Lead-free Halide Double Perovskites Research Articles

Recent Advances in All-Inorganic Lead-Free Three-Dimensional Halide Double Perovskite Nanocrystals

Recent Advances in All-Inorganic Lead-Free Three-Dimensional Halide Double Perovskite Nanocrystals

Potential lead-free small band gap halide double perovskites Cs2CuMCl6 (M\u2009=\u2009Sb, 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.

Stability and optoelectronic property of lead-free halide double perovskite Cs2 B′BiI6 (B′ = Li, Na and K)* *Project supported by the National Natural Science Foundation of China (Grant No. 11864008) and Guangxi Natural Science Foundation, China (Grant Nos. 2018GXNSFAA138185, 2018AD19200, and 2019GXNSFGA245006).

Although lead-based perovskite solar cells have achieved more than 25% power conversion efficiency, the toxicity of lead and instability are still urgent problems faced in industrial application. Lead-free halide double perovskite (DP) materials are promising candidates to resolve these issues. Based on the density functional theory, we explore the geometric stability, thermodynamic stability, mechanical stability, electronic structures, and optical properties of the Cs2 B′BiI6 (B′ = Li, Na and K) DP materials. By analyzing the tolerance factor and octahedral factor, we find the geometric stabilities of Cs2NaBiI6 and Cs2KBiI6 DPs are better than Cs2LiBiI6. By calculating the total energy, formation energy and decomposition energy, we propose that the most favorable structure of Cs2 B′BiI6 is the orthorhombic phase, and Cs2LiBiI6 is less stable relative to the other two counterparts from an energetic viewpoint. Mechanical stability evaluations reveal that the orthorhombic Cs2LiBiI6 material is less stable relative to the isostructural Cs2NaBiI6 and Cs2KBiI6 DPs. The mechanical property calculations indicate that the Cs2 B′BiI6 DPs possess good ductility, which can be used as flexible materials. Electronic structures and optical property calculations show that the orthorhombic Cs2 B′BiI6 DPs have suitable band gap values, weaker exciton binding energies, and excellent optical absorption performance in the visible-light range. Based on the above comprehensive assessments, we can conclude that the orthorhombic Cs2NaBiI6 and Cs2KBiI6 DPs with good stability are promising candidates for solar cell applications.

Band-Gap Engineering of Lead-Free Iron-Based Halide Double-Perovskite Single Crystals and Nanocrystals by an Alloying or Doping Strategy

Lead-free halide double perovskites (DPs) have been considered to be an effective alternative for solving the problem of lead perovskite toxicity and instability. We report a series of lead-free halide DP single crystals (SCs) and nanocrystals (NCs) (Cs2AgxNa1–xFeCl6 (0 ≤ x ≤ 1)). The alloying strategy not only effectively regulates the optical band gap but also significantly improves the air stability and thermal stability. The absorption edge of the NCs is slightly blue-shifted compared to that of the corresponding SCs. Excitingly, we find that Fe3+ ions can be successfully doped into chloride DPs Cs2M′MCl6 (M′ = Na or Ag; M = In, Sb or Bi) and cause the band gap to effectively decrease with universality. In addition, the effect of crystal size on stability is further studied. This work not only explore a series of lead-free transition-metal DPs but also brings about more possibilities for optoelectronic applications in lead-free perovskites.

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.

Chiral Lead-Free Hybrid Perovskites for Self-Powered Circularly Polarized Light Detection.

Metal-halide perovskites are recently emerging as the promising alternative for CPL detection owing to their CPL-sensitive property induced by chiral organics and efficient charge transport of inorganic frameworks. However, most of these reported chiral perovskites involve high concentrations of toxic Pb which will become the potential bottleneck for their further application. Herein, we successfully developed two lead-free halide double perovskites, [(R)-β-MPA]4 AgBiI8 ((R)-β-MPA=(R)-(+)-β-methylphenethylammonium, 1-R), and [(S)-β-MPA]4 AgBiI8 ((S)-β-MPA=(S)-(-)-β-methylphenethylammonium, 1-S). Circular dichroism measurements reveal that these perovskites exhibit notable chirality induced by organic cations to distinguish different polarization states of CPL photons. Significantly, they present unique chiral polar photovoltaic, and resulting self-powered CPL detection without an external power source is unprecedentedly achieved. Furthermore, an anisotropy factor up to 0.3 is acquired for the self-powered CPL detection, reaching the highest value among reported chiral perovskites. This work suggests hybrid double perovskites are promising photoelectronic candidates, and provides a new approach for exploring new "green" circularly polarized light-sensitive materials with high performance.

Lead-Free Halide Double Perovskite Nanocrystals for Light-Emitting Applications: Strategies for Boosting Efficiency and Stability.

Lead‐free halide double perovskite (HDP) nanocrystals are considered as one of the most promising alternatives to the lead halide perovskite nanocrystals due to their unique characteristics of nontoxicity, robust intrinsic thermodynamic stability, rich and tunable optoelectronic properties. Although lead‐free HDP variants with highly efficient emission are synthesized and characterized, the photoluminescent (PL) properties of colloidal HDP nanocrystals still have enormous challenges for application in light‐emitting diode (LED) devices due to their intrinsic and surface defects, indirect band, and disallowable optical transitions. Herein, recent progress on the synthetic strategies, ligands passivation, and metal doping/alloying for boosting efficiency and stability of HDP nanocrystals is comprehensive summarized. It begins by introducing the crystalline structure, electronic structure, and PL mechanism of lead‐free HDPs. Next, the limiting factors on PL properties and origins of instability are analyzed, followed by highlighting the effects of synthesis strategies, ligands passivation, and metal doping/alloying on the PL properties and stability of the HDPs. Then, their preliminary applications for LED devices are emphasized. Finally, the challenges and prospects concerning the development of highly efficient and stable HDP nanocrystals‐based LED devices in the future are proposed.

Lead-Free Alloyed Double-Perovskite Nanocrystals of Cs2(NaxAg1–x)BiBr6 with Tunable Band Gap

Lead-free halide double perovskites (DPs) have emerged as a stable and greener alternative to very toxic lead-based perovskites owing to their outstanding photophysical properties. Here, a series of Cs2(NaxAg1–x)BiBr6 (x = 0, 0.25, 0.5, 0.75, and 1) double-perovskite nanocrystals (NCs) are synthesized, which have not been much explored yet. The formation of highly miscible Cs2(NaxAg1–x)BiBr6 solid solutions is favored because of very low lattice mismatch between Cs2NaBiBr6 and Cs2AgBiBr6. These DP NCs have tunable band gaps, which successively increase with increasing Na/Ag ratio, enabling us to tune their band gap by more than 0.5 eV. The band gap tuning is further confirmed by electronic structure calculations using density functional theory (DFT), which is in line with the band gap estimated from the absorption data. The Cs2(NaxAg1–x)BiBr6 (x = 0, 0.25, 0.5) DP NCs are found to be more stable in the ambient conditions than the Cs2(NaxAg1–x)BiBr6 (x = 0.75, 1.00) DP NCs.

Pressure-induced bandgap engineering of lead-free halide double perovskite (NH4)2SnBr6.

Lead-free halide double perovskites (HDPs) have recently been proposed as potential stable and environment-friendly alternatives to lead-based halide perovskites. Bandgap engineering plays a vital role in the optoelectronic applications of HDP materials. In this study, methods combining high-pressure techniques with density functional theory calculations were employed to implement the bandgap engineering of a classic HDP-based (NH4)2SnBr6. Under high pressure, (NH4)2SnBr6 exhibits a redshift of the bandgap with increasing pressure up to 6.3 GPa and a sudden blueshift up to 20.2 GPa, followed by a redshift at higher pressures, which is relevant to the cubic-tetragonal phase transition, direct-indirect transition, and amorphization, respectively. Our results enrich the understanding of the structural-optical properties of (NH4)2SnBr6 and reveal the special role of NH4+ cations in pressure-induced bandgap engineering, thus providing important information for application in optoelectronic devices and helping to design ideal materials with higher efficiency.

Highly efficient and stable broadband near-infrared-emitting lead-free metal halide double perovskites

A novel lead-free perovskite variant with controllable morphology evolution was developed as a highly efficient and stable broadband near-infrared emitter. High-temperature post-treatment boosted the PLQY of the broadband NIR emission by 13-fold.

Investigation of Structural, Electronic and Optical Properties of Na2InAgCl6, K2InAgCl6, and Rb2InAgCl6 Lead-Free Halide Double Perovskites Regarding with Cs2InAgCl6 Perovskites Cell and a Comparative Study by DFT Functionals

Electronics band structures, the total density of state, the partial density of state and optical properties were investigated by DFT method for A2InAgCl6 (A= Cs, Na, K, and Rb). Moreover, the band gap of Na2InAgCl6, K2InAgCl6, Rb2InAgCl6 and Cs2InAgCl6 were calculated at 1.039 eV, 1.041 eV, 1.039 eV and 1.089 eV by GGA with PBE, 0.784 eV, 0.769 eV, 0.775 eV and 0.770 eV by LDA with CA-PZ and 1.310 eV, 1.152 eV, 1.180 eV and 1.169 eV by GGA with RPBE functionals. The density of states (DOS) and partial density of states (PDOS) were evaluated. Among of used functionals, GGA with RPBE functional has considered the appropriate and acceptable method for calculation of band gap where the 1.310 eV of band gap was reported for Cs2InAgCl6, which is close to experimental value at 1.370 eV. The crystal, Na2InAgCl6, has considered as the best perovskites cell among other four due to low band gap, and the main cause is revealed that it has attached the lightest Na atom to convey the low band gap as lower surface or atomic size atom.

Structural, Electronic, and Optical Properties of Lead-Free Halide Double Perovskite Rb2AgBiI6: A Combined Experimental and DFT Study

Hybrid lead halide perovskites have emerged as an attractive photoactive semiconductor for optoelectronic applications such as photovoltaics. However, their toxicity and stability issues pose significant challenges to its wide-scale applications and hence the need to find alternative perovskites that are stable and environmentally benign. Recently, double perovskites have been suggested as a potential alternative owing to their non-toxicity and high stability. In the present study, we report the first synthesis of Rb2AgBiI6 thin films in a cubic crystal structure using a facile room-temperature single-step solution process synthesis method and explore their potential optoelectronic applications. The structural, thermal, and mechanical stability, electronic, and optical properties are studied using various experimental techniques, and the results are further corroborated by first-principles density functional theory (DFT) calculations. The Rb2AgBiI6 film has an estimated band gap ∼ of 1.98 eV with the demonstrated thermal stability of ∼ 440 oC, suggesting its potential suitability for low-cost thin-film solar cells. The initial fabricated photovoltaic device without optimization of the synthesis conditions and device architecture show power conversion efficiency (PCE) of 0.1 % and an open-circuit voltage (Voc) of 0.46 V. The successful incorporation of Rb in Bi-based double perovskite should open the way to a new class of Rubidium-based perovskites with significant potential for optoelectronic applications.

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.

Investigating the property of water driven lead-free stable inorganic halide double perovskites

Lead free halide double perovskite materials, A2BB́X6 (where A, B and B́ are cations and X is a halide anion) have achieved considerable attention in the field of optoelectronic devices due to their high thermal along with the moisture stability and less toxicity as lead halide perovskites suffer from the stability and toxicity issues which inhibit them to be commercialized. Therefore, synthesis of low cost and stable perovskite materials are the main focus of perovskite family nowadays. Herein, we have reported lead free Cs2AgBiCl6 and Cs2AgBiBr6 double perovskite microcrystals in both organic and a mixture of the aqueous-organic medium. Our studies are not only eradicating the toxicity of lead but also explored towards the stability of perovskite materials in the aqueous medium. Morphology is investigated using SEM and TEM imaging along with the enhancement in emission peak by increasing the content of water.

Capillary-Bridge Controlled Patterning of Stable Double-Perovskite Microwire Arrays for Non-toxic Photodetectors.

Single-crystalline lead halide perovskites with remarkable physical properties offer great potential in integrated optoelectronic applications but are restricted by their instability and toxicity. To address these problems, various strategies including lead-free halide double perovskites with high stabilities of heat, light, and moisture have been developed. However, it still requires an efficient method to pattern single-crystalline, double-perovskite micro-/nanostructures with strict alignment and ordered orientation for the integration of optoelectronic devices. Here, our solution-processing approach employs capillary bridges to control the dewetting dynamics and confine the crystallization in the assembly of non-toxic Cs2AgBiBr6 microwire arrays. We demonstrate the strict alignment, high crystallinity, eliminated grain boundary, and ordered orientation of these as-prepared single-crystalline, double-perovskite microwire arrays. Based on these high-quality microwire arrays, we fabricate high-performance photodetectors with a responsivity of 1,625 A W−1, on/off ratio of 104, and fast response speed of τdecay = 0.04 ms and τrise = 0.28 ms. The long-term crystallographic and spectroscopic stability of Cs2AgBiBr6 microwire arrays has also been demonstrated through the 1 month exposure to air conditioning. Our strategy provides a new perception to fabricate stable perovskite microarrays for the integration of non-toxic optoelectronic devices.

Lead-Free Small-Bandgap Cs2CuSbCl6 Double Perovskite Nanocrystals.

Lead-free halide double perovskites (DPs) have attracted great attention due to their stability, nontoxicity and good photophysical property. In this work, we report a new, small-bandgap Cs2CuSbCl6 DP nanocrystals (NCs) with a bandgap of 1.66 eV, which is the smallest bandgap in reported lead-free three-dimensional DP NCs. Density functional theory calculations confirm that Cs2CuSbCl6 DP has an indirect bandgap of 1.70 eV, in good agreement with the experimental result. The photophysical property of Cs2CuSbCl6 NCs is studied by the combination of femtosecond transient absorption (TA) and nanosecond TA techniques. In addition, the Cs2CuSbCl6 NCs exhibit good stability exposed in the air. These results show that these NCs will have great potential to be used as a light-absorbing material for photovoltaic applications.

Anharmonicity and Ultralow Thermal Conductivity in Lead-Free Halide Double Perovskites.

The lead-free halide double perovskite class of materials offers a promising venue for resolving issues related to toxicity of Pb and long-term stability of the lead-containing halide perovskites. We present a first-principles study of the lattice vibrations in Cs_{2}AgBiBr_{6}, the prototypical compound in this class and show that the lattice dynamics of Cs_{2}AgBiBr_{6} is highly anharmonic, largely in regards to tilting of AgBr_{6} and BiBr_{6} octahedra. Using an energy- and temperature-dependent phonon spectral function, we then show how the experimentally observed cubic-to-tetragonal phase transformation is caused by the collapse of a soft phonon branch. We finally reveal that the softness and anharmonicity of Cs_{2}AgBiBr_{6} yield an ultralow thermal conductivity, unexpected of high-symmetry cubic structures.

Tailorable Indirect to Direct Band-Gap Double Perovskites with Bright White-Light Emission: Decoding Chemical Structure Using Solid-State NMR.

Efficient white-light-emitting single-material sources are ideal for sustainable lighting applications. Though layered hybrid lead-halide perovskite materials have demonstrated attractive broad-band white-light emission properties, they pose a serious long-term environmental and health risk as they contain lead (Pb2+) and are readily soluble in water. Recently, lead-free halide double perovskite (HDP) materials with a generic formula A(I)2B'(III)B″(I)X6 (where A and B are cations and X is a halide ion) have demonstrated white-light emission with improved photoluminescence quantum yields (PLQYs). Here, we present a series of Bi3+/In3+ mixed-cationic Cs2Bi1-xInxAgCl6 HDP solid solutions that span the indirect to direct band-gap modification which exhibit tailorable optical properties. Density functional theory (DFT) calculations indicate an indirect-direct band-gap crossover composition when x > 0.50. These HDP materials emit over the entire visible light spectrum, centered at 600 ± 30 nm with full-width at half maxima of ca. 200 nm upon ultraviolet light excitation and a maximum PLQY of 34 ± 4% for Cs2Bi0.085In0.915AgCl6. Short-range structural insight for these materials is crucial to unravel the unique atomic-level structural properties which are difficult to distinguish by diffraction-based techniques. Hence, we demonstrate the advantage of using solid-state nuclear magnetic resonance (NMR) spectroscopy to deconvolute the local structural environments of these mixed-cationic HDPs. Using ultrahigh-field (21.14 T) NMR spectroscopy of quadrupolar nuclei (115In, 133Cs, and 209Bi), we show that there is a high degree of atomic-level B'(III)/B″(I) site ordering (i.e., no evidence of antisite defects). Furthermore, a combination of XRD, NMR, and DFT calculations was used to unravel the complete atomic-level random Bi3+/In3+ cationic mixing in Cs2Bi1-xInxAgCl6 HDPs. Briefly, this work provides an advance in understanding the photophysical properties that correlate long- to short-range structural elucidation of these newly developed solid-state white-light emitting HDP materials.

Transfer of Direct to Indirect Bound Excitons by Electron Intervalley Scattering in Cs2AgBiBr6 Double Perovskite Nanocrystals

Lead-free halide double perovskites have emerged as a non-toxic alternative to the heavily researched lead-based halide perovskites. However, their optical properties and the initial charge carrier relaxation processes are under debate. In this study, we apply time-resolved photoluminescence and differential transmission spectroscopy to investigate the photo-excited charge carrier dynamics within the indirect band structure of Cs2AgBiBr6 nanocrystals. Interestingly, we observe a high energetic emission stemming from the direct band gap, besides the previously reported emission from the indirect band gap transition. We attribute this emission to the radiative recombination of direct bound excitons. This emission maximum redshifts nearly 1 eV within 10 ps due to electron intervalley scattering, which leads to a transfer of direct to indirect bound excitons. We conclude that these direct bound excitons possess a giant oscillator strength causing not only a pronounced absorption peak at the optical band gap energy but also luminescence to occur at the direct band gap transition in spite of the prevailing intervalley scattering process. These results expand the understanding of the optical properties and the charge carrier relaxation in double perovskites, thus, facilitating the further development of optoelectronic devices harnessing lead-free perovskites.

Lead-free perovskite and double perovskite solar cells

The toxicity and instability of lead-halide perovskite materials in the solar cells are yet demands must be resolved to vie to find alternatives of free lead-based halide perovskites and double perovskite materials. Here we report on the experimental synthesis of a new family of lead-free inorganic halide double perovskites Cesium copper antimony chloride (Cs2CuSbCl6) and perovskite Cesium copper chloride (CsCuCl3) as a based sensitizer of the solar cells and studies of its characterize and optical properties of the Photo-Voltaic (PV) device. The configuration structural of PV devices is (FTO/TiO2/CsCuCl3 or Cs2CuSbCl6/Cu2O/Al electrode). The double perovskite Cs2CuSbCl6 and Perovskite CsCuCl3 are successfully synthesized, performed of structural investigations by X-ray diffraction, and depicted of optical properties via optical absorption of UV−vis measurement. The perovskite (CsCuCl3) belongs to the space grope P6122 and the Hexagonal structure phase. The strongest peeks of the XRD diffraction pattern of the double perovskite Cs2CuSbCl6 are taken at the reflection points (-201), (-202), (-3-11), (220), (-223) and (-5-34). From UV−vis measurements obtained values of a direct gap of both (Cs2CuSbCl6) and (CsCuCl3) are (1.5 and 2.2) eV, respectively. The optimum power conversion efficiency (PCE) of the PV device which used Cs2CuSbCl6 as a sensitizer is 0.94%, the filling factor is 24%, the open-circuit voltage is 582.5 mV and the short circuit current was 2.656 mA. The measurement is carried out under one sun intensity illumination (1000 W/m2).