Organic-inorganic hybrid perovskite quantum dots with high PLQY and enhanced carrier mobility through crystallinity control by solvent engineering and solid-state ligand exchange

How to improve the structural stabilities of halide perovskite quantum dots: review of various strategies to enhance the structural stabilities of halide perovskite quantum dots

Halide perovskites have emerged as promising materials for various optoelectronic devices because of their excellent optical and electrical properties. In particular, halide perovskite quantum dots (PQDs) have garnered considerable attention as emissive materials for light-emitting diodes (LEDs) because of their higher color purities and photoluminescence quantum yields compared to conventional inorganic quantum dots (CdSe, ZnSe, ZnS, etc.). However, PQDs exhibit poor structural stabilities in response to external stimuli (moisture, heat, etc.) owing to their inherent ionic nature. This review presents recent research trends and insights into improving the structural stabilities of PQDs. In addition, the origins of the poor structural stabilities of PQDs and various methods to overcome this drawback are discussed. The structural degradation of PQDs is mainly caused by two mechanisms: (1) defect formation on the surface of the PQDs by ligand dissociation (i.e., detachment of weakly bound ligands from the surface of PQDs), and (2) vacancy formation by halide migration in the lattices of the PQDs due to the low migration energy of halide ions. The structural stabilities of PQDs can be improved through four methods: (1) ligand modification, (2) core-shell structure, (3) crosslinking, and (4) metal doping, all of which are presented in detail herein. This review provides a comprehensive understanding of the structural stabilities and opto-electrical properties of PQDs and is expected to contribute to future research on improving the device performance of perovskite quantum dot LEDs (PeLEDs).

Room-temperature efficient photoluminescence mechanism of indium doping lead-free colloidal MA3Bi2Br9 quantum dots solution

Lead halide perovskite quantum dots (QDs) are promising candidates for future optoelectronic devices due to their excellent photonic and electronic properties. However, poor stability and toxicity problems limit their further development. This work demonstrates the doping tactics to boost the optical properties of lead-free colloidal MA3Bi2Br9 QDs, the indium ion (In3+) doping presented herein is found to be effective in improving the photoluminescence (PL) properties of MA3Bi2Br9 (CH3NH2 = MA) QDs without alerting their favorable electronic structure. It has been elucidated by microscopy and diffraction results that the In3+ doping optimizes the QDs solution octahedron structure, and the PL red-shifted phenomenon coincides well with the analogous red-shifted obtained in the ultraviolet/visible (UV-Vis) absorption spectroscopy, which is due to the quantum confinement effect. And the nanosecond transient absorption (ns-TA) spectroscopy elucidates that the enhanced radiative recombination process contributes to enhanced stability and luminescence. The photoluminescence quantum yield (PLQY) of MA3Bi2Br9 QDs is increased by 60.7%. This work offers a valid strategy for improving the quality of the lead-free perovskite QDs.

Amphiphilic Polymer Capped Perovskite Compositing with Nano Zr-MOF for Nanozyme-Involved Biomimetic Cascade Catalysis

CsPbX3 perovskite nanocrystal (NC) is considered as an excellent optical material and is widely applied in optoelectronics. However, its poor water stability impedes its study in enzyme-like activity, and further inhibits its application in biomimetic cascade catalysis. Herein, for the first time, the oxidase-like and ascorbate oxidase-like activities of an amphiphilic polymer capped CsPbX3 are demonstrated, and its catalytic mechanism is further explored. Furthermore, an all-nanozyme cascade system (multifunctional CsPbBr3 @Zr-metal organic framework (Zr-MOF) and Prussian blue as oxidase-like and peroxidase-like nanozyme) is constructed with a portable paper-based device for realizing the dual-mode (ratiometric fluorescence and colorimetric) detection of ascorbic acid in a point-of-care (POC) fashion. This is the first report on the utilization of all-inorganic CsPbX3 perovskite NC in biomimetic cascade catalysis, which opens a new avenue for POC clinical disease diagnosis.

Emission properties of Pd-doped CsPbBr3 perovskite nanocrystal: Infrared emission due to the Pd-doping

Perovskite-type materials have attracted great attention in recent times due to their interesting characteristics, such as their luminescent properties. The good photoluminescence quantum yields as well as the possibility of tuning the emission wavelength has allowed the study of these materials in several applications, such as sensors or LEDs. As sensors, making nanocrystals of these perovskites emitting in the near infrared (NIR) would open the possibility of using these materials in biomedical applications. In the present work, Pd-doped CsPbBr₃ perovskite nanocrystals (NCs) were synthesized and characterized. We show here Pd-doped NCs synthesized emit in NIR, at about 875 nm, using a laser emitting at 785 nm as the excitation source. This result is really new and promising, because it opens the possibility of using these nanocrystals in many applications as sensor in the field of nanobiomedicine in the future.

Structural, vibrational, photoelectrochemical, and optical properties of two-dimensional Ruddlesden-Popper perovskite BA2PbI4 crystals

Two-dimensional organic-inorganic hybrid Ruddlesden-Popper perovskites have attracted a lot of attention due to their unique photochemical properties and enhanced stability towards photoluminescence devices. Compared with three-dimensional materials, two-dimensional perovskites show great potential for photoelectric applications due to their tunable band gap, great excitation binding energy, and large crystal anisotropy. Although the synthesis and optical properties of BA₂PbI₄ crystals have been extensively studied, the role of their microstructure in photoelectric applications, their electronic structure, and their electron-phonon interaction are still poorly understood. In this paper, based on the preparation of BA₂PbI₄ crystals, the electronic structure, phonon dispersion, and vibrational properties of BA₂PbI₄ crystals were revealed in detail with the help of density functional theory. The BA₂PbI₄ stability diagram of formation enthalpy was calculated. The crystal structure of the BA₂PbI₄ crystals was characterized and calculated with the aid of Rietveld refinement. A contactless fixed-point lighting device was designed based on the principle of an electromagnetic induction coil, and the points with different thicknesses of BA₂PbI₄ crystal were tested. It is proved that the excitation peak of the bulk is 564 nm, and the surface luminescence peak is 520 nm. Phonon dispersion curves and the total and partial phonon densities of states have been calculated for the BA₂PbI₄ crystals. The calculated results are in good agreement with the experimental Fourier infrared spectra. Besides the basic characterization of the BA₂PbI₄ crystals, the photoelectrochemical properties of the materials were also studied, which further proves the excellent photoelectric properties of the BA₂PbI₄ crystals and the broad application prospect.

Continuous manufacturing of highly stable lead halide perovskite nanocrystals via a dual-reactor strategy

Lead halide perovskite nanocrystals possess incredible potential as next generation emitters due to their stellar set of optoelectronic properties. Unfortunately, their instability towards many ambient conditions and reliance on batch processing hinder their widespread utilities. Herein, we address both challenges by continuously synthesizing highly stable perovskite nanocrystals via integrating star-like block copolymer nanoreactors into a house-built flow reactor. Perovskite nanocrystals manufactured in this strategy display significantly enhanced colloidal, UV, and thermal stabilities over those synthesized with conventional ligands. Such scaling up of highly stable perovskite nanocrystals represents an important step towards their eventual use in many practical applications in optoelectronic materials and devices.

Supermolecule-assisted synthesis of perovskite nanorods with high PLQY for standard blue emission

We fabricated CsPbBr₃ nanorods with standard blue emission (462 nm) and a high PLQY of ∼90% with the assistance of supermolecules. The β-CD works as a co-ligand and confine the isotropic growth of the nanocrystals to produce anisotropic nanorods. The strong coordination between β-CD and Pb favors the improvement of the PLQY and stability.

Synthesis and Applications of Halide Perovskite Nanocrystals in Optoelectronics

The perovskites used for optoelectronic devices have been more attractive during recent years due to their wide variety of advantages, such as their low cost, high photoluminescence quantum yield (PLQY), high carrier mobility, flexible bandgap tunability, and high light absorption ability. However, optoelectronic applications for traditional inorganic and organic materials present dilemmas due to their hardly tunable bandgap and instability. On the other hand, there are some more important benefits for perovskite nanocrystals, such as a size-dependent bandgap and the availability of anion exchange at room temperature. Therefore, perovskite NC-based applications are currently favored, offering a research direction beyond perovskite, and much research has focused on the stability issue and device performance. Thus, the synthesis and applications of perovskite NCs need to be thoroughly discussed for the future development of solar cells, light-emitting diodes, photodetectors, and laser research.

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.

Low-Temperature Discrimination of Defect States by Exciton Dynamics in Thin-Film MAPbBr3 Perovskite

Exciton dynamics significantly influences the performance of the optoelectronic devices, which is intensively studied in the light-emitting perovskite of CH₃NH₃PbBr₃ (MAPbBr₃). However, most of the existing investigations have focused on the free excitons. In this study, we investigate the emissive recombination from defect states in MAPbBr₃ using temperature- and excitation-dependent photoluminescence measurements. It is revealed that two emission peaks centered at about 550 and 590 nm are presented at temperatures as low as 10 K, instead of one peak at 535 nm for the observation at room temperature. These two peaks are attributed to the emission of bound excitons after self-absorption and bulk defects, respectively. It is found that the distribution of the bound and trapped excitons is strongly influenced by the morphology of the MAPbBr₃ films. These results provide deep insights into the exciton dynamics in MAPbBr₃, facilitating new physics for the design of related optoelectronic materials and devices.

A Study on Improving the Sensitivity of Indirect X-ray Detectors by Adding Hybrid Perovskite Quantum Dots

In this paper, we demonstrate the enhancement in detection sensitivity of an indirect X-ray detector based on poly(3-hexylthiophene) (P3HT) and fullerene derivatives [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) by adding perovskite quantum dots (PeQDs). The weight ratio of P3HT and PC71BM was fixed at 1:1 (20 mg/mL in chlorobenzene), and different amounts of FAPbBr3 PeQDs of (0–3) mg were added to the P3HT:PC71BM active layer solution. The experimental results show that the detector using P3HT:PC71BM:FAPbBr3 PeQDs (1 mg) achieved a sensitivity of 2.10 mA/Gy∙cm2. To further improve the sensitivity, a ligand exchange experiment was performed on the P3HT:PC71BM:FAPbBr3 PeQDs (1 mg) detector. Under the condition of 12 h ligand exchange time, the detector with P3HT:PC71BM:FAPbBr3 PeQDs (1 mg) showed the highest sensitivity of 2.26 mA/Gy∙cm2, which was increased by 28% compared to the pristine detector with a P3HT:PC71BM active layer.

Amphiphilic Polymer Ligand-Assisted Synthesis of Highly Luminescent and Stable Perovskite Nanocrystals for Sweat Fluorescent Sensing

The weak interfacial binding affinities of the inorganic perovskite core with ligands and high density of surface defect states induce the facile detachment of surface ligands from nanocrystals (NCs), resulting in their poor colloidal stability and fluorescence in aqueous. In this work, a powerful ligand engineering strategy was proposed for eliminating the surface defects and aggregation of the NCs. Using an amphiphilic polymer octylamine-modified polyacrylic acid (OPA) as a capping ligand, the as-synthesized CsPbBr₃ NCs retain high photoluminescence intensity and stability by the modified ligand-assisted reprecipitation method. The increase in the fluorescence lifetime and NC size could also be observed, and how the NC particle size influences fluorescence lifetime was further studied. In addition, the water stability, photostability, and thermal stability were significantly improved, and the fluorescence of NCs can maintain 80.13% of the original value in water for 15 d. We further validated that the strong binding affinity of OPA and oleylamine ligands with CsPbBr₃ NCs leads to a reduction in surface trap states, and a large amount of carboxyl groups of the OPA made the NCs preserve good water solubility. In addition, the OPA has the ability of adjusting the particle size of NCs. Furthermore, a wavelength-shifted colorimetric sensor based on these NCs was constructed for detection of Cl^- in sweat, which enables the rapid and visual detection of Cl^- with high accuracy and stability. Overall, these CsPbBr₃ NCs synthesized by the ligand engineering strategy validated their wide applications in biomedical sensing fields.

Water-Assisted Perovskite Quantum Dots with High Optical Properties

Lead halide perovskite quantum dots (PeQDs) have excellent optical properties, such as narrow emission spectra (FWHM: 18–30 nm), a tunable bandgap (λPL: 420–780 nm), and excellent photoluminescence quantum yields (PLQYs: >90%). PeQDs are known as a material that is easily decomposed when exposed to water in the atmosphere, resulting in causing PeQDs to lower performance. On the other hand, according to the recent reports, adding water after preparing the PeQD dispersion decomposed the PeQD surface defects, resulting in improving their PLQY. Namely, controlling the amount of assisting water during the preparation of the PeQDs is a significantly critical factor to determining their optical properties and device applications. In this paper, our research group discovered the novel effects of the small amount of water to their optical properties when preparing the PeQDs. According to the TEM Images, the PeQDs particle size was clearly increased after water-assisting. In addition, XPS measurement showed that the ratio of Br/Pb achieved to be close to three. Namely, by passivating the surface defect using Ostwald ripening, the prepared PeQDs achieved a high PLQY of over 95%.

Enhanced Air Stability of Perovskite Quantum Dots by Manganese Passivation

Organic-inorganic perovskite quantum dots (PeQDs) have attracted attention due to their excellent optical properties, e.g., high photoluminescence quantum yields (PLQYs; >70%), a narrow full width at half maximum (FWHM; 25 nm or less), and color tunability adjusted by the halide components in an entire tunability (from 450 nm to 730 nm). On the other hand, PeQD stability against air, humidity, and thermal conditions has still not been enough, which disturbs their application. To overcome these issues, with just a focus on the air stability, Mn2+ ion passivated perovskite quantum dots (Mn/MAPbBr3 QDs) were prepared. Mn2+ could be expected to contract the passivating layer against the air condition because the Mn2+ ion was changed to the oxidized Mn on PeQDs under the air conditions. In this research, Mn/MAPbBr3 QDs were successfully prepared by ligand-assisted reprecipitation (LARP) methods. Surprisingly, Mn/MAPbBr3 QD films showed more than double PLQY stability over 4 months compared with pure MAPbBr3 ones against the air, which suggested that oxidized Mn worked as a passivating layer. Improving the PeQD stability is significantly critical for their application.

Phase-Transfer Exchange Lead Chalcogenide Colloidal Quantum Dots: Ink Preparation, Film Assembly, and Solar Cell Construction

Solution-processed colloidal quantum dots (CQDs) are promising candidates for the third-generation photovoltaics due to their low cost and spectral tunability. The development of CQD solar cells mainly relies on high-quality CQD ink, smooth and dense film, and charge-extraction-favored device architectures. In particular, advances in the processing of CQDs are essential for high-quality QD solids. The phase transfer exchange (PTE), in contrast with traditional solid-state ligand exchange, has demonstrated to be the most promising approach for high-quality QD solids in terms of charge transport and defect passivation. As a result, the efficiencies of Pb chalcogenide CQD solar cells have been rapidly improved to 14.0%. In this review, the development of the PTE method is briefly reviewed for lead chalcogenide CQD ink preparation, film assembly, and device construction. Particularly, the key roles of lead halides and additional additives are emphasized for defect passivation and charge transport improvement. In the end, several potential directions for future research are proposed.

Post-synthetic modification mechanism for 1D spin crossover coordination polymers

Suitable solvent os crucial to achieve a quantitative PSM reaction. Then, this method is not restricted to porous materials.

Tunable ionic conductivity and photoluminescence in quasi-2D CH3NH3PbBr3 thin films incorporating sulphur doped graphene quantum dots

Ion migration in hybrid halide perovskites is ubiquitous in all conditions. However, the ionic conductivity can be manipulated by changing the material composition, operating temperature, light illumination, and applied bias as well as the nature of the interfaces of the devices. There have been various reports on electron ion coupling in hybrid perovskite semiconductors which gives rise to anomalous charge transport behavior in these devices under an applied bias. In this investigation, we have synthesized a mixture of 2D/3D perovskites by incorporating sulphur-doped graphene quantum dots (SGQDs) and demonstrated that the optical and electrical properties of the hybrid system can be tuned by controlling the ion conductivity through the active layer. It has been observed that the recombination resistance in undoped CH₃NH₃PbBr₃ perovskites follows an anomalous behavior while the doped CH₃NH₃PbBr₃ perovskite shows a monotonic increase with increasing applied bias due to reduced ionic conductivity. SGQDs at the grain boundaries of 2D/3D perovskites prohibit ion migration through the active layer, and therefore the electronic-ionic coupling is reduced. This results in increased recombination resistance with increasing applied bias.

Highly luminescent and stable quasi-2D perovskite quantum dots by introducing large organic cations

Herein, ultra-stable quasi-two-dimensional perovskite quantum dots (quasi-2D PQDs) are synthesized by introducing the butylamine cation (BA⁺) into the methylamine lead bromide perovskite (MAPbBr₃). By reducing the dimensionality of the perovskite structure, the quasi-2D perovskite (BA)₂(MA) _x-1Pb _x Br_3x+1 presents higher luminescence efficiency and better environmental stability than traditional 3D perovskites, which is mainly because the dimensionality-reduced perovskite has higher exciton binding energy and formation energy. Under an optimal MA : BA ratio of 1 : 1, the quasi-2D perovskite exhibits about four times higher luminescence efficiency (PLQY = 49.44%) than pristine MAPbBr₃; meanwhile it emits stable luminescence in an environment with 80% humidity for 50 days. Most importantly, carbon quantum dot (CQD) doping has also been applied in this work, which effectively passivates the defects of (BA)₂(MA) _x-1Pb _x Br_3x+1 via H-bond interaction, further improving the stability of the perovskite in water. Inspired by the superior performances of the proposed quasi-2D nanomaterial, a novel colorimetric method based on halide ion exchange has been developed for H₂O₂ detection, which also demonstrates that PQDs show significant potential in the field of environmental monitoring.

Energy/Charge Transfer Modulation with Spacer Ligands for Highly Emissive Quantum Dot-Polymer Blend

A blend of perovskite quantum dots (QDs) and a hole transport layer (HTL) is a feasible candidate to solve the long-standing issues in light-emitting diodes (LEDs) such as charge injection, energy state matching, and defect passivation. However, QD:HTL blend structures for QD-based LEDs suffer from fast charge and energy transfers due to an inhomogeneous distribution of QDs and the HTL matrix. Here we report new cross-linkable spacer ligands between QDs and TFB that result in a highly emissive QD:TFB-blended LED device. We synthesize three representative spacer ligands to control the charge and energy transfers between QDs and the HTL. The first spacer ligand is used for controlling the molecular distance between QDs and TFB, and the second spacer ligand is designed to investigate how molecular interaction between QDs and the spacer ligand affects the optical property of the QD:TFB blend. Subsequently, the best spacer ligand, a 10-((2-benzoylbenzoyl)oxy)decanoic acid, is designed to anchor TFB (via a benzophenone group) and simultaneously bond to QDs (with a carboxylic acid functional group). The carboxylic acid group strongly interacts with QDs, dramatically improving the cross-linking rate between QDs and TFB. Due to the direct interaction between QDs and TFB, hole carriers can be effectively injected to perovskite QDs through the conductive backbone of TFB, resulting in the highest luminance values of 10917 cd/m² at 7.4 V due to carrier injection balance. This is at least 10 times better LED performance compared with a pristine QD device.

Tunable Dual-Color Emission Perovskites via Post-Synthetic Modification Strategy for Near-Unity Photoluminescence Quantum Yield

Lead halide perovskites (LHPs) with excellent performance have become promising materials for optoelectrical devices. However, as for the dual-color emission LHPs (DELHPs), the low photoluminescence quantum yield (PLQY) hinders their applications. Herein, a simple low-cost room-temperature post-synthetic modification strategy is used to achieve a near-unity PLQY of DELHPs. It is proven that ZnBr₂ plays an important role as an inorganic ligand in reducing surface defects to induce a 95.4% increase in the radiative decay rate and a 99.5% decrease in the nonradiative decay rate in the treated DELHPs compared with the pristine DELHPs. The performance of the blue emission from the surface lattice is greatly improved via the modification of ZnBr₂. DELHPs with different ratios of blue and green emissions are obtained by changing the specific surface area and ZnBr₂ concentration. The distribution and mechanism of Zn²⁺ are discussed using the research model based on these DELHPs. The first example of the single-layer dual-color perovskite electroluminescence device is realized from DELHPs. This work provides a new perspective for improving the performance of DELHPs, which will greatly accelerate the development of emission materials of LHPs.

The curious case of ion migration in solid-state and liquid electrolyte-based perovskite devices: unveiling the role of charge accumulation and extraction at the interfaces

Electrochemical impedance spectroscopy (EIS) has been extensively used for the detailed investigation and understanding of the plethora of physical properties of variegated electrochemical and solid-state systems. Over the past few years, EIS has revealed many significant findings in hybrid halide perovskite (HHP)-based optoelectronic devices too. Photoinduced ion-migration, negative capacitance, anomalous mid-frequency capacitance, hysteresis, and instability to heat, light and moisture in HHP-based devices are among the few issues addressed by the IS technique. However, performing EIS in perovskite devices presents new challenges related to multilayer solid-state device geometry and complicated material properties. The ions in the perovskite behave in a specified manner, which is dictated by the energy-levels of the transport layer. Electronic-ionic coupling is one of the major challenges to understand ion transport kinetics in solid-state devices. In this work, we have performed impedance measurements in both solid-state (S-S) and liquid-electrolyte (L-E) device geometry to unfold the effect of charge transport layers on the ac ionic conductivity in perovskite materials. We have modelled the impedance spectra using the electrical equivalent circuit (EEC) and compared the behaviour of ions in different controlling environments. It was concluded that the AC as well as dc ionic conductivity and the accumulation of ions in the perovskite material are highly influenced by the nature of the interface in different device geometry. Charge accumulation in the S-S device gives rise to large polarisation, thereby negative capacitance or any inductive loop can be observed in the Nyquist plot while in the L-E device the presence of an electric double layer at the perovskite/electrolyte interface reduces the surface polarisation effect. Ionic conductivity is hopping limited in the low field regime and diffusion limited in the high field regime in the S-S device. Moreover, the perovskite/electrolyte based devices are promising candidates for electrolyte gated field-effect transistors, perovskite-based supercapacitors and electrochemical cells for water splitting or CO₂ reduction.

Materials, photophysics and device engineering of perovskite light-emitting diodes

Here we provide a comprehensive review of a newly developed lighting technology based on metal halide perovskites (i.e. perovskite light-emitting diodes) encompassing the research endeavours into materials, photophysics and device engineering. At the outset we survey the basic perovskite structures and their various dimensions (namely three-, two- and zero-dimensional perovskites), and demonstrate how the compositional engineering of these structures affects the perovskite light-emitting properties. Next, we turn to the physics underpinning photo- and electroluminescence in these materials through their connection to the fundamental excited states, energy/charge transport processes and radiative and non-radiative decay mechanisms. In the remainder of the review, we focus on the engineering of perovskite light-emitting diodes, including the history of their development as well as an extensive analysis of contemporary strategies for boosting device performance. Key concepts include balancing the electron/hole injection, suppression of parasitic carrier losses, improvement of the photoluminescence quantum yield and enhancement of the light extraction. Overall, this review reflects the current paradigm for perovskite lighting, and is intended to serve as a foundation to materials and device scientists newly working in this field.

Antisolvent solvothermal synthesis of MAPbBr3 nanocrystals for efficient solar photodecomposition of methyl orange

Exploring high performance photocatalysts is of great importance to relieve the environment pollution issues. In this paper, we introduce a facile antisolvent solvothermal method to synthesize methylammonium lead tribromide perovskite (MAPbBr₃) nanocrystals and successfully employ them as efficient photocatalysts. Compared to the room temperature synthesized MAPbBr₃ (RT-MAPbBr₃), the antisolvent solvothermal synthesized MAPbBr₃ (AS-MAPbBr₃) has multiple outstanding properties, such as improved crystallinity with lower grain boundary density, enhanced light absorption in visible range, suitable band gap of 2.31 eV and extended photoluminescence (PL) lifetime as long as 2627.82 ns. By taking advantages of the above merits, the AS-MAPbBr₃ exhibits efficient photocatalytic performance by decomposition of methyl orange under solar light. A high apparent rate constant of 101.2 × 10^-3 is achieved along with excellent cyclability, which significantly outperforms the RT-MAPbBr₃ (56.0 × 10^-3) and P25 (16.5 × 10^-3). The underlying mechanism for MO photocatalytic degradation is deeply explored and proposed. Our present study suggests that the antisolvent solvothermal method can be a promising method to synthesize perovskite nanocrystals, and might also provide some insights in developing a series of high performance perovskite based photocatalysts.

Enhanced Stabilities and Production Yields of MAPbBr3 Quantum Dots and Their Applications as Stretchable and Self-Healable Color Filters

Organic-inorganic hybrid CH₃NH₃PbBr₃ (MAPbBr₃) perovskite quantum dots (PQDs) are considered as promising and cost-effective building blocks for various optoelectronic devices. However, during centrifugation for the purification of these PQDs, commonly used polar protic and aprotic non-solvents (e.g., methanol and acetone) can destroy the nanocrystal structure of MAPbBr₃ perovskites, which will significantly reduce the production yields and degrade the optical properties of the PQDs. This study demonstrates the use of methyl acetate (MeOAc) as an effective non-solvent for purifying as-synthesized MAPbBr₃ PQDs without causing severe damage, which facilitates attainment of stable PQD solutions with high production yields. The MeOAc-washed MAPbBr₃ PQDs maintain their high photoluminescence (PL) quantum yields and crystalline structures for long periods in solution states. MeOAc undergoes a hydrolysis reaction in the presence of the PQDs, and the resulting acetate anions partially replace the original surface ligands without damaging the PQD cores. Time-resolved PL analysis reveals that the MeOAc-washed PQDs show suppressed non-radiative recombination and a longer PL lifetime than acetone-washed and methanol-washed PQDs. Finally, it is demonstrated that a composite of the MAPbBr₃ PQDs and a thermoplastic elastomer (polystyrene-block-polyisoprene-block-polystyrene) is feasible as a stretchable and self-healable green color filter for a white light-emitting diode device.

Reducing the Photodegradation of Perovskite Quantum Dots to Enhance Photocatalysis in CO2 Reduction

Solution-processed perovskite quantum dots (QDs) have been intensively researched as next-generation photocatalysts owing to their outstanding optical properties. Even though the intrinsic physical properties of perovskite QDs have been significantly improved, the chemical stability of these materials remains questionable. Their low long-term chemical stability limits their commercial applicability in photocatalysis. In this study, we investigated the photodegradation mechanisms of perovskite QDs and their hybrids via photoluminescence (PL) by varying the excitation power and the ultraviolet (UV) exposure power. Defects in perovskite QDs and the interface between the perovskite QD and the co-catalyst influence the photo-stability of perovskite QDs. Consequently, we designed a stable perovskite QD film via an in-situ cross-linking reaction with amine-based silane materials. The surface ligand comprising 2,6-bis(N-pyrazolyl)pyridine nickel(II) bromide (Ni(ppy)) and 5-hexynoic acid improved the interface between the Ni co-catalyst and the perovskite QD. Then, ultrathin SiO2 was fabricated using 3-aminopropyltriethoxy silane (APTES) to harness the strong surface binding energy of the amine functional group of APTES with the perovskite QDs. The Ni co-catalyst content was further increased through Ni doping during purification using a short surface ligand (3-butynoic acid). As a result, stable perovskite QDs with rapid charge separation were successfully fabricated. Time-correlated single photon counting (TCSPC) PL study demonstrated that the modified perovskite QD film exhibited slow photodegradation owing to defect passivation and the enhanced interface between the Ni co-catalyst and the perovskite QD. This interface impeded the generation of hot carriers, which are a critical factor in photodegradation. Finally, a stable red perovskite QD was synthesized by applying the same strategy and the mixture between red and green QD/Ni(ppy)/SiO2 displayed an CO2 reduction capacity for CO (0.56 µmol/(g∙h)).

Modulating Hierarchical Micro/Mesoporosity by a Mixed Solvent Approach in Al-MOF: Stabilization of MAPbBr3 Quantum Dots

Various hierarchical micro/mesoporous MOFs based on {[Al(μ-OH)(1,4-NDC)]⋅H₂ O} (MOF1) with tunable porosities (pore volume and surface area) have been synthesized by assembling Al^III and 1,4-NDC (1,4-naphthalenedicarboxylate) under microwave irradiation by varying water/ethanol solvent ratio. Water/ethanol mixture has played a crucial role in the mesopore generation in MOF1M₂₅ , MOF1M₅₀ , and MOF1M₇₅ , which is achieved by in situ formation of water/ethanol clusters. By adjusting the ratio of water/ethanol, the particle size, surface area and micro/mesopore volume fraction of the MOFs are controlled. Furthermore, reaction time plays a critical role in mesopore formation as realized by varying reaction time for the MOF with 50 % ethanol (MOF1M₅₀ ). Additionally, hierarchical MOF (MOF1M₅₀ ) has been used as a template for the stabilization of MAPbBr₃ (MA=methylammonium) perovskite quantum dots (PQDs). MAPbBr₃ PQDs are grown inside MOF1M₅₀ , where mesopores control the size of PQDs which leads to quantum confinement.

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.

Perovskite Quantum Dot-Reduced Graphene Oxide Superstructure for Efficient Photodetection

High-performance photodetectors require efficient photogeneration and charge transport. Perovskite quantum dots (PQDs) have received enormous interest for applications in optoelectronics due to their high photogeneration efficiency. However, they offer meager carrier transport. Reduced graphene oxide (RGO) exhibits inferior photoresponse compared to materials such as quantum dots. An effective synthesis protocol to grow PQDs from the RGO lattice may facilitate direct charge transfers from PQDs to RGO, which could not be accomplished by mixing individual PQDs with RGO or making a bilayer. At ambient condition, the photodetector fabricated with the PQD-RGO superstructure showed high responsivity of 1.07 × 10³ A/W, detectivity of 1 × 10¹³ Jones as well as sharp switching in the visible wavelength. After 3 months in an unencapsulated sample, the photocurrent was decreased ∼10% of its initial value while preserving speed and cycle stability at ambient condition.

Establishing charge-transfer excitons in 2D perovskite heterostructures

Charge-transfer excitons (CTEs) immensely enrich property-tuning capabilities of semiconducting materials. However, such concept has been remaining as unexplored topic within halide perovskite structures. Here, we report that CTEs can be effectively formed in heterostructured 2D perovskites prepared by mixing PEA₂PbI₄:PEA₂SnI₄, functioning as host and guest components. Remarkably, a broad emission can be demonstrated with quick formation of 3 ps but prolonged lifetime of ~0.5 μs. This broad PL presents the hypothesis of CTEs, verified by the exclusion of lattice distortion and doping effects through demonstrating double-layered PEA₂PbI₄/PEA₂SnI₄ heterostructure when shearing-away PEA₂SnI₄ film onto the surface of PEA₂PbI₄ film by using hand-finger pressing method. The below-bandgap photocurrent indicates that CTEs are vital states formed at PEA₂PbI₄:PEA₂SnI₄ interfaces in 2D perovskite heterostructures. Electroluminescence shows that CTEs can be directly formed with electrically injected carriers in perovskite LEDs. Clearly, the CTEs presents a new mechanism to advance the multifunctionalities in 2D perovskites.

Forming charge transfer excitons (CTEs) exclusively within perovskite structures remains as an unexplored issue. Here, the authors report the establishment of CTEs for demonstrating broad light emission within quasi-2D perovskite heterostructures, presenting “intermolecular-type” excited states.

Unraveling the antisolvent dripping delay effect on the Stranski–Krastanov growth of CH3NH3PbBr3 thin films: a facile route for preparing a textured morphology with improved optoelectronic properties

Controlled nucleation and growth by delaying the antisolvent dripping time leads to the formation of a textured perovskite thin film morphology with improved optoelectronic properties.

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.

Determination of complex dielectric function of CH3NH3PbBr3 perovskite cubic colloidal quantum dots by modified iterative matrix inversion method

Recent advances in lead halide perovskite quantum dots appeal with their potential in various optoelectronic devices such as photovoltaics, photodetectors, light-emitting diodes (LEDs) and lasers. However, lack of information on the intrinsic optical properties of lead halide perovskite quantum dots (QDs) lags the progress in device performances and further development in various applications. In this letter, the complex dielectric function of CH₃NH₃PbBr₃ perovskite cubic colloidal QDs was determined from the UV-Vis absorption by using a modified iterative matrix inversion (IMI) method. The modified IMI method takes into account the dilute solution with cubic inclusions, while the conventional method only considers spherical or elliptical inclusions by Maxwell-Garnett (MG) effective medium theory. In addition, singly subtractive Kramer Kronig (SSKK) relations have also been considered to compensate for possible errors arising from the finite wavelength range of the experimental absorption data.

Efficient Quantum Dot Light-Emitting Diodes Based on Trioctylphosphine Oxide-Passivated Organometallic Halide Perovskites

Metal halide perovskite quantum dots (QDs) have attracted significant research interest in the next-generation display and solid illumination fields due to their excellent optical properties of high photoluminescence quantum efficiency, high color purity, obvious quantum confinement effect, and large exciton binding energy. A large amount of surface defects and nonradiative recombination induced by these defects are considered as major problems to be resolved urgently for practical applications of perovskite QDs in high-efficiency light-emitting diodes (LEDs). Herein, we report an efficient passivation of green perovskite QD CH₃NH₃PbBr₃ with trioctylphosphine oxide (TOPO). By simply adding the appropriate amount of TOPO into the nonpolar toluene solvent to synthesize CH₃NH₃PbBr₃ QDs, the surface defects of these as-synthesized perovskite QDs are obviously reduced, along with an increased photoluminescence lifetime and suppressed nonradiative recombination. Further investigation indicates that electronegative oxygen from TOPO (Lewis base) bonds with uncoordinated Pb²⁺ ions and labile lead atoms in perovskite. With TOPO passivation, the green perovskite QD LEDs based on CH₃NH₃PbBr₃ show significant performance improvement factors of 93.5, 161.1, and 168.9% for luminance, current efficiency, and external quantum efficiency, respectively, reaching values of 1635 cd m^-2, 5.51 cd A^-1, and 1.64% in the eventual optimized devices. Furthermore, the presence of TOPO dramatically improves stabilities of CH₃NH₃PbBr₃ QDs and related devices. Our work provides a robust platform for the fabrication of low-defect-density perovskite QDs and efficient, stable perovskite QD LEDs.

Photoinduced photoluminescence enhancement in self-assembled clusters of formamidinium lead bromide perovskite nanocrystals

Nanocrystals of formamidinium lead bromide perovskite (FAPbBr3) self-assemble into clusters in powder and film samples and provide a prolonged photoluminescence lifetime, which is attributed to the diffusion of charge carriers through interparticle states formed among nanocrystals. Interestingly, the photoluminescence lifetime decreases and the emission intensity increases for the clusters, which is with the increase in the intensity of excitation light. By doping the nanocrystal clusters with C60, we successfully harvested the photogenerated charge carriers. Nonetheless, at high intensities of excitation, the rate of radiative recombination becomes comparable to that of the electron transfer to C60. Thus, the optimum rate of electron transfer to C60 is accomplished by minimally exciting the self-assembled nanocrystals.

Realizing a highly luminescent perovskite thin film by controlling the grain size and crystallinity through solvent vapour annealing

Organometallic halide perovskite films were treated with novel facile solvent vapour annealing to control crystal grain size as well as the crystallinity of perovskite. As both polarity and vapour pressure of the treatment solvent for perovskite increase, luminance increases and the wavelength of the photoluminescence emission peak decreases due to enhanced crystallinity and reduced grain size.

Investigation of high contrast and reversible luminescence thermochromism of the quantum confined Cs4PbBr6 perovskite solid

Thermochromism of organic/inorganic halide perovskites has attracted particular interest due to their potential applications as photoluminescence (PL)-based temperature sensors. However, despite the outstanding PL characteristics, their use as a thermochromic material in practical temperature ranges has been limited because of their poor thermal stability. In this study, we used the quantum confinement effect and exceptional PL quantum efficiency of the Cs4PbBr6 perovskite to demonstrate their high on/off ratio (20) and reversible PL thermochromism in the solid state in practical temperature ranges including room temperature (RT). Systematic photophysical and optical characterization studies, including exciton-phonon scattering, exciton binding energy, exciton decay dynamics, and crystal structure change, were performed to investigate the origin of this unique thermochromic PL property. The results showed that the efficient and highly reversible thermochromic PL emission of the Cs4PbBr6 perovskite is due to its desirable optical properties such as highly luminescent emission, efficient PL quenching at high temperatures, and thermally reversible structural changes.

Observation and implication of halide exchange beyond CsPbX3 perovskite nanocrystals

Anion exchange between pre-synthesized all-inorganic nanocrystals with a perovskite structure is a promising approach to tune their chemical composition and optical properties. Herein we have reported the first study of internanocrystal anion exchange reactionsin the cesium lead halide family, including CsPbX3, Cs4PbX6 and CsX, and we found that the anion exchange dynamics is highly dependent on their crystalline phase. In stark contrast to the fast rate in CsPbX3, cesium based non-perovskite NCs display much slower halide mobility. The reaction time is increased to several hours in Cs4PbX6 and days in CsX, respectively. Furthermore, we confirm that mixing these NCs with the same halide but different structures will induce halide diffusion from Cs4PbX6 NCs and CsX NCs to CsPbX3 NCs. This feature can be explored to utilize the Cs4PbX6 NCs and CsX NCs as a halide source to improve the photoluminescence efficiency and colloidal stability of CsPbX3 NCs.

Incorporation of rubidium cations into blue perovskite quantum dot light-emitting diodes via FABr-modified multi-cation hot-injection method

Solution-processed lead halide perovskite quantum dots (QDs) are emerging as one of the most promising candidates for emissive display application. Although perovskite QDs with a full spectrum of visible light emissions have been realized for years, realizing the efficient electroluminescence of blue perovskites at room temperature still faces severe challenges. Herein, we demonstrate both the efficient photoluminescence and electroluminescence of the blue perovskite QDs via a simple FABr-modified multi-cation hot-injection (FMMHI) method. The FMMHI method is unique in both the addition of FABr into the PbBr2 precursor solution and the incorporation of small rubidium (Rb+) into the blue perovskite QDs light-emitting diodes (QLEDs). The addition of FABr into the precursor solution can realize strong quantum confinement effect, large exciton binding energy and high-quality perovskite QD films. Besides, the bandgap can be enlarged by the Rb+-induced perovskite octahedral distortion and strong quantum confinement effect. Excellent PLQYs of 64.5% and 49.8% were achieved for the developed greenish-blue QDs (Rb0.33Cs0.67)0.42FA0.58PbBr3 and deep-blue QDs (Rb0.33Cs0.67)0.42FA0.58PbCl1.25Br1.75 in solid film state. Moreover, maximum external quantum efficiencies (EQEs) of 3.6% and 0.61% were also achieved with an electroluminescence peak wavelength at 502 and 466 nm, respectively, indicating that the perovskite QDs incorporated with Rb+ possess great potential for the development of high-performance blue perovskite electroluminescence diodes.

Solvent engineering for high conversion yields of layered raw materials into large-scale freestanding hybrid perovskite nanowires

Hybrid organic-inorganic perovskite nanowires have received considerable attention for applications in optoelectronic fields; however, practical implementation of perovskite nanowires into optoelectronic devices is often hindered by low product yields, scaled-up synthesis, or overall lack of synthetic control. Herein, we report novel insights into large-scale freestanding lead halide perovskite nanowires using a series of solvent effects. A facile pathway for converting layered raw materials to nanowires with high conversion yields can facilitate the large-scale applications of nanowires, which could help us understand the unique aspects of the formation chemistry of the materials. We introduced a two-step process to obtain perovskite nanowires without an impurity phase that first involves the formation of Pb-containing nanowires with the poor solvent drip method. Subsequently, the as-prepared Pb-containing precursors can be used as morphology templates for converting completely into perovskite nanowires with high conversion yields of 83% through a series of solvent effects, such as solvent-driven, solvent-removing, solvent-exchange and solvent-growth. The synthetic strategy was demonstrated to be applicable for other perovskite-based materials, which can offer a comprehensive understanding of the formation mechanism of perovskite-based materials.