Titanium dioxide (TiO2) and zinc oxide (ZnO) semiconductors have similar band gap positions but TiO2 performs better as an anode material in dye-sensitized solar cell applications. We compared two electrodes made of TiO2 nanoparticles and ZnO nanorods sensitized by an aggregation-protected phthalocyanine derivative using ultrafast transient absorption spectroscopy. In agreement with previous studies, the primary electron injection is two times faster on TiO2, but contrary to the previous results the charge recombination is slower on ZnO. The latter could be due to morphology differences and the ability of the injected electrons to travel much further from the sensitizer cation in ZnO nanorods.
Understanding the primary processes
of charge separation (CS) in
solid-state dye-sensitized solar cells (DSSCs) and, in particular,
analysis of the efficiency losses during these primary photoreactions
is essential for designing new and efficient photosensitizers. Phthalocyanines
(Pcs) are potentially interesting sensitizers having absorption in
the red side of the optical spectrum and known to be efficient electron
donors. However, the efficiencies of Pc-sensitized DSSCs are lower
than that of the best DSSCs, which is commonly attributed to the aggregation
tendency of Pcs. In this study, we employ ultrafast spectroscopy to
discover why and how much does the aggregation affect the efficiency.
The samples were prepared on a standard fluorine-doped tin oxide (FTO)
substrates covered by a porous layer of TiO
2
nanoparticles,
functionalized by a Pc sensitizer and filled by a hole transporting
material (Spiro-MeOTAD). The study demonstrates that the aggregation
can be suppressed gradually by using co-adsorbates, such as chenodeoxycholic
acid (CDCA) and oleic acid, but rather high concentrations of co-adsorbate
is required. Gradually, a few times improvement of quantum efficiency
was observed at sensitizer/co-adsorbate ratio Pc/CDCA = 1:10 and higher.
The time-resolved spectroscopy studies were complemented by standard
photocurrent measurements of the same sample structures, which also
confirmed gradual increase in photon-to-current conversion efficiency
on mixing Pc with CDCA.
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