# Publications

2022
Hoffman, J. M. ; Hadar, I. ; Li, X. ; Ke, W. ; Vasileiadou, E. S. ; Strzalka, J. ; Chen, L. X. ; Kanatzidis, M. G. Film formation mechanisms in mixed-dimensional 2D/3D halide perovskite films revealed by in situ grazing-incidence wide-angle X-ray scattering. Chem 2022, 8 1067-1082. Abstract

Summary Mixed-dimensional 2D/3D hybrid halide perovskites retain the stability of 2D perovskites (formula (A′)2(A)n−1PbnI3n+1) and long diffusion lengths of the 3D materials (AMX3), thereby affording devices with extended stability as well as state-of-the art efficiencies approaching those of the 3D materials. These films are made by spin-coating precursor solutions with an arbitrarily large average layer thickness n (⟨n⟩ > 7) to give films with both 2D and 3D phases. Although the 2D and 3D perovskite film formation mechanisms have been studied, little is understood about composite 2D/3D film formation. We used in-situ grazing-incidence wide-angle scattering with synchrotron radiation to characterize the films fabricated from precursor solutions with stoichiometries of (BA)2(MA)n−1PbnI3n+1 (⟨n⟩ = 3, 4, 5, 7, 12, 50, and ∞ (MAPbI3)). Four different mechanisms are seen depending on the stoichiometry in the precursor solution. Kinetic analysis shows faster and earlier growth of the solvate with increasing ⟨n⟩.

He, Y. ; Hadar, I. ; De Siena, M. C. ; Klepov, V. V. ; Pan, L. ; Chung, D. Y. ; Kanatzidis, M. G. Sensitivity and Detection Limit of Spectroscopic-Grade Perovskite CsPbBr3 Crystal for Hard X-Ray Detection. Advanced Functional Materials 2022, 2112925. Abstract

Spectroscopic-grade single crystal detectors can register the energies of individual X-ray interactions enabling photon-counting systems with superior resolution over traditional photoconductive X-ray detection systems. Current technical challenges have limited the preparation of perovskite semiconductors for energy-discrimination X-ray photon-counting detection. Here, this work reports the deployment of a spectroscopic-grade CsPbBr3 Schottky detector under reverse bias for continuum hard X-ray detection in both the photocurrent and spectroscopic schemes. High surface barriers of ≈1 eV are formed by depositing solid bismuth and gold contacts. The spectroscopic response under a hard X-ray source is assessed in resolving the characteristic X-ray peak. The methodology in enhancing X-ray sensitivity by controlling the X-ray energies and flux, and voltage, is described. The X-ray sensitivity varies between a few tens to over 8000 μC Gyair−1 cm−2. The detectable dose rate of the CsPbBr3 detectors is as low as 0.02 nGyair s−1 in the energy discrimination configuration. Finally, the unbiased CsPbBr3 device forms a spontaneous contact potential difference of about 0.7 V enabling high quality of the CsPbBr3 single crystals to operate in “passive” self-powered X-ray detection mode and the X-ray sensitivity is estimated as 14 μC Gyair−1 cm−2. The great potential of spectroscopic-grade CsPbBr3 devices for X-ray photon-counting systems is anticipated in this work.

He, Y. ; Hadar, I. ; Kanatzidis, M. G. Detecting ionizing radiation using halide perovskite semiconductors processed through solution and alternative methods. Nature Photonics 2022, 16, 14-26. Abstract

The direct detection of high-energy radiation such as X-rays and γ-rays by semiconductors at room temperature is a challenging proposition that requires remarkably pure and nearly perfect crystals. The emergence of metal halide perovskites, defect-tolerant semiconductors, is reviving hope for new materials in this field after an almost 20 year hiatus. Metal halide perovskites, which combine exceptional optoelectronic properties, versatile chemistry and simple synthesis, are challenging traditional approaches for the development of novel semiconductors for detecting hard radiation. We discuss the relevant physical properties, promising materials, fabrication techniques and device architectures for high-performance, low-cost detectors by targeting next-generation semiconductors for radiation detection. We also present a perspective on the impact of such advances in future medical imaging applications.

2021
He, Y. ; Petryk, M. ; Liu, Z. ; Chica, D. G. ; Hadar, I. ; Leak, C. ; Ke, W. ; Spanopoulos, I. ; Lin, W. ; Chung, D. Y. ; et al. CsPbBr3 perovskite detectors with 1.4% energy resolution for high-energy γ-rays. NATURE PHOTONICS 2021, 15, 36-42. Abstract

Halide perovskite semiconductors are poised to revitalize the field of ionizing radiation detection as they have done to solar photovoltaics. We show that all-inorganic perovskite CsPbBr3 devices resolve 137Cs 662-keV γ-rays with 1.4% energy resolution, as well as other X- and γ-rays with energies ranging from tens of keV to over 1 MeV in ambipolar sensing and unipolar hole-only sensing modes with crystal volumes of 6.65 mm3 and 297 mm3, respectively. We report the scale-up of CsPbBr3 ingots to up to 1.5 inches in diameter with an excellent hole mobility–lifetime product of 8 × 10−3 cm2 V−1 and a long hole lifetime of up to 296 μs. CsPbBr3 detectors demonstrate a wide temperature region from ~2 °C to ~70 °C for stable operation. Detectors protected with suitable encapsulants show a uniform response for over 18 months. Consequently, we identify perovskite CsPbBr3 semiconductor as an exceptional candidate for new-generation high-energy γ-ray detection.

He, Y. ; Stoumpos, C. C. ; Hadar, I. ; Luo, Z. ; McCall, K. M. ; Liu, Z. ; Chung, D. Y. ; Wessels, B. W. ; Kanatzidis, M. G. Demonstration of Energy-Resolved γ-Ray Detection at Room Temperature by the CsPbCl3 Perovskite Semiconductor. JOURNAL OF THE AMERICAN CHEMICAL SOCIETY 2021, 143, 2068-2077. Abstract

The detection of γ-rays at room temperature with high-energy resolution using semiconductors is one of the most challenging applications. The presence of even the smallest amount of defects is sufficient to kill the signal generated from γ-rays which makes the availability of semiconductors detectors a rarity. Lead halide perovskite semiconductors exhibit unusually high defect tolerance leading to outstanding and unique optoelectronic properties and are poised to strongly impact applications in photoelectric conversion/detection. Here we demonstrate for the first time that large size single crystals of the all-inorganic perovskite CsPbCl3 semiconductor can function as a high-performance detector for γ-ray nuclear radiation at room temperature. CsPbCl3 is a wide-gap semiconductor with a bandgap of 3.03 eV and possesses a high effective atomic number of 69.8. We identified the two distinct phase transitions in CsPbCl3, from cubic (Pm-3m) to tetragonal (P4/mbm) at 325 K and finally to orthorhombic (Pbnm) at 316 K. Despite crystal twinning induced by phase transitions, CsPbCl3 crystals in detector grade can be obtained with high electrical resistivity of ~1.7 X 109 Ω·cm. The crystals were grown from the melt with volume over several cubic centimeters and have a low thermal conductivity of 0.6 W m-1 K-1. The mobilities for electron and hole carriers were determined to ~30 cm2/(V s). Using photoemission yield spectroscopy in air (PYSA), we determined the valence band maximum at 5.66 ± 0.05 eV. Under gamma-ray exposure, our Schottky-type planar CsPbCl3 detector achieved an excellent energy resolution (~16% at 122 keV) accompanied by a high figure-of-merit hole mobility-lifetime product 3.2 x 10-4 cm2/V and a long hole lifetime (16 μs). The results demonstrate considerable defect tolerance of CsPbCl3 and suggest its strong potential for γ-radiation and X-ray detection at room temperature and above.

Sidhik, S. ; Wang, Y. ; Li, W. ; Zhang, H. ; Zhong, X. ; Agrawal, A. ; Hadar, I. ; Spanopoulos, I. ; Mishra, A. ; Traore, B. ; et al. High-phase purity two-dimensional perovskites with 17.3% efficiency enabled by interface engineering of hole transport layer. CELL REPORTS PHYSICAL SCIENCE 2021, 2. Abstract

State-of-the-art p-i-n-based 3D perovskite solar cells (PSCs) use nickel oxide (NiOx) as an efficient hole transport layer (HTL), achieving efficiencies >22%. However, translating this to phase-pure 2D perovskites has been unsuccessful. Here, we report 2D phase-pure Ruddlesden-Popper BA2MA3Pb4I13 perovskites with 17.3% efficiency enabled by doping the NiOx with Li. Our results show that progressively increasing the doping concentration transforms the photoresistor behavior to a typical diode curve, with an increase in the average efficiency from 2.53% to 16.03% with a high open-circuit voltage of 1.22 V. Analysis reveals that Li doping of NiOx significantly improves the morphology, crystallinity, and orientation of 2D perovskite films and also affords a superior band alignment, facilitating efficient charge extraction. Finally, we demonstrate that 2D PSCs with Li-doped NiOx exhibit excellent photostability, with T99 = 400 h at 1 sun and T90 of 100 h at 5 suns measured at relative humidity of 60% ± 5% without the need for external thermal management.

Lin, W. ; He, J. ; McCall, K. M. ; Stoumpos, C. C. ; Liu, Z. ; Hadar, I. ; Das, S. ; Wang, H. - H. ; Wang, B. - X. ; Chung, D. Y. ; et al. Inorganic Halide Perovskitoid TlPbI3 for Ionizing Radiation Detection. ADVANCED FUNCTIONAL MATERIALS 2021, 31. Abstract

Room temperature semiconductor detector (RTSD) materials for γ-ray and X-ray radiation are in great demand for the nonproliferation of nuclear materials as well as for biomedical imaging applications. Halide perovskites have attracted great attention as emerging and promising RTSD materials. In this contribution, the material synthesis, purification, crystal growth, crystal structure, photoluminescence properties, ionizing radiation detection performance, and electronic structure of the inorganic halide perovskitoid compound TlPbI3 are reported on. This compound crystallizes in the ABX3 non-perovskite crystal structure with a high density of d = 6.488 g·cm–3, has a wide bandgap of 2.25 eV, and melts congruently at a low temperature of 360 °C without phase transitions, which allows for facile growth of high quality crystals with few thermally-activated defects. High-quality TlPbI3 single crystals of centimeter-size are grown using the vertical Bridgman method using purified raw materials. A high electrical resistivity of ~1012 Ω·cm is readily obtainable, and detectors made of TlPbI3 single crystals are highly photoresponsive to Ag Kα X-rays (22.4 keV), and detects 122 keV γ-rays from 57Co radiation source. The electron mobility-lifetime product µeτe was estimated at 1.8 x 10-5 cm2·V–1. A high relative static dielectric constant of 35.0 indicates strong capability in screening carrier scattering and charged defects in TlPbI3.

Vasileiadou, E. S. ; Wang, B. ; Spanopoulos, I. ; Hadar, I. ; Navrotsky, A. ; Kanatzidis, M. G. Insight on the Stability of Thick Layers in 2D Ruddlesden-Popper and Dion-Jacobson Lead Iodide Perovskites. JOURNAL OF THE AMERICAN CHEMICAL SOCIETY 2021, 143, 2523-2536. Abstract

Two-dimensional (2D) hybrid organic–inorganic halide perovskites are a preeminent class of low-cost semiconductors whose inherent structural tunability and attractive photophysical properties have led to the successful fabrication of solar cells with high power conversion efficiencies. Despite the observed superior stability of 2D lead iodide perovskites over their 3D parent structures, an understanding of their thermochemical profile is missing. Herein, the calorimetric studies reveal that the Ruddlesden–Popper (RP) series, incorporating the monovalent-monoammonium spacer cations of pentylammonium (PA) and hexylammonium (HA): (PA)2(MA)n-1PbnI3n+1 (n = 2–6) and (HA)2(MA)n-1PbnI3n+1 (n = 2–4) have a negative enthalpy of formation, relative to their binary iodides. In contrast, the enthalpy of formation for the Dion–Jacobson (DJ) series, incorporating the divalent and cyclic diammonium cations of 3- and 4-(aminomethyl)piperidinium (3AMP and 4AMP respectively): (3AMP)(MA)n-1PbnI3n+1 (n = 2–5) and (4AMP)(MA)n-1PbnI3n+1 (n = 2–4) have a positive enthalpy of formation. In addition, for the (PA)2(MA)n−1PbnI3n+1 family of materials, we report the phase-pure synthesis and single crystal structure of the next member of the series (PA)2(MA)5Pb6I19 (n = 6), and its optical properties, marking this the second n = 6, bulk member published to date. Particularly, (PA)2(MA)5Pb6I19 (n = 6) has negative enthalpy of formation as well. Additionally, the analysis of the structural parameters and optical properties between the examined RP and DJ series offers guiding principles for the targeted design and synthesis of 2D perovskites for efficient solar cell fabrication. Although the distortions of the Pb–I–Pb equatorial angles are larger in the DJ series, the significantly smaller I···I interlayer distances lead to overall smaller band gap values, in comparison with the RP series. Our film stability studies on the RP and DJ perovskites series reveal consistent observations with the thermochemical findings, pointing out to the lower extrinsic stability of the DJ materials in ambient air.

Vasileiadou, E. S. ; Hadar, I. ; Kepenekian, M. ; Even, J. ; Tu, Q. ; Malliakas, C. D. ; Friedrich, D. ; Spanopoulos, I. ; Hoffman, J. M. ; Dravid, V. P. ; et al. Shedding Light on the Stability and Structure-Property Relationships of Two-Dimensional Hybrid Lead Bromide Perovskites. CHEMISTRY OF MATERIALS 2021, 33, 5085-5107. Abstract

Spanopoulos, I. ; Hadar, I. ; Ke, W. ; Guo, P. ; Mozur, E. M. ; Morgan, E. ; Wang, S. ; Zheng, D. ; Padgaonkar, S. ; Reddy, G. N. M. ; et al. Tunable Broad Light Emission from 3D Hollow'' Bromide Perovskites through Defect Engineering. JOURNAL OF THE AMERICAN CHEMICAL SOCIETY 2021, 143, 7069-7080. Abstract

Hybrid halide perovskites consisting of corner-sharing metal halide octahedra and small cuboctahedral cages filled with counter cations have proven to be prominent candidates for many high-performance optoelectronic devices. The stability limits of their three-dimensional perovskite framework are defined by the size range of the cations present in the cages of the structure. In some cases, the stability of the perovskite-type structure can be extended even when the counterions violate the size and shape requirements, as is the case in the so-called “hollow” perovskites. In this work, we engineered a new family of 3D highly defective yet crystalline “hollow” bromide perovskites with general formula (FA)1–x(en)x(Pb)1–0.7x(Br)3–0.4x (FA = formamidinium (FA+), en = ethylenediammonium (en2+), x = 0–0.44). Pair distribution function analysis shed light on the local structural coherence, revealing a wide distribution of Pb–Pb distances in the crystal structure as a consequence of the Pb/Br-deficient nature and en inclusion in the lattice. By manipulating the number of Pb/Br vacancies, we finely tune the optical properties of the pristine FAPbBr3 by blue shifting the band gap from 2.20 to 2.60 eV for the x = 0.42 en sample. A most unexpected outcome was that at > 0.33 en incorporation, the material exhibits strong broad light emission (1% photoluminescence quantum yield (PLQY)) that is maintained after exposure to air for more than a year. This is the first example of strong broad light emission from a 3D hybrid halide perovskite, demonstrating that meticulous defect engineering is an excellent tool for customizing the optical properties of these semiconductors.

2020
Cai, S. ; Hao, S. ; Luo, Z. - Z. ; Li, X. ; Hadar, I. ; Bailey, T. ; Hu, X. ; Uher, C. ; Hu, Y. - Y. ; Wolverton, C. ; et al. Discordant nature of Cd in PbSe: off-centering and core-shell nanoscale CdSe precipitates lead to high thermoelectric performance. ENERGY & ENVIRONMENTAL SCIENCE 2020, 13, 200-211. Abstract

We report a novel hierarchical microstructure in the PbSe–CdSe system, which collectively contributes to significant enhancement in thermoelectric performance, with ZTave ∼ 0.83 across the 400–923 K temperature range, the highest reported for p-type, Te-free PbSe systems. We have investigated the local atomic structure as well as the microstructure of a series of PbSe–xCdSe materials, up to x = 10%. We find that the behavior of the Cd atoms in the octahedral rock salt sites is discordant and results in off-center displacement and distortion. Such off-centered Cd in the PbSe matrix creates (1) L–Σ electronic energy band convergence, (2) a flattened L band, both contributing to higher Seebeck coefficients, and (3) enhanced phonon scattering, which leads to lower thermal conductivity. These conclusions are supported by photoemission yield spectroscopy in air (PYSA), solid state 111Cd, 77Se NMR spectroscopy and DFT calculations. Above the solubility limit (>6%CdSe), we also observe endotaxial CdSe nano-precipitates with core–shell architecture formed in PbSe, whose size, distribution and structure gradually change with the Cd content. The nano-precipitates exhibit a zinc blende crystal structure and a tetrahedral shape with significant local strain, but are covered with a thin wurtzite layer along the precipitate/matrix interface, creating a core–shell structure embedded in PbSe. This newly discovered architecture causes a further reduction in lattice thermal conductivity. Moreover, potassium is found to be an effective p-type dopant in the PbSe–CdSe system, leading to an enhanced power factor, a maximum ZT of ∼1.4 at 923 K for Pb0.98K0.02Se–6%CdSe.

Luo, Y. ; Cai, S. ; Hao, S. ; Pielnhofer, F. ; Hadar, I. ; Luo, Z. - Z. ; Xu, J. ; Wolverton, C. ; Dravid, V. P. ; Pfitzner, A. ; et al. High-Performance Thermoelectrics from Cellular Nanostructured Sb2Si2Te6. JOULE 2020, 4 159-175. Abstract

We introduce Sb2Si2Te6 as a high-performance thermoelectric material. Single-crystal X-ray diffraction analysis indicates that Sb2Si2Te6 has a layered two-dimensional structure with Sb3+ cations and [Si2Te6]6− units as building blocks adopting the Fe2P2Se6 structure type. Sb2Si2Te6 is a direct-band-gap semiconductor with valence-band maximum and conduction-band minimum at the Z point in the Brillouin zone, where the band is doubly degenerate. Polycrystalline bulk pellets of Sb2Si2Te6 with randomly packed grains exhibit an intrinsically high thermoelectric figure of merit ZT of ∼1.08 at 823 K. We then create a cellular nanostructure with ultrathin Si2Te3 nanosheets covering the Sb2Si2Te6 grains, which act as a hole-transmitting electron-blocking filter and at the same time cause extra phonon scattering. This dual function of the cellular nanostructure achieves an ultralow thermal conductivity value of ∼0.29 Wm−1K−1 and a high ZT value of ∼1.65 at 823 K for Sb2Si2Te6, along with a high average ZT value of ∼0.98.

Khoury, J. F. ; He, J. ; Pfluger, J. E. ; Hadar, I. ; Balasubramanian, M. ; Stoumpos, C. C. ; Zu, R. ; Gopalan, V. ; Wolverton, C. ; Kanatzidis, M. G. Ir6In32S21, a polar, metal-rich semiconducting subchalcogenide. CHEMICAL SCIENCE 2020, 11, 870-878. Abstract

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 (1.19me) are substantially smaller than the average electron masses near the CBM (2.51me), 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.

Li, X. ; Fu, Y. ; Pedesseau, L. ; Guo, P. ; Cuthriell, S. ; Hadar, I. ; Even, J. ; Katan, C. ; Stoumpos, C. C. ; Schaller, R. D. ; et al. Negative Pressure Engineering with Large Cage Cations in 2D Halide Perovskites Causes Lattice Softening. JOURNAL OF THE AMERICAN CHEMICAL SOCIETY 2020, 142, 11486-11496. Abstract

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.

Spanopoulos, I. ; Hadar, I. ; Ke, W. ; Guo, P. ; Sidhik, S. ; Kepenekian, M. ; Even, J. ; Mohite, A. D. ; Schaller, R. D. ; Kanatzidis, M. G. Water-Stable 1D Hybrid Tin(II) Iodide Emits Broad Light with 36% Photoluminescence Quantum Efficiency. JOURNAL OF THE AMERICAN CHEMICAL SOCIETY 2020, 142, 9028-9038. Abstract
The optical and light emission properties of tin and lead halide perovskites are remarkable because of the robust room-temperature (RT) performance, broad wavelength tunability, high efficiency, and good quenching resistance to defects. These highly desirable attributes promise to transform current light-emitting devices, phosphors, and lasers. One disadvantage in most of these materials is the sensitivity to moisture. Here, we report a new air-stable one-dimensional (1D) hybrid lead-free halide material (DAO)Sn2I6 (DAO, 1,8-octyldiammonium) that is resistant to water for more than 15 h. The material exhibits a sharp optical absorption edge at 2.70 eV and a strong broad orange light emission centered at 634 nm, with a full width at half-maximum (fwhm) of 142 nm (0.44 eV). The emission has a long photoluminescence (PL) lifetime of 582 ns, while the intensity is constant over a very broad temperature range (145–415 K) with a photoluminescence quantum yield (PLQY) of at least 20.3% at RT. Above 415 K the material undergoes a structural phase transition from monoclinic (C2/c) to orthorhombic (Ibam) accompanied by a red shift in the band gap and a quench in the photoluminescence emission. Density functional theory calculations support the trend in the optical properties and the 1D electronic nature of the structure, where the calculated carrier effective masses along the inorganic chain are significantly lower than those perpendicular to the chain. Thin films of the compound readily fabricated from solutions exhibit the same optical properties, but with improved PLQY of 36%, for a 60 nm thick film, among the highest reported for lead-free low-dimensional 2D and 1D perovskites and metal halides.

Hoffman, J. M. ; Strzalka, J. ; Flanders, N. C. ; Hadar, I. ; Cuthriell, S. A. ; Zhang, Q. ; Schaller, R. D. ; Dichtel, W. R. ; Chen, L. X. ; Kanatzidis, M. G. In Situ Grazing-Incidence Wide-Angle Scattering Reveals Mechanisms for Phase Distribution and Disorientation in 2D Halide Perovskite Films. ADVANCED MATERIALS 2020, 32. Abstract
2D hybrid halide perovskites with the formula (A′)2(A)n-1PbnI3n+1 have remarkable stability and promising efficiency in photovoltaic and optoelectronic devices, yet fundamental understanding of film formation, key to optimizing these devices, is lacking. Here, in situ grazing-incidence wide-angle X-ray scattering (GIWAXS) is used to monitor film formation during spin-coating. This elucidates the general film formation mechanism of 2D halide perovskites during one-step spin-coating. There are three stages of film formation: sol–gel, oriented 3D, and 2D. Three precursor phases form during the sol–gel stage and transform to perovskite, first giving a highly oriented 3D-like phase at the air/liquid interface followed by subsequent nucleations forming slightly less oriented 2D perovskite. Furthermore, heating before crystallization leads to fewer nucleations and faster removal of the precursors, improving orientation. This outlines the primary causes of phase distribution and perpendicular orientation in 2D perovskite films and paves the way for rationally designed film fabrication techniques.
Hoffman, J. M. ; Malliakas, C. D. ; Sidhik, S. ; Hadar, I. ; McClain, R. ; Mohite, A. D. ; Kanatzidis, M. G. Long periodic ripple in a 2D hybrid halide perovskite structure using branched organic spacers. CHEMICAL SCIENCE 2020, 11, 12139-12148. Abstract

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.

Ke, W. ; Chen, C. ; Spanopoulos, I. ; Mao, L. ; Hadar, I. ; Li, X. ; Hoffman, J. M. ; Song, Z. ; Yan, Y. ; Kanatzidis, M. G. Narrow-Bandgap Mixed Lead/Tin-Based 2D Dion-Jacobson Perovskites Boost the Performance of Solar Cells. JOURNAL OF THE AMERICAN CHEMICAL SOCIETY 2020, 142, 15049-15057. Abstract

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 MA0.5FA0.5Pb0.5Sn0.5I3 (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.

2019
Williamson, C. B. ; Nevers, D. R. ; Nelson, A. ; Hadar, I. ; Banin, U. ; Hanrath, T. ; Robinson, R. D. Chemically reversible isomerization of inorganic clusters. SCIENCE 2019, 363, 731-735. Abstract

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.

Xu, Y. ; Jiao, B. ; Song, T. - B. ; Stoumpos, C. C. ; He, Y. ; Hadar, I. ; Lin, W. ; Jie, W. ; Kanatzidis, M. G. Zero-Dimensional Cs2TeI6 Perovskite: Solution-Processed Thick Films with High X-ray Sensitivity. ACS PHOTONICS 2019, 6 196-203. Abstract

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.