Nir Kedem Michael Kulbak Thomas Michael Brenner Gary Hodes David Cahen
Materials and Interfaces, Weizmann Institute of Science, Rehovot, Israel

The rapid improvement in halide perovskite (HaP) -based photovoltaic (PV) device efficiency and their prospective use in LED and laser application has resulted in an accelerated world-wide effort for materials characterization. The halide perovskites are ABX3 compounds, with A a monovalent cation (methylammonium, formamidinium or Cs are those found suitable till now), B is a divalent cation (Pb or, possibly Sn) and X is a halide anion. Using a mixture of organic cations and halides, 20% PV conversion efficiency device has been demonstrated in state of the art devices.

Electron microscopy and associated analytical methods are the most common tools to gain information regarding film morphology, chemical composition and electronic properties at high spatial resolution. We use electron beam induced current (EBIC) to learn about the mechanism of HaP-based PV cell action. During EBIC measurement, the high energy electron beam locally excites carriers while measuring the resulting electro-voltaic current. This allows mapping of the collection efficiency across the device cross section. The collection efficiency profile can then be used to determine electronic properties of the device, such as operating mechanism, carrier diffusion lengths and even doping density. Here the use of EBIC for three types of HaP-based PV devices will be demonstrated. Furthermore adding a light as a second source of excitation and biasing the sample allows a more in-depth study of the electronic properties of the materials involved.

With increasing exposure to the electron beam the EBIC signal shape and intensity gradually change, affecting the apparent electronic properties. By understanding the degradation processes in the material/device, we show that the sensitivity of the method towards beam-induced degradation can be used to add to our understanding of the device function. Comparing the EBIC signal degradation of methylammonium lead iodide (MAPbI3) with that of MAPbBr3 PV cells, we uncover quasi-neutral regions in the former. In addition, comparing the degradation of Cs-based devices with MA-based ones helps correlate the induced damage with core level ionization of the ions.

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