A diminutive modification in arylamine electron donors: Synthesis, photophysics and solvatochromic analysis-towards the understanding of dye sensitized solar cell performances

Article abstract of DOI:10.1039/c5cp05338b

An electron rich donor moiety plays an important role in dye sensitized solar cells (DSCs). In order to attain a suitable donor moiety for DSCs, a deeper understanding of the role of a donor moiety in the dye excited state is significant. In this context,

Full text of DOI:10.1039/c5cp05338b

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ARTICLE TYPE
A Diminutive Modification in Arylamine Electron Donors: Synthesis,
Photophysics and Solvatochromic Analysis − Towards the
Understanding of Dye Sensitized Solar Cell Performances
Venkatesan Srinivasan,1 Murugesan Panneer,2 Madhavan Jaccob,2 Nagaraj Pavithra,3 Sambandam
5
Anandan,3 Arunkumar Kathiravan1*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
Electron rich donor moiety plays an important role in dye sensitized solar cells (DSCs). In order to attain a suitable donor moiety for
DSCs, deeper understanding of the role of donor moiety in the dye excited state is significant. In this context, arylamine dyes based
1
10 different electron donor moieties (TRA, CRA and PyRA) were successfully synthesized and well characterized by using H, 13CꢀNMR
and EIꢀMS spectrometry. Their photophysical properties and solvatochromic behavior were studied by using UVꢀVisible absorption,
steady state and time resolved fluorescence spectroscopic techniques. The absorption of arylamine dyes is due to intramolecular charge
transfer (ICT) between the donor and rhodanineꢀ3ꢀacetic acid via π−bridge, which is further confirmed by DFT calculations. Lippertꢀ
Mataga analysis on the solvatochromic data implies that these molecules are more polar in the excited state, which is additional support
15 for ICT. Further, nanocrystalline TiO2ꢀbased dyeꢀsensitized solar cells (DSCs) were fabricated using these dyes to investigate the
influence of donor moieties on their photovoltaic performance. The overall power conversion efficiencies of 2.57%, 1.68% and 1.25%
were obtained for TRA, PyRA and CRA dyes, respectively. The enhanced power conversion efficiency of TRA is due to longer lifetime
of injected electrons as demonstrated by the electrochemical impedance spectroscopy (EIS) measurements.
merocyanine, coumarin, indoline, squaraine, hemicyanine,
phenothiazine, triphenylamine, fluorene, carbazole and
Introduction
tetrahydroquinoline have been reported and thus making organic
dyes profitable in their application to DSCs. In 2010, Wang et al.
50 have demonstrated the substituted arylamine dye with η up to
10.3% for a metalꢀfree organic dye with an iodine/iodide redox
shuttle.27 Later in 2011, Grätzel et al. reported the
phenyldihexyloxyꢀsubstituted TPA (DHOꢀTPA) dye, which
exhibited a power conversion efficiency (η) of 10.3% in
55 combination with a cobalt redox shuttle.28 In 2013, Wang et al.
tailored a metalꢀfree organic dye with the chromophoric core of
cyclopentadithiopheneꢀbenzothiadiazole (CPDTꢀBT), which
displays power conversion efficiencies at various irradiances of
the simulated air mass (AM) 1.5 sunlight of 11.5–12.8%, setting a
60 new benchmark for organic dyeꢀsensitized solar cells.29 This is
the highest conversion efficiency of metal free organic dyes
reported so far. Therefore, arylamine dyes have been found
desirable candidates for organic sensitizers.
20 Dyeꢀsensitized solar cells (DSCs) is a promising lowꢀcost
alternative to traditional siliconꢀbased photovoltaic cells.1 As a
key component of DSCs, sensitizer play a crucial role in light
harvesting and lightꢀtoꢀelectricity conversion; extensive effort
has, therefore, several ruthenium, porphyrin, and organic dyes
25 have been synthesized and investigated in terms of their
performance in DSCs.2ꢀ6 Ruthenium dyes offer an average
efficiency of 11%, but their application might be limited due to
the scarcity of ruthenium metal and environmental concerns.7ꢀ9
Porphyrins are among the most widely studied sensitizers due to
30 their intense absorption in Soret and Q bands to harvest solar
energy efficiently in
a
broad spectral region.10ꢀ18 More
importantly, their optical, photophysical, and electrochemical
properties can be modulated with peripheral modifications or
inner metal coordinations.19ꢀ22 Several porphyrin monomers have
35 been used in DSCs and record high efficiencies of 12% and 13%
were achieved by GY50 and SM315 porphyrin dyes, respectively,
65 We are particularly interested in the design and synthesis of
arylamine dyes to study the role of donor moiety on the excited
state properties. In fact, minor changes in the structure of
arylamine donors can lead to notable enhancement in the
performance of the dye, and its thermal and photochemical
70 stability. For instance, Chen and coꢀworkers reported a family of
triphenylamine dyes with an increased electron density of donor
moiety as an effective way to improve the dye performance.30
Dye (TPAR4) with a CH2=CH− substituted triphenylamine
using
a
cobalt electrolyte under standard oneꢀsun
illumination.23,24
40 Besides, more efforts have been devoted to the development of
metalꢀfree organic sensitizers in recent years for the wide
availability of the raw materials.25 Particularly, organic dyes with
D–π–A configuration are popular for their nonꢀtoxicity, high
molar extinction coefficients, flexible molecular engineering, and
45 material abundance. Various organic molecules,26 such as
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electron donating group gave 5.8% efficiency under AM 1.5 G
irradiation due to the increase of the electron density in the donor
subunit. The corresponding dye (TPAR1)30 without the
CH2=CH− substitution exhibited a lower efficiency of 4.3%.
infrared spectroscopy (ATRꢀFTIR) spectrometer. Nuclear
magnetic resonance (NMR) spectroscopy was performed on a
BRUKER spectrometer operating at 400 MHz for 1H and 100
MHz for 13C and collected at ambient probe temperature with
5
Hagberg et al. developed a very simple modification of arylamine 50 spectra calibrated to either internal tetramethylsilane (TMS)
dyes namely D7 and D11 sensitizers.31 The overall conversion
efficiency of D7 and D11 are 5.43% and 7.03% respectively. The
higher efficiency of D11 sensitizer compared to D7 sensitizer
demonstrates the beneficial influence of alkoxy units on
standard or to a residual protio solvent. Electron ionizationꢀMass
spectra (EIꢀMS) analyses were acquired using JEOL GC mateꢀII
mass spectrometer photomultiplier tube (PMT) detector. Cyclic
voltammetry (CV) and differential pulse voltammetry (DPV)
10 photovoltage and photocurrent, caused by the enhanced red 55 were recorded using a CHIꢀ620B Electrochemical Analyzer, CH
response. The increased photovoltage of D11ꢀsensitized solar cell
compared to that of the D7ꢀsensitized solar cell is mainly due to
the increased lifetime of the conductionꢀband electrons.
Instruments Inc. A threeꢀelectrode singleꢀcompartment cell with a
glassy carbon working electrode, a platinum wire counter
electrode, and Ag/AgCl reference electrode were employed for
the experiments. The glassy carbon electrode was polished with
15 Here, our aim is to develop the arylamine derivatives with very 60 Al2O3 paste, cleaned thoroughly in an ultrasonic water bath, and
simple modification and to investigate the role of donor moieties
on excited state properties and to correlate the solar cell
performance. In the arylamine dyes, taking TRA (diphenylamine
donor) as a prototype, CRA and PyRA dyes were developed. In
washed with ethanol before each measurement. In the present
study, the oxidation potential of arylamine dyes was measured in
acetonitrile (ACN) with tetrabutylammonium hexafluoroꢀ
phosphate (0.1 M) as electrolyte. About 10 ml of 1×10−3
M
20 TRA, the free phenyl groups are closed in order to obtain rigid 65 arylamine dyes in acetonitrile is charged with tetrabutyl
carbazole donor (CRA), whereas, to form pyrrolidin donor
(PyRA) two phenyl groups were detached from the carbazole
structure (Figure 1). To the best of our knowledge, there are no
systematic studies carried out on these molecules. Steady state
ammonium hexafluorophosphate was taken in an electrochemical
cell and degassed by bubbling nitrogen for 15 min before the
experiments. The electronic absorption spectra of samples were
recorded using CARY 100 Bio UVꢀvisible spectrophotometer.
25 and time resolved spectroscopic analysis were performed to 70 The fluorescence spectral measurements were carried out using
understand the influence of donor moieties.
Fluoromaxꢀ4 spectrofluorometer (Horiba Jobin Yvon). For
fluorescence studies, much diluted solutions were used to avoid
spectral distortions due to the innerꢀfilter effect and emission
reabsorption. Time resolved picosecond fluorescence decays were
75 obtained by the timeꢀcorrelated singleꢀphoton counting (TCSPC)
technique with microchannel plate photomultiplier tube
(Hamamatsu, R3809U) as detector and femtosecond laser as an
excitation source. The second harmonics (400 nm) output from
the modeꢀlocked femtosecond laser (Tsunami, Spectra physics)
80 was used as the excitation source. The instrument response
function for TCSPC system is ∼50 ps. The data analysis was
carried out by the software provided by IBH (DASꢀ6), which is
based on deconvolution technique using nonlinear leastꢀsquares
methods. Femtosecond fluorescence decays have been collected
85 using fluorescence upꢀconversion technique. In our femtosecond
upꢀconversion setup (FOG 100, CDP, Russia) the sample was
excited using the second harmonic (400 nm) of a modeꢀlocked
Tiꢀsapphire laser (Tsunami, Spectra physics). The fundamental
beam (800 nm) was frequency doubled in nonlinear crystal (1
90 mm BBO, θ = 25°, ϕ = 90°) and used for the excitation. The
sample was placed inside a 1 mmꢀthick rotating quartz cell. The
fluorescence emitted from the sample was upꢀconverted in a
nonlinear crystal (0.5 mm BBO, θ = 38°, ϕ = 90°) using the
fundamental beam as a gate pulse. The upconverted light is
95 dispersed in a monochromator and detected using photon
counting electronics. The instrument response function of the
apparatus is 300 fs. The femtosecond fluorescence decays were
fitted using a Gaussian shape for the excitation pulse.
Figure 1: Molecular Structure and photograph of TRA, CRA, PyRA dyes
30 Experimental section
Materials
Rhodanineꢀ3ꢀacetic acid, 4ꢀ(diphenylamino)benzaldehyde, Nꢀ(4ꢀ
35 formylphenyl)carbazole, 4ꢀ(1ꢀpyrrolidino)benzaldehyde, and
dimethyl sulfoxideꢀd6 (DMSOꢀd6) were purchased from Sigmaꢀ
Aldrich. Glacial acetic acid and ammonium acetate were
purchased from Sisco Research Laboratories (SRL), India and
were used without further purification unless otherwise noted.
40 The solvents used for photophysical studies were HPLC grade.
100 DFT calculations
Instrumentation
The geometry optimization and harmonic vibrational frequency
calculations of the TRA, CRA, PyRA dyes were carried with the
Infrared spectrometer (IR) spectra were recorded on BRUKER
45 VERTEX 70 Attenuated total reflectionꢀFourier transform
2
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Gaussian09 program package.32 The ground state geometries of
cm−2 light illuminations by using an AUTOLAB12/FRA2
three dyes were optimized by using the DFT method and the 60 electrochemical analyzer.
hybrid B3LYP33ꢀ35 functional and the 6ꢀ311+G** basis set on all
atoms. The vibrational analysis was performed at each stationary
point found, that confirm its identity as an energy minimum on
the potential energy surface and there is no imaginary frequency.
The choice of the B3LYP functional was based on previous
theoretical studies that show that B3LYP is very successful in the
prediction of photophysical properties of this class of
Result and Discussion
5
Molecular design and synthesis
65
The electron donor moiety in organic sensitizers plays a crucial
role in determining the overall power conversion efficiency.40ꢀ42
Therefore, we have targeted three different donor moieties
containing arylamine dyes to investigate the role of electron
10 compounds.36
Quantum yield measurements
70 donor moiety on the HOMOꢀLUMO energy gap, electrochemical,
photophysical and photovoltaic properties. Note that, in the
targeted dyes, π−bridge (benzene) and acceptor/anchoring group
(rhodanineꢀ3ꢀacetic acid) were remained same. The targeted dyes
(TRA, CRA and PyRA) were obtained via Knoevenagel
Fluorescence quantum yield (ϕF) of arylamine derivatives were
15 calculated by using following equation (1)
ϕF = (AR/AS) (IS/IR) (ηS/ηR)2 ϕR
→ (1) 75 condensation
of
the
corresponding
aldehydes
with
rhodanineꢀ3ꢀacetic acid in presence of acetic acid and ammonium
where, the subscripts S and R refers to the samples and the
20 reference, respectively. A is the absorbance at the excitation
wavelength, I is the integrated emission area, and η is the solvent
acetate. The aldehydes used were 4ꢀ(diphenylamino)
benzaldehyde,
Nꢀ(4ꢀformylphenyl)carbazole
and
4ꢀ(1ꢀpyrrolidino)benzaldehyde. The final products were obtained
refraction index. Coumarinꢀ153 (ϕ=0.58)37 used as a standard for 80 in good yields and successfully characterized by FTꢀIR, NMR
CRA and fluorescein (ϕ=0.9)38 was used as a standard for TRA
and PyRA.
and mass spectroscopic techniques. The detailed synthetic route
(Scheme S1) and characterization data were illustrated in the
supporting information (Figure S1ꢀS9). In the arylamine dyes,
taking TRA (diphenylamine donor) as a prototype, CRA and
85 PyRA dyes were developed. In TRA, the free phenyl groups are
closed in order to obtain rigid carbazole donor (CRA), whereas,
to form pyrrolidin donor (PyRA) two phenyl groups were
detached from the carbazole structure. Structures and photograph
of the synthesized arylamine dyes are depicted in figure 1, which
25
Cell assembly and characterization
FTO glass plates were purchased from BHEL, INDIA. The
photoanode39 was prepared by the following procedure: 0.5 g of
30 TiO2 (P25) nanoparticles and 0.016 g of polyethylene glycol were
added to a solution of 1 M HNO3. Then, 0.21 ml of acetyl acetone
and 0.04 ml of TritonꢀX was added to the mixture. Later the 90 clearly reveals that upon changing the donor moieties, colour of
mixture was ultrasonicated for 1 h then stirred for 24 h to get
homogenous TiO2 paste. Before the preparation of electrodes,
35 FTO plates were washed by sonicating subsequently in a soap
solution, distilled water, acetone, and 2ꢀpropanol for 15 min. For
photoanode preparation, the washed FTO plates were pretreated
with 40 mM solution of TiCl4 for 30 min at 70 ⁰C. After TiCl4
treatment, the plates were washed with water and ethanol and
40 then dried at 100 ⁰C on a hot plate for 10 min. Then, the TiO2
paste was coated on FTO plate by doctor blade technique. Then,
the dyes are remarkably intensified, which is due to the reduced
HOMOꢀLUMO band gaps.
Theoretical consideration
95
Density functional theory calculations were employed to
investigate the impact of donor moiety on the structure and
HOMO/LUMO levels of arylamine dyes. Optimized geometries
of TRA, CRA and PyRA along with notable structural
the plates were sintered at 450 °C for 30 min to remove the 100 parameters are shown in figure 2a, which clearly reveals that
binder, solvent and getting an electricallyꢀconnected network of
TiO2 particles by sintering under tubular furnace. After the
45 sintering process, when the temperature of the plate drops to 80
°C, it was immersed into the dye solution in methanol for 24 h.
intramolecular hydrogen bonding interactions facilitates the most
stable conformation of arylamine dyes under investigation. The
Npyr−Cphenyl bond length in PyRA (1.367 Å) is shorter than the
TRA (1.402 Å) and CRA (1.411 Å). But in the case of TRA and
Excess nonꢀadsorbed dye were washed with anhydrous ethanol. 105 CRA, Npyr−Cphenyl bond distance is closer to single bond length of
The platinum catalyst counter electrode was prepared by
deposition of H2PtCl6.6H2O solution (0.005 M in isopropanol)
50 onto FTO glass and then sintering at 450 °C for 30 min. The DSC
device was fabricated by the following method: the Pt cathode
1.47 Å. This leads to decrease the effective delocalization in CRA
and TRA. This suggest that PyRA dye molecule attributed with
partial double bond character, which leads to effective
delocalization through out the molecule than the other two dye
was placed on top of the photoanode and was tightly clipped 110 molecules. In addition to this, PyRA is found to have planar
together. Then, liquid electrolyte 0.05 M I2/0.5 M LiI/0.5 M 4ꢀ
tertbutyl pyridine (TBP) in 3ꢀmethoxypropionitrile was injected
55 in between the two electrodes. The thickness of the electrolyte
layer is maintained about 50µm (approximately thickness of a
structure whereas in TRA and CRA dyes, the donor moiety is
lying perpendicular to the acceptor and π bridge moieties, as a
result effective charge transfer is expected to be more in PyRA
than TRA and CRA. Upon investigating the ground state
tape). Photocurrent–voltage characteristics and electrochemical 115 dipolemoment of three dye molecules, PyRA has relatively larger
impedance spectra (EIS) of the DSCs were done under 85 mW
dipole moment of 11.08 D compared to the others, TRA (8.55 D)
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and CRA (4.53 D). This is the additional support of PyRA is said
to have effective intramolecular charge separation between the
donor and acceptor units.
(rhodanineꢀ3ꢀacetic acid), this is an essential prerequisite for
charge transfer properties to the conduction band of TiO2.
25 Furthermore, the LUMO is mainly concentrated on the carbonyl
and thiocarbonyl group of rhodanine framework and not
extending further to the carboxyl group. Hence, charge transfer
cannot be transfer through carboxylate group, consequently
through space charge transfer likely for the selected dyes.
30
Figure 3. Molecular orbital composition (%) in the ground state for dye
molecules
It is noteworthy to analyze and understand the origin of
35 individual contributions of various segments of the dye molecules
towards HOMO and LUMO levels. So, Chemissian program was
used to compute the individual contributions of various fragments
of the dyes. The entire molecule was fragmented into two groups,
namely donor and acceptor groups figure 3. Major contributions
40 of HOMO in all the dyes are spread on the donor moiety and it is
greater in CRA (96%), TRA (81%) and lower in PyRA (71%)
whereas the delocalization of LUMO electron density on acceptor
unit of PyRA (69%) is higher than the other two dye molecules
(44% and 31% for TRA and CRA respectively). It clearly
45 indicates that the pyrrolidin unit in PyRA facilitates the effective
charge transfer from donor to acceptor. Overall, theory results
enlightens that, PyRA is found to have excellent charge transfer
properties and hance this molecule may show better solar cell
power conversion efficiency than other dyes.
5
Figure 2a: Optimized geometries of the dyes.
Figure 2b: HOMO and LUMO levels of the dyes.
It is very important to analyze the nature of HOMO and LUMO
levels of organic dyes prior to study their photovoltaic properties.
10 The ground state density plots of the HOMO and LUMO of three
dye molecules calculated at the B3LYP/6ꢀ311+G(d,p) level of
theory in THF medium are shown in figure 2b. The HOMO is
mainly delocalized over the donor unit and the LUMO is
originated on the acceptor and spacer units. Hence, the HOMOꢀ
15 LUMO excitation by light irradiation induced the shift of electron
from donor moiety to the acceptor moiety via π−bridge. This
indicates that the HOMO−LUMO transitions bear a significant
intramolecular charge transfer (ICT) character. Notably, the
contribution of HOMO levels are higher in donor part of the TRA
20 and CRA. But, in the case of PyRA, the contribution of HOMO
level are localized in the entire molecule. Moreover, in all the
dyes, the LUMO levels are largely localized in the acceptor part
50
Steadyꢀstate absorption spectroscopy
To obtain a highly efficient DSC, the light harvesting efficiency
of dye in photoanode is the most important and indispensable
55 factor, which is mainly related to the molar extinction coefficient
of the dye.43 Therefore, we intend to measure the molar extinction
coefficient of arylamine dyes by using UVꢀvisible absorption
spectrophotometer. In addition, by measuring absorption spectra
of arylamine dyes (TRA, CRA and PyRA), one can understand
60 the influence of electron donors on the light harvesting properties.
Hence, UVꢀvis absorption spectra of the arylamine dyes were
recorded in methanol, shown in figure 4a.
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30 shown in figure S10ꢀS12. As seen from the figure, the absorption
maximum of the dye is not much shifted with increasing solvent
polarity, which implies that the effect of solvent in the ground
state is not significant.
(a)
5
4
3
2
1
0
TRA
CRA
PyRA
35 In order to understand the light harvesting properties of arylamine
dyes on TiO2 surface, 4 ꢀmꢀTiO2 nanocrystalline films were
prepared from a commercially available TiO2 paste (Solaronix
SA, TiꢀNanoxide HT/SC series) and they were sensitized in a 0.5
mM of arylamine dyes in methanol at room temperature for 1h.
40 The normalized UVꢀvis absorption spectra of arylamine dyes
deposited on transparent TiO2 films are shown in figure 4b. The
maximal absorption peaks for TRA, CRA and PyRA on TiO2
films appeared at 465, 425 and 455 nm, respectively. Moreover,
their absorption spectra on TiO2 film are broadened due to the
45 interaction of the anchoring group of the dyes with TiO2 which
was commonly observed for dye/TiO2 systems.46,47 This spectral
broadening allows the dye molecules to harvest visible light more
efficiently and thus increases the photocurrent response region
with a rise of shortꢀcircuit current density (JSC).48,49 Except PyRA,
50 compared to the spectrum of TRA and CRA in methanol solution,
a red shift of the absorption peak was observed on TiO2 surface,
which can be attributed to the formation of Jꢀaggregate.
Interestingly, PyRA/TiO2 exhibits a blue (at peak maximum) and
red (at peak tail) shift while compared with its spectrum in
55 methanol solution, thus indicating the mixture of H and J−type
aggregation is possible in PyRA/TiO2.50
350
400
450
500
550
600
Wavelength (nm)
(b)
1.2
1.0
0.8
0.6
0.4
0.2
0.0
TRA/TiO2
CRA/TiO2
PyRA/TiO2
350
400
450
500
550
600
Wavelength (nm)
100
80
Figure 4: Absorption spectra of TRA, CRA and PyRA(a) in MeOH (b)
on TiO2 film
The absorption of arylamine dyes are due to intramolecular
charge transfer (ICT) between the donor to the rhodanineꢀ3ꢀacetic
acid via π−bridge. As the concentration of arylamine dyes were
increased, the absorbance of the band regularly increased and at
the same time, Beer−Lambert’s law was obeyed for all the dyes
in the concentration range of 0.5 × 10−5 M to 2.5 × 10−5 M. The
5
60
40
TRA
CRA
PyRA
20
10 maximum absorption peaks and the corresponding molar
extinction coefficient values of the dyes were 456 nm (4.8 × 104
M−1cm−1) for TRA, 403 nm (3.3 × 104 M−1cm−1) for CRA, and
466 nm (4.6 × 104 M−1cm−1) for PyRA, respectively. In
comparison with the conventional ruthenium dye N719 (1.47 ×
15 104 M−1cm−1 at 535 nm),44 the arylamine dye molecules showed
obviously higher absorption coefficients, which could be
beneficial for better light harvesting. By comparing all the dyes,
PyRA absorbs at longer wavelength with high molar absorption
coefficient, due to strong electron donating ability of pyrrolidin
20 unit to the acceptor via benzene π−bridge. On the other hand,
TRA molecule show a hypsochromic shift (10 nm) while
compared with PyRA, which is due to electrons are delocalized in
the phenyl rings and hence electronꢀpush effect was slightly
diminished. However for CRA, a large hypsochromic shift was
25 observed by compared with PyRA, due to the planar nature of
carbazole donor favours the delocalization of the lone pair
electron of the nitrogen atom, which leads to the decrease in the
electronꢀpush effect.45 Further investigation, absorption spectra
of the arylamine dyes were measured in various solvents and
0
350
400
450
500
550
600
Wavelength (nm)
Figure 5: LHE spectra of the TRA, CRA and PyRA adsorbed TiO2 films.
60
65
70
The light harvesting efficiency (LHE)51 can be related to the
molar extinction coefficient of the dye which is given in the
following equation (2),
LHE = 1–10− Γσ(λ)
→ (2)
where, Г (mol cm−2) is the number of moles of dye per square
centimeter of projected surface area of the film and surface
coverage is calculated by using following equation (3),52ꢀ54
Γ = A(λ)/σ(λ)
→ (3)
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where, A(λ) is the absorbance and σ(λ) is the optical cross
section at a given wavelength, respectively. Using equation (4),
one can able to calculate the σ(λ).
The obtained fluorescence decays are fitted with biexponential
function [F(t) = A1 exp (−t/τ1) + A2 exp (−t/τ2)] and the fitted
values are collectively given in table 1. The ultrafast component
45 of the dyes is at ~2 ps, whereas the slow component is slightly
longer at ~20 ps. The ultrafast decay component, τ1 is attributed
to the solvent relaxation of the excited states and τ2 is due to the
lifetime of the relaxed charge transfer state.56 The obtained
average lifetime of the arylamine dyes are in the order of
5
σ (λ) = ε(λ) × 1000 (cm3 L−1)
→ (4)
The surface coverage of arylamine dyes were determined to be
1.46 × 10−8 mol cm−2 for TRA, 1.43 × 10−8 mol cm−2 for CRA,
and 3.1 × 10−8 mol cm−2 for PyRA. Using these surface coverage 50 PyRA>TRA>CRA. However, the obtained lifetimes are in the
10 values, the LHE spectra of the dye/TiO2 films are plotted and
shown in figure 5. From this figure, it is clear that PyRA have
higher LHE than others, hence it is expect that PyRA should
show higher power conversion efficiency.
picosecond time scales, hence the electron injection from these
dyes to TiO2 is expected to be very fast.56ꢀ58
Table 1: Fitting parameters of arylamine derivatives in THF
55
a
15 Fluorescence spectroscopy
Dyes
TRA
A1 (%) A2 (%)
τ
λemi
590
540
550
τ1 (ps) τ2 (ps)
av (ps)
9.8
5.8
1.7
3.2
29.7
8.3
83
74
35
17
26
65
To get further information about the role of donor moieties on the
excited state property of the arylamine dyes, fluorescence spectra
were recorded by exciting at their absorption maximum. For
20 fluorescence studies, samples are measured at very dilute
solutions in order to avoid the reꢀabsorption, excimer and
concentration effect. The fluorescence of arylamine dyes are very
weak and they exhibited a very low quantum yields such as
0.0011 (TRA), 0.0006 (CRA) and 0.0019 (PyRA) respectively. A
25 weak fluorescence of arylamine dyes is due to intramolecular
charge transfer (ICT). It is previously reported that the ICT is a
competitive relaxation process of the singlet excited state and
typically reduces the fluorescence.55 Due to weak fluorescence in
nature, it is very difficult to examine the role of donor moiety
30 from the steady state fluorescence studies, hence the time
resolved fluorescence measurements has been adopted in order
the understand the role of donor moieties. Therefore,
femtosecondꢀfluorescence set up was used to extract the lifetime
of the dyes, since the lifetime of these dyes are very fast almost as
35 fast as our TCSPC instrument response function (<50 ps). Figure
6 display the femtosecond (fs) fluorescence decay of the dyes in
methanol were measured in a 100 ps time window upon
excitation at 400 nm.
CRA
3.41
PyRA
19.1
13.53
a Probing wavelength (nm)
Solvatochromism
60 It is well known that, excited state properties of the ICT based
dyes were found to be very sensitive to the solvent polarity.59
Hence, solvatochromism of arylamine dyes were performed. The
solvatochromic method is based on the shift of the UV–Vis
absorption and fluorescence maxima with varying polarity of the
65 solvent.60 The absorption and fluorescence spectra of dyes with
varying solvents are shown in figure S10ꢀS15 and the
corresponding maxima are listed in table S1. All the dyes show a
structured emission in nonpolar solvents, whereas moderate and
high polarity solvents show a broad and structureless emission.
70 Stokes’ shift is calculated as the difference between absorption
and emission maxima obtained from the corrected spectra on the
wavenumber scale. The obtained large Stokes’ shift is a result of
efficient charge transfer from the arylamine donor moiety to the
acceptor rhodanineꢀ3ꢀacetic acid moiety (cf. above). Moreover,
75 all the dyes were found to be completely insoluble in water and
hence the photophysical parameters could not be obtained in
water (ε=80).
25
20
TRA
Fit
CRA
Fit
PyRA
Solvent dependent spectral shifts are often interpreted in terms of
80 the Lippert equation (5), which describes Stokes’ shift in terms of
the change in dipole moment of the fluorophore upon excitation
and the dependence of energy of the dipole on the dielectric
constant and refractive index of the solvent61 and is given as:
15
10
Fit
5
0
2
n2
2n2
ꢀG
ꢀE
ε
1
1
2
hc
νF
νA
=
+ constant
a3
1
+
ε
2
+1
85
→ (5)
0
20
40
60
80
100
Time(ps)
Where, νA and νF are the wavenumbers (cm−1) of the
40 Figure 6: Time Resolved fluorescence decay of TRA, CRA and PyRA in
absorption and emission, h is Planck’s constant, c is the speed of
90 light, a is the radius of the cavity, ε is the dielectric constant of
MeOH
6
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DOI: 10.1039/C5CP05338B
the medium, n is the refractive index of the solvent and the term 25 Figure 7 shows the linear plot of solvent polarity function versus
ꢁf is the orientation polarizability (eq. 6).
Stokes’ shift. The change in dipole moment (ꢁꢂ=ꢂe−ꢂg) for all
the dyes have been calculated from the slope of the linear plot
and are listed in table 2. The estimated ꢁꢂ values are more
positive (Table 2) and suggestive of a large dipole moment for
30 the molecules in the excited state. Among the dyes, TRA and
CRA exhibit a large change in dipole moment confirming the
more pronounced ICT for these molecules in the excited state.
The emission spectra of dyes are broadened on moving from
nonpolar to the polar solvents. This probably supports the
35 speculation that the initially formed local excited state transforms
into the more polar charge transfer state, which is stabilized by
the polar solvents. Thus significant change in dipole moment on
excitation has been observed.
n2
2n2
ε
1
1
ꢁ
f =
1
+
2
ε
+1
→ (6)
Figure 7 represents the Lippert−Mataga plot of arylamine dyes in
various solvents. Alcoholic solvents were omitted in order get a
linear correlation. Since, these solvents are involved in specific
soluteꢀsolvent interactions.62
5
8000
TRA
CRA
PyRA
6000
4000
2000
40 Table 2: Ground and excited state dipole moment of the dyes
µE (D)c
25.98
21.93
18.07
Radius (Å) µG (D)a
ꢁµ (D)b
14.9
Dyes
TRA
CRA
PyRA
6.17
6.02
5.61
11.08
4.53
8.55
17.4
9.52
0.0
0.1
0.2
0.3
a Estimated from the B3LYP/6ꢀ311+G** calculations
b calculated from the slope of the linear LꢀM plot
45 c µE = ꢁµ + µG
ꢁf
Figure 7: Lippert−Mataga plots for the dyes.
10 A specific solvent interaction is further verified by varying
percentage of alcohol into a nonꢀpolar solvent. For instance, the
emission spectrum of TRA in pure toluene is structured, whereas
in methanol it is weak, broad and structureless (Figure 8). In the
binary mixture as the methanol amount increases, there is a loss
15 in vibrational fine structure of the TRA spectrum and also a
decrease in the overall spectral intensity is observed. Moreover,
increase in methanol there is a gradual redꢀshift in the emission
maximum. The most likely explanation for the above observed
spectral changes could be the preferential solvation of the polar
20 excited ICT state by a more polar solvent. In toluene, due to the
absence of a solvent dipole, the emission is from the locally
excited state showing a structured spectrum.
Electrochemical properties
Electrochemical properties of arylamine dyes were investigated
50 by cyclic voltammetry (CV) and differential pulse voltommetry
(DPV) in deoxygenated ACN solution containing 0.1 M tetraꢀnꢀ
butylammonium hexafluorophosphate (TBAPF6) as supporting
electrolyte (Figure S16). The measurements were carried out
using a typical threeꢀelectrode electrochemical cell, which
55 consists of glassy carbon electrode which is used as the working
electrode while platinum wire and Ag/AgCl were used as the
counter and reference electrodes, respectively. All of the dyes
show an oxidation wave which is attributable to the removal of an
electron from the amine segment. The exact oxidation potential
60 for the arylamine dyes was extracted from the DPV method and
listed in table 3. As shown in this table, the first oxidation
potentials (Eox) correspond to the HOMO levels of the dyes,
which are more positive than the redox potential of the (I/I3)−
redox couple (0.4 V), indicating that reduction of the oxidized
65 dyes with I− ions is thermodynamically feasible. On the other
hand, the excited state oxidation potentials (Eox*), which
correspond to the LUMO level of the dyes and this can be
obtained by the following equation (7):
600
in Toluene
10 %_MeOH
20 %_MeOH
30 %_MeOH
400
40 %_MeOH
50 %_MeOH
60 %_MeOH
70 %_MeOH
80 %_MeOH
90 %_MeOH
in MeOH
200
Eox*
=
Eox − E(0−0)
→ (7)
70 where, E(0ꢀ0) is the excited state energy of the dye, which is
estimated from the intersection between the normalized
absorption and emission spectra. The obtained Eox* of the dyes
are more negative than the conduction band potential of TiO2
(−0.5 V vs NHE), indicating that the electron injection process
75 from the excited dyes to the TiO2 conduction band is
energetically favourable (Scheme 1).
0
500
600
700
800
Wavelength (nm)
Figure 8: Fluorescence spectra of TRA in toluene–methanol mixtures.
This journal is © The Royal Society of Chemistry [year]
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DOI: 10.1039/C5CP05338B
Driving forces for electron injection (ꢁGinj) from the dye excited
7
6
5
4
3
2
1
0
state to the CB of TiO2 and electron recombination (ꢁGrec) from
the CB of TiO2 to the dye radical cation (Eox) are determined
from the following equations and values are shown in table 3.
PyRA
TRA
CRA
5
ꢁGinj = Eox* − ECB
ꢁGrec = ECB − Eox
→ (8)
→ (9)
The obtained ꢁG values are negative, which indicated that both
10 of the processes are thermodynamically feasible and most
importantly injection is dominant process than recombination for
these dyes.
Table 3: Electrochemical Data and Driving Forces for Electron
15 Transfer Processes on TiO2
0
100
200
300
400
500
600
b
c
Dyes aE0ꢀ0/eV Ep/V Eox/eV Eox*/eV
ꢁGinj ꢁGrec
−0.74 −1.73
−0.81 −1.97
−0.87 −1.57
V (mV)
TRA
CRA
PyRA
2.47
2.78
2.44
1.03
1.27
0.87
1.23
1.47
1.07
−1.24
−1.31
−1.37
40
Figure 9: J−V characteristics of the TRA, CRA and PyRA devices.
The parameters such as shortꢀcircuit current density (JSC), openꢀ
circuit voltage (VOC), fill factor (FF) and overall conversion
efficiency (η) were summarized in table 4. From the results, it is
clear that there is a significant role of donor moieties on the
a E0ꢀ0 values were calculated from intersect of the normalized absorption
and the emission spectra
45 power conversion efficiency. For instance, taking TRA as a
prototype, PyRA and CRA molecules were developed in order to
understand the impact of donor moiety on the performance of the
solar cells. There are several groups reported the power
conversion efficiency of TRA63ꢀ65 around 2.5%, here we have also
50 reproduced the same, hence it is meaningful to compare the
newly developed molecules (PyRA and CRA). Under same
condition, the PyRA and CRA sensitized solar cells consequences
the overall power conversion efficiencies (ɳ) of 1.68 % and 1.25
% respectively. It is clear that the photovoltaic performances of
55 DSCs are affected by changing the electron rich donors in organic
dyes. By comparing PyRA and CRA, the photovoltaic
performance of PyRA is significantly improved; this may due to
the higher molar extinction coefficient and high JSC. Contrary to
our expectation, the power conversion efficiency of PyRA is less
60 than TRA. In order to examine why such a difference in power
conversion efficiency occurred, the electrochemical impedance
spectroscopy66 was employed. Figure 10 shows the EIS Nyquist
plots for DSCs based on TRA, CRA and PyRA. Generally, the
largest semicircle in the lower frequency range in the Nyquist
65 plot represents the interfacial charge transfer at the
TiO2/dye/electrolyte interface,67 which is the charge
recombination between injected electrons and electrolyte.
b
Eox was measured in acetonitrile with 0.1 M TBAPF6 as supporting
electrolyte. Potentials measured vs Ag/AgCl were converted to normal
20 hydrogen electrode (NHE) by addition of + 0.2 V. Ep is the peak potential
of DPV, Eox = Ep + 0.2 V.
c The excited state oxidation potential (vs NHE), calculated from Eox−E0ꢀ0
.
ꢀ 1.5
(ꢀ1.37)
(ꢀ1.31)
(ꢀ1.24)
ꢀ 1.0
ꢀ 0.5
0
TiO2
CB
2.44
2.47
2.78
(I/I3)
(0.4)
0.5
1.0
1.5
(1.07)
PyRA
(1.23)
TRA
(1.47)
CRA
Scheme 1: Schematic energy levels for the electron transfer between
25 arylamine dyes and conduction band of TiO2.
Photovoltaic properties
Table 4: Photovoltaic properties of TRA, CRA and PyRA
70 devices.
Overall results clearly enlightens that the dyes developed in
30 this work have a promising light absorption properties and
suitable ground/excited state oxidation potentials to use them as
potential sensitizers for conventional DSCs. In addition, the
results from DFT calculations, photophysical and electrochemical
studies reveal that PyRA may have potential candidate for solar
35 cells. Hence, the photovoltaic studies of the freshly fabricated
cells are measured under AM1.5 solar irradiation (85 mW/cm2)
and the corresponding JꢀV characteristic of the cells are shown in
figure 9.
JSC/mA cm−2
η(%) τ (ms)
Dyes
VOC/V FF
TRA
CRA
PyRA
6.63
2.92
5.65
0.59 0.55 2.57 17.58
0.61 0.59 1.25 13.26
0.51 0.49 1.68 4.29
8
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DOI: 10.1039/C5CP05338B
Hence, the obtained plot in figure 10 looks like a semicircle
conversion efficiency of TRA is due to high JSC and VOC values.
The lower power conversion efficiency of PyRA is due to faster
electron recombination rate as evidenced by electrochemical
impedance spectroscopy. These findings reveal that diminutive
which is assigned to the charge transfer resistance at the
TiO2/dye/electrolyte interface. The diameter of the semicircle is
in the order of: TRA>CRA>PyRA, which means that the electron
5
transfer resistance for the unwanted back reaction is larger for 45 alteration in the structure of donor moiety play a significant role
TRA than that of CRA and PyRA. This may be responsible for
the achieved higher VOC and JSC of TRA. Moreover, the electron
recombination between the electrolyte and TiO2 and is related to
the electron lifetime in the CB of TiO2, which is ultimately
10 related to VOC. Therefore, Bode phase plots (Figure S17) were
used to find out the effective lifetime (τeff) of electrons in the
conduction band of TiO2. τeff were calculated68 by making use of
the following equation (10)
in photovoltaic performance.
Acknowledgment
50 A.K. thanks DST, India for the DSTꢀSERB Project (Ref. No. CSꢀ
316/2012, Dt. 04/12/2012) and M.J. thanks to DST, India for
DSTꢀINSPIRE Faculty Award [IFA13ꢀCHꢀ100].
Notes
1
15
τeff = 1/2πf
→ (10)
National Centre for Ultrafast Processes, University of Madras, Taramani
55 Campus, Chennai – 600 113, Tamil Nadu, India
2
Department of Chemistry, Loyola College, Chennai – 600 034, Tamil
where, f is the frequency of the corresponding peak in the Bode
phase plot. The obtained electron lifetimes for TRA, CRA and
PyRA are 17.58, 13.26 and 4.29 ms, respectively. In principle, a
20 longer electron lifetime indicate improved suppression of back
reactions between the injected electrons and the electrolyte that
lead to improvement of the VOC value, due to the reduced
electron recombination rate.69 Consequently, the observed longer
electron lifetime in case of TRA could explain its high VOC and
25 also the higher efficiency as well as more effective suppression of
back reaction than other dyes (PyRA and CRA).
Nadu, India
3 Nanomaterials and Solar Energy Conversion Laboratory, Department of
Chemistry, National Institute of Technology, Tiruchirappalli ꢀ 620 015,
60 Tamil Nadu, India
∗ Author for correspondence
Eꢀmail address: akathir23@hotmail.com (Dr. A. Kathiravan)
65 Electronic Supplementary Information (ESI) available.
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