Ultrahigh sensitivity of methylammonium lead tribromide perovskite single crystals to environmental gases

Environment-Induced Reversible Modulation of Optical and Electronic Properties of Lead Halide Perovskites and Possible Applications to Sensor Development: A Review

Lead halide perovskites are currently widely investigated as active materials in photonic and optoelectronic devices. While the lack of long term stability actually limits their application to commercial devices, several experiments demonstrated that beyond the irreversible variation of the material properties due to degradation, several possibilities exist to reversibly modulate the perovskite characteristics by acting on the environmental conditions. These results clear the way to possible applications of lead halide perovskites to resistive and optical sensors. In this review we will describe the current state of the art of the comprehension of the environmental effects on the optical and electronic properties of lead halide perovskites, and of the exploitation of these results for the development of perovskite-based sensors.

Highly efficient photoelectric effect in halide perovskites for regenerative electron sources

Electron sources are a critical component in a wide range of applications such as electron-beam accelerator facilities, photomultipliers, and image intensifiers for night vision. We report efficient, regenerative and low-cost electron sources based on solution-processed halide perovskites thin films when they are excited with light with energy equal to or above their bandgap. We measure a quantum efficiency up to 2.2% and a lifetime of more than 25 h. Importantly, even after degradation, the electron emission can be completely regenerated to its maximum efficiency by deposition of a monolayer of Cs. The electron emission from halide perovskites can be tuned over the visible and ultraviolet spectrum, and operates at vacuum levels with pressures at least two-orders higher than in state-of-the-art semiconductor electron sources.

Gas Sensing with Solar Cells: The Case of NH3 Detection through Nanocarbon/Silicon Hybrid Heterojunctions

Photovoltaic (PV) cells based on single-walled carbon nanotube (SWCNT)/silicon (Si) and multiwalled carbon nanotube (MWCNT)/Si junctions were tested under exposure to NH₃ in the 0-21 ppm concentration range. The PV cell parameters remarkably changed upon NH₃ exposure, suggesting that these junctions, while being operated as PV cells, can react to changes in the environment, thereby acting as NH₃ gas sensors. Indeed, by choosing the open-circuit voltage, V_OC, parameter as read-out, it was found that these cells behaved as gas sensors, operating at room temperature with a response higher than chemiresistors developed on the same layers. The sensitivity was further increased when the whole current-voltage (I-V) curve was collected and the maximum power values were tracked upon NH₃ exposure.

Fluorescent methylammonium lead halide perovskite quantum dots as a sensing material for the detection of polar organochlorine pesticide residues

Methylammonium lead halide perovskite quantum dots (MAPB-QDs) have been widely used for photovoltaic devices due to their special electronic structures. In this work, MAPB-QDs were used for the first time to detect polar organochlorine pesticides (OCPs) based on the phenomenon that the fluorescence spectra of MAPB-QDs were blue-shifted in the presence of polar OCPs. Furthermore, ¹H NMR, FTIR, XPS and XRD were performed first to illustrate the sensing mechanism. In the presence of polar OCPs, the MAPB-QDs' capping ligands, oleic acid (OA) and oleylamine (OAm), were replaced with OCPs and then the chlorine element was adequately doped into QDs, resulting in the increase of the MAPB-QDs' bandgap. As result of the insufficient stability of MAPB-QDs in the presence of moisture, MAPB-QDs were mixed with PDMS and used as the colorimetric cards for fast detection of OCPs in real samples.

Halide perovskite nanocrystal arrays: Multiplexed synthesis and size-dependent emission

Halide perovskites have exceptional optoelectronic properties, but a poor understanding of the relationship between crystal dimensions, composition, and properties limits their use in integrated devices. We report a new multiplexed cantilever-free scanning probe method for synthesizing compositionally diverse and size-controlled halide perovskite nanocrystals spanning square centimeter areas. Single-particle photoluminescence studies reveal multiple independent emission modes due to defect-defined band edges with relative intensities that depend on crystal size at a fixed composition. Smaller particles, but ones with dimensions that exceed the quantum confinement regime, exhibit blue-shifted emission due to reabsorption of higher-energy modes. Six different halide perovskites have been synthesized, including a layered Ruddlesden-Popper phase, and the method has been used to prepare functional solar cells based on single nanocrystals. The ability to pattern arrays of multicolor light-emitting nanocrystals opens avenues toward the development of optoelectronic devices, including optical displays.

Inhibition of Phase Segregation in Cesium Lead Mixed-Halide Perovskites by B-Site Doping

The emergence of all-inorganic halide perovskites has shown great potential in photovoltaic and optoelectronic devices. However, the photo-induced phase segregation in lead mixed-halide perovskites has severely limited their application. Herein, by real-time monitoring the photoluminescence (PL) spectra of metal mixed-halide perovskites under light irradiation, we found that the photo-induced phase transition can be significantly inhibited by B-site doping. For pristine mixed-halide perovskites, an intermediate phase of CsPbBr_xI_3-x can only be stabilized under low excitation power. After introducing Sn²⁺ ions, such intermediate phase can be stabilized in nitrogen atmosphere under high excitation power and phase segregation can be started after the exposure in oxygen due to oxidization of Sn²⁺. Replacing Sn²⁺ by Mn²⁺ can further improve the intermediate phase's tolerance to oxygen proving that B-site doping in perovskites structure by Sn²⁺ or Mn²⁺ could effectively minimize the light-induced phase segregation and promote them to serve as promising candidates in photovoltaic and light-emitting devices.

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Phase segregation process of perovskite materials can be real-time monitored by PL

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Sn²⁺/Mn²⁺ doping can significantly improve the phase stability of CsPbI_xBr_3-x

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Mn²⁺ doping brings CsPbI_xBr_3-x higher tolerance to oxygen and moisture

Micro- and Nanopatterning of Halide Perovskites Where Crystal Engineering for Emerging Photoelectronics Meets Integrated Device Array Technology

Tremendous efforts have been devoted to developing thin film halide perovskites (HPs) for use in high-performance photoelectronic devices, including solar cells, displays, and photodetectors. Furthermore, structured HPs with periodic micro- or nanopatterns have recently attracted significant interest due to their potential to not only improve the efficiency of an individual device via the controlled arrangement of HP crystals into a confined geometry, but also to technologically pixelate the device into arrays suitable for future commercialization. However, micro- or nanopatterning of HPs is not usually compatible with conventional photolithography, which is detrimental to ionic HPs and requires special techniques. Herein, a comprehensive overview of the state-of-the-art technologies used to develop micro- and nanometer-scale HP patterns, with an emphasis on their controlled microstructures based on top-down and bottom-up approaches, and their potential for future applications, is provided. Top-down approaches include modified conventional lithographic techniques and soft-lithographic methods, while bottom-up approaches include template-assisted patterning of HPs based on lithographically defined prepatterns and self-assembly. HP patterning is shown here to not only improve device performance, but also to reveal the unprecedented functionality of HPs, leading to new research areas that utilize their novel photophysical properties.

Modulating the optical and electrical properties of MAPbBr3 single crystals via voltage regulation engineering and application in memristors

Defect density is one of the most significant characteristics of perovskite single crystals (PSCs) that determines their optical and electrical properties, but few strategies are available to tune this property. Here, we demonstrate that voltage regulation is an efficient method to tune defect density, as well as the optical and electrical properties of PSCs. A three-step carrier transport model of MAPbBr3 PSCs is proposed to explore the defect regulation mechanism and carrier transport dynamics via an applied bias. Dynamic and steady-state photoluminescence measurements subsequently show that the surface defect density, average carrier lifetime, and photoluminescence intensity can be efficiently tuned by the applied bias. In particular, when the regulation voltage is 20 V (electrical poling intensity is 0.167 V μm-1), the surface defect density of MAPbBr3 PSCs is reduced by 24.27%, the carrier lifetime is prolonged by 32.04%, and the PL intensity is increased by 112.96%. Furthermore, a voltage-regulated MAPbBr3 PSC memristor device shows an adjustable multiresistance, weak ion migration effect and greatly enhanced device stability. Voltage regulation is a promising engineering technique for developing advanced perovskite optoelectronic devices.

A Behavior-Learned Cross-Reactive Sensor Matrix for Intelligent Skin Perception

Mimicking human skin sensation such as spontaneous multimodal perception and identification/discrimination of intermixed stimuli is severely hindered by the difficulty of efficient integration of complex cutaneous receptor-emulating circuitry and the lack of an appropriate protocol to discern the intermixed signals. Here, a highly stretchable cross-reactive sensor matrix is demonstrated, which can detect, classify, and discriminate various intermixed tactile and thermal stimuli using a machine-learning approach. Particularly, the multimodal perception ability is achieved by utilizing a learning algorithm based on the bag-of-words (BoW) model, where, by learning and recognizing the stimulus-dependent 2D output image patterns, the discrimination of each stimulus in various multimodal stimuli environments is possible. In addition, the single sensor device integrated in the cross-reactive sensor matrix exhibits multimodal detection of strain, flexion, pressure, and temperature. It is hoped that his proof-of-concept device with machine-learning-based approach will provide a versatile route to simplify the electronic skin systems with reduced architecture complexity and adaptability to various environments beyond the limitation of conventional "lock and key" approaches.

Influence of Environment and Light-Stress on the Optoelectronic Properties of Triple-Cation Perovskite Thin Films

In this work, we study the transport properties of triple-cation halide perovskite thin films and their evolution when exposed to air or vacuum and after light-soaking. Transport parameters were investigated by steady-state dark and photocurrent methods as well as by the steady-state photocarrier grating experiment (SSPG) from which the ambipolar diffusion length of thin film materials is estimated. Combined with other characterization measurements, such as photoluminescence and Fourier transform photocurrent spectroscopy, these techniques demonstrate that air plays an important role in the passivation of the surface trap states of the perovskite films. The competition between passivation and degradation of the films under light-soaking was also deeply investigated. Moreover, we show that the degradation of the transport parameters upon light-soaking could be linked mainly to a degradation of the carrier mobility instead of their lifetime.

Reducing Detrimental Defects for High-Performance Metal Halide Perovskite Solar Cells

In several photovoltaic (PV) technologies, the presence of electronic defects within the semiconductor band gap limit the efficiency, reproducibility, as well as lifetime. Metal halide perovskites (MHPs) have drawn great attention because of their excellent photovoltaic properties that can be achieved even without a very strict film-growth control processing. Much has been done theoretically in describing the different point defects in MHPs. Herein, we discuss the experimental challenges in thoroughly characterizing the defects in MHPs such as, experimental assignment of the type of defects, defects densities, and the energy positions within the band gap induced by these defects. The second topic of this Review is passivation strategies. Based on a literature survey, the different types of defects that are important to consider and need to be minimized are examined. A complete fundamental understanding of defect nature in MHPs is needed to further improve their optoelectronic functionalities.

Facile and noninvasive passivation, doping and chemical tuning of macroscopic hybrid perovskite crystals

Halide vacancies and associated metallic lead (Pb°) observed at the surface and deep inside macroscopic organolead trihalide perovskite crystals is removed through a facile and noninvasive treatment. Indeed, Br₂ vapor is shown to passivate Br-vacancies and associated Pb° in the bulk of macroscopic crystals. Controlling the exposure time can markedly improve the overall stoichiometry for moderate exposures or introduce excessive bromide for long exposures, resulting in p-doping of the crystals. In the low dose passivation regime, Hall effect measurements reveal a ca. 3-fold increase in carrier mobility to ca. 15 cm²V^-1s^-1, while the p-doping increases the electrical conductivity ca. 10000-fold, including a 50-fold increase in carrier mobility to ca. 150 cm²V^-1s^-1. The ease of diffusion of Br₂ vapor into macroscopic crystals is ascribed to the porosity allowed in rapidly grown crystals through aggregative processes of the colloidal sol during growth of films and macroscopic crystals. This process is believed to form significant growth defects, including open voids, which may be remnants of the escaping solvent at the solidification front. These results suggest that due to the sol-gel-like nature of the growth process, macroscopic perovskite crystals reported in this study are far from perfect and point to possible pathways to improving the optoelectronic properties of these materials. Nevertheless, the ability of the vapor-phase approach to access and tune the bulk chemistry and properties of nominally macroscopic perovskite crystals provides interesting new opportunities to precisely manipulate and functionalize the bulk properties of hybrid perovskite crystals in a noninvasive manner.

Bulk Chiral Halide Perovskite Single Crystals for Active Circular Dichroism and Circularly Polarized Luminescence

Motivated by the chirality research of the hybrid halide perovskite, we reported the controllable growth of single crystals of (R)-, (S)-, and (R,S)-C₆H₅CH(CH₃)NH₃ (MBA)-based lead (Pb) halide perovskites. The crystal structures were redetermined and further refined to clarify the previously ambiguous crystal structure problems. We further investigated the chiral optical properties of these single crystals including nonlinear optical (NLO) properties and photoluminescence (PL) properties. The as-fabricated (R)- and (S)-MBAPbBr₃ single crystals not only show notable circular dichroism (CD) signals in the absorption spectra but also exhibit obvious circularly polarized luminescence (CPL) characteristics. The available chiral hybrid perovskite single crystals open up the possibility to study these intrinsic chirality properties for optoelectronic applications.

Vacuum-Induced Degradation of 2D Perovskites

Two-dimensional (2D) hybrid organic-inorganic perovskites have recently attracted the attention of the scientific community due to their exciting optical and electronic properties as well as enhanced stability upon exposure to environmental factors. In this work, we investigate 2D perovskite layers with a range of organic cations and report on the Achilles heel of these materials—their significant degradation upon exposure to vacuum. We demonstrate that vacuum exposure induces the formation of a metallic lead species, accompanied by a loss of the organic cation from the perovskite. We investigate the dynamics of this reaction, as well as the influence of other factors, such as X-ray irradiation. Furthermore, we characterize the effect of degradation on the microstructure of the 2D layers. Our study highlights that despite earlier reports, 2D perovskites may exhibit instabilities, the chemistry of which should be identified and investigated in order to develop suitable mitigation strategies.

Dynamic Passivation in Perovskite Quantum Dots for Specific Ammonia Detection at Room Temperature

Perovskite structured CsPbX₃ (X = Cl, Br, or I) quantum dots (QDs) have attracted considerable interest in the past few years due to their excellent optoelectronic properties. Surface passivation is one of the main pathways to optimize the optoelectrical performance of perovskite QDs, in which the amino group plays an important role for the corresponding interaction between lead and halide. In this work, it is found that ammonia gas could dramatically increase photoluminescence of purified QDs and effectively passivate surface defects of perovskite QDs introduced during purification, which is a reversible process. This phenomenon makes perovskite QDs a kind of ideal candidate for detection of ammonia gas at room temperature. This QD film sensor displays specific recognition behavior toward ammonia gas due to its significant fluorescence enhancement, while depressed luminescence in case of other gases. The sensor, in turn-on mode, shows a wide detection range from 25 to 350 ppm with a limit of detection as low as 8.85 ppm. Meanwhile, a fast response time of ≈10 s is achieved, and the recovery time is ≈30 s. The fully reversible, high sensitivity and selectivity characteristics make CsPbBr₃ QDs ideal active materials for room-temperature ammonia sensing.

Visualizing Buried Local Carrier Diffusion in Halide Perovskite Crystals via Two-Photon Microscopy

Halide perovskites have shown great potential for light emission and photovoltaic applications due to their remarkable electronic properties. Although the device performances are promising, they are still limited by microscale heterogeneities in their photophysical properties. Here, we study the impact of these heterogeneities on the diffusion of charge carriers, which are processes crucial for efficient collection of charges in light-harvesting devices. A photoluminescence tomography technique is developed in a confocal microscope using one- and two-photon excitation to distinguish between local surface and bulk diffusion of charge carriers in methylammonium lead bromide single crystals. We observe a large dispersion of local diffusion coefficients with values between 0.3 and 2 cm²·s^-1 depending on the trap density and the morphological environment-a distribution that would be missed from analogous macroscopic or surface measurements. This work reveals a new framework to understand diffusion pathways, which are extremely sensitive to local properties and buried defects.

Study of Metal-Semiconductor-Metal CH3NH3PbBr3 Perovskite Photodetectors Prepared by Inverse Temperature Crystallization Method

Numerous studies have addressed the use of perovskite materials for fabricating a wide range of optoelectronic devices. This study employs the deposition of an electron transport layer of C₆₀ and an Ag electrode on CH₃NH₃PbBr₃ perovskite crystals to complete a photodetector structure, which exhibits a metal–semiconductor–metal (MSM) type structure. First, CH₃NH₃PbBr₃ perovskite crystals were grown by inverse temperature crystallization (ITC) in a pre-heated circulator oven. This oven was able to supply uniform heat for facilitating the growth of high-quality and large-area crystals. Second, the different growth temperatures for CH₃NH₃PbBr₃ perovskite crystals were investigated. The electrical, optical, and morphological characteristics of the perovskite crystals were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), ultraviolet-visible spectroscopy, and photoluminescence (PL). Finally, the CH₃NH₃PbBr₃ perovskite crystals were observed to form a contact with the Ag/C₆₀ as the photodetector, which revealed a responsivity of 24.5 A/W.

Understanding the Ligand Effects on Photophysical, Optical, and Electroluminescent Characteristics of Hybrid Lead Halide Perovskite Nanocrystal Solids

There has been a tremendous amount of interest in developing high-efficiency light-emitting diodes (LEDs) based on colloidal nanocrystals (NCs) of hybrid lead halide perovskites. Here, we systematically investigate the ligand effects on EL characteristics by tuning the hydrophobicity of primary alkylamine ligands used in NC synthesis. By increasing the ligand hydrophobicity, we find (i) a reduced NC size that induces a higher degree of quantum confinement, (ii) a shortened exciton lifetime that increases the photoluminescence quantum yield, (iii) a lowering of refractive index that increases the light outcoupling efficiency, and (iv) an increased thin-film resistivity. Accordingly, ligand engineering allows us to demonstrate high-performance green LEDs exhibiting a maximum external quantum efficiency up to 16.2%. The device operational lifetime, defined by the time lasted when the device luminance reduces to 85% of its initial value, LT85, reaches 243 min at an initial luminance of 516 cd m^-2.

The importance of relativistic effects on two-photon absorption spectra in metal halide perovskites

Despite intense research into the optoelectronic properties of metal halide perovskites (MHPs), sub-bandgap absorption in MHPs remains largely unexplored. Here we recorded two-photon absorption spectra of MHPs using the time-resolved microwave conductivity technique. A two-step upward trend is observed in the two-photon absorption spectrum for methylammonium lead iodide, and some analogues, which implies that the commonly used scaling law is not applicable to MHPs. This aspect is further confirmed by temperature-dependent conductivity measurements. Using an empirical multiband tight binding model, spectra for methylammonium lead iodide were calculated by integration over the entire Brillouin zone, showing compelling similarity with experimental results. We conclude that the second upward trend in the two-photon absorption spectrum originates from additional optical transitions to the heavy and light electron bands formed by the strong spin-orbit coupling. Hence, valuable insight can be obtained in the opto-electronic properties of MHPs by sub-bandgap spectroscopy, complemented by modelling.

Unravelling the effects of oxidation state of interstitial iodine and oxygen passivation on charge trapping and recombination in CH3NH3PbI3 perovskite: a time-domain ab initio study

Understanding nonradiative charge recombination mechanisms is a prerequisite for advancing perovskite solar cells. By performing time-domain density functional theory combined with nonadiabatic (NA) molecular dynamics simulations, we show that electron-hole recombination in perovskites strongly depends on the oxidation state of interstitial iodine and oxygen passivation. The simulations demonstrate that electron-hole recombination in CH3NH3PbI3 occurs within several nanoseconds, agreeing well with experiment. The negative interstitial iodine delays charge recombination by a factor of 1.3. The deceleration is attributed to the fact that interstitial iodine anion forms a chemical bond with its nearest lead atoms, eliminates the trap state, and decreases the NA electron-phonon coupling. The positive interstitial iodine attracts its neighbouring lattice iodine anions, resulting in the formation of an I-trimer and producing an electron trap. Electron trapping proceeds on a very fast timescale, tens of picoseconds, and captures the majority of free electrons available to directly recombine with free holes while inhibiting the recombination of free electrons and holes, delaying the recombination by a factor of 1.5. However, the positive interstitial iodine easily converts to a neutral iodine defect by capturing an electron, giving rise to a singly occupied state above the valence band maximum and acting as a hole trap. The photoexcitation valence band hole becomes trapped by the hole trap state very rapidly, followed by acceleration of recombination with the conduction band free electron by a factor of 1.6. Surprisingly, molecular oxygen interacting with interstitial iodine anion forms a stable IO3 -1 species, which inhibits ion migration, stabilizes perovskites, and suppresses the electron-hole recombination by a factor of 2.7. Our simulations reveal the microscopic effects of the oxidation state of interstitial iodine defects and oxygen passivation in perovskites, suggesting an effective way to improve perovskite photovoltaic and optoelectronic devices.

Light-activated inorganic CsPbBr2I perovskite for room-temperature self-powered chemical sensing

Halide perovskite materials are excellent light harvesters that have generated enormous interest for photovoltaic technology and an increasing number of other optoelectronic applications. Very recently, their use for miniaturized chemical sensors has shown a promising room-temperature response. Here, we present some insights on the use of CsPbBr2I (CPBI) perovskites for self-powered room-temperature sensing of several environmentally and medically relevant compounds demonstrating rapid detection of down to concentrations of 1 ppm. Notably, the photocurrent of these self-powered CPBI-based devices increases under exposure to both reducing (e.g. acetone, propane) and oxidizing (e.g. NO2, O2) gas molecules and decreases rapidly upon reverting to an inert atmosphere. In situ photoluminescence (PL) analysis of the CPBI during exposure to oxidizing molecules reveals a strongly increased PL intensity and longer lifetime indicating a prevalent role of CPBI trap states in the sensing mechanism. These findings provide new insights for the engineering of perovskite-based materials for their future chemical sensing applications.

Interfacial Modification in Organic and Perovskite Solar Cells

Organic bulk heterojunction solar cells (OSCs) and hybrid halide perovskite solar cells (PSCs) are two promising photovoltaic techniques for next-generation energy conversion devices. The rapid increase in the power conversion efficiency (PCE) in OSCs and PSCs has profited from synergetic progresses in rational material synthesis for photoactive layers, device processing, and interface engineering. Interface properties in these two types of devices play a critical role in dictating the processes of charge extraction, surface trap passivation, and interfacial recombination. Therefore, there have been great efforts directed to improving the solar cell performance and device stability in terms of interface modification. Here, recent progress in interfacial doping with biopolymers and ionic salts to modulate the cathode interface properties in OSCs is reviewed. For the anode interface modification, recent strategies of improving the surface properties in widely used PEDOT:PSS for narrowband OSCs or replacing it by novel organic conjugated materials will be touched upon. Several recent approaches are also in focus to deal with interfacial traps and surface passivation in emerging PSCs. Finally, the current challenges and possible directions for the efforts toward further boosts of PCEs and stability via interface engineering are discussed.

Defect Activity in Lead Halide Perovskites

The presence of various types of chemical interactions in metal-halide perovskite semiconductors gives them a characteristic "soft" fluctuating structure, prone to a wide set of defects. Understanding of the nature of defects and their photochemistry is summarized, which leverages the cooperative action of density functional theory investigations and accurate experimental design. This knowledge is used to describe how defect activity determines the macroscopic properties of the material and related devices. Finally, a discussion of the open questions provides a path towards achieving an educated prediction of device operation, necessary to engineer reliable devices.

The Physics of Light Emission in Halide Perovskite Devices

Light emission is a critical property that must be maximized and controlled to reach the performance limits in optoelectronic devices such as photovoltaic solar cells and light-emitting diodes. Halide perovskites are an exciting family of materials for these applications owing to uniquely promising attributes that favor strong luminescence in device structures. Herein, the current understanding of the physics of light emission in state-of-the-art metal-halide perovskite devices is presented. Photon generation and management, and how these can be further exploited in device structures, are discussed. Key processes involved in photoluminescence and electroluminescence in devices as well as recent efforts to reduce nonradiative losses in neat films and interfaces are discussed. Finally, pathways toward reaching device efficiency limits and how the unique properties of perovskites provide a tremendous opportunity to significantly disrupt both the power generation and lighting industries are outlined.

Investigation of Oxygen Passivation for High-Performance All-Inorganic Perovskite Solar Cells

Defect passivation using oxygen has been identified as an efficient and convenient approach to suppress nonradiative recombination and improve the photovoltaic performance of hybrid organic-inorganic halide perovskites (HHPs). However, oxygen can seriously undermine the chemical stability of HHPs due to the reaction of superoxide with protonated organic cations such as CH₃NH₃⁺ and [(NH₂)₂CH]⁺, thus hindering the deep understanding of how oxygen affects their defect properties. Here we substitute free-proton inorganic Cs⁺ for organic moiety to avoid the negative effect of oxygen and then systematically investigate the oxygen passivation mechanism in all-inorganic halide perovskites (IHPs) from theory to experiment. We find that, in contrast to conventional oxygen molecule passivation just through physisorption on the surface of perovskites, the oxygen atom can provide a better passivation effect due to its stronger interaction with perovskites. The key point to achieve O-passivated perovskites rather than O₂ is the dry-air processing condition, which can dissociate the O₂ into O during the annealing process. O-passivated IHP solar cells exhibit enhanced power conversion efficiency (PCE) and better air stability than O₂-passivated cells. These results not only provide deep insights into the passivation effect of oxygen on perovskites but also demonstrate the great potential of IHPs for high photovoltaic performance with simplified ambient processing.

Gas Sensing Properties of Perovskite Decorated Graphene at Room Temperature

This paper explores the gas sensing properties of graphene nanolayers decorated with lead halide perovskite (CH3NH3PbBr3) nanocrystals to detect toxic gases such as ammonia (NH3) and nitrogen dioxide (NO2). A chemical-sensitive semiconductor film based on graphene has been achieved, being decorated with CH3NH3PbBr3 perovskite (MAPbBr3) nanocrystals (NCs) synthesized, and characterized by several techniques, such as field emission scanning electron microscopy, transmission electron microscopy and X-ray photoelectron spectroscopy. Reversible responses were obtained towards NO2 and NH3 at room temperature, demonstrating an enhanced sensitivity when the graphene is decorated by MAPbBr3 NCs. Furthermore, the effect of ambient moisture was extensively studied, showing that the use of perovskite NCs in gas sensors can become a promising alternative to other gas sensitive materials, due to the protective character of graphene, resulting from its high hydrophobicity. Besides, a gas sensing mechanism is proposed to understand the effects of MAPbBr3 sensing properties.

Surface Pb-Dimer Passivated by Molecule Oxygen Notably Suppresses Charge Recombination in CsPbBr3 Perovskites: Time-Domain Ab Initio Analysis

Experiments show that excess lead atoms accelerate charge recombination while oxygen passivation can heal the defects and enhance solar cell efficiency. Using ab initio nonadiabatic (NA) molecular dynamics, we demonstrate that an excess lead atom forms a Pb-dimer with a single surface lead atom of CsPbBr₃(001) surface and creates a deep hole trap. The electron-hole recombination is accelerated to over 10 ps via fast hole trapping or bypassing the hole trap compared to the pristine CsPbBr₃, occurring on tens of picoseconds. Pb-dimer passivated with oxygen molecules forms Pb-O bonds, breaks the Pb-dimer, and removes the trap state, leading to a decrease in the recombination and extending excited-state lifetime to over 100 ps. The deceleration arises mainly due to the reduced NA coupling and short decoherence time. The study advances our understanding of excited-state dynamics of all-inorganic perovskites in the presence of excess lead and oxygen atmosphere.

Ultrastable, Deformable, and Stretchable Luminescent Organic-Inorganic Perovskite Nanocrystal-Polymer Composites for 3D Printing and White Light-Emitting Diodes

Organic-inorganic perovskite nanocrystals with excellent optoelectronic properties have been utilized in various applications, despite their stability issues. The perovskite materials are sensitive to environments such as polar solvents, moisture, and heat. Thus, they are not used for extrusion three-dimensional (3D) printing, as it is usually conducted in the ambient environment and requires heating to liquefy the printed materials. In this work, 11 thermoplastic polymers conventionally used for extrusion 3D printing were investigated to test their capability as protective encapsulation materials for perovskite nanocrystals. Three of them exhibited good protective properties, and one (polycaprolactone, PCL) of these three could be blended with perovskite nanocrystals to form perovskite nanocrystal-PCL composites, which were deformable and stretchable once heated. Because of the low melting point of PCL, the perovskite nanocrystals maintained their optical properties after 3D printing, and the printed objects were still having fluorescent behavior. Moreover, fluorescent micrometer-sized fibers based on the perovskite nanocrystal-PCL composites could also be simply prepared using cotton candy makers. Perovskite nanocrystal-PCL composite films with different emission wavelengths were incorporated with blue light-emitting diodes (LEDs) to realize white LEDs with Commission Internationale de l'Éclairage chromaticity coordinates of (0.33, 0.33).

Mechanistic Insight into Surface Defect Control in Perovskite Nanocrystals: Ligands Terminate the Valence Transition from Pb2+ to Metallic Pb0

Organolead halide perovskite nanocrystals (NCs) have emerged as promising materials for various optoelectronic applications. However, their practical applications have been limited due to low structural integrity and poor luminescence stability associated with fast attachment-detachment dynamics of surface capping molecules during postprocessing. At present, a framework for understanding how the functional additives interact with surface moieties of organolead halide perovskites is not available. Methylammonium lead bromide NCs without surfactants on their surface provide an ideal system to investigate the direct interactions of the perovskite with functional molecules. When the oleic acid is used in a combination with n-octylamine, its contribution to surface passivation is significantly increased by protonating the alkyl amine to the corresponding ammonium ion. Our results demonstrate that the Br vacancies at the nonpassivated surface result in a reduction of Pb²⁺ to Pb⁰ by trapping electrons generated from the exciton dissociation, which provides a main pathway for exciton trapping.

Giant reduction of the random lasing threshold in CH3NH3PbBr3 perovskite thin films by using a patterned sapphire substrate

Hybrid organic-inorganic metal halide perovskites are currently arousing enthusiasm and stimulating huge activity across several fields of optoelectronics due to their outstanding properties. In this study, we present the incoherent random lasing (RL) emissions from CH3NH3PbBr3 perovskite thin films on both planar fluorine-doped tin oxide (FTO) substrates and patterned sapphire substrates (PSSs). A detailed examination of the spectral evolution indicates that inelastic exciton-exciton scattering called P-emission is the most plausible mechanism accounting for the lasing emissions. The RL threshold of the perovskite films on PSSs is found to be effectively reduced by more than one order of magnitude from 2.55 to 0.15 μJ per pulse compared to that on FTO substrates. The giant threshold reduction is ascribed to the enhanced random scattering of light and the photon recycling induced by the multireflection processes at the perovskite/PSS interface, which increases the likelihood that the inoperative random rays will re-enter the possible optical loops formed among the perovskite particles, resulting in considerable optical resonance enhancement. The simulation results reveal that the light extraction efficiency on the top facet of the perovskites is significantly increased by approximately 155% by utilizing the PSS instead of the FTO substrate. Moreover, the first direct experimental observation of the multireflection phenomenon of light, as well as the dynamic processes of photon propagation in the composite PSS structure, is presented by Kerr-gate-based time-resolved photoluminescence. Our results provide an effective strategy to achieve high-performance perovskite random lasers and novel light-emitting devices for speckle-free full-field imaging and solid-state lighting applications, by introducing ingeniously designed periodic nano-/microscale optical structures.

Ferroelectricity of the Orthorhombic and Tetragonal MAPbBr3 Single Crystal

Hybrid organic-inorganic halide perovskites (HOIPs) MAPbBr₃ and their ramifications have emerged because of the photovoltaic, optical, and other fascinating performances of HOIPs in recent years. However, many intrinsic properties, such as crystal structure and ferroelectricity, are still controversial. In this work, the ferroelectricity of the orthorhombic and tetragonal MAPbBr₃ single crystal was confirmed through the dielectric behavior versus bias electric field ε( E), the temperature-dependent pyroelectric current with positive/negative poling, and the positive-up-negative-down (PUND) measurements. The electric field dependence of dielectric constant curves shows a butterfly type shape in the orthorhombic and tetragonal phase. The pyroelectric current shows two maxima at 155 and 245 K, corresponding to ferroelectric-ferroelectric and ferroelectric-paraelectric phase transitions, respectively. In particular, the direction of the pyroelectric current can be reversed by a positive or negative poling electric field, which is the assertive evidence of ferroelectricity. The PUND measurements act as the most convincing proof of the ferroelectricity of the MAPbBr₃ single crystal. This work reports new evidence of the ferroelectric properties of the MAPbBr₃ single crystal, which provides the intrinsic property when considering their high power conversion efficiencies.

Environmental Gating and Galvanic Effects in Single Crystals of Organic-Inorganic Halide Perovskites

Understanding the impact of environmental gaseous on the surface of organometal halide perovskites (OMHPs) couples to the electronic and ionic transport is critically important. Here, we explore the transport behavior and origins of the gas sensitivity in MAPbBr₃ single crystals (SCs) devices using impedance spectroscopy and current relaxation measurements. Strong resistive response occurs when crystals are exposed to different environments. It was shown that SC response to the environment is extremely different at the surface as compared to the bulk due to the disorder surface chemistry. The nonlinear transport properties studied using ultrafast Kelvin probe force microscopy (G-KPFM) to unravel spatio-temporal charge dynamics at SC/electrode interface. The relaxation processes observed in pulse relaxation and G-KPFM measurements along with gas sensitivity of crystals suggest the presence of a triple-phase boundary between environment, electrode, and crystal. Results indicate that the environment is a nontrivial component in the operation of OMHP devices which is reminiscent of fuel cell systems. Furthermore, the triple-phase boundary can play a significant role in the transport properties of OMHPs due to the possibility of the redox processes coupled to the concentration of bulk ionic species. Although instrumental for understanding the device characteristics of perovskites, our studies suggest a new opportunity of coupling the redox chemistry of the Br₂-Br^- pair that defines the bulk ionic conductivity of MAPbBr₃ with the redox chemistry of gaseous (or liquid) environment via a suitable electrocatalytic system to enable new class of energy storage devices and gas sensors.

O2 as a molecular probe for nonradiative surface defects in CsPbBr3 perovskite nanostructures and single crystals

Lead halide perovskites, owing to their flexible, scalable chemistry and promising physical properties are attracting increasing attention for solution-processed optoelectronic and photonic technologies. Despite their well-known 'defect tolerant' electronic structure, studies highlighted the active role of shallow and deep defect states, as well as of oxidative environmental conditions, on the optical and electrical behavior of perovskite nanocubes, films and single bulk crystals. To date, however, no in-depth systematic study of the surface trap-mediated processes in perovskite materials of different dimensionality has been conducted. In this work, we aim to bridge this gap by using O2 as a molecular probe for the effects of surface states on the exciton recombination processes of nanocubes (NCs), nanowires (NWs), nanosheets (NSs) and bulk single crystals (SCs) of CsPbBr3 perovskite. Continuous wave and time-resolved photoluminescence (PL) experiments in a controlled O2 atmosphere reveal the opposite optical response of NCs with respect to higher dimensional perovskites directly deriving from the different nature of the material surfaces. Specifically, O2 passivates surface hole-traps in NWs, NSs and SCs, leading to PL brightening with unaltered recombination dynamics. Conversely, NCs appear to be free from such surface hole-traps and exposure to O2 leads to direct extraction of photogenerated electrons that competes with radiative exciton recombination, leading to dimmed PL efficiency in atmospheric conditions. This opposite oxygen PL response demystifies the critical role of surface passivation in perovskite NCs in stark contrast to higher dimensional nanostructures and single crystals.

Halide lead perovskites for ionizing radiation detection

Halide lead perovskites have attracted increasing attention in recent years for ionizing radiation detection due to their strong stopping power, defect-tolerance, large mobility-lifetime (μτ) product, tunable bandgap and simple single crystal growth from low-cost solution processes. In this review, we start with the requirement of material properties for high performance ionizing radiation detection based on direct detection mechanisms for applications in X-ray imaging and γ-ray energy spectroscopy. By comparing the performances of halide perovskites radiation detectors with current state-of-the-art ionizing radiation detectors, we show the promising features and challenges of halide perovskites as promising radiation detectors.

Halide lead perovskites have emerged recently as possible candidates for high performance radiation detectors besides efficient solar cells. Here Wei et al. review the recent progress on perovskite based radiation detectors and suggest that they may compete with the conventional counterparts.

Enhanced X-ray Sensitivity of MAPbBr3 Detector by Tailoring the Interface-States Density

An important factor for the high performance of light-harvesting devices is the presence of surface trappings. Therefore, understanding and controlling the carrier recombination of the organic-inorganic hybrid perovskite surface is critical for the device design and optimization. Here, we report the use of aluminum zinc oxide (AZO) as the anode to construct a p-n junction structure MAPbBr₃ nuclear radiation detector. The AZO/MAPbBr₃/Au detector can tolerate an electrical field of 500 V·cm^-1 and exhibit a very low leakage current of ∼9 nA, which is 1 order of magnitude lower than that of the standard ohmic contact device. The interface state density of AZO/MAPbBr₃ contact was reduced from 2.17 × 10¹⁰ to 8.7 × 10⁸ cm^-2 by annealing at 100 °C under an Ar atmosphere. Consequently, a photocurrent to dark current ratio of 190 was realized when exposed to a green light-emitting diode with a wavelength of 520 nm (∼200 mW·cm^-2). Simultaneously, a high X-ray sensitivity of ∼529 μC·Gy_air^-1 cm^-2 was achieved under 80 kVp X-ray at an electric field of 50 V·cm^-1. These results demonstrate the use of surface engineering to further optimize the performance of MAPbBr₃ detectors, which have many potential applications in medical and security detection with low radiation dose brought to the human body.

Mechanism of Photoluminescence Intermittency in Organic-Inorganic Perovskite Nanocrystals

Lead halide perovskite nanocrystals have demonstrated their potential as active materials for optoelectronic applications over the past few years. Nevertheless, one issue that hampers their applicability has to do with the observation of photoluminescence intermittency, commonly referred to as "blinking", as in their inorganic counterparts. Such behavior, reported for structures well above the quantum confinement regime, has been discussed to be strongly related to the presence of charge carrier traps. In this work, we analyze the characteristics of this intermittency and explore the dependence on the surrounding atmosphere, showing evidence for the critical role played by the presence of oxygen. We discuss a possible mechanism in which a constant creation/annihilation of halide-related carrier traps takes place under light irradiation, with the dominant rate being determined by the atmosphere.

Morphological and chemical tuning of lead halide perovskite mesocrystals as long-life anode materials in lithium-ion batteries

This is the first report stating that excellent cycling stability for 1000 cycles of MAPbBr₃ anodes composed of small sized samples should be attributed to the small size, low defect concentration structure and good interface charge transfer.

Imperfections and their passivation in halide perovskite solar cells

Perovskite solar cells to date are made of polycrystalline films which contain a high density of defects. Imperfection passivation to reduce non-radiative recombination and suppress ion migration could improve device efficiency and device stability.

Layer-edge device of two-dimensional hybrid perovskites

Two dimensional layered organic-inorganic hybrid perovskites (2D perovskites) are potential candidates for next generation photovoltaic device. Especially, the out-of-plane surface perpendicular to the superlattice plane of 2D perovskites (layer-edge surface) has presented several exotic behaviors, such as layer-edge states which are found to be crucial for improving the efficiency of 2D perovskite solar cells. However, fundamental research on transport properties of layer-edge surface is still absent. In this report, we observe the electronic and opto-electronic behavior in layer-edge device of 2D perovskites. The dark and photo currents are demonstrated to strongly depend on the crystallographic orientation in layer-edge device, and such anisotropic properties, together with photo response, are related to the thickness of inorganic layers. Finally, due to the abundant hydroxyl groups, water molecules are easy to condense on the layer-edge surface, and the conductance is extremely sensitive to the humidity environment, indicating a potential application of humidity sensor.

The edge of the thin sheets of two dimensional organic-inorganic hybrid perovskite crystals shows exotic physical properties that remain unexplored. Here Cheng et al. make electronic devices and study their transport properties, photoresponses and humid sensitivities which show large anisotropy.

Efficient Perovskite Light-Emitting Diodes: Effect of Composition, Morphology, and Transport Layers

Organic-inorganic metal halide perovskites are emerging as novel materials for light-emitting applications due to their high color purity, band gap tunability, straightforward synthesis, and inexpensive precursors. In this work, we improve the performance of three-dimensional perovskite light-emitting diodes (PeLEDs) by tuning the emissive layer composition and thickness and by using small-molecule transport layers. Additionally, we correlate PeLED efficiencies to the perovskite structure and morphology. The results show that the PeLEDs containing perovskites with an excess of methylammonium bromide (MABr) to lead bromide (PbBr₂) in a 2:1 ratio and a layer thickness of 80 nm have the highest performance. The optimized device exhibits a peak luminance of 17 600 cd/m² and an external quantum efficiency of 3.9%. Structural and morphological studies reveal a reduction in crystallite size and surface roughness with decreasing perovskite layer thickness and increasing ratio of MABr to PbBr₂. Balanced charge injection, spatial charge confinement, and reduction in nonradiative sites can explain the enhanced performance by virtue of favorable morphology and transport layer choice.

Giant Light-Emission Enhancement in Lead Halide Perovskites by Surface Oxygen Passivation

Surface condition plays an important role in the optical performance of semiconductor materials. As new types of semiconductors, the emerging metal-halide perovskites are promising for next-generation optoelectronic devices. We discover significantly improved light-emission efficiencies in lead halide perovskites due to surface oxygen passivation. The enhancement manifests close to 3 orders of magnitude as the perovskite dimensions decrease to the nanoscale, improving external quantum efficiencies from <0.02% to over 12%. Along with about a 4-fold increase in spontaneous carrier recombination lifetimes, we show that oxygen exposure enhances light emission by reducing the nonradiative recombination channel. Supported by X-ray surface characterization and theoretical modeling, we propose that excess lead atoms on the perovskite surface create deep-level trap states that can be passivated by oxygen adsorption.

Photo-induced dual passivation via Usanovich acid-base on surface defects of methylammonium lead triiodide perovskite

Post-fabrication defect passivation of organometal halide perovskites has become an efficient way to improve their photophysical properties, but the underlying mechanisms are still in debate. In this work, we used p-benzoquinone (p-BQ) to generate surface defects on methylammonium lead triiodide perovskite (MAPbI3), and found that a Usanovich acid-base (O2, acetone or acetonitrile) treatment can effectively passivate those defects and lead to photoluminescence (PL) enhancement. The passivation effect arose from partial neutralization of defect charges via electron transfer between passivation reagents and relevant defects. O2 accepted photo-generated electrons, formed negatively charged oxygen species and attached to the I vacancy site to reduce its PL quenching efficiency by neutralising the defects positive charge. Likewise, acetone accepted photo-generated holes, formed positively charged species and partially neutralised the defects negative charge. The reduced trapping ability of defects caused PL enhancement. In addition, the observed photo-catalysed oxidation of acetone by O2 on the crystal surface supported the single electron transfer mechanism, and showed the potential of MAPbI3 as a photo-catalyst.

Surface Effect on 2D Hybrid Perovskite Crystals: Perovskites Using an Ethanolamine Organic Layer as an Example

Despite the remarkable progress of optoelectronic devices based on hybrid perovskites, there are significant drawbacks, which have largely hindered their development as an alternative of silicon. For instance, hybrid perovskites are well-known to suffer from moisture instability which leads to surface degradation. Nonetheless, the dependence of the surface effect on the moisture stability and optoelectronic properties of hybrid perovskites has not been fully investigated. In this work, the influence of the surface effect of 2D layered perovskites before and after mechanical exfoliation, representing rough and smooth surfaces of perovskite crystals, are studied. It is found that the smooth 2D perovskite is less sensitive to ambient moisture and exhibits a considerably low dark current, which outperforms the rough perovskites by 23.6 times in terms of photodetectivity. The superior moisture stability of the smooth perovskites over the rough perovskites is demonstrated. Additionally, ethanolamine is employed as an organic linker of the 2D layered perovskite, which further improves the moisture stability. This work reveals the strong dependence of the surface conditions of 2D hybrid perovskite crystals on their moisture stability and optoelectronic properties, which are of utmost importance to the design of practical optoelectronic devices based on hybrid perovskite crystals.

From Exceptional Properties to Stability Challenges of Perovskite Solar Cells

The discovery and development of organic-inorganic halide perovskites with exceptional properties has become an active research area in the field of photovoltaics. Perovskite solar cells (PSCs) have attracted much attention in recent years due to various attractive advantages, such as simple solution processing, low manufacturing cost, and high performances with power conversion efficiencies now reaching certified values close to 23% within a very short time frame of five years. Despite this rapid progress, the inferior device stability remains a great challenge. This review focuses on the factors limiting the stability of PSCs, such as humidity, heat, and irradiation, summarizing recent strategies to overcome stability and fabrication obstacles in order to open new perspectives to achieve highly durable perovskite devices toward future industrialization.