We perform ultrafast Faraday holographic imaging to track the magnetization dynamics of perovskites in time and space. This interferometric imaging technique, based on off-axis holography, has the advantage of being shot-noise limited and allows us to get access to both amplitude and phase information of the measured signal. As a result, we can directly retrieve and disentangle the angular momentum and the spin components of the total magnetic moment inside the material. Here, we present our results on Methylammonium Lead Tribromide (MAPbBr3), a prototypical hybrid metal halide perovskite with captivating magnetic properties for future opto-spintronic applications.
Bismuth vanadate (BiVO4) is a key prototypical photocatalyst for water splitting, due to efficient collection of sunlight with an absorption onset at 2.5 eV, close to the maximum flux of the solar spectrum, and high solar to hydrogen conversion efficiency of up to 9.2%. Despite these promising characteristics, the fundamental nature and dynamics of photoexcitations in BiVO4 remain unclear. We now use advances in x-ray pump-probe techniques at sub-picosecond timescales to study the interactions of photo-excitations with the crystal lattice and connected changes of the atomic valency state in BiVO4 thin films. We measure pump-probe X-Ray Diffraction (XRD), X-ray Diffuse Scattering (XDS) and X-ray Absorption Near-Edge (XANES) at EuXFEL and APS to resolve structural and electronic dynamics. We find an unexpected ultrafast photoinduced structure change from monoclinic to tetragonal phases. From dynamics of related electronic valency changes and lattice strain fields, we draw up a detailed mechanistic model of our observations.
The formation of local strain fields is a key aspect in understanding light-induced processes in semiconductors: For instance, electric conductivity is influenced by the formation of polarons, quasiparticles that evolve from the interaction of a charge carrier with the lattice. We performed pump-probe experiments with an X-ray Free-Electron Laser (XFEL) to measure the photoinduced X-ray scattering dynamics of epitaxial BiVO4 with femtosecond time resolution. We then compared this data to simulations of different localized strain fields in a regular quadratic lattice. While the material shows little diffuse scattering, comparison with simulations of an acoustic strain wave indicates that the material is contracting in a concerted motion.
Hybrid semiconductor materials are predicted to lock chirality into place and encode asymmetry into their electronic states, while the softness of their crystal lattice accommodates lattice strain and maintains high crystal quality with the low defect densities necessary for high luminescence yields. The realization of chiral bulk emitters with bright circularly-polarized luminescence from such materials is desired for the design of chiroptical photonic and opto-spintronic applications. Here, we report photoluminescence quantum efficiencies (PLQE) as high as 39%, and degrees of circularly polarized photoluminescence of up to 52%, at room temperature, in the chiral layered hybrid lead-halide perovskites (R/S/Rac)-3BrMBA2PbI4 (3BrMBA = 1-(3-Bromphenyl)-ethylamine). Using x-ray diffraction and density-functional theory we elucidate the detailed chirality transfer mechanism from chiral crystal structures to spin-orbit-split band structures. Using state-of-the-art transient chiroptical spectroscopy, we rationalize the excellent photoluminescence yields from suppression of non-radiative loss channels and very high rates of radiative recombination. We further find that photo-excitations sustain polarization lifetimes that exceed the timescales of radiative decays, which rationalize the high degrees of polarized luminescence. We postulate that the superior optoelectronic properties of the layered hybrid perovskites arise from their special tolerance to crystal structure chirality, which we carefully designed by cation engineering. Our findings pave the way towards high-performance solution-processed photonic systems for chiroptical applications and chiral-spintronic logic at room temperature.
The outstanding optoelectronic properties of hybrid metal halide perovskites, and their strong spin-orbit coupling, enable efficient manipulation of the charge carrier’s angular momentum. In this work, we investigate the dominant spin relaxation mechanism in CH3NH3PbBr3 films with doping levels up to 50% with the transition metal Mn2+. We investigate the spin relaxation times in these paramagnetic hybrid semiconductors with ultrafast circularly polarized broadband transient absorption spectroscopy at cryogenic temperatures. We report extended spin relaxation lifetimes by a factor of three which we explain with motional narrowing effects in the paramagnetic ensemble of Mn2+ impurities. Our work now presents paramagnetic doping of hybrid semiconductors as a promising concept for quantum technologies and spintronic devices.
KEYWORDS: Perovskite, Modulation, Luminescence, Optoelectronics, Process control, Tandem solar cells, Solar energy, Solar cells, Semiconductors, Quantum efficiency
Using transient optical spectroscopies, we study excitation recombination dynamics in manganese-doped cesium lead-halide perovskite nanocrystals. Unexpectedly, we find an increase in the intrinsic excitonic radiative recombination rate upon doping, which is typically a challenging material property to tailor. Supported by ab initio calculations, we can attribute the enhanced emission rates to increased exciton localization through lattice periodicity breaking from Mn dopants, which increases exciton effective masses and overlap of electron and hole wavefunctions and thus the oscillator strength. Our report of a fundamental strategy for improving luminescence efficiencies in perovskite nanocrystals will be valuable for maximizing efficiencies in light-emitting applications.
Functional materials combining the optoelectronic functionalities of semiconductors with control of the spin degree of freedom are highly sought after for the advancement of quantum technology devices and provide exciting avenues for polarized light-emission. Previous work towards this goal introduced small amounts of magnetic elements into crystalline semiconductor, e.g. through vacuum-based deposition, to obtain dilute magnetic semiconductors (DMS).
In my talk, I will present our efforts on gaining control over spin dynamics and spin interactions through compositional and structural tuning in solution-processable hybrid perovskite semiconductors. We aim to exploit the exceptional optoelectronic properties of these hybrid perovskites, together with their tolerance in the electronic states to dopants and defects, to make advances towards high-performance DMS.
Hybrid perovskites have emerged as exceptional semiconductors for optoelectronic applications. Here, we control the cation alloying to push optoelectronic performance through alteration of the charge carrier dynamics in mixed-halide perovskites. In contrast to single-halide perovskites, we find high luminescence yields for photo-excited carrier densities far below solar illumination conditions. Using time-resolved spectroscopy we show that the charge-carrier recombination regime changes from second to first order within the first tens of nanoseconds after excitation. Supported by microscale-mapping of the optical bandgap and elemental composition, electrically-gated transport measurements and first-principles calculations, we demonstrate that spatially-varying energetic disorder in the electronic states causes local charge accumulation, creating p- and n-type photo-doped regions, which unearths a strategy for efficient light emission at low charge-injection in solar cells and LEDs.
Optical pump-terahertz probe spectroscopy is a powerful contact-free technique for probing the electronic properties of novel nanomaterials and their response to photoexcitation. This technique can measure charge carrier transport and dynamics with sub-picosecond temporal resolution. Electrical conductivity, charge carrier lifetimes, mobilities, dopant concentrations and surface recombination velocities can be measured with high accuracy and with considerably higher throughput than achievable with traditional contact-based techniques. We describe how terahertz spectroscopy is revealing the fascinating properties and guiding the development of a number of promising semiconductor materials, with particular emphasis on III-V semiconductor nanowires and devices.
Luis Pazos, Monika Szummilo, Robin Lamboll, Johannes Richter, Micaela Crespo-Quesada, Mojtaba Abdi-Jalebi, Harry Beeson, Milan Vrucinic, Mejd Alsari, Henry Snaith, Bruno Ehrler, Richard Friend, Felix Deschler
We mapped the propagation of photogenerated luminescence and charges from a local photoexcitation spot in thin films of lead tri-iodide perovskites using a confocal microscopy setup with independent excitation and collection objectives. We observed regenerated PL emission at distances as far as 50 micrometers away from photoexcitation. We then made a scratch in the film to increase out-scattering and found that the peak of the internal photon spectrum red-shifts from 765 to ≥800 nanometers. This is caused by the sharp decay of the absorption coefficient at the band tail, which allows longer wavelength photons to travel further between emission and absorption events, originating charges far from excitation.
We then built a lateral-contact solar cell with selective electron- and hole-collecting contacts, using a combination of photolitography and electrodeposition. We used these devices as a platform to study photocurrent propagation and found that charge extraction can be achieved well beyond 50 micrometers away from the excitation.
We connect these two observations by comparing the decay in intensity of the recycled component of the PL (which is around 765 nm) with the decay in photocurrent. Taking into account that PL is proportional to the square of charge density, whilst photocurrent is proportional to charge density.
Photon recycling leads to an increase in internal photon densities, which leads to a build-up of excited charges. This increases the split of quasi-Fermi levels and enhances the achievable open circuit voltage in a solar cell.
Hybrid halide perovskites are at the frontier of optoelectronic research due to their excellent semiconductor properties and solution processability. For this reason, much attention has recently been focused on understanding photoexcited charge-carrier generation and recombination in these materials. Conversely, very few studies have so far been devoted to understanding carrier-carrier and carrier-phonon scattering mechanisms in these materials. This is surprising given that carrier scattering mechanisms fundamentally limit charge-carrier motilities and therefore the performance of photovoltaic devices. We apply linear polarization selective transient absorption measurements to polycrystalline CH3NH3PbBr3 hybrid halide perovskite films as an effective way of studying the scattering processes in these materials. Comparison of the photo induced bleach signals obtained when the linear polarizations of the pump and probe are aligned either parallel or perpendicular to one another, reveal a significant difference in spectral intensity and shape within the first few hundred femtoseconds after photoexcitation.
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