60

Chapter 3

High Molar Extinction Coefficient Heteroleptic Ru(II) Complex for

Dye Sensitized Solar Cell Applications

3.1. Introduction

As we have discussed in the introduction, the widely used sensitizers in DSSC

are Ru(II) polypyridyl complexes.1-4

The most successful Ru(II) based charge transfer

sensitizers are cis-bis(4,4’-dicarboxy-2,2

’-bipyridine)dithiocyanato ruthenium (II) (N3

dye),cis-Bis(isothiocyanato)(2,2’-bipyridyl-4,4’-dicarboxylato)(4,4’-di-nonyl-2’-

bipyridyl)ruthenium(II) (Z-907) and trithiocyanato 4,4’4

”-tricarboxy-2,2

’:6

’,2

”-terpyridine

ruthenium (II) (Black dye) with an efficiency >11%.5,6

In order to further improve the

efficiency of Ru(II) sensitizers one has to improve its absorption in the red and near-IR

region. Even though, Black dye has absorption up to 600 nm but its molar absorption

coefficient is very low. This hinders the utilization of solid redox electrolytes and plastic

substrate in place of glass substrate in which we require thinner TiO2 layer. The optimal

sensitizer for the dye-sensitized solar cell should be panchromatic (i.e., absorb visible

light of all colors). Ideally, all photons below a threshold wavelength of about 920 nm

should be harvested and converted to electric current. This limit is derived from

thermodynamic considerations showing that the conversion efficiency of any single-

junction photovoltaic solar converter peaks at approximately 33 % near threshold energy

of 1.4 eV.7 Another drawback of Black dye is the presence of three –NCS groups in its

molecular structure. The –NCS groups not only detect the properties of the Ru(II)

complexes but also oxidizes to –CN group under irradiation thus imposing a question

mark on their chemical stability and hence durability.8,9

Molecular engineering of ruthenium complexes that can act as panchromatic

charge-transfer sensitizers for TiO2-based solar cells presents a challenging task, as

This work appeared in Journal of Chemical Sciences, 2011, 123, 371-378.

61

several requirements which are very difficult to be met simultaneously have to be

fulfilled by the dye. The lowest unoccupied molecular orbitals (LUMOs) and the highest

occupied molecular orbitals (HOMOs) have to be maintained at levels where photo-

induced electron transfer in the TiO2 conduction band and regeneration of the dye by

iodide can take place at practically 100% yield. This restricts greatly the options available

to accomplish the desired red shift of the metal-to-ligand charge-transfer transitions

(MLCT) to about 900 nm. The spectral properties and stability of the ruthenium

Polypyridyl complexes can be tuned in two ways: first, by enhancing the molar

absorption coefficient as well as broadening of the metal-to-ligand charge transfer band

and, second, by replacement of –NCS groups by the bipyridine ligand, respectively.

Herein the present work, we have synthesized a new type of ruthenium

sensitizers, m-BL-1, having a terpyridine ligand and a bipyridine ligand in its molecular

structure. The two –NCS groups in black dye are replaced by a bipyridine ligand. The

bipyridine ligand is having an electron releasing group 3,5-di-tert-butyl phenyl groups on

extended- conjugation. The presence of extended- conjugation in its molecules

increases the molar absorption coefficient.10,11

This chapter provides details of synthesis,

characterization and phtovoltaic studies of newly synthesised ruthenium sensitizer. The

molecular structure of dye is shown in Figure.3.1.

N

N

N

N

N

Ru

N

C

SHOOC

HOOC

COOH

m-BL-1

Figure.3.1. Molecular structure of m-BL-1

62

3.2. Experimental details

The compounds [2,2’,6’2”]Terpyridine-4,4’,4”-tricaboxylic acid trimethyl ester and and

Black dye were synthesized according to the procedures as reported in the literature.12,13

3.2.1. Synthesis of HRD-1 ligand

This ligand was synthesized by using Wittig reaction. NaH (360 mg, 15 mmol) was

added to a solution of 2,2’-bipyridine-4,4’-diphosphonate (1.5 g, 3.5 mmol) and 3,5-di-

tert-butyl benzaldehyde (1.7 g, 7.8 mmol) in 150 ml of dry tetrahydrofuran (THF). The

resulting mixture was refluxed overnight under nitrogen atmosphere. The reaction

mixture was allowed to cool to room temperature and filtered the compound. The filtrate

is concentrated and the solid is washed with methanol and dried to obtain the desired

product in pure form of 75% yield. Elemental analysis of C42H52N2 (calculated mass % in

parentheses) C, 86.20 (86.25); H, 9.00 (8.96); N, 4.75 (4.79). 1H NMR (300 MHz, 25

oC,

CDCl3) 1.34 (m, 36H), 6.97 (d, 1H), 7.13-7.25 (m, 8H), 7.29 (d, 1H), 8.63 (s, 2H), 8.71

(d, 2H).

3.2.2. Synthesis of m-BL-1

RuCl3.3H2O (624 mg, 2.5 mmol) was dissolved in a mixture of ethanol and chloroform

(100 ml). To this 1.00 g (2.5 mmol) of [2,2’,6’2”]-terpyridine-4,4’,4”-tricaboxylic acid

trimethyl ester was added and the reaction mixture was refluxed under nitrogen

atmosphere. The progress of the reaction was monitored by UV-Vis., spectroscopy. After

4 h the solvent was removed under reduced pressure and the crude compound was dried

under vacuum to get the trichloro derivative of ruthenium complex.

The trichloro derivative of ruthenium complex (200 mg, 0.32 mmol) was dissolved

in 100 ml of dry DMF. To this, 228 mg (0.39 mmol) of 4,4’-bis-[3,5-di-tert-butyl-

phenyl)-vinyl]-[2,2’]bipyridyl was added and reaction mixture was refluxed for 4 h under

nitrogen atmosphere. The reaction mixture was cool to 80 oC and aqueous solution of

ammonium thiocyanate (760 mg, 9.92 mmol) was added and refluxed for further 2 h. The

reaction mixture was further cool to room temperature and triethyl amine and water (2+2

ml) were added and then refluxed for 48 h. Then the solvent was removed under reduced

63

pressure and water was added to get the precipitate. The solid precipitate was filtered,

washed with water and dried under vacuum. The obtained solid material was dissolved in

methanol, which contains TBA (tetrabutyl ammonium hydroxide) and submitted to

sephadex LH-20 column chromatography with methanol as eluent. The main intense

brown color band was collected and solvent was removed under reduced pressure to get

the desired compound. m-BL-1: Anal. calcd. for C61H63N6O6RuS: C, 66.04; H, 5.72; N,

7.58% Found: C, 66.00; H, 5.65; N, 7.50 %. MS: m/z+1TBA 1351. 1H NMR (CD3OD)

(300 MHz) ppm: 9.08 (dd, 4H, J = 4.6 Hz), 8.80 (s, 2H), 8.70 (d, 2H, J = 4.5 Hz), 8.65

(s, 2H), 7.70 (d, 2H, J = 4.0), 7.25 (d, 2H, J = 3.8Hz), 7.20 (s, 2H), 7.13 (s, 4H), 7.10 (d,

2H, J = 6.2 Hz), 6.95 (d, 2H, J = 6.2 Hz), 1.35 (m, 36H). UV-Vis, in ethanol (max, M-

1 cm

-1): 510 (17,300).

N

N

N

H3COOC

COOCH3

COOCH3

RuCl3, EtOH

N

N

N

Ru

ClCl

Cl

H3COOC

H3COOC

COOCH3

N N

t-Bu t-But-Bu t-Bu

NH4NCS, DMF

TEA, H2O

N

N

N

N

N

Ru

HOOC

HOOC

COOH

N

C

S

HRD-1 Ligand

m-BL-1

Scheme 3.1

64

3.3. Results and Discussions

3.3.1. Synthesis and Characterization

The ligand 4,4’-bis-[3,5-di-tert-butyl-phenyl)-vinyl]-[2,2’]bipyridyl (HRD-1 ligand) was

synthesized as per the literature methods.11

The complex m-BL-1 was synthesized by

refluxing RuCl3.3H2O and [2,2’,6’2”]-terpyridine-4,4’,4”-tricaboxylic acid trimethyl

ester in DMF solvent to get the corresponding trichloro complex. To this complex, 4,4’-

bis-[3,5-di-tert-butyl-phenyl)-vinyl]-[2,2’]bipyridyl and ammonium thiocyanate was

refluxed for further four hours. The solvent was removed under vacuo and an obtained

crude material was subjected to sephadex column with methanol as eluent to get the

desired compound. The complex m-BL-1 was completely characterized by CHN

analysis, FT-IR, Mass, UV-Vis., fluorescence spectroscopy as well as cyclic

voltammetry. The elemental analysis is satisfactory and corresponds to the presence of

one TBA group in its molecular structure. The mass spectrum of m-BL-1 consists of a

peak at 1351 m/z, which corresponds to the presence of one TBA molecule in its

molecular structure, Figure 3.2.

Figure 3.2. ESI-MS spectrum of m-BL-1

65

400 500 600 700 800

0

1

2

Ab

sorb

ance

Wavelength (nm)

Figure 3.3 shows the absorption spectrum of m-BL-1 recorded in ethanol and was

compared to that of standard Black dye. The metal to ligand charge transfer transition

(MLCT) of m-BL-1 was centered at 510 nm with molar extinction coefficient () of

17,300 M-1

cm-1

.

Figure 3.3. UV-Vis absorption and emission spectra of m-BL-1 in ethanol solvent

In contrast, black dye has MLCT absorption maximum at 619 nm with of 7,987 M-1

cm-1

.14

The absorption maximum was bathochromically shifted in the case of m-BL-1,

due to the replacement of two –NCS ligands with an ancillary bipyridine ligand. Also the

molar extinction coefficient, value of m-BL-1 has doubled when compared to that of

standard sensitizer, Black dye which is due to the increase in the extended -conjugation

66

on the ancillary bipyridine ligand of m-BL-1 complex.15

The intraligand -* transitions

of terpyridine ligand are located at 320 nm, while that of bipyridine is at 280 nm. The

absorption features of m-BL-1 adsorbed on an opaque TiO2 film (8 m thick) was also

measured and was found to be similar to that of observed in solution except for a slight

red shift in absorption maxima due to interaction of anchoring groups to the

nanocrystalline TiO2 surface. Figure 3.3 also shows the emission spectrum of m-BL-1.

Excitation of lower energy MLCT transition of m-BL-1 in ethanol solvent at room

temperature produces an emission centered at 720 nm. From the absorption and emission

spectra, the singlet state (E0-0) energy of m-BL-1 and Black dye was calculated to be

1.94 & 1.60 eV, respectively. The emission of m-BL-1 was quenched when adsorbed on

to 8m thick TiO2 film layer due to the electron injection from the excited state of Ru(II)

complex to the conduction band of TiO2. The absorption and emission data is shown in

Table 3.1.

Table 3.1. UV-Visible, emission and electrochemical data of m-BL-1

Sensitizer max, nm,

(mol-1

cm-1

)a

em, nmb E1/2 V vs.

SCEc

Oxd Red

E0-0, eVd Eox

*

Black Dye 619 (7,987) 850 0.60 -1.10 1.60 -1.0

m-BL-1 510 (17,300) 720 0.78 -1.33 1.94 -1.16

aSolvent: ethanol, Error limits: max, ±1 nm, ±10%.

bSolvent: ethanol, max, ±1 nm.

cSolvent: DMF, Error limits: E1/2 ±0.03 V, 0.1 M TBAP.

dError limits: 0.05 eV.

eFrom the

reference no. 5

With a view to evaluate HOMO-LUMO levels of new photosensitizer, we have carried

out the electrochemical investigations in DMF solvent using 0.1 M tetrabutyl ammonium

perchlorate as supporting electrolyte. The redox potentials were measured by both cyclic

67

1.0 0.8 0.6 0.4 0.2 0.0

Voltage / V

Figure 3.4. Cyclic ( ) and Differential pulse voltammogram (----) of m-BL-1

and differential pulse voltammetric techniques and the data was compared with that of

the standard sensitizer Black dye. The redox potentials of the new ruthenium sensitizer

and the standard dye were presented in Table 3.1. From the Table 3.1, it is clear that m-

BL-1 undergoes one electron reversible oxidation at 0.78 V vs. SCE. The oxidation

process can be readily assigned to the Ru(II)/Ru(III) redox couple. It also undergoes two

reductions at -1.33 and -1.70 V, corresponding to the reduction of terpyridine acid and

ancillary bipyridine ligands. The excited oxidation potential of m-BL-1 is calculated

using equation 3.1.

E∗ox = Eox − E0−0 (3.1)

68

Where EOX is oxidation potential of sensitizers, and E0-0 is singlet state energy of

sensitizer. The excited state potential of m-BL-1 is found to be 1.16 V, which is above

the conduction band of TiO2.16

To get an insight into the molecular structure and electron distribution of the m-

BL-1 sensitizer, its geometry has been optimized using density functional theory (DFT)

calculations using mPW1PW91 method for the geometry optimization with LANL2DZ

basis function on Ru and 6-31g(d) basis function on C,H,N,O and S. As can be seen from

the Figure 3.5, in HOMO, HOMO-1 & HOMO-2, the electron density is delocalized over

the Ru(II) metal and –NCS ligand. The LUMO, LUMO+1 and LUMO+2 are π* orbitals

delocalized over the terpyridine carboxylic acid ligand facilitating electron injection from

the excited state of m-BL-1 sensitizer to the conduction band of TiO2. These results are

in good agreement with other ruthenium(II) polypyridyl complexes reported in the

literature.17

Figure 3.5. Molecular orbital spatial orientation of m-BL-1

69

3.3.2. Photovoltaic Studies

The performance of newly synthesized m-BL-1 as a sensitizer with a sandwich type

nanocrystalline TiO2 electrode composed of transparent layer was determined from

measurements on photovoltaic cells using three different redox electrolytes i.e., E-01, Ei-

301 & EA-10 and compared its performance with that of standard sensitizer Black dye

under similar test cell conditions. The photocurrent action spectra of both m-BL-1 and

Black dye in all three redox electrolytes were shown in Figure 3.6, where the incident

monochromatic photon-to-current conversion efficiencies (IPCE) values are plotted as a

function of excitation wavelength. The E-01 redox electrolyte contains 0.05 M I2, 0.1 M

LiI, 0.6 M 1,2-dimethyl-3-n-propylimidazolium iodide (DMPI), 0.5 M tert-butyl pyridine

in acetonitrile solvent; Ei-301 redox electrolyte contains 1.0 M LiI, 0.05 M I2 -

butyrolactone and EA-10 redox electrolyte contains 0.1 M I2, 0.6 M DMPI and 0.5 M n-

methylbenzimidazolium iodide (NMBI) in γ- butyrolactone solvent.

Figure 3.6. Photocurrent action spectra of (a) m-BL-1 & (b) black dye using (_____

) Ei-

301, (--) E-01 and (…) EA-10 redox electrolytes

70

We have observed IPCE values of 55, 84 & 35% at 520 nm using E-01, Ei-301 & EA-10,

respectively using m-BL-1 as sensitizer. Under similar test cell conditions, the Black dye

has shown IPCE value of 37, 73 and 9% at 627 nm using E-01, Ei-301 & EA-10,

respectively. From these observations it is clear that DSSC devices using m-BL-1

sensitizer has shown better IPCE values than Black dye. From the Figure 3.6 it is clear

that the photocurrent action spectrum resembles the absorption spectra except for a slight

red shift by ca. 10 nm in both m-BL-1 and Black dye. The photoresponse of thin films

displays a broad spectral response covering the entire visible spectrum up to 850 nm in

case of m-BL-1 and up to 950 nm in case of black dye.

The photovoltaic performance of DSSC based on new ruthenium sensitizer m-

BL-1 with three different redox electrolytes along with standard sensitizer Black dye

under 1 sun (1000 W/m2) irradiation was shown in Table 3.2.

Table 3.2. Photovoltaic performance of m-BL-1 and Black dye.a

Sensitizer Electrolyteb Jsc (mA/cm

2)c Voc (mV)

c ff

c (%)

m-BL-1 E-01 8.56 610 0.72 3.76

Ei-301 13.11 510 0.46 3.11

EA-10 4.82 650 0.68 2.13

Black Dye E-01 10.04 710 0.61 4.47

Ei-301 15.88 560 0.37 3.29

EA-10 2.57 680 0.68 1.20

aPhotoelectrode: TiO2 (8 + 4 m and 0.74 cm

2);

belectrolyte: E-01: 0.05 M I2, 0.1 M LiI,

0.6 M 1,2-dimethyl-3-n-propylimidazolium iodide (DMPI), 0.5 M tert-butyl pyridine in

acetonitrile solvent. Ei-301: 1.0 M LiI, 0.05 M I2 -butyrolactone. EA-10: 0.1 M I2, 0.6

M DMPI and 0.5 M n-methylbenzimidazolium iodide (NMBI) in γ-butyrolactone. cError

limits: JSC: ±20 mA/cm2, VOC = ±30 mV, ff = ±0.03.

71

Figure 3.7. Current-voltage characteristics (a) m-BL-1 & (b) Black dye using (_____

) Ei-

301, (--) E-01 and (……) EA-10 redox electrolytes.

72

The solar energy-to-electricity conversion efficiency, under white-light irradiation was

obtained using the equations 2.2 & 2.3. Figure 3.7 shows photocurrent-voltage of m-BL-

1 sensitizer grafted onto nanocrystalline TiO2 films along with Black dye with three

different redox electrolytes and related parameters are listed in Table 3.2.

We have observed an overall conversion efficiency of 3.72% under 1.0 sun irradiation

(JSC = 8.56 mA/cm2, VOC = 610 mV, ff = 0.72) using E01 redox electrolytes. Under

similar test cell conditions black dye has shown of 4.47% (JSC = 10.04 mA/cm2, VOC =

710 mV, ff = 0.61). The solvent used in this redox electrolyte is acetonitrile, which is

highly volatile and as a results durability of the device will be low.

To overcome these problems, we have used -butyrolactone solvent in Ei-301 and

EA-10 redox electrolytes. The reason for using -butyrolactone lies in its high boiling

point, low volatility, non-toxic and good photochemical stability, making it viable for

practical applications. Using EA-10 redox electrolyte with m-BL-1 sensitizer, we have

observed of 2.13% (JSC = 4.82 mA/cm2, VOC = 650 mV, ff = 0.68). Under similar test

cell conditions, black dye has shown of 1.20% (JSC = 2.57 mA/cm2, VOC = 680 mV, ff

= 0.68). The low efficiency of test cell devices using EA-10 redox electrolyte in both the

sensitizers are due to the high viscous nature of -butyrolactone solvent and is therefore

unable to penetrate into the pores of nanocrystalline TiO2. In order to further improve the

efficiency of test cell devices, we have modified the redox electrolyte composition

keeping the solvent same. We have used excess LiI in Ei-301 redox electrolyte. The

presence of excess concentration LiI, decreases VOC due to positive shift of band edge but

increases electron transport property and acts as the driving force for electron injection.

From the Table 2, it is clear in both the sensitizers, VOC reduced by 15% with an increase

of JSC. As a result the efficiency is also increased in both the sensitizers.

We have observed better efficiency using m-BL-1 sensitizer with EA-10 redox

electrolyte than black dye based devices. In contrast, with the two other redox

electrolytes the efficiency is high with black dye than m-BL-1 sensitizer. This may be

due to the absorption of black dye extends up to 950 nm whereas in m-BL-1 it extends

only 850 nm.

73

3.3.3. Thermal Studies

In order to use for outdoor applications of DSSC devices, the sensitizer should be stable

to higher temperatures. For this reason, we have carried out the thermogravimetric

analysis of m-BL-1 to understand the thermal stability of new sensitizer. Figure 3.8

shows the thermogram of m-BL-1. From the figure it is clear that the thermal behavior of

this sensitizer is stable upto 2500C. The initial weight loss (21%) is attributed to the

removal of carboxyl group. From the thermal data, it is clear that DSSC device based on

this sensitizer is potentially useful for rooftop applications.

Figure 3.8. TG-DTG curves of m-BL-1 sensitizer with heating rate of 100C/min under

nitrogen

3.3.4. Conclusions

In conclusion, we have designed and synthesized a new type of ruthenium polypyridyl

sensitizer that contains a terpyridine, a bipyridine and thiocyanate ligand in its molecular

74

structure. Extended -conjugation has been used to increase the molar absorption

coefficient and terpyridine is incorporated to enhance the stability of the device. The new

sensitizer was completely characterized by elemental analysis, ESI-MS, IR, 1H NMR and

fluorescence spectroscopy as well as cyclic voltammetry. The molar absorption

coefficient of the new sensitizer was found to be higher than the standard Black dye.

From absorption, electrochemistry and fluorescence studies it is clear that the LUMO of

this sensitizer is above the TiO2 conduction band and HOMO lies below the I-/I3

- redox

couple. The new photosensitizers were tested in DSSC by using E-01, Ei-301 and EA-10

redox electrolytes. The photosensitizer m-BL-1 was found more efficient than black dye

using EA-10 redox electrolyte, it has shown of 2.13%. Under similar test cell

conditions, Black dye has shown of 1.20%. Thermal stability studies show that m-BL-

1 was stable upto 2500C and it is therefore quite suitable for rooftop applications.

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