Similar to photoluminescence (PL) imaging, CL imaging allows for mappings and measurements of CL-spectra and lifetimes in materials. The main benefit of CL over PL images is that it can offer much higher resolution, in principle up to 5 nm [1], for some applications. This can easily be understood by realising that for CL the excitation, that induces luminescence from the sample, stems from electrons instead of photons. As a result, the resolution is not bound by the diffraction limit for light waves, but it depends on the electron-beam setup (SEM, TEM, …). Additionally, one must keep in mind that the exciting electrons provide much more energy per excitation event and therefore can excite multiple charge carriers to high energies at once. Ultimately, this means that while PL and CL collect the same signal in the form of photons, CL shouldn’t be seen as a simple extension of PL with higher resolution, but must be considered a complementary optical characterisation technique based on a different excitation mechanism.
Applications in solar cell research
Sub-wavelength-sized features like crystal grains, ultra-thin (~100nm) cell layers, and nanowires can be studied in much more detail, by combining SEM-imaging with CL-spectra. While this can be already helpful for investigation of dislocations [3] in silicon, CL is even more interesting for emerging photovoltaic materials, such as perovskite solar cells and other thin-film materials, due to the much smaller dimensions of the relevant features in those kind of cells. For example, one can use this technique to create a profile of the CL-spectrum along the depth of a CIGS solar cell and by doing so visualize the bandgap-gradient of the absorber layer (see Figure 2).
Tutorial on CL-characterization for photovoltaic materials
References:
1. J. Schefold, S. Meuret, N. Schilder, H. Agrawal, E. Garnett, and A. Polman, Spatial resolution of coherent cathodoluminescence super-resolution microscopy. ACS Photon. 6: p. 1067 (2019)
2. T. Coenen, B.J.M. Brenny, E.J.R. Vesseur, and A. Polman, Cathodoluminescence microscopy. MRS Bull. 40: 359 (2015)
3. Woong Lee, Jun Chen, Bin Chen, Jiho Chang, and Takashi Sekiguchi, Cathodoluminescence study of dislocation-related luminescence from small-angle grain boundaries in multicrystalline silicon. Appl. Phys. Lett. 94: p. 112103 (2009)
4. https://www.delmic.com/en/
5. A.N. Nikolaeva, et al. Fluctuations in net doping and lifetime in Cu(In,Ga)Se2 solar cells. IEEE WCPEC 2018: 2512-2514 (2018)
Further reading/resources:
CL for PV, Delmic: https://www.delmic.com/en/solutions/cathodoluminescence-pv
CL for material science, Gatan:
https://www.gatan.com/cathodoluminescence-insulators-metals-and-plasmonics
CL on wikipedia:
https://en.wikipedia.org/wiki/Cathodoluminescence