Dynamic SEM imaging of surface photovoltages in MAPbI3 perovskites

Secondary electrons (SE) imaging in Scanning Electron Microscope (SEM) is a very versatile and powerful technique to unveil the morphology and surface topology of materials and systems, over a wide range of view fields and down to the nanoscale. Actually, the SE emission yield and image contrast strongly depend on local values of the work function at material surface. In turn, work function not only depends on material composition, but also on local electric field distribution at surface. In this way, SEM imaging is a means for 2D mapping of surface voltage and charge distributions [1,2], in case morphology-related SE contrast is either negligible or it may be decoupled from charge effects. Optical absorption is known to excite photovoltage distributions at semiconductor and insulator surfaces. In principle, therefore, SEM can also be a contactless means to probe photovoltages in shallow layers, by coupling an optical pump beam inside the analysis chamber. Optically excited charge distributions act to modify electrical potentials at surface proximity, which in turn affect the collection of SE contrast signal. Dynamically evolving phenomena can be probed by standard sampling of the photoexcited SE contrast over time. As for optically induced phenomena evolving on faster time scales (up to the kHz range), these can be accessed by using modulated optical and electron beams, properly synchronized. We report on the assembling of laser-assisted dynamic SEM apparatus and on the imaging of evolving surface photovoltages in semiconducting methylammonium lead tri-iodede (MaPbI3) perovskites,an emerging material in hybrid-organic photovoltaics. Our experiments show the generation of a local field due to laser pumping in the visible region, revealed by an evolution in the SE contrast map. These results are consistent with the photo activation and dynamics of trap states inside the material band gap3. A spectral dependence of the contrast signal on the choice of optical pumping wavelength is noticed. The SE contrast distribution evolves on the second time scale. However, under the high vacuum conditions – as those in the SEM chamber- we observe a permanence of the local field up to several hours.   [1] J. Cazaux, “Calculated effects of work function changes on the dispersion of secondary electron emission data: Application for Al and Si and related elements”, J. Appl.Phys.110, 024906 (2011); [2] L. Xu et al., “Secondary Electron Microscopy Dopant Contrast Image (SEMDCI) for Laser Doping”, IEEE J.PHOTOVOLTAICS, 3, 762, (2013) [3] Motti, S. G. , et al. Photoinduced Emissive Trap States in Lead Halide Perovskite Semiconductors, ACS Energy Lett. 2016, 1, 726−730, DOI: 10.1021/acsenergylett.6b00355