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 FA_0.83Cs_0.17Pb(I_1-xClx)₃ films with nanoscale spatial resolution (Fig. 1a). By analysing characteristic phonon signatures in the spectral response, we distinguish domains of the photoactive cubic α-phase from the trigonal δ-phase and PbI₂ nano-islands (Fig. 1b). 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 t_p (Fig. 1c). Notably, the full pump-induced waveforms at different t_p (Fig. 1d) show a deeply subcycle time shift ∆_t, which can be microscopically linked to a diffusion-driven change in the vertical carrier distribution with t_p (Fig. 1e, inset). Combining a straightforward rate equation model with the finite-dipole model, we extract the evolution of the out-of-plane carrier density profile from ∆_t (Fig. 1e). Mapping the out-of-plane diffusion coefficient D along a line across different grains, including α-phase grains and PbI₂ contaminations, we find a homogeneous value of D = (0.2 ± 0.1) cm²s^-1, which is surprisingly immune to compositional and structural variations (Fig. 1f). 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.