Controlled Morphological Growth and Photonic Lasing in Cesium Lead Bromide Microcrystals.

Morphology plays a crucial role in defining the optical, electronic, and mechanical properties of halide perovskite microcrystals. Therefore, developing strategies that offer precise control over crystal morphology during the growth process is highly desirable. This work presents a simple scheme to simultaneously grow distinct geometries of cesium lead bromide (CsPbBr3) microcrystals, including microrods (MR), microplates (MP), and microspheres (MS), in a single chemical vapor deposition (CVD) experiment. By strategically adjusting precursor evaporation temperatures, flux density, and the substrate temperature, we surpass previous techniques by achieving simultaneous yet selective growth of multiple CsPbBr3 geometries at distinct positions on the same substrate. This fine growth control is attributed to the synergistic variation in fluid flow dynamics, precursor substrate distance, and temperature across the substrate, offering regions suitable for the growth of different morphologies. Pertinently, perovskite MR are grown at the top, while MP and MS are observed at the center and bottom regions of the substrate, respectively. Structural analysis reveals high crystallinity and an orthorhombic phase of the as-grown perovskite microcrystals, while persistent photonic lasing manifests their nonlinear optical characteristics, underpinning their potential application for next-generation photonic and optoelectronic devices.

Crown Ethers with Different Cavity Diameters Inhibit Ion Migration and Passivate Defects toward Efficient and Stable Lead Halide Perovskite Solar Cells.

Organic-inorganic hybrid metal halide perovskite solar cells have been considered as one of the most promising next-generation photovoltaic technologies. Nevertheless, perovskite defects and Li+ ionic migration will seriously affect the power conversion efficiency and stability of the formal device. Herein, we designed two crown ether derivatives (PC12 and PC15) with different cavity diameters, which selectively bind to different metal cations. It is found that PC15 in perovskite precursor solution can actively regulate the nucleation and crystallization processes and passivate the uncoordinated Pb2+ ions, while PC12 at the interface between the perovskite layer and hole-transporting layer can effectively inhibit the migration of Li+ ions and reduce nonradiative recombination losses. Therefore, PC12 and PC15 can act as "lubricant" and defect passivators, as well as inhibitors of ion migration, when they are synergistically applied at the surface and bulk of perovskite layer. Consequently, the optimized device achieved a champion efficiency of 24.8% with significantly improved humidity, thermal, and light stability.

Origin of Dopant-Carrier Exchange Coupling and Excitonic Zeeman Splitting in Mn2+-Doped Lead Halide Perovskite Nanocrystals.

Low-dimensional metal halide perovskites have unique optical and electrical properties that render them attractive for the design of diluted magnetic semiconductors. However, the nature of dopant-exciton exchange interactions that result in spin-polarization of host-lattice charge carriers as a basis for spintronics remains unexplored. Here, we investigate Mn2+-doped CsPbCl3 nanocrystals using magnetic circular dichroism spectroscopy and show that Mn2+ dopants induce excitonic Zeeman splitting which is strongly dependent on the nature of the band-edge structure. We demonstrate that the largest splitting corresponds to exchange interactions involving the excited state at the M-point along the spin-orbit split-off conduction band edge. This splitting gives rise to an absorption-like C-term excitonic MCD signal, with the estimated effective g-factor (geff) of ca. 70. The results of this work help resolve the assignment of absorption transitions observed for metal halide perovskite nanocrystals and allow for a design of new diluted magnetic semiconductor materials for spintronics applications.

Near-Full-Spectrum Emission Realized in a Single Lead Halide Perovskite across the Visible-Light Region.

The engineering of tunable photoluminescence (PL) in single materials with a full-spectrum emission represents a highly coveted objective but poses a formidable challenge. In this context, the realization of near-full-spectrum PL emission, spanning the visible light range from 424 to 620 nm, in a single-component two-dimensional (2D) hybrid lead halide perovskite, (ETA)2PbBr4 (ETA+ = (HO)(CH2)2NH3+), is reported, achieved through high-pressure treatment. A pressure-induced phase transition occurs upon compression, transforming the crystal structure from an orthorhombic phase under ambient conditions to a monoclinic structure at high pressure. This phase transition driven by the adaptive and dynamic configuration changes of organic amine cations enables an effective and continuous narrowing of the bandgap in this halide crystal. The hydrogen bonding interactions between inorganic layers and organic amine cations (N-H…Br and O-H…Br hydrogen bonds) efficiently modulate the organic amine cations penetration and the octahedral distortion. Consequently, this phenomenon induces a phase transition and results in red-shifted PL emissions, leading to the near-full-spectrum emission. This work opens a possibility for achieving wide PL emissions with coverage across the visible light spectrum by employing high pressure in single halide perovskites.

Red-Emitting CsPbI3/ZnSe Colloidal Nanoheterostructures with Enhanced Optical Properties and Stability.

Producing heterostructures of cesium lead halide perovskites and metal-chalcogenides in the form of colloidal nanocrystals can improve their optical features and stability, and also govern the recombination of charge carriers. Herein, the synthesis of red-emitting CsPbI3/ZnSe nanoheterostructures is reported via an in situ hot injection method, which provides the crystallization conditions for both components, subsequently leading to heteroepitaxial growth. Steady-state absorption and photoluminescence studies alongside X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy analysis evidence on a type-I band alignment for CsPbI3/ZnSe nanoheterostructures, which exhibit photoluminescence quantum yield of 96% due to the effective passivation of surface defects, and an enhancement in carrier lifetime. Furthermore, the heterostructure growth of ZnSe domains leads to significant improvement in the stability of the CsPbI3 nanocrystals under ambient conditions and against thermal and UV irradiation stress.

Resonant Multiple-Phonon Absorption Causes Efficient Anti-Stokes Photoluminescence in CsPbBr3 Nanocrystals.

Lead halide perovskite nanocrystals, such as CsPbBr3, exhibit efficient photoluminescence (PL) up-conversion, also referred to as anti-Stokes photoluminescence (ASPL). This is a phenomenon where irradiating nanocrystals up to 100 meV below gap results in higher energy band edge emission. Most surprising is that ASPL efficiencies approach unity and involve single-photon interactions with multiple phonons. This is unexpected given the statistically disfavored nature of multiple-phonon absorption. Here, we report and rationalize near-unity anti-Stokes photoluminescence efficiencies in CsPbBr3 nanocrystals and attribute them to resonant multiple-phonon absorption by polarons. The theory explains paradoxically large efficiencies for intrinsically disfavored, multiple-phonon-assisted ASPL in nanocrystals. Moreover, the developed microscopic mechanism has immediate and important implications for applications of ASPL toward condensed phase optical refrigeration.

Scintillation Properties of CsPbBr3 Nanocrystals Prepared by Ligand-Assisted Reprecipitation and Dual Effect of Polyacrylate Encapsulation toward Scalable Ultrafast Radiation Detectors.

Lead halide perovskite nanocrystals (LHP-NCs) embedded in polymeric hosts are gaining attention as scalable and low-cost scintillation detectors for technologically relevant applications. Despite rapid progress, little is currently known about the scintillation properties and stability of LHP-NCs prepared by the ligand assisted reprecipitation (LARP) method, which allows mass scalability at room temperature unmatched by any other type of nanostructure, and the implications of incorporating LHP-NCs into polyacrylate hosts are still largely debated. Here, we show that LARP-synthesized CsPbBr3 NCs are comparable to particles from hot-injection routes and unravel the dual effect of polyacrylate incorporation, where the partial degradation of LHP-NCs luminescence is counterbalanced by the passivation of electron-poor defects by the host acrylic groups. Experiments on NCs with tailored surface defects show that the balance between such antithetical effects of polymer embedding is determined by the surface defect density of the NCs and provide guidelines for further material optimization.

Enhancing Performance and Stability of Sn-Pb Perovskite Solar Cells with Oriented Phenyl-C61-Butyric Acid Methyl Ester Layer via High-Temperature Annealing.

Phenyl-C61-butyric acid methyl ester (PCBM) can be used as a passivation material in perovskite solar cells (PeSCs) in order to reduce the trap site of the perovskite. Here, we show that a thick PCBM layer can form a smoother surface on the SnO2 substrate, improving the grain size and reducing the microstrain of the perovskite. High-temperature annealing treatment of PCBM layer not only increases its solvent resistance to perovskite precursor or antisolvent, but also enhances its molecular alignment, resulting in improved conductivity as an electron transport layer. High-temperature annealed PCBM (HT-PCBM) effectively minimizes trap-assisted nonradiative recombination by reducing trap density in perovskite and improving the electrical properties at the interface between SnO2 and perovskite layers. This HT-PCBM process significantly enhances the performance of the PeSCs, including the open-circuit voltage (VOC) from 0.39 to 0.77 V, fill factor from 52% to 65%, and power conversion efficiency (PCE) from 6.03% to 15.50%, representing substantial improvements compared to devices without PCBM. This PCE is the highest efficiency among conventional (n-i-p) Sn-Pb PeSCs reported to date. Moreover, passivating the trap sites of SnO2 and separating the interface between the Sn-containing perovskite and the substrate effectively have improved the stability of the Sn-Pb perovskite in the n-i-p structure. The optimized best device with HT-PCBM has maintained an efficiency of over 90% for more than 300 h at 85 °C and 5000 h at room temperature in a glovebox atmosphere.

Role of Hydrogen Bonding on the Design of New Hybrid Perovskites Unraveled by Machine Learning.

Selecting a set of reactants to accurately design a new low dimensional hybrid perovskite could greatly accelerate the discovery of materials with great potential in photovoltaics, or solid-state lighting. However, this design is challenging as most hybrid metal halides are not perovskites and no feature is clearly associated to the structural characteristics of the inorganic metal halide network. This work first demonstrates that the organic molecules are key parameters to determine the structure type of the inorganic network (i.e., perovskite versus non-perovskite). Then, machine learning (ML) algorithms are used to identify the key features of the organic cations leading to the perovskite structure type. Using a large dataset of hybrid metal halides, this work extracts the organic molecules of all hybrid lead halide compounds, calculates 2756 molecular descriptors and fingerprints for each of these molecules, and are able to predict through ML techniques if a specific organic amine will lead to the perovskite type with an accuracy up to 88.65%. Descriptors related to hydrogen bonding are identified as important features. Thus, a simple but reliable design principle could be demonstrated: the presence of primary ammonium cation is the primary condition to prepare hybrid lead halide perovskites regardless of their dimensionalities.

Perovskite/silicon tandem solar cells-compositions for improved stability and power conversion efficiency.

Perovskite/Silicon Tandem Solar Cells (PSTSCs) represent an emerging opportunity to compete with industry-standard single junction crystalline silicon (c-Si) solar cells. The maximum power conversion efficiency (PCE) of single junction cells is set by the Shockley-Queisser (SQ) limit (33.7%). However, tandem cells can expand this value to ~ 45% by utilising two stacked solar cells to harvest the solar spectrum more efficiently. 33.9% PCE has already been achieved with PSTSCs. This perspective analyses recent advances in PSTSC technology, with an emphasis on optimal perovskite composition, the problem and mitigation of light-induced halide phase segregation, self-assembled hole transporting monolayers and additives that can improve and stabilise the perovskite. Top-performing compositions show three cationic components (Cs+, FA+, Pb2+) and three anionic (I-, Br-, Cl-) with a bandgap between 1.55 and 1.77 eV and a theoretical maximum of 1.73 eV (717 nm). Anionic additives such as (Br3)- and SCN- reduce trap states and segregation. 2D-perovskite grain boundary interfaces are created with cationic alkylammonium additives such as methyl-phenethylammonium (MPEA) and result in improved performance. 2-, 3- or 4-terminal devices with a (partly) textured silicon heterojunction (SHJ) bottom cell are ideal. An ultra-thin interfacial recombination layer (~ 5 nm) of indium tin oxide (ITO) or indium zinc oxide (IZO) containing a carbazole-based hole transporting self-assembled monolayer (Me-4PACz) is used for optimal 2-terminal tandem devices.

Defect Engineering of Metal Halide Perovskite Nanocrystals via Spontaneous Diffusion of Ag Nanocrystals.

Perovskite nanocrystals (NCs) have emerged as a promising building block for the fabrication of optic-/optoelectronic-/electronic devices owing to their superior characteristics, such as high absorption coefficient, rapid ion mobilities, and tunable energy levels. However, their low structural stability and poor surface passivation have restricted their application to next-generation devices. Herein, a drug delivery system (DDS)-inspired post-treatment strategy is reported for improving their structural stability by doping of Ag into CsPbBr3 (CPB) perovskite NCs; delivery to damaged sites can promote their structural recovery slowly and uniformly, averting the permanent loss of their intrinsic characteristics. Ag NCs are designed through surface-chemistry tuning and structural engineering to enable their circulation in CPB NC dispersions, followed by their delivery to the CPB NC surface, defect-site recovery, and defect prevention. The perovskite-structure healing process through the DDS-type process (with Ag NCs as the drug) is analyzed by a combination of theoretical calculations (with density functional theory) and experimental analyses. The proposed DDS-inspired healing strategy significantly enhances the optical properties and stability of perovskite NCs, enabling the fabrication of white light-emitting diodes.

Ultranarrow Line Width Room-Temperature Single-Photon Source from Perovskite Quantum Dot Embedded in Optical Microcavity.

Ultranarrow bandwidth single-photon sources operating at room-temperature are of vital importance for viable optical quantum technologies at scale, including quantum key distribution, cloud-based quantum information processing networks, and quantum metrology. Here we show a room-temperature ultranarrow bandwidth single-photon source generating single-mode photons at a rate of 5 MHz based on an inorganic CsPbI3 perovskite quantum dot embedded in a tunable open-access optical microcavity. When coupled to an optical cavity mode, the quantum dot room-temperature emission becomes single-mode, and the spectrum narrows down to just ∼1 nm. The low numerical aperture of the optical cavities enables efficient collection of high-purity single-mode single-photon emission at room-temperature, offering promising performance for photonic and quantum technology applications. We measure 94% pure single-photon emission in a single-mode under pulsed and continuous-wave (CW) excitation.

CsPbBr3 Nanocrystals as Efficient Photocatalysts for Dehydrohalogenation: Toward Environmentally Friendly Trichloroethylene Synthesis.

The demand for a benign alternative to energy-intensive industrial chemical transformations is critical. Lead halide perovskites have emerged as promising candidates due to their unique optoelectronic properties, including high absorption coefficients in the visible region, tunable band gaps, and long charge carrier-diffusion lengths. In this study, we present a model reaction to showcase the photocatalytic utility of perovskite nanocrystals (NCs). Specifically, we demonstrate the synthesis of trichloroethylene (TCEt) from 1,1,2,2-tetrachloroethane (TCE) using CsPbBr3 NCs under white light illumination. The band-edge positions of the NCs and the redox potential of TCE enable efficient electron transfer for C-Cl bond activation. Furthermore, while ensuring operational stability, CsPbBr3 NCs undergo light-controlled modification, leading to the formation of mixed-halide perovskite (CsPbBrxCl3-x) NCs during the reaction. This procedure yields a mixed-halide perovskite that maintains stability while containing the desired halide content. Additionally, the reaction produces HBr as a byproduct, serving as a self-cleaning technique to eliminate excess Br- ions from the solution. Ultimately, we achieve nearly 100% conversion of CsPbBr3 to pure CsPbCl3 NCs, with a full width at half-maximum of approximately 11.2 nm. Our clean and efficient approach to synthesizing TCEt using perovskite NCs provides interesting insights into violet light-emitting diode (LED) fabrication and color patterning. This study highlights the promising potential of perovskite materials for sustainable chemical transformations and optoelectronic applications.

Dual-Emissive Mn-Doped Lead Halide Perovskite Nanocrystals as Background-Suppressed Latent Fingerprint Detection Probes.

Diverse strategies have been developed to visualize latent fingerprints (LFPs) that are undetectable by the naked eye. Among them, fluorescence-based approaches have emerged as an attractive method for enabling high-resolution LFP imaging. However, the use of fluorescent probes for LFP detection remains challenging due to cumbersome processing, low selectivity, and high background interference. Here, we demonstrate highly efficient, sensitive, and background-free LFP detection with dual-color emission arising from manganese (Mn)-doped lead halide perovskite (CsPb(Cl1-yBry)3) nanocrystals (NCs). The resulting bright, fluorescent, solid-state nanopowder (NP) permits the visualization of LFP ridge structures and the resolution of level 1-3 LFP features. The dual-color emission of the Mn-doped perovskite NP provides a simple, robust, and effective route to overcome background interference, thereby increasing the resolution and sensitivity of the LFP detection. The combination of the high quantum efficiency and dual emission of Mn-doped perovskite NP offers great potential for forensic science.

Molecularly imprinted fluorescence assay based on lead halide perovskite quantum dots for determination of benzo(a)pyrene.

Molecularly imprinted polymers with methylammonium lead halide perovskite quantum dots (MIP@MAPbBr3 PQDs) have been prepared and applied to the determination of benzo(a)pyrene (BaP) for the first time. The photoluminescence (PL) of MIP@MAPbBr3 PQDs was enhanced due to the surface passivation of defects by BaP. PL excitation and emission spectra, X-ray diffraction, Fourier transform infrared, and time-resolved PL studies suggest that the interaction between MIP@MAPbBr3 PQDs and BaP is a dynamic process. After MIP@MAPbBr3 PQDs were incubated with BaP, the benzene ring in the molecular structure of BaP can interact with MIP@MAPbBr3 PQDs through π electrons, which reduces non-radiative recombination of MIP@MAPbBr3 PQDs and lengthens excited state lifetime. The PL intensity of the MIP@MAPbBr3 PQDs-BaP system was monitored at 520 nm with 375 nm excitation. Under optimized conditions, the PL intensity of MIP@MAPbBr3 PQDs is linear with the concentration of BaP in the 10 to 100 ng·mL-1 range, with a detection limit of 1.6 ng·mL-1. The imprinting factor was 3.9, indicating excellent specificity of MIP@MAPbBr3 PQDs for BaP. The MIP@MAPbBr3 PQDs were subsequently applied to the PL analysis of BaP in sunflower seed oil, cured meat, and grilled fish samples, achieving recoveries from 79.3 to 107%, and relative standard deviations below 10%. This molecularly imprinted fluorescence assay improves the selectivity of BaP in complex mixtures and could be extended to other analytes.

Long-Lived Exciton Coherence in Mixed-Halide Perovskite Crystals.

Compositional engineering of the optical properties of hybrid organic-inorganic lead halide perovskites is crucial for the realization of efficient solar cells and light-emitting devices. We study the effect of band gap fluctuations on coherent exciton dynamics in a mixed FA0.9Cs0.1PbI2.8Br0.2 perovskite crystal by using photon echo spectroscopy. We reveal a narrow homogeneous exciton line width of 16 µeV at a temperature of 1.5 K. The corresponding exciton coherence time T2 = 83 ps is exceptionally long due to the localization of excitons at the scale of tens to hundreds of nanometers. From spectral and temperature dependences of the two- and three-pulse photon echo decay, we conclude that for low-energy excitons pure decoherence associated with elastic scattering on phonons is comparable with the exciton lifetime, while for excitons with higher energies, inelastic scattering to lower energy states via phonon emission dominates.

Recycling and Recovery of Deactivated CsPbBr3 Perovskite after Photocatalytic CO2 Reduction Reaction.

CsPbBr3 perovskite nanocrystals have emerged as promising candidates for photocatalysis. However, their conversion efficiency is hampered by material instability, and the accumulation of deactivated perovskites produced after photocatalytic reactions raises significant environmental concerns. To address this issue, we developed a mechanochemical grinding approach assisted by oleylamine as an additive to restore the optical properties and photocatalytic activity of deactivated CsPbBr3, which was due to aggregation in the photocatalytic CO2 reduction reaction. Upon regeneration, the CsPbBr3 nanocrystals exhibited an average length of 34.21 nm and an average width of 20.86 nm, demonstrating optical properties comparable to those of the pristine CsPbBr3 nanocrystals. Moreover, they achieved a conversion efficiency of 88.7% compared with pristine CsPbBr3 nanocrystals in the photocatalytic CO2 reduction reaction. This method effectively enhanced the utilization of CsPbBr3, offering a novel approach for the recycling and recovery of perovskite materials and thereby minimizing material waste and environmental pollution.

Emissive Dark Excitons in Monoclinic Two-Dimensional Hybrid Lead Iodide Perovskites.

Typically, bright excitons (XB) emit light in two-dimensional (2D) layered hybrid perovskites. There are also dark excitons (XD), for which radiative recombination is spin-forbidden. Application of a magnetic field can somewhat relax the spin-rule, yielding XD emission. Can we obtain XD light emission in the absence of a magnetic field? Indeed, we observe unusually intense XD emission at ∼7 K for (Rac-MBA)2PbI4, (Rac-4-Br-MBA)2PbI4, and (R-4-Br-MBA)2PbI4 (Rac-MBA: racemic methylbenzylammonium), which crystallize in a lower symmetry monoclinic phase. For comparison, orthorhombic (R-MBA)2PbI4 does not exhibit XD emission. XD has a lower energy than XB, with energy difference ΔE. In monoclinic samples, ΔE ∼ 20 meV is large enough to suppress the thermal excitation of XD to XB, at temperatures <30 K. Consequently, XD recombines by emitting light with a long lifetime (∼205 ns). At higher temperatures, the emission switches to the spin-allowed XB (lifetime < 1 ns).

Effect of Passivating Molecules and Antisolvents on Lifetime of Green Dion-Jacobson Perovskite Light-Emitting Diodes.

We investigated the influence of two passivating molecules containing a P═O group on the performance of quasi-2D Dion-Jacobson halide perovskite light-emitting diodes, namely, triphenylphosphine oxide (TPPO) and diphenyl-4-triphenylsilylphenyl phosphine oxide (TSPO1). We found that both passivating molecules lead to increased efficiency compared to control devices, while they had opposite effects on device lifetime, with a decrease observed for TPPO and an increase observed for TSPO1. The two passivating molecules resulted in differences in energy-level alignment, electron injection, film morphology and crystallinity, and ion migration during operation. While TPPO resulted in improved photoluminescence decay times, overall higher maximum external quantum efficiency (EQE) and device lifetime were obtained for TSPO1 compared to TPPO (14.4% vs 12.4% EQE, 341 min vs 42 min T50).

Performance Improvement of Lead-Based Halide Perovskites through B-Site Ion-Doping Strategies.

Owing to their excellent properties, lead halide perovskites have attracted extensive attention in the photoelectric field. Presently, the certified power conversion efficiency of perovskite solar cells has reached 25.7%, the specific detectivity of perovskite photodetectors has exceeded 1014 Jones, and the external quantum efficiency of perovskite-based light-emitting diode has exceeded 26%. However, their practical applications are limited by the inherent instability induced by the perovskite structure due to moisture, heat, and light. Therefore, one of the widely used strategies to address the issue is to replace partial ions of the perovskites with ions of smaller radii to shorten the bond length between halides and metal cations, improving the bond energy and enhancing the perovskite stability. Particularly, the B-site cation in the perovskite structure can affect the size of eight cubic octahedrons and their gap. However, the X-site can only affect four such voids. This review comprehensively summarizes the recent progress in B-site ion-doping strategies for lead halide perovskites and provides some perspectives for further performance improvements.