2 research outputs found
Effect of Hole Transporting Material on Charge Transfer Processes in Zinc Phthalocyanine Sensitized ZnO Nanorods
The photoinduced electron transfer
processes were studied for hybrid
systems consisting of self-assembled monolayer of zinc phthalocyanine
(ZnPc) assembled on ZnO nanorods and a film of organic hole transporting
material (HTM) atop. Polythiophene (P3HT) or Spiro-OMeTAD were used
as HTM. The study was carried out by ultrafast transient absorption
spectroscopy technique with selective excitation of ZnPc at 680 nm
or P3HT at 500 nm. Data analysis revealed that photoexcitation of
ZnPc in the structure ZnO|ZnPc|P3HT results in a fast (1.8 ps) electron
transfer from ZnPc to ZnO, which is followed by a hole transfer from
the ZnPc cation to P3HT roughly in 30 ps. However, in the case of
ZnO|ZnPc|Spiro-OMeTAD structure, the primary reaction upon excitation
of ZnPc is a fast (0.5 ps) hole transfer from ZnPc to Spiro-OMeTAD,
and
the second step is electron injection from the ZnPc anion to ZnO in
roughly 120 ps. Thus, we demonstrate two structurally very similar
hybrid architectures that implement two different mechanisms for photoinduced
charge separation found in dye-sensitized or in organic solar cells
Photoinduced Electron Injection from Zinc Phthalocyanines into Zinc Oxide Nanorods: Aggregation Effects
Phthalocyanines
(Pc) are well-known light-harvesting compounds.
However, despite the tremendous efforts on phthalocyanine synthesis,
the achieved energy conversion efficiencies for Pc-based dye-sensitized
solar cells are moderate. To cast light on the factors reducing the
conversion efficiency, we have undertaken a time-resolved spectroscopy
study of the primary photoinduced reactions at a semiconductor-Pc
interface. ZnO nanorods were chosen as a model semiconductor substrate
with enhanced specific surface area. The use of a nanostructured oxide
surface allows to extend the semiconductor-dye interface with a hole
transporting layer (spiro-MeOTAD) in a controlled way, making the
studied system closer to a solid-state dye-sensitized solar cell.
Four zinc phthalocyanines are compared in this study. The compounds
are equipped with bulky peripheral groups designed to reduce the self-aggregation
of the Pcs. Almost no signs of aggregation can be observed from the
absorption spectra of the Pcs assembled on a ZnO surface. Nevertheless,
the time-resolved spectroscopy indicates that there are inter-Pc charge
separation–recombination processes in the time frame of 1–100
ps. This may reduce the electron injection efficiency into the ZnO
by more than 50%, pointing out to a remaining aggregation effect.
Surprisingly, the electron injection time does not correlate with
the length of the linker connecting the Pc to ZnO. A correlation between
the electron injection time and the ”bulkiness” of the
peripheral groups was observed. This correlation is further discussed
with the use of computational modeling of the Pc arrangements on the
ZnO surface