Real-time dynamical imaging of light induced photo-voltages in hybrid halide perovskites by Scanning Electron Microscopy

Secondary electron (SE) Surface-sensitive imaging detection in Scanning Electron Microscopy (SEM) probes space-charge field distributions and potentials at surfaces and interfaces [1,2]. Recently, its combination with optical excitation has opened wider scenarios [3,4,5,6]. In this work we present the combination of a customized SEM with laser light excitation of the sample under test in a Light-Assisted SEM configuration (LASEM). LASEM relies on the optically induced local modification of SE yield to provide real-time mapping of photo-voltages and charge dynamics, and qualifies as a complementary approach to near-field probe microscopies [7] and nonlinear photoemission spectroscopies [8]. We applied LASEM to thin films of Metal Ammonium Lead Triiodide perovskite (MAPbI3), an outstanding light-sensitive material in solar light harvesting and photovoltaics [9], and also appealing as an active material for light generation [10]. MAPbI3 is excited by illumination at above-bandgap photon energy, while keeping electron beam dose below damaging threshold. Fig. 1 reports temporal evolution of LASEM contrast under illumination and after light removal. A contrast reversal is evident, as the illumination is turned off. The system evolves in dark over several hours and a near complete recovery occurs within days. LASEM contrast pattern depends on the geometry of SE collection, as proven by varying angular orientation of the sample with respect to the in-column detector, (Fig. 2), consistently with the hypothesis of an optically excited charge field at surface. Ray-tracing numerical simulation of SE flight and detection supports the hypothesis. The contrast dynamics, is consistent with the evolution of optically induced structural modifications in MAPbI3, as also experimentally observed, by using complementary techniques [11,12]. As above demonstrated, LASEM is potentially a versatile technique which can be extensively applied to photo-sensitive materials. A real-time temporal resolution in the millisecond range can be envisaged. [1] Cazaux J., Ultramicroscopy 110, 242 (2010) [2] Tsurumi D., Hamada K. and Kawasaki Y., JAP 113,144901 (2013) [3] De Boer, P., Hoogenboom, J. P., Giepmans, B. N. G., 12, 6, 503, (2015) [4] Najafi, E.,et al. Science, 347 (6218) ,164 (2015). [5] Zani M. et al., Ultramicroscopy 187, 93 (2018). [6] Park H. and Zuo J. M., 94, 251103 (2009) [7] Kronik, L. and Shapira, Y. Surface Science Reports 37(1), 1, (1999). [8] Long J.P. et al, PRL, 64, 1158 (1990) [9] Stranks, S. D., et al. Science 342(2013), 341 (2014). [10] D’Innocenzo V. et al., J. Am. Chem. Soc., 136 (51), 17730 (2014) [11] DeQuilettes D. W., et al., Nature Communications 7, 11683 (2016) [12] Gottesman, R., Journal of Phys.Chem. Letters 6, 2332 (2015).