On how electron density affects the redox stability of phenothiazine sensitizers on semiconducting surfaces

Article abstract of DOI:10.1039/C6CC09992K

The stabilities of three organic dyes that differ only by two substituents (-OMe,-H and-Br) about the phenothiazine donor unit were evaluated when immobilized on a semiconductor surface. All three dyes delivered modest power conversion efficiencies (PCEs) in the dye-sensitized solar cell (DSSC), but maintained 75% of their initial PCE over 300 h of sustained simulated sunlight. Electron-donating substituents increased the stability of the phenothiazine radical unit created after light-induced charge injection into the semiconductor; however, this did not translate to higher DSSC stability, which appears to be more sensitive to the basicity of the anchoring group for this series.

Full text of DOI:10.1039/C6CC09992K

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On how electron density affects the redox
stability of phenothiazine sensitizers on
Cite this:DOI: 10.1039/c6cc09992k
semiconducting surfaces†
Received 15th December 2016,
Accepted 17th January 2017
Valerie A. Chiykowski, Brian Lam, Chuan Du and Curtis P. Berlinguette*
DOI: 10.1039/c6cc09992k
rsc.li/chemcomm
The stabilities of three organic dyes that differ only by two sub- radical cationic species that ostensibly participates in deleterious
stituents (–OMe, –H and –Br) about the phenothiazine donor unit reactions that may compromise the sustained performance of
2
4
were evaluated when immobilized on a semiconductor surface. All DSSCs.
three dyes delivered modest power conversion efficiencies (PCEs) in We set out to increase the stability of organic dyes by
the dye-sensitized solar cell (DSSC), but maintained 75% of their preparing a series of dyes bearing a phenothiazine (PTZ) donor
initial PCE over 300 h of sustained simulated sunlight. Electron- unit (Fig. 1). The sulfur tether increases the coplanarity of
donating substituents increased the stability of the phenothiazine the aromatic rings within the PTZ unit, resulting in a more
radical unit created after light-induced charge injection into the delocalized hole that is known to accommodate higher stability
2
4,26
semiconductor; however, this did not translate to higher DSSC in the oxidized form.
The higher radical cation stability of
stability, which appears to be more sensitive to the basicity of the this unit has been exploited for preventing overcharging in
2
7–30
anchoring group for this series.
lithium-ion batteries and near-infrared imaging,
and was
very recently utilized in the DSSC as both a donor and conjugated
3
1–35
Organic chromophores are capable of producing PCEs greater bridge.
We hypothesized that substitution at the C-3 and C-7
1–5
than 10% in the DSSC. High performance metal-free sensitizers positions of the PTZ unit with electron-donating groups (EDGs)
typically contain a ‘‘donor–p-bridge–acceptor’’ electronic structure would engender even higher redox stability (Fig. 1) by stabilizing
where an electron-rich donor group is connected to an electron- the radical cation formed upon one-electron oxidation. To test this
deficient acceptor group through a conjugated bridge. The assertion, methoxy groups were installed at these two positions
acceptor group is most often a carboxylic acid group that func- (Dye-OMe) and the properties were evaluated against the dibromo
tions to covalently link the dye to a semiconductor surface such as (Dye-Br) and unsubstituted (Dye-H) benchmark compounds
6,7
8–10
2
TiO .
This asymmetric electronic structure is imperative for (Fig. 1). It is shown herein that the stability of oxidized dyes in
improves with electron-
This scenario donating substituents; however, this trend did not correlate to
facilitating efficient light-induced charge transfer from the solution and when immobilized on TiO
2
6,7,11,12
organic donor unit to the semiconductor.
leaves the lowest unoccupied b-spin orbital (b-LUSO) for the device stability as no significant difference in stability for the series
photo-oxidized dye localized on the ‘‘donor’’ unit and poised for was observed during accelerated aging tests.
À
II 13–17
regeneration by the redox active mediator species (e.g., I , Co ).
3,4,18–20
While metal-free dyes that yield high PCEs are now known,
21,22
commercial deployment requires robust performance.
Organic
23–25
dyes are notoriously unstable and do not satisfy this criterion.
The discovery of commercially viable organic dyes therefore
hinges on our ability to identify and resolve relevant degrada-
tion processes. In the case of organic dyes, light-induced charge
transfer from the dye to the semiconductor yields a reactive
Departments of Chemistry and Chemical & Biological Engineering, The University of
British Columbia, 2036 Main Mall, Vancouver, BC, V6T 1Z1, Canada.
E-mail: cberling@chem.ubc.ca
†
Electronic supplementary information (ESI) available: Experimental procedures,
1
13
H and C NMR spectra, mass spectra, spectroscopic, spectroelectrochemical and
time-dependent DFT data. See DOI: 10.1039/c6cc09992k
Fig. 1 Molecular representations of Dye-Br, Dye-H and Dye-OMe, which
contain different substituents at the C3 and C7 positions.
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The three novel organic dyes under investigation were Table 1 Summary of spectroscopic and electrochemical properties for
Dye-Br, Dye-H and Dye-OMe in MeCN
accessed through Ullmann condensation of commercially avail-
able or synthetically-derived phenothiazine donor units to
a
b
c
d
+
e
l
max
l
Fluor
E
1/2
E
0–0
E(S /S*)
3
6,37
produce tertiary amines (Scheme S1, ESI†).
bridge was attached through a palladium-catalyzed Suzuki
coupling to the tertiary amines P1-H and P3-OMe intermediates. Dye-H
The thiophene Dye
(nm)
(nm)
(V vs. NHE)
(eV)
(V vs. NHE)
Dye-Br
370 (1.4)
384 (1.1)
411 (1.3)
531
565
572
1.16
1.02
0.78
2.98
2.63
2.56
À1.82
À1.61
À1.78
Dye-OMe
The cyanoacrylic acid anchoring group was added by a Knoevenagel
condensation following previously reported literature procedures
a
Corresponds to maximum of lowest-energy absorption band; e in
4
À1 À1
b
3
8
units of 10 M
of emission band after excitation at lmax
.1 M n-NBu PF
s
indicated in parentheses. Maximum wavelength
to yield final products Dye-Br, Dye-H and Dye-OMe. The
c + 0
ꢀ
1
13
.
PTZ /PTZ redox couple in
structure of each dye was determined by H and C NMR
À1
0
4
6
electrolyte solution at 50 mV s and referenced
d
spectroscopy (Fig. S9–S16, ESI†) as well as high resolution mass against Ag/AgCl in saturated KCl solution (+197 mV vs. NHE). E0–0
determined from intersection of normalized absorption and emission
spectrometry (HR-MS).
e
+
spectra. Calculated with the relationship: E(S /S*) = E1/2 À E0–0
.
The sulfur bridge was shown by density functional theory (DFT)
to maintain the phenothiazine donor unit perpendicular to the
bridge thereby disrupting the conjugation between the HOMO
Cyclic voltammograms recorded for each dye in 0.1 M
PF MeCN solutions (Fig. 3) revealed that oxidation of
and LUMO orbitals (Fig. 2a). The HOMO resides entirely on the n-NBu
4
6
phenothiazine donor unit while the LUMO is located primarily on the PTZ unit occurs at +0.78, +1.02 and +1.16 V vs. NHE for
the cyanoacrylic acid acceptor moiety and partially on the thienyl Dye-OMe, Dye-H and Dye-Br, respectively. This trend confirms
p-bridge. TD-DFT calculations predict a HOMO - LUMO transition that the electron density at the C-3 and C-7 positions has a
at B760 nm, but this was not observed experimentally. The marked shift on the HOMO energy. The chemical stability of
dominant lowest energy absorption band for the PTZ-R series the oxidized dyes was inferred by evaluating the ratio of anodic
at B400 nm (Fig. 2b and Table 1) is instead assigned as a HOMOÀ2 to cathodic currents. Dye-Br was determined to be the least
or HOMOÀ3 - LUMO transition involving charge transfer from chemically stable on the basis that the anodic current was
the p-bridge to the acceptor unit. The wavelength maxima for these significantly larger than that of the cathodic scan. The anodic:
bands are bathochromically shifted from 370 to 411 nm with cathodic current ratio converged on 1 for Dye-OMe implicating
ꢀ
+
increasingly electron-rich substituents. The absorption spectra of higher stability of the PTZ unit. Moreover, 50 successive scans
the dyes adsorbed to mesoporous TiO (Fig. 2b, inset) followed the of dyes in solution produced a nominal decrease in current for
2
same trend, but were collectively blue-shifted by 40 nm.
Dye-OMe, while similar experiments for Dye-Br showed a signifi-
cant diminution in current with subsequent scans corresponding
to a relatively lower oxidative stability in solution.
The stability of each oxidized dye on TiO was also analyzed
2
spectroelectrochemically. The working electrode (Dye-R on TiO2)
was oxidized for 1 min at a potential +0.1 V more positive than
ꢀ
+
the Dye-R /Dye-R redox potential to produce spectroscopic features
ꢀ
+
37
at 500–700 nm that are diagnostic of a PTZ unit (Fig. S2, ESI†).
Fig. 2 (a) Calculated frontier orbitals of optimized structures for the Dye-
X series plotted at an absolute isovalue of 0.001. (b) UV-vis absorption
spectra for the Dye-X series recorded in MeCN. Inset: Absorption spectra
Fig. 3 Cyclic voltammograms for Dye-Br, Dye-H and Dye-OMe recorded
(a) in solution and (b) when immobilized on TiO in 0.1 M n-NBu PF
6
2
4
MeCN solutions at room temperature. Dashed lines indicate E1/2. The first
50 recorded scans are suggestive of oxidative instability for Dye-Br.
2
for the three dyes immobilized on TiO .
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The applied bias was then removed and the intensities of the
ꢀ
+
absorption bands corresponding to PTZ were monitored as a
function of time to quantify the lifetimes of the oxidized forms. The
final spectra were not superimposable with the initial spectra
indicating that the source of signal diminution is due to dye
decomposition rather than electrochemical one-electron reduction
back to the neutral dye (Fig. S2, ESI†). Dye-OMe persisted the
longest in the oxidized form with a half-life (t1/2) of 13 min; the t1/2
values for Dye-H and Dye-Br were measured to be 6 and 3 min, Fig. 5 (a) Comparison of I–V curves for Dye-Br (blue), Dye-H (black) and
Dye-OMe (red) at 0 h (solid) and 300 h (dashed) of sustained 1 Sun
irradiation and constant temperature of 45 1C. (b) Open-circuit potential
respectively (Fig. 4). These results are aligned with our expectation
that the radical cationic form of the dyes are stabilized with
electron-rich substituents.
(V
oc) for dyes Dye-Br, Dye-H and Dye-OMe in a DSSC under constant
illumination at AM 1.5 (1 Sun) and 45 1C for 300 h.
We then set out to examine the performance of the dyes in
the DSSC with an iodide-based electrolyte. The relevant energies
of the dyes that govern the interfacial chemistry were deemed sustained 1 Sun irradiation at a constant temperature of 45 1C.
appropriate for sensitization: the excited-state reduction poten- Current–voltage (I–V) curves were recorded periodically to
+
tials, E(S /S*) (Table 1), were more negative than À0.50 V and assess device stability (Fig. 5b). All three sensitizers retained
therefore capable of injecting into the conduction band of TiO
2
;
475% of the original V s and PCEs over this period, with very
oc
and the HOMO levels were sufficiently positive to participate in little differences in degradation rates across the series. These
the one-electron redox chemistry relevant for regeneration by accelerated aging tests demonstrate that the PTZ unit is indeed
3
9
iodide (Table 1). Note that the HOMO level of Dye-OMe was a stable building block for organic sensitizers. The nominal
measured to be +0.78 V vs. NHE and therefore lies at the differences in device stabilities revealed that there is little
À
À
ꢀ
threshold for regeneration by iodide (I /I
2
B +0.8 V vs. correlation between device stability and stability of the oxidized
1
3
NHE), but effective regeneration at these potentials are dyes for this series. In fact, the highest rate of dye deterioration
4
0,41
known.
collected for devices containing each dye are listed in Table 2. PTZ lifetime.
The open-circuit potentials (V ), short-circuit currents ( J ) and
These results implicate other factors such as dye desorption
Data extracted from current–voltage plots (Fig. 5a) was measured for Dye-OMe, the dye that displayed the longest
ꢀ
+
oc
sc
PCEs were progressively higher for Dye-Br o Dye-H o Dye-OMe. playing a more dominant role in affecting DSSC stability. We
The J values track the relative HOMO energies and the corres- analyzed the relative rates of dye desorption for the series by
sc
ponding incident photon-to-current efficiencies (IPCEs; Fig. S3, immersing sensitized TiO substrates in electrolyte solutions
2
ESI†). The higher minority carrier concentrations are also likely and tracking the changes in dye coverage spectrophotometrically
responsible for the higher Voc values, as the negatively shifted (Fig. S7, ESI†). Dye-OMe desorbed fastest, with merely 32% of the
HOMO level of Dye-OMe did not yield a lower photovoltage.
dye molecules remaining on the semiconductor after 5 days in
The temporal stabilities of the DSSCs with each of the three the electrolyte. Substrates stained with Dye-H and Dye-Br
sensitizers were tested by subjecting each device to 300 h of retained 50% and 80%, respectively, of the dyes on the surface
a
over this same period. This trend tracks the pK s of the dyes that
we measured in DMSO: 2.63, 2.73 and 2.91 for Dye-Br, Dye-H and
Dye-OMe, respectively (Fig. S8, ESI†). These results point to the
PTZ substituents affecting the acidity of the carboxylate unit and
the binding to the TiO . The most acidic Dye-Br likely loses the
2
carboxylic acid proton on the anchoring group and the resulting
carboxylate anion can form a more stable covalent bond with the
TiO than the protonated Dye-OMe that forms an intermolecular
2
hydrogen bond with the semiconductor surface.
Table 2 Characterization parameters of solar cells sensitized by Dye-Br,
Dye-H and Dye-OMe under 1.5 AM (1 Sun)
À2
Dye
Jsc (mA cm
)
Voc (mV)
Fill factor (FF)
Z (%)
Dye-Br
5.2 Æ 0.1
7.3 Æ 0.1
8.7 Æ 0.1
720 Æ 11
740 Æ 11
745 Æ 3
0.78 Æ 0.01
0.75 Æ 0.02
0.72 Æ 0.06
2.7 Æ 0.1
3.8 Æ 0.1
4.3 Æ 0.3
Fig. 4 Temporal change in UV-vis absorption at 580, 520 and 640 nm for Dye-H
Dye-OMe
the one-electron oxidized forms of immobilized Dye-Br, Dye-H and
Dye-OMe, respectively. The dyes were each oxidized by applying a bias
The electrolyte solution contained 1.0 M 1,3-dimethylimidazolium
iodide (DMII), 60 mM I , 0.5 M tert-butylpyridine, 0.05 M NaI and
.1 M GuNCS in a mixed solvent system of MeCN and valeronitrile
corresponds to data recorded immediately after the system is returned (85 : 15, v/v). Averaged parameters with standard deviations from no
to Voc. Dashed lines indicate t1/2
fewer than two different devices are listed.
1
00 mV more positive than the measured E1/2 value for each dye for 1 min
2
in 0.1 mM n-NBu PF in MeCN. The first data points (i.e., t = 0 s)
4
6
0
.
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