Exploring Responses of Contact Kelvin Probe Force Microscopy in Triple-Cation Double-Halide Perovskites

Abstract

Hybrid organic−inorganic perovskite (HOIP) solar cells have undergone unprecedentedly rapid improvement in power conversion efficiencies during the last decade, but controversy remains regarding the role of local phenomena and heterogeneity in grain by grain. These challenges in turn necessitate understanding the fundamental mechanisms operating at individual grains and exploring how these mechanisms give rise to the collective behaviors of the films. Although numerous efforts have targeted local phenomena in these materials using classical scanning probe microscopy modalities, the measurements remain difficult to interpret due to intrinsic coupling between possible ferroelectric, ion migration, bulk and surface electrochemical phenomena, and the effects of light on these behaviors. Here, we explore the electromechanical responses in (Cs 0.05 FA 0.85 MA 0.15 )Pb(I 0.85 Br 0.15 ) 3 perovskite thin film in the dark and under illumination using band excitation-piezoresponse force microscopy (BE-PFM). To identify and separate ferroic, electrostatic, and electrochemical responses, we utilize contact-Kelvin probe force microscopy (cKPFM). The cKPFM responses show weak variability with morphological features in the dark, while such variability significantly increases under illumination. Deconvolution of the cKPFM responses via principal component analysis (PCA) shows heterogeneity in grain to grain possibly due to variation in migration and segregation of halide ions in mixed perovskites at specific grains. Further, we propose an electrostatic mechanism based on the physics of semiconductors, which can directly link the observed cKPFM behaviors with the ionic and electronic density of states (DOS) and their time dynamics. These findings suggest that cKPFM is a powerful probe of local ionic behaviors on the nanoscale, both in HOIPs and other materials.

Light–ferroelectric interaction in two-dimensional lead iodide perovskites

“…As a result, band dispersion depends on the number of data points with lower band dispersion. According to our previous studies, 45,46 larger band dispersion is induced under illumination in 3D MHPs due to the higher density of charge carriers. In fact, the linear curve in the cKPFM responses is associated with pure electrostatic behavior and transition to paraelectricity while the larger band dispersion is induced by a higher density of charge carriers including photogenerated charge carriers and mobile ion species.…”

“…In the cKPFM response, the band dispersion is a measure of the charge injection onto the film surface where larger band dispersion represents a higher density of charge carriers. 48,49 As shown in the cKPFM response under illumination, the narrow band dispersion (Fig. 3i and j) identifies the pure electrostatic nature as non-ferroelectricity.…”

“…Our recent study provides a detailed explanation of the PCA approach in cKPFM data. 45 From the PCA analysis, the 4D cKPFM data ( V read , V write , x , y ) can be deconvoluted to a 3D data set R av ( V read , x , y ) where R av represents the average responses. The cKPFM data set is analyzed by PCA as below:…”

Why it is Unfortunate that Linear Machine Learning “Works” so well in Electromechanical Switching of Ferroelectric Thin Films

“…For example, bipolar triangular waveforms can be used to measure ferroelectric switching (Figures S1 and S2, Supporting Information), [15] First-order reversal curves can measure switching dynamics, [16] and contact-Kelvin probe force microscopy to isolate ionic, electrostatic, ferroelectric contributions. [17][18][19] The challenge is extracting actionable information from data that spans many positions, frequency, voltage, cycle, and time dimensions and contains multichannel information. These challenges are exacerbated when searching for rare spatial-temporal phenomena.…”