Mechanisms of Instability in Ru-Based Dye Sensitization Solar Cells

In-situ infrared studies performed with operating Ru-complex-sensitized wet solar cells using a total reflection technique reveal that the ruthenium complex (both tri- and mononuclear) attached to TiO 2 is photoelectrochemically transformed and irreversibly consumed under conditions of insufficient regeneration by iodide or from the oxide within the nanocrystalline TiO 2 pores. The sensitizer [(Ru(bpy) 2 (CN) 2 ) 2 Ru(bpca) 2 ] 2- (bpy is 2,2‘-bipyridine, bpca is 2,2‘-bipyridine-4,4‘-dicarboxylate) decomposes into fragments; one of them was identified to be Ru(bpy) 2 (CN) 2 . For the sensitizer Ru(bpca) 2 (SCN) 2, it is shown that a molecular fragment (absorbing at 2013 cm -1 ) is generated which is diffusing out of the nanostructured TiO 2 layer. Due to its correlation with the photocurrent density, it is identified as a product of the oxidized sensitizer. Due to a high serial resistance introduced by the total reflection element and the resulting low fillfactor of the sensitization cell during in-situ measurements, only small photocurrents (5−10 μA cm -2 ) could be passed through the sensitizing interface. Since the rate of product formation should be proportional to the ratio of photocurrent density to iodide concentration, the iodide concentration was correspondingly reduced (1−10 mM) as compared to the conditions in a solar cell (10 mA cm -2, 1 M). This spectroscopic technique was developed because efforts to produce stable sensitization solar cells proved to be unsuccessful due to sealing problems. Our experiments do not seem to permit extrapolation to 10 7 −10 8 electron transfer numbers for sensitizing Ru complexes, and real long-term testing is required for reevaluating long-term performance.