Lead-free Double Perovskite Nanocrystals Research Articles

Improvement of optical properties of lead-free double perovskite Cs2AgInCl6 nanocrystals through Bi ion alloying

Lead-free double perovskite nanocrystals (NCs) have garnered significant attention due to their non-toxicity and excellent stability, offering vast potential applications in the field of optoelectronics. They are considered promising substitutes for lead-based perovskite NCs. Herein, we successfully synthesized monodisperse, uniformly sized, cubic-shaped Cs2AgIn1-xBixCl6 (0 ≤ x ≤ 1) NCs via a simple hot injection approach. The Cs2AgInCl6 NCs exhibit a lower photoluminescence quantum yield (PLQY) of 3.9 % due to the presence of parity-forbidden transitions. However, with the introduction of Bi3+ alloying, the parity-forbidden transitions are broken, transforming into direct transitions allowed by parity. This transformation leads to efficient bright yellow emission, with the highest PLQY reaching up to 31.6 % (Cs2AgIn0.90Bi0.10Cl6 NCs). Adjusting the Bi3+ alloying concentration allows tuning the NCs band gap from 3.62 eV to 2.88 eV, and the exciton lifetime can be extended from 5.99 ns to 24.88 ns. Temperature-dependent photoluminescence spectra and density functional theory calculations indicate that Bi3+ alloying increases the exciton binding energy, weakens the electron-phonon coupling, and breaks parity-forbidden transitions. Moreover, Cs2AgIn0.90Bi0.10Cl6 NCs display outstanding stability under ambient conditions, laying a solid foundation for their applications in optoelectronics. The results of this study open a new pathway to enhance the optical performance of lead-free double perovskite nanomaterials.

Unraveling Defect-Mediated Enhancement of Transient Photoconductivity and Slower Carrier's Mobility Decay in Cu-Doped Cs2AgBiBr6 Nanocrystals Using Ultrafast Pump-Probe Spectroscopy.

Lead-free double perovskite nanocrystals (A2B'(III)B″(I)X6 NCs) address the instability and toxicity concerns of lead-based counterparts, but their device performance is limited by subpar absorption and unexplored carrier dynamics. Impurity ion doping offers a route to tune electrical conductivity and charge carrier transport. Herein, we synthesized Cu-doped Cs2AgBiBr6 (CABB) nanocrystals using a hot-injection approach and investigated the charge carrier's dynamics through ultrafast pump-probe spectroscopy. Copper introduction into the CABB lattice enhanced absorption in the near-infrared region and introduced sub-band gap defect states in CABB NCs. The transient absorption study revealed a faster bleach decay with increased copper doping, as a result of charge transfer from the conduction band to copper defect states. Also, an optical pump terahertz probe study displays higher photoconductivity and mobility in Cu-doped CABB NCs. Slower mobility decay in Cu-doped systems was attributed to the charge carrier's entrapment at the defect state. These findings suggest that copper-doped CABB is a superior contender for optoelectronic applications over conventional CABB.

Room temperature synthesis of nanocomposite thin films with embedded Cs2AgIn0.9Bi0.1Cl6 lead-free double perovskite nanocrystals with long-term water stability, wide range pH tolerance, and high quantum yield

Cs2AgIn0.9Bi0.1Cl6 nanocrystals encapsulated with polystyrene or polymethyl methacrylate are described with high quantum yields, long luminescence lifetimes and water stability.

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.

Enhanced optical properties of lead-free double perovskite Cs2AgBiBr6 nanocrystals by doping of Na ions

Lead free double perovskite nanocrystals is an ideal alternate considering its nontoxic and stable property comparing with their counterpart lead halide perovskite NCs. A simple solution synthesis strategy was used to prepare lead-free double perovskite Cs2AgBiBr6 nanocrystals (NCs) doped with Na ions, resulting in NCs with a monodisperse state, homogeneous morphology, and good crystallinity. By adjusting the amount of Na+ doping, the band gap of the NCs was significantly increased from 2.57 eV to 3.00 eV. Furthermore, the relatively low value of photoluminescence quantum yield (PLQY) in the original Cs2AgBiBr6 NCs (∼0.5%) was increased to about 8.5% when doped with 60% Na+. Additionally, the exciton lifetime of the NCs was extended to 16.15 ns compared to the original 2 ns. As a result, the doping of Na + significantly enhanced the PLQY of Cs2AgBiBr6 NCs, indicating their potential value for use in optoelectronic devices.

Improved optical properties of lead-free double perovskite Cs2NaBiCl6 nanocrystal via K ions doping

Double perovskite has shown great potential to solve the toxicity and instability problems of lead halide perovskite in practical applications. However, most lead-free double perovskite nanocrystals have low photoluminescence quantum yield (PLQY). Metal ion doping is usually an effective method to improve the optical properties of lead-free double perovskite nanocrystals (NCs). Here, we have prepared monodisperse Cs2NaBiCl6 NCs with uniform morphology and good crystallinity, and a series of Cs2NaBiCl6 NCs with different K+ doping amounts were prepared by colloid synthesis strategy. As the amount of potassium ion doping increased, the absorption spectra of the perovskites shifted towards the red, and the bandgap decreased from 3.56 eV to 3.50 eV. At the same time, the introduction of K+ improved the optical properties of Cs2NaBiCl6 NCs. The PLQY of Cs2Na0.62K0.38BiCl6 NCs reached 7.06 %, which was 4.38 times higher than that of Cs2NaBiCl6 NCs. The lifetime of exciton was increased from 3.42 ns to 12.02 ns by 3.5 times. Thermogravimetric analysis showed that Cs2Na0.62K0.38BiCl6 NCs had better thermal stability than Cs2NaBiCl6 NCs. Temperature-dependent PL spectra showed that Cs2Na0.62K0.38BiCl6 had higher exciton binding energy and longitudinal optical phonon energy than Cs2NaBiCl6 NCs.

Incorporation sodium ions into monodisperse lead-free double perovskite Cs2AgBiCl6 nanocrystals to improve optical properties

Lead-free double perovskite nanocrystals (NCs) have emerged as a promising candidate in the optical field, owing to their non-toxic, good moist heat and chemical stability. However, their poor optical properties limited their application. To improve the optical properties of lead-free double perovskite NCs, metal ion doping or alloying had been suggested as a promising strategy. Here, we prepared monodisperse, uniformly sized, cubic morphology of Cs2AgBiCl6 NCs with different Na+ incorporation amounts via a simple hot-injection method. The Na+ incorporation broke the parity-forbidden transition by reducing the inversion symmetry of the electron wave function at the Ag site, which changed the parity of the self-trapped exciton wave function and thus allowed radiative recombination. As a result, the photoluminescence quantum yield (PLQY) of Na+-alloyed Cs2AgBiCl6 NCs (12.1%) was higher than that of Cs2AgBiCl6 NCs (2.4%), and the exciton lifetime of Na+-alloyed Cs2AgBiCl6 NCs increased to 36.98 ns from 17.58 ns for Cs2AgBiCl6 NCs. By adjusting the amount of Na+ incorporation, the band gap of Cs2AgBiCl6 NCs can be significantly tuned from ∼2.90 eV to ∼3.50 eV. Furthermore, the temperature-dependent photoluminescence spectra indicated that the Na+-alloyed Cs2AgBiCl6 NCs possessed higher longitudinal optical phonon energy and exciton binding energy compared to Cs2AgBiCl6 NCs. This suggested that there were strong exciton-phonon interactions during exciton recombination, a reduced probability of non-radiative processes, and excellent thermal stability. It offers a promising strategy for improving the optical properties of lead-free double perovskite NCs, and have the potential to replace traditional lead halide perovskite NCs in future optoelectronic applications.

Lead-free double perovskite Rb+, Sb3+-codoped Cs2NaInCl6 nanocrystals with highly efficient and tunable photoluminescence

Lead-free double perovskite nanocrystals (LFDP NCs) are promising materials for illumination and display applications because of their potential to overcome the toxicity and instability of lead-based counterparts. Despite the success in the synthesis of LFDP NCs, tunable control over the photoluminescence (PL) of LFDP NCs is still challenging. Herein, brightly emissive and stable Rb+, Sb3+-codoped Cs2NaInCl6 NCs are synthesized while fulfilling tunable self-trapped exciton (STE) emission in the blue region through the A, B-site codoping strategy. Sb3+ is introduced into the B-site of Cs2NaInCl6 NCs to break the parity-forbidden transition and boost the emission efficiency. By further altering the amount of A-site doped Rb+, the emission peak of the NCs is tunable from 447 to 528 nm, because Rb+ enlarges the electron-phonon coupling energy, which leads to larger Stokes shifts and therefore the continuous red-shift of PL emission. The Rb+, Sb3+-codoped Cs2NaInCl6 NCs exhibit robust PL and phase stability against moisture after one year of storage. A series of LFDP NCs with tunable emission are synthesized according to the codoping strategy and further employed as the color conversion materials for fabricating light-emitting diodes.

Bi3+ and Tb3+ Co-doped Cs2AgInCl6 Lead-free double perovskite nanocrystals for detection of temperature and copper ions

The content of Cu2+ in lubricating oil and lubricant temperature are important indicators predicting mechanical failure. Therefore, developing a nontoxic fluorescence probe is necessary to detect Cu2+ and temperature in lubricating oil. The lead-free inorganic double perovskite nanocrystals (NCs) Cs2AgInCl6 are potential candidates. However, the low fluorescence intensity and the high excitation energy required of Cs2AgInCl6 NCs limit their practical applications. In this study, Bi3+ and Tb3+ were successfully co-doped into Cs2AgInCl6 NCs via the hot-injection method. The doping of Bi3+ produces a broad emission originating from self-trapped excitons and reduces the excitation energy, allowing commercial LEDs as excitation sources. Tb3+ ions doping offers characteristic emission peaks (5D0-7FJ) of Tb3+ ions and improves the fluorescence intensity of Cs2AgInCl6 NCs. Furthermore, the Cs2AgInCl6: Bi3+/Tb3+ NCs have been employed as optical thermometry, which provide a temperature calibration curve with the maximum absolute and relative sensitivities of 2.15% K−1 at 350 K and 2.25% K−1 at 303 K in the temperature range of 303–423 K, respectively. Finally, the nanocrystals have been applied to detect Cu2+ in lubricating oil. The fluorescent probe shows a good detection sensitivity of 8.94 × 10-4 nM−1 and a low detection limit of 14.3 nM in the range of 10–300 nM. This work not merely offers a novel way for improving the luminescence performances of double perovskite NCs Cs2AgInCl6, but broadens their potential for detection of Cu2+ and temperature.

Impact of bismuth-doping on enhanced radiative recombination in lead-free double-perovskite nanocrystals.

Lead-free double-perovskite nanocrystals (NCs) have received considerable attention as promising candidates for environmentally friendly optical applications. Furthermore, double-perovskite nanostructures are known to be physically stable compared to most other inorganic halide perovskites, with a generic chemical formula of ABX3 (e.g., A = Cs+; B = Sn2+ or Ge2+; X = Cl−, Br−, I−, or their combination). However, relevant experimental studies on the photophysical properties are still insufficient for Pb-free double-perovskite NCs. Herein, we synthesized Cs2Ag0.65Na0.35InCl6 NCs doped with bismuth (Bi3+) ions and investigated their photophysical properties to reveal the role of the dopant on the enhanced photoemission properties. Specifically, it was found that the photoluminescence quantum yield (PLQY) increased up to 33.2% by 2% Bi-doping. The optical bandgap of the NCs decreased from 3.47 eV to 3.41 eV as the amount of the dopant increased from 2% to 15%. To find out the effect of Bi-doping, the temperature-dependent PL properties of the undoped and doped NCs were investigated by utilizing steady-state and time-resolved PL spectroscopy. With increasing the temperature from 20 K to 300 K, the PL intensities of the doped NCs decreased slower than the undoped ones. The correlated average PL lifetimes of both the bismuth-doped and undoped NCs decreased with increasing the temperature. The experimental results revealed that all the NC samples showed thermal quenching with the temperature increasing, and the PL quenching was suppressed in bismuth-doped NCs.

Bi Doped Lead-Free Double Perovskite Nanocrystals of Cs2agxli1-Xincl6 with High Quantum Yield

Bi Doped Lead-Free Double Perovskite Nanocrystals of Cs2agxli1-Xincl6 with High Quantum Yield

Robust dual cationic ligand for stable and efficient warm-white light emission in lead-free double perovskite nanocrystals

Bismuth-doped alloyed Cs2Ag1-xNaxInCl6 double perovskite nanocrystals (DP NCs) emerged as a new class of lead-free alternative for single-emitter-based solution-processed white-light-emitting devices (WLEDs). However, their thin-film processing and device fabrication have been limited due to low photoluminescence (PL) quantum yield (QY) and degradation during purification from the as-synthesized crude solution. Moreover, easy reduction of a silver ions by oleylamine also raised concern regarding stability issues of silver based DP NCs. Here, we report a facile synthesis and purification method for shape-pure and monodispersed bismuth-doped alloyed Cs2Ag1-xNaxInCl6 DP NCs with high PL QY and long-term stability. At optimized bismuth-doping, alloyed Cs2Ag0.4Na0.6InCl6 DP NCs showed warm white-light-emission with a PL QY of 40% due to suppressed non-radiative recombination. The synthesis of highly emissive and stable DP NCs was achieved by utilizing a combination of strongly coordinating silver-trioctylphosphine (Ag-TOP) complex along with additional TOP ligand. The Ag-TOP complex served as a highly reactive silver precursor and prevented the reduction of silver ions into metallic silver. While TOP facilitated the nucleophilic reaction with benzoyl chloride during nucleation and growth stage and forms benzoyl trioctylphosphonium chloride intermediate that served as both halide source and surface capping ligand which enabled the formation of high-quality DP NCs. The high tolerance of DP NCs against common antisolvents such as methyl acetate and isopropanol was attributed to the tight binding of dual cationic ligand benzoyl trioctylphosphonium and oleylammonium cations together with oleate anion to the surface of DP NCs.

Third-Order Nonlinear Optical Properties and Saturation of Two-Photon Absorption in Lead-Free Double Perovskite Nanocrystals under Femtosecond Excitation

Lead halide perovskites have been widely explored in the field of photovoltaics, light-emitting diodes, and lasers due to their outstanding linear and nonlinear optical (NLO) properties. But, the p...

Lead-free rare-earth double perovskite Cs2AgIn1−γ−xBixLaγCl6 nanocrystals with highly efficient warm-white emission

Lead-free double perovskite nanocrystals (NCs), such as Cs2AgInCl6, have attracted considerable attention as stable and non-toxic alternatives to lead-based perovskites. However, the low photoluminescence (PL) intensity of pristine Cs2AgInCl6 limits its practical applications. In this study, a series of Cs2AgIn1−γ−xBixLaγCl6 NCs were synthesized to break the parity-forbidden transition and modify the associated optical functionalities. A broadband bright warm-white emission in the visible region was achieved, with an excellent PL quantum yield of 60%. The dynamic mechanism, involving ultrafast transient absorption, suggests that high-efficiency PL is induced by triplet self-trapping exciton emission. The incorporation of La3+-Bi3+ facilitated the singlet-triplet transition by increasing the lifetime and quickening the intersystem crossing process. This finding provides a reliable method for optimizing the optical properties of emerging lead-free halide perovskite NCs.

Transparent and Low-Loss Luminescent Solar Concentrators Based on Self-Trapped Exciton Emission in Lead-Free Double Perovskite Nanocrystals

Luminescent solar concentrators (LSCs) are light-harvesting devices that redirect solar light to an edge-attached photovoltaic cell, and thus, they have high potential to be incorporated directly into buildings’ windows to allow for generating electricity. Perovskite nanocrystals (PNCs) are promising materials for LSCs because their enticing optical properties can be engineered to provide a high photoluminescence (PL) quantum yield (QY) and low overlap between absorption and emission spectra. Replacement of toxic, lead-containing perovskites in LSCs by lead-free PNCs, while retaining high optical efficiency of the device, remains the key challenge, which needs to be overcome to build environmentally friendly solar-harvesting platforms. In this work, we use nanocrystals of Bi-doped Cs2Ag0.4Na0.6InCl6 double perovskites with a self-trapped exciton emission to realize for the first time a transparent, low-reabsorption, lead-free perovskite-based LSC. Fabricated 100 cm2 LSCs show an internal optical quantum efficiency of 21.2% with the corresponding internal concentration factor of 2.7. Monte Carlo (MC) ray-tracing simulations identified the loss caused by nonunity PL QY to be the most significant contribution to the overall efficiency loss. The MC simulations also allowed us to estimate the efficiency of 39.4% for 2,500 cm2 LSCs with hypothetical unity PL. These results demonstrate a significant promise held by Bi-doped lead-free PNCs for LSCs.

Excitation Management of Lead-Free Perovskite Nanocrystals Through Doping.

Despite their low toxicity and phase stability, lead-free double perovskite nanocrystals, Cs2AgInCl6 in specific, have suffered from low quantum yield of photoluminescence. This is mainly due to two reasons, including (i) the quenching effect from metal silver which was usually formed at high temperature from Ag+ reduction in the presence of organic amines and (ii) the parity-forbidden transition of pristine double perovskites. Here, we reported a room-temperature synthesis of Cs2AgInCl6 nanocrystals in an inverse microemulsion system, where Ag+ reduction was largely suppressed. By codoping Bi and Na ions, dark self-trapping excitons (STEs) were converted into bright ones, enabling a bright phosphor of photoluminescence quantum yield up to 56%. Importantly, the doping approach at room temperature relaxed the parity-forbidden transition (1S0 → 3P2) of Bi-6s2 orbitals, revealing a fine structure of a triband excitation profile. Such spin-rule relaxation was ascribed to symmetry breaking of the doped lattice, which was evidenced by Raman spectroscopy. In a proof-of-concept experiment, the bright nanocrystals were used as a color-converting ink, which enabled a stable white light light-emitting diode to operate in various environments, even under water, for long-term service.

Room temperature synthesis and characterization of novel lead-free double perovskite nanocrystals with a stable and broadband emission

We demonstrated the synthesis of ultra small and stable Cs3BiBr6 nanocrystals, ∼1.5–3 nm, via a room-temperature antisolvent method. Red-shift of bandgap was observed in low temperature PL measurements with an activation energy of ∼41 meV.

Colloidal lead-free Cs2AgBiBr6 double perovskite nanocrystals: Synthesis, uniform thin-film fabrication, and application in solution-processed solar cells

Recently developed lead-free double perovskite nanocrystals (NCs) have been proposed for the possible application in solution-processed optoelectronic devices. However, the optoelectronic applications of double perovskite NCs have been hampered due to the structural and chemical instability in the presence of polar molecules. Here, we report a facile strategy for the synthesis and purification of Cs2AgBiBr6 double perovskite NCs that remained stable even after washing with polar solvent. This is realized with our efficient colloidal route to synthesize Cs2AgBiBr6 NCs that involve stable and strongly coordinated precursor such as silver-trioctylphosphine complex together with bismuth neodecanoate, which leads to a significantly improved chemical and colloidal stability. Using layer-by-layer solid-state ligand exchange technique, a compact and crack-free thin film of Cs2AgBiBr6 NCs were fabricated. Finally, perovskite solar cells consisting of Cs2AgBiBr6 as an absorber layer were fabricated and tested.

Self-trapped exciton engineering for white-light emission in colloidal lead-free double perovskite nanocrystals

Intrinsic broadband photoluminescence (PL) of self-trapped excitons (STEs) are systematically studied in lead-free double perovskite nanocrystals (NCs). It is clarified that bandgap (direct/indirect) has important influence on the PL properties of STEs: indirect bandgap NCs exhibit strong exciton-phonon coupling which results in non-radiative STEs, while direct bandgap NCs exhibit moderate exciton-phonon coupling, inducing bright STE PL. Furthermore, by alloying K+ and Li+ ions in Cs2AgInCl6 NCs, the NCs exhibit broadband white-light emission. Charge-carrier dynamics study indicates that the efficient white-light emission originates from the further suppressed non-radiative processes of the STEs in the direct bandgap structure. This work may deepen the understanding of STEs and guide the design of high-performance lead-free perovskites.

Machine-Learning-Accelerated Perovskite Crystallization

Summary Perovskites have seen significant research interest in the last decade. As ternary and quaternary compounds, their chemical space is exceptionally large, yet perovskite development has been limited to a restricted set of chemical constituents often discovered through trial and error. Here, we report a high-throughput experimental framework for the discovery of new perovskite single crystals. We use machine learning (ML) to guide the sequence of ever-improved robotic synthetic trials. We perform high-throughput syntheses of perovskite single crystals with a protein crystallization robot and characterize the outcomes with the aid of convolutional neural network-based image recognition. We then use an ML model to predict the optimal conditions for the synthesis of a new perovskite single crystal, enabling us to report the first synthesis of (3-PLA)2PbCl4.This material exhibits strong blue emission, illustrating the applicability of the method in identifying new optoelectronic materials.