Layered (2D) hybrid perovskite materials have shown great potential as passivating films and as active photovoltaic layers which improve the long-term stability of perovskite solar cells. While understanding the charge transport properties of 2D hybrid perovskites is critical for their effective implementation, characterization of carrier dynamics both in the plane of and transverse to the layer structure has been limited. We explore carrier transport using polarization-dependent terahertz (THz) conductivity measurements in benchmark layered and bulk hybrid perovskites. We have synthesized ~300 nm thick films of bulk methylammonium, n = 1 layered butylammonium, and n=1 layered phenethylammonium lead iodide perovskite semiconductors with respective 2D layers oriented parallel to silicon substrates and with crystalline domain sizes of approximately 1 micron diameter. In the measurements, ~110 femtosecond pulses from a Ti:sapphire laser source were down-converted to THz radiation via two-color plasma emission and transmitted through our samples. Transmission measurements at normal incidence yielded values of complex transmission coefficients which enable determination of the complex dielectric function in the 0.3 - 8 THz range, corresponding to the in-plane complex conductivities of these materials. We will also discuss results of non-normally incident transmission measurements varying s and p THz polarization to probe the differences in ligand-mediated out-of-plane transport between these layered perovskites. We uncover dramatically different conductivities between these intrinsic crystal orientations which cumulatively define charge carrier behavior in these materials. This work expands on the utility of oblique incidence terahertz experiments which are rarely implemented for complex parameter extraction in uniaxial thin films. Furthermore, these results underscore the potential for THz spectroscopy to probe fundamental charge transport phenomena in complex photovoltaic materials with layered structures.

Anisotropic charge carrier dynamics in 2D hybrid perovskites probed by oblique incidence ultrabroadband terahertz spectroscopy

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