Bis (4 ′-Tert-butylbiphenyl-4-yl)aniline (BBA)-substituted A3B zinc porphyrin as light harvesting material for conversion of light energy to electricity

Article abstract of DOI:10.1142/S1088424620500327

Limited synthetic steps via low-cost starting materials are needed to develop large-scale light-Active materials for efficient solar cells. Here, novel bis(4′-Tert-butylbiphenyl-4-yl)aniline (BBA) based A3B zinc porphyrin (GB) is synthesized and applied as a light harvesting/electron injection material in dye-sensitized solar cells. The GB sensitizer was characterized by various spectroscopic techniques and the optimized device shows JSC of 10.98 ± 0.37 mA/cm2 and power conversion efficiency (PCE) of 3.34 ± 0.26%. In addition, performance is enhanced up to 3.9% by the addition of co-Adsorbent 3a,7a-dihydroxy-5b-cholic acid (chenodeoxycholic acid, CDCA) to minimize staking of the planar porphyrin macrocycles. These results demonstrate that novel broad-Absorbing light-Active material (GB) could be used for indoor solar panels.

Full text of DOI:10.1142/S1088424620500327

This is a JAM - Just Accepted Manuscript
Journal of Porphyrins and Phthalocyanines
Article Title:
Bis(4'-tert-butylbiphenyl-4-yl)aniline (BBA)-Substituted A B Zinc Porphyrin
3
as Light Harvesting Material for Conversion of Light Energy to Electricity
Author(s):
DOI:
Naresh Duvva, Suneel Gangada, Raghu Chitta, Lingamallu Giribabu
10.1142/S1088424620500327
To be cited as:
Naresh Duvva, Suneel Gangada, Raghu Chitta, Lingamallu Giribabu, Bis(4'-tert-butylbiphenyl-4-
yl)aniline (BBA)-Substituted A B Zinc Porphyrin as Light Harvesting Material for Conversion of
Light Energy to Electricity, Journal of Porphyrins and Phthalocyanines, doi:
0.1142/S1088424620500327
3
1
Received: 8 June 2020
Accepted: 25 June 2020
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Accepted manuscript to appear in JPP
Bis(4′-tert-butylbiphenyl-4-yl)aniline (BBA)-Substituted A B
3
Zinc Porphyrin as Light Harvesting Material for Conversion of
Light Energy to Electricity
Naresh Duvva, Suneel Gangada, Raghu Chitta, Lingamallu Giribabua,d*
a
b
bc*
a
Polymer and Functional Materials Division, CSIR-Indian Institute of Chemical Technology, Hyderabad 500007,
Telangana, India.
bDepartment of Chemistry, School of Chemical Sciences & Pharmacy, Central University of Rajasthan, Kishangarh,
Dist. Ajmer, Rajasthan-305817, India.
cDepartment of Chemistry, National Institute of Technology Warangal, Hanamkonda, Warangal – 506004, Telangana,
India.
dAcademy of Scientific and Innovative Research (AcSIR), Ghaziabad, New Delhi 201002, India.
Received date (to be automatically inserted after your manuscript is submitted)
Accepted date (to be automatically inserted after your manuscript is accepted)
ABSTRACT: Indeed, limited synthetic steps via low-cost starting materials need to develop large-scale light active
materials for efficient solar cells. Here, Novel Bis(4′-tert-butylbiphenyl-4-yl)aniline (BBA) based A
3
B Zinc porphyrin
(
GB) is synthesised and applied as a light harvesting/electron injection material in dye sensitized solar cell. GB
sensitizer was characterized by various spectroscopic techaniques and optimized device shows JSC of 10.98±0.37
2
mA/cm and power conversion efficiency (PCE) of 3.34±0.26%. In addition, performance is enhanced up to ~3.9% by
the addition of co-adsorbent 3a,7a-dihydroxy-5b-cholic acid (chenodeoxycholic acid, CDCA) to minimize π-π staking
of planar porphyrin marocycle. These results demonstrate that novel broad absorbing light active material (GB) used for
indoor solar panels.
KEYWORDS: porphyrin, sensitizer, efficiency, redox couple, electron transer
*Correspondence to: raghuchitta@nitw.ac.in, giribabu@iict.res.in
1

Accepted manuscript to appear in JPP
1
. INTRODUCTION
Dye-Sensitized Solar Cells (DSSCs) are at verge of commercialization of technology as it is successfully
demonstrated to Building Integrated Photo-Voltaic technology (BIPV) and open up avenues towards transparent solar
cell technology [1-3]. Grätzel et al. in 1991 first reported DSSC with an efficiency of 7.1% using N719 sensitizer which
upon continuous efforts has ramped up to 14.7% using metal-free organic sensitizer very recently [4, 5]. The efficiency
of the device was doubly enhanced mainly due to the use of panchromatic sensitizers that can absorb the sun light over
a broad range of solar spectrum and convert it to electricity. In this regard, a great variety of sensitizers including Ru(II)
polypyridyl complexes, other metal complexes, metal-free organic dyes and pigment based molecules have been
developed and used as light harvesters in DSSC [6-10]. Among various classes of sensitizers employed, porphyrin
based dyes have attracted special attention mainly due to their high molar extinction coefficients, feasibility to
accommodate different chromophores at meso-positions and synthetic viability tune its opto-electronic properties [11].
For this reason, researchers have paid special attention on porphyrin based after outshine the efficiency obtained by
Yella and co-workers using porphyrin based sensitizers in Donor-π-Acceptor fashion [12]. However, D-π-A porphyrin
sensitizers involve tedious synthetic protocols and difficulty to scale up to multi gram quantities which hampers their
use in solar modules for BIPV. For this reason, many research groups including our groups have performed structural
modifications on porphyrin macrocycle with desired architecture such as A
electronic properties with enhanced yields [14-17]. Among these porphyrin architectures, we are particularly interested
in A B type due to loss of symmetry that disturbs the π-delocalized electrons creating a dipole moment, which further
4 3 2 2 2
, A B, A B , and A BC to tune the opto-
3
renders us to modulate the electrochemical and photophysical properties. In addition, this architecture allows
introducing bulky groups at meso- position of the porphyrin which promotes the solubility, hinders π-π stacking that
minimizes the aggregation and recombination phenomena in the device.
3
Imahori et al. designed a series of A B zinc porphyrins with varying meso- substituents and revealed conversion
efficiency of 4.6% using I-/I
-
redox couple [18]. In contrast, electron rich triphenyl amine-based (TPA) donors show
3
effective optical and electrochemical properties and exhibit excellent charge mobilities. A great variety of TPA based
materials were designed and used for various optoelectronic applications such as OLED, PSCs, OFETs and DSSCs [19,
2
0]. Grätzel and co-workers have designed triphenyl amine and ethynylenephenylene appended porphyrins and showed
that they exhibit conversion efficiencies of 3-4.5% [21]. Susana et al. have introduced hexyl thiophene π-bridge
between porphyrin marocycle and anchoring group and reported efficiencies up to 6% in A B type porphyrin sensitizers
14]. Recently, our groups have reported A B type porphyrin sensitizers containing donor hexyl-phenothiazine moieties
3
[
3
appended at three meso- positions of the porphyrin core and an acceptor as either phenyl carboxylic acid (G10) or
thiophene carboxylic acid (G11). The sensitizer with phenyl carboxylic acid acceptor i.e., G11 has shown conversion
efficiency of 5.40% using liquid I-/I
-
redox couple, which is highest among A
B type porphyrin sensitizers, we realized that the
Bis(4′-tert-butylbiphenyl-4-yl)aniline) (BBA) as a better contender than TPA as it has an intriguing ability to stabilize
the charge separated and exhibit the cation radical at 745 nm and 900 nm [22]. In the present work, we have designed a
B porphyrin sensitizer in which the three meso-positions were decorated with donor BBA moieties and the other
3
3
B type porphyrin sensitizers reported till
date [13]. In continuation of our efforts towards the development of A
3
(
A
3
meso-position was functionalized with phenyl carboxylic acid which can play a dual role as acceptor and anchoring
group (Chart 1). Systematic investigation of optical and electrochemical properties of GB revealed a good alignment of
HOMO-LUMO positions of the dye with the TiO
2
/electrolyte as well as absorption in the range of 300-625 nm.
Theoretical studies also demonstrated localization of the HOMO on donor BBA while LUMO orient on terminal
2
benzoic acid moiety indicating that the feasibility of light induced electron transfer via D-π-A group into TiO . More
2

Accepted manuscript to appear in JPP
importantly, bulky BBA unit retards the charge recombination and inhibits the dye aggregations leads to exhibit Jsc of
2
1
1.12 mA/cm and the performance up to ~3.9% in dye-sensitized solar cells using liquid redox electrolyte.
Chart 1.
2
. EXPERIMENTAL SECTION
2
.1. Materials
Commercially available chemicals and needed reagents were procured from TCI and Sigma-Aldrich. All the
reactions were carried out under nitrogen or argon atmosphere using dry and degassed solvents. AR (Analytical
reagent) grade solvents were used for the reactions while LR (laboratory reagent) grade solvents were used for
purifications and column chromatography. ACME silica gel (100-200 mesh) was used in column chromatography and
either gravity or flash chromatography for purification of the compounds. Thin-layer chromatography was performed
on Merck-pre-coated silica gel 60-F254 plates.
2.2. Synthesis
Bis(4′-tert-butylbiphenyl-4-yl)aniline (BBA) and 4-(Bis(4-tert-butylbiphenyl-4-yl)amino)benzaldehyde (BBA-CHO)
were synthesized according literature methods [22,23].
Synthesis 1: 1 was synthesized according to a modified literature procedure [20]. To a solution of BBA-CHO (1.96 g,
3
.65 mmol) and methyl-4-formylbenzoate (0.2 g, 1.218 mmol) in dry CHCl
3
(200 mL) were added pyrrole (0.32 g, 4.87
mmol) and BF ·O(Et) (0.17 g, 1.218 mmol) and the solution was stirred at r.t. for 90 min. DDQ (1.02 g, 4.5 mmol) in
3
2
THF (5 mL) was added and stirred further for 45 min. After the solvent was evaporated, the residue was separated by
1
silica gel column chromatography (hexane-THF 3:1 v/v) to afford 1 (10.2% yield) as dark purple powder. H NMR (δ,
ppm): 9.07-9.04 (m, 4H), 8.82 (d, J = 4.8 Hz, 2H), 8.46 (d, J = 8.3Hz, 2H), 8.33 (d, J =8.2 Hz, 2H), 8.13(d, J = 8.3 Hz,
6
H), 7.66 (dd, J = 8.6, 1.6 Hz, 12H), 7.61-7.55 (m, 18H), 7.52-7.48 (m, 26H), 4.14 (s, 3H), 1.38 (s, 54H), -2.66 (s, 2H).
MALDI-TOF: (m/z) found 1968.85 (calculated 1967.67).
Synthesis of GB: 1 (0.2g, 0.102mmole), LiOH.H
was refluxed overnight. The reaction was quenched with water and extracted with CH
under vacuum and Zn(OAC) (56mg, 0.305mmol) in DCM and MeOH (2:1 v/v) added. The reaction mixture was
stirred at r.t. for 12 h. The reaction was quenched with water; the organic layer was washed with brine and dried over
2
O (17.06mg, 0.406mmol) were taken in THF and the reaction mixture
2
2
Cl . The solvent was removed by
2
anhydrous Na
2
SO
4
. The solvent was evaporated and the residue was subjected to column chromatography on silica gel
1
(
Hexane/THF 5:1, v/v) to afford GB (69% yield) as green purple powder. H NMR: 9.08-9.06 (m, 6H), 8.66 (d, J = 4.2
Hz, 2H), 8.41 (d, J = 8.6 Hz, 2H), 8.15(d, J = 8.5 Hz, 6H), 7.67-7.65 (m, 12H), 7.62-7.59 (m, 18H), 7.52-7.49 (m, 26H),
1
.39 (s, 54H). MALDI-TOF: (m/z) found 2019.65 (calculated 2017.5).
Scheme 1.
2
.2 Instrumentation
1H-NMR spectra were obtained at 400 MHz using a Bruker, 300 Avance NMR spectrometer running X-WIN NMR
3
software. The chemical shifts were taken relative to the TMS (tetramethylsilane) and chloroform-d (CDCl ) was used as
the internal references. The elemental analyses were performed on an Elementar, Vario MICRO CUBE analyzer.
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Accepted manuscript to appear in JPP
Matrix-assisted laser desorption ionization time-of-flight (MALDI–TOF) mass spectrometry performed on Shimadzu
Biotech Axima Performance 2.9.3.20110624: Mode Reflectron-HiRes, Power: 85. Major fragmentations are given as
percentages relative to the base peak intensity. Cyclic voltammetric measurements were performed on a system
controlled CH instruments model CHI 620C electrochemical analyzer. The experiments were performed at 1 mM
sample solution in tetrahydrofuran solvent at a scan rate of 100 mV/s using 0.1 M tetrabutylammonium perchlorate
(
TBAP) as a supporting electrolyte. The working electrode is glassy carbon, standard calomel electrode (SCE) is used
as reference electrode, and platinum wire is a counter electrode. Mettler Toledo TGA/SDTA 851e instrument was used
-
1
for the thermogravimetric measurements at a heating rate 10°C min with 10 mg of sample.
The absorption spectra were recorded on a Shimadzu (Model UV-3600) spectrophotometer at concentrations, 1 × 10-
6
-5
M for soret band and 1 × 10 M for Q band absorption. Spex model Fluorlog-3 was used for recording steady state
fluorescence spectra for solutions with optical density at the wavelength of excitation (λex) ≈ 0.15.
GB dye was optimized to its global minimum energy using B3LYP hybrid functional [24] and 6-31(d,p) basis set as
the input [25]. Further calculations were performed by Gaussian 09 (Revision B.01) ab initio quantum chemical
software on the personal computer [26]. Ground state properties such as energy-minimized structures, frontier
molecular orbitals (FMOs) were implemented by Density Functional Theory (DFT) in the gas phase. Excited state
properties such as percentage of molecular contribution, oscillatory strength, singlet transition energy were executed by
Time Dependent Density Functional Theory (TDDFT) in the tetrahydrofuran solvent. TDDFT calculations to describe
the solvation of the dyes in tetrahydrofuran were carried out by the integral equation formalism polarizable continuum
model (PCM) within the self-consistent reaction field (SCRF) theory [27, 28]. To simulate the major portions of
absorption spectra and interpretation of nature of transactions were carried by employing the software Gauss Sum 2.2.5
[
29, 30]. The contribution percentages of individual units present in the dyes to the respective molecular orbitals were
calculated.
Device fabrication
The details for the preparation of TiO
2
electrode and photovoltaic device fabrication were reported in our previous
nano particle layer (∼20 nm
reports. [31, 32]. Briefly, for the working electrode, the paste composed of transparent TiO
2
diameter) was screen printed on fluorine-doped tin oxide (FTO) conducting glass substrate (transmission >85% in the
2
visible, sheet resistance 10 Ω/square). The required thickness (8μm) and cell area (0.25 cm ) was obtained by repetitive
screen printing. Subsequently, an additional scattering layer made of anatase TiO
nm in diameter) was screen printed on the active transparent layer to improve the device performance. Now, the screen
printed double layer sintered at 450°C for 1 h in air. To coat the TiO film, the dye was dissolved in 1:1 (v/v)
acetonitrile/tert-butyl alcohol at 0.2 mM concentration. The prepared TiO films were immersed in these dye solutions
while TiO films are at 80°C to remove moisture from TiO pores and kept at r.t for 16 h and dye was adsorbed onto
these films. After complete adsorption of the dye onto TiO surface photo-electrode was removed from the solution, and
the non-adsorbed dye was removed by washing with acetonitrile under a stream of dry air or argon. The Pt-counter
2
particles of 5 µm thickness (∼400
2
2
2
2
2
electrode was prepared by the spin coating of H
2
PtCl
6
solution onto FTO conducting glass substrate and heated to
3
85°C temperature for 15 min. The dye-coated TiO
2
film and thermally platinized FTO conducting glass counter
electrode were separated by a Surlyn spacer (40 µm thick) and sealed by heating the polymer frame at 100°C. Finally,
the DSSC cells were assembled by the electrolyte solution consisting of a mixture of 0.6 M dimethylpropyl-
imidazolium iodide, 0.05 M I
2
, 0.1M LiI, and 0.5 M tert-butylpyridine in acetonitrile. A black metal mask with an area
2
-2
of 0.25 cm under AM 1.5 sunlight (100 mW cm , WXS-155S-10: Wacom Denso Co. Japan) were used to measure the
4

Accepted manuscript to appear in JPP
current-voltage characteristics. The incident photon-to-current conversion efficiency (IPCE) spectra were measured
with a monochromatic incident light of 1 x 1016 photons cm in direct current mode (CEP-2000BX, Bunko-Keiki).
-2
3
3
. RESULTS AND DISCUSSION
.1. Synthesis and characterization
The optimized synthetic pathway of the novel GB dye is outlined in Scheme 1. The synthesis of BBA-CHO was
achieved by preparing BBA by arylation of aniline with p-tert-butylbiphenyl followed by Vilsmeyer formylation. The
ester substituted porphyrin, 1 was synthesized by condensation of BBA-CHO and methyl-4-formylbenzoate with
3 2
pyrrole in presence of BF .OEt and DDQ using Lindsey method [20]. Hydrolysis of the ester using LiOH followed by
metalation with zinc acetate yielded GB as green powder in 40% yield (Scheme 1). The sensitizer GB was
1
characterized by various spectroscopic techniques and electrochemical methods. H NMR spectra confirms the
molecular integrity of the GB sensitizer (Figure S3). The MALDI-MS spectrum of GB showed a peak found at 2019.65
(
M+2) (calculated 2017.5) confirms molecular ion peak.
3
.2. Optical and electrochemical studies
Figure 1 demonstrates the optical absorption spectra of GB dye in THF solution as well when anchored onto 2 μM
2
thick TiO layer and the corresponding optical data were presented in Table 1. The optical absorption spectrum displays
four prominent absorption bands in 300-650 nm region in which absorption peak at higher energy i.e., at 340 nm can be
attributed to the donor BBA moiety as in this region porphyrin has minimum absorption [20]. In contrast, two less
intense Q-band absorption appeared in 500-650 nm region arise due to a2u(π)/e
first excited state (S ), and an intense soret band appeared at 441 nm arises due to a1u(π)/e
contributes to the second excited state (S ). The absorption maxima of GB dye is hypsochromic shifted when compared
to phenothiazine substituted A B porphyrins i.e., G10 and G11 sensitizers [13]. On the other hand, when coated on
TiO , the soret and Q bands of the GB dye exhibited a strong bathochromic shift, when compared to solution spectrum,
probably due to the interaction of anchored carboxylic group with nanocrystalline TiO [33]. The absorption band edge
of GB dye was observed at 620 nm in solution and at 701 nm on TiO indicating that the structural geometry when
anchored on TiO might have led to the formation of J-aggregations (π-π stacking) [34], such as larger bathochromic
g
(π*) electronic transition, ascribed to the
1
g
(π*) electronic transition
2
3
2
2
2
2
shift of the sensitizer would lead to greater light capture and hence, increases power conversion efficiency. When
excited at 440 nm in THF, the GB dye exhibited emission with maximum at 621 nm (Fig. 1). The intersection of
absorption and photoluminescence spectra gave band gap of the GB dye of 2.1±0.05eV and the band gap enhanced
when compared to G10 and G11 sensitizers i.e., 1.85±0.05 eV [13]. The quantum yield of GB dye (Table 1) was
enriched in appraisal with reference compound 5,10,15,20-tetraphenyl zinc porphyrin (ZnTPP) [35]. Also, the singlet
excited lifetime was analysed in THF solvent (Figure S6), and it was found to be 1.58 ns (Fig. S6).
Fig. 1.
Table 1.
Subsequently, we measured the redox potentials of GB dye in THF solvent by using cyclic voltammetry technique
which is further used to estimate the HOMO and LUMO positions of the dye. Under the experimental conditions, GB
dye undergoes two one electron, reversible oxidations at 0.82 and 1.16 V vs SCE (The first oxidation peak +0.82V peak
is corresponding to BBA) and a one electron reduction occurred at -0.93 V vs. SCE (Fig. 2a) belongs to porphyrin. The
5

Accepted manuscript to appear in JPP
first oxidation potential of the GB dye was used to calculate HOMO position and LUMO was estimated by subtracting
2
the band gap from its HOMO level [36]. The resulting energy levels are found to be apt for the TiO and electrolyte
energy levels. Both experimental and calculated HOMO and LUMO energy levels (vide infra) are in good agreement to
each other and found to be close to N719 dye energy levels (see Fig. 2b). These results indicated that improved short-
circuit current density (JSC) and open-circuit voltage (VOC) will be exhibited by the GB dye.
Fig. 2.
In DSSC it is very essential to understand the electronic properties of oxidized sensitizers because in case of Ru(II)
polypyridyl complexes, the oxidized species are found to be not stable due to the presence of labile –NCS ligands
leading to the replacement with iodide ligand as a result it loses its optical and electronic properties of the complexe
[
37]. For this reason, we have executed spectroelectrochemical studies of GB sensitizer to understand its electronic
properties of oxidized species. During controlled potential oxidation of GB at +0.90 V vs. SCE, both the soret band and
Q band absorption decreases its intensity while the absorption of BBA at 340 nm enhanced its intensity (Figure 3).
During this process, isosbestic points were observed at 367 nm, 394 nm, 470 nm, and 585 nm and these results
suggested that the oxidation of a single product. These characteristic changes indicated that the oxidation is mainly ring
centred rather than the metal [38]. The oxidation of GB sensitizer was clearly suggested by formation of isosbestic
points at 405, 485, 550 and 598 nm which tell us that there is formation of a single product. On the other hand, the
absorption spectrum of the dye was recovered when applied potential was removed suggesting that the porphyrin
sensitizer is stable under electrochemical potential applied.
Fig. 3.
3
.3. Computational studies
Density functional simulations (DFT) using Gaussian 09 software were performed to gain insight into the
electronic and structural properties of the isolated GB dye. Electron injection ability of the light harvesting material
into TiO ) was investigated through density functional theory (DFT) simulations at B3LYP/6-31G (d, p) level [39].
(
2
Energy optimized structure of GB dye (as shown in Figure 4a) was adopted as zero imaginary frequency with minimum
energy and BBA moiety and anchoring groups are oriented as perpendiculer to the porphyrin ring. Figure 4b represents
the electron density distribution of GB sensitizer suggest that HOMO (-4.98 eV) localized on BBA moiety and partially
on porphyrin, on the other hand LUMO (-2.30 eV) delocalized over porphyrin and anchroing carboxylic acid apposite
2
for efficient electron injection from excited state of dye to conduction of band of TiO . On the other hand, HOMO-1
and HOMO-2 levels distributed on BBA moiety whereas LUMO+1 and LUMO+2 on porphyrin moiety and anchoring
group and thus theoretically calculated HOMO-LUMO band gap of GB sensitizer 2.68 eV, which is high when
compared to G10 2.54 eV) and G11 (2.58 eV) A3B sensitizers [13]. Fig. S5 shows that the spatial distribution of
LUMO+1 is restricted on the porphyrin marcocyle, indicating that an optical trnsition between the HOMO and the
LUMO+1 would be more favourable. Therefore, calculated HOMO-LUMO levels of isolted GB dye shown in partial
agreement with cyclic voltammetric results (see Table 1). Furthermore, the donor BBA make possible minmizing the
aggregation (Figure 3a) that will enhance the PET mechanism in GB dye, which leads to high device efficiency.
Fig. 4.
3
.4. Device Studies
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Accepted manuscript to appear in JPP
Fig. S5 The performance of newly designed dey GB as a sensitizer was determined by performing measurements
on photovoltaic device using a liquid redox couple and compared their results with other literature reported
triphenylamine substituted porphyrin of A
photon-to-current conversion efficiency (IPCE) is plotted as a function of wavelength. The photocurrent action
spectrum resembles the absorption spectra (on TiO , Figure 1) except for slight red shift with a dip at around 550 nm
with IPCE of ~45% at 450 nm (soret band region) and 650 nm (Q band region). The red shift of the absorption is,
probably, due to π-π stacking of porphyrin on TiO surface because of planar nature of porphyrin macrocycle. It is
3
B type. The photocurrent action specrum is shown in Figure 5a, where the
2
2
know in literature that the addition of co-adsorbant 3a,7a-dihydroxy-5b-cholic acid (chenodeoxycholic acid, CDCA)
would reduce the π-π stacking and enhance the device performance [39]. For this reason, we have added 60 mM CDCA
in dye solution and fabricated the device. As a result the dip at around 550 nm region was vanished and continuous
IPCE spectrum from soret band to Q band region with IPCE of 50% was observed. In both the cases, onset of IPCE
extends up to 800 nm suggesting a higher conversion of sunlight into electricity. Figure 5b illustrates the current-
2
voltage characteristics with black metal mask (active area 0.25 cm ) under the standard conditions (AM 1.5G, 100 mW
-
2
cm ). The resultant average photovoltaic parameters (Jsc, Voc, FF, η) of GB sensitizer in liquid redox electrolyte;
2
efficiency (η) = 3.34±0.26%, Jsc = 10.98±0.37 mA/cm , Voc = 0.62±0.002 V, and FF = 0.49±0.10. The device
2
parameters enhanced to (η) = 3.83±0.34%, Jsc = 11.12±0.43 mA/cm , Voc = 0.62±0.002 V, and FF = 0.56±0.10 after
addition of 60 mM concentration of CDCA additive. Thus, it is confirmed that the addition of co-adsorbant, CDCA
further prevented dye-aggregation on TiO
2
surface and enhanced the Jsc and ultimately, the device performance. The
B type porphyrins are
reasons for the low efficiency of the device when compared to other triphenylamine based A
3
might be due to that the presence of extend π-conjugation through heterocyclic aromatic unit between donor
triphenylamine-porphyrin and anchoring group. As a result the HOMO-LUMO gap reduced with absorption maxima
bathochromically shifted which can more photons and converted it into efficiencies up to 4-6% [13, 21].
Fig. 5.
Table 2.
Finally, we have carried out thermal stability of GB sensitizer, which is a pre-requisite for long term DSSC
device applications. For this reason, we have carried out thermo gravimetric analysis (TGA) of GB sensitizer.
TGA/DTG graphs of GB dye showed that it is stable up to 400°C (Figure 5) and initial weight loss of 5.10% observed
between 220°C to 350°C is attributed to the loss of moisture and carboxylic group. Therefore, this BBA donor based
A
3
B zinc porphyrin (GB) sensitizer is thermally stable and generated moderately efficient DSSCs.
Fig. 6.
4
. CONCLUSION
A novel Bis(4′-tert-butylbiphenyl-4-yl)aniline (BBA) donor based A
sensitizer in DSSC applications. DFT and electrochemical energy levels reveal that LUMO of the GB dye is well-suited
for electron injection into TiO conduction band and it’s HOMO for dye-regeneration from the redox couple. GB dye
exhibited strong UV-visible-NIR absorption when anchored onto TiO nanoparticle layer and hence maximized the sun
3
B Zinc porphyrin (GB) was introduced as
2
2
light harvesting capacities. Consequently, the optimized device exhibited a photo-current density of 10.98 ± 0.37 and
7

Accepted manuscript to appear in JPP
efficiency of 3.34 ± 0.26%. Furthermore, upon addition of the co-adsorbent CDCA the device performance was
improved efficiency 3.83 ± 0.34% due to inhibition of the molecular aggregation. Therefore, broad absorbing porphyrin
3
dyes with A B architecture are more beneficial due to ease of synthetic protocols for the dye sensitized solar cells
towards the commercialization in near future.
Acknowledgements
This work was financially supported by the Department of Science and Technology under solar energy research
initiative (DST-CERI, DST/TM/CERI/C228 & DST/TMD/CERI D46) programme. Author RC is grateful to
Department of Science and Technology (DST, CRG/2018/002661). Authors ND thanks to CSIR for senior research
fellowship and Govind Reddy for his support. We thank Director CSIR-IICT for the support (IICT/Pubs./2020/135).
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Chart 1. Structure of GB dye.
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Scheme 1. Synthetic pathway for the newly designed GB dye.
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1
0
0
0
0
.2
.9
.6
.3
.0
16
12
8
Abs. in solution
Abs. on TiO
Em. in solution
GB
2
BBA
ex = 440nm
4
0
800
3
00
400
500
600
700
Wavelength (nm)
Figure 1. Optical absorption spectra of GB dye in THF (black), upon adsorption on 2 μM thick TiO
2
layer (red), and steady state
emission spectra in THF (blue).
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Figure 2. Cyclic voltammograme of GB sensitizer. (b) Molecular energy level diagram with GB dye (in comparison with N719)
electrodes and redox electrolyte.
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1.0
0.8
0.6
0.4
0.2
0.0
GB Dye
3
00
400
500
600
700
800
Wavelength(nm)
Figure 3. Oxidative OTTLE (Full form should be mentioned) studies of GB dye in 0.3 M TBAP/THF at an applied potential of 0.90
V (vs. SCE/KCl).
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Figure 4. Simulated optimized structure (b) energy levels of the GB dye at B3LYP/6-31 G (d, p) level.
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Figure 5. a) IPCE spectra and b) J-V characteristics of GB sensitizer.
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0
0
0
0
0
0
,5
Derivative curve
GB dye
,4
,3
,2
,1
,0
1
00
200
300
400
500
600
700
Temperature (°C)
o
-1
Figure 6. TGA/DTG curves of GB dye at constant heating rate of 10 C min under N
2
atmosphere.
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Table 1. Optical, and electrochemical data.
-
em(Ф)b
c
c
Dye
GB
λ
abs nm(log ɛ M
λ
λ
onset, nm
E
ox
E
red HOMO E
g
LUMO
0.82 -0.93 -5.36 2.10 -3.30
.16
1
-1 a
cm )
340 (4.93)
621 (0.18)
701
4
5
6
41 (5.43)
57 (4.21)
02 (3.96)
1
aSolvent CH
Cl
, Error limits: λmax, ± 1 nm, log ε, ± 10%. Quantum yield of sensitizer is measured with the reference 5,10,15,20-
a
2
2
c
tetra-phenyl zinc porphyrin (ZnTPP). THF, 0.1 M TBAP; Glassy carbon working electrode, Standar calomel electrode is reference
electrode, Pt electrode is auxillary electrode. Error limits, E1/20.01 V,
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Table 2. Device parameters of the GB dye.
Dye
Voc (V)a Jsc (mA/cm2)b
FFc
PCE (%)d Adsorbed amount of dye
-
2
(
mol.cm )
GB
0.62 ± 0.002 10.98 ± 0.37 0.49 ± 0.10 3.34 ± 0.26
1.19 x 10-7
1.03 x 10-7
GB-CDCA 0.62 ± 0.004 11.12 ± 0.43 0.56 ± 0.10 3.83 ± 0.34
a
b
c
d
Open-circuit photovoltage (Voc), Short-circuit photocurrent density (Jsc), Fill factor (FF), Conversion efficiency (η).
2
0