Resolving Ionic Spectra of Lead-Halide Perovskites to the Nanometer

Abstract

Ionic defects play a central role in the function and dysfunction of lead-halide perovskite solar cells yet their distribution and redistribution along the heterogenous thin film is poorly-understood. This is due in part to the limited resolution of conventional optical spectroscopy methods with respect to the nanoscale grain length. Meanwhile, finer-resolution (in particular, scanning probe) methods tend to lack the spectroscopic capability to effectively discern distinct ionic contributions. Here we implement a nanoscale spectroscopic method whereby the capacitance between a conductive atomic force microscopy tip and lead-halide perovskite samples is studied as a function of excitation frequency to yield maps of local distributions of ionic defect spectra in the 100 Hz to 10 kHz regime. We utilize a nonnegative matrix factorization algorithm to disentangle linearly independent contributions to the signal and reveal, in triple-cation lead-halide perovskites, a contrast in the concentration and mobility of ionic species (i) between grains and boundaries, and (ii) between grains. We compare spectra obtained over pristine methylamonnium lead iodide microcrystals versus a decomposed pellet to attribute part of the perovskite capacitive response to phase-segregated lead iodide and conclude with a tentative assignment of capacitive resonances. The vicinity of the lead-iodide signature to the primary kHz-range ionic signature of the perovskite solar cell, along with the variation of the primary resonance across the sample surface, may in part explain the difficulties in reaching an experimental consensus of the iodide vacancy activation energy.