Proceedings of 6th International Conference on Hybrid and Organic Photovoltaics (HOPV14)
Publication date: 1st March 2014
We found coordinated shifts on the order of ~100 meV in the interfacial energy levels of ruthenium dye sensitizers and that of the conduction band of nano-crystalline TiO2 film. The energy shifts were induced by potential-determining additives that were widely used in highly efficient dye-sensitized solar cells. The observation of comparable shifts of interfacial energy levels in dyes and TiO2 film requires deeper understanding of the varied quantum efficiency of interfacial electron injection, which had been derived under the assumption of the band shift only in TiO2. Based on general physical concept and further observations, we attributed the variable electron injection efficiency to the influence of potential-determining additives on the interfacial electronic structures of both the dyes and TiO2. Our results revealed that the electronic coupling between the donor states in photoexcited dyes and the acceptor states in TiO2 was affected by the additional interfacial dipole layer, which led to changes in electron injection efficiency.
We also found the amplitude of energy shifts, in ruthenium dyes and metal-free organic dyes, to be most likely related to the number of COOX groups. This relationship implies that the effect of large coordinated shifts in ruthenium dyes/TiO2 is one of the most probable reasons why these dyes enable higher performance than do metal-free organic dyes, in which the latter only have relatively slight coordinated shifts.
This discovery provides deeper insight into the interfacial electron injection mechanism and points out the importance of the realignment effect on interfacial energy levels. The advanced understanding would be helpful for the design of new additives and sensitizers for highly effcient solar cells.
1. Yang, X; Zhang, S; Zhang, K.; Liu, J.; Qin, C.; Chen, H.; Islam, A.; Han, L. Coordinated shifts of interfacial energy levels: insight into electron injection in highly efficient dye-sensitized solar cells. Energy Environ. Sci. 2013, 6, 3637-3645. 2. Zhang, S; Yang, X; Zhang, K.; Chen, H.; Yanagida, M.; Han, L. Effects of 4-tert-butylpyridine on the quasi-Fermi levels of TiO2 films in the presence of di?erent cations in dye-sensitized solar cells. Phys. Chem. Chem. Phys., 2012, 13, 19310-19313. 3. Zhang, S; Yanagida, M.; Yang, X; Han, L. Effect of 4-tert-Butylpyridine on the Quasi-Fermi Level of Dye-Sensitized TiO2 Films. App. Phys. Express, 2012, 4, 042301. 4. Zhang, K.; Zhang, S; Sodeyama, K.; Yang, X; Chen, H.; Yanagida, M.; Tateyama, Y.; Han, L. A New Factor Affecting the Performance of Dye-Sensitized Solar Cells in the Presence of 4-tert-Butylpyridine. App. Phys. Express, 2012, 5, 042303.
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