Time-resolved 2D mapping of surface photovoltages and charge dynamics in semiconductors by SEM

We introduce laser-assisted Time-Resolved dynamical SEM technique (TR-SEM), to provide 2D mapping of photo-induced surface charge distributions and related local potentials in semiconductors. TR-SEM relies on the optically induced local modifications of Secondary Electron (SE) detection yield and is a means to measure photovoltage, complementary to near-field probe microscopies and photoemission spectroscopies. The technique is hosted in a commercial SEM, equipped with optical viewports to couple light into the specimen chamber and focus it at the sample surface. Real-time imaging of field patterns is provided on timescales compatible with SEM scanning rates, potentially reaching a temporal resolution of milliseconds. TR-SEM patterns are strongly dependent on the geometry of SE collection. The information about the distribution of local potentials at the semiconductor surface can be singled out by comparing experimental contrast maps with numerical modeling of the mechanism of SE detection [1][2]. TR-SEM has been applied to lead-halide hybrid perovskite (MAPbI3) heterostructures typical for photovoltaic applications. It provided dynamical 2D mapping of photogenerated charge fields, under high-irradiance in vacuum conditions (up to 50 W/cm2) and in the absence of any electrical bias [2]. Optical excitation is provided by above-bandgap illumination. Irradiation-induced permanent material damage is avoided by operating the SEM at 5 keV of energy and 1–10 pA of primary current. TR-SEM confirms how the specific nature of charge selective contacts and related interfaces crucially affect the optoelectronic response of MAPbI3 thin films. We will discuss how, according to the type of contact, photogenerated electron-hole pairs may play different roles and trigger different photochemical reactions also affecting the migration of ions within the films, eventually ruling the long term evolution of surface charge distribution and photo-potentials. [1] G. Irde, et al., Micron 2019, 121, 53 [2] M.W. H. Garming, et al. J. Phys. Chem. Lett. 2020, 11, 8880 [3] S. M. Pietralunga, et al., Adv. Mater. Interfaces 2020, 2000297