scCO2 soluble dyes can be accessed by using masked binding
groups, such as anhydrides, for use in DSSC applications. The
solubility of the dye in scCO2 can be further enhanced by
the incorporation of fluoroalkyl substituents, which replace the
analogous alkyl substituents. DOP (8) and DOFP (9) dyes were
studied in liquid DSSC devices, where dyes were deposited by
conventional and under scCO2 conditions in batch mode. In
conventional solvents, DOFP (9) showed slightly better perfor-
mance than the alkyl analogues with an overall improvement of
14% in ECE, which is an advantageous discovery for application
to other dyes in DSSCs. When the dyes were deposited using
scCO2, both dyes produced photovoltaic responses as a result
of the masked anhydride group. Although the solubility of
DOP (8) in scCO2 is improved in the presence of this masked
binding group, it is still low and thus hinders satisfactory dye
adsorption. In contrast, the modified perylene dye DOFP (9) has
greater solubility in scCO2, attributed to the presence of both the
masked binding group and the fluorinated substituent, resulting
in better dye deposition. With the scCO2 deposition method,
DOFP (9) (ECE = 1.25%) showed similar cell performance to
the alkyl analogue DOP (8) (ECE = 1.24%). This performance
is comparable when DOFP (9) is deposited using conventional
solvent (ECE = 1.41%).
In conclusion, the replacement of organic solvents with
scCO2 for depositing dyes onto photoanodes provides several
fabrication benefits. By employing the scCO2 process we have
dramatically reduced dye deposition times from 15 h for
conventional organic solvents to only 2 h. No additional
photoanode rinsing step is required, as conducted in the
conventional deposition step.39 The energy intensive heat re-
activation step (450 ◦C) required with conventional solvents
has been eliminated by use of the scCO2 process. The valuable
dye material can be easily recovered in a solid form after the
scCO2 process simply by venting of the CO2 gas, which can
also be recycled. This is in stark contrast to the costly and
laborious processes with the conventional solvent method, which
generates large quantities of unreacted dye and solvent and
requires separation and disposal of waste solvent. We have
demonstrated an effective dye deposition method for DSSCs
using scCO2 with a comparable photovoltaic response to that
of conventional deposition methods. Current work is directed
at applying these concepts to other dyes and investigating
alternative non-fluorinated CO2-solublizing groups.40
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Acknowledgements
We thank CSIRO Future Manufacturing Flagship for sup-
port and award of a postdoctoral fellowship to SM. ABH
acknowledges the CSIRO Fellow-OCE Science Team and GEC
acknowledges the CSIRO OCE Julius Award. We appreciate
assistance from Dr’s R. Williamson and A. Riches on scCO2
chemistry.
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