Defect Passivation in Lead‐Halide Perovskite Nanocrystals and Thin Films: Toward Efficient LEDs and Solar Cells

CsPbI3 Perovskite Quantum Dot-Based WORM Memory Device with Intrinsic Ternary States

The migration of mobile ionic halide vacancies is usually considered detrimental to the performance and stability of perovskite optoelectronic devices. Taking advantage of this intrinsic feature, we fabricated a CsPbI3 perovskite quantum dot (PQD)-based write-once-read-many-times (WORM) memory device with a simple sandwich structure that demonstrates intrinsic ternary states with a high ON/OFF ratio of 103:102:1 and a long retention time of 104 s. Through electrochemical impedance spectroscopy, we proved that the resistive switching is achieved by the migration of mobile iodine vacancies (VIs) under an electric field to form conductive filaments (CFs). Using in situ conductive atomic force microscopy, we further revealed that the multilevel property arises from the different activation energies for VIs to migrate at grain boundaries and grain interiors, resulting in two distinct pathways for CFs to grow. Our work highlights the potential of CsPbI3 PQD-based WORM devices, showcasing intrinsic multilevel properties achieved in a simple device structure by rationally controlling the drift of ionic defects.

Evaluating Pb-based and Pb-free Halide Perovskites for Solar-Cell Applications: A Simulation Study

Metal halide Pb-based and Pb-free perovskite crystal structures are an essential class of optoelectronic materials due to their significant optoelectronic properties, optical absorption and tuneable emission spectrum properties. However, the most efficient optoelectronic devices were based on the Pb as a monovalent cation, but its toxicity is a significant hurdle for commercial device applications. Thus, replacing the toxic Pb with Pb-free alternatives (such as tin (Sn)) for diverse photovoltaic and optoelectronic applications is essential. Moreover, replacing the volatile methylammonium (MA) with cesium (Cs) leads to the development of an efficient perovskite absorber layer with improved optical & thermal stability and stabilized photoconversion efficiency. This paper discusses the correlation between the experimental and theoretical work for the Pb-based and Pb-free perovskites synthesised using the hot-injection method at different temperatures. Here, simulation is also carried out using the help of SCAPS-1D software to study the effect of various parameters of CsSnI3 and CsPbI3 layers on solar cell performance. This experimental and theoretical comparative study of the Hot-injection method synthesised CsPbI3 and CsSnI3 perovskites is rarely investigated for optoelectronic applications.

Defects of Metal Halide Perovskites in Photocatalytic Energy Conversion: Friend or Foe?

Photocatalytic solar-to-fuel conversion over metal halide perovskites (MHPs) has recently attracted much attention, while the roles of defects in MHPs are still under debate. Specifically, the mainstream viewpoint is that the defects are detrimental to photocatalytic performance, while some recent studies show that certain types of defects contribute to photoactivity enhancement. However, a systematic summary of why it is contradictory and how the defects in MHPs affect photocatalytic performance is still lacking. In this review, the innovative roles of defects in MHP photocatalysts are highlighted. First, the origins of defects in MHPs are elaborated, followed by clarifying certain benefits of defects in photocatalysts including optical absorption, charge dynamics, and surface reaction. Afterward, the recent progress on defect-related MHP photocatalysis, i.e., CO2 reduction, H2 generation, pollutant degradation, and organic synthesis is systematically discussed and critically appraised, putting emphasis on their beneficial effects. With defects offering peculiar sets of merits and demerits, the personal opinion on the ongoing challenges is concluded and outlining potentially promising opportunities for engineering defects on MHP photocatalysts. This critical review is anticipated to offer a better understanding of the MHP defects and spur some inspiration for designing efficient MHP photocatalysts.

Pattern-Matched Polymer Ligands Toward Near-Perfect Synergistic Passivation for High-Performance and Stable Br/Cl Mixed Perovskite Light-Emitting Diodes

Mixed Br/Cl perovskite nanocrystals (PeNCs) exhibit bright pure-blue emission benefiting for fulfilling the Rec. 2100 standard. However, phase segregation remains a significant challenge that severely affects the stability and emission spectrum of perovskite light-emitting diodes (PeLEDs). Here, we demonstrate the optimization of the spacing between polydentate functional groups of polymer ligands to match the surface pattern of CsPbBr1.8Cl1.2 PeNCs, resulting in effective synergistic passivation effect and significant improvements in PeLED performances. The block and alternating copolymers with different inter-functional group spacing are facilely synthesized as ligands for PeNCs. Surprisingly, block copolymers with a higher functional group density do not match PeNCs, while alternating copolymers enable efficient PeNCs with the high photoluminescence intensity, low non-radiative recombination rate and high exciton binding energy. Density functional theory calculations clearly confirm the almost perfect match between alternating copolymers and PeNCs. Finally, pure-blue PeLEDs are achieved with the emission at 467 nm and Commission Internationale de l'Eclairage (CIE) coordinates of (0.131, 0.071), high external quantum efficiency (9.1 %) and record spectral and operational stabilities (~80 mins) in mixed-halide PeLEDs. Overall, this study contributes to designing the polymer ligands and promoting the development of high-performance and stable pure-color PeLEDs towards display applications.

Data-Driven Controlled Synthesis of Oriented Quasi-Spherical CsPbBr3 Perovskite Materials

Controlled synthesis of lead-halide perovskite crystals is challenging yet attractive because of the pivotal role played by the crystal structure and growth conditions in regulating their properties. This study introduces data-driven strategies for the controlled synthesis of oriented quasi-spherical CsPbBr3, alongside an investigation into the synthesis mechanism. High-throughput rapid characterization of absorption spectra and color under ultraviolet illumination was conducted using 23 possible ligands for the synthesis of CsPbBr3 crystals. The links between the absorption spectra slope (difference in the absorbance at 400 nm and 450 nm divided by a wavelength interval of 50 nm) and crystal size were determined through statistical analysis of more than 100 related publications. Big data analysis and machine learning were employed to investigate a total of 688 absorption spectra and 652 color values, revealing correlations between synthesis parameters and properties. Ex situ characterization confirmed successful synthesis of oriented quasi-spherical CsPbBr3 perovskites using polyvinylpyrrolidone and Acacia. Density functional theory calculations highlighted strong adsorption of Acacia on the (110) facet of CsPbBr3. Optical properties of the oriented quasi-spherical perovskites prepared with these data-driven strategies were significantly improved. This study demonstrates that data-driven controlled synthesis facilitates morphology-controlled perovskites with excellent optical properties.

Flexible and Stretchable Light-Emitting Diodes and Photodetectors for Human-Centric Optoelectronics

Optoelectronic devices with unconventional form factors, such as flexible and stretchable light-emitting or photoresponsive devices, are core elements for the next-generation human-centric optoelectronics. For instance, these deformable devices can be utilized as closely fitted wearable sensors to acquire precise biosignals that are subsequently uploaded to the cloud for immediate examination and diagnosis, and also can be used for vision systems for human-interactive robotics. Their inception was propelled by breakthroughs in novel optoelectronic material technologies and device blueprinting methodologies, endowing flexibility and mechanical resilience to conventional rigid optoelectronic devices. This paper reviews the advancements in such soft optoelectronic device technologies, honing in on various materials, manufacturing techniques, and device design strategies. We will first highlight the general approaches for flexible and stretchable device fabrication, including the appropriate material selection for the substrate, electrodes, and insulation layers. We will then focus on the materials for flexible and stretchable light-emitting diodes, their device integration strategies, and representative application examples. Next, we will move on to the materials for flexible and stretchable photodetectors, highlighting the state-of-the-art materials and device fabrication methods, followed by their representative application examples. At the end, a brief summary will be given, and the potential challenges for further development of functional devices will be discussed as a conclusion.

Microstrain and Crystal Orientation Variation within Naked Triple-Cation Mixed Halide Perovskites under Heat, UV, and Visible Light Exposure

The instability of perovskite absorbers under various environmental stressors is the most significant obstacle to widespread commercialization of perovskite solar cells. Herein, we study the evolution of crystal structure and microstrain present in naked triple-cation mixed CsMAFA-based perovskite films under heat, UV, and visible light (1 Sun) conditions by grazing-incidence wide-angle X-ray scattering (GIWAXS). We find that the microstrain is gradient distributed along the surface normal of the films, decreasing from the upper surface to regions deeper within the film. Moreover, heat, UV, and visible light treatments do not interfere with the crystalline orientations within annealed polycrystalline films. However, when subjected to heat, the naked perovskite films exhibit a rapid component decomposition, induced by phase separation and ion migration. Conversely, under exposure to UV and 1 Sun light soaking, the naked perovskite films undergo a self-optimization structure evolution during degradation and develop into smoother films with reduced surface potential fluctuations.

Mn(II)-Activated Zero-Dimensional Zinc(II)-Based Metal Halide Hybrids with Near-Unity Photoluminescence Quantum Yield

As derivatives of metal halide perovskite materials, low-dimensional metal halide materials have become important materials that have attracted much attention in recent years. As one branch, zinc-based metal halides have the potential for practical applications due to their lead-free, low-toxicity and high-stability characteristics. However, pure zinc-based metal halide materials are still limited by their poor optical properties and cannot achieve large-scale practical applications. Therefore, in this work, we report an organic-inorganic hybrid zero-dimensional zinc bromide, (TDMP)ZnBr4, using transition metal Mn2+ ions as dopants and incorporating them into the (TDMP)ZnBr4 lattice. The original non-emissive (TDMP)ZnBr4 exhibits bright green emission under the excitation of external UV light after the introduction of Mn2+ ions with a PL peak position located at 538 nm and a PLQY of up to 91.2%. Through the characterization of relevant photophysical properties and the results of theoretical calculations, we confirm that this green emission in Mn2+:(TDMP)ZnBr4 originates from the 4T1 → 6A1 optical transition process of Mn2+ ions in the lattice structure, and the near-unity PLQY benefits from highly localized electrons generated by the unique zero-dimensional structure of the host material (TDMP)ZnBr4. This work provides theoretical guidance and reference for expanding the family of zinc-based metal halide materials and improving and controlling their optical properties through ion doping.

Polymer-enhanced perovskite oxide-based photocatalysts: a review

Oxide perovskites (OPs) have emerged as promising photocatalysts for numerous applications, such as energy conversion, renewable fuels, and environmental remediation. Although OPs are gaining traction, their efficacies are still hindered by low charge carrier mobility and poor stability. This study investigated the function of polymers actively participating in OP structures to improve the overall characteristics. An overview of the polymer-enhanced perovskite oxide photocatalyst (PEPOP) field was effectively reviewed. These PEPOPs were demonstrated in photovoltaics, pollutant degradation, and gas conversion and reduction. Nonetheless, additional research is needed to explore the potential of PEPOPs to establish their efficacy in photocatalytic applications. The technological improvements of PEPOPs were hindered by significant challenges related to stability and sensitivity. The urgency of this review was apparent due to the fast-paced nature of research in the field of photocatalysis. Recent breakthroughs and emerging applications highlight the need for a comprehensive overview of PEPOPs and their enhanced catalytic capabilities. Consequently, a broad outlook was provided for the current state of PEPOP-related studies, highlighting the potential of these materials for future applications.

Designing complex Pb3SBrxI4-x chalcohalides: tunable emission semiconductors through halide-mixing

Chalcohalides are desirable semiconducting materials due to their enhanced light-absorbing efficiency and stability compared to lead halide perovskites. However, unlike perovskites, tuning the optical properties of chalcohalides by mixing different halide ions into their structure remains to be explored. Here, we present an effective strategy for halide-alloying Pb3SBrxI4-x (1 ≤ x ≤ 3) using a solution-phase approach and study the effect of halide-mixing on structural and optical properties. We employ a combination of X-ray diffraction, electron microscopy, and solid-state NMR spectroscopy to probe the chemical structure of the chalcohalides and determine mixed-halide incorporation. The absorption onsets of the chalcohalides blue-shift to higher energies as bromide replaces iodide within the structure. The photoluminescence maxima of these materials mimics this trend at both the ensemble and single particle fluorescence levels, as observed by solution-phase and single particle fluorescence microscopy, respectively. These materials exhibit superior stability against moisture compared to traditional lead halide perovskites, and IR spectroscopy reveals that the chalcohalide surfaces are terminated by both amine and carboxylate ligands. Electronic structure calculations support the experimental band gap widening and volume reduction with increased bromide incorporation, and provide useful insight into the likely atomic coloring patterns of the different mixed-halide compositions. Ultimately, this study expands the range of tunability that is achievable with chalcohalides, which we anticipate will improve the suitability of these semiconducting materials for light absorbing and emission applications.

Bioinspired, vertically stacked, and perovskite nanocrystal-enhanced CMOS imaging sensors for resolving UV spectral signatures

Imaging and identifying target signatures and biomedical markers in the ultraviolet (UV) spectrum is broadly important to medical imaging, military target tracking, remote sensing, and industrial automation. However, current silicon-based imaging sensors are fundamentally limited because of the rapid absorption and attenuation of UV light, hindering their ability to resolve UV spectral signatures. Here, we present a bioinspired imaging sensor capable of wavelength-resolved imaging in the UV range. Inspired by the UV-sensitive visual system of the Papilio xuthus butterfly, the sensor monolithically combines vertically stacked photodiodes and perovskite nanocrystals. This imaging design combines two complementary UV detection mechanisms: The nanocrystal layer converts a portion of UV signals into visible fluorescence, detected by the photodiode array, while the remaining UV light is detected by the top photodiode. Our label-free UV fluorescence imaging data from aromatic amino acids and cancer/normal cells enables real-time differentiation of these biomedical materials with 99% confidence.

Retina-inspired narrowband perovskite sensor array for panchromatic imaging

The retina is the essential part of the human visual system that receives light, converts it to neural signal, and transmits to brain for visual recognition. The red, green, and blue (R/G/B) cone retina cells are natural narrowband photodetectors (PDs) sensitive to R/G/B lights. Connecting with these cone cells, a multilayer neuro-network in the retina provides neuromorphic preprocessing before transmitting to brain. Inspired by this sophistication, we develop the narrowband (NB) imaging sensor combining R/G/B perovskite NB sensor array (mimicking the R/G/B photoreceptors) with a neuromorphic algorithm (mimicking the intermediate neural network) for high-fidelity panchromatic imaging. Compared to commercial sensors, we use perovskite "intrinsic" NB PD to exempt the complex optical filter array. In addition, we use an asymmetric device configuration to collect photocurrent without external bias, enabling a power-free photodetection feature. These results display a promising design for efficient and intelligent panchromatic imaging.

Nanoimprinted 2D-Chiral Perovskite Nanocrystal Metasurfaces for Circularly Polarized Photoluminescence

The versatile hybrid perovskite nanocrystals (NCs) are one of the most promising materials for optoelectronics by virtue of their tunable bandgaps and high photoluminescence (PL) quantum yields. However, their inherent crystalline chemical structure limits the chiroptical properties achievable with the material. The production of chiral perovskites has become an active field of research for its promising applications in optics, chemistry, or biology. Typically, chiral halide perovskites are obtained by the incorporation of different chiral moieties in the material. Unfortunately, these chemically modified perovskites have demonstrated moderate values of chiral PL so far. Here, a general and scalable approach is introduced to produce chiral PL from arbitrary nanoemitters assembled into 2D-chiral metasurfaces. The fabrication via nanoimprinting lithography employs elastomeric molds engraved with chiral motifs covering millimeter areas that are used to pattern two types of unmodified colloidal perovskite NC inks: green-emissive CsPbBr₃ and red-emissive CsPbBr₁ I₂ . The perovskite 2D-metasurfaces exhibit remarkable PL dissymmetry factors (g_lum ) of 0.16 that can be further improved up to g_lum of 0.3 by adding a high-refractive-index coating on the metasurfaces. This scalable approach to produce chiral photoluminescent thin films paves the way for the seamless production of bright chiral light sources for upcoming optoelectronic applications.

Rapid Data-Efficient Optimization of Perovskite Nanocrystal Syntheses through Machine Learning Algorithm Fusion

With the demand for renewable energy and efficient devices rapidly increasing, a need arises to find and optimize novel (nano)materials. With sheer limitless possibilities for material combinations and synthetic procedures, obtaining novel, highly functional materials has been a tedious trial and error process. Recently, machine learning has emerged as a powerful tool to help optimize syntheses; however, most approaches require a substantial amount of input data, limiting their pertinence. Here, three well-known machine-learning models are merged with Bayesian optimization into one to optimize the synthesis of CsPbBr₃ nanoplatelets with limited data demand. The algorithm can accurately predict the photoluminescence emission maxima of nanoplatelet dispersions using only the three precursor ratios as input parameters. This allows us to fabricate previously unobtainable seven and eight monolayer-thick nanoplatelets. Moreover, the algorithm dramatically improves the homogeneity of 2-6-monolayer-thick nanoplatelet dispersions, as evidenced by narrower and more symmetric photoluminescence spectra. Decisively, only 200 total syntheses are required to achieve this vast improvement, highlighting how rapidly material properties can be optimized. The algorithm is highly versatile and can incorporate additional synthetic parameters. Accordingly, it is readily applicable to other less-explored nanocrystal syntheses and can help rapidly identify and improve exciting compositions' quality.

Deciphering the Role of Water in Promoting the Optoelectronic Performance of Surface-Engineered Lead Halide Perovskite Nanocrystals

Lead halide perovskites are promising candidates for high-performance light-emitting diodes (LEDs); however, their applicability is limited by their structural instability toward moisture. Although a deliberate addition of water to the precursor solution has recently been shown to improve the crystallinity and optical properties of perovskites, the corresponding thin films still do not exhibit a near-unity quantum yield. Herein, we report that the direct addition of a minute amount of water to post-treated formamidinium lead bromide (FAPbBr₃) nanocrystals (NCs) substantially enhances the stability while achieving a 95% photoluminescence quantum yield in a NC thin film. We unveil the mechanism of how moisture assists in the formation of an additional NH₄Br component. Alongside, we demonstrate the crucial role of moisture in assisting localized etching of the perovskite crystal, facilitating the partial incorporation of NH₄⁺, which is key for improved performance under ambient conditions. Finally, as a proof-of-concept, the application of post-treated and water-treated perovskites is tested in LEDs, with the latter exhibiting a superior performance, offering opportunities toward commercial application in moisture-stable optoelectronics.

A Multifunctional Polymer as an Interfacial Layer for Efficient and Stable Perovskite Solar Cells

Metal-cation defects and halogen-anion defects in perovskite films are critical to the efficiency and stability of perovskite solar cells (PSCs). In this work, a random polymer, poly(methyl methacrylate-co-acrylamide) (PMMA-AM), was synthesized to serve as an interfacial passivation layer for synergistically passivating the under-coordinated Pb²⁺ and anchor the I^- of the [PbI₆ ]^4- octahedron. Additionally, the interfacial PMMA-AM passivation layer cannot be destroyed during the hole transport layer deposition because of its low solubility in chlorobenzene. This passivation leads to an enhancement in the open-circuit voltage from 1.12 to 1.22 V and improved stability in solar cell devices, with the device maintaining 95 % of the initial power conversion efficiency (PCE) over 1000 h of maximum power point tracking. Additionally, a large-area solar cell module was fabricated using this approach, achieving a PCE of 20.64 %.

Layer-by-layer assembly of CsPbX3 nanocrystals into large-scale homostructures

Advances in surface chemistry of CsPbX₃ (where X = Cl, Br or I) nanocrystals (NCs) enabled the replacement of native chain ligands in solution. However, there are few reports on ligand exchange carried out on CsPbX₃ NC thin films. Solid-state ligand exchange can improve the photoluminescence quantum yield (PLQY) of the film and promote a change in solubility of the solid surface, thus enabling multiple depositions of subsequent nanocrystal layers. Fine control of nanocrystal film thickness is of importance for light-emitting diodes (LEDs), solar cells and lasers alike. The thickness of the emissive material film is crucial to assure the copious recombination of charges injected into a LED, resulting in bright electroluminescence. Similarly, solar cell performance is determined by the amount of absorbed light, and hence the light absorber content in the device. In this study, we demonstrate a layer-by-layer (LbL) assembly method that results in high quality films, whose thicknesses can be finely controlled. In the solid state, we replaced oleic acid and oleylamine ligands with didodecyldimethylammonium bromide or ammonium thiocyanate that enhance the PLQY of the film. The exchange is carried out through a spin-coating technique, using solvents with strategic polarity to avoid NC dissolution or damage. Exploiting this technique, the deposition of various layers results in considerable thickening of films as proven by atomic force microscope measurements. The ease of handling of our combined process (i.e. ligand exchange and layer-by-layer deposition) enables thickness control over CsPbX₃ NC films with applicability to other perovskite nanomaterials paving the way for a large variety of layer permutations.

Rationalizing the Effect of Polymer-Controlled Growth of Perovskite Single Crystals on Optoelectronic Properties

To improve and modulate the optoelectronic properties of single-crystal (SC) metal halide perovskites (MHPs), significant progress has been achieved. Polymer-assisted techniques are a great approach to control the growth rate of SCs effectively. However, the resultant optoelectrical properties induced by polymers are ambiguous and need to be taken into the consideration. In this study, we have synthesized methylammonium lead triiodide (MAPbI₃) SCs using polyethylene glycol (PEG) and polystyrene (PS) polymers where PEG contains oxygen functionalities and PS does not. We studied the electrical properties of these SCs under dark and illumination conditions. It was observed that PEG-assisted SCs showed few defects with lower photocurrent as compared to the PS-assisted ones because of defect-mediated conductivity. The results are further verified by transient current response, responsivity, and capacitance-frequency measurements. The present study sheds light on the polymer selection for the growth of MHP SCs and their optoelectronic properties.

Colloidal FAPbBr3 perovskite nanocrystals for light emission: what's going on?

Semiconducting nanomaterials have been widely explored in diverse optoelectronic applications. Colloidal lead halide perovskite nanocrystals (NCs) have recently been an excellent addition to the field of nanomaterials, promising an enticing building block in the field of light emission. In addition to the notable optoelectronic properties of perovskites, the colloidal NCs exhibit unique size-dependent optical properties due to the quantum size effect, which makes them highly attractive for light-emitting diodes (LEDs). In the past few years, perovskite-based LEDs (PeLEDs) have demonstrated a meteoritic rise in their external quantum efficiency (EQE) values, reaching over 20% so far. Among various halide perovskite compositions, FAPbBr3 and its variants remain one of the most interesting and sought-after compounds for green light emission. This review focuses on recent progress in the design and synthesis protocols of colloidal FAPbBr3 NCs and the emerging concepts in tailoring their surface chemistry. The structural and physicochemical features of lead halide perovskites along with a comprehensive discussion on their defect-tolerant properties are briefly outlined. Later, the prevalent synthesis, ligand, and compositional engineering strategies to boost the stability and photoluminescence quantum yield (PLQY) of FAPbBr3 NCs are extensively discussed. Finally, the fundamental concepts and recent progress on FAPbBr3-based LEDs, followed by a discussion of the challenges and prospects that are on the table for this enticing class of perovskites, are reviewed.

One-Step Thermal Gradient- and Antisolvent-Free Crystallization of All-Inorganic Perovskites for Highly Efficient and Thermally Stable Solar Cells

All-inorganic perovskites have emerged as promising photovoltaic materials due to their superior thermal stability compared to their heat-sensitive hybrid organic-inorganic counterparts. In particular, CsPbI2 Br shows the highest potential for developing thermally-stable perovskite solar cells (PSCs) among all-inorganic compositions. However, controlling the crystallinity and morphology of all-inorganic compositions is a significant challenge. Here, a simple, thermal gradient- and antisolvent-free method is reported to control the crystallization of CsPbI2 Br films. Optical in situ characterization is used to investigate the dynamic film formation during spin-coating and annealing to understand and optimize the evolving film properties. This leads to high-quality perovskite films with micrometer-scale grain sizes with a noteworthy performance of 17% (≈16% stabilized), fill factor (FF) of 80.5%, and open-circuit voltage (VOC ) of 1.27 V. Moreover, excellent phase and thermal stability are demonstrated even after extreme thermal stressing at 300 °C.

Quantum Efficiency Enhancement of Lead-Halide Perovskite Nanocrystal LEDs by Organic Lithium Salt Treatment

Surface-defect passivation is key to achieving a high photoluminescence quantum yield in lead halide perovskite nanocrystals. However, in perovskite light-emitting diodes, these surface ligands also have to enable balanced charge injection into the nanocrystals to yield high efficiency and operational lifetime. In this respect, alkaline halides have been reported to passivate surface trap states and increase the overall stability of perovskite light emitters. On the one side, the incorporation of alkaline ions into the lead halide perovskite crystal structure is considered to counterbalance cation vacancies, whereas on the other side, the excess halides are believed to stabilize the colloids. Here, we report an organic lithium salt, viz. LiTFSI, as a halide-free surface passivation on perovskite nanocrystals. We show that treatment with LiTFSI has multiple beneficial effects on lead halide perovskite nanocrystals and LEDs derived from them. We obtain a higher photoluminescence quantum yield and a longer exciton lifetime and a radiation pattern that is more favorable for light outcoupling. The ligand-induced dipoles on the nanocrystal surface shift their energy levels toward a lower hole-injection barrier. Overall, these effects add up to a 4- to 7-fold boost of the external quantum efficiency in proof-of-concept LED structures, depending on the color of the used lead halide perovskite nanocrystal emitters.

Multifunctional and Transformative Metaphotonics with Emerging Materials

Future technologies underpinning multifunctional physical and chemical systems and compact biological sensors will rely on densely packed transformative and tunable circuitry employing nanophotonics. For many years, plasmonics was considered as the only available platform for subwavelength optics, but the recently emerged field of resonant metaphotonics may provide a versatile practical platform for nanoscale science by employing resonances in high-index dielectric nanoparticles and metasurfaces. Here, we discuss the recently emerged field of metaphotonics and describe its connection to material science and chemistry. For tunabilty, metaphotonics employs a variety of the recently highlighted materials such as polymers, perovskites, transition metal dichalcogenides, and phase change materials. This allows to achieve diverse functionalities of metasystems and metasurfaces for efficient spatial and temporal control of light by employing multipolar resonances and the physics of bound states in the continuum. We anticipate expanding applications of these concepts in nanolasers, tunable metadevices, metachemistry, as well as a design of a new generation of chemical and biological ultracompact sensing devices.

Sb3+-Er3+-Codoped Cs2NaInCl6 for Emitting Blue and Short-Wave Infrared Radiation

Cs 2 NaInCl 6 double perovskite is stable, environmentally benign, and can be prepared easily. But it has high band gap (5.1 eV), and therefore, does not show optical and optoelectronic properties in the visible and short-wave infrared (SWIR) region. Here we introduce such functionalities in Cs 2 NaInCl 6 by codoping Sb 3+ ( s -electron doping) and Er 3+ ( f -electron doping). Sb 3+ doping introduces optically allowed 5 s 2 → 5 s 1 5 p 1 electronic absorption at the sub band gap level, which then emit blue light with ~93% photoluminescence quantum yield. On the other hand, f-f electronic absorption of Er 3+ is parity forbidden. Codoping Sb 3+ -Er 3+ , leads to transfer of excitation energy from Sb 3+ to Er 3+ , yielding SWIR emission at 1540 nm that falls in the optical fiber communication region. Temperature (6 to 300 K) dependent photoluminescence measurements elucidate the excitation mechanism in the codoped Cs 2 NaInCl 6 . Further, we have fabricated phosphor converted light emitting diode (pc-LED) by coating the codoped sample on commercial ultraviolet LED chips. The pc-LED emit stable blue and SWIR radiation over prolonged operation (over 84 hours) at 5.1 V.

Thermodynamics of nanocrystal–ligand binding through isothermal titration calorimetry

Manipulations of nanocrystal (NC) surfaces have propelled the applications of colloidal NCs across various fields such as bioimaging, catalysis, electronics, and sensing applications.

Tin-based halide perovskite materials: properties and applications

Organic–inorganic hybrid halide perovskite materials have attracted considerable research interest, especially for photovoltaics. In addition, their scope has been extended towards light-emitting devices, photodetectors, or detectors. However, the toxicity of lead (Pb) element in perovskite compositions limits their applications. Therefore, a tremendous research effort on replacing is underway. More specifically, tin-based perovskites have shown the highest potential for this purpose. However, many challenges remain before these materials reach the goals of stability, safety, and eventually commercial application. This perspective considers many aspects and the critical development possibilities of tin-based perovskites, including drawbacks and challenges based on their physical properties. Additionally, it provides insights for future device applications that go beyond solar cells. Finally, the existing challenges and opportunities in tin-based perovskites are discussed.

This perspective presents the current status and prospects of tin-perovskites and the relevant optoelectronic device applications.

Perovskite Nanocrystals with Tunable Fluorescent Intensity during Anion Exchange for Dynamic Optical Encryption

Perovskite nanocrystals (PNCs) have demonstrated their potential use in many applications such as optical encryption because of their excellent optical properties. However, the optical encryption using PNCs is mainly based on the formation of static patterns with luminescence on/off switching. In this work, we demonstrated that the capping ligands play an important role in tuning the luminescence intensity of the PNCs during ion exchange. The surfactant, oleylamine (OAm), is essential in shifting the luminescence color of the PNCs from green to yellow during the ion exchange. In the absence of OAm, the luminescence in the green and yellow regions is quenched during the ion exchange and the luminescence is recovered in the red region by adding trioctylphosphine (TOP) into the ion-exchange solution. On the basis of these findings, we proposed a dynamic optical encryption strategy using PNCs with different capping ligands by tuning the luminescence intensity. The encoded message is hidden in the green pattern at the beginning, shown during the ion exchange, and erased when the pattern is completely converted from a green color to a red color after the ion exchange ends. This dynamic encryption strategy enhances the security level and is compatible with human eye-perceivable patterns and binary coding algorithms.

Radical Form of PbI2: A New Defect Passivator for Efficient Perovskite Solar Cells

PbI₂ is a commonly used passivator for defect passivation in perovskite solar cells (PSCs). However, the poor conductivity nature of PbI₂ may limit the further improvement of device performance. Here, we report a radical form of PbI₂ with high conductivity to passivate defects for efficient PSCs through a combination of N,N,N',N'-tetramethylbenzidine (TMB). When PbI₂ is combined with TMB, 4 orders of magnitude higher conductivity will be achieved owing to the formation of a TMB-PbI₂ radical. As a result, the device performance is impressively increased from 20.48 to 22.63%. In addition, the device stability is also greatly improved and 95% of the initial efficiency is retained after aging at 85 °C for 600 h.

Dimensionality Control of Inorganic and Hybrid Perovskite Nanocrystals by Reaction Temperature: From No-Confinement to 3D and 1D Quantum Confinement

This work focuses on the systematic investigation of the shape, size, and composition‐controlled synthesis of perovskite nanocrystals (NCs) under inert gas‐free conditions and using pre‐synthesized precursor stock solutions. In the case of CsPbBr₃ NCs, we find that the lowering of reaction temperature from ∼175 to 100 °C initially leads to a change of morphology from bulk‐like 3D nanocubes to 0D nanocubes with 3D‐quantum confinement, while at temperatures below 100 °C the reaction yields 2D nanoplatelets (NPls) with 1D‐quantum confinement. However, to our surprise, at higher temperatures (∼215 °C), the reaction yields CsPbBr₃ hexapod NCs, which have been rarely reported. The synthesis is scalable, and their halide composition is tunable by simply using different combinations of precursor solutions. The versatility of the synthesis is demonstrated by applying it to relatively less explored shape‐controlled synthesis of FAPbBr₃ NCs. Despite the synthesis carried out in the air, both the inorganic and hybrid perovskite NCs exhibit nearly‐narrow emission without applying any size‐selective separation, and it is precisely tunable by controlling the reaction temperature.