Hybrid Organic-inorganic Lead Halide Research Articles

Insights from Theoretical Modeling of Cesium-Formamidinium-Based Mixed-Halide Perovskite Solar Cells for Outdoor and Indoor Applications.

This study presents an in-depth computational analysis of hybrid organic-inorganic lead halide perovskite solar cells (PSCs) with a composition of Cs0.17FA0.83Pb(Br0.4I0.6)3 (FA: formamidinium) material under cool and warm light-emitting diodes (LEDs). We propose a novel design of an inverted (p-i-n) PSC to compare the power conversion efficiencies (PCEs) of opaque and semitransparent models under the AM1.5G spectrum and indoor LED lighting. The Shockley-Queisser (SQ) limits were estimated for LEDs with color temperatures of 3000 and 6000 K, revealing significant differences in PCE compared to standard solar radiation. The optical and electrical properties of the perovskite devices were simulated by using the transfer-matrix method and one-dimensional drift-diffusion model. We report a PCE of 15.8% for opaque devices under the AM1.5G spectrum, while the semitransparent devices exhibit PCEs of 12.07% and 10.17% for front and rear illumination, respectively. Under indoor conditions with cool LED lighting, the opaque devices demonstrate a significantly higher PCE of 28.38% and an impressive photovoltage of 1.17 V, surpassing the semitransparent devices, which show efficiencies of approximately 19.5% (front illumination) and 18.3% (rear illumination). While the interface between the hole transport layer and perovskite has a major impact on the device performance of opaque solar cells, the perovskite/electron transport layer junction plays a more critical role in the performance of semitransparent solar cells. The power densities for opaque devices reached up to 106.25 μW/cm2 under cool LED and 97.1 μW/cm2 with warm LED illumination. For semitransparent devices, the power densities exceeded 60.71 μW/cm2 on front-side illumination and 73.66 μW/cm2 on rear illumination under cool LEDs. These results emphasize the significant potential of hybrid PSCs for efficient energy harvesting under various lighting conditions, making them promising candidates for powering low-energy-consumption electronics in indoor environments.

Progress of Hole-Transport Layers in Mixed Sn-Pb Perovskite Solar Cells.

Hybrid organic-inorganic lead halide perovskite solar cells (PSCs) have rapidly emerged as a promising photovoltaic technology, with record efficiencies surpassing 26%, approaching the theoretical Shockley-Queisser limit. The advent of all-perovskite tandem solar cells (APTSCs), integrating Pb-based wide-bandgap (WBG) with mixed Sn-Pb narrow-bandgap (NBG) perovskites, presents a compelling pathway to surpass this limit. Despite recent innovations in hole transport layers (HTLs) that have significantly improved the efficiency and stability of lead-based PSCs, an effective HTL tailored for Sn-Pb NBG PSCs remains an unmet need. This review highlights the essential role of HTLs in enhancing the performance of Sn-Pb PSCs, focusing on their ability to mitigate non-radiative recombination and optimize the buried interface, thereby improving film quality. The distinct attributes of Sn-Pb perovskites, such as their lower energy levels and accelerated crystallization rates, necessitate HTLs with specialized properties. In this study, the latest advancements in HTLs are systematically examined for Sn-Pb PSCs, encompassing organic, self-assembled monolayer (SAM), inorganic materials, and HTL-free designs. The review critically assesses the inherent limitations of each HTL category, and finally proposes strategies to surmount these obstacles to reach higher device performance.

Lead-free Cs3Bi2I9 perovskite hexagonal microplates: A promising material solution-processed for ultraviolet self-powered photodetectors

In recent times, hybrid organic-inorganic lead halide perovskites have garnered significant attention in the realm of optoelectronic devices. Metal halide perovskites are extensively researched for photodetection due to their exceptional optical and electrical properties. However, the toxicity of lead and the unstable nature of organic components are limiting its potential. Lead-free perovskites, on the other hand, have low toxicity and are being explored as a promising material for photodetector applications. However, achieving self-powered perovskite photodetectors with high photoresponse, broad spectral sensitivity, and long-term stability remains challenging Among them, Cs3Bi2I9 perovskites have emerged as standout, garnering attention for their environmentally friendly nature, impressive stability, and heightened photosensitivity. Herein, FTO/ Al-doped ZnO (AZO)/Cs3Bi2I9/P3HT/C self-powered ultra-violet (UV) photodetector. This device demonstrated outstanding self-powered photodetection capabilities with a photoresponsivity of 29 mA/W, a specific detectivity of 1.05×1011 Jones, and an impressive response rise and decay time of 90 ms /140 ms. Furthermore, the PDs device can function as a bias-based UV photodetector.

The Effect of Cesium Incorporation on the Vibrational and Elastic Properties of Methylammonium Lead Chloride Perovskite Single Crystals.

Hybrid organic-inorganic lead halide perovskites (LHPs) have emerged as a highly significant class of materials due to their tunable and adaptable properties, which make them suitable for a wide range of applications. One of the strategies for tuning and optimizing LHP-based devices is the substitution of cations and/or anions in LHPs. The impact of Cs substitution at the A site on the structural, vibrational, and elastic properties of MAxCs1-xPbCl3-mixed single crystals was investigated using X-ray diffraction (XRD) and Raman and Brillouin light scattering techniques. The XRD results confirmed the successful synthesis of impurity-free single crystals, which exhibited a phase coexistence of dominant cubic and minor orthorhombic symmetries. Raman spectroscopy was used to analyze the vibrational modes associated with the PbCl6 octahedra and the A-site cation movements, thereby revealing the influence of cesium incorporation on the lattice dynamics. Brillouin spectroscopy was employed to investigate the changes in elastic properties resulting from the Cs substitution. The incorporation of Cs cations induced lattice distortions within the inorganic framework, disrupting the hydrogen bonding between the MA cations and PbCl6 octahedra, which in turn affected the elastic constants and the sound velocities. The substitution of the MA cations with smaller Cs cations resulted in a stiffer lattice structure, with the two elastic constants increasing up to a Cs content of 30%. The current findings facilitate a fundamental understanding of mixed lead chloride perovskite materials, providing valuable insights into their structural and vibrational properties.

Trace organic–inorganic hybrid lead halide perovskite nanocrystals as photocatalysts for PET-RAFT polymerization with high efficiency

Lead halide perovskite has emerged as a new class of materials in photocatalysis due to its promising photocatalytic performance. Here, we introduce organic–inorganic hybrid lead halide perovskite (OIHP) nanocrystals as photocatalysts (PCs) for efficient photoinduced electron/energy transfer–reversible addition–fragmentation chain transfer (PET-RAFT) polymerization. PET-RAFT polymerization is successfully conducted with 0.025 wt% PC loading under blue light (6 mW/cm2, 460 nm), yielding upwards of 90 % monomer conversion and polymer products with narrow dispersity (≤1.20) within 2 h, demonstrating a fast polymerization rate. Moreover, the oxygen resistance of the polymerization process has been demonstrated. Due to the advantages of low PC usage, fast polymerization rate and oxygen tolerance, OIHP mediated PET-RAFT polymerization is expected to be used for large-scale production and industrial applications.

Solution Processed Hybrid Lead Perovskite Films for White Light Emission and Lasing Applications

Abstract2D organic–inorganic hybrid lead halide perovskite films are fascinating optoelectronic materials for white‐light emission and lasing applications. Here, it is demonstrated that nano‐meter‐thick films of perovskites can be formed at room temperature using Langmuir–Schaefer (LS) deposition technique. In situ X‐ray measurements reveal a self‐assembly process of the 2D perovskite formation at the air–water interface. This formation process requires the presence of stearic acid monolayer on the water subphase having a solution of lead‐perovskite in dimethylformamide. Microscopy and X‐ray measurements of these nanofilms transferred onto substrates show crystalline phase formation that deviates from the known orthorhombic structure of the bulk crystals. The nanofilms exhibit novel broad optical emission with sharp band‐edge transition in temperature‐dependent photoluminescence studies. Also, their band‐edge emission is distinctly blue‐shifted compared to that of thicker 2D perovskite flakes prepared by the slow‐cooling chemical synthesis process. 2D flakes exhibit lasing that cannot be obtained in the LS nanofilms as their thicknesses are lower than the lasing wavelength prohibiting wave‐guide formation. The results show that the 2D single‐crystalline nature of the LS films and flakes are suitable materials to develop broad‐band white light emission across the entire visible range and to obtain lasing without external cavities, respectively.

Coherent Carrier Spin Dynamics in FAPbBr3 Perovskite Crystals.

Coherent spin dynamics of electrons and holes are studied in hybrid organic-inorganic lead halide perovskite FAPbBr3 bulk single crystals using the time-resolved Kerr ellipticity technique at cryogenic temperatures. The Larmor spin precession of the carrier spins in a magnetic field is monitored to measure the Landé g-factors of electrons (+2.44) and holes (+0.41). These g-factors are highly isotropic. The measured spin dephasing times amount to a few nanoseconds, and the longitudinal hole spin relaxation time is 470 ns. The important role of the strong hyperfine interaction between carrier spins and nuclear spins is demonstrated via dynamic nuclear polarization. At low temperatures, electron and hole spin relaxation predominantly occurs via the hyperfine interaction, whose importance significantly decreases at temperatures above 12 K. We overview the spin dynamics in various lead halide perovskite crystals and polycrystalline films and conclude on their common features provided by charge carrier localization at cryogenic temperatures.

Compositional Engineering of Hybrid Organic-Inorganic Lead-Halide Perovskite and PVDF-Graphene for High-Performance Triboelectric Nanogenerators.

Triboelectric nanogenerators (TENGs) have attracted a great deal of attention since they can convert ubiquitous mechanical energy into electrical energy and serve as a continuous power source for self-powered sensors. Optimization of the dielectric material composition is an effective way to improve the triboelectric output performance of TENGs. Herein, the hybrid organic-inorganic lead-iodide perovskite Cs0.05FA0.95-xMAxPbI3 was prepared by blade coating and used as a positive friction layer material. Moreover, PVDF-graphene (PG) nanofibers were prepared as negative friction layer materials by electrostatic spinning. The output performance of the TENG was enhanced by varying the MA content of the pervoskite films and the graphene content of the PG nanofibers. The champion output TENG based on Cs0.05FA0.9MA0.05PbI3/PG-0.15 achieved an open-circuit voltage of 245 V, a short-circuit current of 24 μA, and a charge transfer of 80.2 nC. Meanwhile, a maximum power density of 11.23 W m-2 was obtained at 100 MΩ. Moreover, the device exhibits excellent energy-harvesting properties, including excellent stability and durability, rapidly charges capacitors, and lights commercial LEDs and digital tubes.

Broadband blue light emissions of one-dimensional hybrid Cu(I) halides with ultrahigh anti -water stability.

Low-dimensional organic-inorganic hybrid lead halide perovskites have attracted much interest in solid-state lighting and displays, but the toxicity and instability of lead halide are obstacles to their industrial applications, which must be overcome. As an alternative, Cu(I)-based hybrid metal halides have emerged as a new type of luminescent material owing to their diversified structure, adjustable luminescence, low toxicity and low cost. Herein, we report three one-dimensional (1D) hybrid Cu(I)-based halides with the general formula ACu2Br4 (A = [(Me)4-Pipz]2+ and [BuDA]2+ and [TMEDA]2+). These 1D hybrid Cu(I) halides display stable broadband blue emission with maximum emission peaks in the range of 445-474 nm and the highest photoluminescence quantum yield of 37.8%. Furthermore, in-depth experimental and theoretical investigations revealed that the broadband blue emissions originate from the radiative recombination of self-trapped excitons. Most importantly, there is no structural degradation and attenuation of emission intensity even after continuously soaking these halides in water for at least two months, demonstrating their ultra-high anti-water stability. Hirshfeld surface analysis shows that a large number of weak hydrogen bonds can protect the inorganic skeleton from degradation due to water. This work provides a new strategy for the design of water-stable Cu(I)-based halides with efficient blue emission and wide potential applications in humid environments.

Phase Transitions, Dielectric Response, and Nonlinear Optical Properties of Aziridinium Lead Halide Perovskites.

Hybrid organic-inorganic lead halide perovskites are promising candidates for next-generation solar cells, light-emitting diodes, photodetectors, and lasers. The structural, dynamic, and phase-transition properties play a key role in the performance of these materials. In this work, we use a multitechnique experimental (thermal, X-ray diffraction, Raman scattering, dielectric, nonlinear optical) and theoretical (machine-learning force field) approach to map the phase diagrams and obtain information on molecular dynamics and mechanism of the structural phase transitions in novel 3D AZRPbX3 perovskites (AZR = aziridinium; X = Cl, Br, I). Our work reveals that all perovskites undergo order-disorder phase transitions at low temperatures, which significantly affect the structural, dielectric, phonon, and nonlinear optical properties of these compounds. The desirable cubic phases of AZRPbX3 remain stable at lower temperatures (132, 145, and 162 K for I, Br, and Cl) compared to the methylammonium and formamidinium analogues. Similar to other 3D-connected hybrid perovskites, the dielectric response reveals a rather high dielectric permittivity, an important feature for defect tolerance. We further show that AZRPbBr3 and AZRPbI3 exhibit strong nonlinear optical absorption. The high two-photon brightness of AZRPbI3 emission stands out among lead perovskites emitting in the near-infrared region.

Optical and structural characterization of CsPb(I1-xBrx)3 nanomaterials prepared by the mechanochemical method

Hybrid organic-inorganic lead halide perovskites (LHP) are gaining considerable attention due to their wide range of potential applications, particularly as an active layer in solar cells and light-emitting devices owing to their excellent light-harvesting and photoluminescence emission properties. Considering their high color purity and color-tunability, high photoluminescence quantum yield (PLQY), easy fabrication, and cost-efficiency, LHP are promising candidates to compete with the traditional active layer in solid-state lighting. The PL emission of CsPbX3 (X = I, Br, Cl) covers the entire visible range from 400 to 730 nm and can be easily tuned via halide replacement in the I ↔ Br ↔ Cl series. In this work, photoluminescence (PL), luminescence excitation, UV/VIS/NIR absorption, structural and morphological properties of CsPb(I1-xBrx)3 (x = 0–1) nanosized powders prepared by the mechanochemical synthesis method is investigated at 80K and room temperature. A narrow PL peak with full width at half maximum (FWHM) = 16.5 and 6.6 nm was observed at room temperature and 80K from CsPbBr3, respectively. The relation between structure and optical properties of nanopowders is discussed. It is shown that the halide exchange mechanism is an effective approach for fabrication of defect-free materials.

Introducing Ferroelasticity into 1D Hybrid Lead Halide Semiconductor by Halogen Substitution Strategy.

Organic-inorganic hybrid lead halide perovskites (OLHPs), represented by (CH3 NH3 )PbI3 , are one of the research focus due to their exceptional performance in optoelectronic applications, and ferroelastic domain walls are benign to their charge carrier transport that is confirmed recently. Among them, the 1DOLHPs feature better stability against desorption and moisture, but related 1D ones possessing ferroelasticity are rarely investigated and reported so far. In this work, the 1D ferroelastic semiconductor (N-iodomethyl-N-methyl-morpholinium)PbI3 ((IDMML)PbI3 ) is prepared successfully by introducing successively halogenate atoms from Cl, Br to I into the organic cation of the prototype (N,N-dimethylmorpholinium)PbI3 ((DMML)PbI3 ). Notably, (IDMML)PbI3 shows the narrow bandgap energy (≈2.34eV) according to the ultraviolet-visible absorption spectrum and the theoretical calculation, and possesses the evident photoconductive characteristic with the on/off ratio of current of ≈50 under the 405nm light irradiation. This work provides a new case for the ferroelastic OLHPs and will inspire intriguing research in the field of optoelectronic.

Long-Lived Exciton Coherence in Mixed-Halide Perovskite Crystals.

Compositional engineering of the optical properties of hybrid organic-inorganic lead halide perovskites is crucial for the realization of efficient solar cells and light-emitting devices. We study the effect of band gap fluctuations on coherent exciton dynamics in a mixed FA0.9Cs0.1PbI2.8Br0.2 perovskite crystal by using photon echo spectroscopy. We reveal a narrow homogeneous exciton line width of 16 μeV at a temperature of 1.5 K. The corresponding exciton coherence time T2 = 83 ps is exceptionally long due to the localization of excitons at the scale of tens to hundreds of nanometers. From spectral and temperature dependences of the two- and three-pulse photon echo decay, we conclude that for low-energy excitons pure decoherence associated with elastic scattering on phonons is comparable with the exciton lifetime, while for excitons with higher energies, inelastic scattering to lower energy states via phonon emission dominates.

Minimal Molecular Building Blocks for Screening in Quasi-Two-Dimensional Organic-Inorganic Lead Halide Perovskites.

Layered hybrid organic-inorganic lead halide perovskites have intriguing optoelectronic properties, but some of the most interesting perovskite systems, such as defective, disordered, or mixed perovskites, require multiple unit cells to describe and are not accessible within state-of-the-art ab initio theoretical approaches for computing excited states. The principal bottleneck is the calculation of the dielectric matrix, which scales formally as O(N4). We develop here a fully ab initio approximation for the dielectric matrix, known as IPSA-2C, in which we separate the polarizability of the organic/inorganic layers into minimal building blocks, thus circumventing the undesirable power-law scaling. The IPSA-2C method reproduces the quasi-particle band structures and absorption spectra for a series of Ruddlesden-Popper perovskites to high accuracy, by including critical nonlocal effects neglected in simpler models, and sheds light on the complicated interplay of screening between the organic and inorganic sublattices.

High-performance photodetector based on semi-encompassed CH3NH3PbCl3–ZnO microwire heterojunction with alterable spectral response

Recently, heterojunctions consisting of hybrid organic-inorganic lead (Pb) halide perovskites and other semiconductors have drawn increasing attention for the potential application in photodetectors due to their exceptional performance. However, their performance is usually limited by the relatively low crystalline quality of perovskites, and the response spectra of the devices are difficult to adjust according to the practical requirement. Here, high quality CH3NH3PbCl3 micro-sized crystals have been successfully fabricated on one side of individual ZnO microwire to form heterojunction photodetector by a two-step crystallization method. The heterojunction device presents a low dark current (60 nA at −6 V) along with a rapid response speed (rise time of <20 μs and fall time of ∼500 μs). More interestingly, the modulation of the response spectra and the responsivity can be realized by operating the device under front or back illumination due to the self-filtering properties. Our findings provide a promising method for combining perovskites with other inorganic materials to form high-performance heterojunction photodetectors.

Investigation of thickness dependent efficiency of CsPbX3 (X = I, Br) absorber layer for perovskite solar cells

The drawbacks of solar cells consisting of hybrid organic-inorganic lead halide perovskites led to the development of solar cells containing completely inorganic perovskites. Therefore, here we have carried out detailed density functional theory (DFT) based investigation of cubic phase CsPbX3 (X = I, Br). Absence of imaginary frequencies in phonon dispersion curves and negative cohesive energies of the perovskites depicts stability of our crystal structures. From electronic properties of CsPbI3 and CsPbBr3 we have deduced that the major contributions in their valence band region is due to I-5p and Br-4p orbitals, respectively. The static dielectric constant of CsPbI3 is greater than CsPbBr3 which shows that it consists of low carrier recombination rate. Red shift has also been observed in absorption coefficient plot while going from Br → I in CsPbX3. The thickness of absorber layer present in perovskite solar cells (PSCs) directly influences its efficiency and stability. Therefore, here we have used a detailed framework i.e., spectroscopic limited maximum efficiency (SLME) to predict solar cell parameters of perovskites absorber layer as a function of their thicknesses. Using SLME approach which is thickness dependent it has been predicted that at thicknesses above 2.56 μm and 1.39 μm of absorber layers CsPbI3 and CsPbBr3, respectively, their efficiencies obtained using SLME approach are higher compared to that obtained using Shockley-Queisser (SQ) limit. The efficiencies of CsPbI3 and CsPbBr3 absorber layers calculated using SLME approach at absorber thickness of 20 μm are obtained around 30.41% and 24.53%, respectively.

Challenges and Potential of Perovskite Solar Cells

A solar cell is a device that converts sunlight into electricity. There are different types of solar cells but in this literature mainly focuses on a type of new dominant solar cell material that has the name organo-metal halide perovskite, namely known as perovskite solar cells, in shortly PSCs . In this respect, the efficiency of power conversion is taken into account to replace the dominancy of traditional and second generation solar cell fields by perovskite solar cells. Perovskite solar cell is a type of solar cell including a perovskite structure, usually a hybrid organic-inorganic lead or tin halide- based material. In this review, a comprehensive study of the perspective challenges and their potential has been highlighted for their future application. There are rigorous research efforts in aspects of device engineering, including physical and chemical passivation, and the use of a wide variety of organic and inorganic additives to develop the advanced PSCs.

Revealing the ground‐state geometry, optoelectronic, mechanical and thermodynamic behaviors, and efficiency of formamidinium mixed halide perovskites for solar cell applications

Hybrid organic-inorganic lead halide perovskites are the revolutionary materials that have brought a new era of low-cost and highly efficient solar cells. Generally, high-performing perovskite solar cells incorporate formamidinium-based lead halide perovskites. Using density functional theory, we employed generalized gradient approximation (GGA) and meta-GGA (MGGA) exchange-correlation functionals to investigate the structural, electronic, optical, mechanical, and thermodynamic properties of the perovskite series FAPbI3, FAPbBr2I, FAPbBrI2, and FAPbBr3. Both the functionals successfully determined the most stable structure for each mixed halide perovskite. Electronic properties such as band structure, the density of states, and effective masses have been calculated and discussed. Our estimated band gap values using MGGA functional are more close to the experiments than PBE. Optical properties of the perovskite series such as dielectric function, complex refractive index, absorption coefficient, and reflectivity have been explained in the light of photovoltaic applications. Moreover, we predicted the maximum absorber efficiency via the spectroscopic limited maximum efficiency method. FAPbI3 achieved the highest efficiency by virtue of its suitable band gap and maximum absorption. Mechanical properties have also been studied through elastic constants, elastic moduli, Pugh ratio, Poisson ratio, and other several properties, which reveal the declining flexibility of perovskites with bromine. Further, thermodynamic properties like heat capacity, Debye temperature, vibrational internal energy, vibrational entropy, thermal expansion, and thermal stability have been discussed and analyzed. Among all perovskites, the thermodynamic stability of FAPbBr2I is found superior due to strong Br-I bonds and the tougher attraction of the ionic system in the perovskite lattice. All the results are compared with earlier reported values and found to be in fair agreement.

Photoinduced large polaron transport and dynamics in organic–inorganic hybrid lead halide perovskite with terahertz probes

Organic-inorganic hybrid metal halide perovskites (MHPs) have attracted tremendous attention for optoelectronic applications. The long photocarrier lifetime and moderate carrier mobility have been proposed as results of the large polaron formation in MHPs. However, it is challenging to measure the effective mass and carrier scattering parameters of the photogenerated large polarons in the ultrafast carrier recombination dynamics. Here, we show, in a one-step spectroscopic method, that the optical-pump and terahertz-electromagnetic probe (OPTP) technique allows us to access the nature of interplay of photoexcited unbound charge carriers and optical phonons in polycrystalline CH3NH3PbI3 (MAPbI3) of about 10 μm grain size. Firstly, we demonstrate a direct spectral evidence of the large polarons in polycrystalline MAPbI3. Using the Drude–Smith–Lorentz model along with the Frӧhlich-type electron-phonon (e-ph) coupling, we determine the effective mass and scattering parameters of photogenerated polaronic carriers. We discover that the resulting moderate polaronic carrier mobility is mainly influenced by the enhanced carrier scattering, rather than the polaron mass enhancement. While, the formation of large polarons in MAPbI3 polycrystalline grains results in a long charge carrier lifetime at room temperature. Our results provide crucial information about the photo-physics of MAPbI3 and are indispensable for optoelectronic device development with better performance.

Multilevel halide perovskite memristors based on optical & electrical resistive switching effects

With the aim of addressing the growing challenge of approaching the end of Moore's law and von Neumann bottleneck, the evolution of neuromorphic computing based on memristors that emulate the behaviors of the brain has attracted great attention. Organic-inorganic hybrid lead halide perovskites with inherent hybrid ionic-electronic conduction ability have been proposed as promising candidates for memristors to replace traditional charge-based memory. Here we propose MA0.85Cs0.15PbI3 memristors with high surface crystallinity and stable operation in the atmospheric environment by mixing inorganic cation Cs+ and organic cation MA+. The prepared memristors present excellent resistive switching performances including low operation voltage, high ON/OFF ratio, stable endurance, long retention, and robustness under ambient condition. Besides that, four distinct resistance states have been obtained in the above devices by modifying the operation voltages. Moreover, the resistance switching processes can be controlled both by electrical field and light illumination. Finally, this research provides utilization potentiality in the optical and electrical two-dimension controlled multilevel information devices with a total of five stable resistance states. The present work would offer an effective way to develop multifunctional high-density data-storage applications by storing multiple resistance states in a single unit cell.