Thiocyanate-free ruthenium(II) sensitizers with a bi-imidazole ligand in dye-sensitized solar cells (DSSCs)

Article abstract of DOI:10.1039/c7nj01363a

In this study, tris-heteroleptic Ru(ii) sensitizers denoted as SD-15 and SD-16 with a 4,4′,5,5′-tetramethyl-1H,1′H-2,2′-bis-imidazole moiety as the ancillary ligand were designed, synthesized and characterized using FTIR-ATR, ¹H-NMR and UV-vis spectrophotometry. Structural modulations resulted in a red shift in the low-energy metal-to-ligand charge transfer (MLCT) absorption peak. The novel bis-imidazole based Ru(ii) sensitizer was adsorbed on a TiO₂ surface via -COOH anchoring groups. The band gap, and the ground and excited state oxidation potentials were measured. The photovoltaic performance of the as-fabricated solar cells was measured and showed that the presence of bis-imidazole provided more negative free energy for dye regeneration (-ΔG_regeneration) and less driving force for electron injection (-ΔG_injection) into the conduction band edge of TiO₂. Therefore, the use of SD-15 and SD-16 resulted in lower J_SC values of 10.20 and 3.52 mA cm^-2, respectively, than N719. The open-circuit photovoltage (V_OC) of SD-15 and SD-16 was 0.58 and 0.52 V, respectively, less than N719 under the same experimental conditions due to the decreased eTiO₂ lifetime of 0.71 ms compared to 8.8 ms of N719 for the injected electrons that was calculated using impedance measurements. Our efforts in this research program led to the development of more stable Ru(ii) complexes compared to N719 NCS-based complexes; however, the conversion efficiency was lower. Hence, the fine-tuning of bi-imidazole is needed to identify more efficient NCS-free Ru(ii) sensitizers for higher efficiency DSSCs.

Full text of DOI:10.1039/c7nj01363a

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PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
rsc.li/njc

Page 1 of 16
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ThiocyanateꢀFree Ruthenium(II) Sensitizers with A biꢀimidazole
Ligand in DyeꢀSensitized Solar Cells (DSSCs)
Saba Ashrafa,c, Javeed Akhtarb, Humaira M. Siddiqia, Ahmed ElꢀShafeic*
a Department of Chemistry, QuaidꢀIꢀAzam University, Islamabad 45320, Pakistan
b Department of Physics, Polymers & Materials Synthesis Lab (PMS), COMSATS, Institute of
Information Technology, Chak Shahzad Islamabad, Pakistan
c Department of Textile Engineering, Chemistry & Science, North Carolina State University,
Raleigh, NC, 27695, USA
*Corresponding author eꢀmail: Ahmed_ElꢀShafei@ncsu.edu (Prof. Dr. Ahmed ElꢀShafei)
ABSTRACT
In this study, trisꢀheteroleptic Ru(II) sensitizers denoted as SDꢀ15 and SDꢀ16 with 4,4',5,5'ꢀ
tetramethylꢀ1H,1'Hꢀ2,2'ꢀbisꢀimidazole moiety as ancillary ligand were designed, synthesized and
1
characterized by FTIRꢀ ATR, HꢀNMR and UVꢀVis spectrophotometry. Structural modulations
resulted in red shift in lowꢀenergy metalꢀtoꢀligand charge transfer (MLCT) absorption peak. The
novel bisꢀimidazole based Ru(II) sensitizer was adsorbed on TiO2 surface via –COOH anchoring
groups. Band gap, ground and excited state oxidation potentials were measured. Photovoltaic
performance of asꢀfabricated solar cells was measured and showed that the presence of bisꢀ
imidazole which provided more negative free energy for dye regeneration (–∆Gregeneration) and
less driving force for electron injection (–∆Ginjection) into the conduction band edge of TiO2.
Therefore, SDꢀ15 and SDꢀ16 resulted in lower JSC of 10.20 and 3.52 mA cmꢀ2, respectively, than
N719. Openꢀcircuit photovoltage (VOC) of SDꢀ15 and SDꢀ16 was 0.58 and 0.52 V less than N719
under same experimental conditions due to decreased eTiO2 life time of 0.71 ms compared to 8.8
ms of N719 for injected electrons calculated by impedance measurements. Our effort in this
research program led to the developments of more stable Ru(II) complexes compared to N719
NCSꢀbased complexes; however, the conversion efficiency was lower. Hence, fineꢀtuning of biꢀ
imidazole is needed to identify more efficient NCSꢀfree Ru(II) sensitizers for higher efficiency
DSSCs.

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1. INTRODUCTION
In recent years, efforts have been made to explore the efficient ways to harness the sunlight.
Solar energy is getting an exceptional attention, which indicates quick advancement, particularly
in the field of photovoltaics, opening up new horizon in research and innovation for utilizing
renewable energy sources.1 One of the major advancement in photovoltaics field is dyeꢀ
sensitized solar cells (DSSCs), introduced by O' Regan and Grätzel in 1991.2 They have
captivated deep consideration because of their lowꢀcost fabrication process and ready to work at
more extensive edges and under low light conditions.3 Sensitizer (dye molecules) is a key
component of DSSC, which absorbs light from the visible region and then initiates the
movement of electron from highest occupied molecular orbital (HOMO) to lowest unoccupied
molecular orbital (LUMO) of dyes.4
Although the DSSC showed moderate to good efficiency, but still longꢀterm stability of
solar devices is not promising.5 Sensitizers can undergo degradation quickly owing to the
presence of thiocyanate (ꢀNCS) moiety, thus lowering the lifetime of solar devices.6 A number of
sensitizers have been tested including N719, N3 and black dye7,8 but they also are very unstable
due to labile nature of thiocyanate moiety present in asꢀreported sensitizers.9,10 To enhance the
longꢀterm stability of solar devices, a number of attempts have been made to prepare thiocyanate
(ꢀNCS) free sensitizers.11,12 Such sensitizers offer extended and enhanced structural stability to
devices for example, pyridꢀ2ꢀyl pyrazolate or triazolate,13,14 pyridylꢀazolate,15 pyridꢀ2ꢀ
yltetrazolate,16 8ꢀoxyquinolate,17,18 dianionic bipyrazolate19 based solar devices showed moderate
to good efficiency (1.3 to 9.6 % ), thus paving the way to explore and design a better and
efficient thiocyanate free sensitizers.
Recently, H. Cheema et al. reported thiocyanteꢀfree ruthenium dye (HDꢀ11) with the
addition of 3ꢀtrifluoromehtyl pyrazole10 which showed % η upto 5.2%. Similarly, C. P.
Berlinguette and coꢀworker tested phenylpyridine (ppy)20 as thiocyanateꢀfree sensitizer in DSSC
and the molecular structures of both the dyes are shown in Fig. 1. In this work, we report biꢀ
midazole based thiocyanate free ruthenium sensitizers and to the best of our knowledge, these
has not been investigated in literature for DSSC applications as shown in Fig. 2.

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HDꢀ11
3
Fig. 1. Molecular structures of thiocyanateꢀfree trisꢀheteroleptic Ru(II) sensitizers10,20
1
The asꢀprepared SDꢀ15 and SDꢀ16 compounds were characterized by FTIR and HꢀNMR. The
photophysical and electrochemical characterizations were carried using UVꢀVisible spectroscopy
and cyclic voltammetry. Impedance spectroscopy was used to study interfere charge
recombination resistance and electron life time at dye/TiO2 interface. IPCE and IꢀV graph for
solar cells sensitized with SDꢀ15 and SDꢀ16 were measured under 1 sun illumination with
similar conditions.
SDꢀ15
SDꢀ16
Fig. 2. Chemical Structure of thiocyanateꢀfree Ru(II) dyes.
2. EXPERIMENTAL

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2.1. MATERIALS AND EQUIPMENTS
All chemicals were purchased from SigmaꢀAldrich, Fisher Scientific or TCIꢀAmerica and were
used as received. Silica (230ꢀ400 mesh, Grade 60) for purification of dyes was purchased from
Fischer Scientific. FTꢀIR (ATR) spectra were recorded on a Nicolet Nexus 470 FTIR
spectrometer (Thermo Scientific, USA) and UVꢀVisible spectra were measured by using Cary
300 spectrophotometer. The Mass Spectrometry Analysis was carried out on a highꢀresolution
mass spectrometer – the Thermo Fisher Scientific Executive Plus MS, a bench top fullꢀscan
OrbitrapTM mass spectrometer using Electroꢀspray Ionization (ESI). Samples were dissolved in
methanol, were sonicated for 15 minutes, and then diluted 1:1 with 20 mM ammonium acetate
1
and analyzed via syringe injection into the mass spectrometer at a flow rate of 10 µL/min. Hꢀ
NMR spectra were recorded in a Bruker 500 MHz spectrometer. Fluorologꢀ3 spectrofluorometer
(HORIBA Jobin Yvon Inc.) was used to measure fluorescence at room temperature.
2.2. Synthesis of the thiocyanateꢀfree trisꢀheteroleptic Ru(II) sensitizers
The synthesis of both the ancillary ligands was carried out according to the procedure reported in
literature using Knoevenagel condensation type reaction. Detailed synthetic procedures for the
proposed thiocyanateꢀfree trisꢀheteroleptic ruthenium(II) complexes SDꢀ15 & SDꢀ16 can be
found in the Supporting Information (SI). Typical three step one pot procedure was employed to
synthesize the proposed sensitizers according to reported literature.10 The yield of the crude
products was in the range of 65–70%, which were purified through the Silica column for two
times to get the highly pure product in 45–50% yield. The pure product was then characterized
by FTIR and 1HꢀNMR.
1
Synthesis of sensitizer SDꢀ15. Dark red solid (140 mg, yield 69%). H NMR (500 MHz,
DMSOꢀd6): δ 9.93 (s, 1H), 9.02 (s, 1H), 8.76 (d, J = 5.7 Hz, 1H), 8.53 (s, 1H), 8.34 – 8.29 (m,
2H), 8.22 (d, J = 8.9 Hz, 1H), 7.71 – 7.66 (m, 4H), 7.62 – 7.57 (m, 2H), 7.51 (s, 1H), 7.16 (d, J =
16.3 Hz, 1H), 6.92 (d, J = 15.7 Hz, 1H), 4.50 (q, J = 9.5, 3.7 Hz, 2H), 2.48 (s, 6H), 2.19 (s, 6H),
1.46 (t, J = 8.6 Hz, 3H).
1
Synthesis of sensitizer SDꢀ16. Reddish orange solid (128 mg, yield 66%). H NMR (500 MHz,
DMSOꢀd6): δ 9.78 (s, 1H), 8.90 (d, J = 5.0 Hz, 1H), 8.85 (s, 1H), 8.63 (m, 2H), 8.42 (d, J = 4.6

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Hz, 1H), 7.90 (d, J = 5.1 Hz, 1H), 7.27 (d, J = 14.3 Hz, 1H), 7.15 – 7.06 (m, 4H), 7.03 – 6.94 (m,
10H), 6.89 (d, J = 14.7 Hz, 1H), 2.56 (s, 6H), 2.20 (s, 6H).
2.3. Time correlated single photon counting (TCSPC) measurements
Fluorescence spectra were recorded in a 1 cm path length quartz cell using 2×10ꢀ5 M solutions on
Fluorologꢀ311. The emitted light was detected in the steady state mode using a Hamamatsu
R2658 detector. The emission was measured in the steady state mode by exciting at the MLCT
λmax for each dye with exit and entrance slits set at 11 nm with 0.1 s integration time.
2.4. Ground state oxidation potential (GSOP) and excited state oxidation potential (ESOP)
measurements
The experimental HOMO and E0ꢀ0 energy values for SDꢀ15 and SDꢀ16 were measured using a
cyclic voltammetry (CV) and the absorption onset, respectively. The CV was carried out in DMF
with 0.1M [TBA][PF6]– as an electrolyte at a scan rate of 50 mV sꢀ1. Glassy carbon was used as
the working electrode (WE), Pt wire as counter electrode and Ag/Ag+ in acetonitrile was used as
the reference electrode. Fc/Fc+ was used as an internal reference, voltage measured was
converted to NHE by addition of 0.63 V. Figures 3S–4S (SI) show the CV graphs of all the
sensitizers.
2.5. TiO2 electrode preparation and device fabrication
The photoꢀanodes composed of nanocrystalline TiO2 and counter electrodes were prepared using
literature procedures.21 Fluorineꢀdoped tin oxide (FTO) coated glasses (2.2 mm thickness, sheet
resistance of 8 ꢁ/cm2, TEC 8, Pilkington) were washed with detergent, deionized water, dry
acetone and absolute ethanol, sequentially. After this FTO glass plates were immersed into a 40
mM aqueous TiCl4 solution at 70 ºC for 30 min and washed with water and ethanol. Thin layer
(8ꢀ12 mm thick) of TiO2 (Solaronix, TiꢀNanoxide T/SP) was deposited (active area, 0.18 cm2) on
transparent conducting glass by squeegee printing. After drying the electrodes at 500 ºC for 30
min, scattering layer (5 mm thick) TiO2 particles (Solaronix, TiꢀNanoxide R/SP) were printed.
The TiO2 electrodes were heated under an airflow at 350 ºC for 10 min, followed by heating at
500 ºC for 30 min. After cooling to room temperature, the TiO2 electrodes were treated with 40
mM aqueous solution of TiCl4 at 70 ºC for 30 min and then washed with water and ethanol. The

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electrodes were heated again at 500 ºC for 30 min and left to cool to 80 ºC before dipping into
the dye solution. The dye solutions (0.3 mM) were prepared in 1:1:1 acetonitrile, tertꢀbutyl
alcohol and DMSO. Deoxycholic acid was added to the dye solution as a coadsorbate at a
concentration of 20 mM. The electrodes were immersed in the dye solutions with active area
facing up and then kept at 25 ºC for 20 h to adsorb the dye onto the TiO2 surface.
For preparing the counter electrode, preꢀcut TCO glasses were washed with water followed by
0.1 M HCl in EtOH, and sonication in acetone bath for 10 min. These washed TCO were then
dried at 400 ºC for 15 min. Thin layer of Ptꢀpaste (Solaronix, Platisol T/SP) on TCO was printed
and the printed electrodes were then cured at 450 ºC for 10 mints. The dye sensitized TiO2
electrodes were sandwiched with Pt counter electrodes and the electrolyte (Solaronix, Iodolyte
ANꢀ50) was then injected into the cell, while the two electrodes were held together with the
clips.
2.6. Photoꢀelectrochemical measurements
Photocurrentꢀvoltage characteristics of DSCs were measured using a Keithley 2400 source meter
under illumination of AM 1.5 G solar light from solar simulator (SOL3A, Oriel) equipped with a
450 W xenon lamp (91160, Oriel). The incident light intensity was calibrated using a reference
Si solar cell (Newport Oriel, 91150V) to set 1 Sun (100 mW cmꢀ2). The measurement was fully
controlled by Oriel IV Test Station software. IPCE (incident monochromatic photon to current
conversion efficiency) experiments were carried out using a system (QEX10, PV NO
Measurements, USA) equipped with a 75 W short arc xenon lamp (UXLꢀ75XE, USHIO, Japan)
as a light source connected to a monochromator. Calibration of incident light was performed
before measurements using a silicone photodiode (IF035, PV Measurements). All the
measurements were carried out without the use of antiꢀreflecting film.
2.7. Electrochemical impedance spectroscopy (EIS)
The electrochemical impedance spectra were measured with an impedance analyzer potentiostat
(BioꢀLogic SPꢀ150) under illumination using a solar simulator (SOL3A, Oriel) equipped with a
450 Wꢀxenon lamp (91160, Oriel). EIS spectra were recorded over a frequency range of 100
mHz ꢀ 200 kHz at room temperature. The applied bias voltage was set at the VOC of the DSCs,

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with AC amplitude set at 10 mV. The electrical impedance spectra were fitted using ZꢀFit
software (BioꢀLogic).
3. Results & Discussion: The synthesis of proposed trisꢀheteroleptic ruthenium(II) complexes
was carried out according to already reported procedure10 in a typical three step one pot method.
3.1. Photophysical measurements: UVꢀVis absorption spectra of SDꢀ15, SDꢀ16 and N719,
measured in DMF using a concentration of 2 × 10ꢀ5 M, are given in Fig. 4 and the results are
summarized in Table 1.
Structural modification of N719 by the replacement of NCS with biꢀimidazole along with
carbazole and triphenyl amino ancillary ligand resulted in red shifted absorption spectra of SDꢀ
15 and SDꢀ16 respectively. Red shift in absorption spectra was caused by the stabilizing effect of
biꢀimidazole on both HOMO and LUMO. HOMO destabilization was expected with the
introduction of strong electron donating carbazole and triphenyl amino groups as ancillary
ligand.22,23 the tuning of ground state oxidation potential is depicted in Fig. 4. The dominant role
of biꢀimidazole in stabilizing ground state can be attributed to the substantial delocalization of
HOMO on NCS part as found previously by DFT calculations.
Table 1. Experimental Absorption and emission properties of SDꢀ15, SDꢀ16 and N719
Sensitizers
Absorption λmax (nm)
ε (Mꢀ1 cmꢀ1)
Emission λmax (nm)
300; 392; 532 (d → π*)
302; 413; 523 (d → π*)
310; 381; 529 (d → π*)
62,950; 42,650; 17,000
90,500; 26,250; 21,150
46,100; 14,400; 12,800
728
748
SDꢀ15
SDꢀ16
N719
722

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9
8
7
6
5
4
3
2
1
0
2.5
SD-15
SD-15
SD-16
SD-16
2
1.5
1
0.5
0
280
380
480
580
Wavelength (nm)
680
780
880
Fig. 3. UV–Vis absorption (solidꢀlines) and emission spectra (dashedꢀlines) of complexes SDꢀ15
and SDꢀ16, measured in DMF (2 × 10ꢀ5 M)
Fig. 4. Energy level diagram and comparison between GSOP and ESOP of SDꢀ15, SDꢀ16 and
N719.

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3.2. Electrochemical measurements
Cyclic voltammetry (CV) was used to measure and compare the ground state oxidation potential
(GSOP) of SDꢀ15 and SDꢀ16 in solution. CV graph (SI Fig. S3ꢀS4) was used to calculate the
oxidation onset which is equivalent to the GSOP (ground state oxidation potential. Additionally
E0ꢀ0 was calculated from onset of absorption. The values of E0ꢀ0 and GSOP were used to
calculate the ESOP (excited state oxidation potential, the values in volts (V) against NHE were
converted to electron volt (eV) according to Equation (1).
ꢀ
ꢀ ꢁ
ꢁ
ESOP = [ GSOP V + 4.7 − ꢂꢃꢄꢃ eV
(2)
GSOP and ESOP for SDꢀ15, SDꢀ16 and N719 are compared in Fig. 4, Table 2. GSOP potential
of SDꢀ15 (–5.8 eV) was lower in energy compared to I3/Iꢂ redox couple (–5.2 eV or –0.5 V vs.
NHE),24 thus providing ample driving force for efficient dye regeneration. ESOP of SDꢀ15, SDꢀ
16 and N719 was 0.31, 0.40 and 0.43 eV higher in energy higher in energy than the conduction
band edge of semiconductor nanocrystalline TiO2 (–4.2 eV)25 which is expected to result in
efficient electron injection for N719. It was found that electron injection free energy for SDꢀ15
and SDꢀ16 was 0.31 and 0.4 eV, since the lowest energy difference between ESOP and TiO2
conduction band for efficient electron injection is not precisely determined, but usually expected
to be more than 0.15 eV.26 Hence, electron injection free energies of both trisꢀheteroleptic Ru(II)
complexes were thermodynamically favorable.
Table 2. Excited state oxidation potential (E*), ground state oxidation potential and the lowest
singlet–singlet electronic transitions (E0ꢀ0) for SDꢀ15, SDꢀ16 and N719 experimental.
Sensitizer
E0ꢀ0
GSOP (HOMO)
ESOP (LUMO)
1.95
1.93
1.99
–5.84
–5.73
–5.76
–3.89
–3.80
–3.77
SDꢀ15
SDꢀ16
N719

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3.3. Photovoltaic device characterizations
Photovoltaic characterizations were carried out in order to establish the structure–property
relationship with the replacement of NCS and influence of change in energy levels of SDꢀ15 and
SDꢀ16 on solar cells, Photocurrent action spectra for SDꢀ15 and SDꢀ16 are shown in Fig. 5 and
details about solar cell preparations and assembly are given in SI. Higher IPCE response was
recorded for SDꢀ15 than SDꢀ16 due to the presence of strong electron donating carbazole moiety.
Incident photon to current conversion efficiency (IPCE) response of higher than 40% from 400
nm to 600 nm was exhibited by SDꢀ15. SDꢀ15 showed higher molar absorptivity in solution,
however absorbed photons were not translated to photocurrent which can be attributed to lower
thermodynamic free energy of electron injection. However, this can can concluded based on
IPCE results that electron injection free energy should be higher than 0.15 eV in order to get
efficient electron injection.26
50
SD-15
SD-16
45
40
35
30
25
20
15
10
5
0
300
400
500
600
700
800
Wavelength (nm)
Fig. 5. IPCE obtained with dyes SDꢀ15 and SDꢀ16 anchored on nanocrystalline TiO2 film.
IꢀV characteristics of the solar devices prepared from SDꢀ15 and SDꢀ16 are in Fig. 6. Better
IPCE response of SDꢀ15 translated in term of higher JSC of 10.20 mA cmꢀ2 compared to 3.52 mA
cmꢀ2 of SDꢀ16. Lower JSC for both SDꢀ15 and SDꢀ16 than N719 is attributed to low free energy
for electron injection into the conduction band of TiO2.
Solar cells with SDꢀ15 and SDꢀ16 gave overall power conversion efficiency (% η) of 3.3 with
VOC of 0.58 V and ff of 0.56 and 1.0 with VOC of 0.52 V and ff of 0.58 compared to (% η) of 7.7

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for N719 (VOC = 0.77 and ff = 0.71). The fundamental energy loss or loss of potential in a solar
cell can be calculated by following the relation as discussed previously.27
Potential loss in a solar cell = Energy of charge generating photon – VOC obtained from solar cell.
Energy of charge generating photon can be calculated from the onset of IPCE spectra which was
1.57 eV for N719 and 1.70 and 1.97 eV for SDꢀ15 and SDꢀ16 respectively. The potential loss for
SDꢀ15 and SDꢀ16 based solar cell was 1.12 and 1.45 V compared to 0.80 V of N719. Hence,
though both ruthenium(II) complexes have much better UV/Vis absorption properties (Fig. 3), its
ground and excited state energy levels are not favorably positioned to result in efficient electron
injection and dye regeneration as it is evident from Figures 4 and 5 and potential loss
calculations in solar cell.
12
SD-15
SD-16
10
8
6
4
2
0
0
0.1
0.2
0.3
Voltage / V
0.4
0.5
0.6
Fig. 6. Photocurrentꢀvoltage characteristics of DSSCs sensitized with the Ruꢀcomplexes SDꢀ15
and SDꢀ16.
The photovoltaic parameters including the shortꢀcircuit photoꢀ current density (JSC), openꢀcircuit
voltage (VOC), fill factors (ff) and overall cell efficiencies (%η) are summarized in Table 3.

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Table 3
Photovoltaic characteristics of SDꢀ15 and SDꢀ16
Sensitizersa
JSC (mA cmꢀ2)
VOC (V)
0.58
ff
η (%)
3.32
10.20
0.56
0.58
SDꢀ15
3.52
0.52
1.06
SDꢀ16
a Conditions: coꢀadsorbate, DCA 20 mM; irradiated light, AM 1.5 solar light (100 mW cmꢀ2).
JSC, shortꢀcircuit photocurrent density; VOC, openꢀcircuit photoꢀvoltage; ff, fill factor; η, total
power conversion efficiency.
3.4. Electrochemical impedance spectroscopy characterization
Electrochemical impedance spectroscopy (EIS) successfully models the charge transfer and
chemical capacitance at the interface of TiO2/dye/electrolyte and pt/eletrolyte in DSSC under
operational conditions.10,28–31 Typical EIS measurements result in Nyquist and Bode plots.
In Nyquist plot, high frequency semicircle can be correlated with charge transfer at the counter
electrode and intermediate frequency is related to electron transport through mesoporous TiO2
and back electron transfer from TiO2 to electrolyte.28–32 Gräetzel et al. discussed the sizes of
these semicircles as bigger the radius of middle semicircle, higher the charge recombination
resistance (Rct) from TiO2 to electrolyte corresponding to Nernst impedance, whereas radius of
same semicircle corresponds to resistance of charge diffusion (Rd) under Gerischer
impedance.28,33 In EIS Nyquist plots (Fig. 7), two semicircles were observed. Small semiꢀ circle
at lower frequency models the cathode charge transfer resistance which is directly related with ff.
In efficient solar cell, Nernst impedance or Rct is greater than Rd as observed for N719,28 whereas
Rd is greater than Rct for SDꢀ15 based solar cell whereas SDꢀ16 has only one semicircle.
Intermediate impednace semicircle for SDꢀ15, can be assigned to Gerischer impedance due to the
fact, VOC obtained by employing SDꢀ15 was less than N719, whereas higher Rct results in higher
VOC as observed previously.31

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25
20
15
10
5
SD-15
SD-16
0
0
10
20
30
40
Re (z) / ohm
50
60
70
80
Fig. 7. EIS Nyquist plots for DSCs sensitized with SDꢀ15 and SDꢀ16.
In Bode frequency plot (Fig. 8), lifetime for injected electrons into TiO2 conduction band can be
1
determined by using the formula (ꢅꢆꢇ
=
), where ꢅ is the lifetime of electrons in TiO2 and
ꢊ
2ꢈꢉ
f is mid frequency peak in Bode plots. The frequency peak of the DSSCs based on SDꢀ15 and
SDꢀ16 was 224 Hz whereas only 18 Hz for N719, corresponding to eTiO2 of 0.71 ms and 8.8 ms,
respectively thus resulting in lower VOC for both SDꢀ15 and SDꢀ16 (shorter eTiO2) compared to
N719, which correlates well with the actual VOC reported for the solar cells respectively. Higher
Rd and shorter eTiO2 for SDꢀ15 and SDꢀ16 can be attributed to lower free energy of electron
injection resulting in shorter diffusion compared to N719 and strong electron withdrawing nature
of HOMO due to the presence of biꢀimidazole moiety.
45
SD-15
SD-16
40
35
30
25
20
15
10
5
0
-1
0
1
2
3
4
5
Frequency (Hz)
Fig. 8. EIS Bode plots for DSCs sensitized with SDꢀ15 and SDꢀ16.

New Journal of Chemistry
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DOI: 10.1039/C7NJ01363A
Conclusion
Two novel trisꢀheteroleptic thiocyanateꢀfree Ru(II) complexes based on biꢀimidazole moiety
along with carbazole in SDꢀ15 and triphenyl amino group in SDꢀ16 were successfully
synthesized and characterized for DSSCs. The presence of strong electronꢀdonating carbazole
and triphenyl with biꢀimidazole in ruthenium(II) sensitizers resulted in red shift of the low
energy MLCT and higher molar absorptivity compared to N719. The different ligands in
complexes changed the overall characteristics of the ruthenium complexes. SDꢀ15 gave the
highest solarꢀtoꢀpower conversion efficiency of 3.32% under AM 1.5 irradiation and the
efficiency performance is in good agreement with molar absorptivity data. This study suggests
that biꢀimidazole chromophore opens up an alternative route to afford a good class of NCSꢀfree
Ru(II) complexes for DSSC research based on ruthenium metal. More research needs to be done
to fineꢀtune the molecular structure biꢀimidazole for the developments of higher efficiency
sensitizers.
Supporting Information Available
The supporting information includes the detailed synthetic procedures, spectroscopic data and
fabrication techniques.
Acknowledgments
This work is financially supported by the Higher Education Commission (HEC), Pakistan.
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