Cu2+-Doped CsPbI3 Nanocrystals with Enhanced Stability for Light-Emitting Diodes

Surface Doping to Suppress Iodine Ion Migration for Stable FAPbI3 Perovskite Quantum Dot Solar Cells

Formamidine lead iodide (FAPbI3) quantum dots (QDs) have attracted great attention as a new generation of photovoltaic material due to their long carrier diffusion length, benign ambient stability, and light-harvesting ability. However, its large surface area with inherent thermodynamic instability and highly defective ionic termination are still major obstacles to fabricating high-performance devices. Herein, a metallic ion dopant is developed to post-treat FAPbI3 QDs immediately after their fabrication by using a metal-glutamate salt solution. Both experimental and theoretical results show that alkaline (earth) metal ions (Mg2+, Na+, and K+) in their glutamate salt can not only successfully substitute insulating long-chain ligands to form thinner ligand shells but inhibit the formation of iodine vacancies on the surface of QDs. As a result, the glutamate-Mg based solar cell exhibits a champion efficiency of 13.48%, and the other two solar cells treated by glutamate alkaline metal salts (Na+ and K+) achieve photoelectrical conversion efficiencies of 13.26% and 11.88%, respectively, all of which are higher than of control cell with an efficiency of 11.58%. Therefore, this substantial progress provides intuitive cognition and guidance for the improvement of photoelectric performance and the commercial application of quantum dot solar cells.

Strategies To Achieve Long-Term Stability in Lead Halide Perovskite Nanocrystals and Its Optoelectronic Applications

The lead halide perovskite (LHP) nanocrystals (NCs) research area is flourishing due to their exceptional properties and great potential for a wide range of applications in optoelectronics and photovoltaics. Yet, despite the momentum in the field, perovskite devices are not yet ready for commercialization due to degradation caused by intrinsic phase transitions and external factors such as moisture, temperature, and ultraviolet (UV) light. To attain long-term stability, we analyze the origin of instabilities and describe different strategies such as surface modification, encapsulation, and doping for long-term viability. We also assess how these stabilizing strategies have been utilized to obtain optoelectronic devices with long-term stability. This Mini-Review also outlines the future direction of each strategy for producing highly efficient and ultrastable LHP NCs for sustainable applications.

Ultrafast Rejuvenation of Aged CsPbI3 Quantum Dots and Efficiency Improvement by Sequential 1-Dodecanethiol Post-Treatment Strategy

Metal halide perovskite CsPbI3 quantum dots (QDs) have sparked widespread research due to their intriguing optoelectronic. However, the CsPbI3 QDs undergo inevitable aging and luminescence quenching caused by the weak binding ability of oleate (OA-)/oleylammonium (OAm+), hindering further practical application. Herein, we have realized ultrafast rejuvenation of the aged CsPbI3 QDs that have lost their photoluminescence performance based on a 1-dodecanethiol (DDT) surface ligand to restore the outstanding red light emission with a high photoluminescence quantum yield (PLQY) from 25 to 90%. Furthermore, CsPbI3 QDs with DDT surface treatment maintain a cubic phase and high PLQY value even after 35 days. The DDT ligands can form a strong bond with Pb2+ and passivate I- ion vacancies, enhancing radiative recombination efficiency and thereby improving the PLQY of the QDs. The stable yet easily accessible surface of the DDT-capped CsPbI3 QDs was successfully employed as white LEDs and exhibited considerable enhanced luminous performance, suggesting promising application in solid-state lighting fields.

Efficient open-air synthesis of Mg2+-doped CsPbI3 nanocrystals for high-performance red LEDs

Inorganic CsPbI3 perovskite nanocrystals (NCs) exhibit remarkable optoelectronic properties for illumination. However, their poor stability has hindered the development of light-emitting diodes (LEDs) based on these materials. In this study, we propose a facile method to synthesize Mg2+-doped CsPbI3 NCs with enhanced stability and high photoluminescence (PL) intensity under ambient air conditions. Theoretical calculations confirm that doped NCs possess stronger formation energy compared to undoped NCs. The undoped CsPbI3 NCs emit red light at approximately 653 nm. We optimize the doping ratio to 1/30, which significantly enhances the photoluminescence of single-particle CsPbI3 NCs. Subsequently, we fabricate a red LED by combining the CsPbI3 NCs with a blue chip. The resulting LED, based on the doped CsPbI3 NCs, exhibits excellent performance with a high luminance of 4902.22 cd m-2 and stable color coordinates of (0.7, 0.27). This work not only presents a simple process for synthesizing perovskite NCs but also provides a design strategy for developing novel red LEDs for various applications.

Stability Strategies and Applications of Iodide Perovskites

Iodide perovskites have demonstrated their unprecedented high efficiency and commercialization potential, and their superior optoelectronic properties, such as high absorption coefficient, high carrier mobility, and narrow direct bandgap, have attracted much attention, especially in solar cells, photodetectors, and light-emitting diodes (LEDs). However, whether it is organic iodide perovskite, organic-inorganic hybrid iodide perovskite or all-inorganic iodide perovskite the stability of these iodide perovskites is still poor and the contamination is high. In recent years, scholars have studied more iodide perovskites to improve their stability as well as optoelectronic properties from various angles. This paper systematically reviews the strategies (component engineering, additive engineering, dimensionality reduction engineering, and phase mixing engineering) used to improve the stability of iodide perovskites and their applications in recent years.

Doping Up the Light: A Review of A/B-Site Doping in Metal Halide Perovskite Nanocrystals for Next-Generation LEDs

All-inorganic metal halide perovskite nanocrystals (PeNCs) show great potential for the next generation of perovskite light-emitting diodes (PeLEDs). However, trap-assisted recombination negatively impacts the optoelectronic properties of PeNCs and prevents their widespread adoption for commercial exploitation. To mitigate trap-assisted recombination and further enhance the external quantum efficiency of PeLEDs, A/B-site doping has been widely investigated to tune the bandgap of PeNCs. The bandgap of PeNCs is adjustable within a small range (no more than 0.1 eV) by A-site cation doping, resulting in changes in the bond length of Pb-X and the angle of [PbX6]4. Nevertheless, B-site doping of PeNCs has a more significant impact on the bandgap level through modification of surface defect states. In this perspective, we delve into the synthesis of PeNCs with A/B-site doping and their impacts on the structural and optoelectronic properties, as well as their impacts on the performance of subsequent PeLEDs. Furthermore, we explore the A-site and B-site doping mechanisms and the impact of device architecture on doped PeNCs to maximize the performance and stability of PeLEDs. This work presents a comprehensive overview of the studies on A-site and B-site doping in PeNCs and approaches to unlock their full potential in the next generation of LEDs.

Interfacial alloying between lead halide perovskite crystals and hybrid glasses

The stellar optoelectronic properties of metal halide perovskites provide enormous promise for next-generation optical devices with excellent conversion efficiencies and lower manufacturing costs. However, there is a long-standing ambiguity as to whether the perovskite surface/interface (e.g. structure, charge transfer or source of off-target recombination) or bulk properties are the more determining factor in device performance. Here we fabricate an array of CsPbI3 crystal and hybrid glass composites by sintering and globally visualise the property-performance landscape. Our findings reveal that the interface is the primary determinant of the crystal phases, optoelectronic quality, and stability of CsPbI3. In particular, the presence of a diffusion "alloying" layer is discovered to be critical for passivating surface traps, and beneficially altering the energy landscape of crystal phases. However, high-temperature sintering results in the promotion of a non-stoichiometric perovskite and excess traps at the interface, despite the short-range structure of halide is retained within the alloying layer. By shedding light on functional hetero-interfaces, our research offers the key factors for engineering high-performance perovskite devices.

Simultaneous Enhancement of the Luminescence Intensity and Stability of Deep-Red CsPbI3 Perovskite Quantum Dots Achieved by a Doping Strategy

The phase instability of CsPbI3 perovskite quantum dots (PQDs) restricts their practical applications due to the easy conversion from the luminescent cubic phase to the non-luminescent orthorhombic phase. The elemental doping route has been regarded as one of the most effective strategies to achieve high-quality PQDs-based phosphors. Herein, a stoichiometric amount of nickel chloride (NiCl2) has been effectively doped into the CsPbI3 lattice. The incorporation of Ni2+ ions has little effect on the crystal phase, structure, and morphology of the CsPbI3 PQDs but greatly influences their luminescence properties. The Ni2+ doping not only improves the luminescence performance but also greatly enhances the stability against temperature, storage time, and polar solvent. The formation process and luminescence and stability improvement mechanisms have been discussed. Moreover, the influence of a series of other metal chlorides (KCl, NaCl, MgCl2, ZnCl2, SnCl2, and CaCl2) on the luminescence performance of CsPbI3 PQDs has been systematically investigated, revealing that the luminescence intensity increases by introducing CaCl2, SnCl2, or ZnCl2 but decreases after doping MgCl2, NaCl, or KCl into the CsPbI3 lattice. The as-proposed doping strategy may have a significant impact on tackling the intrinsic instability of all-inorganic CsPbX3 PQDs, shedding light on their future applications in light-emitting diode (LED) devices and solid-state lighting.

Influence of Sr doping on the photoelectronic properties of CsPbX3 (X = Cl, Br, or I): a DFT investigation

To broaden the application of cesium lead halide perovskites, doping technology has been widely proposed. In this study, we calculated a 12.5% concentration of a Sr-doped CsPbX₃ (X = Cl, Br, or I) perovskite via density functional theory. The results showed that the bandgap energy of the perovskite increased by 0.2-0.3 eV. The high symmetry points of the energy band changed from R to Γ after Sr doping because the Sr doping affected the initial distribution of atomic orbital hybridization. In addition, optical absorption spectra after doping showed an obvious blueshift, whereas the absorption coefficient of CsPb_0.875Sr_0.125X₃ had the same magnitude as undoped CsPbX₃. Moreover, the effective masses of electrons and holes changed within a small range (0.01-0.03 m₀) after Sr doping. According to the findings of this study, the CsPb_0.875Sr_0.125X₃ perovskite is expected to become an ideal candidate material for designing photovoltaic and photoelectric devices.

Highly Thermally Sensitive Cascaded Wannier-Mott Exciton Ionization/Carrier Localization in Manganese-Doped Perovskite Nanocrystals

Transition-metal doping in perovskite nanocrystals strongly alters the photophysical properties of these nanocrystals. However, the details of the underlying thermal and optical processes within such an intriguing symmetry-breaking nanosystem are far from clear. Herein, we study the sensitively temperature-dependent and highly competent delocalized exciton and transition-metal ion-captured carrier recombination processes in manganese-doped CsPbBr_0.6Cl_2.4 nanocrystals. The combined experimental and theoretical studies reveal that both the exciton ionization and capture of the band-edge carriers by the manganese ions play the dominant roles in determining the proportion of the manganese ions-dominated recombination process. A density functional theory calculation of the temporal fluctuation of the manganese ions-accommodated localized orbitals further confirms that the thermally enhanced nonadiabatic electron-phonon coupling promotes the probability of the carrier localization. These findings reveal the respective crucial roles of the exciton ionization and carrier capture in the localized recombination process in the transition-metal-doped semiconductor nanocrystals.

Phase-Stable and Highly Luminescent CsPbI3 Perovskite Nanocrystals with Suppressed Photoluminescence Blinking

Despite their low band gap, the utility of CsPbI₃ nanocrystals (NCs) in solar photovoltaic and optoelectronic applications is rather limited because of their phase instability and photoluminescence (PL) intermittency. Herein we show that phase-pure, monodispersed, stable and highly luminescent CsPbI₃ NCs can be obtained by tweaking the conventional hot-injection method employing NH₄I as an additional precursor. Single-particle studies show a significant suppression of PL blinking. Among all NCs studied, 60% exhibit only high-intensity ON states with a narrow distribution of intensity. The remaining 40% of NCs exhibit a much wider distribution of PL intensity with a significant contribution of low-intensity OFF states. Excellent characteristics of these CsPbI₃ NCs are shown to be the result of NH₄⁺ replacing some surface Cs⁺ of an iodide-rich surface of the NCs. These phase-stable and highly luminescent CsPbI₃ NCs with significantly suppressed PL blinking can be useful single-photon emitters and promising materials for optoelectronic and solar photovoltaic applications.

Entropy-Driven Stabilization of Multielement Halide Double-Perovskite Alloys

Currently, a major obstacle restricting the commercial application of halide perovskites is their low thermodynamic stability. Herein, inspired by the high-stability high-entropy alloys, we theoretically investigated a variety of multielement double-perovskite alloys. First-principles calculations show that the entropy contribution to Gibbs free energy, which offsets the positive enthalpy contribution by up to 35 meV/f.u., can significantly enhance the material stability of double-perovskite alloys. We found that the electronic properties of bandgaps (1.04-2.21 eV) and carrier effective masses (0.34 to greater than 2 m₀) of the multielement double-perovskite alloys can be tuned over a wide range. Meanwhile, the parity-forbidden condition of optical transitions in the Cs₂AgInCl₆ perovskite can be broken because of the lower symmetry of the configurational disorder, leading to enhanced transition intensity. This work demonstrates a promising strategy by utilizing the alloy entropic effect to further improve the material stability and optoelectronic performance of halide perovskites.

Room-Temperature, Highly Pure Single-Photon Sources from All-Inorganic Lead Halide Perovskite Quantum Dots

Attaining pure single-photon emission is key for many quantum technologies, from optical quantum computing to quantum key distribution and quantum imaging. The past 20 years have seen the development of several solid-state quantum emitters, but most of them require highly sophisticated techniques (e.g., ultrahigh vacuum growth methods and cryostats for low-temperature operation). The system complexity may be significantly reduced by employing quantum emitters capable of working at room temperature. Here, we present a systematic study across ∼170 photostable single CsPbX3 (X: Br and I) colloidal quantum dots (QDs) of different sizes and compositions, unveiling that increasing quantum confinement is an effective strategy for maximizing single-photon purity due to the suppressed biexciton quantum yield. Leveraging the latter, we achieve 98% single-photon purity (g(2)(0) as low as 2%) from a cavity-free, nonresonantly excited single 6.6 nm CsPbI3 QDs, showcasing the great potential of CsPbX3 QDs as room-temperature highly pure single-photon sources for quantum technologies.

Passivation Layer of Potassium Iodide Yielding High Efficiency and Stable Deep Red Perovskite Light-Emitting Diodes

Perovskite light-emitting diodes (PeLEDs) are promising candidates used for superthin emissive displays with high resolution, high brightness, and wide color gamut, but the CsPbI₃ nanocrystal (NC) based ones usually have an external quantum efficiency (EQE) of less than 20%, which needs further enhancement to minimize the gap between their counterparts. Herein, we propose to improve optical properties of the CsPbI₃:Sr emissive layer (EML) by inserting an additional potassium iodide (KI) passivation layer between the hole transport layer and EML to increase the film quality, photoluminescence quantum yield, and thermal stability of the EML. The KI layer can also increase the carrier mobility to balance the charge injection in PeLEDs, leading to a reduction in Auger recombination and Joule heating. An interesting deep-red-emitting PeLED (λ_em = 687 nm) with a record EQE of 21.8% and a lifetime T₅₀ of 69 min is obtained by applying the additional KI passivation layer. Moreover, a flexible PeLED consisting of the KI layer is also demonstrated to have a record EQE of 12.7%. These results indicate that the use of a functional KI layer is a feasible way to develop high-performance electroluminescent devices.

Structurally Tolerance-Factor-Tuned Metal Halide Nanocrystals for Environmentally Stable and Efficient Red Light-Emitting Diodes

Black phase CsPbI₃, naturally possessing the superiority of high radiative recombination efficiency and narrow emission line width, shows promise for commercial applications of red perovskite light-emitting diodes (PeLEDs). However, the metastable black phase CsPbI₃ with a marginal tolerance factor (t) of 0.81 would easily convert to the nonoptical yellow phase. Herein, we demonstrate the strategy of partial substitution of larger dimethylammonium cation (DMA⁺) for Cs⁺ to achieve the stable tolerance factor of 0.903 for greatly improved Cs_0.7DMA_0.3PbI₃ nanocrystals. These NCs present a superior ultraviolet (UV) irradiation stability by retaining 80% of the initial photoluminescence intensity after 5 h, which is much better than that of its counterparts (retaining 30%). Based on this, the as-developed red PeLEDs demonstrate remarkable luminance of 1258 cd/m² and external quantum efficiency of 3.39%, which are almost 6 times and 3 times that of its counterparts, respectively (203 cd/m² and 1.28%). This strategy may pave the way to improving the stability and efficiency of PeLEDs.

Highly Luminescent, Stable, and Red-Emitting CsMgxPb1-xI3 Quantum Dots for Dual-Modal Imaging-Guided Photodynamic Therapy and Photocatalytic Activity

In this study, for the first time, red-emitting CsMg_xPb_1-xI₃ quantum dots (QDs) are prepared by doping with magnesium (Mg) ions via the one-pot microwave pyrolysis technique. The X-ray diffraction and X-ray photoelectron spectroscopy results have confirmed partial substitution of Pb²⁺ by Mg²⁺ inside the CsPbI₃ framework. The as-synthesized CsMg_xPb_1-xI₃ QDs have exhibited excellent morphology, higher quantum yield (upto ∼89%), better photostability and storage stability than undoped CsPbI₃. Next, the bioavailability of as-synthesized hydrophobic CsMg_xPb_1-xI₃ QDs is improved by encapsulating them into gadolinium-conjugated pluronic 127 (PF127-Gd) micelles through hydrophobic interactions (PQD@Gd). The optical properties of perovskite quantum dots (PQDs) and the presence of Gd could endow the PQD@Gd with fluorescence imaging, magnetic resonance imaging (MRI), and phototherapeutic properties. Accordingly, the MRI contrasting effects of PQD@Gd nanoagents are demonstrated by employing T₁ and T₂ studies, which validated that PQD@Gd nanoagents had superior MR contrasting effect with a r₂/r₁ ratio of 1.38. In vitro MRI and fluorescence imaging analyses have shown that the PQD@Gd nanoagents are internalized into the cancer cells via a caveolae-mediated endocytosis pathway. The PQD@Gd nanoagents have exhibited excellent biocompatibility even at concentrations as high as 450 ppm. Interestingly, the as-prepared PQD@Gd nanoagents have efficiently produced cytotoxic reactive oxygen species in the cancer cells under 671 nm laser illumination and thereby induced cell death. Moreover, the PQD@Gd nanoagent also demonstrated excellent photocatalytic activity toward organic pollutants under visible light irradiation. The organic pollutants rhodamine b, methyl orange, and methylene blue were degraded by 92.11, 89.21, and 76.21%, respectively, under 60, 80, and 100 min, respectively, irradiation time. The plausible mechanism for the photocatalytic activity is also elucidated. Overall, this work proposes a novel strategy to enhance the optical properties, stability, and bioapplicability of PQDs. The multifunctional PQD@Gd nanoagents developed in this study could be the potential choice of components not only for cancer therapy due to dual-modal imaging and photodynamic therapeutic properties but also for organic pollutant or bacterial removal due to excellent photocatalytic properties.

Large-scale continuous preparation of highly stable α-CsPbI3/m-SiO2 nanocomposites by a microfluidics reactor for solid state lighting application

CsPbI3-Mesoporous SiO2 nanocomposites with ultrahigh chemical stability were fabricated by the microfluidic technology for large-scale continuous production.

Enhance luminescence or change morphology: effect of the doping method on Cu2+-doped CsPbBr3 perovskite nanocrystals

The effect of Cu2+ on CsPbBr3 nanocrystals is compared between the hot-injection method and postsynthetic cation-exchange reaction.

Stable and Efficient Red Perovskite Light-Emitting Diodes Based on Ca2+-Doped CsPbI3 Nanocrystals

α-CsPbI₃ nanocrystals (NCs) with poor stability prevent their wide applications in optoelectronic fields. Ca²⁺ (1.00 Å) as a new B-site doping ion can successfully boost CsPbI₃ NC performance with both improved phase stability and optoelectronic properties. With a Ca²⁺/Pb²⁺ ratio of 0.40%, both phase and photoluminescence (PL) stability could be greatly enhanced. Facilitated by increased tolerance factor, the cubic phase of its solid film could be maintained after 58 days in ambient condition or 4 h accelerated aging process at 120°C. The PL stability of its solution could be preserved to 83% after 147 days in ambient condition. Even using UV light to accelerate aging, the T₅₀ of PL could boost 1.8-folds as compared to CsPbI₃ NCs. Because Ca²⁺ doping can dramatically decrease defect densities of films and reduce hole injection barriers, the red light-emitting diodes (LEDs) exhibited about triple enhancement for maximum the external quantum efficiency (EQE) up to 7.8% and 2.2 times enhancement for half-lifetime of LED up to 85 min. We believe it is promising to further explore high-quality CsPbI₃ NC LEDs via a Ca²⁺-doping strategy.

Mg2+-Assisted Passivation of Defects in CsPbI3 Perovskite Nanocrystals for High-Efficiency Photoluminescence

CsPbI₃ perovskite nanocrystals (NCs) are emerging as promising materials for optoelectronic devices because of their superior optical properties. However, the poor stability of CsPbI₃ NCs has become a huge bottleneck for practical applications. Herein, we report an effective strategy of Mg²⁺-assisted passivation of surface defects to obtain high emission efficiency and stability in CsPbI₃ NCs. It is found that the introduced Mg²⁺ ions are mainly distributed on the surface of NCs and then passivate the NC defects, enhancing radiative decay rate and reducing nonradiative decay rate. As a result, the as-prepared Mg²⁺-treated CsPbI₃ (Mg-CsPbI₃) NCs exhibit the highest photoluminescence quantum yield (PLQY) of 95%. The Mg-CsPbI₃ NC colloidal solution retains 80% of its original PLQY after 80 days of atmosphere exposure. The red perovskite light-emitting diodes based on the Mg-CsPbI₃ NCs demonstrate an external quantum efficiency of 8.4%, which shows an almost 4-fold improvement compared to the devices based on the untreated NCs.