Visualizing Buried Local Carrier Diffusion in Halide Perovskite Crystals via Two-Photon Microscopy

Abstract

Halide perovskites have shown great potential for light emission and photovoltaic applications due to their remarkable electronic properties and compatibility with cost-effective fabrication techniques. Although the device performances are promising, they are still limited by microscale heterogeneities in their photophysical properties. In particular, the relation between local heterogeneities and the diffusion of charge carriers at the surface and in the bulk, crucial for efficient collection of charges in a light harvesting device, is not well understood.Here, a photoluminescence tomography technique is developed in a confocal microscope using one-and two-photon excitation to distinguish between local surface and bulk diffusion of charge carriers in methylammonium lead bromide single crystals. The local temporal diffusion is probed at various excitation depths to build statistics of local electronic diffusion coefficients.The measured values range between 0.3 to 2 cm 2 .s -1 depending on the local trap density and the morphological environmenta distribution that would be missed from analogous macroscopic or surface-measurements. Tomographic images of carrier diffusion were reconstructed to reveal buried crystal defects that act as barriers to carrier transport. This work reveals a new framework to understand and homogenise diffusion pathways, which are extremely sensitive to local properties and buried defects.Over the past ten years, halide perovskites have emerged as strong candidates for various light-harvesting and light-emission applications [1][2][3] . The performances of perovskitebased photovoltaics (PV) and light-emitting diodes (LEDs) are now competing with mature, commercial technologies [4] . This rapid development has been made possible by the design of new halide perovskite compositions [5][6][7] which generally share properties of remarkably long carrier diffusion lengths (0.1-1 μm) [8,9] even when simple cost-effective fabrication techniques are employed. However, for halide perovskites to reach their full potential, one has to understand the microscopic heterogeneities that still limit their performances [10,11] . For instance, local defects, both at the surface and inside the bulk, trap charge carriers thus limiting their ability to diffuse through the material. It is therefore critical to investigate the diffusion mechanisms at the local scale to identify these trap sites and elucidate ways to mitigate their influence on carrier diffusion and recombination.Methylammonium lead bromide (MAPbBr3, MA=CH3NH3 + ) single crystals have remarkable photophysical properties as highlighted in recent reports on amplified spontaneous emission [12] and lasing phenomena [13,14] , two-photon absorption [15,16] , extreme sensitivity to environment [17] , excitonic properties [18,19] , and long carrier diffusion lengths [20] . Additionally, their optical properties are well-documented including their refractive index [21,22] and exciton binding energy [23] , and photon reabsorption has been quantified [22,24,25...

Revealing Nanomechanical Domains and Their Transient Behavior in Mixed‐Halide Perovskite Films

“…The excitation power was maintained below 2.5 µJ cm −2 pulse −1 . According to previous references, [ 10,56 ] this fluence was high enough to saturate the traps and locally create Auger non‐radiative effects. Nevertheless, the confocal nature of the measurement may have meant that carriers quickly diffuse and therefore the concentration quickly dilutes down to trap limited regime (which likely reflects the lifetimes one measure).…”

“…Since 2009, halide perovskites have emerged as promising materials for efficient optoelectronic devices such as photovoltaics and light‐emitting diodes (LEDs), [ 1–3 ] with performance in some configurations already exceeding comparable existing commercial technologies. [ 4,5 ] Halide perovskite semiconductors show unique properties such as remarkable defect tolerance, [ 6 ] efficient light absorption [ 7,8 ] and long charge‐carrier diffusion lengths, [ 9–11 ] which put them at the forefront of emerging thin‐film technologies. One advantage they have over conventional semiconductors is that they can be fabricated through facile and cost‐effective preparation techniques (e.g., solution processing), with the precursor components self‐assembling into relatively large entities (domains of ≈0.1 to several micrometers in size, [ 12 ] referred to as grains).…”

Local Energy Landscape Drives Long-Range Exciton Diffusion in Two-Dimensional Halide Perovskite Semiconductors

“…(See SI Section XII and Figures S11 and S12 for Gaussian profiles and fits at selected times.) We find a linear relationship between the quantity [σ 2 ( t ) – σ 2 (0)] and t ( Figure 2 c), indicating that the spatial broadening is due to classical exciton diffusion 23 , 29 and can be related to the exciton diffusion coefficient D using the formula 23 , 29 , 30 Linear fits to the data in Figure 2 c with eq 1 yield a diffusion coefficient of 0.018 cm –2 s –1 for n x = 1.0 × 10 8 cm –2 ( Figure 2 c inset). At this low exciton density, we expect exciton trapping to dominate on the basis of the earlier PL efficiencies, η.…”

Charge Transport Properties of Methylammonium Lead Trihalide Hybrid Perovskite Bulk Single Crystals

“…[ 89 ] Bakr and co‐workers have also used a similar contactless method, the time‐resolved photoluminescence microscopy method for analyzing and visualizing buried heterogeneities, in bulk crystals. [ 99 ] Other contactless methods that have been developed albeit for thin films, also include those based on transient absorption microscopy, which has been implemented to display a spatial distribution map of free charge carriers and excitons in MAPbI 3 , as well as allowing for femtosecond time resolution, as well as ultrafast THz spectroscopy and microwave photoconductivity. [ 100–105 ]…”