Small-Band-Gap Halide Double Perovskites

Halide Perovskites Breathe Too: The Iodide-Iodine Equilibrium and Self-Doping in Cs2SnI6

The response of an oxide crystal to the atmosphere can be personified as breathing-a dynamic equilibrium between O2 gas and O2- anions in the solid. We characterize the analogous defect reaction in an iodide double-perovskite semiconductor, Cs2SnI6. Here, I2 gas is released from the crystal at room temperature, forming iodine vacancies. The iodine vacancy defect is a shallow electron donor and is therefore ionized at room temperature; thus, the loss of I2 is accompanied by spontaneous n-type self-doping. Conversely, at high I2 pressures, I2 gas is resorbed by the perovskite, consuming excess electrons as I2 is converted to 2I-. Halide mobility and irreversible halide loss or exchange reactions have been studied extensively in halide perovskites. However, the reversible exchange equilibrium between iodide and iodine [2I-(s) ↔ I2(g) + 2e-] described here has often been overlooked in prior studies, though it is likely general to halide perovskites and operative near room temperature, even in the dark. An analysis of the 2I-(s)/I2(g) equilibrium thermodynamics and related transport kinetics in single crystals of Cs2SnI6 therefore provides insight toward achieving stable composition and electronic properties in the large family of iodide perovskite semiconductors.

Lanthanide Doping into All-Inorganic Heterometallic Halide Layered Double Perovskite Nanocrystals for Multimodal Visible and Near-Infrared Emission

The introduction of lanthanide ions (Ln3+) into all-inorganic lead-free halide perovskites has captured significant attention in optoelectronic applications. However, doping Ln3+ ions into heterometallic halide layered double perovskite (LDP) nanocrystals (NCs) and their associated doping mechanisms remain unexplored. Herein, we report the first colloidal synthesis of Ln3+ (Yb3+, Er3+)-doped LDP NCs utilizing a modified hot-injection method. The resulting NCs exhibit efficient near-infrared (NIR) photoluminescence in both NIR-I and NIR-II regions, achieved through energy transfer down-conversion mechanisms. Density functional theory calculations reveal that Ln3+ dopants preferentially occupy the Sb3+ cation positions, resulting in a disruption of local site symmetry of the LDP lattices. By leveraging sensitizations of intermediate energy levels, we delved into a series of Ln3+-doped Cs4M(II)Sb2Cl12 (M(II): Cd2+ or Mn2+) LDP NCs via co-doping strategies. Remarkably, we observe a brightening effect of the predark states of Er3+ dopant in the Er3+-doped Cs4M(II)Sb2Cl12 LDP NCs owing to the Mn component acting as an intermediate energy bridge. This study not only advances our understanding of energy transfer mechanisms in doped NCs but also propels all-inorganic LDP NCs for a wider range of optoelectronic applications.

Excitons in metal-halide perovskites from first-principles many-body perturbation theory

Metal-halide perovskites are a structurally, chemically, and electronically diverse class of semiconductors with applications ranging from photovoltaics to radiation detectors and sensors. Understanding neutral electron-hole excitations (excitons) is key for predicting and improving the efficiency of energy-conversion processes in these materials. First-principles calculations have played an important role in this context, allowing for a detailed insight into the formation of excitons in many different types of perovskites. Such calculations have demonstrated that excitons in some perovskites significantly deviate from canonical models due to the chemical and structural heterogeneity of these materials. In this Perspective, I provide an overview of calculations of excitons in metal-halide perovskites using Green's function-based many-body perturbation theory in the GW + Bethe-Salpeter equation approach, the prevalent method for calculating excitons in extended solids. This approach readily considers anisotropic electronic structures and dielectric screening present in many perovskites and important effects, such as spin-orbit coupling. I will show that despite this progress, the complex and diverse electronic structure of these materials and its intricate coupling to pronounced and anharmonic structural dynamics pose challenges that are currently not fully addressed within the GW + Bethe-Salpeter equation approach. I hope that this Perspective serves as an inspiration for further exploring the rich landscape of excitons in metal-halide perovskites and other complex semiconductors and for method development addressing unresolved challenges in the field.

Improving Low-temperature Performance and Stability of Na2 Ti6 O13 Anodes by the Ti-O Spring Effect through Nb-doping

Na2 Ti6 O13 (NTO) with high safety has been regarded as a promising anode candidate for sodium-ion batteries. In the present study, integrated modification of migration channels broadening, charge density re-distribution, and oxygen vacancies regulation are realized in case of Nb-doping and have obtained significantly enhanced cycling performance with 92 % reversible capacity retained after 3000 cycles at 3000 mA g-1 . Moreover, unexpected low-temperature performance with a high discharge capacity of 143 mAh g-1 at 100 mA g-1 under -15 °C is also achieved in the full cell. Theoretical investigation suggests that Nb preferentially replaces Ti3 sites, which effectively improves structural stability and lowers the diffusion energy barrier. What's more important, both the in situ X-ray diffraction (XRD) and in situ Raman furtherly confirm the robust spring effect of the Ti-O bond, making special charge compensation mechanism and respective regulation strategy to conquer the sluggish transport kinetics and low conductivity, which plays a key role in promoting electrochemical performance.

Understanding the evolution of double perovskite band structure upon dimensional reduction

Recent investigations into the effects of dimensional reduction on halide double perovskites have revealed an intriguing change in band structure when the three-dimensional (3D) perovskite is reduced to a two-dimensional (2D) perovskite with inorganic sheets of monolayer thickness (n = 1). The indirect bandgap of 3D Cs2AgBiBr6 becomes direct in the n = 1 perovskite whereas the direct bandgap of 3D Cs2AgTlBr6 becomes indirect at the n = 1 limit. Here, we apply a linear combination of atomic orbitals approach to uncover the orbital basis for this bandgap symmetry transition with dimensional reduction. We adapt our previously established method for predicting band structures of 3D double perovskites for application to their 2D congeners, emphasizing new considerations required for the 2D lattice. In particular, we consider the inequivalence of the terminal and bridging halides and the consequences of applying translational symmetry only along two dimensions. The valence and conduction bands of the layered perovskites can be derived from symmetry adapted linear combinations of halide p orbitals propagated across the 2D lattice. The dispersion of each band is then determined by the bonding and antibonding interactions of the metal and halide orbitals, thus affording predictions of the essential features of the band structure. We demonstrate this analysis for 2D Ag-Bi and Ag-Tl perovskites with sheets of mono- and bilayer thickness, establishing a detailed understanding of their band structures, which enables us to identify the key factors that drive the bandgap symmetry transitions observed at the n = 1 limit. Importantly, these insights also allow us to make the general prediction that direct → indirect or indirect → direct bandgap transitions in the monolayer limit are most likely in double perovskite compositions that involve participation of metal d orbitals at the band edges or that have no metal-orbital contributions to the valence band, laying the groundwork for the targeted realization of this phenomenon.

Synthesis and Optimization of Cs2B'B″X6 Double Perovskite for Efficient and Sustainable Solar Cells

Hybrid perovskite materials with high light absorption coefficients, long diffusion lengths, and high mobility have attracted much attention, but their commercial development has been seriously hindered by two major problems: instability and lead toxicity. This has led to lead-free halide double perovskite becoming a prominent competitor in the photovoltaic field. For lead-free double perovskites, Pb2+ can be heterovalent, substituted by non-toxic metal cations as a double perovskite structure, which promotes the flexibility of the composition. However, the four component elements and low solubility in the solvent result in synthesis difficulties and phase impurity problems. And material phase purity and film quality are closely related to the number of defects, which can limit the photoelectric performance of solar cells. Therefore, based on this point, we summarize the synthesis methods of Cs2B'B″X6 double perovskite crystals and thin films. Moreover, in the application of solar cells, the existing research mainly focuses on the formation process of thin films, band gap adjustment, and surface engineering to improve the quality of films and optimize the performance of devices. Finally, we propose that Cs2B'B″X6 lead-free perovskites offer a promising pathway toward developing highly efficient and stable perovskite solar cells.

Triangulating Dopant-Level Mn(II) Insertion in a Cs2NaBiCl6 Double Perovskite Using Magnetic Resonance Spectroscopy

Lead-free metal halide double perovskites are gaining increasing attention for optoelectronic applications. Specifically, doping metal halide double perovskites using transition metals enables broadband tailorability of the optical bandgap for these emerging semiconducting materials. One candidate material is Mn(II)-doped Cs₂NaBiCl₆, but the nature of Mn(II) insertion on chemical structure is poorly understood due to low Mn loading. It is critical to determine the atomic-level structure at the site of Mn(II) incorporation in doped perovskites to better understand the structure-property relationships in these materials and thus to advance their applicability to optoelectronic applications. Magnetic resonance spectroscopy is uniquely qualified to address this, and thus a comprehensive three-pronged strategy, involving solid-state nuclear magnetic resonance (NMR), high-field dynamic nuclear polarization (DNP), and electron paramagnetic resonance (EPR) spectroscopies, is used to identify the location of Mn(II) insertion in Cs₂NaBiCl₆. Multinuclear (²³Na, ³⁵Cl, ¹³³Cs, and ²⁰⁹Bi) one-dimensional (1D) magnetic resonance spectra reveal a low level of Mn(II) incorporation, with select spins affected by paramagnetic relaxation enhancement (PRE) induced by Mn(II) neighbors. EPR measurements confirm the oxidation state, octahedral symmetry, and low doping levels of the Mn(II) centers. Complementary EPR and NMR measurements confirm that the cubic structure is maintained with Mn(II) incorporation at room temperature, but the structure deviates slightly from cubic symmetry at low temperatures (<30 K). HYperfine Sublevel CORrelation (HYSCORE) EPR spectroscopy explores the electron-nuclear correlations of Mn(II) with ²³Na, ¹³³Cs, and ³⁵Cl. The absence of ²⁰⁹Bi correlations suggests that Bi centers are replaced by Mn(II). Endogenous DNP NMR measurements from Mn(II) → ¹³³Cs (<30 K) reveal that the solid effect is the dominant mechanism for DNP transfer and supports that Mn(II) is homogeneously distributed within the double-perovskite structure.

Tuning the Band Gap in the Halide Perovskite CsPbBr3 through Sr Substitution

The ability to continuously tune the band gap of a semiconductor allows its optical properties to be precisely tailored for specific applications. We demonstrate that the band gap of the halide perovskite CsPbBr₃ can be continuously widened through homovalent substitution of Sr²⁺ for Pb²⁺ using solid-state synthesis, creating a material with the formula CsPb_1-xSr_xBr₃ (0 ≤ x ≤ 1). Sr²⁺ and Pb²⁺ form a solid solution in CsPb_1-xSr_xBr₃. Pure CsPbBr₃ has a band gap of 2.29(2) eV, which increases to 2.64(3) eV for CsPb_0.25Sr_0.75Br₃. The increase in band gap is clearly visible in the color change of the materials and is also confirmed by a shift in the photoluminescence. Density-functional theory calculations support the hypothesis that Sr incorporation widens the band gap without introducing mid-gap defect states. These results demonstrate that homovalent B-site alloying can be a viable method to tune the band gap of simple halide perovskites for absorptive and emissive applications such as color-tunable light-emitting diodes, tandem solar cells, and photodetectors.

Halide Double-Perovskite Semiconductors beyond Photovoltaics

Halide double perovskites, A₂M^IM^IIIX₆, offer a vast chemical space for obtaining unexplored materials with exciting properties for a wide range of applications. The photovoltaic performance of halide double perovskites has been limited due to the large and/or indirect bandgap of the presently known materials. However, their applications extend beyond outdoor photovoltaics, as halide double perovskites exhibit properties suitable for memory devices, indoor photovoltaics, X-ray detectors, light-emitting diodes, temperature and humidity sensors, photocatalysts, and many more. This Perspective highlights challenges associated with the synthesis and characterization of halide double perovskites and offers an outlook on the potential use of some of the properties exhibited by this so far underexplored class of materials.

Rare-Earth Doping in Nanostructured Inorganic Materials

Impurity doping is a promising method to impart new properties to various materials. Due to their unique optical, magnetic, and electrical properties, rare-earth ions have been extensively explored as active dopants in inorganic crystal lattices since the 18th century. Rare-earth doping can alter the crystallographic phase, morphology, and size, leading to tunable optical responses of doped nanomaterials. Moreover, rare-earth doping can control the ultimate electronic and catalytic performance of doped nanomaterials in a tunable and scalable manner, enabling significant improvements in energy harvesting and conversion. A better understanding of the critical role of rare-earth doping is a prerequisite for the development of an extensive repertoire of functional nanomaterials for practical applications. In this review, we highlight recent advances in rare-earth doping in inorganic nanomaterials and the associated applications in many fields. This review covers the key criteria for rare-earth doping, including basic electronic structures, lattice environments, and doping strategies, as well as fundamental design principles that enhance the electrical, optical, catalytic, and magnetic properties of the material. We also discuss future research directions and challenges in controlling rare-earth doping for new applications.

Lead-free all-inorganic halide double perovskite materials for optoelectronic applications: progress, performance and design

The geometrical and electronic structures of all-inorganic halide double perovskites and their applications in optoelectronic devices are reviewed. Novel design methods are desirable to develop this type of perovskite with superior performance.

Perovskite White Light Emitting Diodes: Progress, Challenges, and Opportunities

As global warming, energy shortages, and environment pollution have intensified, low-carbon and energy-saving lighting technology has attracted great attention worldwide. Light emitting diodes (LEDs) have been around for decades and are considered to be the most ideal lighting technology currently due to their high luminescence efficiency (LE) and long lifespan. Besides, along with the development of modern technology, lighting technologies with higher performance and more functions are desired. Perovskite based LEDs (PeLEDs) have recently emerged as ideal candidates for lighting technology owing to the extraordinary photoelectric properties of perovskite, such as high photoluminescence quantum yields (PLQYs), easy wavelength tuning, and low-cost synthesis. Herein, we open this review by introducing the background of white LEDs (WLEDs), including their light-emitting mechanism, typical characteristics, and key indicators in applications. Then, four main approaches to fabricate WLEDs are discussed and compared. After that, in accordance with the four categories, we focus on the recent progress of white PeLEDs (Pe-WLEDs), followed by the challenges and opportunities for Pe-WLEDs in practical application. Meanwhile, some pertinent countermeasures to their challenges are put forward. Finally, the development promise of Pe-WLEDs is explored.

From Zero- to One-Dimensional, Opportunities and Caveats of Hybrid Iodobismuthates for Optoelectronic Applications

The association of the electron acceptor 4,4'-amino-bipyridinium (AmV²⁺) dication and BiI₃ in an acidic solution affords three organic-inorganic hybrid materials, (AmV)₃(BiI₆)₂ (1), (AmV)₂(Bi₄I₁₆) (2), and (AmV)BiI₅ (3), whose structures are based on isolated BiI₆^3- and Bi₄I₁₆^4- anion clusters in 1 and 2, respectively, and on a one-dimensional (1D) chain of trans-connected corner-sharing octahedra in 3. In contrast with known methylviologen-based hybrids, these compounds are more soluble in polar solvents, allowing thin film formation by spin-coating. (AmV)BiI₅ exhibits a broad absorption band in the visible region leading to an optical bandgap of 1.54 eV and shows a PV effect as demonstrated by a significant open-circuit voltage close to 500 mV. The electronic structure of the three compounds has been investigated using first-principles calculations based on density functional theory (DFT). Unexpectedly, despite the trans-connected corner-shared octahedra, for (AmV)BiI₅, the valence state shows no coupling along the wire direction, leading to a high effective mass for holes, while in contrast, the strong coupling between Bi 6p_x orbitals in the same direction at the conduction band minimum suggests excellent electron transport properties. This contributes to the low current output leading to the low efficiency of perovskite solar cells based on (AmV)BiI₅. Further insight is provided for trans- and cis-MI₅ 1D model structures (M = Bi or Pb) based on DFT investigations.

Optoelectronic Properties of Low-Bandgap Halide Perovskites for Solar Cell Applications

Riding on the coat tails of rapid developments in single-junction halide perovskite solar cells, all-perovskite multijunction solar cells have recently garnered significant attention, with the highest power-conversion efficiency already reaching 25.6%. Much of this progress has been fueled by the rapid rise in the photovoltaic performance of low-bandgap halide perovskite absorbers, materials, which, to date, have only been achievable by the partial or complete substitution of lead with tin. However, much room still exists to develop a more critical understanding of key material properties in these low-bandgap perovskites. Herein, the key optoelectronic properties of absorption, carrier generation, recombination, and transport in these tin-containing perovskites are discussed, showing that intrinsic doping distinctively impacts many of these properties, thereby rendering this class of halide perovskites unique within the family. Current understanding of the mechanisms that degrade optoelectronic performance in these materials and the corresponding devices are also summarized. These collective results highlight an important interplay between doping, defects, and degradation that will need to be controlled. Finally, the current gaps in understanding of these low-bandgap perovskites are outlined, thereby providing guidelines for further research, which will unlock their full potential for realizing a plethora of high-performance optoelectronic devices.

Alloying a single and a double perovskite: a Cu+/2+ mixed-valence layered halide perovskite with strong optical absorption

Introducing heterovalent cations at the octahedral sites of halide perovskites can substantially change their optoelectronic properties. Yet, in most cases, only small amounts of such metals can be incorporated as impurities into the three-dimensional lattice. Here, we exploit the greater structural flexibility of the two-dimensional (2D) perovskite framework to place three distinct stoichiometric cations in the octahedral sites. The new layered perovskites AI 4[CuII(CuIInIII)0.5Cl8] (1, A = organic cation) may be derived from a CuI-InIII double perovskite by replacing half of the octahedral metal sites with Cu2+. Electron paramagnetic resonance and X-ray absorption spectroscopy confirm the presence of Cu2+ in 1. Crystallographic studies demonstrate that 1 represents an averaging of the CuI-InIII double perovskite and CuII single perovskite structures. However, whereas the highly insulating CuI-InIII and CuII perovskites are colorless and yellow, respectively, 1 is black, with substantially higher electronic conductivity than that of either endmember. We trace these emergent properties in 1 to intervalence charge transfer between the mixed-valence Cu centers. We further propose a tiling model to describe how the Cu+, Cu2+, and In3+ coordination spheres can pack most favorably into a 2D perovskite lattice, which explains the unusual 1 : 2 : 1 ratio of these cations found in 1. Magnetic susceptibility data of 1 further corroborate this packing model. The emergence of enhanced visible light absorption and electronic conductivity in 1 demonstrates the importance of devising strategies for increasing the compositional complexity of halide perovskites.

Potential lead-free small band gap halide double perovskites Cs2CuMCl6 (M = Sb, Bi) for green technology

Explorations of stable lead-free perovskites have currently achieved substantial interest to overcome the instability and avoid toxicity related issue faced with the lead-based perovskites. In this study, we have comprehensively studied the stability, nature and origin of electronic, transport and optical properties of inorganic halide double perovskites, which could provide a better understanding of their possible potential applications. The density functional theory is used to investigate the different physical properties of these materials. The stability of these cubic materials is validated by optimizing the structure, tolerance factor, mechanical stability test. The materials are small band gap semiconductors with outshining optoelectronic performance. Due to high optical absorption, high conductivity and low reflectivity they have great potential to be used for optoelectronic application purpose. Because of small band gap we have also investigated the variation of various transport parameters with chemical potential. The semiconducting nature of materials results in ZT close to unity predicting its excellent application in thermoelectric technology.

Phonon-Limited Mobility and Electron-Phonon Coupling in Lead-Free Halide Double Perovskites

Lead-free halide double perovskites have attracted considerable attention as complements to lead-based halide perovskites in a range of optoelectronic applications. Experiments on Cs₂AgBiBr₆ indicate carrier mobilities in the range of 0.3-11 cm²/(V s) at room temperature, considerably lower than in lead-based perovskites. The origin of low mobilities is currently unclear, calling for an atomic-scale investigation. We report state-of-the-art ab initio calculations of the phonon-limited mobility of charge carriers in lead-free halide double perovskites Cs₂AgBiX₆ (X = Br, Cl). For Cs₂AgBiBr₆, we obtain room-temperature electron and hole mobilities of 17 and 14 cm²/(V s), respectively, in line with experiments. We demonstrate that the cause for the lower mobility of this compound, compared to CH₃NH₃PbI₃, resides in the heavier carrier effective masses. A mode-resolved analysis of scattering rates reveals the predominance of Fröhlich electron-phonon scattering, similar to lead-based perovskites. Our results indicate that, to increase the mobility of lead-free perovskites, it is necessary to reduce the effective masses, for example by cation engineering.

Gold-Cage Perovskites: A Three-Dimensional AuIII-X Framework Encasing Isolated MX63- Octahedra (MIII = In, Sb, Bi; X = Cl-, Br-, I-)

The Cs₈Au^III₄M^IIIX₂₃ (M = In³⁺, Sb³⁺, Bi³⁺; X = Cl^-, Br^-, I^-) perovskites are composed of corner-sharing Au-X octahedra that trace the edges of a cube containing an isolated M-X octahedron at its body center. This structure, unique within the halide perovskite family, may be derived from the doubled cubic perovskite unit cell by removing the metals at the cube faces. To our knowledge, these are the only halide perovskites where the octahedral sites do not bear an average 2+ charge. Charge compensation in these materials requires a stoichiometric halide vacancy, which is disordered around the Au atom at the unit-cell corner and orders when the crystallization is slowed. Using X-ray crystallography, X-ray absorption spectroscopy, and pair distribution function analysis, we elucidate the structure of this unusual perovskite. Metal-site alloying produces further intricacies in this structure, which our model explains. Compared to other halide perovskites, this class of materials shows unusually low absorption onset energies ranging between ca. 1.0 and 2.4 eV. Partial reduction of Au³⁺ to Au⁺ affords an intervalence charge-transfer band, which redshifts the absorption onset of Cs₈Au₄InCl₂₃ from 2.4 to 1.5 eV. With connected Au-X octahedra and isolated M-X octahedra, this structure type combines zero- and three-dimensional metal-halide sublattices in a single material and stands out among halide perovskites for its ordering of homovalent metals, ordering of halide vacancies, and incorporation of purely trivalent metals at the octahedral sites.

All-Inorganic Lead-Free Perovskite(-Like) Single Crystals: Synthesis, Properties, and Applications

Due to their nontoxicity, stability, and unique optoelectronic properties, all-inorganic lead-free halide semiconductors with perovskite and perovskite-like structures have successfully emerged as promising optoelectronic materials for various applications, such as solar cells, light-emitting diodes (LEDs), photodetectors, and X-ray detectors. To further explore their practical potentials, researchers have paid more attention in all-inorganic lead-free perovskite (-like) (ILFP) single crystals. For these single crystals, the advantages of large sizes, uniform surface morphology, and few defects can facilitate their excellent performances and practical applications. Besides, compared with the low dimensional and polycrystalline ILFP materials, the ILFP single crystals feature enhanced performances, including a longer carrier diffusion length and a larger light absorption coefficient, which attract a great deal of attention. Therefore, focus is on the researching progress of ILFP single crystals and the development of their preparation methods, as well as the novel properties of ILFP single crystals. In addition, the reported applications of ILFP single crystals are proposed to highlight their practical importance. With the perspective of the evolution and challenges, the current limitations of the materials and devices are discussed, followed by an inspirational outlook on their future development directions.

Why Hybrid Tin-Based Perovskites Simultaneously Improve the Structural Stability and Charge Carriers' Lifetime: Ab Initio Quantum Dynamics

Much effort has been dedicated to boost the development of lead-free perovskite solar cells. However, their performance and stability are still much less competitive to the lead-bearing counterparts. By exploiting a mixed Sn-Ge cation strategy for the development of lead-free perovskites, we perform ab initio electronic structure calculations and quantum dynamics simulations on MASn_0.5Ge_0.5I₃ and compare them to MASnI₃. The calculations demonstrate that the hybrid cation strategy can improve simultaneously the perovskite stability and the lifetime of charge carriers. The stability increases due to a larger space of possible structures within the favorable range of the structural parameters, such as the Goldschmidt tolerance and octahedron factors. By exploring the larger structure space, mixed perovskites find stable configurations with lower free energies and better fitting components that exhibit reduced fluctuations around the equilibrium geometries. Charge carriers live longer in mixed perovskites because cation mixing results in an additional and moderate disorder that separates electrons and holes, reducing their interactions while still maintaining efficient band-like charge transport. These general and fundamental principles established by the analysis of the simulation results are useful for the design of advanced materials for solar energy and construction of optoelectronic devices.

Chemically Localized Resonant Excitons in Silver-Pnictogen Halide Double Perovskites

Halide double perovskites with alternating silver and pnictogen cations are an emerging family of photoabsorber materials with robust stability and band gaps in the visible range. However, the nature of optical excitations in these systems is not yet well understood, limiting their utility. Here, we use ab initio many-body perturbation theory within the GW approximation and the Bethe-Salpeter equation approach to calculate the electronic structure and optical excitations of the double perovskite series Cs₂AgBX₆, with B = Bi³⁺, Sb³⁺ and X = Br^-, Cl^-. We find that these materials exhibit strongly localized resonant excitons with energies from 170 to 434 meV below the direct band gap. In contrast to lead-based perovskites, the Cs₂AgBX₆ excitons are computed to be non-hydrogenic with anisotropic effective masses and sensitive to local field effects, a consequence of their chemical heterogeneity. Our calculations demonstrate the limitations of the Wannier-Mott and Elliott models for this class of double perovskites and contribute to a detailed atomistic understanding of their light-matter interactions.

Recent Advances and Optoelectronic Applications of Lead-Free Halide Double Perovskites

Organic-inorganic metal halide perovskites (most notably CH₃ NH₃ PbI₃ ) have demonstrated remarkable physical attributes for photovoltaic and diverse optoelectronic applications, whereas concerns about toxicity owing to the use of lead in the chemical composition still motivate further exploration of new, nontoxic candidates. Lead-free halide double perovskites (HDPs), designed by the rational chemical substitution of Pb²⁺ with other nontoxic candidate elements, have recently attracted interest as a fascinating alternative to their Pb-based counterparts. Herein, recent advances in crystal structures, physical properties, and versatile optoelectronic applications of lead-free HDPs, such as solar cells, photodetectors, X-ray detectors, and light-emitting diodes, are reviewed. Perspectives to improve the physical and photoelectric properties of existing HDP materials are also discussed and will favor future development of new, lead-free HDP candidates.

How the Structures and Properties of Pristine and Anion Vacancy Defective Organic-Inorganic Hybrid Double Perovskites MA2AgIn(BrxI1-x)6 Vary with Br Content x

This work is dedicated to theoretically investigating the mixed-halide direct band gap organic-inorganic hybrid double perovskites (OIHdPs), MA₂AgIn(Br_xI_1-x)₆, with and without anion vacancy point (AVP) defects. We calculate their structural and optoelectronic properties with different halide compositions and find that the effect of halide composition on the properties of MA₂AgIn(Br_xI_1-x)₆ is quite different from that on lead-bearing perovskites. All the vacancy-free I-bearing systems (x ≠ 1) have nearly the same direct band gap width and carrier activity with MAPbI₃. The Br-rich systems (x > 0.50) are relatively thermodynamical stable and not prone to spontaneous anion segregation and show a strong "self-tolerance" feature toward the inherit defects as well. With these distinguished properties, we are able to conclude that MA₂AgIn(Br_xI_1-x)₆ with 0.50 < x < 1 are promising candidates for Pb-free photovoltaic materials. This Letter provides a detailed microscopic understanding of the vacancy-induced band distortion in lead-free heterovalent substitution OIHdPs and has some guiding significance for molecular design of nontoxic photovoltaic materials.

Recent Progress in Metal Halide Perovskite-Based Tandem Solar Cells

Metal halide perovskite (MHP)-based tandem solar cells are a promising candidate for use in cost-effective and high-performance solar cells that can compete with fossil fuels. To understand the research trends for MHP-based tandem solar cells, a general introduction to single-junction and multiple-junction MHP solar cells and the configuration of tandem devices is provided, along with an overview of the recent progress regarding various MHP-based tandem cells, including MHP/crystalline silicon, MHP/CuInGaS, MHP/organic photovoltaic, MHP/quantum dot, and all-perovskite tandem cell. Future research directions for MHP-based tandem solar cells are also discussed.

Solid-State NMR and NQR Spectroscopy of Lead-Halide Perovskite Materials

Two- and three-dimensional lead-halide perovskite (LHP) materials are novel semiconductors that have generated broad interest owing to their outstanding optical and electronic properties. Characterization and understanding of their atomic structure and structure-property relationships are often nontrivial as a result of the vast structural and compositional tunability of LHPs as well as the enhanced structure dynamics as compared with oxide perovskites or more conventional semiconductors. Nuclear magnetic resonance (NMR) spectroscopy contributes to this thrust through its unique capability of sampling chemical bonding element-specifically (1/2H, 13C, 14/15N, 35/37Cl, 39K, 79/81Br, 87Rb, 127I, 133Cs, and 207Pb nuclei) and locally and shedding light onto the connectivity, geometry, topology, and dynamics of bonding. NMR can therefore readily observe phase transitions, evaluate phase purity and compositional and structural disorder, and probe molecular dynamics and ionic motion in diverse forms of LHPs, in which they can be used practically, ranging from bulk single crystals (e.g., in gamma and X-ray detectors) to polycrystalline films (e.g., in photovoltaics, photodetectors, and light-emitting diodes) and colloidal nanocrystals (e.g., in liquid crystal displays and future quantum light sources). Herein we also outline the immense practical potential of nuclear quadrupolar resonance (NQR) spectroscopy for characterizing LHPs, owing to the strong quadrupole moments, good sensitivity, and high natural abundance of several halide nuclei (79/81Br and 127I) combined with the enhanced electric field gradients around these nuclei existing in LHPs as well as the instrumental simplicity. Strong quadrupole interactions, on one side, make 79/81Br and 127I NMR rather impractical but turn NQR into a high-resolution probe of the local structure around halide ions.

A2AgCrCl6 (A = Li, Na, K, Rb, Cs) halide double perovskites: a transition metal-based semiconducting material series with appreciable optical characteristics

We have theoretically examined the geometries, electronic density of states and band structures of cubic and hexagonal A₂AgCrCl₆ (A = Cs, Rb, K, Na, Li) using meta-GGA SCAN-rVV10. The optimized lattice density was found to vary between 2.68 and 4.08 g cm^-3 for cubic-A₂AgCrCl₆, with the fundamental electronic bandgap (direct) in the range of 0.66-0.69 eV. The cell density of hexagonal A₂AgCrCl₆ was between 2.97 and 3.93 g cm^-3, but with an indirect bandgap of 0.93-1.02 eV. The valence band maximum and the conduction band minimum of A₂AgCrCl₆ were confirmed to be essentially of Cr(3d) character, but the contributions from the orbital states of Cl(3p) to the VBM were also appreciable. Cubic A₂AgCrCl₆ (A = Cs, Rb, K) was identified to possess genuine perovskite stoichiometry, evaluated using various geometry-based indices (viz. octahedral factor, tolerance factor, and global instability index). This was not so for A₂AgCrCl₆ (A = Na, Li), and was due to the small size of Na and Li cations that caused the critical strain of CrCl₆ octahedra and a significant decrease in the cell volume. However, all the five A₂AgCrCl₆ displayed nearly similar optical properties, including the nature of the oscillator peaks in the dielectric function, absorption coefficient, photoconductivity, reflectivity, and Tauc spectra. The zero-limit of the refractive index was calculated around 2.25 and 2.00 for cubic and hexagonal A₂AgCrCl₆, respectively, and the extinction coefficient was very small for all cases. The nature of the optical bandgap and transition peaks discussed in this study of cubic and hexagonal Cs₂AgCrCl₆ agreed well with the experiment. The examination of phonon band dispersion led to the conclusion that cubic-A₂AgCrCl₆ (A = Cs, Rb) are the only halide double perovskites of the entire series that are dynamically stable.

The Cs2AgRhCl6 Halide Double Perovskite: A Dynamically Stable Lead-Free Transition-Metal Driven Semiconducting Material for Optoelectronics

A-Site doping with alkali ions, and/or metal substitution at the B and B'-sites, are among the key strategies in the innovative development of A 2BB'X6 halide double perovskite semiconducting materials for application in energy and device technologies. To this end, we have investigated an intriguing series of five halide-based non-toxic systems, A 2AgRhCl6 (A = Li, Na, K, Rb, and Cs), using density functional theory at the SCAN-rVV10 level. The lattice stability and bonding properties emanating from this study of A 2AgRhCl6 matched well with those that have already been synthesized, characterized and discussed [viz. Cs2AgBiX6 (X = Cl, Br)]. Exploration of traditional and recently proposed tolerance factors has enabled us to identify A 2AgRhCl6 (A = K, Rb and Cs) as stable double perovskites. The band structure and density of states calculations suggested that the electronic transition from the top of the valence band [Cl(3p)+Rh(4d)] to the bottom of the conduction band [(Cl(3p)+Rh(4d)] is inherently direct at the X-point of the first Brillouin zone. The (non-spin polarized) bandgap of these materials was found in the range 0.57-0.65 eV with SCAN-rVV10, which were substantially smaller than those computed with hybrid HSE06 and PBE0, and quasi-particle GW methods. This, together with the appreciable refractive index and high absorption coefficient in the region covering the range 1.0-4.5 eV, enabled us to demonstrate that A 2AgRhCl6 (A = K, Rb, and Cs) are likely candidate materials for photoelectric applications. The results of our phonon calculations at the harmonic level suggested that the Cs2AgRhCl6 is the only system that is dynamically stable (no imaginary frequencies found around the high symmetry lines of the reciprocal lattice), although the elastic moduli properties suggested all five systems examined are mechanically stable.

Halide Mixing and Phase Segregation in Cs2AgBiX6 (X = Cl, Br, and I) Double Perovskites from Cesium-133 Solid-State NMR and Optical Spectroscopy

All-inorganic double perovskites (elpasolites) are a promising potential alternatives to lead halide perovskites in optoelectronic applications. Although halide mixing is a well-established strategy for band gap tuning, little is known about halide mixing and phase segregation phenomena in double perovskites. Here, we synthesize a wide range of single- and mixed-halide Cs₂AgBiX₆ (X = Cl, Br, and I) double perovskites using mechanosynthesis and probe their atomic-level microstructure using ¹³³Cs solid-state MAS NMR. We show that mixed Cl/Br materials form pure phases for any Cl/Br ratio while Cl/I and Br/I mixing is only possible within a narrow range of halide ratios (<3 mol % I) and leads to a complex mixture of products for higher ratios. We characterize the optical properties of the resulting materials and show that halide mixing does not lead to an appreciable tunability of the PL emission. We find that iodide incorporation is particularly pernicious in that it quenches the PL emission intensity and radiative charge carrier lifetimes for iodide ratios as low as 0.3 mol %. Our study shows that solid-state NMR, in conjunction with optical spectroscopies, provides a comprehensive understanding of the structure-activity relationships, halide mixing, and phase segregation phenomena in Cs₂AgBiX₆ (X = Cl, Br, and I) double perovskites.

Carrier Transport Limited by Trap State in Cs2AgBiBr6 Double Perovskites

Understanding the photoinduced carrier dynamics in Cs₂AgBiBr₆ double perovskites is essential for their application in optoelectronic devices. Herein, we report an investigation on the temperature-dependent carrier dynamics in a Cs₂AgBiBr₆ single crystal (SC). The time-resolved photoluminescence (TRPL) measurement indicates that the majority of carriers (>99%) decay through a fast trapping process at room temperature, and as the temperature decreases to 123 K, the population of carriers with a slow fundamental decay kinetics rises to ∼50%. We show that the carrier diffusion coefficient (theoretical diffusion length) varies from 0.020 ± 0.003 cm² s^-1 (0.70 μm) at 298 K to 0.11 ± 0.010 cm² s^-1 (2.44 μm) at 123 K. However, in spite of the long diffusion length, the population of carriers that can perform long-distance transport is restricted by the trap state, which is likely a key reason limiting the performance of Cs₂AgBiBr₆ optoelectronic devices.

Dimensional reduction of the small-bandgap double perovskite Cs2AgTlBr6

Quantum confinement effects in lower-dimensional derivatives of the ABX3 (A = monocation, X = halide) single perovskites afford striking optical and electronic changes, enabling applications ranging from solar absorbers to phosphors and light-emitting diodes. Halide double perovskites form a larger materials family, known since the late 1800s, but lower-dimensional derivatives remain rare and prior work has revealed a minimal effect of quantum confinement on their optical properties. Here, we synthesize three new lower-dimensional derivatives of the 3D double perovskite Cs2AgTlBr6: 2D derivatives with mono- (1-Tl) and bi-layer thick (2-Tl) inorganic sheets and a quasi-1D derivative (1'-Tl). Single-crystal ellipsometry studies of these materials show the first clear demonstration that dimensional reduction can significantly alter the optical properties of 2D halide double perovskites. This large quantum confinement effect is attributed to the substantial electronic delocalization of the parent 3D Ag-Tl perovskite. Calculations track the evolution of the electronic bands with dimensional reduction and the accompanying structural distortions and show a direct-to-indirect bandgap transition as the 3D perovskite lattice is thinned to a monolayer in 1-Tl. This bandgap transition at the monolayer limit is also evident in the calculations for 1-In, an isostructural, isoelectronic analogue to 1-Tl in which In3+ replaces Tl3+, underscoring the orbital basis for the direct/indirect nature of the bandgap. Thus, in complement to the massive compositional diversity of halide double perovskites, dimensional reduction may be used as a systematic route for harnessing electronic confinement effects and obtaining new electronic structures.

Lead-Free Cs4CuSb2Cl12 Layered Double Perovskite Nanocrystals

Concerns about the toxicity of lead-based perovskites have aroused great interest for the development of alternative lead-free perovskite-type materials. Recently, theoretical calculations predict that Pb²⁺ cations can be substituted by a combination of Cu²⁺ and Sb³⁺ cations to form a vacancy-ordered layered double perovskite structure with superior optoelectronic properties. However, accessibilities to this class of perovskite-type materials remain inadequate, hindering their practical implementations in various applications. Here, we report the first colloidal synthesis of Cs₄CuSb₂Cl₁₂ perovskite-type nanocrystals (NCs). The resulting NCs exhibit a layered double perovskite structure with ordered vacancies and a direct band gap of 1.79 eV. A composition-structure-property relationship has been established by investigating a series of Cs₄Cu_xAg_2-2xSb₂Cl₁₂ perovskite-type NCs (0 ≤ x ≤ 1). The composition induced crystal structure transformation, and thus, the electronic band gap evolution has been explored by experimental observations and further confirmed by theoretical calculations. Taking advantage of both the unique electronic structure and solution processability, we demonstrate that the Cs₄CuSb₂Cl₁₂ NCs can be solution-processed as high-speed photodetectors with ultrafast photoresponse and narrow bandwidth. We anticipate that our study will prompt future research to design and fabricate novel and high-performance lead-free perovskite-type NCs for a range of applications.

Inorganic Halide Double Perovskites with Optoelectronic Properties Modulated by Sublattice Mixing

All-inorganic halide double perovskites have emerged as a promising class of materials that are potentially more stable and less toxic than lead-containing hybrid organic-inorganic perovskite optoelectronic materials. In this work, 311 cesium chloride double perovskites (Cs₂BB'Cl₆) were selected from a set of 903 compounds as likely being stable on the basis of a statistically learned tolerance factor (τ) for perovskite stability. First-principles calculations on these 311 double perovskites were then performed to assess their stability and identify candidates with band gaps appropriate for optoelectronic applications. We predict that 261 of the 311 Cs₂BB'Cl₆ compounds are likely synthesizable on the basis of a thermodynamic analysis of their decomposition to competing compounds (decomposition enthalpy <0.05 eV/atom). Of these 261 likely synthesizable compounds, 47 contain no toxic elements and have direct or nearly direct (within 100 meV) band gaps between 1 and 3 eV, as computed with hybrid density functional theory (HSE06). Within this set, we identify the triple-alkali perovskites Cs₂[Alk]⁺[TM]³⁺Cl₆, where Alk is a group 1 alkali cation and TM is a transition-metal cation, as a class of Cs₂BB'Cl₆ double perovskites with remarkable optical properties, including large and tunable exciton binding energies as computed by the GW-Bethe-Salpeter equation (GW-BSE) method. We attribute the unusual electronic structure of these compounds to the mixing of the Alk-Cl and TM-Cl sublattices, leading to materials with small band gaps, large exciton binding energies, and absorption spectra that are strongly influenced by the identity of the transition metal. The role of the double-perovskite structure in enabling these unique properties is probed through an analysis of the electronic structures and chemical bonding of these compounds in comparison with other transition-metal and alkali transition-metal halides.

Recent Advancements in Crystalline Pb-Free Halide Double Perovskites

Lead halide perovskites have gained more and more attention because of their ease of synthesis and excellent photoelectric properties including a large absorption coefficient, long carrier lifetime, long carrier diffusion length, and high carrier mobility. However, their toxicity, instability, and phase degradation in ambient environments impede their large-scale applications. To address these concerns, it is desirable to find stable alternative halide perovskites without toxicity and with comparable optoelectronic properties to lead-based perovskites. Over the years, a considerable number of lead-free halide perovskites have been added to this family of materials, including A2B’B’’X6, A2BX6, and A3B2X9 type perovskites. Among these, double perovskites with the general formula A2B’B’’X6 are deemed to be a potential alternative to lead halide perovskites as they possess good stability under ambient conditions and excellent optoelectronic properties. In this review, recent progress in exploring Pb-free halide double perovskites is highlighted. The synthesis, composition-tuning, physical properties, and applications of representative 3D, 2D, and nanocrystal A2B’B’’X6 double perovskites are introduced. In addition, perspectives about current challenges and solutions in this field are also provided.

Homo- and Heterovalent Doping-Mediated Self-Trapped Exciton Emission and Energy Transfer in Mn-Doped Cs2Na1-xAgxBiCl6 Double Perovskites

Double perovskites exhibit low toxicity, intrinsic thermodynamic stability, and small carrier effective mass. Herein, a novel doping route was adopted to incorporate Mn ions into Cs₂Na_1-xAg_xBiCl₆ double perovskites for engineering the band gap and tailoring the energy transfer. The as-prepared Cs₂Na_1-xAg_xBiCl₆ (0 < x < 1) exhibited excellent photoluminescence and a broad self-trapped exciton (STE) band from 500 to 900 nm, which exhibited an abnormal emission peak blue shift with increasing temperature. For Mn-doped Cs₂Na_1-xAg_xBiCl₆, the two photoluminescence (PL) bands from d-d transition emission of Mn ions and STEs were always observed simultaneously in the PL window. The distinct energy-transfer channel from the Mn²⁺ ion guest to the double-perovskite host resulted in the dominant Mn²⁺ emission. Our results will be helpful for further understanding the nature of the photophysics of double perovskites.

(C3H9NI)4AgBiI8: a direct-bandgap layered double perovskite based on a short-chain spacer cation for light absorption

A new iodide layered double perovskite (C₃H₉NI)₄AgBiI₈ (IPAB) has been developed based on a short-chain spacer cation, which is the first homologous compound in iodide double perovskites that adopt the Ruddlesden–Popper structure type.

A pencil-and-paper method for elucidating halide double perovskite band structures

Halide double perovskites are an important emerging alternative to lead-halide perovskites in a variety of optoelectronic applications. Compared to ABX3 single perovskites (A = monovalent cation, X = halide), A2BB'X6 double perovskites exhibit a wider array of compositions and electronic structures, promising finer control over physical and electronic properties through synthetic design. However, a clear understanding of how chemical composition dictates the electronic structures of this large family of materials is still lacking. Herein, we develop a qualitative Linear Combination of Atomic Orbitals (LCAO) model that describes the full range of band structures for double perovskites. Our simple model allows for a direct connection between the inherently local bonding between atoms in the double perovskite and the resulting delocalized bands of the solid. In particular, we show how bands in halide double perovskites originate from the molecular orbitals of metal-hexahalide coordination complexes and describe how these molecular orbitals vary within a band. Our results provide both an enhanced understanding of known perovskite compositions and predictive power for identifying new compositions with targeted properties. We present a table, which permits the position of the conduction band minimum and valence band maximum in most double perovskites to be immediately determined from the frontier atomic orbitals of the B-site metals. Using purely qualitative arguments based on orbital symmetries and their relative energies, the direct/indirect nature of the bandgap of almost all halide double perovskites can thus be correctly predicted. We hope that this theory provides an intuitive understanding of halide double perovskite band structures and enables lessons from molecular chemistry to be applied to these extended solids.