Project Details
Projekt
Coherent exciton dynamics in lead-free double perovskites
Subject Area
Experimental Condensed Matter Physics
Term
since 2022
Project identifier
Deutsche Forschungsgemeinschaft (DFG) - Project number 506623857
The ideal solution for future optoelectronic and photovoltaic applications based on perovskites is to find stable, inherently non-toxic, earth-abundant and at the same time efficiently light-absorbing, charge-transporting, and energetically tunable material systems. However, the search for competitive lead-free perovskite semiconductor systems for energy conversion and light emission proved more complex than initially anticipated. Lead-free double perovskites are regarded as an important platform for this, but their three-dimensional prototype Cs2AgBiBr6 has so far shown only moderate performance when used in solar cells. The physical reasons for this are currently the subject of controversial discussions. The aim of our proposal is to explore and understand the potential of lead-free double perovskites of different dimensionality (3D, 2D/3D, 2D) for photovoltaic and photonic applications and to provide a conceptual understanding of the details of the energy fine structure and excited states and their dependence on the symmetry and dimensionality of the crystalline host systems. The starting point of our project will be single crystals and films of the reference material Cs2AgBiBr6, whose synthesis was well established in the project in the first funding period of SPP2196. To investigate the effect of dimensionality, we will also fabricate and study 2D and multidimensional 2D/3D double perovskites. Finally, we not only want to avoid toxic elements (e.g., Pb), but rather use earth-abundant and cheap elements. Therefore, we will also grow double perovskites Cs2AgFeCl6 and Cs2NaFeCl6, which we recently found to have intriguing thermochromic and magnetic properties but are largely unexplored. We will investigate the properties of neutral (excitons) and charged (electrons/holes) excited states and their interactions in double perovskite materials of different dimensionality and composition using advanced optical spectroscopy. More specifically, we will use coherent nonlinear optical spectroscopy, in particular, time-resolved, polarization-dependent pump-probe and photon echo methods, which can provide rich information about the energy structure and relaxation processes despite strong inhomogeneous broadening of optical transitions in the systems under investigation. Most importantly, the exciton itself will serve as a probe for the interaction with the crystal lattice, local potential fluctuations, other excitons and charge carriers in our studies. We will study the exciton and photoexcited carrier lifetimes, their binding energy, the symmetry-induced optical anisotropy and the effects of localization (self-trapping) of excitons, charge carriers and more complex quasiparticles. The expected results will not only be of fundamental interest but will also help in the development of new non-toxic and stable perovskite materials for photovoltaics and photonics, but also beyond, for example in quantum applications where coherence effects are important.
DFG Programme
Priority Programmes
