The Nanoscale Photovoltaics Laboratory on a Tip

Solar cells based on metal halide perovskite (MHP) materials will enable cheaper and more energy-efficient photovoltaic and optoelectronic devices compared to current silicon-based technologies. To advance MHP technology further, however, will require a better understanding of the fundamental processes leading to energy losses, unstable operation conditions and premature aging. The macroscopically observed properties of optoelectronic MHP materials and devices are the result of the complicated interplay between nanoscale structure and function. Thus, the key to understanding MHP materials is the micro- and nanoscale characterization of the many nano- and microscale structures; from sub-granular twin domains, over grain boundaries and interfaces to lateral variations in crystal grains orientations and facets. For such nanoscale photovoltaic measurements, scanning force microscopy methods are ideal, as they allow correlated measurements of structure and function.<br/>In this presentation, I will present some of our recent activities in the development of specialized scanning probe microscopy methods to study hybrid perovskite materials. Based on static and time-resolved Kelvin probe force microscopy (KPFM), we have developed unique techniques for mapping the surface potential and photovoltage. Using cross sectional measurements, we map and record the potential distribution across different layers of solar cell devices under operating conditions [1–4]. Here, the key to a reliable cross-sectional KPFM measurement is the connection of a high-resolution quantitative KPFM operation mode [5] and the preparation of a smooth surface through the solar cell. We use a combination of mechanical fracturing of the cells with a complex polishing process using ion beams. Using a pointwise spectroscopy technique, we can record and map the surface photovoltage (SPV) dynamics with 10-20 nm lateral resolution. With this Nano-SPV technique, we revealed local SPV overshoots in the vicinity of grain boundaries following an illumination pulse. The overall aim of our research is to address some of the key challenges of MHP research, such as phase segregation, degradation and interface heterogeneity, to enable a deeper understanding of the different loss mechanisms and intrinsic instabilities that currently limit the application of MHP solar cells.<br/><br/>[1] V. W. Bergmann, S. A. L. Weber, F. Javier Ramos, M. K. Nazeeruddin, M. Grätzel, D. Li, A. L. Domanski, I. Lieberwirth, S. Ahmad, and R. Berger, <i>Real-Space Observation of Unbalanced Charge Distribution inside a Perovskite-Sensitized Solar Cell</i>, Nat Commun <b>5</b>, 6001 (2014).<br/>[2] V. W. Bergmann, Y. Guo, H. Tanaka, I. M. Hermes, D. Li, A. Klasen, S. A. Bretschneider, E. Nakamura, R. Berger, and S. A. L. Weber, <i>Local Time-Dependent Charging in a Perovskite Solar Cell</i>, ACS Appl. Mater. Interfaces <b>8</b>, 19402 (2016).<br/>[3] S. A. L. Weber, I. M. Hermes, S.-H. Turren-Cruz, C. Gort, V. W. Bergmann, L. Gilson, A. Hagfeldt, M. Graetzel, W. Tress, and R. Berger, <i>How the Formation of Interfacial Charge Causes Hysteresis in Perovskite Solar Cells</i>, Energy Environ. Sci. <b>11</b>, 2404 (2018).<br/>[4] I. M. Hermes, Y. Hou, V. W. Bergmann, C. J. Brabec, and S. A. L. Weber, <i>The Interplay of Contact Layers: How the Electron Transport Layer Influences Interfacial Recombination and Hole Extraction in Perovskite Solar Cells</i>, J. Phys. Chem. Lett. <b>9</b>, 6249−6256 (2018).<br/>[5] A. Axt, I. M. Hermes, V. W. Bergmann, N. Tausendpfund, and S. A. L. Weber, <i>Know Your Full Potential: Quantitative Kelvin Probe Force Microscopy on Nanoscale Electrical Devices</i>, Beilstein J. Nanotechnol. <b>9</b>, 1809 (2018).