Imaging Carrier Transport Properties in Halide Perovskites using Time‐Resolved Optical Microscopy

Abstract

Halide perovskites have remarkable properties for relatively crudely processed semiconductors, including large optical absorption coefficients and long charge carrier lifetimes. Thanks to such properties, these materials are now competing with established technologies for use in cost-effective and efficient light harvesting and light emitting devices. Nevertheless, our fundamental understanding of the behaviour of charge carriers in these materialsparticularly on the nano-to micro-scalehas on the whole lagged behind the empirical device performances. Such understanding is essential to control charge carriers, exploit new device structures, and push devices to their performance limits. Among other tools, optical microscopy and spectroscopic techniques have revealed rich information about charge carrier recombination and transport on important length scales. In this Progress Report, we detail the contribution of time-resolved optical microscopy techniques to our collective understanding of the photophysics of these materials. We discuss ongoing technical developments in the field that are overcoming traditional experimental limitations in order to visualise transport properties over multiple time and length scales. Finally, we propose strategies to combine optical microscopy with complementary techniques in order to obtain a holistic picture of local carrier photophysics in state-of-the-art perovskite devices.2 Over the last decade, there has been a dramatic rise in the efficiencies of light harvesting [1,2] , and light emitting [3,4] devices based on halide perovskite materials, with performances already rivalling existing commercial technologies. Perovskites are high-quality semiconductors with remarkable optoelectronic properties including an apparent defect tolerance [5] , long chargecarrier lifetimes [6] , long charge-carrier diffusion lengths [7,8] and sharp absorption edges [9] . However, they also exhibit complex heterogeneous morphological, chemical, structural and photo-physical properties across multiple length scales [10] arising from combinations of their hybrid organic-inorganic nature [11,12] , mixed chemical compositions [13] and their polycrystalline structure [14] , presenting challenges for rigorous charaterisation. A broad range of experimental techniques [10] has been employed to provide insight into their structural, chemical, morphological and macroscopic device operation properties, including electrical devices, diffraction, and electron microscopy characterization approaches, but these tools fall short in elucidating critical processes impacting device operation, namely the dynamics of photoexcited species and the transport of energy on all length scales. Optical spectroscopy techniques [15][16][17][18][19] directly probe the photophysics of materials and devices (see Figure 1a and b) making them powerful tools to establish and contextualise important properties such as absorption and photoluminescence [15] (PL) spectra [20,21] , exciton binding energies [22] , PL or device quantum efficien...

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

“…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.…”

Emerging Perovskite Solar Cell Technology: Remedial Actions for the Foremost Challenges

“…Several researchers have put effort into understanding these fundamentals through spatial and time resolved techniques such as microwave impedance microscopy, c‐AFM, optical microscopy using a home built microscope, and femtosecond time‐resolved transient absorption spectroscopy. [ 316–319 ] Although various methods and techniques are available to investigate local properties in perovskites, still it requires precise experimental studies to simultaneously probe lifetime, transport time, and diffusion length for different microstructures. For the first time, Bahrami et al.…”