Two-dimensional hybrid organic-inorganic perovskites with strongly bound excitons and tunable structures are desirable for optoelectronic applications. Exciton transport and annihilation are two key processes in determining device efficiencies; however, a thorough understanding of these processes is hindered by that annihilation rates are often convoluted with exciton diffusion constants. Here we employ transient absorption microscopy to disentangle quantum-well-thickness-dependent exciton diffusion and annihilation in twodimensional perovskites, unraveling the key role of electron-hole interactions and dielectric screening. The exciton diffusion constant is found to increase with quantum-well thickness, ranging from 0.06 ± 0.03 to 0.34 ± 0.03 cm 2 s −1 , which leads to long-range exciton diffusion over hundreds of nanometers. The exciton annihilation rates are more than one order of magnitude lower than those found in the monolayers of transition metal dichalcogenides. The combination of long-range exciton transport and slow annihilation highlights the unique attributes of two-dimensional perovskites as an exciting class of optoelectronic materials.

Two-dimensional perovskites that could be regarded as natural organic–inorganic hybrid quantum wells (HQWs) are promising for light-emitting diode (LED) applications. High photoluminescence quantum efficiencies (approaching 80%) and extremely narrow emission bandwidth (less than 20 nm) have been demonstrated in their single crystals; however, a reliable electrically driven LED device has not been realized owing to inefficient charge injection and extremely poor stability. Furthermore, the use of toxic lead raises concerns. Here, we report Sn­(II)-based organic–perovskite HQWs employing molecularly tailored organic semiconducting barrier layers for efficient and stable LEDs. Utilizing femtosecond transient absorption spectroscopy, we demonstrate the energy transfer from organic barrier to inorganic perovskite emitter occurs faster than the intramolecular charge transfer in the organic layer. Consequently, this process allows efficient conversion of lower-energy emission associated with the organic layer into higher-energy emission from the perovskite layer. This greatly broadened the candidate pool for the organic layer. Incorporating a bulky small bandgap organic barrier in the HQW, charge transport is enhanced and ion migration is greatly suppressed. We demonstrate a HQW-LED device with pure red emission, a maximum luminance of 3466 cd m–2, a peak external quantum efficiency up to 3.33%, and an operational stability of over 150 h, which are significantly better than previously reported lead-free perovskite LEDs.

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.

Controlling grain growth is of great importance in maximizing the charge carrier transport for polycrystalline thin-film electronic devices. The thin-film growth of halide perovskite materials has been manipulated via a number of approaches including solvent engineering, composition engineering, and post-treatment processes. However, none of these methods lead to large-scale atomically flat thin films with extremely large grain size and high charge carrier mobility. Here, we demonstrate a novel π-conjugated ligand design approach for controlling the thin-film nucleation and growth kinetics in twodimensional (2D) halide perovskites. By extending the π-conjugation and increasing the planarity of the semiconducting ligand, nucleation density can be decreased by more than 5 orders of magnitude. As a result, wafer-scale 2D perovskite thin films with highly ordered crystalline structures and extremely large grain size are readily obtained. We demonstrate high-performance field-effect transistors with hole mobility approaching 10 cm 2 V −1 s −1 with ON/OFF current ratios of ∼10 6 and excellent stability and reproducibility. Our modeling analysis further confirms the origin of enhanced charge transport and field and temperature dependence of the observed mobility, which allows for clear deciphering of the structure−property relationships in these nascent 2D semiconductor systems.

Successful implementation of hot carrier solar cells requires preserving high carrier temperature as carriers migrate through the active layer. Here, we demonstrated that addition of alkali cations in hybrid organic-inorganic lead halide perovskites led to substantially elevated carrier temperature, reduced threshold for phonon bottleneck, and enhanced hot carrier transport. The synergetic effects from the Rb, Cs, and K cations result in ~900 K increase in the effective carrier temperature at a carrier density around 1018 cm−3 with an excitation 1.45 eV above the bandgap. In the doped thin films, the protected hot carriers migrate 100 s of nanometers longer than the undoped sample as imaged by ultrafast microscopy. We attributed these improvements to the relaxation of lattice strain and passivation of halide vacancies by alkali cations based on x-ray structural characterizations and first principles calculations.