Effective suppression of interfacial charge recombination by a 12-crown-4 substituent on a double-anchored organic sensitizer and rotating disk electrochemical evidence

Article abstract of DOI:10.1039/c6ta11091f

Mono/double-anchored phenothiazine-based dyes (DCE1-DCE4) containing various N-substituents (12-crown-4-substituted phenyl, 4-hexoxyphenyl, and bare phenyl) were synthesized and applied as sensitizers for dye-sensitized solar cells (DSSCs). The incorporat

Full text of DOI:10.1039/c6ta11091f

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Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
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Effective Suppression of Interfacial Charge Recombination by 12-
Crown-4 Substituent on Double-Anchored Organic Sensitizer and
Rotating Disk Electrochemical Evidence
Received 00th January 20xx,
Accepted 00th January 20xx
Chun–Ting Li,a Feng–Ling Wu,a Chia–Jung Liang,a Kuo–Chuan Ho,b and Jiann T. Lina,*
DOI: 10.1039/x0xx00000x
Mono/double-anchored phenothiazine-based dyes (DCE1–DCE4) containing various N-substituents (12-crown-4-
substituted phenyl, 4-hexoxyphenyl, and bare phenyl) have been synthesized and applied as the sensitizers for dye-
sensitized solar cells (DSSCs). 12-Crown-4-substituted phenyl entity on the double-anchored dye effectively retards the
dark current via Li+ chelation in the case of LiI redox mediator or steric blockade of Co-phen3+ in the case of cobalt redox
mediator. The retarded dark current was further confirmed from the recombination rate constant for the reduction of the
oxidized redox mediators at the photoanode/electrolyte interface by rotating disk electrode technique. The DCE2-based
DSSC with iodide-based electrolyte exhibited a good cell efficiency up to 8.82%, which surpassed that of the N719-based
DSSC (8.20%). With cobalt-based electrolyte, higher cell efficiencies of 10.12% and 11.17% were obtained at 1 sun and 0.1
sun, respectively, using a Pt-free and TCO-free counter electrode.
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the dark current, two approaches are frequently used: (i)
grafting alkyl/bulky groups at the donor moiety or the π bridge
of metal-free dyes11-14 and (ii) having appropriate amount of
anchors (double or multi-anchors).15-17 Therefore, the oxidized
redox mediator can be effectively blocked or the TiO2 surface
can be effectively protected.
1. Introduction
Dye-sensitized solar cells play an important role in the
development of the third generation solar cells, including
Miyasaka’s pioneer work on perovskite solar cell,1 which later
evolved into high performance mesoscopic pervoskite solar
cells.2-4 As a heart material in dye-sensitized solar cells (DSSCs),
plenty types of dyes were developed, e.g., ruthenium dyes
(CYC-B11, 11.5%),5 zinc dyes (SM315, 13.0%),6 and metal-free
organic dyes (C281, 13.0%;7 the co-sensitization of ADEKA-1
and LEG4, 14.3%8). Among them, metal-free organic dyes
stood out due to their flexibility in molecular design, high
molar extinction coefficients, and high efficiency. In a DSSC,
the major energy loss occurs at the photoanode/electrolyte
interface due to the recombination reactions of the photo-
induced electrons at the TiO2 surface with two kinds of
materials, i.e., the oxidized dye and the oxidized redox
mediators.9, 10 The recombination reaction with the oxidized
mediator will cause the dark current, decrease the electron
density as well as the Fermi level of TiO2, and thus lead to a
lower open-circuit voltage (VOC) of the DSSC, as the VOC value is
determined by the potential difference between the Fermi
level of TiO2 and the redox potential of the mediator. To tackle
We recently found that phenothiazine-based dyes with
double anchors (D(–π–A)2) not only more efficiently
suppressed the dark current, but also provided more electron
extraction pathways, compared to their congeners with mono-
anchor.18-20 Besides, the N-substituent of the phenothiazine
entity offered the opportunity for alleviation of dark current as
well as dye aggregation; the pertinent DSSCs thus reached
higher cell efficiency than the N719-based standard cell.
Adsorption of lithium cation (Li+), frequently used counter-ion
of the iodide or the supporting electrolyte for the cobalt redox
mediator, was known to enhance dark current.21-23 Prevention
of Li+ from its electrostatic interaction with the photo-induced
electrons on the TiO2 surface was deemed a promising
approach to suppress dark current.24, 25 In previous researches,
the incorporation of oligo(ether)-based segment in dye
–
molecular helped with chelating Li+ and quarantining I3 from
the TiO2 surface.26-29 In addition, the fettered I– ion by the Li+
close to the regenerating site of the dye may benefit dye
regeneration. Crown ethers have high affinity for alkali metal
ions via the lone pair electrons at the oxygen atoms,30-32 and
12-crown-4 is particularly suitable for Li+ binding due to its
specific size of cavity.33, 34 White et al.35 and Uemura et al.36
synthesized the crown ether-substituted ruthenium dye
(RC730, 1.87%) and carbazole dye (MK-70, 5.50%), respectively.
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However, these crown ether-substituted dyes gave cell from
1 through a Vilsmeier–Haack reaction (step ii) followed
efficiencies similar to those of their analogues without the by a bromination reaction (step iii), where these steps are
crown ether entity. The role of crown ether in those dyes referred to the reported procedures.26 Subsequently,
a
toward the DSSCs’ performance is still blurry up-to-date. Our triethyleneoxide methyl ether (TEOME)-substituted phenyl
earlier success of using oligo(ether) substituent in entity is attached to the molecule through the C-3 (or C-7)
phenothiazine-based mono-anchored dyes for the DSSCs with position of the phenothiazine by a palladium-catalyzed Suzuki–
high photovoltage and high efficiency26 prompted us to Miyaura C–C coupling reaction39 between TEOME-substituted
explore the possibility of double-anchored phenothiazine dye phenyl bromide and dioxaborolyl phenothiazine (step v). Lastly,
with a 12-crown-4 substituent at the nitrogen atom of the desired product, DCE1, is obtained via Knoevenagel
phenothiazine entity. Herein we report new sensitizers of the condensation of the appropriate aldehyde
type and their highly efficient DSSCs. The role of the crown is cyanoacetic acid (step vi).
also elucidated via different physical studies including rotation
(DCE1a) with
disk electrode electrochemical measurements.
2. Results and discussion
2.1. Synthesis of DCE dyes
The synthetic routes for new DCE dyes are depicted in
Scheme 1
.
Figure 1 shows that DCE1 is a mono-anchored dye
DCE4 are double-anchored dyes with different N-
and DCE2
−
substituents at their phenothiazine entities (including 12-
crown-4-substituted phenyl, 4-hexoxyphenyl, and bare phenyl
substitutes). At first (step i), the 12-crown-4-substituted
phenyl (or 4-hexoxyphenyl, or bare phenyl entity) was
introduced at the nitrogen atom of the phenothiazine core
through Pd(dba)2 (dba = dibenzylideneacetone)-catalyzed C–N
coupling reaction37,
38
with 10H-phenothiazine. The key
Figure 1. Molecular structures of the DCE dyes.
intermediate, 5, of the mono-anchored DCE1 dye is prepared
Scheme 1.
Synthetic route of the DCE dyes. (i) 10H-phenothiazine, Pd(dba)2 (where dba= dibenzylideneacetone), Pt(Bu)3, NaOtBu, toluene, 120 oC, 20 h; (ii) POCl3, DMF, 60 oC,
18 h; (iii) N-bromosuccinimide (NBS), CH2Cl2, 18 h; (iv) bis(pinacolato)diboron, KOAc, PdCl2(dppf) (where dppf = 1,1’-bis(diphenylphosphino)ferrocene), 1-4-dioxane, 120 oC, 20 h; (v)
Pd(PPh3)4, 2M K2CO3, 1-bromo-2,4-bis{2-[2-(2-methoxyethoxy)ethoxy]ethoxy}benzene, toulene, 120 oC, 20 h; (vi) cyanoacetic acid, NH4OAc, AcOH, 120 oC.
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Figure 2. Absorption spectra of DCE dyes (a) dissolved in THF (10 mM) and (b) adsorbed on TiO2 film (4 μm).
To synthesize the double-anchored dyes, the key between the phenothiazine and the N-phenyl substituent (vide
intermediates, , and , are prepared from , and infra). Red shift of the ICT band for the DCE dyes-adsorbed
6,
7
8
1,
2
3,
respectively; through a bromination reaction (step iii) followed TiO2 films (Figure 2b) and tailing of the absorption band edges
by a Suzuki–Miyaura C–C coupling reaction between the (λonsetfilm) beyond 650 nm reflect J-aggregation of the dyes on
dibromo phenothiazine and the appropriate thiophene the TiO2 films.
dioxaborolanes (step v). Finally, the desired products, DCE2 to
Comparable oxidation potentials (EOX) of the dyes measured
DCE4, are obtained via the Vilsmeier–Haack reaction (step ii) by cyclic voltammograms (Figure S1) indicate that the variation
followed by a Knoevenagel condensation reaction.18-20,
40
of the N-substituent of phenothiazine barely influences the
Detailed synthetic information for the DCE dyes are depicted HOMO level of the dye. This result is substantiated by density
in the Supporting Information.
functional theory (DFT) and time-dependent DFT (TDDFT)
calculations. Selected frontier orbitals and low-lying transitions
are shown in Figure S2 and Table S1, respectively. All dyes
exhibit comparable HOMO level suitable for dye regeneration
and appropriate LUMO level for electron injection (Figure S3).
Nearly 100% S0 → S1 (HOMO → LUMO) transiꢀon in all dyes
2.2. Optical properties and DFT calculations of DCE dyes
The double-anchored dyes
(
DCE2
−
DCE4
)
have the
intramolecular charge-transfer transition (ICT) band absorption
peak (λabsTHF) at the wavelength of 477 to 488 nm, which is
about 20−30 nm larger than that of DCE1
(
Table S1) indicates prominent charge transfer from the
(Figure 2a and Table
phenothiazine donor to the anchor. Broader ICT band in
double-anchored dyes than in DCE1 is attributed to the mixing
of S0 → S1 with low energy S0 → S2 transitions. Both anchors on
the double-anchored dyes can function as electron injection
channels based on the following observations: (1) they both
contribute to the LUMO orbital (Figure S3), and (2) they both
possess significant negative charges at the excited state, as
shown in the Mulliken charge plots (Figure S4).
1
). This can be attributed to the extension of π–conjugation
length in the double-anchored dyes.18 Accordingly, the optical
HOMO/LUMO gaps (Egopt) for double- and mono-anchored
dyes are about 2.1−2.2 eV and 2.42 eV, respecꢀvely. The
double-anchored dye with an electron-donating substituent
THF
(i.e., DCE2 or DCE3) exhibits only slightly larger λabs
and
molar extinction coefficient (ε) than its congener with a bare
phenyl substituent (DCE4), owing to the large twist angle
Table 1. Optical and electrochemical data of the dyes, and their photovoltaic parameters for iodide-based DSSCs
Dye
Optical and electrochemical properties[a]
Photovoltaic properties[f]
opt
Eg
film
*
λabsTHF [nm]
λonset
EOX
E0-0
VOC
JSC
FF
η
Dye loading
[mol cm-2]
(ε×10-4 M-1 cm-1)
[eV][b]
[nm][c]
[V][d]
[V][e]
[mV]
[mA cm-2]
[%]
DCE1
DCE2
DCE3
DCE4
457 (0.80)
487 (1.14)
488 (1.01)
477 (0.82)
2.42
2.15
2.09
2.16
650
730
700
675
1.08
1.06
1.09
1.07
-1.34
-1.09
-1.00
-1.09
658.8±1.41
706.0±2.44
680.1±2.34
666.5±1.41
11.95±0.10
18.19±0.03
18.16±0.04
15.64±0.20
0.69±0.00
0.69±0.00
0.65±0.00
0.67±0.00
5.44±0.04
8.82±0.01
7.99±0.01
7.02±0.10
3.94×10-7
2.38×10-7
2.26×10-7
2.88×10-7
[a]Based on THF solution at 298 K. [b]Calculated via Egopt = 1240/λ0-0, where λ0-0 is read from the cross point of absorption and emission spectra). [c]Based on 4 μm-thick
TiO2 film. [d]Adjusted to be versus to normal hydrogen electrode (NHE) using ferrocene/ferrocenium as an internal reference. [e]Calculated via E0-0* = EOX−Egopt (vs. NHE).
[f]Measured at AM 1.5G based on five DSSCs with I–/I3– electrolyte.
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Figure 3. Plots of photovoltaic performance for the DSSCs with various DCE dyes using I–/I3– redox mediator: (a) the current density–voltage curves, measured under simulated AM
1.5G illumination or dark condition, (b) the extracted electron density–photovoltage curves, measured under an intensity-modulated (10 to 300 W m-2) white-light-emitting diode,
(c) and (d) the electrochemical impedance spectra (Nyquist and Bode plots, respectively), measured under dark at an applied bias of -0.70V.
The dihedral angle between the phenothiazine and the N- The VOC value of a DSSC is influenced by the conduction band
substituted phenyl ring falls in the range from 82.6o to 83.9o edge (ECB) of the dye-adsorbed TiO2 film and the retardation
23
(
Figure S5), which may result in minute electronic ability of the dye against dark current.22,
Via charge-
communication between these two entities. Consequently, the extraction technique, the electron density (dn) on the DCE dye-
ICT bands of DCE2
−
DCE4 have similar absorption maxima and adsorbed TiO2 film decreases in an order of DCE2
DCE4 DCE1 Figure 3b), indicating that all double-anchored
DCE dyes exhibit a downward shift of the ECB of TiO2 film
> DCE3 >
intensity.
>
(
2.3. Photovoltaic performance of the cells with various DCE dyes compared with mono-anchored DCE1 41,
.
The outcome is
42
attributed to the release of two protons from the double-
anchored dye upon dye adsorption on the TiO2 surface.18, 43
Therefore, the higher VOC values for the double-anchored DCE
dyes imply that the dark current suppression efficiency
predominates over the ECB shift in these cells. This is consistent
with previous findings that double-anchored dyes are more
effective in suppressing the dark current (vide infra).18 It is
commonly found that efficient dark current suppression leads
to higher VOC value of the cells.7, 14, 44, 45 Among the three
substituents in double-anchored DCE dyes, 12-crown-4-
substituted phenyl moiety is the most sterically congested, and
phenyl moiety is the least, suggesting that steric factor plays
an important role in suppressing the dark current. We
previously also found that more sterically hindered N-
substituent in double-anchored phenothiazine dyes could
Photocurrent density–voltage curves (Figure 3a) of the DSSCs
coupled with various DCE dyes were measured under
–
simulated AM 1.5G illumination using I–/I3 redox mediator. In
Table 1, all double-anchored dyes (DCE2−DCE4) have higher
cell efficiencies (η: 7.31−8.41%) than the mono-anchored DCE1
(6.31%), owing to the higher open-circuit voltage (VOC) and
short-circuit current density (JSC). Although DCE1 possesses the
largest dye-loading amount on TiO2 thin film, the most blue-
shifted spectral range and lowest absorptivity result in the
least JSC value of its DSSC. The smaller JSC for DCE4 than DCE2
and DCE3 is also attributed to the narrower and weaker
absorption for DCE4 (both in solution and on the TiO2 film).
The VOC values for DCE dyes decrease in the order of DCE2
(706 mV) > DCE3 (680 mV) > DCE4 (667 mV) ≈ DCE1 (658 mV).
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suppress the dark current more effectively:40 bis(2- the retardation ability of the substituent at the phenothiazine
ethylhexyloxy)phenyl moiety (750 mV) > bis(n-hexyloxy)phenyl entity of DCE dyes decreases in the order of 12-crown-4-
moiety (730 mV) > 2-ethylhexyl moiety (680 mV).
Moreover, DCE2 has high affinity toward Li+ via its 12- moiety. Under 1 sun illumination (100 mW cm-2) with an open-
crown-4-substituted phenyl moiety, preserving the ECB of TiO2 circuit condition, charge transfer resistance (Rct2 Figure S7a
DCE1, which
substituted phenyl moiety > 4-hexoxyphenyl moiety > phenyl
,
)
in addition to the retardation of charge recombination;33, 34, 42, increase in the order of DCE2
≈ DCE3 < DCE4 <
light
46 thus, DCE2 gives the highest VOC. The VOC value as well as the agrees well with their JSC values. The values of τe
(Figure S7b)
dark
retardation ability against the dark current for the mono- also agree with the τe values. In addition, it is notable that
light
anchored DCE1 is the least among all DCE dyes, even though the values of VOC, dark currents, Rrec, τedark, and τe
,
DCE1 has the most favorable ECB of TiO2 and the beneficial 12- irrespective of their different measurement techniques, show
crown-4 entity. This most probably stems from more severe a perfect consistency with one another.
dye aggregation of DCE1, as evidenced from the clear blue-
shift in its incident photon-to-current conversion efficiency 2.5. Highly efficient DCE2-based DSSC with the cobalt-based
(IPCE, Figure S6a) spectra after using chenodeoxycholic acid
(CDCA) as the co-adsorbent. The argument on the correlation
of VOC value with dark current suppression is verified via the
dark current density–voltage curves shown in the lower part of
Figure 3a. Under dark condition, the higher onset bias, where
the dark current of the DSSC is raised, implies better
suppression of charge recombination and thereby higher VOC
for the DSSC.9, 46 The onset bias of the cells shows the trend of
electrolyte and the novel Pt-free counter electrode
When the best DCE2-based DSSC is performed in a cobalt-
based electrolyte (Figure 4), a higher η of 9.52% is obtained
with a much higher VOC of 856.8 mV, a lower JSC of 15.41 mA
cm–2, and a better FF of 0.72 (Table 2), compared to the cell
parameters measured in iodide-based electrolyte. The higher
VOC (~150 mV higher) and lower JSC for cobalt-based electrolyte
are attributed to the more positive redox potential and the
mass transfer limit of cobalt complex, respectively.49
DCE2
> DCE3 > DCE4 ≈ DCE1, indicating a great consistency
between the results read from the VOC values (under 1 sun)
and dark currents (under dark) of the pertinent DSSCs. In
Figure S6a, the IPCE spectra (400 to 800 nm) of the cells show
their absorption edge decreasing as DCE2
≈ DCE3 > DCE4 ≈
DCE1, which agrees well with absorption spectra of the DCE
dyes-adsorbed TiO2 films. With the highest JSC, IPCE, and VOC
values, the cell of DCE2 exhibits the highest η of 8.82%, which
is higher than that of N719-based cell (η = 8.68%, Figure S6b).
2.4. Electrochemical impedance spectra analysis
Electrochemical impedance spectra (EIS) analysis is used to
investigate the impedance properties at the photoanode/
electrolyte interface in the iodide-based DSSC (Table S2).47, 48
Under dark condition with an applied voltage of –0.70 V, the
values of charge recombination resistance (Rrec
,
Figure 3c) and
Figure 3d) decrease in
an order of DCE2 (26.55 Ω, 11.65 ms) > DCE3 (22.49 Ω, 6.74 ms)
DCE4 (19.09 Ω, 5.13 ms) > DCE1 (18.70 Ω, 5.13 ms). The
dark
electron recombination lifetime (τe
,
>
results provide further support of the previous conclusions: (1)
double-anchored dyes indeed have better recombination
retardation abilities than that of the mono-anchored dye; (2)
Figure 4. Photocurrent density–voltage curves of the DSSCs with DCE2 and DCE4 dyes,
measured in a cobalt-based electrolyte using the counter electrodes of platinum/FTO
and TiO1.1Se0.9/carbon cloth at different incident light intensities.
Table 2. Photovoltaic parameters of the cobalt-based DSSCs with DCE2 and DCE4 dyes[a]
Dye
Light intensity [mW cm-2]
VOC [mV]
JSC [mA cm-2]
FF
η [%]
Counter electrode
DCE2
100 (1.00 sun)
100 (1.00 sun)
57 (0.57 sun)
12 (0.12 sun)
100 (1.00 sun)
100 (1.00 sun)
856.8±7.21
848.3±2.89
840.0±0.00
795.0±0.00
805.0±0.00
830.0±0.00
15.41±0.12
15.96±0.01
10.14±0.01
2.32±0.01
0.72±0.01
0.75±0.00
0.75±0.00
0.73±0.00
0.57±0.01
0.63±0.00
9.52±0.08
10.12±0.02
11.05±0.04
11.17±0.03
5.17±0.09
7.81±0.06
Pt/FTO[b]
TiO1.1Se0.9/CC[c]
TiO1.1Se0.9/CC[c]
TiO1.1Se0.9/CC[c]
Pt/FTO[b]
DCE4
11.26±0.09
14.83±0.09
TiO1.1Se0.9/CC[c]
[a]Based on five DSSCs with tris(1,10-phenanthroline)cobalt(II/III) trifluoromethanesulfonimide salts (Co-phen2+/Co-phen3+). [b]A platinum (Pt) film coated on a fluorine–
doped tin oxide (FTO) conducting glass substrate. [c]A titanium oxide-selenide (TiO1.1Se0.9) ternary composite film coated on a flexible carbon cloth (CC) conducting
substrate.
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Figure 5. Plots of i–1 vs. ω–0.5 for the films of TiO2/DCE2 and TiO2/DCE4, measured in the (a) iodide-based and (b) cobalt-based electrolytes.
Scheme 2. The sketches of the I3– reduction behaviors at (a) TiO2/DCE2/electrolyte and (b) TiO2/DCE4/electrolyte interfaces.
Compared to the DCE4-based DSSC (5.17%), the DCE2-based rotating disk electrode (RDE) technique. Here, RDE analysis
DSSC gives a much higher η; there is no doubt that the 12- aims to simultaneously measure two key parameters, the
crown-4 substituent in DCE2 also effectively suppresses the intrinsic heterogeneous rate constant (k0) and the effective
recombination reaction in the cobalt-based electrolyte. In active area (Ae), for the reduction of the oxidized redox species.
addition, Pt-free counter electrodes are economical alternative When scanning from positive to negative potential at a certain
components of DSSCs application and attract considerable rotating rate (ω), more and more negative charges (or current,
interests in recent years.50-52 Hence, a newly designed Pt-free i) accumulate at the TiO2/DCE dye/electrolyte interface to
and TCO-free counter electrode, i.e., a titanium oxide-selenide trigger the reduction of the oxidized redox species. According
(TiO1.1Se0.9) ternary composite film coated on a flexible carbon to the simplified Koutecký–Levich equation53 (Experimental
cloth (CC) conducting substrate (denoted as TiO1.1Se0.9/CC) was Section), plots of i-1 vs. ω-0.5 were used to calculate values of k0
synthesized. With this new counter electrode, the DCE2-based and Ae, measured in an electrolyte solely containing the
DSSC reaches 10.12% at 1 sun (100 mW cm-2), and this cell oxidized redox species (I3– for Figure 5a or Co-phen3+ for Figure
efficiency is even higher than that obtained using traditional 5b). In the iodide-based electrolyte, based on the same slope
counter electrode of Pt/FTO. Similarly, the DCE4-based DSSC (i.e., same Ae of 0.02 cm2), the TiO2/DCE2 film exhibits lower k0
has better performance using TiO1.1Se0.9/CC (7.81%) than that of 2.93×10-4 cm s-1 than the TiO2/DCE4 film (k0 = 9.45×10-4 cm
–
using Pt/FTO (5.17%). The new TiO1.1Se0.9/CC electrode is s-1), indicating a slower recombination rate of I3 ion with the
certainly
a great substitution for the traditional Pt/FTO accumulated negative charges at the TiO2/DCE2/electrolyte
electrode; the detailed information about TiO1.1Se0.9/CC interface. It is most likely that chelation of Li+ by DCE2 dye at
electrode will be published elsewhere. Last but not least, at its 12-crown-4-substituted phenyl moiety, which traps I3– apart
dim light illumination, the best DCE2-based DSSC renders from the dye-uncovered TiO2 surface due to electrostatic
11.05% at 0.57 sun and 11.17% at 0.12 sun in the cobalt-based interaction between Li+ and I3 , and thus leads to slower
–
electrolyte using the new TiO1.1Se0.9/CC counter electrode recombination rate (Scheme 2). In contrast, for the cobalt-
(
Table 2), indicating the infinite potential of this DSSC for based electrolyte, the more sterically congested 12-crown-4-
indoor applications.
substituted phenyl moiety (with or without Li+-chelation) in
DCE2 may deter the bulky oxidized redox species from getting
close to the TiO2 surface. Based on the same measured Ae of
0.54 cm2, TiO2/DCE2 film also possesses a much lower k0
(4.60×10-4 cm s-1) than the k0 (14.17×10-4 cm s-1) of TiO2/DCE4
film. Apparently, 12-crown-4 moiety-containing DCE2 more
effectively restrains the recombination reactions in both
iodide-based and cobalt-based electrolytes.
2.6. Rotating disk electrode analysis for charge recombination
rate constant at the TiO2/DCE/electrolyte interface
To explore the effect of 12-crown-4 moiety on the electron
transfer at the photoanode/electrolyte interface, films of
TiO2/DCE2 (with crown ether) and TiO2/DCE4 (without crown
ether) were subjected to electrochemical studies using
6 | J. Name., 2012, 00, 1-3
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ARTICLE
solvent of acetonitrile/Nafion® 117 solution (9/1, in volume
ratio). Here, the Nafion® 117 solution worked as a film-forming
agent to make the films coating on their GCE substrates. The
loading amount of TiO2/DCE2 or TiO2/DCE4 film on GCE was
about 0.1 mg. The obtained films of TiO2/DCE2 and TiO2/DCE4
were dried at 70 oC before use.
Conclusions
The introduction of the 12-crown-4-substituted phenyl entity
into phenothiazine-based dye with double anchors is
a
–
promising approach to reduce the charge recombination for I3
reduction due to Li+-chelation or Co-phen3+ reduction due to
steric barrier effect, at the photoanode/electrolyte interface,
i.e., to suppress the dark current in a DSSC. In addition to dark
current measurements and electrochemical impedance
spectra analyses, better dark current suppression was
confirmed by rotating disk electrode technique for the first
time. The best iodide-based DSSC with DCE2 reaches an
efficiency of 8.82%, which is higher than that with DCE4 by
~21% and is also higher than that of N719-based cell. In a
cobalt-based electrolyte, highest cell efficiencies of 10.12%
and 11.17% are obtained at 1 and 0.12 sun, respectively.
At a certain rotating speed (50, 100, 150, 200, 250, or 300
rpm),
a linear sweep voltammetry (LSV) curve for the
TiO2/DCE2 or TiO2/DCE4 film is scanned from 0.2 V to –0.3 V;
in other word, more and more negative electrons accumulate
at the TiO2/DCE/electrolyte interface to trigger the reduction
–
of the oxidized redox species (I3 or Co-phen3+). Thus, six LSV
curves were obtained at different rotating speeds to get six
values of the reciprocal currents (i-1) at the formal potential (E0’)
–
of I–/I3
(
Figure S8a and Figure S8b) or Co-phen2+/Co-phen3+
(
Figure S8c and Figure S8d). As shown in Figure S8, the
background current signal of the GCE substrate is in an order
10-8 A, which can be considered to have negligible effect
toward the signals of TiO2/DCE films.55 In accordance with the
simplified Koutecký–Levich equation below,53 the values of k0
Experimental Section
DSSCs’ fabrication. A home-made transparent TiO2 paste and a
home-made scattering TiO2 paste was prepared according to
the published procedures54. For the iodide-based electrolyte, a
20 μm-thick TiO2 film, containing a 15 μm-thick transparent
layer (Solarnix transparent paste) and a 5 μm-thick scattering
layer (home-made scattering paste), was used. For the cobalt-
based electrolyte, a 12 μm-thick TiO2 film, containing an 8 μm-
thick transparent layer (home-made transparent paste) and a 4
μm-thick scattering layer (another home-made scattering
paste), was prepared via the same procedure. The detailed
information for the TiO2 films was depicted in the Supporting
and Ae can be obtained via the plots of plots of i-1 vs. ω-0.5
.
1
1
1
=
+
ͤ
0.62nFAeD2 /3 υ-1 /6 ω1/2
where i is the specific current obtained from the LSV curve at
C
i
nFAe
͟ C
–
the formal potential of the redox mediator (I–/I3 or Co-
phen2+/Co-phen3+), n is the number of electrons transferred, F
is Faraday constant, C is the concentration of the oxidized
–
redox species (I3 or Co-phen3+), D is the apparent diffusion
coefficient of the oxidized redox species, ν is the kinetic
viscosity of electrolyte, and ω is the angular velocity converted
from the rotating speed.
Information. The sintered TiO2 film was immersed in 3×10−4
M
dye solution with 10 mM of chenodeoxycholic acid (CDCA) at
room temperature for at least 12 h using a mixed solvent of
acetonitrile/tert-butanol (ACN/TBA=1:1, in volume ratio).
Finally, a dye-adsorbed TiO2 film was assembled with a
sputtered-platinum/FTO counter electrode using a 25 μm-thick
Surlyn® (SX1170-25, Solaronix S. A., Switzerland) as the cell
spacer. The iodide-based electrolyte, containing 0.1 M lithium
iodide (LiI), 1.0 M 1,2-dimethyl-3-propylimidazolium iodide
(DMPII), 0.04 M iodine (I2), and 0.5 M 4–tert–butylpyridine
(TBP) in acetonitrile/3–methoxypropionitrile (ACN/MPN = 8:2,
in volume ratio), was injected into the cell gap between these
two electrodes by capillarity. Similarly, the cobalt-based
electrolyte, containing 0.35 M tris(1,10-phenanthroline)cobalt
For RDE analysis, the iodide-based electrolyte contained 1.0
–
mM of tetrabutylammonium triiodide (I3 ) and 0.1 M lithium
perchlorate (LiClO4) in acetonitrile, and the cobalt-based
electrolyte included 1.0 mM of tris(1,10-phenanthroline)cobalt
tris(trifluoromethanesulfonimide) (Co-phen3+) and 0.1 M of
lithium perchlorate in acetonitrile. The formal potential of Co-
phen2+/Co-phen3+ and the diffusion coefficient of Co-phen3+
were measured, respectively, by the cyclic voltammetry
(
Figure S9) and another RDE analysis (Figure S10) in
accordance with the Levich equation.53 The corresponding
experimental details were depicted in the Supporting
Information.
bis(trifluoro-methanesulfonimide),
0.05
M
tris(1,10-
Acknowledgements
phenanthroline)cobalt tris(trifluoromethanesulfonimide), 0.8
M TBP, and 0.1 M lithium perchlorate (LiClO4) in acetonitrile,
was used.
We acknowledge the Support of the Academia Sinica (AS), the
Ministry of Science and Technology of Taiwan, National Taiwan
University, and Instrumental Center of Institute of Chemistry,
AS.
Rotating disk electrode (RDE) analysis. The RDE analysis was
performed in a three-electrode electrochemical system using a
glassy carbon electrode (GCE, with a geometric area of 0.2 cm2)
coated with TiO2/DCE2 or TiO2/DCE4 film as the working
electrode. The films of TiO2/DCE2 or TiO2/DCE4 was coated on
the GCE via the drop-coating process using a precursor
solution (10 μL), which contained 10 wt% of TiO2 nanoparticle
(P25), 3×10−4 M of DCE dye, and 10 mM of CDCA in a mixing
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Journal of Materials Chemistry A
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Table of Content
Effective Suppression of Interfacial Charge Recombination by
12-Crown-4 Substituent on Double-Anchored Organic
Sensitizer and Rotating Disk Electrochemical Evidence
‡
Chun–Ting Li,† Feng–Ling Wu,† Chia–Jung Liang,† Kuo–Chuan Ho, and Jiann T. Lin*,†
†Institute of Chemistry, Academia Sinica, No. 128, Sec. 2, Academia Road, Nankang District, Taipei
‡
11529, Taiwan; Department of Chemical Engineering, National Taiwan University, No. 1, Sec. 4,
Roosevelt Road, Taipei 10617, Taiwan
*Corresponding authors: Tel: +886-2-2789-8522; Fax: +886-2-2783-1237; E–mail:
jtlin@chem.sinica.edu.tw (Jiann T’suen Lin)
Crown-ether-substituted double-anchored organic dye surpasses the charge recombination with
iodide and cobalt mediators to reach 10.12% (1 sun) and 11.17% (0.1 sun).