Research proposal

1. DANIEL MORALES SALAZAR
04/10/2013
CHEM 4803/6171
Synthesis of Low Spin Iron Complex
as Potential Redox Mediator for Dye
Sensitized Solar Cells

2. Representation of a Dye Sensitized Solar Cell
-LUMO potential of the dye
must be more negative than
the potential of the TiO2
Conduction Band edge (-0.5
V).
-HOMO potential of the dye
must be more positive than
Redox potential of electrolyte
for efficient dye regeneration.

3. Important electron processes in a DSSC
Regeneration and Recombination kinetics
(2) 100 fs to 100 ps
(3) μs range
(5) μs to ms range
(6) ms to s
(7) ns

4. I−/I3
− Redox Couple


5. Open-circuit Voltage (Voc)
- Increasing the redox potential of the electrolyte
increases Voc and reduces overpotential losses.

6. Important Requirements for New
Electrolytes
 Positive redox potential to maximize/optimize Voc (1.2 V), yet allowing
enough driving force to regenerate photo-oxidized dye (0.18 V-0.4 V vs.
NHE).
 Slow recombination kinetics (5) and (6).
 Negligible light absorption in the visible region.
Alternative Redox Mediators
Co(II/III)(bpy)3, 0.535 V Co(II/III)(bpy-pz)2, 0.86 V Fc/Fc+, 0.62 V
• Fast recombination
kinetics, poor dye
regeneration
• Not optimal with Pt-
coated counter electrode
• Very fast
recombination kinetics

7. Proposed Research
 Hypothesis: Synthesis of low spin Fe(II/III) iron
complexes, with a fast self-exchange electron rate,
and a large redox potential, as alternative redox
mediators in DSSCs with a high Voc.
1. ([Fe(bphen)3]2+/3+)
2. ([Fe(phen)3]2+/3+)
R=Me, Ph, OR2
1. 2.

8. Motivations
 Potential reversibility between the Fe2+/3+ species and
possibility to tune the redox potential through ligand
modifications.
 Larger self-exchange electron rate than cobalt analogs due to
smaller reorganizational energy going from presumably low
spin d6 to low spin d5 electron configuration. Dye regeneration.
 Bulky electron donating groups can function as insulating
spacers, slowing down dark current.
k11= 1.0 x 106 M-1s-1 k11= 1.0 x 10-9 M-1s-1

9. Motivations
Generally, the observed trends in the redox potentials are dependent on
geometrical distortions caused by the introduced ligands.

10. Potential Drawbacks
 The redox potential of the parent complex
([Fe(phen)3]2+/3+) (1.22 V vs SHE) must be lowered
by 0.2 V to allow dye regeneration.
 Light absorption in the visible spectrum (400-700
nm) might reduce the photocurrent.
 Ligand substitutions at the 5,6 positions are barely
described in the literature.
 Faster self-exchange electron rates are also related
to faster recombination kinetics.

11. Synthesis of benzo[f][1,10]phenanthroline ligand
The syntheses of ligands as well as the ferrous complexes are
described in the literature. Low yields are reported for the ligands,
ranging from 15 to 30% due to multiple steps
Skraup
synthesis

12. Characterization of the complexes
 1H-NMR, 13C-NMR, ESR (Fe3+), IR spectroscopy,
and high resolution mass spectroscopy can be used
to characterize the ligands and complexes.
 Magnetic susceptibility measurements through the
Evans method can be used to corroborate the low
spin nature of the [Fe(bphen)3]2+/3+ complexes.
 UV/Vis/NIR absorption spectra are obtained.
 Cyclic voltammetry experiments are
performed to fulfill the reversibility criteria

13. Characterization of the complexes
 Single-crystal X-ray diffraction
 NMR line-broadening technique is used to measure
the electron transfer self-exchange rate. Faster self-
exchange translates into faster regeneration kinetics.

14. Preparation of electrolytes
 Typical example of an electrolyte preparation: “The
electrolyte consists of 0.22 M [Fe(bphen)3]2+ , 0.05 M
[Fe(bphen)3]3+ , 0.1 M LiClO4, and 0.2 M 4-tert-
butylpyridine in acetonitrile.”
Decreases
photocurrent,
increases
photovoltage
Increases
photocurrent
, decreases
photovoltage

15. Experiments with electrolytes
 The redox potential of the electrolytes is calculated using
the Nernst equation and the formal potentials obtained
from cyclic voltammetry.
 YD2-o-C8 porphyrin dye (1.29 V vs. NHE)
 J/V and IPCE spectra.
- Octyloxy groups reduce charge
recombination
- 2.12 x 105 M-1 cm-1 (400 to 500 nm)

16. References
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