Over the last several years, there has been tremendous progress in the development of nanoscale halide perovskite materials and devices that possess a wide range of band gaps and tunable optical and electronic properties. Particularly, the emerging two-dimensional (2D) forms of halide perovskites are attracting more interest due to the long charge carrier lifetime, high photoluminescence quantum efficiency, and great defect tolerance. Interfacing 2D halide perovskites with other 2D materials including graphene and transition metal dichalcogenides (TMDs) significantly broadens the application range of the 2D materials and enhances the performance of the functional devices. The synthesis and characterization of 2D halide perovskite nanostructures, the interface of the 2D halide perovskites with other 2D materials, and the integration of them into high-performance optoelectronic devices including solar cells, photodetectors, transistors, and memory devices are currently under investigation. In this article, we review the progress of the above-mentioned topics in a timely manner and discuss the current challenges and future promising directions in this field.

Surface passivation is an effective way to boost the efficiency and stability of perovskite solar cells (PSCs). However, a key challenge faced by most of the passivation strategies is reducing the interface charge recombination without imposing energy barriers to charge extraction. Here, a novel multifunctional semiconducting organic ammonium cationic interface modifier inserted between the light‐harvesting perovskite film and the hole‐transporting layer is reported. It is shown that the conjugated cations can directly extract holes from perovskite efficiently, and simultaneously reduce interface non‐radiative recombination. Together with improved energy level alignment and the stabilized interface in the device, a triple‐cation mixed‐halide medium‐bandgap PSC with an excellent power conversion efficiency of 22.06% (improved from 19.94%) and suppressed ion migration and halide phase segregation, which lead to a long‐term operational stability, is demonstrated. This strategy provides a new practical method of interface engineering in PSCs toward improved efficiency and stability.

Electroluminescence efficiencies and stabilities of quasi-two-dimensional halide perovskites are restricted by the formation of multiple-quantum-well structures with broad and uncontrollable phase distributions. Here, we report a ligand design strategy to substantially suppress diffusion-limited phase disproportionation, thereby enabling better phase control. We demonstrate that extending the π-conjugation length and increasing the cross-sectional area of the ligand enables perovskite thin films with dramatically suppressed ion transport, narrowed phase distributions, reduced defect densities, and enhanced radiative recombination efficiencies. Consequently, we achieved efficient and stable deep-red light-emitting diodes with a peak external quantum efficiency of 26.3% (average 22.9% among 70 devices and cross-checked) and a half-life of ~220 and 2.8 h under a constant current density of 0.1 and 12 mA/cm2, respectively. Our devices also exhibit wide wavelength tunability and improved spectral and phase stability compared with existing perovskite light-emitting diodes. These discoveries provide critical insights into the molecular design and crystallization kinetics of low-dimensional perovskite semiconductors for light-emitting devices.

Quasi‐2D hybrid halide perovskites have drawn considerable attention due to their improved stability and facile tunability compared to 3D perovskites. The expansiveness of possibilities has thus far been limited by the difficulty in incorporating large ligands into thin‐film devices. Here, a bulky bi‐thiophene 2T ligand is focused on to develop a solvent system around creating strongly vertically‐aligned (2T)2(MA)6Pb7I22 (n = 7) quasi‐2D perovskite films. By starting with a poorly coordinating solvent (gamma‐butyrolactone) and adding a small amount of dimethylsulfoxide and methanol, it is found that vertical orientation and z‐uniformity is greatly improved. These are carefully examined and verified using grazing‐incidence wide‐angle X‐ray scattering analysis and advanced optical characterizations. These films are incorporated into champion solar cells that achieve a power conversion efficiency of 13.3%, with a short‐circuit current density of 18.9 mA cm‐2, an open‐circuit voltage of 0.96 V, and a fill factor of 73.8%. Furthermore, the quasi‐2D absorbing layers show excellent stability in moisture, remaining unchanged after hundreds of hours. In addition, 2T is compared with the more common ligands butylammonium and phenylethylammonium in this solvent system to develop heuristics and deeper understanding of how to incorporate large ligands into stable photovoltaic devices.

Quasi-2D halide perovskites have attracted much interest as a promising material for light-emitting diodes (LEDs) due to their tunability in quantum confinement and halide alloy formation to modulate the energy bandgap and emission color. However, two-factor phase separations with respect to heterogeneous quantum-well thicknesses and halide segregation are still crucial issues in quasi-2D perovskite LEDs, leading to low external quantum efficiencies (EQEs) and color shifts. Herein, we compare quasi-2D perovskite films using different cations to unveil the key contributions from the chemical design of organic cations. While mixing halide ions in conventional quasi-2D perovskite films induces micrometerscale heterogeneity, new extended and twisted conjugated cations suppress the two-factor phase separations, leading to high EQEs of over 25% and controllable emission wavelengths across red and near-infrared regions. The fundamental insights in this work will provide guidance for advancing materials design and device performance in the future.