Two-dimensional perovskites, in which inorganic layers are stabilized by organic spacer molecules, are attracting increasing attention as a more robust analogue to the conventional three-dimensional metal-halide perovskites. However, reducing the perovskite dimensionality alters their optoelectronic properties dramatically, yielding excited states that are dominated by bound electron-hole pairs known as excitons, rather than by free charge carriers common to their bulk counterparts. Despite the growing interest in two-dimensional perovskites for both light harvesting and light emitting applications, the full impact of the excitonic nature on their optoelectronic properties remains unclear, particularly regarding the spatial dynamics of the excitons within the two-dimensional (2D) plane.Here, we present direct measurements of in-plane exciton transport in single-crystalline layered perovskites. Using time-resolved fluorescence microscopy, we show that excitons undergo an initial fast, intrinsic normal diffusion through the crystalline plane, followed by a transition to a slower subdiffusive regime as excitons get trapped. Interestingly, the early intrinsic exciton diffusivity depends sensitively on the choice of organic spacer. We find a clear correlation between the stiffness of the lattice and the diffusivity, suggesting exciton-phonon interactions to be dominant in determining the spatial dynamics of the excitons in these materials. Our findings provide a clear design strategy to optimize exciton transport in these systems. lead, tin), X is a halide anion (chloride, bromide, iodide), L is a long organic spacer molecule, and n is the number of octahedra that make up the thickness of the inorganic layer. The separation into fewatom thick inorganic layers yields strong quantum and dielectric confinement effects. 38 As a result, the exciton binding energies in 2D perovskites can be as high as several hundreds of meVs, which is around an order of magnitude larger than those found in bulk perovskites. [39][40][41] The excitonic character of the excited state is accompanied by an effective widening of the bandgap, an increase in the oscillator strength, and a narrowing of the emission spectrum. [40][41][42] The strongest confinement effects are observed for n = 1, where the excited state is confined to a single B-X-octahedral layer (see Figure 1a).Light harvesting using 2D perovskites relies on the efficient transport of excitons and their subsequent separation into free charges. 43 This stands in contrast to bulk perovskites in which free charges are generated instantaneously thanks to the small exciton binding energy. 39 Particularly, with excitons being neutral quasi-particles, the charge extraction becomes significantly more challenging as they cannot be guided to the electrodes through an external electric field. 44 Excitons need to diffuse to an interface before the electron and hole can be efficiently separated into free charges. 45 On the other hand, for light emitting applications the spatial displacement is ...

Two-dimensional layered perovskites are attracting increasing attention as more robust analogues to the conventional three-dimensional metal-halide perovskites for both light harvesting and light emitting applications. However, the impact of the reduced dimensionality on the optoelectronic properties remains unclear, particularly regarding the spatial dynamics of the excitonic excited state within the two-dimensional plane. Here, we present direct measurements of exciton transport in single-crystalline layered perovskites. Using transient photoluminescence microscopy, we show that excitons undergo an initial fast diffusion through the crystalline plane, followed by a slower subdiffusive regime as excitons get trapped. Interestingly, the early intrinsic diffusivity depends sensitively on the choice of organic spacer. A clear correlation between lattice stiffness and diffusivity is found, suggesting exciton-phonon interactions to be dominant in the spatial dynamics of the excitons in perovskites, consistent with the formation of exciton-polarons. Our findings provide a clear design strategy to optimize exciton transport in these systems.

Transient microscopy is of vital importance in understanding the dynamics of optical excited states in optoelectronic materials, as it allows for a direct visualization of the movement of energy carriers in space and time. Important information on trap‐state dynamics can be obtained using this technique, typically observed as a slow‐down of energy transport as carriers are trapped at defect sites. To date, however, studies of the trap‐state dynamics have been mostly limited to phenomenological descriptions of the early time‐dynamics. Here, it is shown how long‐acquisitiontime transient photoluminescence microscopy can be used to provide a detailed map of the trapstate landscape in 2D perovskites, in particular when used in combination with transient spectroscopy. An anomalous evolution of the studied exciton distribution is observed, which cannot be explained with existing models for trap limited exciton transport that only account for a single trap type. Instead, using a continuous diffusion model and performing Brownian dynamics simulations, it is shown that this behavior can be explained by accounting for a distinct distribution of traps in this material. These results highlight the value of transient microscopy as a complementary tool to more common transient spectroscopy techniques in the characterization of excited state dynamics in semiconductors.

Distinction between mobile and trapped tracers in disordered media reveals a simple phenomenological law for the subdiffusive exponent which reproduces the behaviour observed in a wide range of obstacles structures.

Halide mixing is one of the most powerful techniques to tune the optical bandgap of metal-halide perovskites. However, halide mixing has commonly been observed to result in phase segregation, which reduces excited-state transport and limits device performance. While the current emphasis lies on the development of strategies to prevent phase segregation, it remains unclear how halide mixing may affect excited-state transport even if phase purity is maintained. Here, we study exciton transport in phase pure mixed-halide 2D perovskites of (PEA) 2 Pb(I 1– x Br x ) 4 . Using transient photoluminescence microscopy, we show that, despite phase purity, halide mixing inhibits exciton transport. We find a significant reduction even for relatively low alloying concentrations. By performing Brownian dynamics simulations, we are able to reproduce our experimental results and attribute the decrease in diffusivity to the energetically disordered potential landscape that arises due to the intrinsic random distribution of alloying sites.

Previous research has shown that gold nanoparticles immersed in water in an optical vortex lattice formed by the perpendicular intersection of two standing light waves with a π/2 rad phase difference will experience enhanced dispersion that scales with the intensity of the incident laser. We show that flexible nanoscale dumbbells (created by attaching two such gold particles by means of a polymer chain) in the same field display different types of motion depending on the chain length and field intensity. We have not disregarded the secondary optical forces due to light scattering. The dumbbells may disperse, rotate or remain trapped. For some values of the parameters, the (enhanced) dispersion possesses a displacement distribution with exponential tails, making the motion anomalous, though Brownian.Optofluidics faces the challenging problem of understanding the interactions of light waves, electrons, and fluid and solid matter at the micro-and nanoscale. Previous work has shown that it can provide a way to control the transport properties of nanoscale objects, and it has already been applied to the guiding and sorting of particles in microfluidic flows [1,2].Numerical experiments have shown how to control the magnitude of the mean square displacement in a dilute suspension of gold nanoparticles in water by creating a stationary optical field at the intersection of two coherent laser beams with wavelengths close to the plasmon resonance (λ ≈ 395 nm) [3,5]. In particular, Albaladejo et al. demonstrated that perpendicular beams with a phase lag of π/2 rad enhance the dispersion of nanoparticles by a factor proportional to the power density of the laser [3].We aim here to provide a method for tuning the dispersion properties of nanoscale dumbbells created by attaching two identical 50 nm-radius gold spheres by means of a polymer strand [4]. Even though the setup in [3] enhances the dispersion for gold nanoparticles, the mean square displacement for dumbbells depends critically on the field intensity and the length of the connecting strand compared to the wavelength, as we shall show below. We will use the term diffusion to refer to thermal diffusion caused by random molecular collisions, and dispersion to refer to the combined effect of thermal fluctuations, optical forces and hydrodynamic coupling. SIMULATION SETUPFollowing Ref.[3], we began with a nonconservative optical field generated in water by the intersection of two perpendicular coherent laser beams with a π/2 rad phase difference polarised along the z axis. The resulting force field acting on gold nanoparticles along the xy plane, F opt (x, y), an optical vortex lattice (Fig. 1), corresponded to the equation below, F opt =2α ′ n c I ∇(sin(kx) + sin(ky)) 2 + 2α ′′ n c I ∇ × (2 cos(kx) cos(ky) e z ). (1) The refractive index was set to n = √ 1.8. We assumed that the particle radii are small enough compared to the incident wavelength to treat the particles as electric dipoles with a moment given by p = ǫǫ 0 αE, with complex electric polarisability α = α ′ + iα...

This cover image illustrates a transient microscopy measurement of the excited state energy transport in a perovskite lattice. An initial Gaussian population of excitations flows outwards, here illustrated by the flow of water. The energy transport is obstructed by defects along the way and transient microscopy measurements can therefore be used to accurately map out the trap‐state landscape in the perovskite lattice. For further information, see article number 2001875 by Ferry Prins and co‐workers.

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