Spatial Inhomogeneity Analysis of Cesium-Rich Wrinkles in Triple-Cation Perovskite

Abstract

Chemical composition engineering in metal halide perovskite leads to enhanced stability and better transport properties, opening the gate to their integration in competitive photovoltaic tandem devices or LEDs. However, triple-cation perovskites show morphological, chemical, optical, and optoelectronic heterogeneities. In this study, we focus on micrometric spatial inhomogeneities where we observe wrinkle formation in the fabrication process linked to the cesium addition. Electron-dispersive spectroscopy and photoluminescence (PL) hyperspectral imaging designate these morphological features as Cs-rich and N-poor, which underlines the role of long-range chemical migration in the perovskite formation. We also study charge-carrier diffusion and recombination using time-resolved PL imaging under wide-field illumination, which we correlate with 3D drift diffusion modeling. We underline the impact of wrinkles on local optoelectronic properties such as the bandgap opening due to Cs enrichment and the longer charge carrier lifetime due to lower trap density.

Heterogeneity at multiple length scales in halide perovskite semiconductors

“…2b), the interfaces between grains and their overall 3D distribution throughout the microstructure of a halide perovskite sample undoubtedly influence non-radiative recombination events, ion migration and the infiltration of species such as oxygen and moisture [39][40][41] . Some of the encompassing long-range disorder (in the range of tens of micrometres or larger) may stem from these nanoscale and mesoscale heterogeneities; however, undesirable disorder can also arise from unoptimized fabrication practices 42 , sub-optimal interfaces 43 or poor heat-transfer management that promotes widespread strain or wrinkling 44,45 (Fig. 2c).…”

“…Long-range variations in the properties of semiconductor materials are frequently observed using optical microscopy (reflection or transmission) 40,75,109 , wide-field PL and electroluminescence (EL) 44,110 or low-magnification SEM imaging modes 111 . To alleviate long-range non-uniformities in material properties, there have been notable advances in device fabrication and synthesis techniques to increase the connectivity of the perovskite grains for improved film quality 83,[112][113][114][115][116][117][118][119] .…”

Monitoring Charge Carrier Diffusion across a Perovskite Film with Transient Absorption Spectroscopy

“…Charge carrier mobility is one of the most important parameters for materials in applications such as solar cells, but it is notoriously difficult to measure. New contact-free measurement techniques have been sought in order to avoid the problems faced by contact methods − (the need to apply voltage across the sample, diffusion of ions due to the voltage, and electrode–film interface complications), but so far the optical methods − have mainly been able to measure intragrain mobility with some exceptions for measuring mobility parallel to the film surface. , This is problematic because mobility perpendicular to the film surface is often more relevant for practical applications: The intragrain mobility and mobility over multiple grain boundaries can differ by orders of magnitude, , while the perpendicular mobility is somewhere between these two extremes. Our new contact-free method presented in this Letter allows us to determine this mobility across thin films.…”

Substance and shadow of formamidinium lead triiodide based solar cells

“…[40] The entropy enhancement and reduction of annealing temperature upon Cs inclusion which was optimized over a range of 3 -20% to balance the bandgap widening and the increase of density of states (DOS) below the valance band maxima (VBM). [41] In addition to the phase stability, different imaging or mapping techniques such as Synchrotron-based nanoscale X-ray fluorescence (n-XRF) [42] , photoluminescence (PL), hyperspectral imaging [43] showed that inorganic Cs doping homogenizes the halide distribution between Cl, I and Br and reduces the heterogeneity in the charge carrier dynamics [42,43] which eventually highlighted the ability of the triple-cationmixed halide perovskites to function as capable light harvesters. The addition of dopants as additives in the precursor solution, a non-uniform distribution at perovskite surface and bulk has been reported in thin films which significantly affects the structural, optoelectronic and device properties.…”