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).

Ultrafast photo-induced carrier dynamics of perovskite quantum dots during structural degradation.

In this study, the ultrafast photo-induced carrier dynamics of red-emitting PQDs during structural degradation was investigated using time-resolved transient absorption spectroscopy. The spectroscopic analysis revealed how the carrier dynamics varied when PQDs were exposed to a polar solvent. Three decay modes (carrier trapping, radiative carrier recombination and trap-assisted non-radiative recombination) were proposed to analyze the carrier dynamics of PQDs. The light-emitting property of PQDs is primarily influenced by radiative carrier recombination. This study demonstrates that structural degradation induced halide migration within PQDs and the formation of defects within the crystal lattice, leading to a proliferation of carrier trapping states. The increased trap states led to a reduction in carriers undergoing radiative carrier recombination. Additionally, PQDs degradation accelerated radiative carrier recombination, indicating a faster escape of carriers from excited states. Consequently, these factors hinder carriers remaining in excited states, leading to a decline in the light-emitting property of PQDs. Nevertheless, increasing an excitation fluence could reduce the carrier trapping mode and increase the radiative carrier recombination mode, suggesting a diminishment of the impact of carrier trapping. These findings offer a more comprehensive understanding of structural degradation of PQDs and can contribute to the development of PQDs with high structural stability.

Reusable, Ultrasensitive, Patterned Conjugated Polyelectrolyte-Surfactant Complex Film with a Wide Detection Range for Copper Ion Detection.

Conjugated polyelectrolytes (CPEs) are emerging as promising materials in the sensor field because they enable high-sensitivity detection of various substances in aqueous media. However, most CPE-based sensors have serious problems in real-world application because the sensor system is operated only when the CPE is dissolved in aqueous media. Here, the fabrication and performance of a water-swellable (WS) CPE-based sensor driven in the solid state are demonstrated. The WS CPE films are prepared by immersing a water-soluble CPE film in cationic surfactants of different alkyl chain lengths in a chloroform solution. The prepared film exhibits rapid, limited water swellability despite the absence of chemical crosslinking. The water swellability of the film enables the highly sensitive and selective detection of Cu2+ in water. The fluorescence quenching constant and the detection limit of the film are 7.24 × 106 L mol-1 and 4.38 nM (0.278 ppb), respectively. Moreover, the film is reusable via a facile treatment. Furthermore, various fluorescent patterns introduced by different surfactants are successfully fabricated by a simple stamping method. By integrating the patterns, Cu2+ detection in a wide concentration range (nM-mM) can be achieved.

Sequential structural degradation of red perovskite quantum dots and its prevention by introducing iodide at a stable gradient concentration into the core-shell red perovskite quantum dots.

Perovskite quantum dots (QDs) have been extensively studied as emissive materials for next-generation optoelectronics due to their outstanding optical properties; however, their structural instabilities, specifically those of red perovskite QDs, are critical obstacles in realizing operationally reliable perovskite QD-based optoelectronic devices. Accordingly, herein, we investigated the sequential degradation mechanism of red perovskite QDs upon their exposure to an electric field. Via electrical and chemical characterization, we demonstrated that degradation occurred in the following order: anion-defect-assisted halide migration, cation-defect-assisted migration of I-/Cs+ ions, defective gradient I ion distribution, structural distortion, and ion transport/I2 vaporization with defect proliferation. Among these steps, the defective gradient I ion distribution is the key process in the structural degradation of perovskite QDs. Based on our findings, we designed perovskite/SiO2 core-shell QDs with stable gradient I concentrations. Most notably, the operational stabilities of perovskite QD-light-emitting diodes (PeLEDs) fabricated using the perovskite/SiO2 core-shell QDs were approximately 5000 times those of the PeLEDs constructed using pristine perovskite QDs.

Disordered-Layer-Mediated Reverse Metal-Oxide Interactions for Enhanced Photocatalytic Water Splitting.

In heterogeneous catalysts, metal-oxide interactions occur spontaneously but often in an undesired way leading to the oxidation of metal nanoparticles. Manipulating such interactions to produce highly active surface of metal nanoparticles can warrant the optimal catalytic activity but has not been established to date. Here we report that a prior reduced TiO2 support can reverse the interaction with Pt nanoparticles and augment the metallic state of Pt, exhibiting a 3-fold increase in hydrogen production rate compared to that of conventional Pt/TiO2. Spatially resolved electron energy loss spectroscopy of the Ti valence state and the electron density distribution within Pt nanoparticles provide direct evidence supporting that the Pt/TiO2/H2O triple junctions are the most active catalytic sites for water reduction. Our reverse metal-oxide interaction scheme provides a breakthrough in the stagnated hydrogen production efficiency and can be applied to other heterogeneous catalyst systems composed of metal nanoparticles with reducible oxide supports.

Synthesis of Chemically Stable Ultrathin SiO2-Coated Core-Shell Perovskite QDs via Modulation of Ligand Binding Energy for All-Solution-Processed Light-Emitting Diodes.

Recently, perovskite quantum dots (QDs) have attracted intensive interest due to their outstanding optical properties, but their extremely poor chemical stability hinders the development of the high-performance perovskite QD-based light-emitting diodes (PeLEDs). In this study, chemically stable SiO2-coated core-shell perovskite QDs are prepared to fabricate all-solution-processed PeLEDs. When the SiO2 shell thickness increases, the chemical stability of perovskite QDs is dramatically improved, while the charge injection efficiency is significantly decreased, which becomes the biggest obstacle for PeLED applications. Thus, controlling the SiO2 thickness is essential to obtain core-shell perovskite QDs optimal for PeLEDs in an aspect of chemical and optoelectrical properties. The 3-aminopropyl-triethoxysilane (APTES)/oleylamine (OAm) volume ratio is found to be a critical factor for obtaining an ultrathin SiO2 shell. Optimization of the APTES/OAm ratio affords A-site-doped CsPbBr3 QDs with an ultrathin SiO2 shell (A-CsPbBr3@SiO2 QDs) that exhibit longer radiative lifetimes and smaller shallow trap fraction than those without A-site doping, resulting in a higher photoluminescence quantum yield. A-CsPbBr3@SiO2 QDs also demonstrate long-term superior chemical stability in polar solvents without loss of optical properties due to passivation by the SiO2 shell and less defects via A-site doping. Consequently, all-solution-processed PeLED is successfully fabricated under ambient conditions, facilitating perovskite QD utilization in low-cost, large-area, flexible next-generation displays.

Design of Chemically Stable Organic Perovskite Quantum Dots for Micropatterned Light-Emitting Diodes through Kinetic Control of a Cross-Linkable Ligand System.

Perovskite quantum dot (QD) light-emitting diodes (PeLEDs) are ideal for next-generation display applications because of their excellent color purity, high efficiency, and cost-effective fabrication. However, developing a technology for high-resolution multicolor patterning of perovskite QDs remains challenging, owing to the chemical instability of these materials. To overcome these issues, in this work, the generation of surface defects is prevented by controlling the ligand-binding kinetics using a stable ligand system (Stable LS). The crystalline reconstruction of perovskite QDs after addition of the Stable LS results in an ≈18% increase in their photoluminescence quantum yield in solution and it also improves the ambient stability of the perovskite QD solution. Moreover, the perovskite QDs with Stable LS can undergo cross-linking under UV irradiation. The tightly bridged perovskite QDs effectively prevent moisture-assisted ligand dissociation in film state due to the increased hydrophobicity and restricted movement of the cross-linked surface ligands. Thus, the cross-linked perovskite QD film shows improved chemical/environmental stability without substantial deterioration in optoelectrical properties. As a result, a white electroluminescent device with high resolution (≈1 µm) is successfully fabricated by inkjet printing using green and red perovskite QDs.

Unusual piezochromic fluorescence of a distyrylpyrazine derivative crystals: phase transition through [2 + 2] photocycloaddition under UV irradiation.

The piezochromic fluorescence (FL) of a distyrylpyrazine derivative, 2,3-diisocyano-5,6-distyrylpyrazine (DSP), was investigated in this study. Depending on the recrystallization method, DSP afforded two different crystals with green and orange FL emission. The orange color FL emission crystal (O-form) was easily converted to the green color FL emission one (G-form) by manual grinding. The G-form was also converted to a slightly different orange color FL emission crystal (RO-form) by a weak UV irradiation. When the RO-form was ground again, the G-form was regenerated. The FL colors changed between the G- and RO-forms over several ten times by repeated mechanical grinding and UV irradiation. The FL, UV-visible, 1H-NMR and XRD results showed that the O (or RO)-to-G transformation induced by mechanical stress results from the change of degree of molecular stacking from dense molecular stacking structure to relatively loose molecular stacking structure, whereas the G-to-RO reconversion by UV irradiation results from return to dense molecular stacking structure again due to lattice movement (lattice slipping) allowed by photocycloaddition in solid-state.

Retarding Ion Exchange between Conducting Polymers and Ionic Liquids for Printable Top Electrodes in Semitransparent Organic Solar Cells.

Semitransparent organic solar cells (ST-OSCs) are considered to be an influential tool for aesthetic and economic building-integrated photovoltaics, which can be fabricated by the printing technology. A poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) and ionic liquid (IL) composite has been considered as an electrode for ST-OSCs because of its high electrical conductivity, high transparency, and printability. However, we found that the introduction of IL into the PEDOT:PSS solution for enhancing its electrical conductivity results in (1) nonreliable printing of PEDOT:PSS/IL composite films because of gradual gelation of the mixture solution and (2) the production of chemically reactive ion pairs during ion exchange between PSS and IL, which induces the oxidation of the underlying organic semiconductors during printing. To solve these problems, we developed a sequential printing method using pristine PEDOT:PSS and IL solutions to retard ion exchange, thus preventing chemical doping of organic semiconductors by newly generated ion pairs. Finally, by using only solution processes, we demonstrate efficient ST-OSCs with a printed PEDOT:PSS/IL composite as the top electrode, exhibiting a power conversion efficiency of 6.32% at an average visible transmittance of 35.4%.

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.

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

Correction for 'Investigation of high contrast and reversible luminescence thermochromism of the quantum confined Cs4PbBr6 perovskite solid' by Jong H. Kim et al., Nanoscale, 2019, DOI: 10.1039/c8nr10223f.

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.

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.

Improvement of perovskite crystallinity by omnidirectional heat transfer via radiative thermal annealing.

As promising photo-absorbing materials for photovoltaics, organic-inorganic hybrid perovskite materials such as methylammonium lead iodide and formamidinium lead iodide, have attracted lots of attention from many researchers. Among the various factors to be considered for high power conversion efficiency (PCE) in perovskite solar cells (PSCs), increasing the grain size of perovskite is most important. However, it is difficult to obtain a highly crystalline perovskite film with large grain size by using the conventional hot-plate annealing method because heat is transferred unidirectionally from the bottom to the top. In this work, we presented radiative thermal annealing (RTA) to improve the structural and electrical properties of perovskite films. Owing to the omnidirectional heat transfer, swift and uniform nuclei formation was possible within the perovskite film. An average grain size of 500 nm was obtained, which is 5 times larger than that of the perovskite film annealed on a hot-plate. This perovskite film led to an enhancement of photovoltaic performance of PSCs. Both short-circuit current density and PCE of the PSCs prepared by RTA were improved by 10%, compared to those of PSCs prepared by hot-plate annealing.

Temperature-Dependent Photoluminescence of CH3NH3PbBr3 Perovskite Quantum Dots and Bulk Counterparts.

Organic-inorganic lead halide perovskite is emerging as a potential emissive material for light emitting devices, such as, light emitting diodes (LEDs) and lasers, which has emphasized the necessity of understanding its fundamental opto-physical properties. In this work, the temperature-dependent photoluminescence of CH3NH3PbBr3 perovskite quantum dots (QDs), polycrystalline thin film (TF), and single crystal (SC) has been studied. The optophysical properties, such as exciton-phonon scattering, exciton binding energy, and exciton decay dynamics, were investigated. The exciton-phonon scattering of perovskite is investigated, which is responsible for both PL line width broadening and nonradiative decay of excitons. The exciton binding energy of QDs, TF, and SC were estimated to be 388.2, 124.3, and 40.6 meV, respectively. The observed main exciton decay pathway for QDs is the phonon assisted thermal escape, while that for TF and SC was the thermal dissociation due to low exciton binding energy.

Multiple Heterojunction in Single Titanium Dioxide Nanoparticles for Novel Metal-Free Photocatalysis.

Despite a longstanding controversy surrounding TiO2 materials, TiO2 polymorphs with heterojunctions composed of anatase and rutile outperform individual polymorphs because of the type-II energetic band alignment at the heterojunction interface. Improvement in photocatalysis has also been achieved via black TiO2 with a thin disorder layer surrounding ordered TiO2. However, localization of this disorder layer in a conventional single TiO2 nanoparticle with the heterojunction composed of anatase and rutile has remained a big challenge. Here, we report the selective positioning of a disorder layer of controlled thicknesses between the anatase and rutile phases by a conceptually different synthetic route to access highly efficient novel metal-free photocatalysis for H2 production. The presence of a localized disorder layer within a single TiO2 nanoparticle was confirmed for the first time by high-resolution transmission electron microscopy with electron energy-loss spectroscopy and inline electron holography. Multiple heterojunctions in single TiO2 nanoparticles composed of crystalline anatase/disordered rutile/ordered rutile layers give the nanoparticles superior electron/hole separation efficiency and novel metal-free surface reactivity, which concomitantly yields an H2 production rate that is ∼11-times higher than that of Pt-decorated conventional anatase and rutile single heterojunction TiO2 systems.

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

The photoluminescence quantum yield (PLQY) and charge carrier mobility of organic-inorganic perovskite QDs were enhanced by the optimization of crystallinity and surface passivation as well as solid-state ligand exchange. The crystallinity of perovskite QDs was determined by the Effective solvent field (Esol) of various solvents for precipitation. The solvent with high Esol could more quickly countervail the localized field generated by the polar solvent, and it causes fast crystallization of the dissolved precursor, which results in poor crystallinity. The post-ligand adding process (PLAP) and post-ligand exchange process (PLEP) increase the PLQY of perovskite QDs by reducing non-radiative recombination and the density of surface defect states through surface passivation. Particularly, the post ligand exchange process (PLEP) in the solid-state improved the charge carrier mobility of perovskite QDs in addition to the PLQY enhancement. The ligand exchange with short alkyl chain length ligands could improve the packing density of perovskite QDs in films by reducing the inter-particle distance between perovskite QDs. The maximum hole mobility of 6.2 × 10-3 cm2 V-1 s-1, one order higher than that of pristine QDs without the PLEP, is obtained at perovskite QDs with hexyl ligands. By using PLEP treatment, compared to the pristine device, a 2.5 times higher current efficiency in perovskite QD-LEDs was achieved due to the improved charge carrier mobility and PLQY.

Fluorescent Molecular Rotors for Viscosity Sensors.

Fluorescent molecular rotors (FMRs) can act as viscosity sensors in various media including subcellular organelles and microfluidic channels. In FMRs, the rotation of rotators connected to a fluorescent π-conjugated bridge is suppressed by increasing environmental viscosity, resulting in increasing fluorescence (FL) intensity. In this minireview, we describe recently developed FMRs including push-pull type π-conjugated chromophores, meso-phenyl (borondipyrromethene) (BODIPY) derivatives, dioxaborine derivatives, cyanine derivatives, and porphyrin derivatives whose FL mechanism is viscosity-responsive. In addition, FMR design strategies for addressing various issues (e.g., obtaining high FL contrast, internal FL references, and FL intensity-contrast trade-off) and their biological and microfluidic applications are also discussed.

High-Efficiency Polycrystalline Perovskite Light-Emitting Diodes Based on Mixed Cations.

We have achieved high-efficiency polycrystalline perovskite light-emitting diodes (PeLEDs) based on formamidinium (FA) and cesium (Cs) mixed cations without quantum dot synthesis. Uniform single-phase FA1- xCs xPbBr3 polycrystalline films were fabricated by one-step formation with various FA:Cs molar proportions; then the influences of chemical composition on film morphology, crystal structure, photoluminescence (PL), and electroluminescence (EL) were systematically investigated. Incorporation of Cs+ cations in FAPbBr3 significantly reduced the average grain size (to 199 nm for FA:Cs = 90:10) and trap density; these changes consequently increased PL quantum efficiency (PLQE) and PL lifetime of FA1- xCs xPbBr3 films and current efficiency (CE) of PeLEDs. Further increase in Cs molar proportion from 10 mol % decreased crystallinity and purity, increased trap density, and correspondingly decreased PLQE, PL lifetime, and CE. Incorporation of Cs also increased photostability of FA1- xCs xPbBr3 films, possibly due to suppressed formation of light-induced metastable states. FA1- xCs xPbBr3 PeLEDs show the maximum CE = 14.5 cd A-1 at FA:Cs = 90:10 with very narrow EL spectral width (21-24 nm); this is the highest CE among FA-Cs-based PeLEDs reported to date. This work provides an understanding of the influences of Cs incorporation on the chemical, structural, and luminescent properties of FAPbBr3 polycrystalline films and a breakthrough to increase the efficiency of FA1- xCs xPbBr3 PeLEDs.

Molecular Viscosity Sensors with Two Rotators for Optimizing the Fluorescence Intensity-Contrast Trade-Off.

A series of fluorescent molecular rotors obtained by introducing two rotational groups ("rotators"), which exhibit different rotational and electron-donating abilities, are discussed. Whereas the control molecular rotor, PH, includes a single rotator (the widely used phenyl group), the PO molecular rotors consist of two rotators (a phenyl group and an alkoxy group), which exhibit simultaneous strongly electron-donating and easy rotational abilities. Compared with the control rotor PH, PO molecular rotors exhibited one order of magnitude higher quantum yield (fluorescence intensity) and simultaneously exhibited significantly higher fluorescence contrast. These properties are directly related to the strong electron-donating ability and low energy barrier of rotation of the alkoxy group, as confirmed by dynamic fluorescence experiments and quantum chemical calculations. The PO molecular rotors exhibited two fluorescence relaxation pathways, whereas the PH molecular rotor exhibited a single fluorescence relaxation pathway. Cellular fluorescence imaging with PO molecular rotors for mapping cellular viscosity was successfully demonstrated.