Stabilizing Cesium Lead Halide Perovskite Lattice through Mn(II) Substitution for Air-Stable Light-Emitting Diodes

Improved ultraviolet radiation stability of Mn2+-doped CsPbCl3 nanocrystals via B-site Sn doping

A facile and effective B-site Sn doping strategy is used to grow Mn²⁺-doped CsPbCl₃ (Mn:CsPbCl₃) nanocrystals with enhanced ultraviolet irradiation stability and luminescence properties.

Thermally stable and hydrophilic CsPbBr3/mPEG-NH2 nanocrystals with enhanced aqueous fluorescence for cell imaging

Hydrophilic CsPbBr₃/mPEG-NH₂ nanocrystals with enhanced fluorescence for cell imaging.

Luminescent perovskites: recent advances in theory and experiments

This review summarizes previous research on luminescent perovskites, including oxides and halides, with different structural dimensionality. The relationship between the crystal structure, electronic structure and properties is discussed in detail.

Highly Stable Silica-Wrapped Mn-Doped CsPbCl3 Quantum Dots for Bright White Light-Emitting Devices

As an outstanding less-Pb candidate, doping Mn²⁺ ions into perovskite quantum dots (QDs) has received significant interest in the application of light-emitting diodes (LEDs). However, their further applications are impeded by poor chemical instability. Here, the silica-wrapped Mn-doped CsPbCl₃ QDs are fabricated via hydrolyzing (3-aminopropyl) triethoxysilane with improved operational stability. Also, the photoluminescence quantum yield as high as 55.4% for the CsPbMnCl₃@SiO₂ composite is achieved. Silica wrapping can protect the perovskite QDs from damage by temperature and humidity as well as anion exchange. Furthermore, white LED devices are prepared by employing the mixture of green CsPbBr₃ QDs and orange-red CsPbMnCl₃@SiO₂ composites. The as-obtained white LED device operated at a forward current of 20 mA exhibits bright natural light with a high luminous efficiency of 77.59 lm/W, and the corresponding color rendering index of 82 and color temperature (CCT) of 3950 K are obtained. Additionally, the electroluminescence spectrum shows nearly no variation after 24 h operation. This work will promote the Mn-doped CsPbCl₃ QDs material to the practical application in solid-state LEDs.

Rational chemical doping of metal halide perovskites

Metal halide perovskites benefit from the combination of wide absorption, high carrier mobility, defect tolerance, moderate exciton binding energies, and versatility of solution processes, showing great promise in photovoltaics and optoelectronics. However, the issues of long-term instability and toxicity of lead are supposed to limit their further practical applications. Chemical doping of an impurity into metal halide perovskites was reported to be a relatively effective approach to solving these issues while providing additional tunable physical and chemical properties. In an attempt to boost the research field further, it is imperative to summarize the recent significant work on metal halide doped perovskites, disclosing the underlying structure-property relationships to provide useful insights into applications of these perovskites with high performance. In this review, we highlight the rational design of doped perovskites by both theoretical and experimental efforts as well as their potential application spanning various fields.

Improved stability of CsPbBr3 perovskite quantum dots achieved by suppressing interligand proton transfer and applying a polystyrene coating

All-inorganic lead halide perovskite quantum dots (PQDs) have shown great promise for optoelectronic applications due to their unique optical and electrical properties. However, their poor stability against moisture, UV light or thermal attacks greatly hinders their practical applications. One of the reasons for this instability is the proton transfer between oleic acid (OA) and oleylamine (OLA), which induces serious ligand loss. The idea of this work is to prevent the interligand proton transfer by replacing OLA with cetyl trimethylammonium bromide (CTAB) that cannot be protonated. The synthesized CsPbBr₃ PQDs in solution show high photoluminescence quantum yields up to 71% and also exhibit higher stability against acetone than the counterparts synthesized using oleylammonium bromide (OLABr). Subsequently, CsPbBr₃ PQDs with a peak wavelength of 515 nm (g-CsPbBr₃) synthesized using CTAB are further composited with a polymer of carboxyl-functionalized polystyrene (cPS). The composite shows enhanced thermal and moisture stability. It is demonstrated that this green-emitting composite can produce high color gamut (130%) white light-emitting diodes when combined with the K₂SiF₆:Mn⁴⁺ phosphor and a blue InGaN chip, enabling its use in display backlights.

Simultaneous Strontium Doping and Chlorine Surface Passivation Improve Luminescence Intensity and Stability of CsPbI3 Nanocrystals Enabling Efficient Light-Emitting Devices

A method is proposed to improve the photo/electroluminescence efficiency and stability of CsPbI₃ perovskite nanocrystals (NCs) by using SrCl₂ as a co-precursor. The SrCl₂ is chosen as the dopant to synthesize the CsPbI₃ NCs. Because the ion radius of Sr²⁺ (1.18 Å) is slightly smaller than that of Pb²⁺ (1.19 Å) ions, divalent Sr²⁺ cations can partly replace the Pb²⁺ ions in the lattice structure of perovskite NCs and cause a slight lattice contraction. At the same time, Cl^- anions from SrCl₂ are able to efficiently passivate surface defect states of CsPbI₃ nanocrystals, thus converting nonradiative trap states to radiative states. The simultaneous Sr²⁺ ion doping and surface Cl^- ion passivation result in the enhanced photoluminescence quantum yield (up to 84%), elongated emission lifetime, and improved stability. Sr²⁺ -doped CsPbI₃ NCs are employed to produce light-emitting devices with a high external quantum yield of 13.5%.

Impurity Ions Codoped Cesium Lead Halide Perovskite Nanocrystals with Bright White Light Emission toward Ultraviolet-White Light-Emitting Diode

White light-emitting diodes (WLEDs) based on all-inorganic perovskite CsPbX₃ (X = Cl, Br, I) nanocrystals (NCs) have attracted extensive interests. However, the native ion exchange among halides makes them extremely difficult to realize the white emission. Herein, we demonstrate a novel strategy to obtain WLED phosphors based on the codoping of different metal ion pairs, such as Ce³⁺/Mn²⁺, Ce³⁺/Eu³⁺, Ce³⁺/Sm³⁺, Bi³⁺/Eu³⁺, and Bi³⁺/Sm³⁺ into stable CsPbCl₃ and CsPbCl _xBr_{3- x} NCs. Notably, by the typical anion exchange reaction, the highly efficient white emission of Ce³⁺/Mn²⁺-codoped all-inorganic CsPbCl_1.8Br_1.2 perovskite NCs was achieved, with an optimal photoluminescence quantum yield of 75%, which is much higher than the present record of 49% for single perovskite phosphors. Moreover, the WLED with a luminous efficiency of 51 lm/W based on the 365 nm ultraviolet chip and CsPbCl_1.8Br_1.2:Ce³⁺/Mn²⁺ nanophosphor was achieved. This work represents a novel device for perovskite-based phosphor-converted WLEDs.

Enhanced luminescence and energy transfer in Mn2+ doped CsPbCl3-xBrx perovskite nanocrystals

Manganese ion (Mn²⁺) doped CsPbCl_3-xBr_x nanocrystals (NCs) with dual emissions have emerged as a promising candidate for white light emitting devices. The luminescence properties of Mn²⁺ doped CsPbCl_3-xBr_x NCs were studied by steady-state and time-resolved photoluminescence (PL) spectroscopy at temperature ranging from 80 to 300 K. The Mn²⁺ doped NCs with varied Br compositions were synthesized by means of Cl-to-Br anion exchange. The obtained doped NCs exhibited a tunable narrow band-edge emission band from 405 to 500 nm with a lengthened PL lifetime and a wide Mn²⁺ emission band at 600 nm with a shortened PL lifetime as the Br composition increases. It was interestingly found that PL intensity of the Mn²⁺ emission band was significantly enhanced by replacing Cl ions with Br and reached the maximum value in Mn²⁺:CsPbCl_2.15Br_0.85 NCs. Especially at low temperature (80 K), the intensity ratios of Mn²⁺ emission to band-edge emission increased ∼80 times in Mn²⁺:CsPbCl_2.15Br_0.85 NCs, compared with Mn²⁺:CsPbCl₃ ones. The temperature-dependent energy-transfer efficiencies of Mn²⁺:CsPbCl₃ and Mn²⁺:CsPbCl_2.15Br_0.85 NCs from excitons in NCs to Mn²⁺ ions were obtained. The PL enhancement of Mn²⁺ in CsPbCl_3-xBr_x NCs was attributed to Br composition-dependent exciton energy transfer to Mn²⁺.

Room temperature precipitated dual phase CsPbBr3-CsPb2Br5 nanocrystals for stable perovskite light emitting diodes

Although the efficiency of metal halide perovskite light emitting diodes (PeLEDs) has been improved to an attractive level, the poor stability of perovskite emitting layers is a major concern for the application of PeLEDs. Herein, we report a facile ligand-assisted precipitation synthesis of stable dual-phase CsPbBr₃-CsPb₂Br₅ nanocrystals (NCs) for improving the stability of PeLEDs. In our synthetic process, the bromide-rich circumstance is beneficial to generate high quality dual-phase perovskite NCs with PLQY as high as 92% and a narrow emission linewidth (19 nm). More importantly, as-synthesized dual phase perovskite NCs exhibit extremely high thermal stability in heating tests in air with a considerable humidity of 30%-55% in comparison with previously reported single phase CsPbBr₃ NCs. The aforementioned advantages of our synthesized dual-phase CsPbBr₃-CsPb₂Br₅ NCs allow for the fabrication of light emitting layers of PeLEDs under ambient conditions. The fabricated green PeLED based on CsPbBr₃-CsPb₂Br₅ NCs shows a low turn-on voltage of 2.5 V and a high brightness of 8383 cd m^-2 at 8 V. Owing to the high stability of dual-phase CsPbBr₃-CsPb₂Br₅ NCs, the fabricated PeLED also exhibits better operational stability in comparison with those PeLEDs based on single phase CsPbBr₃ NCs. Our work presents a new route to fabricate stable perovskite light-emitting diodes using room temperature precipitated dual-phase CsPbBr₃-CsPb₂Br₅ NCs as emitting layer materials.

Template-Free Synthesis of High-Yield Fe-Doped Cesium Lead Halide Perovskite Ultralong Microwires with Enhanced Two-Photon Absorption

Doping in perovskite is challenging and competitive due to the inherently fast growth mechanism of perovskite structure. Here, we demonstrate successful synthesis of high-yield Fe-doped cesium lead halide perovskite ultralong microwires (MWs) that have diameters up to ∼5 μm and lengths up to millimeters via an antisolvent vapor-assisted template-free method. Microstructure characterization confirms the uniformly doped Fe in the high-quality crystal perovskite MWs. Significantly, doping the Fe(III) concentration can affect both the MW morphology and photoluminescence (PL). The band edge emission of the MW at variable excitation has been accounted for by the superposition and combination of optical transitions of nearby singlet, triplet, and magnetic polaronic excitons. High-quality two-photon PL emission and the enhanced nonlinear absorption coefficient of Fe-doped MWs have been experimentally demonstrated. This superhigh nonlinear absorption coefficient and high-quality optical properties endow it with promising applications in spin-related optical switching and optical limiting devices.

Yb3+ and Yb3+/Er3+ Doping for Near-Infrared Emission and Improved Stability of CsPbCl3 Nanocrystals

Lead halide perovskite nanocrystals (NCs) exhibit excellent tunable emissions covering the entire visible spectral region, but they do not emit near-infrared (NIR) light. We synthesized rare earth element doped CsPbCl3 NCs for NIR emission. The Yb3+ doped CsPbCl3 NCs emitted strong 986 nm NIR light; the Yb3+/Er3+ co-doped CsPbCl3 NCs emitted at 1533 nm. The total photoluminescence quantum yield (PL QY) of the CsPbCl3 NCs changed from 5.0% to 127.8% upon incorporating 2.0% Yb3+, a factor of 25.6 times enhancement. The material's stability was tested under continuous ultraviolet (365 nm) irradiation. The doped CsPbCl3 NCs exhibited a better stability than the undoped one. The PL intensity of the undoped CsPbCl3 NCs dropped to 20% of the initial value in 27 h, while the doped one took 85 h.

Methodologies toward Efficient and Stable Cesium Lead Halide Perovskite-Based Solar Cells

In an attempt to replace thermally vulnerable organic perovskites, considerable research effort has recently been focused on the development of all-inorganic perovskites in the field of photovoltaics. The preceding studies demonstrated that cesium lead halide perovskites are promising candidates for thermally stable and efficient solar cell materials. Here, the recent progress in cesium lead halide perovskite-based solar cells is summarized. Whether organic cations are essential for the superiority of halide perovskites is controversial. However, more than 13% efficient solar cells have been successfully fabricated by employing cesium lead halide perovskites in a short amount of time. The state-of-the-art materials engineering techniques will help to achieve a remarkable photovoltaic performance comparable to that of organic perovskites. In addition, improved understanding of the intrinsic photophysical behaviors will provide new insights that will facilitate further improvements in solar cell applications.

Confining Mn2+-Doped Lead Halide Perovskite in Zeolite-Y as Ultrastable Orange-Red Phosphor Composites for White Light-Emitting Diodes

CsPbX₃ (X = Cl, Br, I) perovskite quantum dots (QDs) have emerged as competitive candidate luminescent materials in the photoelectric fields due to their superior luminescence properties. However, the major drawback such as poor resistance to temperature, moisture, and irradiation of light, especially for the red QDs with I^-, hinders their practical applications. Herein, we synthesized Mn²⁺-doped CsPbCl₃ embedded in the cage of zeolite-Y as a new orange-red phosphor for the white light-emitting diode (WLED). The composites have significantly improved resistance to both elevated temperature and water over the bare Mn²⁺-doped QDs. The former exhibits little degradation whereas the latter shows apparent decline upon the irradiation of lights in the orange LED devices, which are fabricated by employing each material as a color-conversion phosphor coated on a 365 nm UV chip. A WLED is also achieved with a 365 nm UV chip coated with a CsPb(Cl_0.5,Br_0.5)₃-Y blue phosphor and a CsPb_0.75Mn_0.25Cl₃-Y orange phosphor. The device possesses a Commission Internationale de l'Éclairage coordinate of (0.34, 0.36), a correlated color temperature of 5336 K and a color rendering index of 81.

Highly efficient and stable blue-emitting CsPbBr3@SiO2 nanospheres through low temperature synthesis for nanoprinting and WLED

Inorganic perovskite quantum dots (QDs) have attracted wide attention in display and solid-state lighting because of their easily tunable band-gaps and high photoluminescence quantum yields (PLQY) of green light emission. However, some drawbacks limit their practical applications, including the low PLQY of blue light emission and the instability in the moisture environment. In this work, efficient blue-light emitting CsPbBr₃ perovskite QDs with PLQY of 72% were developed through a bandgap engineering approach. The achieved blue-light emitting PLQY is much higher than the values acquired in the inorganic perovskite QDs in the literature. And the emission color of the as-prepared QDs can be facially tuned by only adjusting the reaction temperature. Further, the mono-dispersed perovskite QDs@SiO₂ composites were constructed benefiting from the low temperature synthesis. The optical performance of the QDs could be well persisted even in the moisture environment. Finally, the as-prepared QDs@SiO₂ composite was fabricated as the QD ink on the anti-counterfeit printing technology, from which the obtained pattern would emit varied color under UV lamp. And the as-prepared composites was also applied for fabricating WLED, with Commission Internationale de l'Eclairage (CIE) color coordinates of (0.33, 0.38) and power efficiency of 32.5 lm W^-1, demonstrating their promising potentials in solid-state lighting.

Spontaneous Silver Doping and Surface Passivation of CsPbI3 Perovskite Active Layer Enable Light-Emitting Devices with an External Quantum Efficiency of 11.2

Lead halide perovskite nanocrystals are currently under intense investigation as components of solution-processed light-emitting devices (LEDs). We demonstrate LEDs based on Ag doped-passivated CsPbI3 perovskite nanocrystals with external quantum efficiency of 11.2% and an improved stability. Ag and trilayer MoO3/Au/MoO3 structure were used as cathode and anode, respectively, which reduce the electron injection barrier and ensure the high transparency and low resistance of the anode. Silver ions diffuse into perovskite film from the Ag electrode, as confirmed by the elemental mapping, the presence of Ag 3d peaks in the X-ray photoelectron spectrum, and the peak shift in the X-ray diffraction patterns of CsPbI3. In addition to doping, silver ions play the beneficial role of passivating surface defect states of CsPbI3 nanocrystals, which results in increased photoluminescence quantum yield, elongated emission lifetime, and improved stability of perovskite films.

MnII-Doped Cesium Lead Chloride Perovskite Nanocrystals: Demonstration of Oxygen Sensing Capability Based on Luminescent Dopants and Host-Dopant Energy Transfer

The design of photoluminescence-quenching probes for molecular oxygen (O₂) is always a large space to explore. Luminescent semiconductor nanocrystals (NCs) have been proposed as emerging oxygen-responsive probes, but the inherent O₂ sensing of phosphorescent semiconductor NCs has not been reported so far. Here, we demonstrate the O₂ sensing capability of Mn^II-doped CsPbCl₃ nanocrystals (Mn:CsPbCl₃ NCs) and reveal the role of O₂ on the optical de-excitation process of such perovskite nanocrystals (PNCs). By adjusting the amount and distribution of Mn^II dopants, as well as the host-dopant energy transfer process in PNCs, we highlight that O₂ can reversibly quench the Mn^II emission due to the temporary disturbance to the ligand field of near-surface Mn^II dopants in PNCs. In phosphorescence mode, the photoluminescence intensity of the Mn:CsPbCl₃ NCs is quenched by 53% on increasing O₂ concentration from 0 to 100%. The Stern-Volmer plot shows a good linear in the 0-12% O₂ concentration range. High sensing reversibility and rapid signal response are also achieved. In our perception, the mechanism study makes our PNCs candidates for the optical probes of O₂, and it is enlightening to explore more possibilities of the inherent O₂ sensing based on the semiconductor-doped NCs (not restricted to Mn^II-doped PNCs) with phosphorescence emission.

Humidity-Induced Photoluminescence Hysteresis in Variable Cs/Br Ratio Hybrid Perovskites

Hybrid organic-inorganic perovskites containing Cs are a promising new material for light-absorbing and light-emitting optoelectronics. However, the impact of environmental conditions on their optical properties is not fully understood. Here, we elucidate and quantify the influence of distinct humidity levels on the charge carrier recombination in Cs _xFA_{1- x}Pb(I _yBr_{1- y})₃ perovskites. Using in situ environmental photoluminescence (PL), we temporally and spectrally resolve light emission within a loop of critical relative humidity (rH) levels. Our measurements show that exposure up to 35% rH increases the PL emission for all Cs (10-17%) and Br (17-38%) concentrations investigated here. Spectrally, samples with larger Br concentrations exhibit PL redshift at higher humidity levels, revealing water-driven halide segregation. The compositions considered present hysteresis in their PL intensity upon returning to a low-moisture environment due to partially reversible hydration of the perovskites. Our findings demonstrate that the Cs/Br ratio strongly influences both the spectral stability and extent of light emission hysteresis. We expect our method to become standard when testing the stability of emerging perovskites, including lead-free options, and to be combined with other parameters known for affecting material degradation, e.g., oxygen and temperature.

Solvent-Assisted Surface Engineering for High-Performance All-Inorganic Perovskite Nanocrystal Light-Emitting Diodes

All-inorganic cesium halide perovskite nanocrystals have attracted much interest in optoelectronic applications for the sake of the readily adjustable band gaps, high photoluminescence quantum yield, pure color emission, and affordable cost. However, because of the ineluctable utilization of organic surfactants during the synthesis, the structural and optical properties of CsPbBr₃ nanocrystals degrade upon transforming from colloidal solutions to solid thin films, which plagues the device operation. Here, we develop a novel solvent-assisted surface engineering strategy, producing high-quality CsPbBr₃ thin films for device applications. A good solvent is first introduced as an assembly trigger to conduct assembly in a one-dimensional direction, which is then interrupted by adding a nonsolvent. The nonsolvent drives the adjacent nanoparticles connecting in a two-dimensional direction. Assembled CsPbBr₃ nanocrystal thin films are densely packed and very smooth with a surface roughness of ∼4.8 nm, which is highly desirable for carrier transport in a light-emitting diode (LED) device. Meanwhile, the film stability is apparently improved. Benefiting from this facile and reliable strategy, we have achieved remarkably improved performance of CsPbBr₃ nanocrystal-based LEDs. Our results not only enrich the methods of nanocrystal surface engineering but also shed light on developing high-performance LEDs.

Dendrimer ligands-capped CH3NH3PbBr3 perovskite nanocrystals with delayed halide exchange and record stability against both moisture and water

CH₃NH₃PbBr₃ perovskite nanocrystals (NCs) suffer from poor stability because of their high sensitivity to environmental moisture and water. To solve this problem, previous works mainly focus on embedding perovskite NCs into water-resistant matrix to form large composites (size of microns or larger). As an alternative solution without serious changing of NC size, enhancing the stability of perovskite NCs themselves by ligand engineering is rarely reported. In this work, we used hyperbranched polyamidoamine (PAMAM) dendrimers with two different generations (G0 and G4) to synthesize CH₃NH₃PbBr₃ perovskite NCs with high photoluminescence (PL) quantum yields (QY) above 70% and a new record stability. A novel dendrimers generation-dependent stability of perovskite NCs was observed. The water-resistance time is 18 h (27 h) for perovskite NCs capped by G0 (G4) generation of PAMAM, which is 7 times (11 times) longer than that of traditional oleic acid-capped NCs. Similar PAMAM generation-related stability is also observed in moisture-resistance tests. The stability time against moisture is 500 h (800 h) for G0 (G4) generation of PAMAM-capped perovskite NCs, which is a new record stability time against moisture for CH₃NH₃PbBr₃ perovskite NCs. In addition, our results also indicate that PAMAM ligands outside perovskite NCs can dramatically slow down the speed of halide exchange. Even for the mixture of perovskite NCs with two different halide composition, the original luminescence properties of PAMAM-capped perovskite NCs can retain after mixing. In view of slow halide exchange speed, excellent water and moisture stability, PAMAM dendrimers-capped perovskite NCs and their mixture are available as color conversion single layer in fabrication of light-emitting diodes (LED).

All-inorganic perovskite quantum dot/TiO2 inverse opal electrode platform: stable and efficient photoelectrochemical sensing of dopamine under visible irradiation

CsPbX3 (X = Cl, Br or I) perovskite quantum dots (PQDs) have attracted tremendous attention due to their extraordinarily excellent optical properties. However, there is still an obstacle for their bio-application, which is limited by their water-instability. In this work, we have designed a novel visible light triggered photoelectrochemical (PEC) sensor for dopamine (DA) based on CsPbBr1.5I1.5 PQD immobilized three-dimensional (3D) TiO2 inverse opal photonic crystals (IOPCs). Supported by the TiO2 IOPCs, the water-stability of the PQDs as well as that of the PEC sensor was considerably improved. Furthermore, employed as a photoactive material in PEC sensor, CsPbBr1.5I1.5 PQDs can expand the photocurrent response of the PEC sensor to the whole visible region. In addition, the modulation of the photonic stop band effect of TiO2 IOPCs on the incident light and the emission of PQDs could further enhance the photocurrent response. Such a PEC sensor demonstrates sensitive detection of DA in phosphate buffer saline solution and serum, with a good linear range from 0.1 μM to 250 μM and a low detection limit of approximately 0.012 μM. Our strategy opens an alternative horizon for PQD based PEC sensing, which could be more sensitive, convenient and inexpensive for clinical and biological analysis.

Genesis, challenges and opportunities for colloidal lead halide perovskite nanocrystals

Lead halide perovskites (LHPs) in the form of nanometre-sized colloidal crystals, or nanocrystals (NCs), have attracted the attention of diverse materials scientists due to their unique optical versatility, high photoluminescence quantum yields and facile synthesis. LHP NCs have a 'soft' and predominantly ionic lattice, and their optical and electronic properties are highly tolerant to structural defects and surface states. Therefore, they cannot be approached with the same experimental mindset and theoretical framework as conventional semiconductor NCs. In this Review, we discuss LHP NCs historical and current research pursuits, challenges in applications, and the related present and future mitigation strategies explored.

High Br- Content CsPb(Cl yBr1- y)3 Perovskite Nanocrystals with Strong Mn2+ Emission through Diverse Cation/Anion Exchange Engineering

The unification of tunable band edge (BE) emission and strong Mn²⁺ doping luminescence in all-inorganic cesium lead halide perovskite nanocrystals (NCs) CsPbX₃ (X = Cl and Br) is of fundamental importance in fine tuning their optical properties. Herein, we demonstrate that benefiting from the differentiation of the cation/anion exchange rate, ZnBr₂ and preformed CsPb_{1- x}Cl₃: xMn²⁺ NCs can be used to obtain high Br^- content Cs(Pb_{1- x- z}Zn _z)(Cl _yBr_{1- y})₃: xMn²⁺ perovskite NCs with strong Mn²⁺ emission, and the Mn²⁺ substitution ratio can reach about 22%. More specifically, the fast anion exchange could be realized by the soluble halide precursors, leading to anion exchange within a few seconds as observed from the strong BE emission evolution, whereas the cation exchange instead generally required at least a few hours; moreover, their exchange mechanism and dynamics process have been evaluated. The Mn²⁺ emission intensity could be further varied by controlling the replacement of Mn²⁺ by Zn²⁺ with prolonged ion exchange reaction time. White light emission of the doped perovskite NCs via this cation/anion synergistic exchange strategy has been realized, which was also successfully demonstrated in a prototype white light-emitting diode (LED) device based on a commercially available 365 nm LED chip.

Lead-Free Perovskite Nanocrystals for Light-Emitting Devices

Lead halide perovskites with nanoscale geometries have received recent attention due to the defect-tolerant high photoluminescence quantum yield at tunable emission wavelengths and the possibility of room-temperature synthesis that does not compromise the physical properties of the materials. These characteristics offer opportunities to advance displays that cover the widest perceivable color. However, lead toxicity obstructs the commercialization of this technology. Therefore, recent efforts have investigated lead-free halide perovskite nanocrystals. Here, we provide our perspectives on the most exciting achievements in the materials design and photophysical properties of lead-free perovskite nanocrystals, particularly for applications in light-emitting devices. This Perspective includes a short summary on the characteristic features of halide perovskite nanocrystals; discussion on the candidate elements to replace lead; methods to prepare colloidal lead-free perovskite nanocrystals; methods to control and enhance the optical properties; a recent demonstration of utilizing lead-free perovskite nanocrystals in light-emitting devices; and an outlook on the field.

Benzoyl Halides as Alternative Precursors for the Colloidal Synthesis of Lead-Based Halide Perovskite Nanocrystals

We propose here a new colloidal approach for the synthesis of both all-inorganic and hybrid organic-inorganic lead halide perovskite nanocrystals (NCs). The main limitation of the protocols that are currently in use, such as the hot injection and the ligand-assisted reprecipitation routes, is that they employ PbX₂ (X = Cl, Br, or I) salts as both lead and halide precursors. This imposes restrictions on being able to precisely tune the amount of reaction species and, consequently, on being able to regulate the composition of the final NCs. In order to overcome this issue, we show here that benzoyl halides can be efficiently used as halide sources to be injected in a solution of metal cations (mainly in the form of metal carboxylates) for the synthesis of APbX₃ NCs (in which A = Cs⁺, CH₃NH₃⁺, or CH(NH₂)₂⁺). In this way, it is possible to independently tune the amount of both cations and halide precursors in the synthesis. The APbX₃ NCs that were prepared with our protocol show excellent optical properties, such as high photoluminescence quantum yields, low amplified spontaneous emission thresholds, and enhanced stability in air. It is noteworthy that CsPbI₃ NCs, which crystallize in the cubic α phase, are stable in air for weeks without any postsynthesis treatment. The improved properties of our CsPbX₃ perovskite NCs can be ascribed to the formation of lead halide terminated surfaces, in which Cs cations are replaced by alkylammonium ions.

Benzoyl Halides as Alternative Precursors for the Colloidal Synthesis of Lead-Based Halide Perovskite Nanocrystals

We propose here a new colloidal approach for the synthesis of both all-inorganic and hybrid organic-inorganic lead halide perovskite nanocrystals (NCs). The main limitation of the protocols that are currently in use, such as the hot injection and the ligand-assisted reprecipitation routes, is that they employ PbX2 (X = Cl, Br, or I) salts as both lead and halide precursors. This imposes restrictions on being able to precisely tune the amount of reaction species and, consequently, on being able to regulate the composition of the final NCs. In order to overcome this issue, we show here that benzoyl halides can be efficiently used as halide sources to be injected in a solution of metal cations (mainly in the form of metal carboxylates) for the synthesis of APbX3 NCs (in which A = Cs+, CH3NH3+, or CH(NH2)2+). In this way, it is possible to independently tune the amount of both cations and halide precursors in the synthesis. The APbX3 NCs that were prepared with our protocol show excellent optical properties, such as high photoluminescence quantum yields, low amplified spontaneous emission thresholds, and enhanced stability in air. It is noteworthy that CsPbI3 NCs, which crystallize in the cubic α phase, are stable in air for weeks without any postsynthesis treatment. The improved properties of our CsPbX3 perovskite NCs can be ascribed to the formation of lead halide terminated surfaces, in which Cs cations are replaced by alkylammonium ions.

Boosting the efficiency of inverted quantum dot light-emitting diodes by balancing charge densities and suppressing exciton quenching through band alignment

We report an inverted and multilayer quantum dot light emitting diode (QLED) which boosts high efficiency by tuning the energy band alignment between charge transport and light emitting layers. The electron transport layer (ETL) was ZnO nanoparticles (NPs) with an optimized doping concentration of cesium azide (CsN₃) to effectively reduce electron flow and balance charge injection. This is by virtue of a 0.27 eV upshift of the ETL's conduction band edge, which inhibits the quenching of excitons and preserves the superior emissive properties of the quantum dots due to the insulating characteristics of CsN₃. The demonstrated QLED exhibits a peak current efficiency, power efficiency and external quantum efficiency of up to 13.5 cd A^-1, 10.6 lm W^-1 and 13.4% for the red QLED, and correspondingly 43.1 cd A^-1, 33.6 lm W^-1 and 9.1% for green, and 4.1 cd A^-1, 2.0 lm W^-1 and 6.6% for the blue counterparts. Compared with QLEDs without optimization, the performance of these modified devices shows drastic improvement by 95.6%, 39.4% and 36.7%, respectively. This novel device architecture with heterogeneous energy levels reported here offers a new design strategy for next-generation high efficiency QLED displays and solid-state lighting technologies.

CsPbX3/Cs4PbX6 core/shell perovskite nanocrystals

The synthesis of 3D/0D core/shell lead halide perovskite nanocrystals has been realized using the seeded growth approach for the first time.

New complex phosphates Cs3MIIBi(P2O7)2 (MII – Ca, Sr and Pb): synthesis, characterization, crystal and electronic structure

New phosphates Cs₃M^IIBi(P₂O₇)₂ (M^II – Ca, Sr and Pb) with the original framework topology are discussed as prospective hosts for luminescent ions.

Trap state passivation and photoactivation in wide band gap inorganic perovskite semiconductors

CsPbCl₃ is a promising material to construct future short wavelength optoelectronic devices based on inorganic perovskite semiconductors.

Mass production of Mn2+-doped CsPbCl3 perovskite nanocrystals with high quality and enhanced optical performance

A novel microwave-assisted hot injection strategy for mass production of high quality Mn²⁺:CsPbCl₃ NCs was reported.

Interfacial Synthesis of Highly Stable CsPbX3/Oxide Janus Nanoparticles

The poor stability of CsPbX₃ (X = Cl, Br, I) nanocrystals (NCs) has severely impeded their practical applications. Although there are some successful examples on encapsulating multiple CsPbX₃ NCs into an oxide or polymer matrix, it has remained a serious challenge for the surface modification/encapsulation using oxides or polymers at a single particle level. In this work, monodisperse CsPbX₃/SiO₂ and CsPbBr₃/Ta₂O₅ Janus nanoparticles were successfully prepared by combining a water-triggered transformation process and a sol-gel method. The CsPbBr₃/SiO₂ NCs exhibited a photoluminescence quantum yield of 80% and a lifetime of 19.8 ns. The product showed dramatically improved stability against destruction by air, water, and light irradiation. Upon continuous irradiation by intense UV light for 10 h, a film of the CsPbBr₃/SiO₂ Janus NCs showed only a slight drop (2%) in the PL intensity, while a control sample of unmodified CsPbBr₃ NCs displayed a 35% drop. We further highlighted the advantageous features of the CsPbBr₃/SiO₂ NCs in practical applications by using them as the green light source for the fabrication of a prototype white light emitting diode, and demonstrated a wide color gamut covering up to 138% of the National Television System Committee standard. This work not only provides a novel approach for the surface modification of individual CsPbX₃ NCs but also helps to address the challenging stability issue; therefore, it has an important implication toward their practical applications.

Improving the Stability and Size Tunability of Cesium Lead Halide Perovskite Nanocrystals Using Trioctylphosphine Oxide as the Capping Ligand

Recently, all-inorganic cesium lead halide (CsPbX₃, X = Cl, Br, and I) nanocrystals (NCs) have drawn wide attention because of their excellent optoelectronic properties and potential applications. However, one of the most significant challenges of such NCs is their low stability against protonic solvents. In this work, we demonstrate that by incorporating a highly branched capping ligand, trioctylphosphine oxide (TOPO), into the traditional oleic acid/oleylamine system, monodisperse CsPbX₃ NCs with excellent optoelectronic properties can be achieved at elevated temperatures (up to 260 °C). The size of such NCs can be varied in a relatively wide range. The capping of TOPO on NCs has been verified through Fourier transform infrared spectroscopy measurement. More importantly, the presence of TOPO can dramatically improve the stability of CsPbX₃ NCs against ethanol treatment. After ethanol treatment for 100 min, the emission intensity of the TOPO-capped sample dropped only 5%, whereas that of non-TOPO-capped NCs dropped up to 86%. This work may shed some light on the preparation and application of CsPbX₃ NCs with higher stability.

Effects of Organic Cation Length on Exciton Recombination in Two-Dimensional Layered Lead Iodide Hybrid Perovskite Crystals

In recent years, 2D layered organic-inorganic lead halide perovskites have attracted considerable attention due to the distinctive quantum confinement effects as well as prominent excitonic luminescence. Herein, we show that the recombination dynamics and photoluminescence (PL) of the 2D layered perovskites can be tuned by the organic cation length. 2D lead iodide perovskite crystals with increased length of the organic chains reveal blue-shifted PL as well as enhanced relative internal quantum efficiency. Furthermore, we provide experimental evidence that the formation of face-sharing [PbI₆]^4- octahedron in perovskites with long alkyls induces additional confinement for the excitons, leading to 1D-like recombination. As a result, the PL spectra show enhanced inhomogeneous broadening at low temperature. Our work provides physical understanding of the role of organic cation in the optical properties of 2D layered perovskites, and would benefit the improvement of luminescence efficiency of such materials.

Fluorescent Alloy CsPb x Mn1-x I3 Perovskite Nanocrystals with High Structural and Optical Stability

CsPbI₃ nanocrystals are still limited in their use because of their phase instability as they degrade into the yellow nonemitting δ-CsPbI₃ phase within a few days. We show that alloyed CsPb _x Mn_1-x I₃ nanocrystals have essentially the same optical features and crystal structure as the parent α-CsPbI₃ system, but they are stable in films and in solution for periods over a month. The stabilization stems from a small decrease in the lattice parameters slightly increasing the Goldsmith tolerance factor, combined with an increase in the cohesive energy. Finally, hybrid density functional calculations confirm that the Mn²⁺ levels fall within the conduction band, thus not strongly altering the optical properties.