Proceedings of nanoGe Fall Meeting19 (NFM19)
DOI: https://doi.org/10.29363/nanoge.nfm.2019.284
Publication date: 18th July 2019
Metal-halide hybrid perovskites exhibit excellent optoelectronic properties – apart from their rather moderate charge carrier mobilities.
The origin of these moderate mobilities has been attributed to several (contradicting) effects, such as the formation of large and small polarons, dynamic disorder due to the soft nature of these materials, slow rotational modes of the organic molecules, as well as to the confinement of charge carriers in grains, ferroelectric domains or nanostructures.
To clarify the nature of the charge carrier transport, we probed different hybrid and inorganic halide perovskites thin films and nano-crystals by Optical-Pump Terahertz-Probe (OPTP) spectroscopy.
Interestingly, we find very similar sum mobilities of approximately 60 cm2/Vs as well as similar phonon spectra for MAPbI3, (Cs,FA)PbI3, CsPbI3 [1] and (Cs,FA,MA)Pb(I,Br)3, thin films [1,2]. This finding excludes a significant impact of the organic cation rotation and supports the dominance of the Pb-I cage on the optoelectronic properties.
Additionally, OPTP reveals the frequency-dependence of the charge carrier mobility at THz-frequencies, which indicates very short scattering times at room temperature. Such large scattering impedes transport and limits the charge carrier mobilities.
Temperature-dependent OPTP measurements on the same materials show a strongly increasing mobility with lower temperature, thus excluding small polaron formation and hopping transport. Instead, this behavior can be modeled by conventional large polaron theory and Fröhlich-type electron-phonon scattering.
Thin films of CsPbI3-nanocrystals exhibit even smaller charge carrier mobilities of < 1cm2/Vs compared to their polycrystalline counterparts. Therefore, their charge transport is limited by the coupling between the nano-crystals and different approaches are shown to improve this coupling.
Further, we discuss the impact of phonon modes on the OPTP signal that overlay the mobility-correlated signal. Such phonon contributions are especially relevant for nano-crystals and at low temperatures.
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