Scanning Probe Microscopy (some recent examples )​
We use various scanning probe techniques to study the properties of semiconductors at the nanoscale. We use atomic force microscopy (AFM) to measure the topography of thin film solar cells. An example is shown in Figure 1, top panel for the case of halide perovskites [1].
In addition to the topography, we use Kelvin probe force microscopy (KPFM) to measure the work function of the thin films with nanometer resolution. This allows us to identify inhomogeneities on the nanoscale. An example is the PbI2 secondary phase on halide perovskite surfaces (see Figure 1 lower panel). In addition, grain boundaries, which need to be passivated to make efficient solar cells can be studied with KPFM. Here, we aim to measure the localized change accumulation or depletion at these crystallography imperfections.
We use scanning tunneling microscopy (STM) and spectroscopy (STS) to investigate the local density of states of materials with nanometer resolution. This spectroscopic technique allows us to measure the position of the Fermi-level at the surface, combined with the position of the valence and conduction band. An example is shown in Figure 2 where the local density of states of Cu(In,Ga)Se2 after different processing conditions were analyzed [2]. The surface bandgap, and the Fermi-level position could be deduced.
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Since the work function and the density of states at the surface are very sensitive to contamination, most of the studies in our group are carried out in ultra-high vacuum. In addition, the solar cell absorbers are often very rough and therefore, we use frequency modulation KPFM to minimize measurement artifacts. We showed that this is essential to obtain reliable results [3].
Sometimes it is necessary to remove the topmost layer(s) and measure the electrical properties below. Therefore, we use conductive AFM combined with wear-resistant tips. An example of a monolayer MoS2 on Al2O3 is shown in Figure 3 [4]. Here the difference in conductivity shows that the AFM tip can be used to selectively remove the MoS2 monolayer. The Al2O3 is insulating, showing no current whereas the MoS2 is, in the present case conductive.
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​References:
[1] Hieulle, J., Krishna, A., Boziki, A., Audinot, J. N., Farooq, M. U., Machado, J. F., Mladenović, M., Phirke, H., Singh, A., Wirtz, T., Tkatchenko, A., Graetzel, M., Hagfeldt, A., & Redinger, A. (2023). Understanding and decoupling the role of wavelength and defects in light-induced degradation of metal-halide perovskites. Energy and Environmental Science, 17(1), 284–295. https://doi.org/10.1039/d3ee03511e
[2] Boumenou, C. K., Phirke, H., Rommelfangen, J., Audinot, J. N., Nishiwaki, S., Wirtz, T., Carron, R., & Redinger, A. (2023). Nanoscale Surface Analysis Reveals Origins of Enhanced Interface Passivation in RbF Post Deposition Treated CIGSe Solar Cells. Advanced Functional Materials, 33(30). https://doi.org/10.1002/adfm.202300590
[3] Lanzoni, E. M., Gallet, T., Spindler, C., Ramírez, O., Boumenou, C. K., Siebentritt, S., & Redinger, A. (2021). The impact of Kelvin probe force microscopy operation modes and environment on grain boundary band bending in perovskite and Cu(In,Ga)Se2 solar cells. Nano Energy, 88 (April). https://doi.org/10.1016/j.nanoen.2021.106270
[4] J. Rommelfangen PhD thesis, Analysis of electronic and vibrational properties in mono-layer MoS2: Exploring sulfur vacancies, oxidation and strain-induced effects, University of Luxembourg 2024