Introduction

Lead halide perovskites exhibit remarkable photovoltaic performance, characterized by long carrier lifetime and extraordinary defect tolerance. The highly dynamical lattice of the perovskites is believed to play a key role by forming a large polaron (Fig. 1A), which arises from electron-phonon coupling across multiple unit cells and stabilizes the photoinduced carriers. On the other hand, the perovskite films are known to be spatially non-uniform in their optoelectronic response, yet the fundamental origin of such disorder, particularly in relation to the elementary electronphonon coupling, has remained elusive.

Here, we develop ultrafast heterodyne infrared nano-imaging [1] to provide access to the full spatio-temporal-spectral response dynamics and apply to a triple cation perovskite film [2] to reveal its polaron dynamics on the nanoscale. By directly probing the pump-induced mid-infrared "polaron absorption" (Fig. 1B) with nanometer spatial resolution [3], we resolve the heterogeneity in the polaron formation and its dynamics on the picosecond scale. By taking advantage of the combined spatial, temporal, and spectral resolution, we also probe a nano-localized transient molecular vibrational response that signifies the coupling between polaron and molecular cations. We thus characterize the nanoscale disorder in perovskites from the perspective of the fundamental electron-phonon coupling, the control of which is critical for the engineering of perovskite-based optoelectronic devices.

Ultrafast heterodyne infrared nano-imaging system

Fig. 1C shows the schematic of our ultrafast heterodyne infrared nano-imaging and -spectroscopy system [1]. The optical pump and mid-infrared probe pulses are collinearly focused to a metallic tip apex in an atomic force microscope, inducing a nanoscale pump-probe signal [4]. The near-field pump-probe signal is interfered with a reference pulse for heterodyne detection, implementing phase and spectral resolution (Fig. 1D,1E) and also eliminating contributions arising from the uncontrolled interference of the near-field signal with the far-field background scatter [5].

Polaronic heterogeneity in a lead halide perovskite

We study a triple cation perovskite (Fig. 2A), [(FA 0.83 MA 0.17 ) 0.95 Cs 0.05 ]Pb(I 0.83 Br 0.17 ) 3 , as a highly stable model system of the perovskites. Fig. 2B shows conventional far-field visible-pump/mid-infrared-probe decay profiles with pronounced dependence on the pump fluence, suggesting that the observed carrier dynamics are dominated by the second and higher order carrier recombination. For a given fluence, we acquire pump-probe spectra (Fig. 2C), demonstrating the peak-like structure around 1100-1200 cm -1 with tail on the high-frequency side, consistent with the spectral response expected for polaron absorption [3]. The time evolution of the spectral line shape signifies increasing polaron size as the pump-probe waiting time is increased. Fig. 2D shows a spatio-temporal mapping of such polaron absorption by ultrafast heterodyne infrared nano-imaging, demonstrating the heterogeneity both in polaron formation and polaron recombination dynamics [2]. To further characterize the disordered nature of the coupling between the photoinduced carriers and perovskite lattice, we measure a transient vibrational response arising from the coupling between polaron and molecular cation at the nanoscale (Fig. 2E). The spatial variation in the transient vibrational peak position (Fig. 2F) demonstrates the non-uniform charge-cation coupling, suggesting that the observed heterogeneity in polaron absorption likely relates to the disorder in the dynamical lattice elasticity [6].

Conclusions

In this work, we establish ultrafast heterodyne infrared nano-imaging as a tool to characterize the heterogeneity in polaron dynamics, as well as an enabling method for transient vibrational nano-spectroscopy which reveals disordered charge-lattice coupling. The technique is generally applicable to address disorder in electron-phonon coupling, which has been identified in a variety of systems including two-dimensional materials and superconductors.