Design and synthesis of new ruthenium complex for dye-sensitized solar cells

Article abstract of DOI:10.1039/c6ra10881d

A series of ruthenium complexes (MC-1-MC-3), incorporated with carbazole, fluorene and phenothiazine units with dipyrido[3,2-a:2′,3′-c]phenazine are synthesized, characterized and their optical, electrochemical and photovoltaic properties are investigated. The observed differences in light-harvesting ability of the sensitizers are associated with the electron donor strength of the ancillary ligand used for preparing complexes. The dye-sensitized solar cell fabricated from complex MC-1 exhibited a power conversion efficiency of 6.18%. It is demonstrated that new molecular design and increase in molar absorption coefficient of the sensitizer improved the device performance.

Full text of DOI:10.1039/c6ra10881d

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Design and Synthesis of New Ruthenium Complex for DyeꢀSensitized
Solar Cells
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M. G. Murali,a Xingzhu Wang,b,c Qing Wang,b,c Suresh Valiyaveettil,a,b*
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a Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore
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b NanoCoreꢀNUSNNI, TꢀLab Building, National University of Singapore, 5A Engineering Drive
1, Singapore 117411
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c Department of Materials Science and Engineering, Faculty of Engineering, National University
of Singapore, Singapore 117574
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Abstract
A series of ruthenium complexes (MC1−MC3), incorporated with carbazole, fluorene and
phenothiazine units with dipyrido[3,2ꢀa:2',3'ꢀc]phenazine is synthesized, characterized and their
optical, electrochemical and photovoltaic properties are investigated. The observed differences in
lightꢀharvesting ability of the sensitizers are associated with the electron donor strength of the
ancillary ligand used for preparing complexes. The dyeꢀsensitized solar cell fabricated from
complex MC–1 exhibited a power conversion efficiency of 6.18%. It is demonstrated that new
molecular design and increase in molar absorption coefficient of the sensitizer improved the
device performance.
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Introduction
Dyeꢀsensitized solar cells (DSSCs) have attracted considerable attention as a potential source of
renewable energy owing to their lowꢀcost of production and readily available components.1,2
DSSCs were fabricated by adsorbing the dye molecules on a mesoporous oxide films such as
TiO2, ZnO or Nb2O5. The dye molecules acted as a sensitizer which broaden the light absorption
window of the photoanode in the solar spectral region.3–6 A few ruthenium complexes,7–10 metal
free organic dyes11–15 and metalloporphyrins12,16–18 have been developed and used as sensitizers
in DSSCs. However, low power conversion efficiencies and shorter life time need to be
improved for practical applications of DSSCs.19
Ruthenium complexes exhibit broad absorption and enhanced molar absorptivity with
longꢀterm stability.20 One approach to improve the lightꢀharvesting efficiency of the ruthenium
complex is via extending the absorption range in the visible region via increasing the conjugation
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length of the ligands used in metal complexes.21,22 For example, replacing one of the 2,2'ꢀ
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bipyridylꢀ4,4'ꢀdicarboxylic acid ligand (dcbpy) in the ruthenium compound of N719 and N3 with
an extended πꢀconjugated ancillary ligand containing thiophene or alkoxy benzene derivatives,
the absorption maximum was extended up to the longer wavelength region.23,24 The compound
dcbpy was used as a common anchoring group for metal complexes on TiO2 substrate.25
In the present study, the area of coverage on the TiO2 surface was extended by the design
of linear dye molecules with metal coordinating sites which form complexes with ruthenium
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(Ru) metal ions. The 2,2'ꢀbipyridineꢀ4,4'ꢀdicarboxylic acid is used as anchoring group for the
metal complex. Such linear dye helps to cover a large area on the electrode, reduce aggregation,
improve the stability, and enhance the efficiency of energy capture and transfer processes. The
conceptual diagram is given in Figure 1.
Linear conjugated
antenna group
Metal bridge
M
Anchoring group with
multiple functional groups
TiO2
ITO
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Figure 1 Conceptual diagram for the DSSC dyes designed and investigated in this study
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Molecular structures of three new ruthenium complexes (MC1 – MC3) with linear light
harvesting units such as carbazole, fluorene and phenothiazine attached to the dipyrido[3,2ꢀ
a:2',3'ꢀc]phenazine units are given in Figure 2. The optical, electrochemical properties, electron
distribution of frontier orbital of the metal complexes and energy conversion performance of the
synthesized ruthenium complex are investigated in detail.
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Figure 2 Target metal complexes (MC1 − MC3) synthesized and used for the DSSC studies.
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Experimental section
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Materials and instruments
The starting materials, 3ꢀbromocarbazole, 2ꢀbromofluorene, 3ꢀbromoꢀ10Hꢀphenothiazine and
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4,7ꢀdibromobenzo[c]ꢀ1,2,5ꢀthiadiazole were purchased from Sigma Aldrich and used as
received. 3ꢀBromoꢀ9ꢀ(2ꢀethylhexyl)ꢀ9Hꢀcarbazole (2A)26, 9ꢀ(2ꢀethylhexyl)ꢀ3ꢀ(4,4,5,5ꢀtetra
methylꢀ1,3,2ꢀdioxaborolaneꢀ2ꢀyl)ꢀ9Hꢀcarbazole (3A)26, 4,7ꢀbis(9ꢀ(2ꢀethylhexyl)ꢀ9Hꢀcarbazolꢀ3ꢀ
yl)benzo[c][1,2,5]thiadiazole27 and 1,10ꢀphenanthrolineꢀ5,6ꢀdione28 were synthesized according
to the reported procedure. All other chemicals and reagents were obtained from Sigma Aldrich
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Ltd and used without further purification. Reactions were monitored by thin layer
chromatography (TLC) using silica gel plates and products were purified on silica gel column
using different solvents.
1H and 13C NMR spectra were recorded using a Bruker Avance AV300 (300/500/75
MHz) NMR spectrometer with TMS as internal reference. The following abbreviations were
used to explain the signal multiplicities: s = singlet, d = doublet, t = triplet, m = multiplet, br =
broad. ESI/EI/APCI mass spectrometry was used to confirm the mass of compound. UVꢀVis
absorption spectra were measured using a UVꢀ1800 Shimadzu UVꢀVIS spectrophotometer with
an optical filter that is calibrated at a bandwidth of 1 nm. Structural optimization and frontier
orbital energy levels were calculated using density functional (DFT) model at B3LYP/6ꢀ31G(d)
level. The LUMO and HOMO electron density plots were generated from the optimized
structures using GaussView 5. Electrochemical studies of the metal complex were carried out
using a CHI electrochemical analyzer. Cyclic voltammograms were recorded using a threeꢀ
electrode cell system, with dye sensitized photoanode as working electrode, platinum wire as
counter electrode and Ag/AgNO3 as reference electrode (0.1M electrolyte solution with 0.01M
AgNO3 in CAN). Photocurrentꢀvoltage characteristics under AM 1.5G illumination at 95 mW
cm−2 were carried out with a Keithley 2400 source meter and PVIV software package from
Newport. Sunlight was obtained from a solar simulator (450 W, Newportclass A), and power
was calibrated using a silicon reference cell. The photocurrent density – voltage traces of metal
complex were measured under ambient conditions.29 The active area of the cells was 0.119 cm2
defined by a mask.
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Preparation of TiO2 electrode and DSSC device fabrication
Fluorineꢀdoped indium tin oxide (FTO with a sheet resistivity of 8 − 10 ꢀ square–1) coated glass
plate was cleaned using 5% Decon 90 solution, deionized water and ethanol sequentially in an
ultrasonic bath sequentially. On FTO plate, nanocrystalline TiO2 (20 nm particle size) layers
were deposited by successive screen printing using a TiO2 paste (Dyesol, DSL 18ꢀNR). A layer
of larger TiO2 particles (400 nm particle size, WERꢀO paste, Dyesol) was deposited on top of the
transparent layer to increase light scattering on the TiO2 layer. Thereafter, the electrode was
gradually heated at 125 °C for 10 min, at 325 °C for 10 min, at 375 °C for 10 min, at 450 °C for
15 min and at 500 °C for 15 min. The electrodes were soaked in 40 mM solution of TiCl4 in
distilled water for 30 min at 75 ºC, washed with water and dried under ambient condition. The
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prepared electrodes were immersed in a sensitizer (0.3 mmol) solution in DMF : tꢀBuOH mixture
(3 : 1 v/v) for 24 h, followed by washing with DMF prior to cell assembly. A platinum thin film
(thickness = 100 nm) deposited on a FTO coated glass plate was used as the counter electrode.
The electrolyte composed of 0.5 M lithium iodide, 0.05 M iodine and 0.5 M 4ꢀtertꢀbutylpyridine
in CH3CN, was injected through a hole at the platinized FTO in a partial vacuum. Finally, the
device was used for testing the performance of DSSCs.
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Synthesis of ligands and the metal complex
Synthesis and characterization data of compounds, 2A – 2C, 3A – 3C and 4A – 4C are achieved
using reported procedures and the details are summarized in the supplementary information. The
synthetic route for compound 6A and target sensitizer, MC1 is given in Scheme 1. Similar
synthetic schemes were used for MC2 and MC3 (see supporting information, Scheme 2 and
Scheme 3).
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Scheme 1. Synthetic route for MC1; (I) KOH, RBr, DMF, room temperature, 12 h; (ii)
Bis(pinacolato)diboron, CH3COOK, Pd(dppf)Cl2.CH2Cl2, 1,4ꢀdioxane, 80 °C, 18 h, (iii) 4,7ꢀ
dibromoꢀ2,1,3ꢀbenzothiadiazole, K2CO3, Pd(PPh3)4, THF:H2O, reflux, 24 h; (iv) Zn, acetic acid,
70 °C, 6 h; (v) 1,10ꢀphenanthrolineꢀ5,6ꢀdione, acetic acid, 70 °C, 12 h, (vi) Dichloro(pꢀ
cymene)ruthenium(II) dimer, 2,2'ꢀbipyridineꢀ4,4'ꢀdicarboxylic acid, NH4NCS, DMF, reflux.
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Synthesis of Compound 6A
Compound 4A (0.28 g, 0.4 mmol) was dissolved in 10 ml of glacial acetic acid, followed by
adding zinc dust (1.02 g, 16 mmol) in one portion, stirred at 70 °C for 6 h and filtered to remove
the solid residue. The obtained crude compound, diamine 5A was treated with 1,10ꢀ
phenanthrolineꢀ5,6ꢀdione (85 mg, 0.4 mmol) and the resulting solution was stirred at 70 °C for
12 h. Reaction mixture was poured into ice water and extracted with chloroform (3 x 25 ml). The
organic phase was washed with water several times, dried over anhydrous Na2SO4 and
concentrated. The crude product was purified using silica gel column chromatography with
dichloromethane: methanol (99:1) as eluent to obtain compound 6A as a red solid (0.2 g, yield 62
%). Similar procedure was followed for the synthesis of ligand 6B and 6C.
Compound 6A: 1H NMR (CDCl3, 300 MHz, δ ppm): 9.4 (d, 2H), 9.2 (d, 2H), 8.8 (d, 2H), 8.24
(s, 2H), 8.19 (d, 2H), 8.1 (d, 2H), 7.66 (d, 4H), 7.54 (d, 4H), 7.32 (d, 2H), 4.34 (d, 4H), 2.27–
2.20 (m, 2H), 1.52–1.25 (m, 16H), 1.01 (t, 6H), 0.92 (t, 6H). 13C NMR (CDCl3, 75 MHz,
δ ppm): 152.08, 148.36, 141.41, 140.71, 140.64, 139.66, 139.56, 133.80, 130.65, 129.08, 127.87,
125.77, 124.01, 123.35, 123.21, 122.85, 120.26, 119.13, 109.26, 108.44, 47.67, 39.59, 31.16,
28.93, 24.51, 23.19, 14.16, 11.02. HRꢀMS (APCI, m/z): calcd. for C58H57N6 ([M+1]), 837.4639;
found, 837.4646.
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Compound 6B: H NMR (CDCl3, 300 MHz, δ ppm): 9.4 (d, 2H), 9.24 (d, 2H), 8.08 (d, 2H),
7.96 (d, 2H), 7.82 (d, 4H), 7.68 (d, 2H), 7.44 (d, 4H), 7.32 (d, 4H), 2.1 (t, 8H), 0.72 (br s, 4H),
0.9ꢀ0.86 (m, 32H), 0.59 (t, 12H), 0.46 (t, 12H). 13C NMR (CDCl3, 75 MHz, δ ppm): 152.44,
150.93, 148.25, 141.14, 140.01, 137.10, 134.27, 130.71, 128.20, 126.92, 126.62, 125.70, 124.45,
119.89, 119.16, 55.21, 44.81, 35.04, 34.08, 32.22, 31.42, 29.82, 28.46, 27.33, 22.85, 14.14,
14.07, 10.57. HRꢀMS (APCI, m/z): calcd. for C76H91N4 ([M+1]), 1059.7238; found, 1059.7245
(error: ꢀ0.7 ppm).
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Compound 6C: H NMR (CDCl3, 300 MHz, δ ppm): 9.09 (t, 5H), 7.85 (s, 2H), 7.64–7.59 (m,
8H), 7.0 (br, s, 7H), 3.98 (t, 4H), 1.96 (br, s, 4H), 1.41–1.25 (m, 12H), 0.95 (t, 6H). 13C NMR
(CDCl3, 75 MHz, δ ppm): 151.60, 146.66, 139.37, 138.49, 133.71, 129.82, 127.23, 127.11,
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124.16, 115.38, 114.48, 66.94, 60.24, 31.44, 29.55, 26.66, 22.53, 14.06, 13.92. HRꢀMS (APCI,
m/z): calcd. for C54H49N6S2([M+1]), 845.3455; found, 845.3452.
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Synthesis of target compound MC–1
Compound 6A (0.1 g, 0.11 mmol) and dichloro(pꢀcymene)ruthenium(II) dimer (36.5 mg, 0.055
mmol) were dissolved in 5 ml anhydrous DMF under inert atmosphere. The reaction mixture was
heated at 80 °C for 4 h, followed by the addition of 2,2′ꢀbipyridineꢀ4,4′ꢀdicarboxylic acid (26
mg, 0.11 mmol). Reaction mixture was heated at 150 °C for another 4 h under dark, excess
NH4NCS (91 mg, 1.2 mmol) was added, stirred for 5 h, poured into ice water and the black
residue was filtered off. The excess NH4NCS was removed by washing with water, followed by
methanol. The crude product was further purified by repeated reꢀprecipitation from chloroform:
methanol (1:9) mixture to obtain the target compound MC–1 as a black powder (60 mg, yield 38
%). MC–2 and MC–3 were synthesized using similar procedures.
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MC–1: H NMR (DMF, 500 MHz, δ ppm): 9.88 (s, 1H), 9.58 (s, 1H), 9.31 (s, 1H), 9.1 (s, 1H),
8.77 (s, 1H), 8.48 (d, 1H), 8.29 (d, 2H), 8.12 (s, 2H), 7.87 (d, 2H), 7.81 (d, 2H), 7.73 (d, 4H),
7.65–7.6 (m, 4H), 7.53 (t, 4H), 7.26 (t, 2H), 4.55 (d, 4H), 1.99–1.93 (m, 2H), 1.44–1.4 (m, 16H),
0.9–0.82 (m, 12H). MS (ESI): (Mw=1298.5) found m/z = 1297.2 [M+]
MC–2: 1H NMR (DMF, 500 MHz, δ ppm): 9.85 (s, 1H), 9.8 (d, 1H), 9.61 (d, 1H), 9.33 (s, 1H),
9.15 (s, 1H), 8.56 (d, 1H), 8.51 (d, 1H), 8.54–8.51 (m, 4H), 8.01 (d, 1H), 7.85–7.81 (m, 4H),
7.72 (d. 2H), 7.66 (d. 2H), 7.59 (s, 2H), 7.54 (t. 2H), 7.70–7.42 (m, 4H), 2.3–2.24 (m, 8H), 0.98–
0.9 (m, 32H), 0.76 (br s, 4H), 0.68–0.64 (m, 12H), 0.61–0.56 (m, 12H). MS (ESI): (Mw=1521)
found m/z = 1519.7
MC–3: 1H NMR (DMF, 500 MHz , δ ppm): 9.79 (s, 1H), 9.5 (s, 1H), 9.17 (s, 1H), 9.07–8.97
(m, 4H), 8.37 (s, 1H), 8.10 (d, 2H), 7.79–7.71 (m, 4H), 7.44–7.36 (m, 4H), 7.28–7.24 (m, 4H),
7.19–7.09 (m. 6H), 4.04 (t, 4H), 1.86–1.80 (m, 4H), 1.26–1.16 (m, 12H), 0.82–0.74 (t, 6H). MS
(ESI): (Mw=1306.5) found m/z = 1305.1 [M+].
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Results and Discussion
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Synthesis of target molecules
Three new precursor compounds, 6A – 6C, were synthesized using established procedures and
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used to prepare the target dye molecules, MC1 – MC3. In a typical procedure, Nꢀalkylation of 3ꢀ
bromocarbazole (1A) using KOH as a base in DMF solvent at room temperature to get 3ꢀbromoꢀ
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9ꢀ(2ꢀethylhexyl)ꢀ9Hꢀcarbazole 2A. Subsequent Miyaura borylation reaction26 of compound 2A
(2B for MC–2 and 2C for MC–3) with bis(pinacolato)diborane was carried out using KOAc as a
base and Pd(dppf)Cl2.CH2Cl2 as a catalyst in 1,4ꢀdioxane to obtain compound 3A in good yield.
Compounds 4A was prepared via Suzuki Miyaura coupling reaction27 of corresponding mono
boronic acid pinacol ester compound 3A and 4,7ꢀdibromobenzo[c]ꢀ1,2,5ꢀthiadiazole The
.
compound 4A was reduced with zinc dust in glacial acetic acid to yield corresponding diamine,
5A. Since the diamine was unstable, it was used as such for the next step and treated with 1,10ꢀ
phenanthrolineꢀ5,6ꢀdione in glacial acetic acid at 70 ºC for 12 h to yield ligand 6A. Similarly,
ligand 6B and 6C were prepared from appropriate starting materials. All intermediate
1
compounds and ligands were purified using silica column chromatography. H NMR and mass
spectra of free ligands are presented in the supplementary data (Figure S1 and S2).
To prepare the target molecules, MC1 – MC3, dichloro(pꢀcymene)ruthenium(II) dimer
and 2,2′ꢀbipyridineꢀ4,4′ꢀdicarboxylic acid were treated with appropriate amounts of the ligands
(6A – 6C) in anhydrous DMF under inert atmosphere (Scheme 1).20,22 The crude complexes were
further purified by successive reꢀprecipitation using chloroform: methanol solvent mixture. The
products were partially soluble in solvents such as chloroform, dichloromethane, THF, methanol,
DMSO and completely soluble in DMF. The sensitizers (MC1 – MC3) were characterized using
1H NMR and electrospray ionization (ESI) mass spectrometry.
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Optical properties
The absorption and emission spectra of compounds 6A – 6C in DMF solution are given in Figure
3. The ligands exhibited two high energy absorption bands between 300 – 390 nm region (at 295
and 383 nm for 6A, 310 and 385 nm for 6B, 337 and ~382 for 6C) are assigned to the intraꢀ
ligand π–π* transitions of electron donor chromophores,30 whereas, the absorption maxima
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observed at 472, 436 and 490 nm in the longer wavelength region, are assigned to nꢀπ*
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absorption bands.20,22 Further, nꢀπ* absorption band of compound 6C is red shifted by 18 and 54
nm as compared to that of compound 6A and 6B, respectively. The observed red shift could be
attributed to the stronger electron donating ability of phenothiazine unit in compound 6C.31 Also,
emission maxima were observed at 632, 565 and 614 nm, for 6A, 6B and 6C, respectively.
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1.0
0.5
0.0
12
10
8
400 500 600
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4
2
0
300 400 500 600 700 800
Wavelength (nm)
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Figure 3 UVꢀVis absorption (left) and emission (right) spectra of free ligands, 6A – 6C in DMF,
6A, 6B, 6C, inset: part of absorption spectra from 400 – 600 nm is enlarged
for clarity).
(
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The absorption and emission spectra of the complexes MC1 – MC3 in DMF solution are
given in Figure 4, and summarized in Table 1. As shown in Figure 4, the sensitizers showed
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broader absorption maxima in the region of 300 to 400 nm owing to the intraꢀligand π–π*
transition.20 Further, the absorption spectra of the compound 6A – 6C are dominated by metal to
ligand charge transfer transition and the maxima are centered at 505, 471 and 513 nm for MC1 –
MC3, respectively.22 The lowest energy metal to ligand charge transfer transition band of MC–3
is red shifted about 8 and 42 nm as compared to that of MC–1 and MC–2, respectively. Here, the
stronger electron donor unit phenothiazine in the ancillary ligand 6C of MC–3 lowers the metal
to ligand charge transfer transition between metal center and the anchoring ligand. MLCT is the
dominant factor for determining the efficiency of photosensitizer in DSSCs and increasing the
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strength of the electron donor in the ancillary ligand of the metal complex, its MLCT transition
will be driven to a lower energy and the dye will have a higher efficiency when used in DSSCs.22
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On the other hand, the molar absorption coefficient of the intraꢀligand π–π* and MLCT band of
MC–3 is lower than those of MC–1 and MC–2 (Table 1). These results indicate that enhancing
the donor strength of the ancillary ligand does not increase the intensity of MLCT band, but shift
to a lower energy region. This may lead to the decrease in device performance for MC–3. The
emission maxima of target molecules were obtained by exciting at the respective low energy
MLCT absorption bands, emission maxima appeared at 685, 600 and 560 nm, respectively, for
MC1 – MC3.
1.5
10
1.0
0.5
8
0.0
400 500 600 700
6
4
2
0
300 400 500 600 700 800
Wavelength (nm)
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Figure 4 UVꢀVis absorption (left) and emission spectra (right) of MC1–MC3 in DMF (
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MC–1,
MC–2,
MC–3, inset: part of absorption spectra from 400 – 600 nm is
enlarged for clarity).
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The target molecular complexes MC1 – MC3 were adsorbed on a 2.5 ꢁm transparent
TiO2 thin film which showed features similar to those of the corresponding absorption spectra in
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solution state, but exhibited a blueꢀshift as a result of deprotonation of −COOH group on TiO2
surface and presumably Hꢀaggregate formation (Figure 5).32
MC-1
MC-2
1.0
MC-3
0.8
0.6
0.4
0.2
0.0
400
500
600
Wavelength (nm)
Figure 5 UVꢀVis absorption spectra of MC1 – MC3 adsorbed on transparent mesoporous TiO2
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films
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Electrochemical properties
To use dye molecule as a sensitizers in DSSCs, its band structure should match with the energy
level of the semiconductor anode and the redox electrolyte or the hole conductor. That is to say,
to minimize energetic losses during the electronꢀtransfer reaction, the energy level of the excited
state of dye should be matched with the TiO2 conduction band. Also its redox potential should be
sufficiently positive that it can be regenerated via electron donation from a redox electrolyte,
–
such as I3 /I–. Cyclic voltammograms (CV) of MC1 – MC3 were recorded on TiO2 film with 0.1
M tetrabutylammonium hexafluorophosphate as the electrolyte at a scan rate of 100 mV sꢀ1. For
calibration, the redox potential of ferrocene/ferrocenium (Fc/Fc+) was measured under the same
condition and observed at 0.14 V vs Ag/AgNO3 electrode. It was assumed that the redox
potential of Fc/Fc+ has an absolute energy level of −4.80 eV in vacuum.33,34 The HOMO and
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LUMO energy levels of sensitizers were derived from oxidation/reduction potentials of the target
molecules and summarized in Table 1.35
Two oxidative peaks are observed for the metal complex (Figure S4, supplementary
data). The first oxidation peak is assigned to the oxidation of the metal center, and the second
peak comes from the oxidation of free ligand.36 The oxidation potential (Eoxd) of the metal center
is estimated to be 0.57 and 0.68 V, respectively, for MC–1 and MC–2, whereas, the oxidation
peak of metal centre observed for MC–3 at 0.45 V is cathodically shifted by 0.21 V indicating
the electronꢀrich nature of the compound 6C. The second oxidation potential is situated at 0.81,
1.02 and 0.9 V, for MC−1, MC−2 and MC−3, respectively. It was observed that CV of the metal
complexes is not electrochemically reversible. The reduction potentials (Ered) were determined
by subtracting the optical energy band gap of the metal complex on TiO2 film (Eg) from Eoxd and
are found to be −1.3 V for MC–1, −1.18 V for MC–2 and −1.41 V for MC–3. The HOMO and
LUMO energy levels are estimated to be −5.01 and −3.15 eV for MC–1, −5.1 and −3.24 eV for
MC–2 and −4.92 and −3.06 eV for MC–3.
256
Table 1.Optical and electrochemical data of MC1−MC3
Sensitizer
Eoxd of
Ru III/II
[V]
Eoxdon(V)/
HOMO
(eV)
Ered(V)/
LUMO
(eV)
λ
max/nm
λ
emm/nm
(ε/L mol-1 cm-1)
MC–1
685
0.57
298 (107180 ± 80)
341 (52600 ± 60)
505 (16300 ± 30)
313 (78600 ± 70)
471 (15510 ± 50)
300 (61880 ± 90)
346 (34600 ± 60)
513 (12620 ± 45)
0.35/−5.01
−1.3/−3.15
MC–2
MC–3
600
560
0.68
0.45
0.44/−5.1
−1.18/−3.24
−1.41/−3.06
0.26/−4.92
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258
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Ered was calculated by the formula: Ered = Eoxd – Eg. The absorption onset of the each metal
complex on TiO2 film was used to calculate the energy gap (Eg). HOMO and LUMO energy
levels were calculated using formula, HOMO [eV] = Eoxdon–EFc/Fc+ + 4.8; LUMO [eV] = HOMO
– Eg. Eoxdon is the onset oxidation potential. i.e potential at which oxidation of the metal complex
starts, whereas Eoxd is the oxidation potential at specific voltage.
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–
Energy level diagrams of MC1 – MC3, TiO2 and I3 /I– electrolyte are illustrated in Figure
6. The position of the HOMO level of the metal complex is sufficiently below the energy level of
iodide/triiodide couple redox mediator (−4.8 eV) allowing dye regeneration. Nevertheless, the
position of the LUMO level is closer to the TiO2 conduction band (−4.0 eV) to facilitate efficient
electron transfer from the excited dye to TiO2.20
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Figure 6 Energy level diagrams of MC1 – MC3, TiO2 and I3 /I–
–
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Density functional theory calculations
In order to get insight into the geometrical configuration and electron distribution of the frontier
orbitals of the sensitizer, density functional theory (DFT) calculations were carried out using a
B3LYP/6ꢀ31G level program.37 The optimized ground state molecular structure and orbital
profiles for the HOMO and LUMO of the target complex MC–1 are presented in Figure 7. It is
observed that the HOMO of the target dyes is distributed on the ruthenium metal and the NCS
ligands. Within the NCS ligands, the amplitude is primarily located on the sulfur atom. The
LUMO and HOMO–1 of MC–1 are localized homogeneously on the dcbpy ligands, facilitating
electron injection from the excited Ru metal complex to the TiO2. Molecular orbital profile of all
metal complexes is similar in structure and distribution (Figure 7 and Supporting Information,
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Figure S5 for MC–2 and MC–3). The charge transfer between the metal and ligands is not
expected to contribute to the conversion efficiency of the DSCs.22 It is observed that LUMO+1of
MC–1 and LUMO of MC–2 and MC–3 are localized more on their ligand part. This leads to
decrease in the strength of the MLCT transition between the metal and the dcbpy ligand, which
is further supported by the absorption studies of the sensitizer, where decrease in intensity of
MLCT band of metal complexes was observed.
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Figure 7 Graphical representation of the frontier orbitals of MC–1calculated at the B3LYP/6ꢀ
31G (d,p) level of theory. Atoms in red, yellow, brown, blue, and gray correspond to oxygen,
sulfur, carbon, nitrogen, and ruthenium, respectively.
Solar cell performance
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The photocurrent density–voltage traces of MC1 – MC3 incorporated solar cells are shown in
Figure 8 and the detailed device data are summarized in Table 2. The sensitized solar cell of
MC–1 exhibit a short–circuit photocurrent density (Jsc) of 11.49 mA cm–2, an open–circuit
potential (Voc) of 0.71 V and a fill factor (FF) of 0.75, corresponding to an overall conversion
efficiency (
η) of 6.18 % under AM 1.5 sunlight illumination. The power conversion efficiency of
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MC–2 and MC–3 based solar cell is 3.57 and 1.62 %, respectively. The MC–2 based solar cell
showed low device performance as compared to that of MC–1 and this could be attributed to its
weak light harvesting ability in the longer wavelength region (Figure 4). A decrease in
photocurrent density was observed for MC–2 (7.27 mA cm–2) than that of MC–1. On the other
hand, MC–3 based device showed a low device performance, though its MLCT band is at lower
energy region than those of other two metal complexes. The obtained results agree with the fact
that the molar absorption coefficient of MLCT band in MC–3 is lower than those of MC–1 and
MC–2 (Table 1). The open–circuit voltage value of MC–1 and MC–2 based device is almost
similar, but the value for MC–3 is lower than those of the other two complexes. This could be
contributed to poor device performance for MC–3 based DSSC devices. It is known that the
open–circuit voltage of the device depends on the Fermi level energy of TiO2. Higher the proton
concentration on the surface, the lower the Fermi level energy of the TiO2 nanocrystallite.22,38
The low efficiency for MC–3 is mainly attributed to the lower Jsc value and molar absorption
coefficient.
12
MC-1
MC-2
MC-3
10
8
6
4
2
0
0.0
0.2
0.4
0.6
0.8
Voltage (V)
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313
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315
Figure 8 Current density–voltage characteristics of photovoltaic devices with MC1 – MC3 as
sensitizer under AM 1.5 simulated sunlight (95 mW cm–2) illumination. (Cell active area: 0.119
cm2)
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Table 2. Photocurrent density (Jsc), open–circuit potential (Voc), fill factor (FF) and power
conversion efficiency ( ) of MC1–MC3 sensitized devices
η
Sensitizer
MC–1
Jsc [mA/cm2]
11.49
Voc [V]
0.71
FF
0.75
0.73
0.75
η
(%)
6.18
MC–2
7.27
0.66
3.57
1.62
MC–3
3.55
0.60
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Conclusions
In conclusion, a new series of ruthenium complexes, MC1–MC3 incorporating carbazole,
fluorene and phenothiazine units attached to dipyrido[3,2ꢀa:2',3'ꢀc]phenazine were synthesized,
characterized and employed as sensitizers in DSSCs. Increasing the electron donor strength of
the ancillary ligand improved the overall spectral response of the sensitizer in the longer
wavelength region. The device based on MC–1 sensitizer showed a short–circuit photocurrent
density (Jsc) of 11.49 mA cmꢀ2, an open–circuit voltage (Voc) of 0.71 V, and a fill factor (FF) of
0.75, corresponding to an overall conversion efficiency (η) of 6.18 %. Such metal complexes are
useful towards understanding the mechanism of DSSCs and enhance the performance of such
devices in the future.
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332
Acknowledgements
The authors wish to thank the Ministry of Education for funding support (Rꢀ143ꢀ000ꢀ570ꢀ112)
and department of chemistry at the National University of Singapore for technical support.
333
References:
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Design and Synthesis of New Ruthenium Complex for Dye-
Sensitized Solar Cells
M. G. Murali,a Xingzhu Wang,b,c Qing Wang,b,c Suresh Valiyaveettil,a,b*
Graphical Abstract
Linear conjugated
antenna group
M
Metal bridge
Anchoring group with
multiple functional groups
TiO2
ITO