Light-dependent Impedance Spectra and Transient Photoconductivity in a Ruddlesden-Popper 2D Lead-halide Perovskite Revealed by Electrical Scanned Probe Microscopy and Accompanying Theory

TL;DR

This study uses electric force microscopy to investigate the light-dependent impedance and transient photoconductivity of a 2D Ruddlesden-Popper perovskite, revealing slower photoconductivity dynamics compared to 3D counterparts and extending theoretical models to time-resolved measurements.

Contribution

The paper introduces a time-dependent transfer function extension to the impedance model, enabling analysis of time-resolved electric force microscopy signals in 2D perovskites.

Findings

Impedance spectrum shows modest change under illumination.

Photoconductivity rise and decay times are ~100 μs, slower than electron-hole recombination.

Extended impedance model to include time-resolved measurements.

Abstract

Electric force microscopy was used to record the light-dependent impedance spectrum and the probe transient photoconductivity of a film of butylammonium lead iodide, BA$_{2}$PbI$_{4}$, a 2D Ruddlesden--Popper perovskite semiconductor. The impedance spectrum of BA$_{2}$PbI$_{4}$ showed modest changes as the illumination intensity was varied up to 1400 mW/cm$^{2}$, in contrast with the comparatively dramatic changes seen for 3D lead-halide perovskites under similar conditions. BA$_{2}$PbI$_{4}$'s light-induced conductivity had a rise time and decay time of $\sim$ 100 $\mu$s, 10$^{4}$ slower than expected from direct electron-hole recombination and yet 10$^{5}$ faster than the conductivity-recovery times recently observed in 3D lead-halide perovskites and attributed to the relaxation of photogenerated vacancies. What sample properties are probed by electric force microscope measurements remains an open question. A Lagrangian-mechanics treatment of the electric force microscope experiment was recently introduced by Dwyer, Harrell, and Marohn which enabled the calculation of steady-state electric force microscope signals in terms of a complex sample impedance. Here this impedance treatment of the tip-sample interaction is extended, through the introduction of a time-dependent transfer function, to include time-resolved electrical scanned probe measurements. It is shown that the signal in a phase-kick electric force microscope experiment, and therefore also the signal in a time-resolved electrostatic force microscope experiment, can be written explicitly in terms of the sample's time-dependent resistance (i.e., conductivity).