Effects of triphenyl phosphate as an inexpensive additive on the photovoltaic performance of dye-sensitized nanocrystalline TiO2 solar cells

Article abstract of DOI:10.1039/c5ra06849e

The effect mechanism of triphenyl phosphate (TPP) is studied as an effective and inexpensive additive in the electrolyte of dye-sensitized solar cells (DSSCs) performance. The N719- and TPA-sensitized devices with a modified electrolyte show high efficiencies of 7.04% and 2.73% under 100 mW cm^-2 light illumination, respectively. The enhancement in V_oc arises from increasing electron density in the conduction band (CB) of TiO₂ that leads to a shift in the Fermi level (E_F) and thereby a suppression in electron recombination occurs. Electrochemical and spectroscopic data exhibit slower electron recombination and indicates that TPP forms a charge transfer complex with iodine in the electrolyte. This complexation decreases the concentration of free triiodide and limits the electron recombination so that it improves J_sc. Furthermore, the results reveal that TPP is a suitable inexpensive alternative for 4-tert-butylpyridine (TBP) additive in the DSSCs. We replaced TBP with TPP in DSSC based on TPA dye and observed an 73% increase in η compared to the blank electrolyte (without additive). Also, we investigate the effects of substituents in the phosphate additive structure on the photovoltaic properties by comparing our findings with previous work and found that the aliphatic phosphate additives exhibit a better performance compared to aromatic phosphate additives in the DSSCs.

Full text of DOI:10.1039/c5ra06849e

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ARTICLE TYPE
Effects of triphenyl phosphate as inexpensive additive on the
photovoltaic performance of dyeꢀsensitized nanocrystalline TiO solar
2
cells
Malihe Afrooz and Hossein Dehghani*
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
The effect mechanism of triphenyl phosphate (TPP) is studied as effective and inexpensive additive in the
electrolyte on the dyeꢀsensitized solar cells (DSSCs) performance. The N719ꢀ and TPAꢀsensitized devices
ꢀ
2
with modified electrolyte show high efficiencies of 7.04% and 2.73% under 100 mW cm light
illumination, respectively. The enhancement in Voc arises from increasing electron density in the
conduction band (CB) of TiO that leads to a shift in the Fermi level (E ) and thereby a suppression in
electron recombination occurs. Electrochemical and spectroscopic data exhibits slower electron
recombination and indicates that TPP forms a charge transfer complex with iodine in the electrolyte. This
complexation decreases concentration of free triiodide and limits the electron recombination so that it
1
1
2
0
5
0
2
F
improves J . Furthermore, the results reveal that TPP is a suitable inexpensive alternative for 4ꢀtertꢀ
sc
butylpyridine (TBP) additive in the DSSCs. We replaced TBP with TPP in DSSC based on TPA dye and
observed an 73% increase in η compared to blank electrolyte (without additive). Also, we investigate the
effects of substituents in phosphate additive structure on the photovoltaic properties by comparing our
findings with previous work and found that the aliphatic phosphate additives exhibit a better performance
compared to aromatic phosphate additives in the DSSCs.
1
Introduction
45 in organic dyes and attained an improvement in DSSC
17
performance. The triarylamine unit in the dye structure acts as a
1
8
During the foregone decades, dyeꢀsensitized solar cells
good electron donor and has a nonꢀplanar structure. Also, the
TPA unit prevents unfavorable dye aggregation at the TiO2
surface. So, a large number of triarylamine dyes have been served
as efficient light harvesters for DSSCs that have an electronꢀ
accepting group (carboxylic group) to bounding to the titania
(DSSCs) have received a worldwide attention in many
1ꢀ7
2
3
3
4
5
0
5
0
investigations. The common choices of photosensitizers for
DSSCs based on liquid electrolyte are ruthenium polypyridine
complexes such as cisꢀbis(isothiocyanato)bis (2,2′ꢀbipyridylꢀ4,4′ꢀ
dicarboxylato) ruthenium(II) bisꢀtetrabutylammonium (N719),
cisꢀbis(isothiocyanato)
5
5
6
6
0
5
0
5
8
ꢀ11
19ꢀ22
surface.
In comparison with the great studies that are focused
bis(2,2′ꢀbipyridylꢀ4,4′ꢀdicarboxylato)
and tri(thiocyanato)(4,4′,4″ꢀtricarboxyꢀ
on the design and application of different dyes with high
performance in DSSCs, extensive research have not been devoted
to modified redox mediators. Since Gratzel and O’Regan reported
a conversion efficiency of 11% for dyeꢀsensitized solar cell by
1
2
ruthenium(II) (N3)
1
3
2
,2′:6′,2″ꢀterpyridine) ruthenium(II) (black dye) because of their
broad absorption spectra and favorable photovoltaic performance.
In the DSSCs based on ruthenium sensitizer, dye is the one of the
expensive components because ruthenium is a rare metal. If
Ru(II) polypyridyl complexes are replaced with organic
sensitizers, then the cost of the device will be drastically low. So,
in the past, N719 dye was the traditional choice as the
photosensitizer for DSSC, but today, in numerous studies for
optimizing electrolyte systems and electrodes, organic dyes apply
as sensitizer because of their simple synthesis methods, low cost
2
3
using iodide/triiodide electrolyte in the 1991,
in many
investigations, this redox couple is used as mediator with
24ꢀ28
multitude of solvents and various additives.
It is important to
note that the unique properties of the iodine electron mediator
rely on sluggish electron recombination reaction and fast dye
regeneration.
In DSSCs, electron recombination is one of the great
challenges, because it diminishes the electron transfer which
leads to decrease in the photovoltaic performance. The main
electron recombination path is the back reaction of electron from
conduction band (CB) of TiO2 to the oxidized species in the
electrolyte solution. Adding additives into the standard electrolyte
1
4ꢀ16
and high molar extinction coefficients.
Among the metal free
organic sensitizers, triarylamine dyes have excellent electronic
properties for applying as photosensitizer in DSSCs. In 2004,
Yanagida et al. introduced the triarylamine unit as electron donor
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is a simple approach to suppress the electron recombination
process. Additives can play a momentous role to enhance short
circuit current (J ), open circuit voltage (V ) and overall
(transmission >90% in the visible, sheet resistance 8 Ω per
square, 2.3 mm thickness), N719 dye and surlyn spacer (30 µm)
were obtained from Dyesol (Australia).
sc
oc
photoelectric conversion efficiency (η) of DSSCs. Additives can
60
5
0
5
0
5
0
5
0
5
0
change the position of the Fermi level (E ) of semiconductor or
2.2 Instrumentation
UVꢀvis spectra of the solutions were measured on a GBC cintra 6
UVꢀvisible spectrophotometer. FTꢀIR spectra were recorded
using the KBr pellets on a Magna 550 Nicolet spectrometer. H
65 NMR spectrum of the dye was obtained using Bruker DPXꢀ400
F
redox couple potential, interfacial charge transfer and
recombination kinetic. Also, it is believed that these compounds
act as Lewis base and increase the Voc when they adsorb onto the
1
29,30
surface of the TiO by shifting the CB in TiO semiconductor.
2
2
1
1
2
2
3
3
4
4
5
Thus, the negative shift of the CB arises from accumulation of
MHz NMR spectrometer with DMSOꢀd as solvent.
6
electrons in the CB of TiO . Also, it is confirmed that some
Cyclic voltammetry measurements were carried out with a
Sama 500 potentiostat (Isfahan, Iran) using a three electrode
system consist of a glass carbon working electrode (2 mm
2
additives can form a charge transfer complex with iodine
2
4,29
molecule and shifts the redox couple potential.
Since 4ꢀtertꢀ
butylpyridine (TBP) was used as additive for iodide/triiodide 70 diameter), a Ag/AgCl/KCl(sat.) reference electrode and a Pt
electrolyte, during the last years, various efforts have been
intensively paid to apply some functional organic compounds as
counter electrode. A solution of 0.1 M of Lithium perchlorate
(SigmaꢀAldrich, USA) in dry acetonitrile was used as supporting
3
1ꢀ35
additives into the electrolyte systems.
phosphate (TBPP) has been introduced as efficient additive for
iodide/triiodide system to modify the dye/TiO /electrolyte 75 under AM 1.5 G simulated light (Luzchem). The EIS spectra
Recently, tributhyl
electrolyte.
The
EIS
were
measured
using
potantiostat/galvanostat (PGSTAT 100, Autolab, EcoꢀChemie)
2
36
interface. In this paper, we have focused on the study of the
effects of triphenyl phosphate (TPP) in the iodide/triiodide
electrolyte, as another phosphate additive, using two different
dyes (2ꢀcyanoꢀ3ꢀ(4ꢀ(diphenylamino)phenyl)acrylic acid (TPA)
were scanned in a frequency range from 0.01 Hz to 500 kHz with
an AC amplitude of 5 mV at room temperature. Photovoltaic
measurements using a Keithley model 2400 digital source meter
(Keithley, USA). The impedance spectra were fitted by Zꢀview
and N719) on the DSSC performance. Based on this modified 80 software (v2.9c, Scribner Associates Inc) using equivalent circuit
electrolyte, the efficiency of the N719ꢀsensitized and organic dye
TPAꢀsensitized DSSCs under standard illumination condition
diagram. The samples were analyzed by PhilipsXLꢀ30ESEM
scanning electron microscope (SEM) equipped with energy
dispersive spectrometer (EDS).
ꢀ
2
(
100 mW cm ) is high 7.04% and 2.73%, respectively. Also,
when TBP is replaced with TPP in the DSSCs based on TPA dye,
a significant increase in η (18%) is observed compared to the cell 85 2.3 Synthesis of 2ꢀcyanoꢀ3ꢀ(4ꢀ(diphenylamino)phenyl)acrylic
with electrolyte containing only TBP additive. Our results reveal
that TPP is a suitable inexpensive alternative for TBP in the
DSSCs.
We carefully investigated details and discussed on the influence
of TPP additive on the electron recombination process in the 90 drops of piperidine (1.12 mmol, 95 mg) in acetonitrile (15 mL)
electrode/dye/electrolyte interface. The origin of the increase in
Voc and Jsc is explained by electrochemical impedance
spectroscopy (EIS) and cyclic voltammetry (CV) measurements.
Based on UVꢀvis and FTꢀIR results, it guessed TPP can form a
charge transfer complex with iodine molecule in the electrolyte.
On the other hand, in this study, in order to introduce another
phosphate additive in the iodide/triiodide redox couple for
DSSCs, we investigate the effects of substituents in phosphate
additive structure on the photovoltaic properties. The findings for
acid (TPA dye)
(4ꢀDiphenylamino)benzaldehyde was purchased SigmaeꢀAldrich
Company. A solution of (4ꢀdiphenylamino)benzaldehyde (0.74
mmol, 205 mg), cyanoacetic acid (1.49 mmol, 127 mg) and a few
was heated at reflux temperature for 12 h at argon atmosphere.
After cooling to room temperature, the resulting precipitate was
extracted with dichloromethane and 0.1 M HCl solution. Organic
phase was collected and purified by silica gel column
95 chromatography with dichloromethane/methanol (6:1, v:v) as
eluent to give TPA dye (146 mg, yield 71%) as a yellow powder
(Fig. 1). UVꢀvis (THF), λmax (nm): 416, 836; IR (KBr), ν (cm ):
3443 (νO–H, s), 2214 (νC≡N, s), 1619 (νC=O, s), 1585 (νC=C, s), 1496
(νC=C, s), 1184 (νC–O, s), 756 (s), 698 (s), 621 (s); H NMR
ꢀ
1
1
TPP additive are compared to our data in previous work for 100 (DMSOꢀd
), δ (ppm): 7.87 (1H, s, –CH=), 7.59 (2H, d, Ar–H),
6
3
7
TBPP. It is found that the phosphate additives with substituent
of aliphatic group may exhibit a better effect on the cell
performance compared to aromatic phosphate additives. This
study is an effort to get a deeper understanding of the phosphate
additives performance and can be useful for the manufacturing of
low cost and high efficiency DSSCs.
7.39 (4H, t, Ar–H), 7.14 (6H, m, Ar–H), 6.88 (2H, d, Ar–H).
ꢀ
+
COO TBA
HOOC
HOOC
N
N
N
NCS
NCS
Ru
N
N
OH
O
2
Experimental
NC
ꢀ
+
2
.1 Materials
COO TBA
TPA dye
All the reagents and solvents, unless otherwise specified, were
obtained from Merck Company. Acetonitrile was dried over 4 Å
molecular sieves for 24 h prior to use. The FTO glass substrates
N719
55
Fig. 1. Molecular structures of TPA and N719 dyes.
2
| Journal Name, [year], [vol], 00–00
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DOI: 10.1039/C5RA06849E
2
.4 Preparation of photovoltaic devices
dye aggregates on the TiO
2
surface and the electron
To prepare TiO paste, 1 g of TiO (P25, Degussa, Germany) was
added into a mixture of acetic acid (0.2 mL), deionized water (2
mL), ethanol (10 mL), terpineol (2 g) and ethyl cellulose (0.5 mL,
recombination from CB to triiodide ions in the electrolyte
solution. Also, Fig. 2 show the frontier orbital plots of the
highest occupied molecular orbital (HOMO) and the lowest
2
2
39
5
0
5
0
5
0
5
10 wt% solution in ethanol) to form a slurry solution and then 60 unoccupied molecular orbital (LUMO) of the TPA dye. At the
followed by sonication for 15 min. To concentrate the paste, it
was vigorously stirred at room temperature for two days to form a
ground state of the dye (HOMO), the electron density
homogeneously distributes on electronꢀdonor moiety
viscous white TiO paste. After cleaning the FTO glass, it was
(triphenylamine unit), whereas at the LUMO, electron density
locates at the electronꢀacceptor moiety (cyanoacrylic acid).
2
treated with a 40 mM aqueous TiCl solution at 70 ˚C for 30 min.
4
1
1
2
2
3
3
Then a TiO paste was coated onto the FTO surface using the 65 EHOMO and ELUMO values of TPA dye are wellꢀmatched with the
2
doctor blade technique. In this study, two layers of TiO particles
were deposited onto FTO glass and used as photoelectrode. The
TiO2 films were sintered at 500 ˚C for 30. Afterwards, the
EF of the TiO2 semiconductor and electrolyte potential. All
spectral and electrochemical properties of TPA dye are
summarized in Table 1 from reference 21.
2
resulting TiO films were immersed into an aqueous solution of
2
21
Table 1 Electrochemical Data of TPA dye.
40 mM aqueous TiCl solution in an oven at 70 ˚C for 30 min,
a
a
b
4
Dye
λ
(
max
abs)
nm]
λ
(em)
max
E
[V vs.
ox
λmaxon
E
0ꢀ0
abs/em
ox
E ꢀ
E0–0
followed by rinsing with deionized water and then calcining at
TiO2
[nm]
[
[nm]
NHE]
[V vs. NHE]
5
00 ˚C for 30 min. The prepared thin film electrodes were
[V]
immersed in the respective dye solutions (0.3 mM of TPA dye in
THF for 2 h and 0.4 mM for N719 in a mixture of tertꢀbutanol
(tꢀBuOH) and acetonitrile (AN) (1 : 1 v/v) for 24 h) at room
TPA
381
515
1.33
405
2.63
ꢀ1.30
a
ꢀ5
Absorption and emission spectral data of the dye (2×10 M).
The oxidation potential of the dye.
b
temperature in the dark. After dye loading, the TiO films were
2
7
7
8
0
5
0
rinsed with THF and assembled with Pt counter electrodes that
were prepared by thermal decomposition of H PtCl solution (2
2
6
mg in 1 mL of ethanol) and heating at 400 ˚C for 15 min. To
fabricate the DSSCs, the dyeꢀcovered TiO2 electrode and Pt
counter electrode were assembled into a cell of sandwich type
and sealed with a 30 µm Surlyn spacer. The electrolytes
employed were a solution of 0.1 M LiI, 0.05 M I , 0.5 M TBP
2
(
SigmaꢀAldrich, USA), 0.6 M 1ꢀbutylꢀ2,3ꢀdimethylimidazolium
iodide (BDMII) and different concentrations (0, 0.05, 0.1, 0.2,
.5, 1 and 2 M) of TPP additive in dry acetonitrile. The TiO /TPP
0
2
film for EDS was prepared in the following way. The TiO film
2
was immersed in a 1 M TPP acetonitrile solution for 2 h and then
rinsed with acetonitrile two times and dried at room temperature.
Fig. 2. Optimized ground state geometry and frontier molecular
orbital of the HOMO and LUMO of the TPA dye calculated with DFT
on a B3LYP/6ꢀ 31G* level in vacuum.
3
Results and discussion
85
3
.1 TPA dye as an organic sensitizer
3
.2 Photovoltaic performance of DSSCs
For designing an efficient organic sensitizer in DSSCs, dye
aggregation on the semiconductor surface is essential basic
characteristic that should be attention. We can prevent
All devices were fabricated using the TPA and N719 sensitizers,
a thin transparent doubleꢀlayer TiO (P25) (about 14 ꢁm) as the
9
0
2
4
4
5
5
0
5
0
5
photoanode, the iodide/triiodide redox shuttle consist of TPP
additive and Ptꢀcoated FTO glass as cathode. The effect of TPP
concentration on the photovoltaic performance of the devices
made from the TPA dye was studied (at seven concentrations of
TPP within a broad range (0ꢀ2 M)). Fig. 3 shows the currentꢀ
voltage (IꢀV) curves for all concentrations of TPP in the
electrolyte solution. The detailed output photovoltaic parameters
of open circuit voltage, short circuit current density, fill factor
unfavorable dye aggregation on the TiO semiconductor surface
2
through optimization of dye structure by anchoring special groups
in their structure or addition of coꢀadsorber to the dye solution.
Cyanoacrylic acid group acts as a suitable acceptor anchoring
group to constitute the DꢀπꢀA structure in the photosensitizer that
95
3
8
has a strong bind onto the TiO surface. Also, the electronic and
2
steric properties of triphenylamine unit in the dye structure
18,39
prevent dye aggregation on the semiconductor surface.
(FF) and photovoltaic conversion efficiency (η) are listed in
In our study, we sensitized a triphenylamine dye as a low cost
dye with a short synthesis route and simple structure for using in
DSSCs. To gain further insight into the electronic properties of
the TPA dye, DFT (density functional theory) calculations were
performed at the B3LYP/6ꢀ31G* level. The optimized geometry
of TPA (Fig. 2) indicates that the three phenyl rings in the TPA
cores are nonꢀplanar, which is beneficial for restraint from the
1
00 Table 2. From the results for DSSCs based on TPA dye, it is clear
that the increase of TPP concentration in the electrolyte solution
ꢀ2
improves Voc and Jsc under simulated sunlight at 100 mW cm .
The J , V and FF of the DSSC based on electrolyte with 0.05 M
sc
oc
ꢀ2
of TPP are 5.57 mA cm , 0.68 V and 0.670, respectively,
05 affording a η of 2.54%.
1
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7
6
5
DSSC based on N719 with modified electrolyte shows a high
improvement in power conversion efficiency that not only dues to
the increase in Jsc but also in V . For N719, 1 M TPP increases
oc
4
5
5
5
0
5
the photovoltaic performance significantly: J augments from
sc
TPA dye
ꢀ2
Standard
5
0
5
4
3
2
1
0
13.85 to 18.70 mA cm , V from 0.69 to 0.71 V and the power
oc
0.05 M
conversion efficiency enhances from 6.03% to 7.04%.
0.1 M
7
.0
0.75
0.70
0.65
0.60
0
0
1
2
.2 M
.5 M
M
(
b)
(a)
6
.5
6.0
1
M
5
5
4
.5
.0
.5
0
0.2
0.4
0.6
0.8
Voltage (V)
Fig. 3. Photocurrent voltage characteristic plots of the DSSCs based on
TPA dye at various TPP concentrations in iodide/triiodide electrolyte
systems under 100 mW cm illumination (AM 1.5 G).
0.0
1.0
2.0
0
.0
0.5
1.0
1.5
2.0
Concentration of TPP (M)
ꢀ2
Concentration of TPP (M)
1
2
2
2
2
.8
.6
.4
.2
2
(
c)
Table 2. Photovoltaic performance of DSSCs based on TPA and
N719 dyes at various TPP concentrations under AM 1.5
−
2
2
(
100 mW cm ) illumination with an active area of 0.18 cm .
Dye
Concentration
of TPP (M)
J
sc
V
[V]
oc
FF
η
[%]
60
ꢀ2
[mA cm ]
TPA
TPA
TPA
TPA
TPA
TPA
TPA
0
0.05
0.1
0.2
0.5
1
5.28
0.65
0.643
0.670
0.675
0.666
0.621
0.592
0.621
2.21
2.54
2.60
2.61
2.55
2.73
2.36
0
.0
1.0
2.0
5.57
5.68
5.68
5.86
6.41
5.28
0.68
0.68
0.69
0.70
0.72
0.72
Concentration of TPP (M)
65
Fig. 4. Dependence of DSSC parameters on the TPP concentration
present in the electrolyte system: (a) open circuit voltage (Voc); (b) short
circuit current (Jsc); (c) energy conversion efficiency (η).
20
15
10
5
0
70
2
N719
N719
0
1
13.85
18.70
0.68
0.71
0.640
0.530
6.03
7.04
Among the above tested concentrations for TPA dye, the DSSC
performance reaches the optimal value (2.73%) when the TPP in
1 M concentration is used as additive. In this concentration of
N719
75
Standard
Modified electrolyte
20
25
30
35
40
TPP, an 21% increase in J along with an 11% increase in Voc are
sc
observed. These increases are responsible for obtained
improvement which is about 24% increase in efficiency (2.73%)
compared to standard electrolyte (2.21%). Although the further
addition of TPP in liquid electrolyte from 1 to 2 M causes a
decrease in the energy conversion efficiency of DSSC from
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Voltage (V)
80
Fig. 5. Photocurrentꢀvoltage characteristic plots of the DSSCs based on
N719 dye at 0 and 1 M of TPP in the iodide/triiodide electrolyte under
00 mW cm illumination (AM 1.5 G).
ꢀ
2
1
2
.73% to 2.36%. The cell based on electrolyte with 2 M of TPP
In the recent years, many researchers focus on the replacement
additive shows a Voc of 0.72 V, a FF of 0.621 and a lower
efficiency (2.36%), which this decrease in η is attributed to lower
Jsc of 5.28 mA cm . High concentration of TPP (2 M) leads to a
decrease in the concentration of the other components. This
8
5
pyridine derivatives with inexpensive efficient additives. Thus, in
this manuscript, we study the effect mechanisms of TPP and TBP
additives on the cell performance. The electrolyte are composed
of TBP (0, 0.5 M) and TPP (0, 1 M). The IꢀV curves for the
DSSCs with electrolyte A (without additive), electrolyte B or
standard electrolyte (with 0.5 M TBP), electrolyte C (with 1 M
TPP) and electrolyte D (with 0.5 M TBP and 1 M TPP) measured
ꢀ
2
decrease diminishes the electron transfer and reduces J , as well.
sc
Dependence of DSSC photovoltaic parameters on the TPP
concentration present in the electrolyte are shown in Fig. 4.
Interestingly, an increase in both Jsc and Voc is observed for
electrolytes containing TPP less than 2 M (Figs. 4a and 4b)
compared to standard electrolyte.
For comparison, the DSSCs based on N719 dye with 0 and 1 M
of TPP in the electrolyte were fabricated and their IꢀV curves are
shown in Fig. 5. We applied the best concentration of TPP
additive (1 M) for DSSC based on N719. As listed in Table 2,
90
−2
at a light intensity of 100 mW cm are presented in Fig. 6 and
their photovoltaic data are summarized in Table 3. The addition
of TBP into the electrolyte leads to an 36% increase in Voc and an
95
14% improve in J , that is responsible for the obtained efficiency
sc
enhancement (from 1.51% (electrolyte A) to 2.21% (with
electrolyte B)).
4
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the higher frequency (left) region reflects the resistance at the
interface between the counter electrode and electrolyte (RCE) and
the larger semicircle in the lower frequency (right) region is
attributed to the charge transfer resistance at the dyedꢀ
8
7
6
5
4
3
2
1
0
5
55 TiO /electrolyte interface resistance (RCT). RCT is related to
2
charge recombination process from the CB of TiO to the
2
40
oxidized ions in the electrolyte solution. The RCT value is
estimated by the real component radius of the fitted larger
semicircle. These estimated and calculated EIS parameters have
60 been listed in Table 4. In the equivalent circuit for EIS, CPE1 is
TPA dye
Electr
olyte
A
1
0
attributed to the dyedꢀTiO /electrolyte interface.
2
0
0.2
0.4
0.6
0.8
1
1
0
9
8
7
6
5
4
3
2
1
0
Voltage (V)
TPA dye
0.05M
0.1 M
0.2 M
Fig. 6. Photocurrentꢀvoltage characteristic of DSSCs constructed with
TPA dye and tested under simulated AM1.5 solar irradiation (100 mW
cm ) for electrolytes containing different amounts of TBP and TPP
additives.
1
2
5
0
6
5
ꢀ
2
0.5 M
1 M
2
M
Standard
Table 3. Photovoltaic performance of DSSCs based on TPA dye at
electrolytes containing different amounts of TBP and DEOX additives
under AM 1.5 (100 mW cm ) illumination with an active area of 0.18
ꢀ2
2
70
cm .
J
sc
V
oc
FF
η
Electrolyte
A
ꢀ2
[
mA cm ]
[V]
0.53
[%]
1.51
4.62
0.616
0.643
0
5
10
15
20
25
B
5.28
0.65
2.21
Z' (Ohm cm2)
75
C
D
7.14
6.41
0.71
0.78
0.515
0.592
2.61
2.73
Fig.7. Nyquist plots from the EIS spectra for the DSSCs based on TPA
dye at various TPP concentrations in the iodide/triiodide electrolyte. The
inset shows equivalent circuit diagram for fitting the EIS data.
3
Addition of TBP leads to an 46% increase in η compare to
blank cell. When TBP is replaced with TPP, a significant
improvement in both Jsc and Voc is observed compared to the cell
with electrolyte A. The J , V and FF of the DSSC based on
electrolyte C are 7.14 mA cm , 0.71 V and 0.515, respectively,
affording a η of 2.61%. We observe an 54% increase in Jsc along
N719
8
0
2
1
0
.5
2
Standard
Modified electrolyte
2
3
3
4
4
5
5
0
5
0
5
0
sc
oc
ꢀ
2
.5
1
85
with an 34% enhancement in V , thereby, a 73% increase in η
oc
compared to the cell with electrolyte A (without any additives).
These results shows that TPP has dramatically effect on Jsc and
.5
0
V . As a comparison, from Table 3, it can be noted that the
oc
electrolyte containing only TBP presents a η value (2.21%) lower
than that of device based on the electrolyte containing only TPP
additive (2.61%) and we observe a significant increase in η
0
2
4
6
8
10
12
90
Z'(Ohm cm2)
Fig. 8. Nyquist plots from the EIS spectra for the DSSCs based on N719
dye at 0 and 1 M of TPP in the iodide/triiodide electrolyte. The inset
shows equivalent circuit diagram for fitting the EIS data.
(18%). Our results show that TPP is effective and inexpensive
alternative additive for TBP in the DSSCs.
95
Also, we can determine some important parameters for the
devices by using EIS measurements.
electron lifetime in the CB of TiO (τ) that is expressing the
electron recombination between the TiO and electrolyte can be
00 extracted from the angular frequency (ωmax) at the second
semicircles in the Nyquist plots of the DSSCs (Equation (1)):
3
.3 Electrochemical impedance spectroscopy
40ꢀ42
For example, the
Electrochemical impedance spectroscopy was studied to further
understand the effect of TPP additive content in the electrolyte
solution on the electron recombination and interfacial charge
transport processes in the DSSCs. The EIS spectra were measured
at a forward bias of ~ 0.70 V near the open circuit voltage. Fig. 7
shows Nyquist plots and equivalent circuit diagram (inset) of the
DSSCs based on TPA dye with standard electrolyte and modified
electrolytes in different concentrations of TPP. For N719 dye, the
EIS spectra of the device with the best concentration of additive
and standard electrolyte were studied (Fig. 8). Each of the
Nyquist plots contains two semicircles. The smaller semicircle at
2
2
1
40
τ = 1/ωmax (1)
The keff is the reaction rate constant for the electron
recombination process with triiodide ion in the electrolyte that
05 can be expressed by Equation (2):
1
keff = 1/τ
(2)
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Table 4. Electrochemical parameters obtained by fitting the EIS of DSSCs with different concentrations of
60
TPP additive using an equivalent circuit for TPA and N719 dyes.
Concentration
of TPP (M)
RCE
(Ω cm )
RCT
(Ω cm )
k
(s )
D
τ
(s)
ns
(cm )
eff
ꢀ1
eff
2
Dye
2
2
ꢀ1
ꢀ3
(cm s )
5
0
5
0
5
0
5
TPA
TPA
TPA
TPA
TPA
TPA
TPA
N719
N719
0
0.05
0.1
0.2
0.5
1
0.54
0.64
0.68
1.48
1.32
1.44
3.68
0.20
0.30
9.85
12.00
11.72
13.38
14.20
16.62
17.08
5.40
10
3.56Eꢀ4
1.89Eꢀ4
1.69Eꢀ4
9.12Eꢀ5
1.08Eꢀ4
8.39Eꢀ5
3.36Eꢀ5
5.30Eꢀ4
2.37Eꢀ4
0.100
0.195
0.195
0.195
0.195
0.270
0.270
0.100
0.195
5.18E+18
9.78E+18
8.66E+18
8.33E+18
7.87E+18
9.32E+18
3.63E+18
1.11E+19
1.91+E19
6
5
5.13
5.13
5.13
5.13
3.70
3.70
10
1
1
2
2
3
3
70
2
0
1
7.09
5.13
Also Deff and n are effective diffusion coefficient of electrons
s
75
To investigate the chemical interactions between TiO2 and
TPP, we study EDS of TiO /TPP (Fig. 10). This spectrum reveals
the presence C and P elements and confirms the TPP layer on the
surface of the TiO2 film. EDS spectrum is a evidence that
confirms TPP can have an interaction with the TiO surface.
and electron density in the CB of TiO , respectively, that obtain
2
2
4
0,43
from EIS parameters.
As shown in Table 4, for TPA and
N719 dyes, the values of RCT for DSSCs in presence of TPP
additive in the electrolyte are more than standard cell. In the
2
DSSC based on TPA dye, the R value increases from ~ 9.8 Ω
CT
80
2
2
cm to ~ 16.6 Ω cm when standard electrolyte is replaced by
modified electrolyte with TPP in concentration of 1 M. It means
that the electron recombination process is further suppressed by
adding more TPP. Also, for N719 dye, the fitted value of RCT for
2
device with modified electrolyte is found ~ 7.1 Ω cm , while the
corresponding value for cell with standard electrolyte is
85
2
~
5.4 Ω cm .
When TPP is added to the electrolyte, the τ and n values are
s
larger compared to the standard electrolyte (Figs. 9a and 9b).
These results indicate that the electron recombination is
effectively suppressed by the adsorption of TPP on the TiO2
surface. As can be seen, the presence of TPP additive in the
electrolyte solution in the DSSCs induces slow recombination.
For DSSCs, it shows similar recombination lifetimes, at light bias
90
2
Fig. 10. The EDS spectrum of TiO /TPP film.
equal to V , the recombination is much slower in presence of 95 So, the higher CPE1 at interfacial of TiO2 and electrolyte is
oc
TPP additive.
originated from the interaction between the phosphate group and
0
.30
.20
.10
.00
titanium (IV) of TiO that leads to an arrangement of positive
charge (on phosphorous atom in TPP) and negative charge (on
2
(
a)
0
0
0
titanium center in TiO
2
) (Fig. 11).
40
45
50
55
100
eꢀ
O
O
O
O
O
O
O
P
O
P
TiO2
TiO
2
TiO
2
0.0
0.5
1.0
1.5
2.0
Concentration of TPP (M)
7
6
6
5
5
4
.0
.5
.0
.5
.0
.5
(c)
Fig. 11. Interaction between the oxygen in TPP and the orbitals of
titanium (IV) in the TiO and formation a chemical capacitance at the
2
dyedꢀTiO /electrolyte interface.
2
1
05
The results in Table 4 clearly show that one of the major
reasons for the increase in η for TPA and N719 based DSSCs is
the slower recombination dynamics that dues to the presence of
9
11
13
15
17
37
TPP additive. In our previous work, a similar result for the
RCT (Ω cm2)
Fig. 9. Dependence of EIS parameters on the TPP concentration
present in the electrolyte system with TPA dye: (a) τ (s); (b) n (cm );
s
1
10 devices based on iodide/triiodide electrolyte with tributyl
phosphate additive was obtained.
ꢀ3
ꢀ
2
2
and (c) dependence of Jsc (mA cm ) on the RCT (Ω cm ).
6
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3
.5 Cyclic voltammetry and UVꢀvisible spectra analysis
55
Among the mixture solutions containing TPP additive in Fig.
3, in the mixture of iodine and TPP in acetonitrile, the
1
The influence of TPP additive on the kinetics between the
triiodide and iodide ions in the electrolyte was studied by CV
measurement. Fig. 12 represents the CV curves of the standard
and modified electrolytes that is referred to the equilibrium
potential of the iodide/triiodide couple. The CV curves were
scanned from 0 to 1.6 V. The redox peaks at more positive
potentials is attributed to the reaction of Equation (3) which is
assigned to oxidation of iodide, and the redox peaks at less
positive potentials is related to Equation (4) which is correlated to
disappearance of the peak at 455 nm dues to the formation of
44,45
[
TPP.I ] complex.
We suggest that the following reactions
2
5
occur in the modified electrolyte according to Equations (5) and
(6):
60
I3ꢀ
Iꢀ + I2
(5)
(6)
TPP +I2
TPP.I2
1
0
0
0
.49
.39
Iodide
Iodine
Triiodide
6
7
7
8
5
0
5
0
4
3
reduction of triiodide:
Iꢀ
I3ꢀ + 2e
TPP
3
(3)
(4)
Iodide+TPP
Iodine+TPP
Triiodide+TPP
I3ꢀ
0.29
2
3I + 2e
2
It is seen when the concentration of TPP in the electrolyte
increases, the anodic peak currents decrease while the anodic and
cathodic peak potentials shift positively. It shows that the
interaction existing between the TPP and redox couple can effect
on the redox reaction so that, in the modified electrolyte, the
oxidation of iodide ions and reduction of triiodide ions become
difficult. These interactions may thus be a reason for decrease in
concentration of free redox ions in the electrolyte solution.
We guess phenyl ring in the TPP additive has an interaction
with redox couple to formation a charge transfer complex,
because, in the previous study, we used tributyl phosphate as
additive and observed an enhancement in efficiency, but the CV
results indicated that TBPP molecule has any interaction with
0
0
.19
.09
1
2
2
3
5
0
5
0
ꢀ0.01
275
375
475
575
675
Wavelength (nm)
Fig. 13. UVꢀvis spectra of solutions with different species of the
electrolyte and TPP additive in acetonitrile at room temperature.
The formation of charge transfer complex decreases the free
concentration of triiodide and thereby a decrease in concentration
of iodine occurs. Therefore, the presence of TPP not only shifts
3
7
redox couple.
85
the E position but also interacts with the redox couple that leads
F
to a decrease in electron recombination and an increase in RCT
,
1
1
1
4
2
0
8
6
4
2
0
2
4
Standard electrolyte
Modified electrolyte
which can have positive effect on the Jsc value. It is seen there is
an interaction between the πꢀsystem (phenyl ring) or oxygen atom
*
in the P=O group of TPP as electron donor and σ of iodine as
9
0
electron acceptor that leads to formation a charge transfer
complex (Fig. 14).
I
I
3
5
ꢀ
ꢀ
95
O
O
O
O
0
.0
0.5
1.0
1.5
O
O
O
P
O
P
Voltage (V)
I
I
Fig. 12. Cyclic voltammograms for the standard electrolyte and
4
0
modified electrolyte with TPP in concentration of 1 M.
(
a)
Fig. 14. Two possible structures for charge transfer complex
TPP.I ].
(b)
1
00
In order to further indication for interaction between the TPP
and redox ions, we study the UVꢀvis spectra of solutions with
different species of the electrolyte. Fig. 13 shows the absorption
spectra of various compounds in acetonitrile at room temperature.
The solution of iodide ion (shown as yellow line) shows no
absorption band in the UVꢀvisible region (275ꢀ700 nm) whereas
for the iodine solution, three absorption bands are observed at
[
2
4
5
The formation of this charge transfer complex can be proved
1
05 by comparing the UVꢀvis spectra of solution containing iodine
molecule and TPP additive. It shows that the phenyl ring in
additive structure has a main effect on the formation of charge
transfer complex. So, we found that the structure of phosphate
additive is effective on the properties of the electrolyte and the
10 DSSC performance.
The UVꢀvis results agree with the CV data. In the following
section, we study the FTꢀIR spectra to determine the structure of
charge transfer complex.
2
97, 367 and 460 nm. In the UVꢀvis spectra of iodide/iodine
5
0
mixture solution, absorption bands are observed at 288 and 361
nm (two peaks). The triiodide ion is characterized by these
absorption bands in UVꢀvis region that are related to electron
transitions in this ion. The disappearance of the peak at 463 nm
shows that the triiodide ion and iodide produce.
1
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3
.4 FTꢀIR results for [TPP.I ] complex
80 3.5 Effect of the TPP additive on the DSSC photovoltaic
2
To further investigate the interaction between TPP additive and
iodine molecule, we study the FTꢀIR spectra of the TPP (Fig.
parameters
The most common additives used for iodide/triiodide electrolyte
are pyridine and its derivatives.
1
5a) and the mixture consist of TPP and iodine (Fig. 15b). The
4
5,53,54
Most studies on DSSCs
5
0
5
FTꢀIR spectrum of TPP shows the typical two bands of aromatic
show adding these additives to the electrolytes enhances open
circuit voltage of the DSSCs, while the short circuit current is
much less affected. This fact dues to a position change in the EF
of semiconductor or redox couple potential. It is well known that
Voc is essentially determined by the difference between redox
ꢀ
1
C=C stretching vibration (1300 to 1660 cm ) and CꢀH stretching
8
9
9
5
0
5
ꢀ
1
ꢀ1
modes (3000 to 3100 cm ). A band at 1293 cm can be
contributed to the stretching band of P=O and a strong band at
1
ꢀ
1
46,47
178 cm reflects PꢀOꢀC stretch.
For the mixture of TPP and
1
iodine, the stretching bands of P=O and PꢀOꢀC shift to 1300 and
couple potential in the electrolyte and the EF level of TiO2
ꢀ
1
1
188 cm , respectively (Fig. 15b). Based on the results of FTꢀIR,
semiconductor. Therefore, the possible explanation for the
enhancement of Voc is related to the shifting the E and redox
we can consider two possible interactions between the TPP and
iodine molecule; oxygen of P=O or phenyl groups of TPP can act
as electron donors to the iodine molecules as electron acceptor
(Fig. 14). Since a shift towards higher wavenumber is found, a
phenylꢀhalogen interaction between the TPP and the halogen
molecule is guessed (Fig. 14b).
F
potential. In general, pyridine derivatives form stable molecular
complexes with the iodide/triiodide ions in the electrolyte that
dues to the electron donating properties of the nitrogen lone pair
in these compounds.
compounds such
1
55,56
Recently, many organic nitrogenous
as pyrimidine, aminopyridine,
20
25
30
35
40
45
50
55
pyrrolidinopyridine and phenothiazine dendrimers have been used
as additives to enhance the Voc and further results in an increased
τ which eventually increases the efficiency of the DSSC.
00 When we use the TPP additive, an improvement in both Jsc and
Voc is observed for modified electrolytes (Tables 2 and 3).
Therefore, we can consider an interaction of TPP with titanium
5
7ꢀ59
1
1
1
(IV) in the TiO and iodine molecules in the electrolyte solution
2
(Scheme 1). The binding TPP to the TiO surface and iodine can
2
05 explain the photovoltaic and electrochemical results. The possible
resonance structures for TPP molecule show the π bond of the
phosphoryl group can break and its electron pair moves into the
oxygen where it acts as electron donor to titanium (IV) in the
TiO which is a Lewis acid. So, the positive charge places on
2
10 phosphorus atom in the phosphate group. The interaction between
the lone electron pair of oxygen atom in phosphate group and
titanium (IV) forms P–O–Ti bond. This interaction leads to an
arrangement of positive charge (on phosphorous atom in TPP)
and negative charge (on titanium center in TiO ) that increases
2
Fig. 15. FTꢀIR spectra of TPP (a) and [TPP.I
2
] (b).
115 the CPE1 at interfacial of TiO and electrolyte (Fig. 11). The
2
combined effects of electron donation of phosphoryl group and
slower charge recombination lead to accumulation of electrons in
60
65
70
75
As shown in Fig. 16, there are two resonance structures for TPP
molecule. With interaction of the πꢀsystem as electron donor and
σ orbital of iodine as electron acceptor, structure (I) in Fig. 14 is
the CB of TiO . Electron recombination process has main role in
*
2
reducing the n in the CB of TiO . According to EIS analysis,
s
2
further forming. Thus, the P=O and PꢀOꢀC bonds become
stronger and the ν (P=O) and ν (PꢀCꢀO) increase. Also we
1
1
1
20 adding TPP to the electrolyte can enhance the τ and n in the CB
s
of TiO for DSSCs, which dues to a reduction in the rate of
ꢀ
1
2
observe a shoulder at the 1178 cm , next to the main peak at the
electron recombination from the EF of TiO2 to triodide ions.
Increasing the n for DSSCs can shift the E in TiO and as a
ꢀ
1
1
188 cm , that it proves presence of the free phenyl rings in TPP
s
F
2
molecule that have not any interactions with iodine molecule.
Noncovalent interactions of aryl or phenyl donors with
halogens (X2 = Br , I ) and structures of these molecular
result improves V . The higher τ and n reflect lower the electron
oc
s
25 recombination. Thus, the RCT increases in presence of TPP that
shows a decrease in electron recombination in DSSCs that
2
2
complexes have been investigated in many experimental and
improves Jsc.
4
8ꢀ52
theoretical studies.
On the other hand, CV and UVꢀvis data show the TPP forms a
Based on the FTꢀIR result, it is deduced that a πꢀcomplex in the
modified electrolyte with TPP additive forms. These results are
according to the CV and UVꢀvis findings.
charge transfer complex with iodine in the electrolyte solution.
30 This complexation leads to a decrease in concentration and
activity of redox ions. So, it reduces the electron recombination
rate in the CB of TiO2 with triiodide ions in the electrolyte
O
O
O
O
O
P O
O
P O
(Scheme 1). We can consider two structures for charge transfer
complex (Fig. 14). For the [TPPꢀI ] complex, according to the
2
(I)
(II)
Fig. 16. Resonance structures of triphenyl phosphate molecule.
8
| Journal Name, [year], [vol], 00–00
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FTꢀIR data, we guess formation of a πꢀcomplex in the modified
electrolyte (Fig. 14b).
modified electrolyte is attributed to the slow electron
60 recombination. It is confirmed that the TPP additive is effective
Our findings show TPP can improve DSSC performance
through several ways: (1) the TPP can adsorb onto the TiO2
surface and acts as Lewis base that it increases the Voc by shifting
the E of TiO semiconductor, (2) by adsorbing of TPP onto the
on the dyedꢀTiO /electrolyte interface and the electrolyte
2
solution. This impact in dyedꢀTiO /electrolyte is on the surface
2
5
charge and in the electrolyte solution is arose in charge transfer
complex formation with iodine molecule. TPP decreases the
F
2
TiO2 surface, the electron recombination from the TiO2 to 65 recombination of electrons in the CB of TiO with triiodide in the
2
triiodide in the electrolyte decreases by preventing triiodide
electrolyte by complexation with iodine in the electrolyte and by
access to the semiconductor surface that can improve V and
preventing triiodide access to the TiO surface.
oc
2
35
1
0
J , (3) TPP forms a complex with redox couple in the
As we demonstrate herein, the results indicate that phosphate
compounds are a promising generation of additives, which
70 deserve further studies. This kind of simple electrolyte
modifications allows us to tune the redox potential of the
electrolyte and are very instructive in the future studies to design
a variety of inexpensive additives with electron donor groups to
obtain more efficient DSSCs.
sc
electrolyte solution and a decrease in electron recombination is
observed (Scheme 1) that increases Jsc.
1
5
Dye
eꢀ
eꢀ
Dye
eꢀ
E'
F
F
TPP
+
TPP
75
E
eꢀ
eꢀ
I
ꢀ
Acknowledgements
The authors gratefully acknowledge from the University of
Kashan for supporting this project by Grant No. 159183/18.
E
F
TiO
2
Iꢀ
TiO
2
ꢀ
3
I
2
0
I
2
TPP
(
a)
(
b)
80 Notes and references
Scheme 1. The interactions of triiodide and iodine near the
Department of Inorganic Chemistry, Faculty of Chemistry,
University of Kashan, P.O. Box 87317ꢀ51167, Kashan, I. R. Iran;
Eꢀmail: dehghani@kashanu.ac.ir; Fax: +98ꢀ31ꢀ55912397;
Tel: +98ꢀ31ꢀ55912386.
2
5
TiO /dye/electrolyte interface in DSSCs: (a) without TPP additive and (b)
2
with TPP additive.
Also, if we compare our results for TPP additive to the
3
7
85
influence of TBPP in previously work, we will find that there is
a correlation between the phosphate additive structure and the
DSSC performance. When the phenyl group is applied in
phosphate structure, a charge transfer complex with iodine
molecule can form that decreases the activity of redox ions. Thus,
phosphate additives with substituent of aliphatic group show a
better effect on the cell performance compared to aromatic
phosphate additives.
1
A. Mishra, M. K. R. Fischer and P. Bauerle, Angew. Chem. Int.
Ed., 2009, 48, 2474ꢀ2499.
A. Abbotto, N. Manfredi, C. Marinzi, F. D. Angelis, E.
3
3
4
4
5
5
0
5
0
5
0
5
2
Mosconi, J. H. Yum, Z. Xianxi, M. K. Nazeeruddin and M.
Gratzel, Energy Environ. Sci., 2009, 2, 1094ꢀ1101.
3
J. Phys. Chem. C, 2012, 116, 11241ꢀ11250.
4
4
5 J. Lim, H. A. Kim, B. H. Kim, C. H. Hand and Y. Jun, RSC
Adv., 2014, 4, 243ꢀ247.
9
0
X. Zong, M. Liang, C. Fan, K. Tang, G. Li, Z. Sun and S. Xue,
Y. Ooyama and Y. Harima, ChemPhysChem, 2012, 13, 4032ꢀ
080.
4
Conclusions
95
To summarize, we investigate the effects of the triphenyl
phosphate additive on the performance of DSSCs. Spectroscopic
and electrochemical measurements indicate that applying of TPP
additive to the redox electrolyte of DSSCs leads to an increase in
6
X. Qian, L. Lu, Y. Z. Zhu, H. H. Gao and J. Y. Zheng, Dyes
Pigments, 2015, 113, 737ꢀ742.
D. Kuanga, P. Comte, S. M. Zakeeruddin, D. P. Hagberg, K. M.
7
the Jsc and V . After optimization of the TPP concentration in
oc
1
1
1
00 Karlsson, L. Sunb, M.K. Nazeeruddina and M. Gratzel, Solar
Energy, 2011, 85, 1189ꢀ1194.
8
Jirousek, P. Liska, N. Vlachopoulos, V. Shklover, C. H. Fischer
and M. Gratzel, Inorg. Chem., 1999, 38, 6298ꢀ6305.
05 9 A. R. Andersen, J. Halme, T. Lund, M. I. Asghar, P. T. Nguyen,
K. Miettunen, E. Kemppainen and O. Albrektsen, J. Phys. Chem.
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the electrolyte, the conversion efficiency of the DSSCs based on
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