energy gaps. There is a diffusionless, displacive reconfiguration of the inorganic core (solid-solid transformation) with first order (isomerization-like) transformation kinetics. Driven by a distortion of the ligand-binding motifs, the presence of hydroxyl species changes the surface energy via physisorption, which determines “phase” stability in this system. This reaction possesses essential characteristics of both solid-solid transformations and molecular isomerizations and bridges these disparate length scales.

We demonstrate a potential candidate, the 0D “all-inorganic” perovskite material Cs₂TeI₆, as a sensitive all-inorganic X-ray photoconductor for the development of the new generation of direct photon-to-current conversion flat-panel X-ray imagers. Cs₂TeI₆ consists of high atomic number elements, has high electrical resistance, and exhibits high air and moisture stability, making it suitable as a sensitive X-ray photoconductor. In addition, we identify that Cs₂TeI₆ film can be prepared under a low-temperature process using electrostatic-assisted spray technique under atmospheric conditions and achieved resistivity of 4.2 × 10¹⁰ Ω·cm. The resulting air- and water-stable Cs₂TeI₆ device exhibits a strong photoresponse to X-ray radiation. An electron drift length on the order of 200 μm is estimated under an applied electrical field strength of 400 V·cm^–1. A high sensitivity for Cs₂TeI₆ thick film device is realized, with the value of 192 nC·R^–1cm^–2 under 40 kVp X-rays at an electrical field of 250 V·cm^–1, which is ∼20 times higher than that of the hybrid 3D perovskite polycrystalline film X-ray detectors. X-ray imaging based on Cs₂TeI₆ perovskite films will require lower radiation doses in many medical and security check applications.

We show an example of hierarchically designing electronic bands of PbSe toward excellent thermoelectric performance. We find that alloying 15 mol % PbTe into PbSe causes a negligible change in the light and heavy valence band energy offsets (ΔE_V) of PbSe around room temperature; however, with rising temperature it makes ΔE_V decrease at a significantly higher rate than in PbSe. In other words, the temperature-induced valence band convergence of PbSe is accelerated by alloying with PbTe. On this basis, applying 3 mol % Cd substitution on the Pb sites of PbSe_0.85Te_0.15 decreases ΔE_V and enhances the Seebeck coefficient at all temperatures. Excess Cd precipitates out as CdSe_1–yTe_y, whose valence band aligns with that of the p-type Na-doped PbSe_0.85Te_0.15 matrix. This enables facile charge transport across the matrix/precipitate interfaces and retains the high carrier mobilities. Meanwhile, compared to PbSe the lattice thermal conductivity of PbSe_0.85Te_0.15 is significantly decreased to its amorphous limit of 0.5 W m^–1 K^–1. Consequently, a highest peak ZT of 1.7 at 900 K and a record high average ZT of ∼1 (400–900 K) for a PbSe-based system are achieved in the composition Pb_0.95Na_0.02Cd_0.03Se_0.85Te_0.15, which are ∼70% and ∼50% higher than those of Pb_0.98Na_0.02Se control sample, respectively.

The chalcohalide compound Hg₃Se₂I₂ with a defect anti-perovskite structure has been demonstrated to be a promising semiconductor for room temperature X- and γ-ray detection. In this work, we use transport agents during the vapor growth of Hg₃Se₂I₂ crystals under gradient temperature profiles to dramatically improve the size and yield of Hg₃Se₂I₂ single crystals. Various growth conditions with combinations of organic polymer (polyethylene) with elemental Hg, Se, or I₂ are compared. The largest single crystals (with size up to 7 × 5 × 3.5 mm³) were obtained using both polyethylene and excess I₂ as the transport agents. The as-prepared detector devices based on these crystals have excellent photo response to a series of radiation sources, including low flux X-ray source, alpha particles, and γ-rays. The X-ray induced photocurrent of Hg₃Se₂I₂ detectors is 3 orders of magnitude higher than the dark current, indicating excellent X-ray photosensitivity. Under ²⁴¹Am α particle source (5.5 MeV), the best energy resolution obtained is ∼8.1%. The Hg₃Se₂I₂ device also shows improved detector performance under ⁵⁷Co and ¹³⁷Cs γ-ray sources. The improved crystal growth and detector performance using this polymer additive during the vapor transport process further confirms the great potential for the development of Hg₃Se₂I₂ for radiation detection.

The power conversion efficiency (PCE) of halide perovskite solar cells is now comparable to that of commercial solar cells. These solar cells are generally based on multication mixed-halide perovskite absorbers with nonideal band gaps of 1.5–1.6 eV. The PCE should be able to rise further if the solar cells could use narrower-band gap absorbers (1.2–1.4 eV). Reducing the Pb content of the semiconductors without sacrificing performance is also a significant driver in the perovskite solar cell research. Here, we demonstrate that mixed Pb/Sn-based perovskites containing the oversized ethylenediammonium (en) dication, {en}FA_0.5MA_0.5Sn_0.5Pb_0.5I₃ (FA = formamidinium, MA = methylammonium), can exhibit ideal band gaps of 1.27–1.38 eV, suitable for the assembly of single-junction solar cells with higher efficiencies. The use of en dication creates a three-dimensional (3D) hollow inorganic perovskite structure, which was verified through crystal density measurements and single-crystal X-ray diffraction structural analysis as well as nuclear magnetic resonance measurements. The {en}FA_0.5MA_0.5Sn_0.5Pb_0.5I₃ structure has massive Pb/Sn vacancies and much higher chemical stability than the same structure without en and vacancies. This new property reduces the dark current and carrier trap density and increases the carrier lifetime of the Pb/Sn-based perovskite films. Therefore, solar cells using {en}FA_0.5MA_0.5Sn_0.5Pb_0.5I₃ light absorbers have substantially enhanced air stability and around 20% improvement in efficiency. After overlaying a thin MABr top layer, we found that the {5% en}FA_0.5MA_0.5Sn_0.5Pb_0.5I₃ material gives an optimized PCE of 17.04%. The results highlight the strong promise of 3D hollow mixed Pb/Sn perovskites in achieving ideal band gap materials with higher chemical stability and lower Pb content for high-performance single-junction solar cells or multijunction solar cells serving as bottom cells.

Two-dimensional (2D) hybrid halide perovskites are promising in optoelectronic applications, particularly solar cells and light-emitting devices (LEDs), and for their increased stability as compared to 3D perovskites. Here, we report a new series of structures using propylammonium (PA⁺), which results in a series of Ruddlesden–Popper (RP) structures with the formula (PA)₂(MA)_n−1Pb_nI_3n+1 (n = 3, 4) and a new homologous series of “step-like” (SL) structures where the PbI₆ octahedra connect in a corner- and face-sharing motif with the general formula (PA)_2m+4(MA)_m−2Pb_2m+1I_7m+4 (m = 2, 3, 4). The RP structures show a blue-shift in bandgap for decreasing n (1.90 eV for n = 4 and 2.03 eV for n = 3), while the SL structures have an even greater blue-shift (2.53 eV for m = 4, 2.74 eV for m = 3, and 2.93 eV for m = 2). DFT calculations show that, while the RP structures are electronically 2D quantum wells, the SL structures are electronically 1D quantum wires with chains of corner-sharing octahedra “insulated” by blocks of face-sharing octahedra. Dark measurements for RP crystals show high resistivity perpendicular to the layers (10¹¹ Ω cm) but a lower resistivity parallel to them (10⁷ Ω cm). The SL crystals have varying resistivity in all three directions, confirming both RP and SL crystals’ utility as anisotropic electronic materials. The RP structures show strong photoresponse, whereas the SL materials exhibit resistivity trends that are dominated by ionic transport and no photoresponse. Solar cells were made with n = 3 giving an efficiency of 7.04% (average 6.28 ± 0.65%) with negligible hysteresis.

PbSe is an attractive thermoelectric material due to its favorable electronic structure, high melting point, and lower cost compared to PbTe. Herein, the hitherto unexplored alloys of PbSe with NaSbSe₂ (NaPb_mSbSe_m+2) are described and the most promising p-type PbSe-based thermoelectrics are found among them. Surprisingly, it is observed that below 500 K, NaPb_mSbSe_m+2 exhibits unorthodox semiconducting-like electrical conductivity, despite possessing degenerate carrier densities of ≈10²⁰ cm⁻³. It is shown that the peculiar behavior derives from carrier scattering by the grain boundaries. It is further demonstrated that the high solubility of NaSbSe₂ in PbSe augments both the thermoelectric properties while maintaining a rock salt structure. Namely, density functional theory calculations and photoemission spectroscopy demonstrate that introduction of NaSbSe₂ lowers the energy separation between the L- and Σ-valence bands and enhances the power factors under 700 K. The crystallographic disorder of Na⁺, Pb²⁺, and Sb³⁺ moreover provides exceptionally strong point defect phonon scattering yielding low lattice thermal conductivities of 1–0.55 W m^-1 K^-1 between 400 and 873 K without nanostructures. As a consequence, NaPb₁₀SbSe₁₂ achieves maximum ZT ≈1.4 near 900 K when optimally doped. More importantly, NaPb₁₀SbSe₁₂ maintains high ZT across a broad temperature range, giving an estimated record ZT_avg of ≈0.64 between 400 and 873 K, a significant improvement over existing p-type PbSe thermoelectrics.

There is strong interest in improving the environmental stability of hybrid perovskite solar cells while maintaining high efficiency. Here, we solve this problem by using epilayers of a wide-band-gap 1D lead iodide perovskitoid structure, based on a short organic cation, namely, thiazole ammonium (TA) in the form of lead iodide (TAPbI₃). The 1D capping layer serves to passivate three-dimensional (3D) perovskite films, which promotes charge transport, improves carrier lifetime, and prevents iodide ion migration of the 3D (MA,FA)PbI₃ film (MA = methylammonium, FA = formamidinium). Furthermore, the corresponding device achieved considerable efficiency and better environmental stability than the -based analogue, delivering a champion PCE value of 18.97% while retaining 92% of this efficiency under ambient conditions in air for 2 months. These findings suggest that utilization of a 1D perovskitoid is an effective strategy to improve the environmental stability of 3D-based perovskite solar cell devices maintaining at the same time their high efficiency.

The first solid-state solar cells, fabricated ≈140 years ago, were based on selenium; these early studies initiated the modern research on photovoltaic materials. Selenium shows high absorption coefficient and mobility, making it an attractive absorber for high bandgap thin film solar cells. Moreover, the simplicity of a single element absorber, its low-temperature processing, and intrinsic environmental stability enable the utilization of selenium in extremely cheap and scalable solar cells. In this paper, a detailed study of selenium solar cell fabrication is presented, and the key factors that affect the selenium film morphology and the resulting device efficiency are presented. Specifically, the crystallization process from amorphous film into functional crystalline device is studied. The importance of controlling the process is shown, and methods to align the growth orientation are suggested. Finally, the crystallization process under illumination, which has general importance for the fabrication of thin film photovoltaics, is investigated. Specifically for selenium, the illumination significantly improves the film morphology and leads to device efficiency of 5.2%, with open-circuit voltage of 0.911 V, short-circuit current density of 10.2 mA cm⁻², and fill factor of 55.0%. These findings form a solid foundation for future improvements of the photovoltaic material and device architecture.

Semiconductors possessing both magnetic and optoelectronic properties are rare and promise applications in opto-spintronics. Here we report the mixed-anion semiconductor BaFMn_0.5Te with a band gap of 1.76 eV and a work function of 5.08 eV, harboring both antiferromagnetism (AFM) and strong red photoluminescence (PL). The synthesis of BaFMn_0.5Te in quantitative yield was accomplished using the “panoramic synthesis” technique and synchrotron radiation to obtain the full reaction map, from which we determined that the compound forms upon heating at 850 °C via an intermediate unknown phase. The structure refinement required the use of a (3+1)-dimensional superspace group Cmme(α01/2)0ss. The material crystallizes into a ZrCuSiAs-like structure with alternating [BaF]⁺ and [Mn_0.5Te]⁻ layers and has a commensurately modulated structure with the q-vector of 1/6a* + 1/6b* + 1/2c* at room temperature arising from the unique ordering pattern of Mn²⁺ cations. Long-range AFM order emerges below 90 K, with two-dimensional short-range AFM correlations above the transition temperature. First-principles calculations indicate that BaFMn_0.5Te is an indirect band gap semiconductor with the gap opening between Te 5p and Mn 3d orbitals, and the magnetic interactions between nearest-neighbor Mn²⁺ atoms are antiferromagnetic. Steady-state PL spectra show a broad strong emission centered at ∼700 nm, which we believe originates from the energy manifolds of the modulated Mn²⁺ sublattice and its defects. Time-resolved PL measurements reveal an increase in excited-state lifetimes with longer probe wavelengths, from 93 ns (at 650 nm) to 345 ns (at 800 nm), and a delayed growth (6.5 ± 0.3 ns) in the kinetics at 800 nm with a concomitant decay (4.1 ± 0.1 ns) at 675 nm. Together, these observations suggest that there are multiple emissive states, with higher energy states populating lower energy states by energy transfer.