Laser Interferometric Rate Measurements of Light-Induced Deformation of Lead-Halide Perovskite Crystals Provide Insights into Ionic Vacancy Formation and Diffusion

Light-induced deformation is one of many unique, poorly understood properties of lead-halide perovskites (LHPs). We study LHP crystal nucleation and growth as well as transient ionic and electronic photoconductivity in thin-films. In a range of LHP thin-films, prior scanning probe microscopy (SPM) work in our group showed that light appears to create ionic vacancies [1–2]. The films’ light-induced conductivity decayed in 10 to 100 seconds, 10⁹ slower than expected from direct electron–hole recombination. The relaxation was generally temperature dependent, with an activation energy of 0.5 eV. These observations suggest that light creates ionic carriers that subsequently recombine slowly in the dark.

Failed SPM studies of LHP crystals led us to delve into the photodeformation of these crystals. Single crystals are, in many ways, an ideal system for studying the effects of chemical composition as they are more uniform and reproducible than thin-films, and crystals are relatively immune to variations in processing conditions as they (ideally) lacks grain boundaries and should have defect densities close to the thermodynamic limit. While “inverse-temperature” crystallization, by which many LHPs crystals are grown, allows for the rapid growth of well faceted crystals, they are often not single crystals [3]. Controversy around a high-profile paper notwithstanding [4–6], it is established in the literature that LHP crystals undergo “photostriction” distinct from thermal-induced expansion; lacking, however, is an adequate microscopic explanation for the origins of the effect given its magnitude, its dependence on chemical composition, and the presence of slow and fast dynamics. Using a fiber-optic laser interferometer from our SPM, we are able to make first-of-their-kind interferometric, temperature-controlled measurements of this light-induced deformation with picometer distance and sub-microsecond temporal resolution.

These ongoing interferometric measurements generally reproduce the magnitude (~100 ppm) and dynamics (several seconds fast regime followed by tens of seconds slow regime) of the deformation seen in the literature but with an entirely new measurement system. Our non-contact, cantilever-free technique disproves the widespread conclusion that the slow dynamics observed are due to cantilever heating. Furthermore, depending on the crystal, we see either light-induced expansion or contraction. X-ray rocking curve, Laue, and light scattering measurements show that these are polycrystals and that the magnitude of photodeformation correlates with crystal quality. Current work focuses on extending these measurements to true single crystals.

We propose that the photodeformation is primarily the result of light-induced defects—both vacancies and interstitials. We observe the dynamics of photocarrier generation spatio-temporally coupled with defect formation and diffusion (where vacancies and interstitials show different dynamics). To test our hypothesis, we are performing a series of time-resolved, variable-wavelength, and variable-temperature measurements on a diverse set of LHP crystals. We believe our unique technique provides a more direct, robust measurement of the photodeformation with improved temporal resolution and more control of experimental parameters. By combining the unique insights provided by these crystal measurements with our existing thin-film SPM work and theories, we hope to expand our understanding of vacancy formation, diffusion, and mechanical stability in these material systems.

References:
[1] Tirmzi, et al. ACS Energy Lett. 2017. DOI: 10.1021/acsenergylett.6b00722
[2] Tirmzi, et al. J. Phys. Chem. C 2019. DOI: 10.1021/acs.jpcc.8b11783
[3] Amari, et al. Cryst. Growth Des. 2020. DOI: 10.1021/acs.cgd.9b01429
[4] Tsai, et al. Science 2018. DOI: 10.1126/science.aap8671
[5] Rolston, et al. Science 2020. DOI: 10.1126/science.aay8691
[6] Tsai, et al. Science 2020. DOI: 10.1126/science.aba6295