High-Efficiency and Emission-Tunable Inorganic Blue Perovskite Light-Emitting Diodes Based on Vacuum Deposition.

The fabrication of perovskite light-emitting diodes (PeLEDs) with vacuum deposition shows great potential and commercial value in realizing large-area display panel manufacturing. However, the electroluminescence (EL) performance of vacuum-deposited PeLEDs still lags behind the counterparts fabricated by solution process, especially in the field of blue PeLEDs. Here, the fabrication of high-quality CsPbBr3- x Clx film through tri-source co-evaporation is reported to achieve high photoluminescence quantum yield (PLQY). Compared with the conventional traditional dual-source co-evaporation, the tri-source co-evaporation method allows for freely adjustable elemental ratios, enabling the introduction of the lattice-matched Cs4 Pb(Br/Cl)6 phase with the quantum-limited effect into the inorganic CsPb(Br/Cl)3 emitter. By adjusting the phase distribution, the surface defects of the emitter can be effectively reduced, leading to better blue emission and film quality. Further, the effects of Cs/Pb ratio and Br/Cl ratio on the PLQY and carrier recombination dynamics of perovskite films are investigated. By optimizing the deposition rate of each precursor source, spectrally stable blue PeLEDs are achieved with tunable emission ranging from 468 to 488 nm. Particularly, the PeLEDs with an EL peak at 488 nm show an external quantum efficiency (EQE) of 4.56%, which is the highest EQE value for mixed-halide PeLEDs fabricated by vacuum deposition.

Intermediate States Directed Chiral Transfer on a Silver Surface.

Chiral synthesis on surfaces has acquired tremendous interest. We herein report a novel approach of two-dimensional chiral transfer directed by metal-organic intermediate states on a silver surface. With initial deposition at low temperature, the achiral 4,4'-dihydroxybiphenyl molecules self-assemble into large scale two-dimensional networks with 4-fold symmetry via intermolecular hydrogen bonding. Fine controlled annealing, however, leads to the formation of tetramer-like chiral metal-organic hybrids, which self-organize into enantiomeric islands on the Ag(100) surface. Subsequent ortho C-C couplings of the reactants lead to dimer products. Of great importance, the chirality expressions of the dimer products are observed to be transferred directly from that of the tetramer intermediate states. The detailed reaction pathways are rationalized by DFT calculations and synchrotron-based XPS experiments, demonstrating the mechanisms of the chiral transfer.

Deprotonation-Induced Phase Evolutions in Co-Assembled Molecular Structures.

In this work, we systematically studied the co-assembly behavior of 1,3,5-tris(4-carboxyphenyl)benzene (TCPB) and 4,4″-diamino- p-terphenyl (DATP) on a silver surface. Due to the thermal instability of carboxylic acids, the co-assembled structure exhibits temperature-dependent evolutions on Ag(111). The level of the deprotonation reactions of TCPB are clarified by the characteristic self-assembled footprints. Aided by these footprints, we are able to identify the structures of the complex co-assembly of TCPB and DATP entities at each stage. Finally, the conclusions are further evidenced by density functional theory calculations.

Step-Edge Assisted Direct Linear Alkane Coupling.

Direct coupling of alkanes via C-H activation of terminal methyl groups has acquired tremendous interests both scientifically and technically. Herein we present the results of linear alkane-coupling at the step edges of Cu surfaces at modulated temperatures. Combining the observations of scanning tunneling microscopy (STM) with density functional theory plus dispersion (DFT-D) calculations, we elucidate the mechanism of the reaction and demonstrate that the low activation barrier relies on heterogeneous catalysis at the upper step edges, where low-coordinated surface atoms are present. We further reveal the generality of the reaction, so that it can be applied on the step edges of different facets of surfaces.

The Synergy of the Buried Interface Surface Energy and Temperature for Thermal Evaporated Perovskite Light-Emitting Diodes.

Multisource coevaporation is such a promising method for the preparation of perovskite films. However, there is limited research about the effects of the buried interface on thermal-evaporated perovskite light-emitting diodes (PeLEDs). In this study, the effects of buried interfaces on thermal-evaporated all-inorganic perovskite films are systematically investigated. It is found that the low-surface-energy buried interface promotes the formation of columnar grain by suppressing heterogeneous nucleation, and functional groups on the high-surface-energy interface have a significant effect on the actual element ratio of the film. The substrate temperature can affect the nucleation and film-formation kinetics of the columnar grains. As a result of the synergistic strategy, a peak external quantum efficiency (EQE) of 8.6% is achieved in the green PeLEDs with a stable emission peak at 516 nm, which is among the best thermal-evaporated PeLEDs reported. This work provides an insight into the preparation of perovskites by thermal evaporation and builds the groundwork for future studies.

A Demulsification-Crystallization Model for High-Quality Perovskite Nanocrystals.

A room-temperature technique with all-nonpolar-solvent, which circumvents the sensitivity of ionic perovskite to polar solvent, has become attractive for the synthesis of metal halide perovskite nanocrystals (PNCs). However, the lack of understanding of the inner mechanism, especially for the state of the precursor and the crystallization process of the PNCs, hinders further development of this technique. Here, through systematic study of the Pb precursor and in situ characterization of the PNCs, it is revealed that the reverse micelle nature of the Pb precursor exactly creates a novel demulsification-crystallization (D-C) model, namely, a two-stage nucleation is divided by a demulsification process for the PNCs. On this basis, a top efficiency for green light-emitting diodes based on PNCs is obtained with a maximum external quantum efficiency of 22.5% through tailoring the D-C model using a multiple-acid-anion synergistic assisted strategy to obtain high-quality PNCs. Beyond the high efficiency, the work paves the way for diverse ideas in PNC synthesis.

CsPb(Br/Cl)3 Perovskite Nanocrystals with Bright Blue Emission Synergistically Modified by Calcium Halide and Ammonium Ion.

Colloidal cesium lead halide (CsPbX3, X = Cl, Br, and I) perovskite nanocrystals (NCs) demonstrate supreme optical properties in the spectra region of infrared, red, and green. High-performance blue-emitting counterparts are still eagerly required for next-generation full-color displays. However, it is challenging to obtain efficient blue perovskite NCs, especially in a deep blue region with an emission wavelength of around 460 nm or shorter. Herein, calcium halide and ammonium ions are applied simultaneously to modify the CsPb(Br/Cl)3 NCs in situ to reduce surface defects, finally remarkably enhancing the photoluminescence quantum yield (PLQY) from 13% to 93% with an emission peak at 455 nm and the Commission Internationale de l'Eclairage (CIE) coordinates at (0.147, 0.030), which is close to the requirement of the Rec.2020 standard and also meets the requirement of blue emission in DCI-P3. Bright white emission and a wide color gamut are also achieved by combining the commercial red-emitting and green-emitting phosphors. The combination of time-resolved PL spectra and femtosecond transient absorption results discloses the reason for PLQY improvement as suppressing the nonradiative recombination.