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 (sub)microsecond dynamics with spatial heterogeneity at nanometer lengthscales. These processes range from 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, all of which are also 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 time-resolved 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 sub-microsecond time resolution dynamics by applying time-frequency analysis to the cantilever’s motion in response to a transient event.[2-5]. We have used both neural networks[6] and a data-driven mode decomposition methods[7] to improve the signal:noise ratio in trEFM as well. Here, we spatially resolve photocarrier dynamics in hybrid organic-inorganic halide perovskites, a state-of-the-art photovoltaic material, using trEFM to map surface potential equilibration during photoexcitation. We show that perovskite materials can reflect variations from grain centers to grain boundaries due to ion motion that correlate with longer trEFM potential equilibration times. Following treatment with several different surface passivation agents, we show that trEFM probes dynamics directly related to surface recombination velocity and carrier lifetimes correlated with time-resolved photoluminescence. Our results reveal nanoscale variations in recombination dynamics following surface passivation, and we also observe heterogeneity in surface potential equilibration times dependent upon perovskite film morphology. To validate these results, we combine wavelength- and intensity-dependent measurements with drift-diffusion simulations to disentangle the influence of carrier recombination and ion migration on surface potential equilibration. These results demonstrate that we can use mechanical detection to image electronic carrier recombination dynamics in perovskites far below the optical diffraction limit, while at the same time also showing the potential for future improvements in heterogeneity of surface passivation. Additionally, the correlation showing the linear relationship between trEFM and trPL indicates that we can use our method as predictive of surface recombination velocity in a way that screens materials but with high spatial resolution.[8] 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) 8. M. D. Breshears, J. Pothoof, et al. arXiv:2412.04423 [cond-mat.mtrl-sci]