Two-dimensional (2D) halide perovskites have emerged as outstanding semiconducting materials thanks to their superior stability and structural diversity. However, the ever-growing field of optoelectronic device research using 2D perovskites requires systematic understanding of the effects of the spacer on the structure, properties, and device performance. So far, many studies are based on trial-and-error tests of random spacers with limited ability to predict the resulting structure of these synthetic experiments, hindering the discovery of novel 2D materials to be incorporated into high-performance devices. In this review, we provide guidelines on successfully choosing spacers and incorporating them into crystalline materials and optoelectronic devices. We first provide a summary of various synthetic methods to act as a tutorial for groups interested in pursuing synthesis of novel 2D perovskites. Second, we provide our insights on what kind of spacer cations can stabilize 2D perovskites followed by an extensive review of the spacer cations, which have been shown to stabilize 2D perovskites with an emphasis on the effects of the spacer on the structure and optical properties. Next, we provide a similar explanation for the methods used to fabricate films and their desired properties. Like the synthesis section, we will then focus on various spacers that have been used in devices and how they influence the film structure and device performance. With a comprehensive understanding of these effects, a rational selection of novel spacers can be made, accelerating this already exciting field.

The 2D Halide Perovskite Rulebook: How the Spacer Influences Everything from the Structure to Optoelectronic Device Efficiency.

Quasi-two-dimensional (quasi-2D) perovskites have attracted extraordinary attention due to their superior semiconducting properties and have emerged as one of the most promising materials for next-generation light-emitting diodes (LEDs). The outstanding optical properties originate from their structural characteristics. In particular, the inherent quantum-well structure endows them with a large exciton binding energy due to the strong dielectric- and quantum-confinement effects; the corresponding energy transfer among different n-value species thus results in high photoluminescence quantum yields (PLQYs), particularly at low excitation intensities. The review herein presents an overview of the inherent properties of quasi-2D perovskite materials, the corresponding energy transfer and spectral tunability methodologies for thin films, as well as their application in high-performance LEDs. We then summarize the challenges and potential research directions towards developing high-performance and stable quasi-2D PeLEDs. The review thus provides a systematic and timely summary for the community to deepen the understanding of quasi-2D perovskite materials and resulting LED devices.

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High-performance quasi-2D perovskite light-emitting diodes: from materials to devices.

While there has been extensive investigation into modulating quasi-2D perovskite compositions in light-emitting diodes (LEDs) for promoting their electroluminescence, very few reports have studied approaches involving enhancement of the energy transfer between quasi-2D perovskite layers of the film, which plays very important role for achieving high-performance perovskite LEDs (PeLEDs). In this work, a bifunctional ligand of 4-(2-aminoethyl)benzoic acid (ABA) cation is strategically introduced into the perovskite to diminish the weak van der Waals gap between individual perovskite layers for promoting coupled quasi-2D perovskite layers. In particular, the strengthened interaction between coupled quasi-2D perovskite layers favors an efficient energy transfer in the perovskite films. The introduced ABA can also simultaneously passivate the perovskite defects by reducing metallic Pb for less nonradiative recombination loss. Benefiting from the advanced properties of ABA incorporated perovskites, highly efficient blue PeLEDs with external quantum efficiency of 10.11% and a very long operational stability of 81.3 min, among the best performing blue quasi-2D PeLEDs, are achieved. Consequently, this work contributes an effective approach for high-performance and stable blue PeLEDs toward practical applications.

High-Performance Blue Perovskite Light-Emitting Diodes Enabled by Efficient Energy Transfer between Coupled Quasi-2D Perovskite Layers.

The research on metal halide perovskite light-emitting diodes (PeLEDs) with green and infrared emission has demonstrated significant progress in achieving higher functional performance. However, the realization of stable pure-blue (≈470 nm wavelength) PeLEDs with increased brightness and efficiency still constitutes a considerable challenge. Here, a novel acid etching-driven ligand exchange strategy is devised for achieving pure-blue emitting small-sized (≈4 nm) CsPbBr3 perovskite quantum dots (QDs) with ultralow trap density and excellent stability. The acid, hydrogen bromide (HBr), is employed to etch imperfect [PbBr6 ]4- octahedrons, thereby removing surface defects and excessive carboxylate ligands. Subsequently, didodecylamine and phenethylamine are successively introduced to bond the residual uncoordinated sites of the QDs and attain in situ exchange with the original long-chain organic ligands, resulting in near-unity quantum yield (97%) and remarkable stability. The QD-based PeLEDs exhibit pure-blue electroluminescence at 470 nm (corresponding to the Commission Internationale del'Eclairage (CIE) (0.13, 0.11) coordinates), an external quantum efficiency of 4.7%, and a remarkable luminance of 3850 cd m-2 , which is the highest brightness reported so far for pure-blue PeLEDs. Furthermore, the PeLEDs exhibit robust durability, with a half-lifetime exceeding 12 h under continuous operation, representing a record performance value for blue PeLEDs.

Perovskite Quantum Dots with Ultralow Trap Density by Acid Etching-Driven Ligand Exchange for High Luminance and Stable Pure-Blue Light-Emitting Diodes

Interfacial Potassium-Guided Grain Growth for Efficient Deep-Blue Perovskite Light-Emitting Diodes

Metal halide perovskites have received much attention for their application in light-emitting diodes (LEDs) in the past several years Rapid progress has been made in efficient green, red, and near

Jingwei Xu

Papers

Rearranging Low-Dimensional Phase Distribution of Quasi-2D Perovskites for Efficient Sky-Blue Perovskite Light-Emitting Diodes.