Hybrid Organic-inorganic Metal Halide Perovskites Research Articles

Anisotropic Electron-Phonon Interactions in 2D Lead-Halide Perovskites.

Two-dimensional (2D) hybrid organic-inorganic metal halide perovskites offer enhanced stability for perovskite-based applications. Their crystal structure's soft and ionic nature gives rise to strong interaction between charge carriers and ionic rearrangements. Here, we investigate the interaction of photogenerated electrons and ionic polarizations in single-crystal 2D perovskite butylammonium lead iodide (BAPI), varying the inorganic lamellae thickness in the 2D single crystals. We determine the directionality of the transition dipole moments (TDMs) of the relevant phonon modes (in the 0.3-3 THz range) by the angle- and polarization-dependent THz transmission measurements. We find a clear anisotropy of the in-plane photoconductivity, with a ∼10% reduction along the axis parallel with the transition dipole moment of the most strongly coupled phonon. Detailed calculations, based on Feynman polaron theory, indicate that the anisotropy originates from directional electron-phonon interactions.

Strain in Hybrid Organic-Inorganic Metal Halide Perovskites Microstructures by Numerical Simulations.

Hybrid organic-inorganic metal halide perovskites (HOIPs) are promising materials for optoelectronics applications. Their optical and electrical properties can be controlled by strain engineering, that results from application of local elastic deformation or deposition on pre-patterned substrates acquiring a conformal 3D shape. Most interesting, their mechanical properties depend on their crystal structure, composition and dimensionality. We explore by numerical simulations the deformation of a selection of HOIPs comprising a broad range of elastic properties. We consider an axial symmetry with the formation of microdomes on flakes. Radial and vertical forces are considered, finding that the radial force is more effective to obtain large deformation. Large vertical displacement and strain is obtained for HOIPs with low stiffness. The layered nature of HOIPs, that are formed by inorganic layers of different thickness and organic spacers, is also investigated, revealing a non-monotonous trend with the proportion of inorganic to organic part.

Circularly Polarized Photoluminescence in Chiral Hybrid Organic–Inorganic Manganese Halide Perovskites: From Bulk Materials to Exfoliated Flakes

AbstractLayered hybrid organic–inorganic metal‐halide perovskites (HOIPs) can acquire chirality from molecules incorporated into their structure showing exciting possibilities in optoelectronics and spintronics. However, researchers have focused their attention on the study of bulk compounds, namely unstable and toxic Pb HOIPs. Herein, the chiroptical properties of R‐ and S‐β‐methylphenethylammonium Mn chloride HOIPs are reported in both the bulk and the few‐layer limit as exfoliated flakes. Red photoluminescence (PL) emission originating from the octahedrally coordinated Mn2+ is observed, with a PL shift to longer wavelengths when passing from bulk to flakes. Circular dichroism and circularly polarized PL mirrored‐signals for enantiomers are found in bulk samples, while in flakes, a degree of circularly polarized emission (P) of up to 17% at 80 K is achieved, comparable to Pb‐based HOIPs. Moreover, angle‐resolved PL measurements on flakes show that the PL emission and P are isotropic. These findings highlight the potential of these HOIPs from bulk to flakes for spin‐optoelectronic applications.

Dimension-Controlled Synthesis of Hybrid-Mixed Halide Perovskites for Solar Cell Application.

Despite the rapid improvement of photovoltaic (PV) efficiency in hybrid organic-inorganic metal halide perovskites (HOIPs), the fabrication procedure of a compact thin film in a large-area application is still a tedious work. Apart from the quality of the thin film, the stability of the perovskite materials and the expensive organic hole transport layer (HTL) within the HOIP-based PV device are the major issues that need to be addressed prior to their commercialization. Herein, a unique glass rod-based facile fabrication technique for producing a compact and stable thin film utilizing a mixed-halide-based perovskite precursor solution is demonstrated. The fabricated devices deliver high photoconversion efficiency (PCE) without the use of any HTL and show an excellent stability under ambient conditions. By varying the organic CH3NH3I (MAI) and inorganic PbBr2 content, perovskite materials with different dimensions, i.e., 3D, 2D, and 1D, are synthesized to produce an active layer for PV devices. Although a 2D single-halide perovskite is reported earlier, herein two different mixed-halide 2D perovskites, i.e., MA2PbI2Br2 and MAPb2IBr4, are synthesized successfully, and their performance is compared in detail along with that of 1D and 3D mixed-halide perovskites. The facile synthesized mixed-halide 2D-based MA2PbI2Br2 perovskite shows a PCE of 10.14% with a high stability of 92% after 100 days without encapsulation, which is much superior as compared to that of the mixed-halide 3D MAPbIBr2. The semiconducting behavior as well as the nature of the bandgap of the synthesized compounds is examined by pursuing density functional theory calculations. Specifically, the role of iodine doping to modify the electronic band structure is investigated, and introduction of iodine is found to reduce the effective masses of both electrons and holes in the perovskite material.

Synthesizing and characterization of Lead Halide Perovskite Nanocrystals solar cells from reused car batteries

With the rapid increase of efficiency up to 23.7% during the past few years, hybrid organic-inorganic metal halide perovskite solar cells (PSCs) have become a research “hot spot” for many solar cell researchers. The perovskite materials show various advantages due to unique characteristics of perovskite materials, such as high photo-to-electric conversion efficiency, direct band gap, high light absorption coefficient, high charge-carrier mobility and long electron-hole electron transport distance. The low-cost fabrication techniques together with the high efficiency makes PSCs comparable with Si-based solar cells. This paper begins with the discussion of crystal structures of perovskite based on recent research findings. The following part of this paper discussion of synthetic process of lead iodide perovskite materials from lead-acid battery and Harvesting material from the anodes and cathodes of car battery; synthesizing PbI2 from the collected materials and compare with pure Lead iodide to know the absolute by XRD peak, depositing lead iodide perovskite nanocrystals. Efficient flexible PSCs are fabricated onto FTO glass substrate by a two-step coating method under ambient condition. By adjusting the concentration of precursor CH3NH3I (MAI), the morphology and thickness of perovskite layer is effectively tailored, according to SEM analysis and using TiO2 as electron transport layer instead of ZnO and CuI instead of spiro-OMeTAD as hole transport layer.

Achieving a Highly Stable Perovskite Photodetector with a Long Lifetime Fabricated via an All-Vacuum Deposition Process

Hybrid organic-inorganic metal halide perovskites (HOIP) have become a promising visible light sensing material due to their excellent optoelectronic characteristics. Despite the superiority, overcoming the stability issue for commercialization remains a challenge. Herein, an extremely stable photodetector was demonstrated and fabricated with Cs0.06FA0.94Pb(I0.68Br0.32)3 perovskite by an all-vacuum process. The photodetector achieves a current density up to 1.793 × 10-2 A cm-2 under standard one sun solar illumination while maintaining a current density as low as 8.627 × 10-10 A cm-2 at zero bias voltage. The linear dynamic range (LDR) and transient voltage response were found to be comparable to the silicon-based photodetector (Newport 818-SL). Most importantly, the device maintains 95% of the initial performance after 960 h of incessant exposure under one sun solar illumination. The achievements of these outstanding results contributed to the all-vacuum deposition process delivering a film with high stability and good uniformity, which in turn delays the degradation process. The degradation mechanism is further investigated by impedance spectroscopy to reveal the charge dynamics in the photodetector under different exposure times.

Implications of Electron Transport Layer and Back Metal Contact Variations in Tin–Lead Perovskite Solar Cells Assessed by Spectroscopic Ellipsometry and External Quantum Efficiency

The structural and optical properties of hybrid organic-inorganic metal halide perovskite solar cells are measured by spectroscopic ellipsometry to reveal an optically distinct interfacial layer among the back contact metal, charge transport, and absorber layers. Understanding how this interfacial layer impacts performance is essential for developing higher performing solar cells. This interfacial layer is modeled by Bruggeman effective medium approximations (EMAs) to contain perovskite, C60, BCP, and metal. External quantum efficiency (EQE) simulations that consider scattering, electronic losses, and the formation of nonparallel interfaces are created with input derived from ellipsometry structural-optical models and compared with experimental EQE to estimate optical losses. This nonplanar interface causes optical losses in short circuit current density (JSC) of up to 1.2 mA cm-2. A study of glass/C60/SnO2/Ag or Cu and glass/C60/BCP/Ag film stacks shows that C60 and BCP mix, but replacing BCP with SnO2 can prevent mixing between the ETLs to prevent contact between C60 and back contact metal and enable the formation of a planar interface between ETLs and back contact metals.

Performance and stability enhancement of perovskite photodetectors by additive and interface engineering using a dual-functional PPS zwitterion.

Hybrid organic-inorganic metal halide perovskites (HOIPs) have gained significant research interest due to their tunable optoelectronic properties and ease of fabrication. Enhancing the stability and efficiency of perovskite materials can be achieved through the passivation of defective surfaces and the improvement of interfacial properties. In this study, we introduce a zwitterionic compound, PPS (3-(1-pyridinio)-1-propanesulfonate), as a bifunctional material that serves as an additive and an interlayer. Incorporating PPS into the perovskite film effectively reduces both positively and negatively charged defects, leading to improved surface morphology and a reduction in undesired charge carrier recombination. Additionally, the formation of a PPS interlayer on SnO2 improves the SnO2/perovskite interfacial characteristics, thereby enhancing charge carrier extraction. As a result, the photodetector exhibits a low dark current of 6.05 × 10-11 A, an excellent responsivity of 5.93 A W-1, a detectivity of 1.51 × 1013 J, and an on/off ratio of 1.2 × 104 under open-air conditions. Moreover, the device demonstrates outstanding stability, retaining 80% of its original responsivity in an ambient environment. This work highlights the great potential of dual-functional materials for defect passivation in future optoelectronic devices, emphasizing the importance of surface modification and interface engineering for improved performance and stability.

How Low Can You Go? Defect Quantification at the 1015 cm–3 Level in Mixed-Cation Perovskites Using Differential Pulse Voltammetry

Reactive defects in hybrid organic–inorganic metal halide perovskites that limit material functionality and durability, factors which ultimately dictate (opto)electronic device performance, are difficult to probe. Herein, we expand an electrochemical methodology for near-valence defect quantification, using a solid-state electrolyte “top contact,” to increase energy resolution and sensitivity via a differential pulse protocol. A new low level of detection of ca. 2 × 1015 cm–3 is reported for reactive defects in a triple-cation system. We confirmed that these defects are associated with mobile iodide ions in grain boundaries and at interfaces by using iodide and bromide spiked electrolytes. The detection limit for this electrochemical method is estimated to be ca. 1014 cm–3, well below that of many electrical or spectroscopic approaches. We predict this methodology lends itself to operando defect characterization during and after processing, at scale, of extremely low defect density perovskites and related optoelectronic platforms, ultimately providing an in-line approach to real-time performance and stability optimization.

Magnetic Properties of Layered Hybrid Organic‐Inorganic Metal‐Halide Perovskites: Transition Metal, Organic Cation and Perovskite Phase Effects

AbstractUnderstanding the structural and magnetic properties in layered hybrid organic‐inorganic metal halide perovskites (HOIPs) is key for their design and integration in spin‐electronic devices. Here, a systematic study is conducted on ten compounds to understand the effect of the transition metal (Cu2+, Mn2+, Co2+), organic spacer (alkyl‐ and aryl‐ammonium), and perovskite phase (Ruddlesden‐Popper and Dion‐Jacobson) on the properties of these materials. Temperature‐dependent Raman measurements show that the crystals’ structural phase transitions are triggered by the motional freedom of the organic cations as well as by the flexibility of the inorganic metal‐halide lattice. In the case of Cu2+ HOIPs, an increase of the in‐plane anisotropy and a reduction of the octahedra interlayer distance is found to change the behavior of the HOIP from that of a 2D ferromagnet to that of a quasi‐3D antiferromagnet. Mn2+ HOIPs show inherent antiferromagnetic octahedra intralayer interactions and a phenomenologically rich magnetism, presenting spin‐canting, spin‐flop transitions, and metamagnetism controlled by the crystal anisotropy. Co2+ crystals with non‐linked tetrahedra show a dominant paramagnetic behavior irrespective of the organic spacer and the perovskite phase. This study demonstrates that the chemical flexibility of HOIPs can be exploited to develop novel layered magnetic materials with tailored magnetic properties.

CTAB assisted solvent-free mechanosynthesis of MAPbX3 nanocrystals: Stability, and photoresponse

In this study, we have used only multifunctional cetyl trimethylammonium bromide (CTAB) with a mechano-synthesis approach, which involves hand grinding to grow hybrid organic-inorganic metal halide perovskite, HPs, (MAPbX3, X ​= ​Br, I). Simplicity, swiftness, reproducibility, and harmony with green chemistry credentials make the mechano-synthesis process a preferable approach to the synthesis of HPs. We have prepared HPs using an equimolar amount of MAX and PbX2 with a mmol amount of CTAB. Steady-state photoluminescence, X-ray diffraction (XRD), and time correlate single-photon count (TCSPC) were performed to investigate the optical and structural features of mechanochemically produced perovskite. The thermogravimetric analysis informs about the thermal stability of the nanocrystals. X-ray diffraction analysis reveals that mechanochemically synthesized HPs show high crystallinity and phase purity with stability over a period of time. The photoinduced current response of the perovskite reveals an excellent increase in photoconductivity when exposed to light (with a 1 ​V bias). The device fabricated of perovskite witnessed the prompt photoresponse under light illumination and temporally-stable photocurrent over the multiple cycles of light irradiation. The synthesized materials are useful for photovoltaic applications because of their phase purity, low bandgap, nanoscale size, long-term stability, and easy-to-produce approach.

Factors influencing halide vacancy transport in perovskite solar cells

Ion migration in inorganic and hybrid organic-inorganic metal halide perovskites causes unusual phenomena in photovoltaic devices, such as current-voltage hysteresis and photoinduced phase transformations. It is now well established that metal halide perovskites are mixed ionic-electronic conductors and halide ions are mobile at room temperature. The effect of various physical stimuli like light, heat, pressure, and applied bias can affect the distribution and movement of ions. However, the impact of each stimulus is not understood as the measured response includes a combination of factors. We explore, using first-principles density functional theory (DFT), halide migration mediated by vacancies in model cubic phases of CsPbX3 (X = Cl, Br, I) and systematically control the electronic, chemical, and mechanical conditions. We assess the potential energy landscape for ion migration and how device-relevant conditions and compositional engineering can influence the physical behaviour of halide perovskites.

Raman spectroscopy in layered hybrid organic-inorganic metal halide perovskites

The continuous progress in the synthesis and characterization of materials in the vast family of hybrid organic-inorganic metal halide perovskites (HOIPs) has been pushed by their exceptional properties mainly in optoelectronic applications. These works highlight the peculiar role of lattice vibrations, which strongly interact with electrons, resulting in coupled states affecting the optical properties. Among these materials, layered (2D) HOIPs have emerged as a promising material platform to address some issues of their three-dimensional counterparts, such as ambient stability and ion migration. Layered HOIPs consist of inorganic layers made of metal halide octahedra separated by layers composed of organic cations. They have attracted much interest not only for applications, but also for their rich phenomenology due to their crystal structure tunability. Here, we give an overview of the main experimental findings achieved via Raman spectroscopy in several configurations and set-ups, and how they contribute to shedding light on the complex structural nature of these fascinating materials. We focus on how the phonon spectrum comes from the interplay of several factors. First, the inorganic and organic parts, whose motions are coupled, contribute with their typical modes which are very different in energy. Nonetheless, the interaction between them is relevant, as it results in low-symmetry crystal structures. Then, the role of external stimuli, such as temperature and pressure, which induce phase transitions affecting the spectrum through change in symmetry of the lattice, octahedral tilting and arrangement of the molecules. Finally, the relevant role of the coupling between the charge carriers and optical phonons is highlighted.

Tailoring Photoluminescence by Strain-Engineering in Layered Perovskite Flakes.

Strain is an effective strategy to modulate the optoelectronic properties of 2D materials, but it has been almost unexplored in layered hybrid organic-inorganic metal halide perovskites (HOIPs) due to their complex band structure and mechanical properties. Here, we investigate the temperature-dependent microphotoluminescence (PL) of 2D (C6H5CH2CH2NH3)2Cs3Pb4Br13 HOIP subject to biaxial strain induced by a SiO2 ring platform on which flakes are placed by viscoelastic stamping. At 80 K, we found that a strain of <1% can change the PL emission from a single peak (unstrained) to three well-resolved peaks. Supported by micro-Raman spectroscopy, we show that the thermomechanically generated strain modulates the bandgap due to changes in the octahedral tilting and lattice expansion. Mechanical simulations demonstrate the coexistence of tensile and compressive strain along the flake. The observed PL peaks add an interesting feature to the rich phenomenology of photoluminescence in 2D HOIPs, which can be exploited in tailored sensing and optoelectronic devices.

Improved water repellency and environmental stability of perovskite solar cells by encapsulating with paraffin wax

Hybrid organic-inorganic metal halide perovskite solar cells are the fastest growing cost-effective photovoltaic technology because of their solution processability and lightweight properties. However, the poor stability of perovskite materials in presence of moisture is the main hindrance to their outdoor applications. Here, paraffin wax is explored as an encapsulating material for improving the water repellency of perovskite solar cells. For this purpose, the drop of paraffin from a burning candle is placed on the device area composed of methylammonium lead iodide as a perovskite material and doped poly(3-hexylthiophene) as a hole-transporting layer. The encapsulation does not affect the photovoltaic performance of the tested devices in ambient conditions, exhibiting the same performance as in an inert atmosphere. Moreover, the photovoltaic performance of the encapsulated solar cells remains unchanged even after 2160 h under ambient conditions with continuous exposure to the relative humidity in the range of 70–85%. The encapsulation is so robust that the photovoltaic performance of the cells does not change even after immersing the encapsulated cell in water or after heating at 85 °C. This performance of paraffin coating is superior to many reported sealing agents, which are deposited using costly instruments or by multistep processes. Thus, the paraffin exhibits the potential to be used as an alternative low-cost coating material for perovskite solar cells.

Advances in Flexible Memristors with Hybrid Perovskites.

Hybrid organic-inorganic metal halide perovskite (HOIP)-based memristors have captured strong attention not only as an emerging candidate for next-generation high-density information storage technology but also for use in healthcare technology and the Internet of Things (IoT) because of their unique properties: low weight, flexibility, compatibility, stretchability, and low power consumption. In this Perspective, we review the recent advances of various aspects of flexible memristors focusing on the selection of the flexible substrates, materials, interfaces, several resistive switching mechanisms, and different methodologies of perovskite growth. The current state of the art of the memristor as an artificial synapse, light-induced resistive switching, and logic gates is comprehensively and systematically reviewed. Finally, we briefly discuss the stability factors of perovskites and present the conclusion with a broad outlook on the progress and challenges in the field of perovskite-based flexible memristors.

Ultralow thermal conductivity in 1D and 2D imidazolium-based lead halide perovskites

Low-dimensional hybrid organic–inorganic metal halide perovskites are rapidly emerging as a fascinating sub-class of the three-dimensional parent structures, thanks to their appealing charge and thermal transport properties, paired to better chemical and thermal stabilities. Extensive investigations of the thermal behavior in these systems are of paramount relevance to understand their optoelectronic and thermoelectric applications. Herein, we present a complete thermophysical characterization of imidazolium lead iodide, (IMI)PbI3, a 1D pseudo-perovskite with chains of face-sharing octahedra, and histammonium lead iodide, (HIST)PbI4, a 2D layered perovskite with corner-sharing octahedra. Upon heating, the two compounds show highly anisotropic thermal expansion effects and high thermal stability until 250–300 °C. The thermal diffusivity of pelletized powders was measured with the laser flash technique from room temperature up to 225 °C. To account for the reduced density of the pelletized powders with respect to the bulk, the diffusivity data in different atmospheres were modeled as a function of the volume fraction and dimensionality of the pores, allowing to extrapolate the thermal conductivity of the bulk materials. The two compounds exhibit an ultralow thermal conductivity of 0.15 W/m K, two to three times lower than that reported on 3D MAPbI3 using the same technique. This finding suggests the primary role of the organic molecules within the hybrid systems, regardless of the octahedra connectivity and dimensionality.

Aqueous phase synthesis of trimethylsulfoxonium lead triiodide for moisture-stable perovskite solar cells

Organosulfonium cations have attracted growing attention over conventional organoammonium cations for the development of moisture-stable hybrid organic–inorganic metal halide perovskite solar cells (PSCs). Herein, the synthesis of a moisture-stable trimethylsulfoxonium lead triiodide ((CH3)3SOPbI3 or TMSOPbI3) perovskite is described via a two-step solution process in an aqueous medium. The synthesized TMSOPbI3 exhibits a one-dimensional nanorod array with an optical bandgap of 2.30 eV and a hexagonal crystal structure. In addition, the fabricated fluorine-doped tin oxide/compact-TiO2/mesoporous-TiO2/TMSOPbI3/CuSCN/Au PSC device generates a maximum power conversion efficiency (PCE) of 2.23% with a good moisture stability at ambient temperature and relative humidity (50%) with no PCE loss during 336 h and no change in the crystal structure during 50 days. The high moisture stability of the device is attributed to the absence of hydrogen bonding between the trimethylsulfoxonium (TMSO+) cation and the H2O molecules along with strong electrostatic interactions between the TMSO+ and [PbI6]4- polyhedra in the TMSOPbI3. This research has demonstrated that TMSO+ is suitable for fabricating a stable perovskite-like material with good optoelectronic properties and is a promising material for practical applications.

Electro-Optic Modulation Using Metal-Free Perovskites.

Electro-optic (EO) modulation is of interest to impart information onto an optical carrier. Inorganic crystals-most notably LiNbO3 and BaTiO3-exhibit EO modulation and good stability, but are difficult to integrate with silicon photonic technology. Solution-processed organic EO materials are readily integrated but suffer from thermal degradation at the temperatures required in operating conditions for accelerated reliability studies. Hybrid organic-inorganic metal halide perovskites have the potential to overcome these limitations; however, these have so far relied on heavy metals such as lead and cadmium. Here, we report linear EO modulation using metal-free perovskites, which maintain the crystalline features of the inorganic EO materials and incorporate the flexible functionality of organic EO chromophores. We find that, by introducing a deficiency of cations, we reduce the symmetry in the perovskite crystal and produce thereby an increased EO response. The best-engineered perovskites reported herein showcase an EO coefficient of 14 pm V-1 at a modulation frequency of 80 kHz, an order of magnitude higher than in the nondefective materials. We observe split peaks in the X-ray diffraction and neutron diffraction patterns of the defective sample, indicating that the crystalline structure has been distorted and the symmetry reduced. Density functional theory (DFT) studies link this decreased symmetry to NH4+ deficiencies. This demonstration of EO from metal-free perovskites highlights their potential in next-generation optical information transmission.

Photocurrent in Metal-Halide Perovskite/Organic Semiconductor Heterostructures: Impact of Microstructure on Charge Generation Efficiency.

Hybrid organic-inorganic metal-halide perovskites have emerged as versatile materials for enabling low-cost, mechanically flexible optoelectronic applications. The progress has been commendable; however, technological breakthroughs have outgrown the basic understanding of processes occurring in bulk and at device interfaces. Here, we investigated the photocurrent at perovskite/organic semiconductor interfaces in relation to the microstructure of electronically active layers. We found that the photocurrent response is significantly enhanced in the bilayer structure as a result of a more efficient dissociation of the photogenerated excitons and trions in the perovskite layer. The increase in the grain size within the organic semiconductor layer results in reduced trapping and further enhances the photocurrent by extending the photocarriers' lifetime. The photodetector responsivity and detectivity have improved by 1 order of magnitude in the optimized samples, reaching values of 6.1 ± 1.1 A W-1, and 1.5 × 1011 ± 4.7 × 1010 Jones, respectively, and the current-voltage hysteresis has been eliminated. Our results highlight the importance of fine-tuning film microstructure in reducing the loss processes in thin-film optoelectronics based on metal-halide semiconductors and provide a powerful interfacial design method to consistently achieve high-performance photodetectors.