Dyes of characteristically different composition have been tested with respect to long-term stability in operating standardized dye sensitized cells during a time period of up to 3600 hours. Selective solar illumination, the use of graded filters, and imaging of photocurrents revealed that degradation is linked to the density of photocurrent passed. Photoelectrochemical degradation was observed with all sensitizers investigated. Sensitization was less efficient and sensitizers were less photostable with nanostructured ZnO compared to nanostructured TiO2. The best performance was confirmed for cis- Ru II (dcbpyH 2) 2 (NCS) 2 on TiO2. However, it was 710 times less stable under other identical conditions on ZnO. Stability is favored by carboxylate anchoring and metal-centred electron transfer. In presence of TiO2, it is enhanced by formation of a stabilizing charge-transfer complex between oxidized Ru dye and back-bonding interfacial Ti 3+ states. This is considered to be the main reason for the ongoing use of expensive Ru complexes in combination with TiO2. The local surface chemistry of the nanocrystalline TiO2 turned out to be a crucial factor for sensitizer stability and requires further investigation.
Tributsch, H., Barkschat, A., Moehl, T., & MacHt, B. (2008). The function of TiO 2 with respect to sensitizer stability in nanocrystalline dye solar cells. International Journal of Photoenergy, 2008. https://doi.org/10.1155/2008/814951