Junzhi Ye

Sb ₂ S ₃ is a promising material for low-toxicity, high-stability next-generation photovoltaics, but its device performance is constrained by large open-circuit voltage ( V _OC ) deficits. From recent spectroscopic investigations, it was hypothesized that this arises from self-trapping, limiting V _OC s to approximately 800 mV, which is indeed the level nearly all Sb ₂ S ₃ solar cells have asymptotically approached. Herein, it is revealed through temperature-dependent mobility measurements that band-like transport, rather than self-trapping, occurs in Sb ₂ S ₃ . By lowering the defect density in Sb ₂ S ₃ thin films, the 800 mV threshold is surpassed to achieve a V _OC of 824 mV. This is accomplished by adding citrate ligands to the precursor solution used for chemical bath deposition, lowering the grain boundary density in Sb ₂ S ₃ films from 1114 ± 52 nm μm⁻ ² to 586 ± 11 nm μm⁻ ² . The likely performance-limiting defects in Sb ₂ S ₃ are identified to be S vacancies or Sb on S anti-sites by comparing deep level transient spectroscopy measurements with defect calculations. This work addresses the debate in the field around whether Sb ₂ S ₃ is limited by defects or self-trapping, showing that it is possible to improve the performance towards the radiative limit through careful defect engineering.

Strongly‐confined perovskite nanoplatelets (PeNPLs) offer opportunities not found in conventional isotropic nanocubes, especially in producing linearly polarized light, as well as enhancing outcoupling through control over the transition dipole moment. But this requires ultrathin nanoplatelets with three or fewer monolayers of PbI ₆ octahedra across the thickness, which are challenging to synthesise uniformly, and their luminescence is strongly affected by surface defects. Together, these limit the performance of ultrathin PeNPLs in light‐emitting diodes (LEDs). Here, we address these challenges with an ancillary ligand engineering strategy. We demonstrate that ligands with phosphoryl functional groups strongly bind to the perovskite surface, while having an organic backbone that is not sterically bulky ensures high ligand density. By modulating nucleation and growth, these ancillary ligands lead to monodisperse PeNPLs that stack more uniformly when self‐assembled into superlattices, with suppressed agglomeration. As a result, from edge‐up PeNPL superlattices, we achieve an enhanced degree of polarization, while from face‐down PeNPL superlattices, we achieve enhanced outcoupling that results in LEDs with 13.1% external quantum efficiency, the highest reported for ultrathin PeNPL LEDs. This work establishes ancillary ligand‐induced synthesis as a decisive route to achieve uniform nanoplatelets with robust orientation control, enabling full utilization of the multifunctionality of anisotropic PeNPLs.

CsPbCl ₃ perovskite nanocrystals (NCs) are promising violet emitters owing to their narrow emission and high color purity, but their low defect tolerance demands careful passivation to achieve high photoluminescence quantum yield (PLQY), and typically only for fresh CsPbCl ₃ NCs. Here, we report a delayed dual‐passivation pathway in CsPbCl ₃ NCs induced by the halide‐rich molecular reagent phosphorus oxychloride (POCl ₃ ), which unexpectedly yields a strong time‐dependent PLQY enhancement instead of the rapid degradation usually observed. POCl ₃ gradually decomposes into P‐ and Cl‐containing species, enabling a controlled release of excess halides that autonomously passivates halide vacancies in a self‐regulated manner. This dynamic self‐healing process boosts the PLQY of colloidal CsPbCl ₃ NCs by over 40‐fold relative to pristine samples and sustains high violet emission efficiencies for more than 2 months of storage under ambient conditions. Spectroscopic measurements and calculations indicate that both liberated Cl ⁻ and in situ—formed phosphonic species passivate halide vacancies and Pb ^{2 +} dangling bonds, suppressing mid‐gap defect states. The resulting self‐passivated NCs deliver a luminance of 409 cd m ^{− 2} , the highest reported for CsPbCl ₃ ‐based violet emitters. These results establish halide‐rich dual passivators such as POCl ₃ as powerful tools for long‐term defect control in chloride perovskite NCs and for robust, bright violet‐LEDs.

CsPbCl ₃ perovskite nanocrystals (NCs) are promising violet emitters owing to their narrow emission and high color purity, but their low defect tolerance demands careful passivation to achieve high photoluminescence quantum yield (PLQY), and typically only for fresh CsPbCl ₃ NCs. Here, we report a delayed dual‐passivation pathway in CsPbCl ₃ NCs induced by the halide‐rich molecular reagent phosphorus oxychloride (POCl ₃ ), which unexpectedly yields a strong time‐dependent PLQY enhancement instead of the rapid degradation usually observed. POCl ₃ gradually decomposes into P‐ and Cl‐containing species, enabling a controlled release of excess halides that autonomously passivates halide vacancies in a self‐regulated manner. This dynamic self‐healing process boosts the PLQY of colloidal CsPbCl ₃ NCs by over 40‐fold relative to pristine samples and sustains high violet emission efficiencies for more than 2 months of storage under ambient conditions. Spectroscopic measurements and calculations indicate that both liberated Cl ⁻ and in situ—formed phosphonic species passivate halide vacancies and Pb ^{2 +} dangling bonds, suppressing mid‐gap defect states. The resulting self‐passivated NCs deliver a luminance of 409 cd m ^{− 2} , the highest reported for CsPbCl ₃ ‐based violet emitters. These results establish halide‐rich dual passivators such as POCl ₃ as powerful tools for long‐term defect control in chloride perovskite NCs and for robust, bright violet‐LEDs.

Lead-halide perovskite nanocrystals (NCs) have gained attention for optoelectronics, but careful selection of the antisolvent used for purification is essential to achieve high monodispersity and yield while minimizing surface damage. Current understanding indicates that this requires lowering the relative polarity of the antisolvent, yet high-polarity antisolvents are widely used for purification, as we confirm through data mining. We show that polarity alone is insufficient for antisolvent selection by comparing ethyl acetate and acetonitrile for CsPbI₃ NC purification. Despite its higher polarity, acetonitrile yields improved colloidal stability compared to ethyl acetate. Using ¹H NMR, FTIR, and XPS measurements, alongside DFT calculations, we demonstrate that acetonitrile acts as a stronger Lewis base, binding to and passivating the NC surface. Coordination of acetonitrile to the perovskite NC surface enhances stability and improves their performance in light-emitting diodes. These findings establish a mechanistic framework for antisolvent selection to realize bright and stable halide perovskite NCs.

The anisotropy of perovskite nanoplatelets (PeNPLs) opens up many opportunities in optoelectronics, including enabling the emission of linearly polarized light. But the limited stability of PeNPLs is a pressing challenge, especially for red-emitting CsPbI ₃ . Herein, we address this limitation by alloying formamidinium (FA) into the perovskite cuboctahedral site. Unlike Cs/FA alloying in bulk thin films or nanocubes, FA incorporation in nanoplatelets requires meticulous control over the reaction conditions, given that nanoplatelets are obtained in kinetically-driven growth regimes instead of thermodynamically-driven conditions. Through in-situ photoluminescence (PL) measurements, we find that excess FA leads to uncontrolled growth, where phase impurities and nanoplatelets of multiple thicknesses co-exist. Restricting the FA content to up to 25% Cs substitution enables monodisperse PeNPLs, and increases the PL quantum yield (from 53% to 61%), exciton lifetime (from 18 ns to 27 ns), and stability in ambient air (from ~2 days to >7 days) compared to CsPbI ₃ . This arises due to hydrogen bonding between FA and the oleate and oleylammonium ligands, anchoring them to the surface to improve optoelectronic properties and stability. The reduction in non-radiative recombination, improvement in the nanoplatelet aspect ratio, and higher ligand density lead to FA-containing PeNPLs more effectively forming edge-up superlattices, enhancing the PL degree of linear polarization from 5.1% (CsPbI ₃ ) to 9.4% (Cs _0.75 FA _0.25 PbI ₃ ). These fundamental insights show how the stability limitations of PeNPLs could be addressed, and these materials grown more precisely to improve their performance as polarized light emitters, critical for utilizing them in next-generation display, bioimaging, and communications applications.

Insulating nanomaterials have large energy gaps and are only electrically accessible under extreme conditions, such as high-intensity radiation and high temperature, pressure or voltage ^1,2 . Lanthanide-doped insulating nanoparticles (LnNPs) are widely studied owing to their exceptional luminescence properties, including bright, narrow-linewidth, non-blinking and non-bleaching emission in the second near-infrared (NIR-II) range ^3,4 . However, it has not been possible to electrically generate excited states in these insulating nanomaterials under low biases and, therefore, not possible to fabricate optoelectronic devices from these systems. Here we report an electrical excitation pathway to obtain emission from LnNPs. By forming LnNP@organic molecule nanohybrids, in which the recombination of electrically injected charges on the organic molecule is followed by efficient triplet energy transfer (TET) to the LnNP, it is possible to turn on LnNPs under a low operating bias. We demonstrate this excitation pathway in light-emitting diodes (LEDs), with low turn-on voltages of about 5 V, very narrow electroluminescence (EL) spectra and a peak external quantum efficiency (EQE) greater than 0.6% in the NIR-II window ⁵ . Our LnNP-based LEDs (LnLEDs) also allow for widely tunable EL properties, by changing the type and concentration of lanthanide dopants. These results open up a new field of hybrid optoelectronic devices and provide new opportunities for the electrically driven excitation sources based on lanthanide nanomaterials for biomedical and optoelectronic applications.

Doping CsPbCl ₃ perovskite nanocrystals (NCs) with Mn ²⁺ has gained attention due to their interesting emission properties. However, the photoluminescence (PL) spectra of these NCs display both dopant and exciton emission peaks. It is critical to achieve color purity for light–emitting diode (LED) applications, but the factors that govern this remain unclear. Herein, a systematic investigation of the factors determining the exciton‐to‐dopant energy transfer process in Mn ² ⁺‐doped CsPbCl ₃ NCs is presented to reveal how the exciton‐to‐Mn ² ⁺ emission ratio can be maximized. These findings indicate that this process is not only affected by dopant concentration and halide (Cl/Br) composition, but also by the co‐dopants used, as well as surface passivation. These factors can potentially account for the discrepancies in the exciton‐to‐dopant emission ratios across the literature. These results show that post‐synthetic surface passivation of Mn ² ⁺‐doped CsPbCl ₃ NCs with quaternary ammonium salt, such as dimethyldidodecylammonium chloride (DDACl), drastically enhances the exciton‐to‐Mn ² ⁺ emission ratio, achieving a two‐fold increase. Ultrafast pump‐probe spectroscopy surprisingly reveals that the passivation can introduce shallow trap states that enhance energy transfer to dopant sites and influence overall luminescence efficiency through non‐radiative decay processes. This study sets guidelines for maximizing dopant emission in doped perovskite NCs.