Melike Karakus, Wen Zhang, Hans Joachim Räder, Mischa Bonn, Enrique Cánovas,
http://pubsdc3.acs.org/doi/10.1021/acsami.7b08986
The long-term stability of dye sensitized solar cells (DSSCs) is determined to a large extent by the photodegradation of their sensitizers. Understanding the mechanism of light induced decomposition of dyes sensitizing a mesoporous oxide matrix may therefore contribute to solutions to increase the life span of DSSCs. Here, we investigate, using ultrafast terahertz photoconductivity measurements, the evolution of interfacial electron transfer (ET) dynamics in N3 dye-sensitized mesoporous TiO2 electrodes upon dye photodegradation. Under inert environment, interfacial ET dynamics do not change over time, indicating that the dye is stable and photodegradation is absent; the associated ET dynamics are characterized by a sub-100 fs rise of the photo-conductivity, followed by long-lived (>>1 ns) electrons in the oxide electrode. When the N3-TiO2 sample is exposed to air under identical illumination conditions, dye photodegradation is evident from the disappearance of the optical absorption associated with the dye. Remarkably, approximately half of the sub-100 fs ET is observed to still occur, but is followed by very rapid (~10 ps) electron-hole recombination. Laser desorption/ionization mass spectrometry, attenuated total reflection FTIR and terahertz photoconductivity analyses reveal that the photo-degraded ET signal originates from the N3 dye photodegradation product as dcbpy (4,4’- dicarboxylic acid - 2,2’-bipyridine), which remains bonded to the TiO2 surface via either bidentate chelation or bridging type geometry.
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