Characterization and Analytics
- Advanced Characterization of Tandem Solar Cells: The combination of several subcells requires adjusted characterization methods to take the specific properties of tandem solar cells into account. The performance of series-connected subcells (in the most commonly investigated two-terminal configuration) depends on the current generation in these subcells. For a correct measurement of device performance with current-voltage characteristics it is therefore important to precisely control the illumination conditions over a wide spectral range. In this context, we investigated the effect of current mismatch on device performance by deliberately manipulating the spectral illumination. Also for the long-term stability testing of tandem solar cells, it is important to control the illumination of both subcells, which we achieve with a dedicated aging setup (see also Stability). Furthermore, the measurement of the external quantum efficiency in tandem solar cells requires the optical characterization of both subcells. This can be done in a dedicated setup, which was developed in collaboration with the University of Ljubljana.
- Photoluminescence: in close collaboration with the group of Thomas Unold, we extensively employ absolute photoluminescence methods to quantify power losses at interfaces in the solar cells and use transient methods (e.g., transient photoluminescence and photoconductivity measurements) to investigate charge collection losses in perovskite solar cells.
- Photoelectron spectroscopy
For further improvement of solar cell devices a deep understanding of the electronic properties of the relevant interfaces, especially towards carrier selective contacts, is needed. Photoelectron spectroscopy (PES) is a key technique to investigate the chemical structure (composition, chemical binding configuration) and electronic properties (work function, valence band maximum, defect states) of semiconductors.
The principle is based on the photoelectric effect: Photons, with a known energy are emitted from a light source, impinge onto a sample, and excite photo electrons. If the excited electrons reach the sample surface and their kinetic energy is sufficient to overcome the work function barrier, they can exit the sample into the vacuum. Using an energy-selective detector, their kinetic energy is measured. From this, the initial energy of the photoelectron in the sample can be calculated.
Measurement set-up: Our photoelectron spectroscopy set-up is equipped for X-Ray PES (XPS) and He-UPS. In addition to these standard techniques, it features a light source for low energy excitation with variable photon energy up to 7eV. We can hence measure PES in constant final state mode (CFSYS)[1,2], where the energy of the photons impinging on the sample is the variable and the kinetic energy where the analyzer detects the electrons is the fixed quantity (in conventional UPS, this is the other way round). We can observe occupied states in the valence band over a high dynamic range of up to seven orders of magnitude and investigate the valence band maximum, band tails and defect states in the band gap up to the Fermi energy. By using near-UV light, the mean free path length of the photo electrons is enhanced, which enables a very high information depth of up to 10 nm, while conventional He-UPS probes only the topmost ~0.5 nm.
- Korte,L.; Laades,A.; Schmidt,M.: Electronic states in a-Si:H/c-Si heterostructures. J. Non-Cryst. Sol. 352 (2006) 1217-20,
- Menzel,D.; Tejada,A.; Al-Ashouri,A.; Levine, I.; Guerra,J.A.; Rech;B.; Albrecht,S.; Korte,L.: Revisiting the Determination of the Valence Band Maximum and Defect Formation in Halide Perovskites for Solar Cells: Insights from Highly Sensitive Near–UV Photoemission Spectroscopy. ACS Appl. Mater. Interfaces 2021, doi: 10.1021/acsami.1c10171