Interfacial charge transport studies and fabrication of high performance DSSC with ethylene cored unsymmetrical dendrimers as quasi electrolytes

Article abstract of DOI:10.1016/j.molliq.2018.05.119

In this paper, authors focus the synthesis of conjugated unsymmetrical stilbenoid dendrimers by Heck and Horner-Wadsworth-Emmons coupling. In UV–visible absorption spectrum, the intensity of the absorption increases with increase in the generation of dendrimers. Further, bathochromic shift is observed on increasing the generation of the dendrimer from zero to first due to the greater widening of the energy gap between π-π* orbitals of the dendrimer system. Interfacial charge transport kinetics such as resistance, chemical capacitance and relaxation lifetime of the fabricated dye-sensitized solar cells (DSSC) are investigated using Nyquist and Bode phase plots by electrochemical impedance spectroscopy. Reduced electron relaxation lifetime (τe) of 1.83 ms (LiI + 7) and 1.04 ms (LiI + 8) provides efficient charge injection and thus reducing recombination process in the device. The performance of DSSC fabricated using unsymmetrical conjugated dendrimers with iodide electrolyte shows higher power conversion efficiency (PCE) than standard LiI based device. Two fold increments are achieved in PCE with first generation unsymmetrical dendrimers compared to their zeroth counterpart. The first generation unsymmetrical dendrimer 8 shows better PCE of 9.037% than all other synthesized dendrimers in the newly fabricated DSSC.

Full text of DOI:10.1016/j.molliq.2018.05.119

Accepted Manuscript
Interfacial charge transport studies and fabrication of high
performance DSSC with ethylene cored unsymmetrical
dendrimers as quasi electrolytes
Mahalingam Ravivarma, Kaliamurthy Ashok Kumar, Perumal
Rajakumar, Arumugam Pandurangan
PII:
DOI:
Reference:
S0167-7322(18)31198-X
doi:10.1016/j.molliq.2018.05.119
MOLLIQ 9177
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
7 March 2018
23 May 2018
27 May 2018
Please cite this article as: Mahalingam Ravivarma, Kaliamurthy Ashok Kumar, Perumal
Rajakumar, Arumugam Pandurangan , Interfacial charge transport studies and fabrication
of high performance DSSC with ethylene cored unsymmetrical dendrimers as quasi
electrolytes. Journal of Molecular Liquids(2017), doi:10.1016/j.molliq.2018.05.119
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ACCEPTED MANUSCRIPT
Interfacial charge transport studies and fabrication of high performance
DSSC with ethylene cored unsymmetrical dendrimers as quasi electrolytes
Mahalingam Ravivarmaa, Kaliamurthy Ashok Kumarb, Perumal Rajakumara* and
Arumugam Panduranganb
aDepartment of Organic Chemistry, University of Madras, Chennai-600 025, Tamil Nadu, India
bDepartment of Chemistry, Anna University, Chennai-600 025, Tamil Nadu, India
Phone: +91-44-2220 2814, Email: perumalrajakumar@gmail.com
Abstract
In this paper, authors focus the synthesis of conjugated unsymmetrical stilbenoid
dendrimers by Heck and Horner-Wadsworth-Emmons coupling. In UV-visible absorption
spectrum, the intensity of the absorption increases with increase in the generation of
dendrimers. Further, bathochromic shift is observed on increasing the generation of the
dendrimer from zero to first due to the greater widening of the energy gap between π - π*
orbitals of the dendrimer system. Interfacial charge transport kinetics such as resistances,
chemical capacitance and relaxation lifetime of the fabricated dye-sensitized solar cells (DSSC)
are investigated using Nyquist and Bode phase plots by electrochemical impedance
spectroscopy. Reduced electron relaxation lifetime (τe) of 1.83 ms (LiI+7) and 1.04 ms (LiI+8)
provides efficient charge injection and thus reducing recombination process in the device. The
performance of DSSC fabricated using unsymmetrical conjugated dendrimers with iodide
electrolyte shows higher power conversion efficiency (PCE) than standard LiI based device.
Two fold increments are achieved in PCE with first generation unsymmetrical dendrimers
compared to their zeroth counterpart. The first generation unsymmetrical dendrimer 8 shows
better PCE of 9.037% than all other synthesized dendrimers in the newly fabricated DSSC.
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Keywords: Dendrimer, DSSC, Electrochemical impedance spectroscopy and Photophysical
properties.
1. Introduction
Dye-sensitized solar cells (DSSCs) are expected to be potential candidate for energy
resources due to their low cost, non-toxicity, flexibility and impressive photovoltaic
performance [1]. However, various parameters in DSSC such as photo-anode materials, dye-
sensitizers, counter electrode materials and redox electrolytes have crucial influence on device
performance. Among various factors, electrolyte originates the primary photo-induced reaction
in redox electrolyte interfacial system and plays a major role in enhancing the device efficiency.
However, conventionally used LiI or KI solutions as electrolytes in DSSC have limitations
such as leakage and sublimation problems. Hence, a metal-free organic dendrimer compounds
-
are used as additive in iodide (I-/I3 ) redox electrolyte to make quasi-gel type electrolyte which
is one of the key components of DSSCs, play a focal role in light harvesting and electron
injection processes. Among the various organic compounds, phenothiazine based compounds
have electron-rich nitrogen and sulfur atoms and embrace easily tunable structures with low
cost and low environmental pollution [2]. Nakashima et al. [3], reported an improved power
conversion efficiency (PCE) of 5.8% using half-dendric phenylazomethine core (Half-DPA
G5) as additive in redox electrolyte system. Our previous work based on dendrimer electrolyte
reported an enhanced PCE of 7.1% using dendrimer with benzothiazole and triazole moieties
[4] and 8.6% was realized in triazole based phenothiazine dendrimers [5] as additive in iodide
redox system for DSSC fabrication.
Dendrimers are among the most attractive supramolecules exhibiting an iterative
architecture and happens to be one of the major themes in recent chemistry. Their synthesis
with precisely tuned structural components like core, branches and surface groups afford
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materials possessing particular physical properties with specific applications [6]. The
exploration of new core structures and branching units to achieve interesting properties for the
dendrimers remains as an ever-important task for synthetic chemists [7]. Development of new
dendritic materials with diverse applications include light harvesting [8], energy transfer [9],
nanocatalysis [10], medical diagnostics [11], transfection vectors [12] and controlled drug
delivery systems [13].
Dendrimers are known as good candidate for light-harvesting applications. Indeed,
numerous light-harvesting dendrimers have been reported [14] with dendritic branches as
scaffolds on which light-absorbing chromophores are anchored [15]. The conjugated
dendrimers are a promising new class of material for organic LEDs. They can be thought of
as consisting of a conjugated light-emitting core, conjugated branches, and surface groups
[16]. To date, there is no report of conjugated unsymmetrical dendrimers for light harvesting
application. The ethylene skeleton in the form of stilbene unit could be an excellent choice for
a central chromophore to construct new photo responsive materials. On the basis of their unique
properties, monodisperse stilbenoid dendritic materials emerge as attractive candidates for
photonic applications.
Most of the dendrimers possess a single type of terminal/surface groups, because they
are easy to synthesize and they are classified as symmetrical dendrimers. However, dendrimers
with two (or more) types of terminal groups is highly desirable to combine several applications
in a single molecule [17]. The surface group of the dendrimer forms the multivalent system
which contributes to a greater extent the photophysical, electrochemical and biological
properties of the dendrimer. By attaching various types of surface group, the single dendrimer
can be expected to show a spectacular change in physical, chemical and biological properties.
The core is chosen in such a way that it has two or more anchoring units with difference in
capacity. Hence in unsymmetrical dendrimer at least two different types of surface group are
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attached to a single core unit. Fig. 1 shows the schematic representation of the unsymmetrical
dendrimer.
Fig. 1 Schematic representation of an unsymmetrical dendrimer
The first unsymmetrical conjugated dendrimer with light harvesting property has been
reported by Zhonghua Peng et al. [18] and Fan et al. [19] reported the chiral diphosphane-
functionalized unsymmetrical dendrimer with sixteen BINAP units and Frechet-type poly(aryl
ether) dendron as the soluble support.
Unsymmetrical dendrimers also could act as an extremely efficient light-harvesting
material [20] by funneling a broad range of wave length of light to the energy trap at the locus.
We believe that unsymmetrical dendrimers would exhibit better applications than symmetrical
dendrimers in material science if the signal communication occurs efficiently between the
peripheral segments and the core of the dendrimers. Hence, our synthetic approach for
unsymmetrical dendrimers is fixed with phenothiazine unit on one side and the other side is
functionalized with different surface units such as ferrocene, pyrene, diphenylamine and
carbazole.
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Ferrocene containing dendrimers are of great interest due to multi-electron transfer and
phenothiazine based conjugated polymers are recently [21] used as a light-emitting material.
Phenothiazine ring system is electron rich by nature and hence functions as an excellent light
harvesting molecular dendritic wedge [22] and dendrimers with carbazole system can emit
intense luminescence [23]. By systematically modifying the structures of the peripheral groups
and the branching unit of the dendritic wedge, the overall energy transfer efficiency of
electroluminescence devices can be fine-tuned around the core [24].
Pyrene-based organic and polymeric materials commonly possess outstanding carrier
transporting capabilities, high photoluminescence quantum efficiency, long fluorescence
lifetime, and high propensity for forming excimers [25] and pyrene-cored dendrimers have
been employed in organic optoelectronic application [26]. The diphenylamine-based dendrons,
which display stronger electron donating abilities than the triphenylamine-based dendrons and
the carbazole based counterparts, are ideal electron donors with low aggregation tendency, high
carrier mobility, and high thermal and photochemical stability [27]. Additionally, the dendritic
diphenylamine derivatives have better solubility in common organic solvents relative to
carbazole-based dendrons due to increased flexibility [28]. The present investigation focus on
the synthesis of novel unsymmetrical conjugated dendrimers 1-8 with ethylene core and with
two different surface groups such as phenothiazine, ferrocene, pyrene, diphenylamine and
carbazole (Fig. 2) and their interfacial charge transport studies as well as their application in
improving the efficiency of DSSC.
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Fig. 2 Molecular structure of unsymmetrical conjugated dendrimers 1-8
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2. Results and Discussions
2.1 Dendrimer synthesis
Wittig
reaction
of
10-ethyl-10H-phenothiazine-3-carbaldehyde
9
with
methyltriphenylphosphonium bromide in the presence of potassium tert-butoxide afforded 10-
ethyl-3-vinyl-10H-phenothiazine 10 in 68% yield. The 1H NMR spectrum of 10-ethyl-3-vinyl-
10H-phenothiazine 10 displayed quartet at δ 3.85 (J = 7.0 Hz) for the N-methylene protons,
two doublets at δ 5.10 (J = 10.8 Hz) and δ 5.58 (J = 17.7 Hz) and a doublet of doublet at δ 6.55
(J = 17.7 Hz, J = 10.8 Hz) for the vinylic protons in addition to the signals for the other aliphatic
and aromatic protons (SI; Fig. S19). In 13C NMR spectrum, 10-ethyl-3-vinyl-10H-
phenothiazine 10 showed signals for the N-methylene carbon at δ 41.9, the vinylic carbon at δ
115.1 and 135.6 in addition to the other aliphatic and aromatic carbon signals
(SI; Fig. S20).
Scheme 1 (i) 1 equiv. Methyl triphenylphosphonium bromide, KOtBu, THF, 12 h; (ii)
Pd(OAc)2, TBAB, K2CO3, DMF, 90 oC, 24 h.
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Heck coupling of 1.0 equiv. of each of 1-bromoferrocene 11, 1-bromopyrene 12, 4-
bromo-N-ethyl-N-phenylaniline 13 and 3-bromo-9-ethyl-9H-carbazole 14 with 1.0 equiv. of
10-ethyl-3-vinyl-10H-phenothiazine 10 using Pd(OAc)2 in the presence of K2CO3 and
tetrabutylammonium bromide resulted in the formation of the unsymmetrical conjugated
dendrimers 1-4 in 72%, 66%, 69% and 67% yields, respectively (Scheme 1).
1
The H NMR spectrum of the unsymmetrical conjugated dendrimer 1 displayed a
quartet at δ 3.90 (J = 6.8 Hz) for the N-methylene proton, three singlets at δ 4.11, 4.25 and 4.42
for the protons of the ferrocenyl unit and two doublets at δ 6.56 (J = 16.2 Hz) and 6.71 (J =
15.9 Hz) for the trans olefinic protons in addition to the signals for the other aliphatic and
aromatic protons (SI; Fig. S1). The 13C NMR spectrum of the unsymmetrical stilbenoid
dendrimer 1 showed signal at  41.9 for the N-methylene carbon and the ferrocenyl carbon at
 66.7, 68.9, 69.2, 83.6; the olefinic carbon at δ 115.1 and 132.5 in addition to the signals for
the other aliphatic and aromatic carbons (SI; Fig. S2). The mass spectrum (HRMS) of 1 showed
the molecular ion peak at m/z 437.0904 (SI; Fig. S3). The structure of the dendrimer 1 was
further confirmed from elemental analysis. Similarly, the structure of dendrimers 2-4 was also
confirmed from spectral and analytical data (SI; Fig. S4-S9).
Scheme 2 Pd(OAc)2, TBAB, K2CO3, DMF, 90 oC, 24 h.
Encouraged by the facile synthesis of the zeroth generation dendrimers, the same
strategy has been employed for the synthesis of the first generation unsymmetrical conjugated
dendrimer. In order to synthesize the first generation dendron 16, 1.0 equiv. 3,5-
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dibromophosphonate ester 15 [29] was reacted with 2.1 equiv. of 10-ethyl-3-vinyl-10H-
phenothiazine 10 under Heck coupling conditions to afford the first generation dendritic
phosphonate ester 16 in 67% yield (Scheme 2).
The 1H NMR spectrum of the first generation dendritic phosphonate ester 16 displayed
two doublets at  3.17 (J = 21.3 Hz) and δ 6.97 (J = 11.1 Hz) for the phosphonate benzylic
proton and for the trans olefinic proton, respectively, in addition to the signals for the other
aliphatic and aromatic protons (SI; Fig. S21). The 13C NMR spectrum of 16 showed a doublet
at  33.8 (J = 136.5 Hz) for the benzylic carbon adjacent to phosphorus, in addition to the other
aliphatic and aromatic carbon signals (SI; Fig. S22).
Scheme 3 (i) Methyl triphenylphosphonium bromide (1.0 equiv.), KOtBu, THF, 12 h. (*
denotes the corresponding position of aldehyde and vinyl functionalities).
Further, Wittig reaction of the aldehyde derivatives 17-20
with methyl
triphenylphosphonium bromide in the presence of potassium tert-butoxide afforded the vinyl
compounds 21, 22, 23 and 24 in 65%, 70%, 66% and 62% yields, respectively (Scheme 3).
The 1H NMR spectrum of the vinylic compound 24 displayed two doublets at δ 5.18 (J = 10.8
Hz) and δ 5.76 (J = 17.7 Hz) and a doublet of doublet at δ 6.89 (J = 17.7 Hz, J = 11.1 Hz) for
the protons of the vinyl unit in addition to the signals for the other aliphatic and aromatic
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protons (SI; Fig. S29). In the 13C NMR spectrum, the vinyl compound 24 showed vinylic
carbon at δ 118.5 and at δ 137.5 in addition to other aliphatic and aromatic carbon signals (SI;
Fig. S30). Similarly, the structure of vinyl compounds 21-23 has been also confirmed from the
spectral data (SI; Fig. S23-S28).
In a similar manner, Heck coupling of 2.1 equiv. of the first generation dendritic vinyl
compounds 21-24 with 1.0 equiv of 3,5-dibromobenzaldehyde 25, in the presence of Pd(OAc)2,
o
K2CO3 and tetrabutylammonium bromide in DMF at 90 C for 12 h afforded the
corresponding unsymmetrical dendritic aldehyde 26-29 in good yields (Scheme 4).
Scheme 4 (i) Pd(OAc)2, TBAB, K2CO3, DMF, 90 oC, 24 h.
The 1H NMR spectrum of the dendritic aldehyde 26 displayed three singlets at δ 4.16,
4.33, 4.49 for the ferrocenyl protons and the trans olefinic protons at δ 6.74 (J = 16.2 Hz) and
7.03 (J = 16.2 Hz) in addition to the signals for the aromatic protons (SI; Fig. S31). The 13C
NMR spectrum of the dendritic aldehyde 26 displayed the ferrocennyl carbons at δ 67.1, 69.3,
69.4 and 82.6, the carbonyl carbon at δ 192.6 in addition to the signals for the aromatic carbons
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(SI; Fig. S32). Similarly, the structure of the dendritic aldehyde 27-29 was also confirmed from
spectral and analytical data (SI; Fig. S33-S38).
Scheme 5 (i) NaH, THF, rt, 12 h.
Encouraged by the facile synthesis of the first generation conjugated phosphonate ester
16 and the aldehydic dendron 26-29, we focused our attention on the convergent synthesis of
the first generation unsymmetrical dendrimers 5-8 through Horner-Wadsworth-Emmons
coupling as shown in Scheme 5. The reaction of 1.0 equiv. of the phosphonate ester 16 with
1.0 equiv. of each of the dendritic aldehyde 26-29 in the presence of NaH in THF at room
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temperature for 12 h gave the unsymmetrical conjugated dendrimer 5-8 in 63%, 52%, 60% and
62% yields, respectively.
1
The H NMR spectrum of the unsymmetrical dendrimer 5 showed a quartet at  3.93
(J = 7.1Hz) for the N-methylene proton, three singlets for the ferrocenyl protons at δ 4.17, 4.32,
4.51 and a doublet at  7.21 (J = 9.9 Hz) for the ethylenic protons in addition to the other
aliphatic and aromatic proton signals (SI; Fig. S10). The 13C NMR spectrum of the
unsymmetrical dendrimer 5 displayed the N-methylene carbon at  41.9, the ferrocennyl
carbons at  66.9, 69.2, 69.3 and 83.3 in addition to other aliphatic and aromatic carbons signals
(SI; Fig. S11). In the mass spectrum (MALDI-TOF) the unsymmetrical dendrimer 5 showed
(SI; Fig. S12) the molecular ion peak at m/z 1102.2 and the structure was also further confirmed
from analytical data. Similarly, the structure of the unsymmetrical dendrimers 6-8 was
confirmed from spectral and analytical data (SI; Fig. S13-S19).
2.2 Photophysical properties of unsymmetrical conjugated dendrimers
The UV-Vis absorption spectrum (Fig. 3) of the unsymmetrical conjugated lower
generation dendrimers 1-4 and conjugated higher generation dendrimers 5-8 was recorded in
DMSO. Two absorption maximum were observed in the range of 292 to 313 nm and 329 to
391 nm. The absorption at lower λmax could be due to π-π transition and the absorption at
higher λmax could be n-π* transition. Zeroth generation dendrimers 1-4 with phenothiazine unit
on one dendritic arm and with one ferrocene/pyrene/diphenylamine/carbazole units on the other
side of the dendritic arm displayed λmax at 309 and 366; 292 and 373; 345 and 391; 306 and
329 nm, respectively. First generation dendrimer with two phenothiazine and two
ferrocene/pyrene/diphenylamine/carbazole unit displayed λmax at 313 and 375 nm; 305 and
381; 309 and 376; 307 and 335 nm.
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Table 1 Photophysical data for dendrimers 1-8 in DMSO (1×10-5 mol/L)
λabs max
(nm)
ε
λem max
(nm)
480
Dendrimers
(104 M-1 cm-1)
309, 366
4.24, 2.49
1
2
3
4
5
6
7
8
292, 373, 415 3.54, 3.09, 2.53
310, 391 1.76, 2.29
306, 329, 375 3.50, 2.81, 2.23
580
508
497
313, 375
305, 381
309, 376
307, 335
14.90, 6.11
9.32, 9.24
535
424
10.45, 9.59
15.55, 14.12
543
545
0.5
0.4
0.3
0.2
0.1
0.0
5
6
7
8
1
2
3
4
1.5
1.0
0.5
0.0
300
350
400
450
500
300
350
400
450
500
550
Wavelength (nm)
wavelength (nm)
Fig. 3 UV-Vis absorption spectra of the dendrimers 1-8 in DMSO (110-5 M)
700
600
1
2
3
4
5
6
7
500
400
300
200
100
8
400
450
500
550
600
650
Wavelength (nm)
Fig. 4 Emission spectra of the dendrimers 1-8 in DMSO (110-5 M)
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As the dendrimer generation increases from zero to first generation, molar absorption
coefficient also increases due to the presence of more number of phenothiazine and
ferrocene/pyrene/diphenylamine/carbazole chromophore units. Bathochromic effect was
observed on increasing the generation of the dendrimer from zeroth to the first, which indicates
that greater widening of the energy gap between - orbitals in the dendrimer system from 1-
4 and 5-8. The photophysical data is summarized in Table 1.
The fluorescence spectra of 1-8 in DMSO showed emission band around 424-580 nm
(Fig. 4) on excitation at 380 nm and the λem maximum values are listed in Table 1. Around 40
nm red shift was observed for the first generation dendrimers 5, 7 and 8 when compared with
corresponding zeroth generation dendrimers 1, 3 and 4 due to increase in the number of
phenothiazine units and also other chromophores on the surface.
2.3 Electrochemical impedance spectroscopy - Nyquist plot
Interfacial charge transport resistance for the fabricated DSSCs was evaluated using
Nyquist plots from electrochemical impedance spectroscopy (EIS). Measurements were
taken in two electrode device configuration for an applied open circuit potential (OCP) in
the frequency region of 1 MHz to 0.1 Hz under dark condition. It reveals double semicircle
behavior for the fabricated DSSCs as shown in Fig. 5a. Presence of two semicircle confirms
the existence of two interfacial resistances in those devices. Intersect of semicircle arc in
the real axis of impedance gives the charge transport resistances at every interface. High
frequency semicircle gives charge transport resistance at Pt/electrolyte interfaces (Rct1) and
low
frequency
semicircle
provides
charge
transport
resistance
at
TiO2/N719/LiI+Dendrimer interfaces (Rct2). Presence of high frequency semicircle is
further evidenced with Bode phase plot of EIS measurements (Fig. 5b) which shows phase
shift in high frequency region. The obtained semicircles were fitted with an equivalent
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circuit model (shown as inset of Fig. 5a) and the fit parameters were summarized in Table
2. Here, Rs be the series resistance that contributes the sheet resistance of fluorine doped
tin oxide (FTO) which appears to be same for all the devices and Cμ be the chemical
capacitance ensued due to photo anode of fabricated DSSCs and τe is the electron relaxation
lifetime expressed in milliseconds [30].
5b
5a
Fig. 5 EIS: (a) Nyquist plot [Inset shows equivalent circuit model for fitting] (b) Bode phase
plot
It is noted from the Table 2 that standard LiI solution electrolyte has high Rct1 and Rct2
owing to the sublimation of iodine at short period. It increases the resistances at every interface
and reduces the charge transportation. In the case of DSSCs fabricated with LiI and dendrimer
1-6 as quasi electrolyte, it shows reduced Rct1 and Rct2 compared to standard LiI electrolyte
which reveals efficient charge transportation at Pt/electrolyte interface and reduced
recombination at TiO2/N719/LiI+Dendrimer interfaces in those devices. Eventhough, the Rct1
gets increased for dendrimer 7 and 8, the Rct2 seems to be very low owing to increased number
of phenothiazine and other surface groups such as diphenylamine and carbazole which can act
as efficient charge injection moiety. Increased protonation of diphenylamine and carbazole unit
in dendrimer electrolyte (7 and 8) shifts their redox potential towards positive potential (upper
shift) and reduced the charge transportation (high Rct1) at Pt/electrolyte interfaces. In addition,
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shift in redox potential also prompt to minimize the potential difference between Fermi TiO2
and HOMO level of redox electrolyte and reduced the Voc of the devices.
Table 2 Electrochemical parameters of DSSCs obtained by fitting the Nyquist plots of EIS
measuremets
Samples
Rs (Ω) Rct1 (Ω) Rct2 (Ω) Cμ (μF) τe(ms) χ2 (x 10-4)
TiO2/N719/LiI/Pt
TiO2/N719/LiI/1/Pt
TiO2/N719/LiI/2/Pt
TiO2/N719/LiI/3/Pt
TiO2/N719/LiI/4/Pt
TiO2/N719/LiI/5/Pt
TiO2/N719/LiI/6/Pt
TiO2/N719/LiI/7/Pt
TiO2/N719/LiI/8/Pt
25.09
25.2
326.8
251.2
243.3
268.7
252.6
253.2
294.9
412
5792
5075
4765
4397
4023
2164
2070
1107
987.3
2.012
3.523
3.837
3.342
3.121
3.785
2.753
1.693
1.2
9.95
17.49
17.49
13.16
13.16
7.51
2.03
1.17
1.03
1.31
4.24
1.58
2.05
1.04
6.52
25.42
24.97
28.29
22.7
21.89
21.82
18.15
5.66
1.83
512.4
1.04
In addition, the electron relaxation lifetime was estimated from the Bode phase plots in
EIS measurements. Fig. 4b, clearly indicates the existence of two interfacial resistances (2
semicircle). The electron relaxation lifetime (τe) were calculated from the peak frequency of
mid frequency phase shift using the relation τe = 1/2πfmid and summarized in Table 2. It is
noteworthy that standard LiI electrolyte based DSSC showed τe of 9.95 ms owing to high Rct2
value. Addition of dendrimer 1-4 with standard LiI electrolyte increases the relaxation lifetime
to very high owing to its high chemical capacitance [31]. High τe has more possibility for charge
recombination rather than charge transportation. It is responsible for high recombination
probability in dendrimer 1-4 based DSSCs, while in the case of DSSCs fabricated using
dendrimer 5-8 resulted in reduced τe which tends to suppress the recombination as evidenced
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from high FF. Especially in DSSCs fabricated using dendrimer 7 and 8 reveals very low τe due
to reduced Rct2 and Cμ which enhance efficient charge injection and reduce the recombination.
2.4 J-V characteristics of fabricated DSSCs
Fig. 6 Photocurrent-Voltage curves of DSSCs fabricated with dendrimer 1-8 as additives under
simulated solar light at 50 mW/cm2 with AM 1.5G filter
Fig. 6 shows the J-V characteristics of fabricated DSSCs with standard iodide
electrolyte (LiI) and quasi-gel polymer electrolyte. Measurements were carried out under
standard test condition using Keithley source meter (2400 pico-ammeter) at 0.5 sun intensity
(Halogen lamp of 50 mW/cm2 with AM 1.5G filter) calibrated using lux meter. The
photovoltaic parameters are evaluated from the J-V curve and summarized in Table 3 using
the following relation:
Imax Vmax
P
Isc V
max
FF(%) 

Isc V
oc
oc
Isc V FF
oc
(%)
P
in
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Where, Imax and Vmax represent the maximum current and voltage in a I-V loop, Isc and Voc
corresponds to short circuit current and open circuit voltage respectively, FF denotes the fill
factor of a device and Pin is the power of the incident light photons.
Table 3 photovoltaic parameters of fabricated DSSCs with unsymmetrical dendrimers 1-8
Electrolyte System/
Voc (V)
Jsc (mA/cm2) FF (%)
ɳ (%)
Dendrimer
TiO2/N719/LiI/Pt
TiO2/N719/LiI/1/Pt
TiO2/N719/LiI/2/Pt
TiO2/N719/LiI/3/Pt
TiO2/N719/LiI/4/Pt
TiO2/N719/LiI/5/Pt
TiO2/N719/LiI/6/Pt
TiO2/N719/LiI/7/Pt
TiO2/N719/LiI/8/Pt
0.467
0.716
0.758
0.745
0.730
0.717
0.747
0.669
0.672
5.22
3.22
3.44
4.75
4.77
5.72
5.57
10.74
10.94
43.18
51.10
53.67
50.77
54.34
52.18
61.46
59.07
61.46
2.108
2.352
2.804
3.592
3.788
4.280
5.116
8.494
9.037
PCE for standard LiI electrolyte based DSSC is (Table 3) very low (2.108%)
compared to that of DSSCs with dendrimer based electrolytes. Enhancement in Jsc, FF and
Voc was observed for the fabricated DSSC with dendrimer based quasi electrolyte which is
responsible for the improved PCE [32]. Addition of dendrimer 1-8 in LiI gives a gel type
quasi solid electrolyte which reduces the leakage and evaporation of iodine in the
electrolyte. Increased FF for all DSSC with dendrimer base has resulted in reduced electron
recombination in the device. Especially dendrimer 6 and 8 based device shows increased
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FF to 60% owing to the presence of pyrene and carbazole moiety respectively. Further, Voc
increases for all dendrimer based DSSCs owing to deprotonation while anchoring on N719
dye surfaces. Deprotonation of dendrimer electrolyte downshift the HOMO level and
increase the potential difference between Fermi level of TiO2 and redox potential of the
electrolyte as evidenced with CV results of standard LiI and dendrimer electrolyte (SI; Fig.
S39).
DSSC with dendrimer 1-6 as base shows increased Voc and the PCE reaches a
maximum of 5%. Eventhough, the obtained Voc of the dendrimer 7 and 8 based DSSCs is
low, the PCE is 8.5% and 9% respectively owing to higher current density (Jsc). Surface
groups such as, diphenyl amine and carbazole in dendrimer 7 and 8 acts as an extended π-
conjugation link that improves the extinction co-efficient of polymer electrolyte which
further improves the Jsc of the device [33]. Further, the electron injection efficiency is
higher in DSSC with dendrimer 5 compared to the device with the dendrimer 1, due to the
existence of ferrie ion which exposes metal to ligand charge transfer (MLCT) process
efficiently and enhances the Jsc to 5.72 mA/cm2. In summary, two fold increment in PCE
for the DSSC with the first generation dendrimers 5-8 is observed whereas with the zeroth
generation dendrimers 1-4 the PCE of 2.3% to 3.7% is observed. Enhanced efficiency of
9.037% is achieved as maximum from DSSC with dendrimer 8 owing to the higher Jsc, Voc
and FF values.
3. Experimental
General Information: All reagent grade chemicals are commercially available such as
Phenothiazine, ferrocenecarboxaldehyde, pyrene, bromoferrocene, Potassium tert-butoxide,
Pd(OAc)2, tetrabutylammonium bromide, triethyl phosphate were purchased from Sigma-
Aldrich and used without purification. Methyltriphenylphosphonium bromide and NaH were
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purchased from Loba Chemie Pvt Ltd., and diphenylamine, carbazole, K2CO3, THF and DMF
were purchased from Sisco Research Laboratories Pvt. Ltd., India and used as received. All
melting points were determined using a Toshniwal meltingpoint apparatus by open capillary
13
tube method and are uncorrected.1H NMR and CNMR spectra were recorded on a Bruker
300 MHz instrument. The IR spectra were recorded on FTIR-8300 spectrophotometer and UV-
Vis spectra were recorded on Agilent 8453 spectrophotometer. The emission spectra were
recorded on HORIBA JOBIN YVON Fluoromax-4 spectrofluorometer. MALDI-TOF mass
spectra were recorded on Voyager-DE PRO mass spectrometer using a -cyano-4-hydroxy
cinnamic acid (CHCA) matrix and HRMS spectra on Water XEVO-G2SQTOF mass
spectrometer. Cyclic voltammetry of dendrimer systems and electrochemical impedance
spectra of fabricated DSSCs were examined using an electrochemical workstation
(AUTOLAB-PGSTAT302N). J-V characteristics of the fabricated devices were measured
using simulator setup with halogen lamp at 0.5 sun intensity (50 mW/cm2) calibrated using lux
meter with AM 1.5G filter. Column chromatography was performed on silica gel (ACME, 100
to 200 mesh). Routine monitoring of the reaction was made using thin layer chromatography
developed on glass plates coated with silica gel-G (ACME) of 25 mm thickness and visualized
with iodine.
General procedure for Wittig reaction (Procedure A): To a suspension of
methyltriphenylphosphonium bromide (1 equiv.) in dry THF (100 mL), under nitrogen
atmosphere at room temperature was added potassium tert-butoxide (7.0 equiv.). The solution
was stirred for 1 h and then a solution of the aldehyde (1 equiv.) in dry THF (30 mL) was added
slowly. The mixture was stirred at room temperature for 12 h and was evaporated to dryness.
The unreacted potassium tert-butoxide was quenched with saturated NH4Cl solution (10 mL).
The reaction mixture was then extracted with CHCl3 (200 mL), washed with water (2 x 200
mL), brine (100 mL) and then dried over anhydrous Na2SO4. Evaporation of the organic layer
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gave a residue, which was purified by column chromatography using hexane as the eluting
solvent to give the corresponding vinyl compounds.
General procedure for Heck Coupling (Procedure B): A stirred mixture of bromo
compound (1.0 equiv.), Pd(OAc)2 (0.1/ 0.2 equiv.) in dry DMF (25 mL) under nitrogen was
successively treated with K2CO3 (3.0/ 6.0 equiv.) and tetrabutylammonium bromide (0.1/ 0.2
equiv.)and stirred for 30 minutes. The vinyl dendron (1.0/ 2.0 equiv.)was then added and the
resulting mixture was stirred at 90 oC for 12 h, cooled and filtered. The filtrate was evaporated
to dryness in vacuo. The residue was extracted with CHCl3 (3x100 mL), washed with water
(3x100 mL) and dried over anhydrous Na2SO4. Evaporation of the organic layer afforded the
crude product, which was purified by column chromatography using the eluent as mentioned
under each compound to afford the corresponding conjugated dendrimers.
General procedure for Horner-Wadsworth-Emmons coupling reaction (Procedure C):
To a stirred suspension of NaH (2.0/3.0 equiv.) in dry THF (100 mL), a solution of the
phosphonate ester (2.0/3.0 equiv.) in dry THF (50 mL) was added dropwise under nitrogen
atmosphere. The mixture was stirred for 30 minutes and then added dropwise to a solution of
aldehyde (1.0 equiv.) in dry THF (20 mL) over a period of one hour and then stirred for 12 h
at room temperature. The excess NaH was quenched with ice-cold water and the solvent was
removed under reduced pressure. The crude product obtained was purified by column
chromatography, using the eluent as mentioned under each compound.
Fabrication of DSSCs:
Fluorinated tin oxide (FTO) glass substrates with defined active area of 4 x 4 mm2 were
coated with anatase TiO2 paste (Degussa-P25) using doctor blade technique in the thickness
range of 20-25 μm. After calcination at 450 ºC the prepared photo anodes were sensitized with
N719 dye (0.3 mM of ethanolic dye solution) for 12 hours in dark condition. Now, the dye-
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sensitized photo anodes were washed with ethanol to remove unanchored dye molecules from
the surface of TiO2. For the preparation of standard LiI electrolyte (liquid), 0.3 M of LiI and
0.03 M of I2 are dissolved in the mixed solvent of acetonitrile and tert-butyl alcohol in (1:1
volume ratio). Finally, 0.1 g of prepared dendrimers 1-8 were added with 1 ml of standard LiI
electrolyte separately to prepare quasi-gel type dendrimer electrolyte which is casted on the
surface of TiO2 photoanode and sandwiched with commercially purchased platinum counter
electrode using binder clips.
4. Conclusion
Synthesis
of
unsymmetrical
conjugated
dendrimers
with
ferrocene/pyrene/diphenylamine/carbazole and phenothiazine as surface units with ethylene as
core unit has been achieved through Heck and Horner-Wadsworth-Emmons coupling. The
photophysical studies indicate that on increasing the generation of the dendrimers the
absorption coefficient increases with bathochromic shift in λmax. DSSC fabricated with
dendrimer 7 and 8 showed high charge injection and reduced recombination. Further, the
efficiency of fabricated DSSCs with quasi electrolyte (LiI+dendrimers) shows enhanced PCE,
owing to its high Jsc and Voc when compared to standard LiI electrolyte based DSSC. DSSC
with unsymmetrical dendrimer 8 as the electrolyte exhibited better power conversion efficiency
(η) of 9.037% than the DSSC with the zeroth and the other first generation unsymmetrical
dendrimers as electrolyte.
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Acknowledgement
The authors thank DST-FIST New Delhi for providing NMR and High Resolution Mass
Spectrum facility to the Department of Organic Chemistry, University of Madras. KA thanks
DST-SERB for the financial support through N-PDF (File No: PDF/2016/003878; Dt: 23-06-
2017) and also thank DST-FIST for providing instrumentation facility at the Department of
Chemistry, Anna University, Chennai, India.
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Graphical Abstract
Interfacial charge transport studies and fabrication of high performance DSSC
with ethylene cored unsymmetrical dendrimers as quasi electrolytes
Mahalingam Ravivarmaa, Kaliamurthy Ashok Kumarb, Perumal Rajakumara* and
Arumugam Panduranganb
aDepartment of Organic Chemistry, University of Madras, Chennai-600 025, Tamil Nadu, India
bDepartment of Chemistry, Anna University, Chennai-600 025, Tamil Nadu, India
Phone: +91-44-2220 2814, Email : perumalrajakumar@gmail.com
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Highlights
 A novel unsymmetrical dendrimers are synthesized with heterocyclic compounds.
 The fabricated DSSCs are evaluated using Nyquist and Bode phase plots of EIS.
 Phenothiazine and Carbazole based dendrimer showed enhanced PCE of 9.037%.
29