Optical Spectroscopy
Über
Die zerstörungsfreie Charakterisierung der elektronischen und optischen Eigenschaften von Filmen, Schichtstapeln und kompletten Bauelementen kann mit verschiedenen Arten der optischen Spektroskopie durchgeführt werden. Die optische Spektroskopie umfasst in erster Linie lumineszenzbasierte Techniken wie transiente und stationäre Photolumineszenz. Der wesentliche Vorteil dieser Techniken besteht darin, dass sie die gleiche Art von Messungen an einer Vielzahl unterschiedlicher Proben (von Schichten bis zu Zellen) mit unterschiedlichen Grenzflächen durchführen können. Dadurch lassen sich Verluste im Volumen und an Grenzflächen quantifizieren, und die Messung der Ladungsträgerdynamik verschiedener Proben ermöglicht Einblicke in den chemischen Ursprung von Effizienz mindernenden Defekten, was wiederum als Leitfaden für das Design und die Optimierung von Bauelementen dient.
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Photothermal Deflection Spectroscopy (PDS)

Photothermal deflection spectroscopy is based on illuminating a sample that is placed inside a cuvette filled with a liquid. The light heats up the sample and in consequence also the liquid. The liquid has to be chosen in a way that its refractive index depends on temperature. Thus, we now have a light induced temperature gradient in the liquid as shown in Fig. 2 that can be detected by using a laser directed normal to the monochromatic light used to excite the sample. This laser beam is then deflected by the refractive index gradient which will be detected by a photodetector. The signal detected by the photodetector is directly proportional to the amount of absorbed light and therefore to the absorptance of the sample. If the thickness is known, also the absorption coefficient (usually the parameter of interest) may be obtained with a high dynamic range of around 4 orders of magnitude.
Transient photoluminescence

The insights obtained from transient photoluminescence (PL) measurements have contributed to an improved understanding of recombination and transport in a wide range of semiconductors. Transient photoluminescence is attractive because it allows contactless measurements of films on glass, layer stacks or complete devices while studying processes on different time- and length-scales. In particular, it permits analysis of the various recombination processes that happen in photovoltaic absorber materials and that may reduce the open-circuit voltage and subsequently the efficiency of solar cells made from these materials. However, analyzing the transients is challenging because of the multitude of (non-linear) effects that contribute to the shape of the PL transient. Recent work was focussed on combining transient photovoltage and transient photoluminescence measurements to generate an understanding of the general meaning of decay times and of the differences and similarities between electrically and optically detected transients [1].
[1] Krückemeier, L., Liu, Z., Krogmeier, B., Rau, U., & Kirchartz, T. (2021). Consistent Interpretation of Electrical and Optical Transients in Halide Perovskite Layers and Solar Cells. Advanced Energy Materials, 11(n/a), 2102290. doi:https://doi.org/10.1002/aenm.202102290