Walter Schottky Institute
Center for Nanotechnology and Nanomaterials

Deschler - Research
Group leader: Dr. Felix Deschler - Emmy Noether Fellow


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Research Focus



A recent material focus of our work are hybrid semiconductors, in particular the hybrid perovskites, which represent an exciting class of sustainable semiconductors with a range of surprising physical characteristics. We fabricate these materials in-house using solution-based self-assembly, which creates new opportunities for tailoring electronic states and interfaces for opto-electronic applications.

Employing our expertise in solid state physics and materials science, and driven by a passion to answer fundamental physical questions, we have recently made key discoveries on the physical origins of the exceptional properties of the hybrid perovskites, e.g. their high luminescence yields. Beyond the hybrid perovskites, we have considerable interest and made high impact contributions in emerging energy materials exhibiting strong localization (i.e. organic molecular semiconductors), local probes of excitation dynamics in nanostructures, and novel architectures for opto-electronic devices.

Cross-cutting these themes is the development and application of novel tools for characterization of nanoscopic interactions in hybrid materials, in which disorder is inevitable and often governs function. These advanced methods, ranging from near-field approaches in high-resolution optical spectroscopy to facility-based time-resolved scattering measurements, are used to elucidate elementary steps in functional materials across various length, time, and energy scales. 



We are currently exploiting the opportunities arising from the versatile perovskite material platform to address fundamental knowledge gaps and aim to achieve breakthroughs in material physics in the following areas:


A.    
Overcoming fundamental challenges associated with charge localization in novel semiconductors to maximize control of luminescent recombination in materials for optoelectronics
B.
Synthesizing and characterizing hybrid magnetic semiconductors to answer open questions regarding spin-interactions within functional materials for green spintronics and information storage
C.
Understanding fundamentals steps of charge-to-ion conversion in photoactive battery electrode materials and operating electrochemical devices







Research Highlights


Photodoping Enables Efficient Luminescence in Alloyed Perovskites


  Nature Photonics (2020)

We reported that mixed-halide perovskites show high luminescence yields for carrier densities far below solar illumination conditions. Supported by microscale mapping of the optical bandgap, 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 densities. Contrary to common believes, disorder in the alloyed perovskites proves beneficial for their optoelectronic properties.



Probing Ultrafast Carrier Thermalization with 2D Electronic Spectroscopy


 Nature Communications (2017)


In band-like semiconductors, charge carriers form a thermal energy distribution rapidly after optical excitation. In hybrid perovskites, the cooling of such thermal carrier distributions occurs on timescales of 300 fs via carrier-phonon scattering. However, the initial build-up of thermal distributions proved challenging to resolve with pump–probe techniques due to the requirement of high resolution in time and pump energy. Here, we use two- dimensional electronic spectroscopy with sub-10 fs resolution to directly observe the carrier interactions that lead to a thermal carrier distribution. We find that thermalization occurs dominantly via carrier-carrier scattering under the investigated fluences and report the dependence of carrier scattering rates on excess energy and carrier density.





P
hoton Recycling in Hybrid Perovskites



 Science (2016)

We have shown that photon recycling, as seen previously in highly efficient gallium arsenide solar cells, contributes to the exceptional performance of hybrid perovskites. In most solar cells, the recombination of photogenerated charge carriers (electrons and holes) wastes all of the energy. In the perovskites, recombination of generated charges emits a photon that can be reabsorbed and create further charge carriers.

We mapped the propagation of photogenerated luminescence and charges from a local photoexcitation spot in thin films of lead tri-iodide perovskites. We observed light emission at distances of ≥50 micrometers and found that the peak of the internal photon spectrum red-shifts from 765 to ≥800 nanometers. Thus, energy transport is not limited by diffusive charge transport but can occur over long distances through multiple absorption-diffusion-emission events.







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