Benzyl Alcohol-Treated CH3NH3PbBr3 Nanocrystals Exhibiting High Luminescence, Stability, and Ultralow Amplified Spontaneous Emission Thresholds.

We report the high yield synthesis of about 11 nm sized CH3NH3PbBr3 nanocrystals with near-unity photoluminescence quantum yield. The nanocrystals are formed in the presence of surface-binding ligands through their direct precipitation in a benzyl alcohol/toluene phase. The benzyl alcohol plays a pivotal role in steering the surface ligands binding motifs on the NC surface, resulting in enhanced surface-trap passivation and near-unity PLQY values. We further demonstrate that thin films from purified CH3NH3PbBr3 nanocrystals are stable >4 months in air, exhibit high optical gain (about 520 cm-1), and display stable, ultralow amplified spontaneous emission thresholds of 13.9 ± 1.3 and 569.7 ± 6 µJ cm-2 at one-photon (400 nm) and two-photon (800 nm) absorption, respectively. To the best of our knowledge, the latter signifies a 5-fold reduction of the lowest reported threshold value for halide perovskite nanocrystals to date, which makes them ideal candidates for light-emitting and low-threshold lasing applications.

Stable and luminescent cesium copper halide nanocrystals embedded in flexible polymer fibers for fabrication of down-converting WLEDs.

Recently, CsPbX3 (X = I, Br, Cl) perovskite nanocrystals (NCs) have drawn wide attention owing to their outstanding photophysical and optoelectronic properties. However, the toxicity of such NCs remained a big challenge for further commercialization. Herein, we adopt facile methods for synthesizing green-emissive Cs3Cu2Cl5 and blue-emissive Cs3Cu2Br2.5I2.5 perovskite NCs that exhibit broad emission spectra with large Stokes shifts. These NCs showed photoluminescence quantum yields (PLQY) up to 65% (Cs3Cu2Cl5 NCs) and 32% (Cs3Cu2Br2.5I2.5 NCs) with limited stabilities. To further improve the stability, the NCs were blended with a hydrophobic polymer poly-methylmethacrylate (PMMA) and embedded inside the polymer fiber by an electrospinning process to form composite fibers. The as-prepared Cs3Cu2Cl5@PMMA and Cs3Cu2Br2.5I2.5@PMMA fiber films demonstrated good surface coverage and better thermal stability, and even retained their emission properties when dispersed in water. The emissive fibers were also deposited on flexible polyethylene terephthalate (PET) substrates that displayed high resistance towards bending and twisting with no signs of breakage, damage, or loss of optical properties. Finally, UV-pumped phosphor-converted WLEDs fabricated by using these blue and green-emitting fibers revealed CIE chromaticity coordinates at (0.27, 0.33) with a maximum luminous efficiency of 69 Lm W-1 and correlated color temperature (CCT) value of 8703 K. These outcomes can be beneficial for the development of futuristic flexible display technologies.

Highly water-stable, luminescent, and monodisperse polymer-coated CsPbBr3 nanocrystals for imaging in living cells with better sensitivity.

Recently, CsPbX3 (X= Cl, Br, I) nanocrystals (NCs) have evolved as a potential contender for various optoelectronic applications due to some of their excellent photophysical properties. Their superior non-linear optical properties enable them to take part in bioimaging applications due to their longer penetration depth and less scattering effect in living cells. However, the poor stability of perovskite NCs in aqueous media still remains a great challenge for practical usage. Comparatively stable silica-coated NCs have a tendency to agglomerate among other NCs and transform into bigger particles. Such big particles clog the inside of narrow channels during the uptake and can't effectively reach the targeted cells. To tackle such issues, we introduce a fast and reproducible synthesis process of CsPbBr3 NCs that are coated with different long-chained organic ligands/polymers and compared their photophysical properties. Among them, polyvinylpyrrolidone (PVP) encapsulated NCs are highly luminescent in the green spectral region and showed a maximum photoluminescence quantum yield (PLQY) of up to 84%. The incorporation of n-isopropyl acrylamide (NIPAM) along with PVP further improves the stability of the PVP-coated NCs against heat and moisture. These NCs exhibit higher water stability compared to silica-coated NCs and maintained their emission properties for about one week in DI water. The smaller particle size, uniform size distribution, higher structural stability, and better dispersivity of polymer-coated NCs in the aqueous media enable them to perform as fluorescent probes for live cell imaging in mammalian Chinese Hamster Ovary (CHO-K1) cells. There is no adverse affect in the cells' viability and morphology even after long incubation periods (∼72 hours). The dosage of Pb-ions contained in the polymer-coated NCs is calculated as below 5 µg mL-1, which is suitable for live cell imaging. This work provides insight for expanding the use of these NCs significantly into bioimaging applications with higher sensitivity.

Understanding the charge transfer mechanism in CsPbBr3 nanocrystals and nitrogen-doped carbon quantum dot heterostructures: effect of nanocrystal encapsulation.

Recently, lead halide perovskite nanocrystal (NC)-based heterostructures have demonstrated significant promise in various research areas, including solar cells, CO2 reduction, and photocatalysis. These hybrid structures have also played a crucial role in advancing our fundamental conception of charge transfer mechanisms occurring at the interface. A thin shell around the NCs is not suitable for the formation of stable and luminescent materials. However, such NCs are suitable for solar cells, LEDs, CO2 reduction, and photocatalytic applications due to higher carrier mobility. Thick-shelled NCs are highly stable but hinder charge transport among the NCs which is beneficial for bio-imaging and color-converted LED fabrication. So, understanding the mechanism of charge transport among the NCs dependent on the shell materials is important. Here, we synthesized CsPbBr3 NCs with various coating materials to vary the effective distance between the perovskite and nitrogen-doped carbon quantum dots (NCQDs) to understand the charge transfer process among them. We encapsulated the NCs with different coating materials (i.e., oleic acid, oleylamine, polyvinylpyrrolidone, and silica) such that the thickness of the NCs' shell can differ. We observed that the charge transfer rate between thick-shelled NCs and NCQDs is slow. The faster charge transfer among the thinner-shelled NCs and NCQDs is feasible due to the bonding of the N-state of NCQDs with Pb-atoms of the CsPbBr3 structure. The density functional theory (DFT) calculations of the heterostructure indicate that the electron acceptor state of the N-atom in NCQDs lies below the conduction band of perovskite NCs, which is accountable for such charge transfer. This study has immense significance as it provides crucial insights into the design and application of heterostructures, which can be extended to various novel opportunities for progress and innovation.

Vacuum-Processed Metal Halide Perovskite Light-Emitting Diodes: Prospects and Challenges.

In less than a decade, organic-inorganic metal halide perovskites (MHPs) have shown tremendous progress in the field of light-emitting applications. Perovskite light-emitting diodes (PeLEDs) have reached external quantum efficiencies (EQE) exceeding 20 % and they have been recognized as a potential contender of the commercial display technologies. However, perovskite thin films in PeLEDs are generally deposited via a spin-coating process, which is not favourable for large area device fabrication. Despite the great success of solution-processed PeLEDs, very few articles have been reported on vacuum processed PeLEDs and the improvements in their optoelctronic performances are also progressing slowly. On the other hand, vacuum processing techniques are mostly used in organic LED technology as they can guarantee (i) the absence of solvent during thin-film growth, (ii) process scalability over large area substrates, and (iii) precise thin-film thickness control. This thin-film growth process is suitable for application in the advancement of a large variety of display technologies. In this Review, we present an overview of current research advances in the field of perovskite thin films grown via vacuum techniques, a study of their photophysical properties, and integration in PeLEDs for the generation of different colors. We also highlight the current challenges and future prospects for the further development of vacuum processed PeLEDs.

Exciton Relaxation Dynamics in Perovskite Cs4PbBr6 Nanocrystals.

Lower-dimensional metal halide perovskites have been recognized as an efficient white light emitter. The broad band emission spectrum originates from the recombination of excited charge carriers through free excitons (FEs), self-trapped excitons (STEs), and defect-trapped excitons. However, the emission properties of zero-dimensional (0-D) perovskites have not been explored extensively. Here, in this work, we have performed low-temperature absorbance, photoluminescence (PL), PL excitation (PLE), PL lifetime, and Raman measurements to understand the exciton relaxation processes in Cs4PbBr6 NCs. Our experimental observations indicate that two distinct UV light spectra evolved from the photoexcited carrier recombination through FE and STE states. We emphasize that such UV light sources can be beneficial for various applications, like curing of materials, disinfection of viruses, hygiene control, etc.

Broadband emission from zero-dimensional Cs4PbI6 perovskite nanocrystals.

To overcome the drawbacks in three-dimensional (3D) perovskites, such as instability, surface hydration, and ion migration, recently researchers have focused on comparatively stable lower-dimensional perovskite derivatives. All-inorganic zero-dimensional (0D) perovskites (e.g., Cs4PbX6; X = Cl-, Br-, I-) can be evolved as a high performing material due to their larger exciton binding energy and better structural stability. The clear understanding of carrier recombination process in 0D perovskites is very important for better exploitation in light-emitting devices. In this work, we comprehensively studied the light emission process in 0D Cs4PbI6 nanocrystals (NCs) and interestingly we observe intense white light emission at low temperatures. According to our experimental observations, we conclude that the white light emission contains an intrinsic exciton emission at 2.95 eV along with a broadband emission covering from 1.77 eV to 2.6 eV. We also confirm that the broadband emission is related to the carrier recombination of both self-trapped excitons (STE) and defect state trapped excitons. Our investigations reveal the carrier recombination processes in Cs4PbI6 NCs and provide experimental guidelines for the potential application of white light generation.

Cesium Oleate Passivation for Stable Perovskite Photovoltaics.

Despite their emergence as promising materials for low-cost and efficient energy power generation technology, the instability of hybrid organic-inorganic lead-halide perovskites toward moisture and heat stress remains a serious obstacle that needs to be tackled for commercialization. Here, we show improved moisture and thermal stability through the use of cesium oleate to modify the perovskite/hole transporting material interface. Passivation using cesium oleate does not induce the formation of any low-dimensional perovskites, suggesting that the organic species only passivate the perovskite's surface and grain boundaries. As a result, enhanced hydrophobic character of the perovskite film is realized upon passivation, evidenced by a large water contact angle of 107.4° and improved stability at ambient conditions (a relative humidity of ∼70%, room temperature). Concomitantly, the proposed passivation strategy leads to an increased amount of cesium concentration within the films, resulting in beneficial enhanced thermal stability of the film at 85 °C. By maintaining the three-dimensional (3D) structure of the solar absorber while concurrently passivating the interfacial defects and vacancies, improved open-circuit voltage (Voc) and unsacrificed short-circuit current density (Jsc) were obtained from the treated devices, leading to power conversion efficiencies of more than 18%. When stored in a humid environment (a relative humidity of ∼55%), devices with cesium oleate passivation maintain 88% of their initial power conversion efficiency after 720 h, degrading two times slower than those of the control. This work offers a strategy of coating 3D perovskites with a unique combination of inorganic cations and long-chain organics to provide hydrophobicity and moisture stability to the solar absorber layer while maintaining good device performances.

Inducing Isotropic Growth in Multidimensional Cesium Lead Halide Perovskite Nanocrystals.

A new two-step synthetic protocol to yield monodisperse spherical zero-dimensional (0D) Cs4 PbX6 nanocrystals (NCs) and three-dimensional (3D) CsPbX3 NCs is described. The first step of the reaction involves the colloidal synthesis of spherical PbX2 seed NCs, which are subsequently converted to Cs4 PbX6 and CsPbX3 NCs through hot injection of a Cs precursor at the desired reaction temperatures. By employing less reactive Pb and halide precursors, the reaction time was extended from several seconds to about five minutes, thereby allowing greater control during the crystallization and growth stages. The adjustment of halide ratios allows color tuning over a wide spectral range (411-669 nm) for CsPbX3 NCs, with high photoluminescence quantum yields (6-65 %) and narrow emission line widths (ca. 13-30 nm). We envisage our spherical NCs to become a starting point for shell growth (e.g., ZnS, CdS, PbS) by overcoming the difficulty of shell growth around thermodynamically unfavorable (i.e., high surface free energy) cuboid-shaped NCs.

Slow cooling and highly efficient extraction of hot carriers in colloidal perovskite nanocrystals.

Hot-carrier solar cells can overcome the Schottky-Queisser limit by harvesting excess energy from hot carriers. Inorganic semiconductor nanocrystals are considered prime candidates. However, hot-carrier harvesting is compromised by competitive relaxation pathways (for example, intraband Auger process and defects) that overwhelm their phonon bottlenecks. Here we show colloidal halide perovskite nanocrystals transcend these limitations and exhibit around two orders slower hot-carrier cooling times and around four times larger hot-carrier temperatures than their bulk-film counterparts. Under low pump excitation, hot-carrier cooling mediated by a phonon bottleneck is surprisingly slower in smaller nanocrystals (contrasting with conventional nanocrystals). At high pump fluence, Auger heating dominates hot-carrier cooling, which is slower in larger nanocrystals (hitherto unobserved in conventional nanocrystals). Importantly, we demonstrate efficient room temperature hot-electrons extraction (up to ∼83%) by an energy-selective electron acceptor layer within 1 ps from surface-treated perovskite NCs thin films. These insights enable fresh approaches for extremely thin absorber and concentrator-type hot-carrier solar cells.

Giant five-photon absorption from multidimensional core-shell halide perovskite colloidal nanocrystals.

Multiphoton absorption processes enable many technologically important applications, such as in vivo imaging, photodynamic therapy and optical limiting, and so on. Specifically, higher-order nonlinear absorption such as five-photon absorption offers significant advantages of greater spatial confinement, increased penetration depth, reduced autofluorescence, enhanced sensitivity and improved resolution over lower orders in bioimaging. Organic chromophores and conventional semiconductor nanocrystals are leaders in two-/three-photon absorption applications, but face considerable challenges from their small five-photon action cross-sections. Herein, we reveal that the family of halide perovskite colloidal nanocrystals transcend these constraints with highly efficient five-photon-excited upconversion fluorescence-unprecedented for semiconductor nanocrystals. Amazingly, their multidimensional type I (both conduction and valence band edges of core lie within bandgap of shell) core-shell (three-dimensional methylammonium lead bromide/two-dimensional octylammonium lead bromide) perovskite nanocrystals exhibit five-photon action cross-sections that are at least 9 orders larger than state-of-the-art specially designed organic molecules. Importantly, this family of halide perovskite nanocrystals may enable fresh approaches for next-generation multiphoton imaging applications.

Corrigendum: Slow cooling and highly efficient extraction of hot carriers in colloidal perovskite nanocrystals.

This corrects the article DOI: 10.1038/ncomms14350.

Highly stable, luminescent core-shell type methylammonium-octylammonium lead bromide layered perovskite nanoparticles.

A new protocol for the synthesis of a highly stable (over 2 months under ambient conditions) solution-processed core-shell type structure of mixed methylammonium-octylammonium lead bromide perovskite nanoparticles (5-12 nm), having spherical shape, color tunability in the blue to green spectral region (438-521 nm) and a high photoluminescence quantum yield (PLQY) of up to 92% is described. The color tunability, high PLQY and stability are due to the quantum confinement imparted by the crystal engineering associated with core-shell nanoparticle formation during growth.

Inorganic Halide Perovskites for Efficient Light-Emitting Diodes.

Lead-halide perovskites have transcended photovoltaics. Perovskite light-emitting diodes (PeLEDs) emerge as a new field to leverage on these fascinating semiconductors. Here, we report the first use of completely inorganic CsPbBr3 thin films for enhanced light emission through controlled modulation of the trap density by varying the CsBr-PbBr2 precursor concentration. Although pure CsPbBr3 films can be deposited from equimolar CsBr-PbBr2 and CsBr-rich solutions, strikingly narrow emission line (17 nm), accompanied by elongated radiative lifetimes (3.9 ns) and increased photoluminescence quantum yield (16%), was achieved with the latter. This is translated into the enhanced performance of the resulting PeLED devices, with lower turn-on voltage (3 V), narrow electroluminescence spectra (18 nm) and higher electroluminescence intensity (407 Cd/m(2)) achieved from the CsBr-rich solutions.

Light-emitting diodes based on solution-processed nontoxic quantum dots: oxides as carrier-transport layers and introducing molybdenum oxide nanoparticles as a hole-inject layer.

We report fabrication and characterization of solution-processed quantum dot light-emitting diodes (QDLEDs) based on a layer of nontoxic and Earth-abundant zinc-diffused silver indium disulfide (AIZS) nanoparticles as an emitting material. In the QDLEDs fabricated on indium tin oxide (ITO)-coated glass substrates, we use layers of oxides, such as graphene oxide (GO) and zinc oxide (ZnO) nanoparticles as a hole- and electron-transport layer, respectively. In addition, we introduce a layer of MoO3 nanoparticles as a hole-inject one. We report a comparison of the characteristics of different device architectures. We show that an inverted device architecture, ITO/ZnO/AIZS/GO/MoO3/Al, yields a higher electroluminescence (EL) emission, compared to direct ones, for three reasons: (1) the GO/MoO3 layers introduce barriers for electrons to reach the Al electrode, and, similarly, the ZnO layers acts as a barrier for holes to travel to the ITO electrode; (2) the introduction of a layer of MoO3 nanoparticles as a hole-inject layer reduces the barrier height for holes and thereby balances charge injection in the inverted structure; and (3) the wide-bandgap zinc oxide next to the ITO electrode does not absorb the EL emission during its exit from the device. In the QDLEDs with oxides as carrier inject and transport layers, the EL spectrum resembles the photoluminescence emission of the emitting material (AIZS), implying that excitons are formed in the quaternary nanocrystals and decay radiatively.