Prof. Dr. Johan Hofkens
The Max Planck Fellowship has enabled a structurally embedded collaboration between KU Leuven and MPI-P, reflected in more than one hundred publications associated primarily with Dept. Bonn and affiliated groups. These activities integrate Hofkens’ expertise in single-molecule and super-resolution spectroscopy with MPI-P’s core strengths in ultrafast and non-linear spectroscopy, particularly in the areas of charge-carrier dynamics and interfacial photophysics. The collaboration has centered on understanding and controlling light–matter interactions in complex materials, with emphasis on metal–halide perovskites and related semiconductors, and on advancing optical methodologies that connect MPI-P’s ultrafast spectroscopy strengths with single‑particle and super‑resolution approaches.
A major thrust of the joint work has been the elucidation of charge-carrier generation, localization, and transport in lead-halide and related perovskite materials, including all-inorganic and mixed-metal compositions. Key contributions include the identification and quantitative characterization of large polaron formation, the dimensional tuning of polaron states, and the demonstration that a stable Mott-polaron–like state imposes an intrinsic upper bound on achievable carrier densities in these materials.
These insights have been obtained by combining time-resolved optical spectroscopies developed at MPI-P with spatially resolved and single-particle photoluminescence methods from the Leuven/Hofkens environment, enabling correlation of local microstructure with polaronic transport and recombination pathways. The resulting framework links microscopic carrier self-trapping and lattice response to device-relevant parameters such as photoconductivity, radiative efficiency, and operational stability in optoelectronic architectures.
Another central theme has been to resolve the spatial and temporal heterogeneity of photophysical processes under operating or near-operating conditions in perovskite and related semiconductor devices. Joint work has established in‑operando, locally resolved optical probes that reveal how local composition, strain, and defect landscapes govern non-radiative losses and long-term stability at the sub-micrometer scale.
Single-particle and super-resolution measurements, coupled with MPI-P’s expertise in ultrafast and terahertz-domain spectroscopies, have made it possible to directly connect local trap formation, carrier funneling, and phase segregation with macroscopic device performance metrics. This multiscale approach—ranging from nanoscopic emitters to full devices—embodies the added value of the Max Planck Fellowship as it leverages complementary infrastructures and know-how across the two institutions.
The collaborative research has significantly strengthened MPI-P’s portfolio in emerging optoelectronic materials, adding single-molecule, wide-field, and super-resolution photophysics to an already strong base in vibrational and ultrafast electronic spectroscopy. The joint work contributes to MPI-P’s strategic themes of charge-carrier transport and interfacial structure–function relationships.
Beyond publications, the fellowship has fostered bidirectional mobility of PhD students and postdocs, co-supervision arrangements, and shared project development, including European and national funding initiatives that explicitly link KU Leuven and MPI-P. These activities help train a new generation of researchers fluent in both advanced optical microscopy and ultrafast spectroscopy, thereby amplifying the long-term scientific and institutional impact of the Max Planck Fellowship at MPI-P.