The SERS signature of PbS quantum dot oxidation

Lead chalcogenide quantum dots (QDs) are promising candidates for the use as sunlight absorbers in sensitized solar cells. While conversion efficiencies of QD solar cells have reached potentially commercially interesting values in the order of 10%, the devices are prone to fast oxidative degradation whose mechanism is not understood in detail yet. Furthermore, O2 and/or H2O present during fabrication has been observed to strongly influence device performance. Analytical tools to monitor the chemical state of QDs in situ and on-line, e.g. during fabrication and operation, are sought for. In this work, we demonstrate that surface-enhanced Raman spectroscopy (SERS) provides the chemical selectivity and high sensitivity required to investigate the oxidation behavior of QDs. We investigate the SER signature of PbS QDs that is commonly employed as sensitizer material in solar cells because of their tunable absorption range and simple and cheap fabrication. We observe SERS fingerprints for a set of three model PbS QD samples with different oxidation degrees. Intact PbS QDs exhibit a strong phonon mode around 196 cm−1. With increasing PbS oxidation, a relative band intensity increase can be observed in the 250-380 cm−1 region. At strong oxidation levels, additional Pb-(sulf)oxide marker bands appear at 614 and 980 cm−1. Based on our results, we suggest to employ PbS-oxidation SER spectral markers as monitoring tools during photovoltaic device fabrication.