Lead-free Metal Halide Perovskites Research Articles

Design and efficiency enhancement of FTO/PC60BM/CsSn0.5Ge0.5I3/Spiro-OMeTAD/Au perovskite solar cell utilizing SCAPS-1D Simulator

The poisoning potential of lead, which is the main component of the absorber layer of lead halide (Pb) perovskites, as well as the stability problems of the manufactured devices, constitute a major obstacle to the industrialization of this technology. As a result, recent research is concentrating on lead-free metal halide perovskites. Unfortunately, current lead-free perovskites suffer from poor performance, hence the interest of our study. The research presented here shows that optimizing several variables related to the performance of each layer of a perovskite solar cell (PSC) constructed from lead-free inorganic materials provides an efficiency of 18.13%. We designed a structure with outstanding performance using the FTO/PC60BM/CsSn0.5Ge0.5I3/Spiro-OMeTAD/Au configuration. The impact of various relevant factors, such as the thickness and defect density of the absorber layer their doping densities, the back contact work, and the operating temperature, have been thoroughly investigated to boost the performance of the proposed device. The performance of cesium-tin-germanium triiodide (CsSn0.5Ge0.5I3) solar cells with different electron transport materials, including ZnO, TiO2, CdS, C60; Cd0.5Zn0.5S, IGZO, has also been examined. It has been demonstrated that using ZnO as an electron transport layer improves electron extraction and, therefore, performance. The best outcomes are obtained after optimizing all the factors mentioned above, namely: Jsc of 28.70 mA/cm2, Voc of 1.115 V, FF of 87.86%, and PCE of 18.13%. Additionally, the explored structure may be an excellent candidate for the future development of lead-free perovskite solar cells.

Perovskite superlattices with efficient carrier dynamics.

Compared with their three-dimensional (3D) counterparts, low-dimensional metal halide perovskites (2D and quasi-2D; B2An-1MnX3n+1, such as B = R-NH3+, A = HC(NH2)2+, Cs+; M = Pb2+, Sn2+; X = Cl-, Br-, I-) with periodic inorganic-organic structures have shown promising stability and hysteresis-free electrical performance1-6. However, their unique multiple-quantum-well structure limits the device efficiencies because of the grain boundaries and randomlyoriented quantum wells in polycrystals7. In single crystals, the carrier transport through the thickness direction is hindered by the layered insulating organic spacers8. Furthermore, the strong quantum confinement from the organic spacers limits the generation and transport of free carriers9,10. Also, lead-free metal halide perovskites have been developed but their device performance is limited by their low crystallinity and structural instability11. Here we report a low-dimensional metal halide perovskite BA2MAn-1SnnI3n+1 (BA, butylammonium; MA, methylammonium; n = 1, 3, 5) superlattice by chemical epitaxy. The inorganic slabs are aligned vertical to the substrate and interconnected in a criss-cross 2D network parallel to the substrate, leading to efficient carrier transport in three dimensions. A lattice-mismatched substrate compresses the organic spacers, which weakens the quantum confinement. The performance of a superlattice solar cell has been certified under the quasi-steady state, showing a stable 12.36% photoelectric conversion efficiency. Moreover, an intraband exciton relaxation process may have yielded an unusually high open-circuit voltage (VOC).

Temperature-activated dielectric relaxation in lead-free halide perovskite single crystals

Lead-free metal-halide perovskites have recently appeared as a promising candidate in optoelectronics and photovoltaics because of their non-toxicity, stability, and unique photophysical properties. Much scientific research has been done on optoelectronic characteristics and photovoltaic applications of lead-free perovskites, but the dielectric characteristics and insight into the relaxation phenomenon remain elusive. Here, we study the dielectric relaxation and conduction mechanism in the single crystalline (SC) A3Bi2X9 (A = MA+/FA+) perovskite using temperature-dependent electrochemical impedance spectroscopy in correlation with the modulus spectroscopy. With increasing temperature, the peak of −Z″(ω) shifts toward a high-frequency regime which specifies the thermally dependent relaxation mechanism in both crystals. The activation energy was estimated as 381 meV for MA3Bi2I9 (MBI) crystal and 410 meV for the FA3Bi2I9 (FBI) crystal suggesting hopping of mobile ions between lattice sites. The connected orientational polarization with the thermal motion of molecules leads to the enhancement in the dielectric constant (ϵ′) with temperature. The ϵ″(ω) in these crystals shows the significant ionic conductivity with a typical 1/fγ type characteristics (in the low-frequency regime) where γ is found to be in the range of 0.93–1.0 for MBI crystal and 0.88–0.98 for FBI crystal. The correlated imaginary part of impedance (−Z″) and modulus (M″) demonstrate the temperature-activated delocalized relaxation (non-Debye toward the Debye type) in these crystals. Stevels model suggests that the contribution of traps reduces with temperature rise and therefore conductivity enhances. Our study provides a comprehensive analysis and in-depth knowledge about the dielectric and conductivity relaxation mechanism in these lead-free perovskite SCs, which will help to implement efficient energy storage devices using these materials.

Activating photocatalytic hydrogen generation on inorganic lead-free Cs2AgBiBr6 perovskite via reversible Cu2+/Cu+ redox couple

Photocatalytic hydrogen evolution from hydrohalic acid by lead-free metal halide perovskite holds great potential but still faces ineffective charge separation and low solar-to-hydrogen conversion efficiency challenges. In the present study, a reversible Cu2+/Cu+ redox couple is introduced into the inorganic lead-free perovskite Cs2AgBiBr6/(HBr + H3PO2)-based photocatalytic H2 evolution system to achieve fast hole extraction and effective charge separation. The rapid Cu+ oxidation by photogenerated holes with more favorable thermodynamics and faster kinetics overcomes the intrinsic slow Br− oxidation half-reaction, leading to rapid extraction and consumption of holes, which dramatically alleviates the charge recombination and improves the utilization efficiency of electrons for protons reduction. As a result, up to 70 times enhancement of H2 production is achieved on Cs2AgBiBr6 photocatalyst with the introduction of Cu2+/Cu+ redox couple.

Piezo-phototronic effect in highly stable lead-free double perovskite Cs2SnI6-PVDF nanocomposite: Possibility for strain modulated optical sensor

Metal halide perovskites (MHPs) are highly emerging materials for their applications in the field of photovoltaics and optoelectronics. However, beyond these applications, the MHPs have been proven to be excellent candidates for the fabrication of self-powered nanogenerators on their own or in conjunction with piezoelectric polymers such as poly(vinylidene-fluoride) (PVDF). More so, lead-free MHPs are the need of hour, which opens up the possibility of a sustainable future. Additionally, in piezoelectric composite films containing MHP, it is possible to utilize the coupling of photoexcitation, semiconducting and piezoelectric properties, which leads to the piezo-phototronic effect. Here, we report the fabrication of Cs2SnI6-PVDF (CSIP) composite, where the synthesis of Cs2SnI6 nanoparticles was realized in an in-situ approach. Notably, the Cs2SnI6 nanoparticles nucleates 100% electroactive β- and γ-phases in PVDF which gives rise to the intrinsic piezoelectric nature of the lead free composite film. Furthermore, to decipher the interfacial interaction between Cs2SnI6 and PVDF first principle DFT study was performed which unravels the non-bonding interaction between the Cs2SnI6 and PVDF. The piezo-phototronic effect was successfully realized in the CSIP film indicating its promising use in the piezotronics sensor. Moreover, the CSIP films were utilized as a piezoelectric nanogenerator, which shows superior piezoelectric output voltage of 9 V and current of 5 μA. It can also be utilized in human physiological signal monitoring, as well as a pressure sensor. Hence, the combined properties of semiconducting, photoexcitation and piezoelectricity in the CSIP films can be utilized in both mechanical energy harvesting and piezotronics sensing applications.

Highly stable metal halides Cs2ZnX4 (X = Cl, Br) with Sn2+ as dopants for efficient deep-red photoluminescence

The development of deep-red emitting lead-free metal-halide perovskites with high photoluminescence quantum yields (PLQYs) and outstanding stability remains a major challenge for displays and deep-tissue bioimaging. In this work, we report a facile and convenient solvothermal method to synthesize metal halides Cs2ZnX4 (X = Cl, Br) that however is PL innert at room temperature. Upon composition engineering utilizing Sn2+ as the dopant, the resulting Cs2ZnCl4:Sn not only emits strong deep-red PL peaked at 700 nm with the highest 99.4% PLQY among the similar materials so far, but also exhibits excellent structure stability in air (PLQY remains 96% after one year exposure to the atmosphere). Detailed experimental characterizations and theoretical calculations reveal that the deep-red emission stems from self-trapped excitons induced by the Sn2+ dopant. Particularly, triplet emission (3P2→1S0) from Sn-5s2 orbitals has been observed at low temperature due to the break of parity-forbidden transition. This work provides an important guidance for the development of deep-red light-emitting materials with low price, high efficiency and excellent stability.

Deep-blue emissive Cs3Cu2I5 perovskites nanocrystals with 96.6% quantum yield via InI3-assisted synthesis for light-emitting device and fluorescent ink applications

Lead-free metal halide perovskites are promising materials for future optoelectronic application, which have recently attracted great attention for their environmental-friendliness and excellent light emission properties. However, the development of deep-blue emission lead-free halide perovskite nanocrystal is still far behind in performance in comparison with its red/green counterparts. Herein, the lead-free Cs3Cu2I5 nanocrystals are prepared via a modified hot-injection strategy. By adding InI3 as precursor, the synthesized Cs3Cu2I5 single-phase nanocrystals show a bright deep-blue emission peaked at ~440 nm, with a record high photoluminescence quantum yield (PL QY) up to 96.6% at an optimized InI3 content of 0.15 mmol. The InI3 additive contributes to the high PL QY through both iodide vacancy reparation of I- ions and size-controlled growth of In3+ ions. First principle calculations reveal that the broadband blue emission originates from self-trapped excitons in Cs3Cu2I5, which are formed due to the excited-state lattice expansion and Cu-I bond lengthening of [Cu2I5]3- cluster under light excitation. The good stabilities of these Cs3Cu2I5 nanocrystals against heat, water, UV irradiation and environmental storage are also investigated experimentally and theoretically in detail. For application demonstration, light-emitting devices are manufactured by using these Cs3Cu2I5 nanocrystals powder as phosphor, which exhibit good working stability with a half-lifetime over 300 h. Besides, the Cs3Cu2I5 nanocrystals solution is also utilized as fluorescent ink, and the printed patterns can maintain anti-counterfeiting features even after treatment by heating and long-term environmental storage, indicating the robustness of these Cs3Cu2I5 nanocrystals for potential anti-counterfeiting application.

Highly crystalline lead-free Cs3Sb2Br9 perovskite microcrystals enable efficient and selective photocatalytic oxidation of benzyl alcohol

Selective oxidation of alcohols to the corresponding aldehydes constitutes a great challenge in organic and green chemistry, which typically requires environmentally unfriendly oxidants/catalysts or relatively harsh conditions (high temperature & pressure). Herein, we report an eco-friendly perovskite-based photocatalyst, in which all-inorganic perovskite Cs3Sb2Br9 microcrystals with high crystallinity were prepared using a facile antisolvent method at various temperatures. For the photocatalytic oxidation of benzyl alcohol, the Cs3Sb2Br9 microcrystal photocatalysts exhibited up to 77% conversion and 88% selectivity toward benzaldehyde under oxygen atmosphere and visible-light illumination. More significantly, the comparative experimental results in air were close to those in oxygen, with the observation of 96% conversion and 94% selectivity for the optimized conditions under air atmosphere. Notably, the conversion of our best-performing photocatalyst was 32 and 6.4 times higher than that of the lead-based CsPbBr3 and FAPbBr3 (FA: formamidinium) perovskites, respectively. Furthermore, the underlying photocatalytic oxidation mechanism was proposed by the electron spin resonance (ESR) and the controlled experiments using specific scavengers. This study sheds light on promising new photocatalytic applications of the lead-free metal halide perovskites.

Post-doping induced morphology evolution boosts Mn2+ luminescence in the Cs2NaBiCl6:Mn2+ phosphor.

As we know, defects caused in the synthetic process of metal halide perovskite are the most difficult to overcome, and greatly limit their photoelectric performances. Herein, a post-doped strategy was utilized to achieve an interesting morphology evolution from a standard octahedron to a snowflake-like sheet during the Mn2+-doped Cs2NaBiCl6 process, which realizes the obvious photoluminescence quantum efficiency (PLQY) enhancement of the Cs2NaBiCl6:Mn2+ phosphor. This surprising evolution is ascribed to the morphology collapse and reconstruction induced by Mn2+ exchange. The obtained phosphor exhibits enhanced 31.56% PLQY, which is two-fold higher than that synthesized by the traditional co-precipitation method, with broad emission spectrum and good PL color stability at 150 °C. Combined with the Cs2SnCl6 : 1mol%Bi3+ phosphor to fabricate the phosphor-converted light-emitting diode, bright white light emission with Ra = 88, CCT = 4320 K, CIE (0.36, 0.33) and a good application potential in high-resolution PL imaging agents was obtained. This work provides a possible effective strategy to improve the PL performance for impurity-doped lead-free metal halide perovskite.

Incipient geometric lattice instability of cubic fluoroperovskites

Inorganic metal halide perovskites are promising materials for next-generation technologies due to a plethora of unique physical properties, many of which cannot be observed in the oxide perovskites. On the other hand, the search for ferroelectricity and multiferroicity in lead-free inorganic halide perovskites remains a challenging research topic. Here, we experimentally show that cubic fluoroperovskites exhibit proximity to incipient ferroelectrics, which manifested in the softening of the low-frequency polar phonons in the Brillouin zone center at cooling. Furthermore, we reveal the coupling between harmonic and anharmonic force constants of the softening phonons and their correlation with the perovskite tolerance factor. Next, using first-principles calculations, we examine the lattice dynamics of the cubic fluoroperovskites and disclose the incipient lattice instability at which the harmonic force constants of low-lying phonons tend to decrease with a reduction of tolerance factor at all high-symmetry points of the Brillouin zone. The correlations with the tolerance factor indicate the geometric origin of observed incipient lattice instability in the cubic fluoroperovskites caused by the steric effect due to the volume filling of the unit cell by different radius ions. These results provide insights into the lattice dynamics and potential ferroelectric properties of inorganic lead-free metal halide perovskites, relevant to further design and synthesis of new multifunctional materials.

Phase control in the synthesis of cesium copper iodide compounds for their photoluminescence and radioluminescence study

Lead-free metal halide perovskites have attracted much attention owing to their non-toxicity and excellent optical properties. Especially, cesium copper iodide compounds exhibit advantages of good environmental stability and low cost, rendering promising potential applications in optoelectronic devices field. Here, we use an anti-solvent assisted crystallization method to realize phase control syntheses of CsCu2I3 and Cs3Cu2I5 by varying the concentrations of precursor solution. Using high precursor solution concentrations, Cs3Cu2I5 with strong blue emission peak at 440 nm is prepared. While the CsCu2I3 is fabricated at low precursor solution concentrations, with the yellow emission peak of 580 nm. Furthermore, the UV-pumped light-emitting diodes (UV-LEDs) are realized by adopting CsCu2I3 and Cs3Cu2I5 powders as phosphors, and we also obverse the scintillation characteristic of the cesium copper iodide powders, showing potential applications for lighting and X-ray detection devices.

Common Defects Accelerate Charge Carrier Recombination in CsSnI3 without Creating Mid-Gap States.

Lead-free metal halide perovskites are environmentally friendly and have favorable electro-optical properties; however, their efficiencies are significantly below the theoretical limit. Using ab initio nonadiabatic molecular dynamics, we show that common intrinsic defects accelerate nonradiative charge recombination in CsSnI3 without creating midgap traps. This is in contrast to Pb-based perovskites, in which many defects have little influence on and even prolong carrier lifetimes. Sn-related defects, such as Sn vacancies and replacement of Sn with Cs are most detrimental, since Sn removal breaks the largest number of bonds and strongly perturbs the Sn-I lattice that supports the carriers. The defects increase the nonadiabatic electron-vibrational coupling and interact strongly with free carrier states. Point defects associated with I atoms are less detrimental, and therefore, CsSnI3 synthesis should be performed in Sn rich conditions. The study provides an atomistic rationalization of why lead-free CsSnI3 exhibits lower photovoltaic efficiency compared to some lead-based perovskites.

(Invited) Lead-Free Halide Perovskites for Hydrogen Evolution

Metal halide perovskites (MHPs) semiconductors possess unique optical properties, such as high optical absorption in the visible region, tunable band-gap and long carrier lifetime, which have been extensively exploited in photovoltaics and optoelectronic applications.[1] More recently there has been a growing interests towards novel applications of MHPs, in particular in the field of solar-driven photocatalysis.[2] However, some of the most interesting photocatalytic reactions such as H2O splitting, CO2 and N2 reduction, dye degradation, require the use of water or alternative polar solvents, this represent a conundrum foreseeing MHPs utilization. In this talk we report clear evidence of water stability for two lead-free metal halide perovskites, namely dimethylammonium (DMA) DMASnBr3 and phenylethylammonium (PEA) PEA2SnBr4 obtained by means of diffraction, optical and x-ray photoelectron spectroscopy. The water stability for the 2D material is somehow expected due to the water-repulsive nature of the symmetry breaking organic cation, some computational work is addressing such improved water stability, showing also that the same mechanism is thought to protect tin from +2 to +4 oxidation. The stability in water of the 3D analogous represent instead an important innovation of the field. Such unprecedented water-stability has been applied to promote photocatalysis in aqueous medium, in particular by devising a novel composite material by coupling DMASnBr3 to g-C3N4, taking advantage from the combination of their optimal photophysical properties. The prepared composites provide an impressive hydrogen evolution rate >1700 μmol g-1 h-1 generated by the synergistic activity of the two composite constituents. DFT calculations provide insight into this enhancement deriving it from the favorable alignment of interfacial energy levels of DMASnBr3 and g-C3N4. The demonstration of an efficient photocatalytic activity for lead-free metal halide perovskites in water paves the way to a new class of light-driven catalysts working in aqueous environments.

Recent advances toward environment-friendly photodetectors based on lead-free metal halide perovskites and perovskite derivatives.

Recently, metal-halide perovskites have emerged as promising materials for photodetector (PD) applications owing to their superior optoelectronic properties, such as ambipolar charge transport characteristics, high carrier mobility, and so on. In the past few years, rapid progress in lead-based perovskite PDs has been witnessed. However, the critical environmental instability and lead-toxicity seriously hinder their further applications and commercialization. Therefore, searching for environmentally stable and lead-free halide perovskites (LFHPs) to address the above hurdles is certainly a worthwhile subject. In this review, we present a comprehensive overview of currently explored LFHPs with an emphasis on their crystal structures, optoelectronic properties, synthesis and modification methods, as well as the PD applications. LFHPs are classified into four categories according to the replacement strategies of Pb2+, including AB(ii)X3, A3B(iii)2X9, A2B(i)B(iii)'X6, and newly-emerging perovskite derivatives. Then, we give a demonstration of the preliminary achievements and limitations in environment-friendly PDs based on such LFHPs and perovskite derivatives, and also discuss their applications in biological synapses, imaging, and X-ray detection. 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 LFHPs and perovskite derivatives for a broader range of fundamental research and practical applications.

All-inorganic lead-free metal halide perovskite quantum dots: progress and prospects

Lead halide perovskite quantum dots have drawn worldwide attention due to their quantum confinement effect and excellent optical gain properties. It is worth noting that due to the toxicity of lead ions and the inherent instability of organic groups, research on all-inorganic lead-free metal halide perovskite quantum dots (ILFHPQDs) has become a hot spot in recent years. This paper summarizes the latest research progress of ILFHPQDs, analyzes the sources and limitations affecting the performance of ILFHPQDs, and provides the improvement methods. Firstly, the typical synthesis strategies of ILFHPQDs are discussed, followed by a focus on the structural characteristics, optoelectronic properties and stability of each type of ILFHPQD. Next, the applications of ILFHPQDs in devices are investigated. Finally, the challenges, solutions and future application directions of ILFHPQDs are prospected.

Study of charge transfer mechanism and dielectric relaxation of all-inorganic perovskite CsSnCl3.

In the field of commercialization, lead-free metal halide perovskite materials are becoming more popular these days because of their prospective use in solar cells and also in other optoelectronic applications. In this paper, a non-toxic CsSnCl3 metal halide is successfully synthesized via the slow evaporation solution growth technique. Such systematic characterizations as differential scanning calorimetry (DSC) measurements, dielectric measurements, and variable-temperature structural analyses indicate that CsSnCl3 goes through a reversible phase transformation at T = 391/393 K from the monoclinic to the cubic system. Optical measurements of CsSnCl3 reveal a direct band-gap value of about 3.04 eV. The study of the charge transfer mechanism of CsSnCl3 is carried out based on Elliott's theory. The conduction mechanism in CsSnCl3 is interpreted through the following two approaches: the non-overlapping small polaron tunneling (NSPT) model (monoclinic phase) and the overlapping large polaron tunneling (OLPT) model (cubic phase). Moreover, the high dielectric constant of CsSnCl3 which is associated with a low dielectric loss makes it a possible candidate for energy harvesting devices.

Hierarchical computational screening of layered lead-free metal halide perovskites for optoelectronic applications

A hierarchical computational screening method is used to find layered lead-free metal halide perovskites with high stability and outstanding optoelectronic properties.

Progress of Lead‐Free Halide Perovskites: From Material Synthesis to Photodetector Application

AbstractLead halide perovskite (LHP) has been widely researched in the photovoltaic field due to its highly attractive optoelectronic properties. Among the LHP‐based devices, the detectivity of the photodetector is as high as 1015 Jones. However, their practical application is limited by the toxicity of lead in perovskite and the inherent instability induced by the perovskite structure against moisture, heat, and light. To address these issues, tremendous efforts have been made to replace Pb2+ with other environmentally friendly metal cations such as Sn2+, Bi3+, Cu2+, Sb3+, and Ge2+. Thus, considerable breakthroughs in device performance and stability using lead‐free metal halide perovskite (LFMHP) have been made in recent years. In this review, the synthesis methods and strategies are focused for enhancing the material quality and photoelectric properties of LFMHPs and the recent research progress of LFMHP‐based photodetectors is summarized. This research provides some promising perspectives for high‐performance LFMHP photodetectors to achieve a broader range of practical applications in the future.

Defect Origin of Emission in CsCu2I3 and Pressure-Induced Anomalous Enhancement.

Lead-free metal halide perovskites CsCu2X3 (X = Cl, Br, I) with a high photoluminescence quantum yield are promising materials for optoelectronic devices. However, the origin of photoluminescence (PL) emission is still under debate, and the anomalous dependence of PL on pressure is unclear. Here, we systemically study the effects of high pressure on the structural, electronic, and optical properties of CsCu2I3 using a diamond anvil cell (DAC) and first-principles calculations. We argue that the ground state structure of CsCu2I3 belongs to the pnma phase rather than the cmcm phase under ambient conditions. There is a structural phase transition from the pnma to the cmcm phase for CsCu2I3 at ∼5 GPa. The optical band gap derivative from absorption spectra increases from 3.57 to 3.62 eV within a pressure range of 0 to 4.03 GPa, and it then decreases over 4.03 GPa. There are two major PL emissions peaks at 2.11 and 2.32 eV, which are attributed to the intrinsic defect related trap states in CsCu2I3. Interestingly, there is an anomalous dependence of both PL emissions on pressure, such that PL peaks show a blueshift and the PL intensity is enhanced from 0 to ∼4 GPa, with redshifting and decreasing at pressures above ∼4 GPa. The anomalous evolution of the two PL emissions also suggests a defect origin of emissions.

Water-Stable DMASnBr3 Lead-Free Perovskite for Effective Solar-Driven Photocatalysis.

Water-stable metal halide perovskites could foster tremendous progresses in several research fields where their superior optical properties can make differences. In this work we report clear evidence of water stability in a lead-free metal halide perovskite, namely DMASnBr3 , obtained by means of diffraction, optical and X-ray photoelectron spectroscopy. Such unprecedented water-stability has been applied to promote photocatalysis in aqueous medium, in particular by devising a novel composite material by coupling DMASnBr3 to g-C3 N4 , taking advantage from the combination of their optimal photophysical properties. The prepared composites provide an impressive hydrogen evolution rate >1700 μmol g-1 h-1 generated by the synergistic activity of the two composite costituents. DFT calculations provide insight into this enhancement deriving it from the favorable alignment of interfacial energy levels of DMASnBr3 and g-C3 N4 . The demonstration of an efficient photocatalytic activity for a composite based on lead-free metal halide perovskite in water paves the way to a new class of light-driven catalysts working in aqueous environments.