Table 2 Photophysical properties (in CH3CN) and photovoltaic
performance data of various derivatives of GFP core chromophores
red Kaede chromophores suited for application in e.g. DSSCs.
Using the –OH form for 1a or –COOH group for 2 and 3 to
coordinate TiO2, moderate conversion efficiency has been
achieved. These, together with the feasibility in its derivatization,
should attract great attention as well as pave a new avenue for
researchers working on bio-organic molecules and/or energy
relevant subjects. At the current stage, the efficiencies are
lower than those reported for other organic,8 ruthenium9
and certain porphyrin10 dyes applied in DSSCs. Further work
in this area will aim to match the performance of these
other dyes.
Dye
lab/nm
Jsc/mA cmÀ2
Voc/mV
FF
Z (%)
1a
1b
1c
1d
2
435a, 495b
436a, 497b
438a, 498b
495a, 512b
470a
6.6
640
472
466
496
453
526
0.72
0.51
0.44
0.68
0.65
0.69
3.04
0.07
0.06
0.43
1.95
2.34
0.31
0.29
1.26
6.63
6.47
3
517a
a
b
Neutral from. Anionic form.
photovoltaic parameters were measured to be 4.42 mA cmÀ2
(Jsc), 613 mV (Voc), 0.651 (FF), and 1.76% (Z). The results
provide evidence of decent stability of the Kaede core
chromophore upon exploitation as a dye for the solar cell.
The DSSC performance of 1b–1d was also examined under
the same cell configuration. Unfortunately, as shown in
Table 2, the photovoltaic parameters are all significantly lower
than that of 1a. We believe that this inferiority is mainly due to
the low solubility of 1b–1d in CH3CN, as evidenced by low
absorbance (o1.0 at lab) for 1b–1d under saturated CH3CN
(or MeOH) and consequently the pale yellow color appearance
on the prepared cell. The lower FF and Voc values also point
to a possible aggregation effect for 1b–1d. Attempts have been
made to increase dye solubility in CH3CN by adding tetrabutyl
ammonium hydroxide; unfortunately the cell performance was
not improved. Although formation of 1b–1d/ammonium salt
increases the solubility of 1b–1d, the deprotonation of hydroxy
group caused lower dye uptake onto TiO2 surface.7d To
optimize the solar cell performance by using other analogues,
we then performed the similar condensation method between
the methoxylated GFP core (p-MBDI, see Scheme 1) and
R = thiophene-2,5-dicarbaldehyde, followed by attachment
of 2-cyanoacetic acid in the presence of piperidine to make
compound 2. We also subsequently reacted 3a (see Scheme 1)
via a Sonogarshira reaction, followed by condensation with
2-cyanoacetic acid, to afford 3 with a high yield of 83% (see
ESI for details).w Both 2 and 3 possess carboxylic groups and
can thus anchor on TiO2. The resulting I–V curve and DSSC
data are listed in Fig. 1 and Table 2, respectively. Clearly,
although 2 holds a Jsc similar to that of 1a, both Voc and FF
are inferior, giving lower conversion efficiency of 1.95%.
Compared with 2, diphenylamine substituted 3 shows a
1.16-fold increase in voltage and a conversion efficiency of
2.34%. The results reveal the effectiveness upon incorporating
a bulky diphenylamine group, which might decrease the
p–p stacking onto the TiO2 surface, rendering a larger Voc
(cf. 2, see Table 2).
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We thus present herein a facile, one-step reaction protocol
to synthesize red Kaede core chromophores, 1a–d, 2 and 3,
with a product yield of 470%. We also demonstrated its
versatility in strategic design of various kinds of functionalized
ꢀc
This journal is The Royal Society of Chemistry 2009
6984 | Chem. Commun., 2009, 6982–6984