J. Ko et al.
À
tion efficiency, followed by the IÀ/I3 system with over 94%
(inset in Figure 4a), although the recombination for the
CoII/CoIII system is slightly faster (Figure 4b). For the JK-
302 and JK-304 based DSSCs using the CoII/CoIII and the IÀ/
À
I3 systems, similar results were observed as with the JK-303
based DSSCs (Figures S4 and S5 in the Supporting Informa-
tion).
In summary, we have designed and synthesized three
highly efficient organic sensitizers with sterically hindered
fluorenyl units and planar indenothiophene derivatives. The
photovoltaic performance is quite sensitive to structural
modifications of the bridged framework. The devices based
À
on JK-303 using IÀ/I3 , CoII/CoIII, polymer gel, or solid state
electrolytes gave conversion efficiencies of 8.69, 9.04, 7.27,
and 5.82%, respectively. These efficiencies are some of the
highest values reported for DSSCs based on organic sensitiz-
ers. We believe that the development of highly efficient or-
ganic sensitizers comparable to ruthenium dyes is possible
through meticulous molecular engineering of dyes, and stud-
ies directed this goal are now in progress.
Experimental Section
DSSC device fabrication: Fluorine doped Tin Oxide (FTO) glass plates
(Pilkington TEC Glass-TEC 8, Solar, 2.3 mm thickness) were cleaned in
a detergent solution using an ultrasonic bath for 30 min and then rinsed
with water and ethanol. The FTO glass plates were immersed in TiCl4
(40 mm) at 708C for 30 min and washed with water and ethanol. A trans-
parent nanocrystalline layer on the FTO glass plates was prepared by
screen printing TiO2 paste (Dyesol, 18NR-T) and then drying at 1208C.
The TiO2 electrodes were gradually heated under an air flow at 3258C
for 5 min, at 3758C for 5 min, at 4508C for 15 min, and at 5008C for
15 min. A paste for the scattering layer containing 400 nm anatase parti-
cles (CCIC, PST-400C) was deposited by screen printing and then dried
for 1 h at 1208C. The TiO2 electrodes were gradually heated under an air
flow at 3258C for 5 min, at 3758C for 5 min, at 4508C for 15 min, and at
5008C for 15 min. The resulting layer was composed of a 6 mm thick
transparent layer and a 3 mm thick scattering layer. The TiO2 electrodes
were treated again with TiCl4 at 708C for 30 min and sintered at 5008C
for 30 min. The TiO2 electrodes were immersed into the dye solution
(0.3 mm in THF/EtOH=1:2 containing 30 mm chenodeoxycholic acid,
CDCA) and kept at room temperature for 12 h. The FTO plate for the
counter electrode was subsequently cleaned with an ultrasonic bath in
H2O, acetone, and HCl (2m, aq). Counter electrodes were prepared by
coating with a drop of H2PtCl6 solution (2 mg of Pt in 1 mL of ethanol)
on a FTO plate and heating at 4008C for 15 min. The dye-adsorbed TiO2
electrode and Pt counter electrode were assembled into a sealed sand-
wich-type cell by heating at 808C with a hot-melt film (60 mm thickness,
Surlyn) as a spacer between the electrodes. A drop of electrolyte solution
(iodine-based electrolyte: DMPImI (0.6m), I2 (0.05m), LiI (0.1m), TBP
Figure 4. a) Transport resistance (Rt), recombination resistance (Rct), and
À
chemical capacitance (Cm) of the JK-303-based DSSCs for IÀ/I3 and CoII/
CoIII under dark condition. Inset shows the charge collection efficiency of
the IÀ/I3À- and the CoII/CoIII-based DSSCs as a function of applied bias
À
potential. b) The electron lifetime and the transport time for IÀ/I3 and
CoII/CoIII as a function of forward bias potential.
dark condition, as calculated from the capacitance of the
EIS,[7b] shows a shift of about 190 mV (Figure S6 in the Sup-
porting Information). Moreover, it is noted that the recom-
À
bination resistance in both the CoII/CoIII and the IÀ/I3 sys-
tems dramatically decreases as the applied bias potential in-
creases, because of the increase in charge carriers injected
by the dye into the conduction band of TiO2. On the other
hand, the transport resistance for the CoII/CoIII system de-
À
creased more smoothly than that of the IÀ/I3 system. The
(0.8m) in MeCN; cobalt-based electrolyte: [Co
ACHTUGNTREN(NUNG bpy)3](B(CN)4)2 (0.22m),
electron lifetime (tr =Cm ꢁRct) observed for the CoII/CoIII
[Co(bpy)3](B(CN)4)3 (0.05m), LiClO4 (0.1m), and TBP (0.8m) in MeCN;
AHCTUNGTRENNUNG
polymer gel electrolyte: DMPImI (0.6m), I2 (0.05m), LiI (0.1m), NMBI
(0.8m), PVDF-HFP (5 wt%) in MeCN) was placed on the drilled hole in
the counter electrode of the assembled cell and was driven into the cell
through vacuum backfilling. Finally, the hole was sealed using additional
Surlyn and a cover glass.
system decreased more smoothly than that observed for the
À
IÀ/I3 electrolyte, whereas the changes in the electron trans-
port time (tt =Cm ꢁRt) are comparable over the range of ap-
plied bias potentials.[14] The photovoltaic performance is re-
flected by the charge collection efficiency (hcc) derived from
hcc =(1+Rt/Rct)À1 or hcc =(1+tt/tr)À1.[7e] The hcc for the CoII/
CoIII system shows a relatively higher value at high forward
ssDSSCs device fabrication: The ssDSSCs were constructed by drop cast-
ing the HTM solution onto the photoelectrode and covering with a Pt-
coated counter electrode, as described below. The Pt counter electrodes
were prepared by drop-casting the H2PtCl6 solution on a FTO plate and
heating at 4008C for 15 min. The conventional compact TiO2 layer (CC-
TiO2 film), with a 200 nm thickness, was prepared by spin coating a tita-
À
bias potentials compared with that for the IÀ/I3 system. The
hcc of the CoII/CoIII system shows over 95% charge collec-
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