Lead-Free Silver-Bismuth Halide Double Perovskite Nanocrystals

Autonomous nanomanufacturing of lead-free metal halide perovskite nanocrystals using a self-driving fluidic lab

Lead-based metal halide perovskite (MHP) nanocrystals (NCs) have emerged as a promising class of semiconducting nanomaterials for a wide range of optoelectronic and photoelectronic applications. However, the intrinsic lead toxicity of MHP NCs has significantly hampered their large-scale device applications. Copper-base MHP NCs with composition-tunable optical properties have emerged as a prominent lead-free MHP NC candidate. However, comprehensive synthesis space exploration, development, and synthesis science studies of copper-based MHP NCs have been limited by the manual nature of flask-based synthesis and characterization methods. In this study, we present an autonomous approach for the development of lead-free MHP NCs via seamless integration of a modular microfluidic platform with machine learning-assisted NC synthesis modeling and experiment selection to establish a self-driving fluidic lab for accelerated NC synthesis science studies. For the first time, a successful and reproducible in-flow synthesis of Cs3Cu2I5 NCs is presented. Autonomous experimentation is then employed for rapid in-flow synthesis science studies of Cs3Cu2I5 NCs. The autonomously generated experimental NC synthesis dataset is then utilized for fast-tracked synthetic route optimization of high-performing Cs3Cu2I5 NCs.

Controlled Synthesis of Lead-Free Double Perovskite Colloidal Nanocrystals for Nonvolatile Resistive Memory Devices

Although lead-free double perovskites such as Cs2AgBiBr6 have been widely explored, they still remain a daunting challenge for the controlled synthesis of lead-free double perovskite nanocrystals with highly tunable morphology and band structure. Here, we report the controlled synthesis of lead-free double perovskite colloidal nanocrystals including Cs2AgBiBr6 and Cs2AgInxBi1-xBr6 via a facile wet-chemical synthesis method for the fabrication of high-performance nonvolatile resistive memory devices. Cs2AgBiBr6 colloidal nanocrystals with well-defined cuboidal, hexagonal, and triangular morphologies are synthesized through a facile wet-chemical approach by tuning the reaction temperature from 150 to 190 °C. Further incorporating indium into Cs2AgBiBr6 to synthesize alloyed Cs2AgInxBi1-xBr6 nanocrystals not only can induce the indirect-to-direct bandgap transition with enhanced photoluminescence but also can improve its structural stability. After optimizing the active layers and device structure, the fabricated Ag/polymethylene acrylate@Cs2AgIn0.25Bi0.75Br6/ITO resistive memory device exhibits a low power consumption (the operating voltage is ∼0.17 V), excellent cycling stability (>10 000 cycles), and good synaptic property. Our study would enable the facile wet-chemical synthesis of lead-free double perovskite colloidal nanocrystals in a highly controllable manner for the development of high-performance resistive memory devices.

Stable and luminescent cesium copper halide nanocrystals embedded in flexible polymer fibers for fabrication of down-converting WLEDs

Recently, CsPbX3 (X = I, Br, Cl) perovskite nanocrystals (NCs) have drawn wide attention owing to their outstanding photophysical and optoelectronic properties. However, the toxicity of such NCs remained a big challenge for further commercialization. Herein, we adopt facile methods for synthesizing green-emissive Cs3Cu2Cl5 and blue-emissive Cs3Cu2Br2.5I2.5 perovskite NCs that exhibit broad emission spectra with large Stokes shifts. These NCs showed photoluminescence quantum yields (PLQY) up to 65% (Cs3Cu2Cl5 NCs) and 32% (Cs3Cu2Br2.5I2.5 NCs) with limited stabilities. To further improve the stability, the NCs were blended with a hydrophobic polymer poly-methylmethacrylate (PMMA) and embedded inside the polymer fiber by an electrospinning process to form composite fibers. The as-prepared Cs3Cu2Cl5@PMMA and Cs3Cu2Br2.5I2.5@PMMA fiber films demonstrated good surface coverage and better thermal stability, and even retained their emission properties when dispersed in water. The emissive fibers were also deposited on flexible polyethylene terephthalate (PET) substrates that displayed high resistance towards bending and twisting with no signs of breakage, damage, or loss of optical properties. Finally, UV-pumped phosphor-converted WLEDs fabricated by using these blue and green-emitting fibers revealed CIE chromaticity coordinates at (0.27, 0.33) with a maximum luminous efficiency of 69 Lm W-1 and correlated color temperature (CCT) value of 8703 K. These outcomes can be beneficial for the development of futuristic flexible display technologies.

Precise Hue Control in a Single-Component White-Light Emitting Perovskite Cs2 SnCl6 through Defect Engineering Based on La3+ Doping

Single-component white light emitters based on the all-inorganic perovskites will act as outstanding candidates for applications in solid-state lighting thanks to their abundant energy states for self-trapped excitons (STE) with ultra-high photoluminescence (PL) efficiency. Here, a complementary white light is realized by dual STEs emissions with blue and yellow colors in a single-component perovskite Cs₂ SnCl₆ :La³⁺ microcrystal (MC). The dual emission bands centered at 450 and 560 nm are attributed to the intrinsic STE1 emission in host lattice Cs₂ SnCl₆ and the STE2 emission induced by the heterovalent La³⁺ doping, respectively. The hue of the white light can be tunable through energy transfer between the two STEs, the variation of excitation wavelength, and the Sn⁴⁺ /Cs⁺ ratios in starting materials. The effects of the doping heterovalent La³⁺ ions on the electronic structure and photophysical properties of the Cs₂ SnCl₆ crystals and the created impurity point defect states are investigated by the chemical potentials calculated using density functional theory (DFT) and confirmed by the experimental results. These results provide a facile approach to gaining novel single-component white light emitter and offer fundamental insights into the defect chemistry in the heterovalent ions doped perovskite luminescent crystals.

Revealing the Mechanism of Pressure-Induced Emission in Layered Silver-Bismuth Double Perovskites

Pressure-induced emission (PIE) associated with self-trapping excitons (STEs) in low-dimensional halide perovskites has attracted great attention for better materials-by-design. Here, using 2D layered double perovskite (C₆ H₅ CH₂ CH₂ NH₃ ⁺ )₄ AgBiBr₈ as a model system, we advance a fundamental physicochemical mechanism of the PIE from the perspective of carrier dynamics and excited-state behaviors of local lattice distortion. We observed a pressure-driven STE transformation from dark to bright states, corresponding a strong broadband Stokes-shifted emission. Further theoretical analysis demonstrated that the suppressed lattice distortion and enhanced electronic dimensionality in the excited-state play an important role in the formation of stabilized bright STEs, which could manipulate the self-trapping energy and lattice deformation energy to form an energy barrier between the potential energy curves of ground- and excited-state, and enhance the electron-hole orbital overlap, respectively.

Lead-Free Metal Halide Perovskite Nanocrystals: From Fundamentals to Applications

Lead (Pb) halide perovskite nanocrystals, with the general formula APbX₃ , where A=CH₃ NH³⁺ , CH(NH₂ )²⁺ , or Cs⁺ and X=Cl^- , Br^- , or I^- , have emerged as a class of materials with promising properties due to their remarkable optical properties and solar cell performance. However, important issues still need to be addressed to enable practical applications of these materials, such as instability, mass production, and Pb toxicity. Recent studies have carried out the replacement of Pb by various less-toxic cations as Sn, Ge, Sb, and Bi. This variety of chemical compositions provide Pb-free perovskite and metal halide nanostructures with a wide spectral range, in addition to being considered less toxic, therefore having greater practical applicability. Highlighting the necessity to address and solve the toxicity problems related to Pb-containing perovskite, this review considers the prospects of the Pb-free perovskite, involving synthesis methods, and properties of them, including advantages, disadvantages, and applications.

Unraveling the Gas-Sensing Mechanisms of Lead-Free Perovskites Supported on Graphene

Lead halide perovskites have been attracting great attention due to their outstanding properties and have been utilized for a wide variety of applications. However, the high toxicity of lead promotes an urgent and necessary search for alternative nanomaterials. In this perspective, the emerging lead-free perovskites are an environmentally friendly and harmless option. The present work reports for the first time gas sensors based on lead-free perovskite nanocrystals supported on graphene, which acts as a transducing element owing to its high and efficient carrier transport properties. The use of nanocrystals enables achieving excellent sensitivity toward gas compounds and presents better properties than those of bulky perovskite thin films, owing to their quantum confinement effect and exciton binding energy. Specifically, an industrially scalable, facile, and inexpensive synthesis is proposed to support two different perovskites (Cs₃CuBr₅ and Cs₂AgBiBr₆) on graphene for effectively detecting a variety of harmful pollutants below the threshold limit values. H₂ and H₂S gases were detected for the first time by utilizing lead-free perovskites, and ultrasensitive detection of NO₂ was also achieved at room temperature. In addition, the band-gap type, defect tolerance, and electronic surface traps at the nanocrystals were studied in detail for understanding the differences in the sensing performance observed. Finally, a comprehensive sensing mechanism is proposed.

High-throughput screening of perovskite inspired bismuth halide materials: toward lead-free photovoltaic cells and light-emitting diodes

Toxicity is the main bottleneck for the commercialization of Pb halide perovskites. Bi has been considered a promising metal cation to replace Pb because of its comparable electronic structures with Pb and better stability. Although experimental and theoretical studies have proposed various Bi-based halides, the present achievements in photovoltaic cells and other photoelectronic device fields do not compete with Pb analogs. Thermodynamic stability, bandgap control, and enhancement of carrier transport are fundamental challenges in the context of intrinsic material properties for developing highly efficient Bi-based devices. This study evaluates the potential of Bi-based halide compounds with good stability and electronic properties through high-throughput density functional theory calculations. Lattice structures and compositions are selected based on previous reports and an open material database. Then, we expanded our dataset to cover all possible compositional variations of A- and X-sites and alloying to B-sites. We examined over six-hundred candidates and found ten new candidates that have not been reported previously. Rb₃SbBiI₉exhibits the best-expected efficiency for high-efficiency solar cells among selected compounds, and other compounds can be used as visible-light-generation sources. Analysis of the screening procedure revealed that vacancy-ordered (A₃B₂X₉)-type Bi-halides exhibit significantly favorable characteristics when compared with those of double perovskites and rudorffite-like structures for Bi-based photoelectronic devices.

Bright Triplet Self-Trapped Excitons to Dopant Energy Transfer in Halide Double-Perovskite Nanocrystals

For inorganic semiconductor nanostructure, excitons in the triplet states are known as the "dark exciton" with poor emitting properties, because of the spin-forbidden transition. Herein, we report a design principle to boost triplet excitons photoluminescence (PL) in all-inorganic lead-free double-perovskite nanocrystals (NCs). Our experimental data reveal that singlet self-trapped excitons (STEs) experience fast intersystem crossing (80 ps) to triplet states. These triplet STEs give bright green color emission with unity PL quantum yield (PLQY). Furthermore, efficient energy transfer from triplet STEs to dopants (Mn²⁺) can be achieved, which leads to white-light emitting with 87% PLQY in both colloidal and solid thin film NCs. These findings illustrate a fundamental principle to design efficient white-light emitting inorganic phosphors, propelling the development of illumination-related applications.

Mixed-Halide Double Perovskite Cs2 AgBiX6 (X=Br, I) with Tunable Optical Properties via Anion Exchange

Lead-free double perovskites, A2 M+ M'3+ X6 , are considered as promising alternatives to lead-halide perovskites, in optoelectronics applications. Although iodide (I) and bromide (Br) mixing is a versatile tool for bandgap tuning in lead perovskites, similar mixed I/Br double perovskite films have not been reported in double perovskites, which may be due to the large activation energy for ion migration. In this work, mixed Br/I double perovskites were realized utilizing an anion exchange method starting from Cs2 AgBiBr6 solid thin-films with large grain-size. The optical and structural properties were studied experimentally and theoretically. Importantly, the halide exchange mechanism was investigated. Hydroiodic acid was the key factor to facilitate the halide exchange reaction, through a dissolution-recrystallization process. In addition, the common organic iodide salts could successfully perform halide-exchange while retaining high mixed-halide phase stability and strong light absorption capability.

Organic Dye/Cs2AgBiBr6 Double Perovskite Heterojunction Solar Cells

The photovoltaic performance of Cs₂AgBiBr₆ perovskite is limited by its light-harvesting ability owing to its broad bandgap. Here, we introduced three indoline dyes, D102, D131, and D149, to sensitize the TiO₂ electron transport layer that was employed in the Cs₂AgBiBr₆ perovskite solar cells (PSCs). The perovskite-indoline dye hybrid cells worked with higher power conversion efficiencies (PCEs) than the corresponding dye-sensitized solar cells and the PSC. Extended absorption resulted in a higher short-circuit current density, up to 8.24 mA cm^-2, and a maximum PCE of 4.23% in the case of D149, for instance. The double perovskite worked as a p-type interlayer between the dyes and spiro-OMeTAD to convey the holes from the former to the latter, resulting in enhancement in the overall performance.

Ultrafast Dynamics of Self-Trapped Excitons in Lead-Free Perovskite Nanocrystals

Lead-free halide perovskite nanocrystals (NCs) have received increasing attention owing to their low toxicity and high stability. Localized charge distribution and strong carrier-phonon coupling in lead-free perovskite NCs facilitates the formation of self-trapped excitons (STEs), which typically give a broadband photoluminescence (PL) emission with a large Stokes shift. In this Perspective, we highlight how PL modulations can give rise to an efficient white-light emission by understanding and tuning the ultrafast dynamics of STEs in lead-free perovskite NCs. We then present the exciton energy transfer mediated by STEs to provide an efficient thermally activated delayed fluorescence and dopant PL. We also illustrate promising directions for future applications based on STEs. We hope that this Perspective can provide a new viewpoint for researchers to understand the ultrafast dynamics of STEs and promote lead-free perovskite NCs for optoelectronic applications.

Bismuth species in the coordination sphere of transition metals: synthesis, bonding, coordination chemistry, and reactivity of molecular complexes

This contribution is focused on bismuth species in the coordination sphere of transition metals. In molecular transition metal complexes, three types of Bi-M bonding are considered, namely dative Bi→M interactions (with Bi acting as a donor), dative Bi←M interactions (with Bi acting as an acceptor) and covalent Bi-M interactions (M = transition metal). Synthetic routes to all three classes of compounds are outlined, the Bi-M bonding situation is discussed, trends in the geometric parameters and in the coordination chemistry of the compounds are addressed, and common spectroscopic properties are summarized. As an important part of this contribution, the reactivity of bismuth species in the coordination sphere of transition metal complexes in stoichiometric and catalytic reactions is highlighted.

Experimental and Theoretical Investigation of the Structural and Opto-electronic Properties of Fe-Doped Lead-Free Cs2 AgBiCl6 Double Perovskite

Lead-free double perovskites have emerged as stable and non-toxic alternatives to Pb-halide perovskites. Herein, the synthesis of Fe-doped Cs2 AgBiCl6 lead-free double perovskites are reported that display blue emission using an antisolvent method. The crystal structure, morphology, optical properties, band structure, and stability of the Fe-doped double perovskites were investigated systematically. Formation of the Fe-doped Cs2 AgBiCl6 double perovskite is confirmed by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analysis. XRD and thermo-gravimetric analysis (TGA) shows that the Cs2 AgBiCl6 double perovskite has high structural and thermal stability, respectively. Field emission scanning electron microscopy (FE-SEM) analysis revealed the formation of dipyramidal shape Cs2 AgBiCl6 crystals. Furthermore, energy-dispersive X-ray spectroscopy (EDS) mapping shows the overlapping of Cs, Bi, Ag, Fe, and Cl elements and homogenous incorporation of Fe in Cs2 AgBiCl6 double perovskite. The Fe-doped Cs2 AgBiCl6 double perovskite shows a strong absorption at 380 nm. It extends up to 700 nm, suggesting that sub-band gap states transition may originate from the surface defect of the doped perovskite material. The radiative kinetics of the crystals was studied using the time-correlated single-photon counting (TCSPC) technique. Lattice parameters and band gap value of the Fe-doped Cs2 AgBiCl6 double perovskites predicted by the density functional theory (DFT) calculations are confirmed by XRD and UV/Visible spectroscopy analysis. Time-dependent photo-response characteristics of the Fe-doped Cs2 AgBiCl6 double perovskite show fast response and recovery time of charge carriers. We believe that the successful incorporation of Fe in lead-free, environmentally friendly Cs2 AgBiCl6 double perovskite can open a new class of doped double perovskites with significant potential optoelectronics devices fabrication and photocatalytic applications.

Lead-Free Halide Perovskites for Light Emission: Recent Advances and Perspectives

Lead-based halide perovskites have received great attention in light-emitting applications due to their excellent properties, including high photoluminescence quantum yield (PLQY), tunable emission wavelength, and facile solution preparation. In spite of excellent characteristics, the presence of toxic element lead directly obstructs their further commercial development. Hence, exploiting lead-free halide perovskite materials with superior properties is urgent and necessary. In this review, the deep-seated reasons that benefit light emission for halide perovskites, which help to develop lead-free halide perovskites with excellent performance, are first emphasized. Recent advances in lead-free halide perovskite materials (single crystals, thin films, and nanocrystals with different dimensionalities) from synthesis, crystal structures, optical and optoelectronic properties to applications are then systematically summarized. In particular, phosphor-converted LEDs and electroluminescent LEDs using lead-free halide perovskites are fully examined. Ultimately, based on current development of lead-free halide perovskites, the future directions of lead-free halide perovskites in terms of materials and light-emitting devices are discussed.

Completely Amine-Free Open-Atmospheric Synthesis of High-Quality Cesium Lead Bromide (CsPbBr3 ) Perovskite Nanocrystals

Cesium lead halide perovskite nanocrystals (NCs) CsPbX₃ (X=Cl, Br, and I) have been prominent materials in the last few years due to their high photoluminescence quantum yield (PLQY) for light-emitting diodes and other significant applications in photovoltaics and optoelectronics. In colloidal CsPbX₃ synthesis, the most commonly used ligands are oleic acid and oleylamine. The latter plays an important role in surface passivation but may also be responsible for poor colloidal stability as a result of facile proton exchange leading to the formation of labile oleylammonium halide, which pulls halide ions out of the NC surface. Herein, a facile, efficient, completely amine-free synthesis of cesium lead bromide perovskite nanocrystals using hydrobromic acid as halide source and tri-n-octylphosphane as ligand under open-atmospheric conditions is demonstrated. Hydrobromic acid serves as labile source of bromide ion, and thus this three-precursor approach (separate precursors for Cs, Pb, Br) gives more control than a conventional single-source precursor for Pb and Br (PbBr₂ ). The use of HBr paved the way to eliminate oleylamine, and thus the formation of labile oleylammonium halide can be completely excluded. Various Cs:Pb:Br molar ratios were studied and optimum conditions for making very stable CsPbBr₃ NCs with high PLQY were found. These completely amine-free CsPbBr₃ perovskite NCs synthesized under bromine-rich conditions exhibit good stability and durability for more than three months in the form of colloidal solutions and films, respectively. Furthermore, stable tunable emission across a wide spectral range through anion exchange was demonstrated. More importantly, this work reports open-atmosphere-stable CsPbBr₃ NCs films exhibiting strong PL, which can be further used for optoelectronic device applications.

The effects of monovalent metal cations on the crystal and electronic structures of Cs2MBiCl6 (M = Ag, Cu, Na, K, Rb, and Cs) perovskites

Lead-halide perovskites have attracted much attention over the past decade, while two main issues, i.e., the lead-induced toxicity and materials' instability, limit their further practice in widespread applications. To overcome these limitations, an effective alternative is to design lead-free perovskite materials with the substitution of two divalent lead ions with a pair of monovalent and trivalent metal ions. However, fundamental physics and chemistry about how tuning material's composition affects the crystal phase, electronic band structures, and optoelectronic properties of the material have yet to be fully understood. In this work, we conducted a series of density functional theory calculations to explore the mechanism that how various monovalent metal ions influence the crystal and electronic structures of lead-free Cs₂MBiCl₆ perovskites. We found that the Cs₂MBiCl₆ (M = Ag, Cu, and Na) perovskites preferred a cubic double perovskite phase with low carrier effective masses, while the Cs₂MBiCl₆ (M = K, Rb, and Cs) perovskites favored a monoclinic phase with relatively high carrier effective masses. The different crystal phase preferences were attributed to the different radii of monovalent metal cations and the orbital hybridization between the metal and Cl ions. The calculation showed that all Cs₂MBiCl₆ perovskites studied here exhibited indirect bandgaps. Smaller bandgap energies for the perovskites with a cubic phase were calculated than those of the monoclinic phase counterparts. Charge density difference calculation and electron localization functional analysis were also conducted and revealed that the carrier mobility can be improved via changing the characteristics of metal-halide bonds through compositional and, thus, crystal structure tuning. Our study shown here sheds light on the future design and fabrication of various lead-free perovskite materials for optoelectronic applications.

Lead-Free, Water-Stable A3 Bi2 I9 Perovskites: Crystal Growth and Blue-Emitting Quantum Dots [A=CH3 NH3 + , Cs+ , and (Rb0.05 Cs2.95 )+ ]

Despite the great success in the increase in the power conversion efficiency of lead halide perovskite solar cells, the toxicity of lead and the unstable nature of the materials are still major concerns for their wider implementation at the industrial level. Herein, large-size single crystals (SCs) are developed in HI solution by using a temperature lowering method and nanocrystals (NCs) of A₃ Bi₂ I₉ perovskites [where A=CH₃ NH₃ ⁺ (MA)⁺ , Cs⁺ , and (Rb_0.05 Cs_2.95 )⁺ ] are formed in ethanol (EtOH) and toluene (TOL). The stability of A₃ Bi₂ I₉ perovskite is investigated by immersing the SCs for 24 h and pellets for 12 h in water. Moreover, the A₃ Bi₂ I₉ perovskite NCs displays a promising photoluminescence quantum yield of 17.63 % and a long lifetime of 8.20 ns.

Halide Pb-Free Double–Perovskites: Ternary vs. Quaternary Stoichiometry

In view of their applicability in optoelectronics, we review here the relevant structural, electronic, and optical features of the inorganic Pb-free halide perovskite class. In particular, after discussing the reasons that have motivated their introduction in opposition to their more widely investigated organic-inorganic counterparts, we highlight milestones already achieved in their synthesis and characterization and show how the use of ab initio ground and excited state methods is relevant in predicting their properties and in disclosing yet unsolved issues which characterize both ternary and quaternary stoichiometry double-perovskites.

Synthesis and Localized Photoluminescence Blinking of Lead-Free 2D Nanostructures of Cs3 Bi2 I6 Cl3 Perovskite

Two-dimensional (2D) lead-free halide perovskites have generated enormous perception in the field of optoelectronics due to their fascinating optical properties. However, an in-depth understanding on their shape-controlled charge-carrier recombination dynamics is still lacking, which could be resolved by exploring the photoluminescence (PL) blinking behaviour at the single-particle level. Herein, we demonstrate, for the first time, the synthesis of nanocrystals (NCs) and 2D nanosheets (NSs) of layered mixed halide, Cs₃ Bi₂ I₆ Cl₃ , by solution-based method. We applied fluorescence microscopy and super-resolution optical imaging at single-particle level to investigate their morphology-dependent PL properties. Narrow emission line widths and passivation of non-radiative defects were evidenced for 2D layered nanostructures, whereas the activation of shallow trap states was recognized at 77 K. Interestingly, individual NCs were found to display temporal intermittency (blinking) in PL emission. On the other hand, NS showed temporal PL intensity fluctuations within localized domains of the crystal. In addition, super-resolution optical image of the NS from localization-based method showed spatial inhomogeneity of the PL intensity within perovskite crystal.

Colloidal Synthesis of Ternary Copper Halide Nanocrystals for High-Efficiency Deep-Blue Light-Emitting Diodes with a Half-Lifetime above 100 h

Currently, the blue perovskite light-emitting diodes (PeLEDs) suffer from a compromise in lead toxicity and poor operation stability, and most previous studies have struggled to meet the crucial blue NTSC standard. In this study, electrically driven deep-blue LEDs (∼445 nm) based on zero-dimensional (0D) Cs₃Cu₂I₅ nanocrystals (NCs) were demonstrated with the color coordinates of (0.16, 0.07) and a high external quantum efficiency of ∼1.12%, comparable with the best-performing blue LEDs based on lead-halide perovskites. Encouraged by the remarkable stability of Cs₃Cu₂I₅ NCs against heat and environmental oxygen/moisture, the proposed device was operated in a continuous current mode for 170 h, producing a record half-lifetime of ∼108 h. The device stability was further verified by an aggressive thermal cycling test (300-360-300 K) and a 35-day storage test. Together with the eco-friendly features and facile colloidal synthesis technique, the 0D Cs₃Cu₂I₅ NCs can be therefore regarded as a promising candidate for deep-blue LEDs applications.

A Supported Bismuth Halide Perovskite Photocatalyst for Selective Aliphatic and Aromatic C-H Bond Activation

Direct selective oxidation of hydrocarbons to oxygenates by O2 is challenging. Catalysts are limited by the low activity and narrow application scope, and the main focus is on active C-H bonds at benzylic positions. In this work, stable, lead-free, Cs3 Bi2 Br9 halide perovskites are integrated within the pore channels of mesoporous SBA-15 silica and demonstrate their photocatalytic potentials for C-H bond activation. The composite photocatalysts can effectively oxidize hydrocarbons (C5 to C16 including aromatic and aliphatic alkanes) with a conversion rate up to 32900 μmol gcat -1  h-1 and excellent selectivity (>99 %) towards aldehydes and ketones under visible-light irradiation. Isotopic labeling, in situ spectroscopic studies, and DFT calculations reveal that well-dispersed small perovskite nanoparticles (2-5 nm) possess enhanced electron-hole separation and a close contact with hydrocarbons that facilitates C(sp3 )-H bond activation by photoinduced charges.

Phase transition and mechanical properties of cesium bismuth silver halide double perovskites (Cs2AgBiX6, X = Cl, Br, I): a DFT approach

Double perovskite-based silver and bismuth Cs₂AgBiX₆ (X = Cl, Br, I) have shown a bright future for the development of low-risk photovoltaic devices due to their high stability and non-toxicity of their elements, unlike Pb-based perovskites. Despite the great focus on the optoelectronic properties of Cs₂AgBiX₆ double perovskites, there are limited studies on the behavior of their structural properties. Herein, we carefully examined the cubic structure of Cs₂AgBiX₆ double perovskites, identifying a pseudo-cubic (ps-cubic) phase, which is similar to the initial cubic phase. The observed pseudo-cubic phase is more consistent with previous experimental results demonstrating higher elastic properties, which are useful for designing optoelectronic devices.

Carrier Multiplication and Hot-Carrier Cooling Dynamics in Quantum-Confined CsPbI3 Perovskite Nanocrystals

Carrier multiplication (CM) is an effective mechanism that makes it possible to use hot carriers (HCs) to bypass the Shockley-Queisser limit for solar-cell efficiency. In this paper, we present a detailed study of both CM and HC cooling dynamics in quantum-confined CsPbI₃ perovskite nanocrystals (NCs), using femtosecond transient absorption spectroscopy. Our results show that barrierless CM, with an efficiency exceeding 90%, can be achieved in strongly confined NCs on a time scale of ≪200 fs. A low CM efficiency (∼40%), however, is observed in weakly confined NCs. HC cooling dynamics suggests the absence of an intrinsic phonon bottleneck in strongly confined NCs. Furthermore, the biexciton Auger rate increased 4-fold in strongly confined NCs compared to that in weakly confined NCs. These results suggest that the enhanced CM in strongly confined NCs likely originates from enhanced Coulomb coupling and relaxed momentum conservation.

Synthesis of lead-free Cs3Sb2Br9 perovskite alternative nanocrystals with enhanced photocatalytic CO2 reduction activity

A synthetic method for uniform and pure Cs3Sb2Br9 NCs has been developed. Cs3Sb2Br9 NCs exhibit a 10-fold increase in activity for the photocatalytic CO2 reduction reaction compared to CsPbBr3 NCs, achieving 510 μmol CO g-1 cat. after 4 h. Density functional theory shows that Cs3Sb2Br9 surfaces sufficiently expose Sb to allow reactivity, as opposed to the unreactive CsPbBr3 surface.

Room Temperature Synthesis of All Inorganic Lead-Free Zero-Dimensional Cs4SnBr6 and Cs3KSnBr6 Perovskites

Lead halide perovskites are excellent candidates for photoelectronic and photovoltaic applications, but the toxicity from lead is extremely concerning. Recently, Sn-based zero-dimensional lead-free perovskites synthesized using solid-state reaction techniques have become a new focus in the field. Here, we report a simple room temperature antisolvent method for the synthesis of all inorganic lead-free green emissive Cs₄SnBr₆ (emission at 524 nm) and cyan emissive Cs₃KSnBr₆ (emission at 500 nm) zero-dimensional perovskites. Their photoluminescence quantum yields reach 20% and 35%, respectively. In addition, they maintain their emission for 46 and 55 h in the air, respectively, compared to only 5 min of CsSnBr₃. This method provides a convenient way to do the research and apply these highly emissive perovskites.

Heating-up synthesis of cesium bismuth bromide perovskite nanocrystals with tailored composition, morphology, and optical properties

This study represents the heating-up synthesis of lead-free cesium bismuth bromide perovskite nanocrystals (NCs). CsBr and BiBr₃ precursors are used to synthesize uniform and phase-pure cesium bismuth bromide NCs, and the reaction is performed via an injection-free, heating-up method in the presence of a solvent mixture with a high boiling point. The size and composition of cesium bismuth bromide NCs are readily controlled by changing the reaction time, temperature, and amount of surfactant added to the reaction mixture. Upon heating, sequential phase evolution occurs, resulting in the formation of kinetically stable BiOBr in the early reaction stages, which transformed into the thermodynamically stable Cs₃BiBr₆ and Cs₃Bi₂Br₉ with an increase in either the reaction time or the reaction temperature. Furthermore, the absorption and photoluminescence properties of Cs₃BiBr₆ and Cs₃Bi₂Br₉ NCs are characterized to investigate their composition-dependent optical properties. This work provides the potential to synthesize various types of lead-free perovskite NCs by tailoring the size and compositions.

Lead-free cesium bismuth bromide perovskite nanocrystals are synthesized via the heating-up method with tailored morphology and optical properties.

Lead-Free Halide Perovskite Nanocrystals: Crystal Structures, Synthesis, Stabilities, and Optical Properties

In recent years, there have been rapid advances in the synthesis of lead halide perovskite nanocrystals (NCs) for use in solar cells, light emitting diodes, lasers, and photodetectors. These compounds have a set of intriguing optical, excitonic, and charge transport properties, including outstanding photoluminescence quantum yield (PLQY) and tunable optical band gap. However, the necessary inclusion of lead, a toxic element, raises a critical concern for future commercial development. To address the toxicity issue, intense recent research effort has been devoted to developing lead-free halide perovskite (LFHP) NCs. In this Review, we present a comprehensive overview of currently explored LFHP NCs with an emphasis on their crystal structures, synthesis, optical properties, and environmental stabilities (e.g., UV, heat, and moisture resistance). In addition, strategies for enhancing optical properties and stabilities of LFHP NCs as well as the state-of-the-art applications are discussed. With the perspective of their properties and current challenges, we provide an outlook for future directions in this rapidly evolving field to achieve high-quality LFHP NCs for a broader range of fundamental research and practical applications.

Charge-Carrier Dynamics of Lead-Free Halide Perovskite Nanocrystals

Lead halide perovskite nanocrystals (NCs) have been widely studied for application in optoelectronic devices due to their excellent optical properties and low-cost synthesis. However, the toxicity of lead and the poor stability of the NCs hindered their practical applications. Sn²⁺-based perovskite with low toxicity was first developed; however, the Sn²⁺-based perovskite NCs are unstable in air and oxidize easily. Recently, air-stable lead-free perovskite NCs have been developed and received increasing attention. Unfortunately, the optical and optoelectronic properties of these lead-free halide perovskite NCs are generally far worse than those of lead-perovskite NCs. Understanding the charge-carrier dynamics of semiconductors is crucial to improve their optical properties. In this Account, we mainly review our recent research progress on the study of charge-carrier dynamics in air-stable lead-free perovskite NCs. The exciton trapping followed by nonradiative recombination was the major carrier relaxation pathway and resulted in a low photoluminescence quantum efficiency (PLQE). A feasible route for passivating surface traps and tuning the self-trapped excitons from "dark" (nonradiative) to "bright" (radiative) was proposed. Through this strategy, the PLQE could be increased over 100-fold. In addition, we have compared several photophysical properties of lead-free perovskite NCs with that of lead perovskite NCs, such as charge-carrier relaxation, exciton-phonon coupling, and hot-carrier cooling. In 2017, we reported the synthesis, optical properties, and charge-carrier dynamics of Cs₃Bi₂X₉ (X: Cl, Br, I) NCs. The Cs₃Bi₂Br₉ NCs exhibited clear exciton trapping processes with time scales in the range of 2-20 ps. The fast trapping processes could be passivated via the use of surfactants (such as oleic acid), and the PLQE increased over 20-fold (from 0.2% to 4.5%). The low PLQE may be due to the reduced dimensionality of Cs₃Bi₂Br₉ (2D) compared with the 3D cubic perovskite structure of CsPbBr₃. We next reported double perovskite Cs₂AgSb_1-yBi_yX₆ (X: Br, Cl; 0 ≤ y ≤ 1) NCs, which exhibited a similar 3D cubic perovskite structure to that of the lead-perovskite NCs. The charge-carrier dynamics indicated that the sub-band-gap exciton trapping processes were dominated by ultrafast (∼1-2 ps) intrinsic self-trapping and trapping at surface defects (∼50-100 ps). While trapping at surface defects can be passivated using surfactants, the self-trapping processes is due to the giant carrier-phonon coupling effect. By designing direct band gap double perovskite NCs to tune the sub-band-gap trapping processes, bright dual-color emission was achieved. Furthermore, the violet PLQE could be improved to 36.6%, which is comparable to that in lead halide perovskite NCs. We hope this Account will deepen the understanding of the charge-carrier dynamics in lead-free perovskite NCs and guide the design of high-performance lead-free perovskites.

From Lead Halide Perovskites to Lead-Free Metal Halide Perovskites and Perovskite Derivatives

Despite the exciting progress on power conversion efficiencies, the commercialization of the emerging lead (Pb) halide perovskite solar cell technology still faces significant challenges, one of which is the inclusion of toxic Pb. Searching for Pb-free perovskite solar cell absorbers is currently an attractive research direction. The approaches used for and the consequences of Pb replacement are reviewed herein. Reviews on the theoretical understanding of the electronic, optical, and defect properties of Pb and Pb-free halide perovskites and perovskite derivatives are provided, as well as the experimental results available in the literature. The theoretical understanding explains well why Pb halide perovskites exhibit superior photovoltaic properties, but Pb-free perovskites and perovskite derivatives do not.