Ultrafast Nanoscopy of Carrier Dynamics and Nanoscale Morphology in Metal Halide Perovskites

The rise of metal halide perovskites has prompted a revolution in optoelectronics research [1]. Yet a comprehensive understanding of the vertical charge transport that underlies the soaring efficiencies of perovskite-based solar cells has remained challenging owing to the materials' nanocrystalline structure [2] and competing crystal phases [3]. Here, we simultaneously probe the intrinsic out-of-plane charge-carrier diffusion and the nanoscale morphology by pushing depth-sensitive terahertz near- field nano-spectroscopy [4] to extreme subcycle timescales [5]. Evanescent terahertz fields at the apex of a metallic tip probe ultrafast dynamics of photocarriers in <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{FA}_{0}.{}_{83}\text{Cs}_{0.17}\text{Pb}(\mathrm{I}_{1}{}_{-\mathrm{x}}\mathrm{C}1_{\mathrm{x}})_{3}$</tex> films with nanoscale spatial resolution (Fig. 1 a). By analysing characteristic phonon signatures in the spectral response, we distinguish domains of the photo active cubic <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\alpha$</tex> -phase from the trigonal δ-phase and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{PbI}_{2}$</tex> nano-islands (Fig. 1 b). To examine the impact of these nanoscale inhomogeneities on carrier dynamics, we monitor the peak pump-induced signal as a function of pump delay time <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$t_{\mathrm{p}}$</tex> (Fig. 1c). Notably, the full pump-induced waveforms at different <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$t_{\mathrm{p}}$</tex> (Fig. 1d) show a deeply subcycle time shift <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\Delta t$</tex>, which can be microscopically linked to a diffusion-driven change in the vertical carrier distribution with <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$t_{\mathrm{r}}$</tex> (Fig. 1e, inset). Com-bining a straightforward rate equation model with the finite-dipole model, we extract the evolution of the out-of-plane carrier density profile from <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\Delta t$</tex> (Fig. 1e). Mapping the out-of-plane diffusion coefficient <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$D$</tex> along a line across different grains, including <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\alpha$</tex> -phase grains and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{PbI}_{2}$</tex> contaminations, we find a homogeneous value of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$D=(0.2\pm 0.1)\ \text{cm}^{2}\mathrm{s}^{-1}$</tex>, which is surprisingly immune to compositional and structural variations (Fig. 1 f). This robustness of vertical diffusion may contribute to the exceptional performance of perovskite-based devices. Linking in situ carrier transport with nanoscale morphology and chemical composition could introduce a paradigm shift for the analysis and optimization of next-generation optoelectronics that are based on nanocrystalline materials.