R. Giridharagopal, M.D. Breshears, J. Pothoof, D.S. Ginger
University of Washington,
United States
Keywords: photovoltaics, electrostatic force microscopy, signal processing
Summary:
Many materials exhibit microsecond dynamics with spatial heterogeneity at nanometer lengthscales. Minority carrier lifetimes in solar cells, ion motion processes in mixed ionic-electronic conductors, dynamic charging in bio-active materials, and exciton separation in two-dimensional devices are all dynamic events that are dependent upon local morphology. Probing this relationship is difficult with traditional scanning probe microscopy (SPM) methods, especially at timescales faster than milliseconds, yet recent advances have improved our ability to capture information and subsequently extract fast dynamics of interest. In electrostatic force microscopy, changes in an AFM cantilever’s oscillating motion can reflect information about the sample due to the electrostatic effects between the metal tip and the substrate.[1] By recording the cantilever position and processing the time-dependent information, it is possible to extract new temporal insights with sub-microsecond time resolution. Time-resolved electrostatic force microscopy (trEFM), for instance, measures dynamic information with sub-microsecond resolution by applying time-frequency analysis to the cantilever’s motion in response to a transient stimulus.[2-4]. We have used both neural networks[5] and a data-driven mode decomposition method[6] to drastically improve the signal:noise ratio in trEFM, and indeed a wide range of signal processing methods can be used in trEFM signal analysis. Here, we show how tEFM can measure microsecond and sub-microsecond dynamic effects normally captured in bulk optical measurements. In particular, we use trEFM to map microsecond-scale carrier dynamics in state-of-the-art perovskite solar cell materials. Under illumination, carriers in a solar cell will eventually recombine; the average time for this event is related to the radiative efficiency of the material and the defect density.[4] We show that processing methods that change the bulk carrier recombination lifetime, like passivating layers and transport interlayers, also exhibit local changes in the charging and discharging dynamics. For example, passivating chemicals like (3-aminopropyl)trimethoxysilane (APTMS) significantly improve the bulk lifetime of halide perovskite solar cells,[7] and this effect is reflected locally in trEFM maps. Indeed, APTMS both reduces the trEFM charging time and narrows the distribution of charging times across grains, consistent with the defect passivating nature of this surface treatment. These data show that AFM-based methods for measuring dynamics can capture the sub-microsecond phenomena measured by optical techniques with the nanoscale resolution of scanning probes. While these data are specific to photovoltaics, the approach to measuring time-dependent information is generalizable to measuring other dynamics with AFM. References: 1. R. Giridharagopal, P. A. Cox, and D. S. Ginger. Acc. Chem. Res. 49, 1769 (2016) 2. R. Giridharagopal, et al. Nano Lett. 12, 893 (2012) 3. D. U. Karatay, et al. Rev. Sci. Instrum. 87, 053702 (2016) 4. R. Giridharagopal, et al. ACS Nano 13, 2812 (2019) 5. M. D. Breshears, et al. J. Chem. Inf. Model. 62, 4342-4350 (2022) 6. D. E. Shea, et al. IEEE Access 9, 83453 (2021) 7. J. Pothoof, et al. J. Phys. Chem. Lett. 14, 6092-6098 (2023)