Interfacial electron transfer across perovskite-electron acceptor heterojunctions plays a significant role in the power-conversion efficiency of perovskite solar cells. Thus, electron donor−acceptor thin films of halide perovskite nanocrystals receive considerable attention. Nevertheless, understanding and optimizing distance-and thickness-dependent electron transfer in perovskite-electron acceptor heterojunctions are important. We reveal the distance-dependent and diffusion-controlled interfacial electron transfer across donor−acceptor heterojunction films formed by formamidinium or cesium lead bromide (FAPbBr 3 /CsPbBr 3 ) perovskite nanocrystals with TiO 2 /C 60 . Self-assembled nanocrystal films prepared from FAPbBr 3 show a longer photoluminescence lifetime than a solution, showing a long-range carrier migration. The acceptors quench the photoluminescence intensity but not the lifetime in a solution, revealing a static electron transfer. Conversely, the electron transfer in the films changes from dynamic to static by moving toward the donor−acceptor interface. While radiative recombination dominates the electron transfer at 800 μm or farther, the acceptors scavenge the photogenerated carriers within 100 μm. This research highlights the significance of interfacial electron transfer in perovskite films.

Mechanically modulating optical properties of semiconductor nanocrystals and organic molecules are valuable for mechano-optical and optomechanical devices. Halide perovskites with excellent optical and electronic properties are promising for such applications. We report the mechanically changing excitons and photoluminescence of self-assembled formamidinium lead bromide (FAPbBr3) quantum dots. The as-synthesized quantum dots (3.6 nm diameter), showing blue emission and a short photoluminescence lifetime (2.6 ns), form 20–300 nm 2D and 3D self-assemblies with intense green emission in a solution or a film. The blue emission and short photoluminescence lifetime of the quantum dots are different from the delayed (ca. 550 ns) green emission from the assemblies. Thus, we consider the structure and excitonic properties of individual quantum dots differently from the self-assemblies. The blue emission and short lifetime of individual quantum dots are consistent with a weak dielectric screening of excitons or strong quantum confinement. The red-shifted emission and a long photoluminescence lifetime of the assemblies suggest a strong dielectric screening that weakens the quantum confinement, allowing excitons to split into free carriers, diffuse, and trap. The delayed emission suggests nongeminate recombination of diffusing and detrapped carriers. Interestingly, the green emission of the self-assembly blueshifts by applying a lateral mechanical force (ca. 4.65 N). Correspondingly, the photoluminescence lifetime decreases by 1 order of magnitude. These photoluminescence changes suggest the mechanical dissociation of the quantum dot self-assemblies and mechanically controlled exciton splitting and recombination. The mechanically changing emission color and lifetime of halide perovskite are promising for mechano-optical and optomechanical switches and sensors.

Halide perovskites are materials for future optical displays and solar cells. Electron donor-acceptor perovskite heterostructures with distinguishing halide compositions are promising for transporting and harvesting photogenerated charge carriers. Combined ebeam lithography and anion exchange are promising to develop such heterostructures but challenging to prepare multiple heterojunctions at desired locations in single crystals. We demonstrate swift laser trappingassisted band gap engineering at the desired locations in MAPbBr 3 microrods, microplates, or nanocrystal thin films. The built-in donor-acceptor double and multiheterojunction structures let us transport and trap photogenerated charge carriers from wide-band gap bromide to narrow-band gap iodide domains. We discuss the charge carrier transport and trapping mechanisms from the viewpoints of engineered bands and band continuity. This work offers a convenient method for designing single-, double-and multi-heterojunction donor-acceptor halide perovskites for photovoltaic, photonic, and electronic applications.

Halide vacancies cause lattice degradation and nonradiative losses in halide perovskites. In this study, we strategically fill bromide vacancies in CsPbBr 3 perovskite nanocrystals with NaBr, KBr, or CsBr at the organic− aqueous interface for hydrophobic ligand-capped nanocrystals or in a polar solvent (2-propanol) for amphiphilic ligand-capped nanocrystals. Energydispersive X-ray spectra, powder X-ray diffraction data, and scanning transmission electron microscopy images help us confirm vacancy filling and the structures of samples. The bromide salts increase the photoluminescence quantum yield (98 ± 2%) of CsPbBr 3 by decreasing the nonradiative decay rate. Single-particle studies show the quantum yield increase originates from the poorly luminescent nanocrystals becoming highly luminescent after filling vacancies. Furthermore, we tune the optical band gap (ultraviolet−visible− near-infrared) of the hydrophobic ligand-capped nanocrystals by halide exchange at the toluene−water interface using saturated NaCl or NaI solutions, which completes in about 60 min under continuous mixing. In contrast, the amphiphilic ligand accelerates the halide exchange in 2-propanol, suggesting ambipolar functional groups speed up the ion-exchange reaction. The bromide vacancy-filled or halide-exchanged samples in a toluene−water biphasic solvent show higher stability than amphiphilic ligand-capped samples in 2-propanol. This strategy of defect passivation, ion exchange, and ligand chemistry to improve quantum yields and tune band gaps of halide perovskite nanocrystals can be promising for designing stable and water-soluble perovskite samples for solar cells, light-emitting diodes, photodetectors, and photocatalysts.

Bandgap-engineered inorganic and hybrid halide perovskite (HP) films, nanocrystals, and quantum dots (PQDs) are promising for solar cells. Electron-acceptor-engineered HP surfaces offer donor-acceptor (D-A) interfaces for efficient energy harvesting throughout...