chapter 4: synthesis of colorants for dye-sensitized solar...

Chapter 4: Synthesis of colorants for dye-sensitized solar cells derived from nitrogen heterocycles

Synthesis of novel colorants for dye-sensitized solar cells and use of greener protocols for heterocyclic synthesis Page 98

4.1 INTRODUCTION

Dye-sensitized solar cells (DSSCs) have attracted attention as noteworthy low-cost

alternatives for the conventional solid p–n junction photovoltaic devices (Gratzel

2001; Gratzel 2004). These solar cells have most dominantly used metal complex

sensitizers involving ruthenium poly-pyridyl complexes especially because of their

high power conversion efficiencies and long term stability (Wang et al. 2003; Liang

et al. 2007). Although such sensitizers have their own advantages, organic dyes

possess wide scope owing to their ease of synthesis, high molar extinction

coefficient, tuneable absorption spectral response from the visible to the near

infrared (NIR) region, environmental friendliness and inexpensive

production techniques.

The photoconversion efficiencies of dye-sensitized solar cells fabricated using

metal-free organic dye molecules commonly contain structural framework involving

donor (D), conjugating π bridge and acceptor (A) groups. This type of arrangement

is important due to the effective photoinduced intramolecular charge transfer

propertyof such systems (Chang et al. 2009; Wu et al. 2012).

Heterocyclic rings prove to be very efficient when incorporated as donors in

molecules for dye-sensitized solar cells. Various nitrogen containing heterocycles

have been explored in the past. These include rings such as tetrahydroquinoline

(Chen et al. 2007a; Chen et al. 2007b), indoline (Ito et al. 2006; Ito et al. 2008),

phenothiazine (Tian et al. 2007), julolidine (Choi et al. 2007), coumarin (Hara et al.

2007; Wang et al. 2007), etc. The nitrogen atom ensures better flow of electrons

from the donor to acceptor units thereby improving the current efficiency (Jsc)

values. Some examples of such systems are shown in the figure below.



Jsc = 10.9 mA.cm–2

Voc = 0.71 V

Efficiency = 5.5 %

Jsc = 10.6 mA.cm–2

Voc = 0.49 V

Efficiency = 2.3 %

Jsc = 8.5 mA.cm–2

Voc = 0.6 V

Efficiency = 3.2 %

Amongst various heterocyclic donor system, carbazole is a moeity that has been

explored well in DSSCs mainly due to their excellent electron donating and hole-

transporting ability (Ning et al. 2009; Chen et al. 2009; Zhang et al. 2009) . This has

led to its demand even in opto and electroactive materials auch as organic light-

emitting diodes (OLEDs) and solid-state DSSCs (Li et al. 2006 ; Ohkita et al.

2004). Carbazole has been employed in different interesting ways in DSSCs. Wang



et al. have used it as a donor with several thiophene units attached from the 3-

position of carbazole in D-π-A structural unit (Wang et al. 2008). In another report,

Srinivas et al. and Teng et al. synthesized molecule by extending the linker and

acceptor part from the nitrogen atom of carbazole (Srinivas et al. 2011; Teng et al.

2009).

Another advantage of carbazole is the presence of two active positions (3 and 6)

which leads to di-substituted derivatives (Ramkumar et al. 2012). Ths type of di-

substitution assists in improving the photo-induced intra-molecular charge transfer

(ICT) from donor to acceptor and increases the electron injection ability from the

LUMO level of the dye molecule to conduction band of TiO2 surface (Yang et al.

2010; Ooyama et al. 2011).

Other important nitrogen heterocycles that we have explored includes indole and

julolidene. Although indole has been taken as a donor in many functional

applications such as non-liner optics (Li et al. 2008; Liet al. 2009a), however there

are few reports on its use for solar cells (Li et al. 2009b; Inoue et al. 2010). Similarly

julolidine is another heterocycle which has also been very less explored (Choi et al.

2007a, Choi et al. 2007b).



Hence, we aimed to utilize these heterocycles as efficient donors for synthesis of

molecules for solar cell applications. The colorants were tested for their photo-

physical and thermal properties. After successful synthesis and purification, these

colorants were successfully used to construct dye-sensitized solar cells. The various

parameters including short-circuit current density (Jsc), short-circuit voltage (Voc)

and cell efficiency (η) were measured.

N

CN

CN

CN

COOH

COOH

NC

N SCOOH

CN



4.2 RESULTS AND DISCUSSION

4.2.1 Synthesis of styryl dyes for dye sensitized solar cell

The indole based novel styryl colorants [7a-7b] were prepared by classical

Knoevenagel condensation of 1-butyl-1H-indole-3-carbaldehyde [4] with

cyanoacetic acid [5] or 2-cyano-3-(p-tolyl)acrylic acid [6] in ethanol using

piperidine as a solvent. The same method was also used for condensation of other

carbazole based sensitizers [12a-12c]. The intermediates 1-butyl-1H-indole-3-

carbaldehyde [4], 9-hexyl-9H-carbazole-3-dicarbaldehyde [11a] and 3, 6-

dicarbaldehyde [11b] were synthesized in a sequence of two steps including

alkylation and formylation. The alkylation step was performed using phase-transfer

catalysis in toluene wherein butyltriethylammonium chloride was taken as the

quarternary ammonium catalyst. The longer alkyl chains ensure good solubility and

also avoid charge recombination process that could lower the efficiency value (He et

al. 2011). The formylation was done by conventional Vilsmeier Haack method using

dimethylformamide and phosphorus oxychloride. The dicarbaldehyde derivative of

carbazole was synthesized using higher equivalents of formylating agent and longer

refluxing. The intermediates and colorants were characterized by 1H-NMR, 13C-

NMR and Mass spectroscopy.



Scheme 4.1: Synthesis of N-butyl-3-formylindole [4]

Scheme 4.2: Synthesis of indole based sensitizers [7a-7b]

+ H3C Br PTC,Toluene

40% NaOH solution N

CH3

NH

[1] [2] [3]

N

CH3

DMF

POCl3

H

O

N

CH3

[3] [4]

where PTC = Butyltriethylammonium chloride (BTEAC)

H

O

N

CH3

[4]

+Ethanol

PiperidineReflux

CN

HOOC[6]

CN

COOH

[5]

Dye 7a

Dye 7b



Scheme 4.3: Synthesis of mono-carbaldehyde [11a] and di-carbaldehyde

derivatives of carbazole [11b]

Scheme 4.4: Synthesis of mono substituted carbazole based sensitizers (12a-12b)

+ BrH3CPTC,Toluene

40% NaOH solution

DMF

POCl3

N

H

O

[8] [9]

N

H

OO

H

N

CH3

NH

[10]

[11a]

[11b]

N

CH3

[10]

N

[11a]

+

Ethanol

PiperidineReflux

CN

HOOC

[6]

CN

COOH

[5]

Dye 12a

Dye 12b

O

H



Scheme 4.5: Synthesis of di substituted carbazole based sensitizers (12c)

Table 4.1: Physical properties of dyes

Dye No. Molecular formula Molecular

weight

Yield in

%

M.P.

DYE 7a C16H16N2O2 268 92 146

DYE 7b C24H22N2O2 370 61 152

DYE 12a C22H22N2O2 346 95 140

DYE 12b C30H28N2O2 449 60 156

DYE 12c C42H35N3O4 646 49 192

[11b]

+Ethanol

PiperidineRefluxCN

HOOC[6]

Dye 12cN

CH3

OO

H H



Figure 4.1: Structure of final dyes


Synthesis of novel colorants for dye-sensitized solar cells and use of greener protocols for heterocyclic synthesis 7

4.2.2 Spectral characteristics of the dyes The linear absorption spectra of the synthesized heterocyclic sensitizers were

measured for concentrations of 1×10-3 M in chloroform. The path length of the cell

was 1 cm whereby the influences of the quartz cuvette and the solvent have been

subtracted. The dyes show a bathochromic shift on changing the donor from indole

to carbazole group and also on extending the conjugation by means of phenyl bridge

(Table 4.2). This could be owing to the better conjugation characteristics

incorporated by use of phenyl linker. The basic spectral characteristics of the dyes

such as the absorption maxima (λmax), emission maxima (λem) and extinction

coefficient (ε) were measured in chloroform and are presented in Table 4.2.

Table 4.2: UV-Visible and emission data of dyes

Dye

No.

Absorption in nm (CHCl3)

Emission in nm

(CHCl3)

Stokes

Shift in nm

DYE 7a 350 385 35

DYE 7b 372 410 38

DYE 12a 392 425 33

DYE 12b 396 436 40

DYE 12c 416 459 43



4.2.3 Thermal properties of the dyes

The dyes were subjected to the thermogravimetric analysis in order to investigate

their thermal stability. The thermo gravimetric analysis (TGA) was carried out in the

temperature range 25-600 °C under nitrogen gas at a heating rate of 10 °C min-1. The

TGA curves revealed that most of the dyes hold extremely good thermal stability

with majority of dyes showing stability above 250 °C as revealed in Table 4.3. It

was observed that the incorporation of benzene bridge improved the thermal stability

of the molecules in comparison to their counterparts without linker groups. The

sensitizer 12c showed the best thermal stability amongst all which could be

attributed to the rigidity of the di-substituted system. The higher value of thermal

stability is very much desirable in high-technological applications like dye-

sensitized solar cells.

Table 4.3: Thermal stability of sensitizers

Dye No.

Temperature stability (°C)

(at 4% product decomposition)

7a 242

7b 262

12a 288

12b 312

12c 325



4.2.4 Application for dye sensitized solar cell The synthesized colorants were applied onto dye-sensitized solar cells. For the

preparation of cells, doctor blading method was employed. After making the films

they were annealed at 450°C for 30 min. For sensitization, the films were

impregnated with 0.5 mM N719 dye in ethanol for 24 h at room temperature. The

samples were then rinsed with ethanol to remove excess dye on the surface and were

air dried at room temperature. This was followed by redox electrolyte addition and

top contact of Pt coated FTO. The electrolyte used was 1 M 1-hexyl-2, 3-dimethyl-

imidazolium iodide, 0.05 M LiI, 0.05 M I2, and 0.5 M 4-tert-butylpyridine in

acetonitrile. The other details concerning the construction of solar cell and its

application process is described in chapter 3.

The dyes 7a, 7b and 12b were applied on DSSC and the Photocurrent density vs.

voltage curves were derived as shown in figures 4.2-4.4. These curves indicate that

compound 7a gave the best efficiency amongst other dyes. This was mainly due to

the enhanced values of short-circuit photocurrent density (Jsc). This increase could

be mainly attributed to the improved injection efficiency of electrons into the

conduction band of TiO2 in the case of dye 7a since the donor group is directly

connected to the acceptor group leading to direct passage of electrons. Amongst

benzene bridged moieties 7b and 12b, the dye 12b gave better efficiency and better

Jsc values which could be owing to the better donating ability of carbazole.



Figure 4.2: Photocurrent density vs. voltage curves for DSSCs based on dye-7a

Name Voc (V) Jsc(mA/cm2) FF (%) η (%)

1st 0.59 2.45 67.9 0.98

2nd 0.59 2.50 66.8 0.99



Figure 4.3: Photocurrent density vs. voltage curves for DSSCs based on dye-7b


1st 0.49 0.41 68.7% 0.14

N

CH3

COOH

CN



Figure 4.4: Photocurrent density vs. voltage curves for DSSCs based on dye-12b


1st 0.51 0.6 64.6 0.19



4.3 EXPERIMENTAL

4.3.1 Materials and equipments

All the solvents and chemicals were procured from S D fine chemicals, Sigma-

Aldrich and were used without further purification. The reactions were monitored by

TLC using 0.25 mm E-Merck silica gel 60 F254 precoated plates, which were

visualized with UV light. UV – Visible absorption spectra were recorded on

Spectronic genesis 2 spectrophotometer instruments from dye solutions (~ 10-3 M) in

chloroform. The 1H NMR spectra were recorded on 400 MHz on Varian mercury

plus spectrometer. Chemical shifts are expressed in δ ppm using TMS as an internal

standard. Mass spectral data were obtained with micromass-Q-Tof (YA105)

spectrometer. Elemental analysis was done on Harieus rapid analyzer. Melting

points measured and thermogravimetric analysis was carried out on SDT Q600 v8.2

Build 100 model of TA instruments.

4.3.2 Synthesis of key intermediates and compounds

4.3.2.1 Synthesis of N-butyl indole [3]

In a 500 ml round bottomed flask fitted with a mercury sealed stirrer, indole [1] (5g,

42 mmol), 50% aqueous sodium hydroxide solution (17.5 ml) and toluene (10 ml)

was heated to 50-55 ºC for 15 minutes. This was followed by addition of butyl

triethyl ammonium chloride (0.17g, 0.03 mole) to the reaction mixture and heating

was continued at 70-75 ºC for 30 minutes. The addition of 1-bromobutane [2] (8.7 g,

6.9 ml, 64 mmol) was done slowly through an addition funnel and reaction mass was

stirred for 3 hours at 70-75 ºC. The progress of the reaction was monitored by thin

layer chromatography. After completion of reaction, the reaction mass was poured



into hot water and left overnight. The product was filtered through suction vacuum

pump, washed with water, dried and recrystallised from ethanol with a yield of (6.2

g, 85 %).

4.3.2.2 Synthesis of N-butyl-3-formylindole [4]

In a three necked 500ml round bottom flask fitted with a mercury sealed stirrer,

addition dropping funnel topped by calcium chloride guard tube and reflux

condenser also topped by calcium chloride guard tube. N, N-dimethyl formamide

(d=0.944, 5.0 g, 5.3 ml, 69.3 mmol) was taken and cooled to 0-5°C with stirring. To

the above solution phosphorous oxychloride (d=1.645, 7.0 g, 4.3 ml, 46.2 mmol)

was added drop wise maintaining the temperature of the reaction mass at 0-5°C. The

DMF - POCl3 complex so formed was stirred for further 15 minutes and N-butyl

indole [3] (4g, 23.1 mmol) was added in lots (15-25 minutes) to the complex. The

reaction mixture was stirred at 0-5°C for 3 hrs and then allowed to attain room

temperature. The mixture was then vigorously stirred under vigorously stirring and

heated to 75°C for 6 h. This solution was then cooled to room temperature, poured in

to ice water, and neutralized to pH 6-7 by drop wise addition of saturated aqueous

sodium hydroxide solution. The mixture was extracted with dichloromethane. The

organic layer was dried with anhydrous NaSO4 and then concentrated on rotary

evaporator. The crude product on purification by column chromatography (mobile

phase- toluene and silica gel 60-120 mesh) afforded as a yellow powder after drying

with 72% of yield (3.3 g).



4.3.2.3 Synthesis of N-hexyl-carbazole [10]

In a 500 ml round bottomed flask fitted with a mercury sealed stirrer, carbazole [8]

(5g, 30 mmol), 50% aqueous sodium hydroxide solution (12.5 ml) and toluene (10

ml) was heated to 50-55 ºC for 15 minutes. This was followed by addition of butyl

triethyl ammonium chloride (0.12g, 0.02 mole) to the reaction mixture and heating

was continued at 70-75 ºC for 30 minutes. The addition of 1-bromohexane [9] (7.4 g,

6.3 ml, 45 mmol) was done slowly through an addition funnel and reaction mass was

stirred for 4 hours at 70-75 ºC. The progress of the reaction was monitored by thin

layer chromatography. After completion of reaction, the reaction mass was poured

into hot water and left overnight. The product was filtered through suction vacuum

pump, washed with water, dried and recrystallised from ethanol to get white powder

with a yield of (6.2 g, 82 %); M.P. = 64 °C

4.3.2.4 Synthesis of N-hexyl-3-formyl-carbazole [11a]

In a three necked 500ml round bottom flask fitted with a mercury sealed stirrer,



(d=0.944, 4.65g, 4.92ml, 63.7 mmol) was taken and cooled to 0-5°C with stirring.

To the above solution phosphorous oxychloride (d=1.645, 7.3g, 4.4 ml, 47.8 mmol)


DMF - POCl3 complex so formed was stirred for further 15 minutes and N-butyl-

carbazole [10] (4g, 15.9 mmol) was added in lots (15-25 minutes) to the complex.

The reaction mixture was stirred at 0-5°C for 3 hrs and then allowed to attain room

temperature. The mixture was then vigorously stirred under vigorously stirring and



heated to 75°C for 8 h. This solution was then cooled to room temperature, poured in

to ice water, and neutralized to pH 6-7 by drop wise addition of saturated aqueous

sodium hydroxide solution. The mixture was extracted with dichloromethane. The

organic layer was dried with anhydrous Na2SO4 and then concentrated on rotary

evaporator. The crude product on purification by column chromatography (mobile

phase- toluene and silica gel 60-120 mesh) afforded as a yellow powder after drying.

Yield = 3 g (69 %); M.P. = 60 °C.

4.3.2.5 Synthesis of N-hexyl-3, 6-diformyl-carbazole [11b]

In a three necked 500ml round bottom flask was fitted with a mercury sealed stirrer,



(d=0.944, 9.30 g, 9.8 ml, 127 mmol) was taken and cooled to 0-5°C with stirring. To

the above solution phosphorous oxychloride (d=1.645, 14.6 g, 8.89 ml, 95.6 mmol)


Vilsmeier complex so formed was stirred for further 15 minutes and N-butyl-

carbazole [10] (4g, 15.9 mmol) was added in lots (15-25 minutes) to the complex.

The reaction mixture was stirred at 0-5°C for 2 hrs and then allowed to attain room

temperature. The mixture was then vigorously stirred and heated to 90 °C for 12

hours. This solution was then cooled to room temperature, poured in to ice water,

and neutralized to pH 6-7 by drop wise addition of saturated aqueous sodium

hydroxide solution. The mixture was extracted with dichloromethane. The organic

layer was dried with anhydrous Na2SO4 and then concentrated on rotary evaporator.

The crude product on purification by column chromatography (mobile phase-



toluene and silica gel 60-120 mesh) afforded a white solid was obtained. Yield = 2.6

g (54 %); M.P. = 138 °C.

4.3.2.6 Synthesis of (Z)-2-cyano-3-(p-tolyl)acrylic acid [6] is described in chapter 2,

section 2.3.2.5

4.3.2.6 Synthesis of (Z)-3-(1-butyl-1H-indol-3-yl)-2-cyanoacrylic acid [7a]

In a three necked 100ml round bottom flask fitted with a mercury sealed stirrer, a

suspension of cyanoacetic acid [5] (0.84 g, 9.9 mmoles) and 1-butyl-1H-indole-3-

carbaldehyde [4] (1.0g, 4.9 mmoles) were heated together in ethanol (10ml, 10 vol)

at reflux in presence of catalytic amount of piperidine for 5 hrs. The completion of

the reaction was monitored by thin layer chromatography. After cooling the reaction

mass, the mixture was poured into water and extracted using ethyl acetate. The ethyl

acetate layer was evaporated under vacuum using rotary evaporator. The obtained

residue was purified by column chromatography (toluene, 60 – 120 mesh silica gel)

to obtain final product [7a]. Yield = 1.2 g (89 %); M.P. = 220 °C.

Analysis of dye [7a]:

A. Mass spectra of the compound showed ion peak at m/z = 269 which

corresponds to molecular weight of [7a]

B. The compound was further confirmed by which showed following signals

[7a].

1H NMR (CDCl3, 300 MHz): δ (ppm) 8.65-8.60 (m, 1H, aromatic CH); 7.84-

7.80 (m, 1H, vinylic CH); 7.43-7.40 (m, 1H, aromatic CH); 7.39-7.32 (m,

2H, aromatic CH); 7.20 (s, 1H, aromatic CH); 4.24-4.20 (m, 2H, aliphatic



CH2); 2.0-1.86 (m, 2H, aliphatic CH2); 1.45-1.36 (m, 2H, aliphatic CH2), 1.0-

0.96 (t, 3H, aliphatic CH3).

C. 13C NMR (CDCl3, 300 MHz): δ (ppm) 147.3, 136.4, 134.7, 128.5, 124.1,

123.0, 118.6, 110.8, 110.2, 47.7, 31.9, 20.1, 13.6.

4.3.2.7 Synthesis of 3-(4-((E)-2-(1-butyl-1H-indol-3-yl)vinyl)phenyl)-2-

cyanoacrylic acid [7b] was synthesized by the same procedure as that of compound

[7a] except that 2-cyano-3-(p-tolyl)acrylic acid [6] was used instead of cyanoacetic

acid. Yield = 1.1 g (61 %); M.P. = 232 °C.

Analysis of dye [7b]:

A. Mass spectra of the compound showed ion peak at m/z = 297, 269 which

corresponds to molecular weight of [7b] after suitable fragmentation

(removal of alkyl chain and COOH group).


[7b].

1H NMR (CDCl3, 300 MHz): δ (ppm) 8.51 (s, 1H, aromatic CH); 7.82-7.79

(m, 1H, vinylic CH); 7.43-7.39 (m, 2H, aromatic CH); 7.36-7.26 (m, 6H,

aromatic CH); 7.25-7.16 (m, 2H, vinylic CH); 4.21-4.17 (m, 2H, aliphatic

CH2); 1.90-1.82 (m, 2H, aliphatic CH2); 1.41-1.36 (m, 2H, aliphatic CH2),

1.0-0.93 (t, 3H, aliphatic CH3).


129.2, 128.5, 123.8, 122.6, 118.6, 118.4, 110.7, 109.9, 93.5, 61.8, 47.5, 31.8,

20.1.



4.3.2.8 Synthesis of 2-cyano-3-(9-hexyl-9H-carbazol-3-yl)acrylic acid [12a]


suspension of cyanoacetic acid [5] (0.60 g, 7.1 mmoles) and that N-hexyl-3-formyl-

carbazole [11a] (1.0g, 3.5 mmoles) were heated together in ethanol (10ml, 10 vol) at

reflux in presence of catalytic amount of piperidine for 7 hrs. The completion of the

reaction was monitored by thin layer chromatography. After cooling the reaction

mass, the mixture was poured into water and extracted using ethyl acetate. The ethyl

acetate layer was evaporated under vacuum using rotary evaporator. The obtained

residue was purified by column chromatography (toluene, 60 – 120 mesh silica gel)

to obtain final product [12a]. Yield = 1.12 g (90 %); M.P. = 202 °C.

4.3.2.9 Synthesis of 2-cyano-3-(4-(2-(9-hexyl-9H-carbazol-3-

yl)vinyl)phenyl)acrylic acid [12b] was synthesized by the same procedure as that of

compound [12b] except that 2-cyano-3-(p-tolyl)acrylic acid [6] was used instead of

cyanoacetic acid. Yield = 0.97 g (60 %); M.P. = 228 °C.

Analysis of dye [12b]:

A. Mass spectra of the compound showed ion peak at m/z = 444 which

corresponds to molecular weight of [12b]


[12b].

1H NMR (CDCl3, 300 MHz): δ (ppm) 10.5 (s, 1H, COOH); 8.74 (s, 1H,

aromatic CH); 8.40 (s, 1H, aromatic CH); 8.24-8.12 (m, 2H, vinylic CH);

7.54-7.50 (m, 2H, aromatic CH); 7.46-7.40 (m, 2H, aromatic CH); 7.30-7.24

(m, 6H, aromatic CH); 7.20 (m, 1H, vinylic CH); 4.40-4.26 (m, 2H, aliphatic



CH2); 1.90-1.80 (m, 2H, aliphatic CH2); 1.30-1.20 (m, 2H, aliphatic CH2),

0.90-0.80 (t, 3H, aliphatic CH3).


126.9, 125.4, 123.5, 122.8, 122.6, 126.9, 126.6, 117.0, 109.5, 104.4, 97.7,

62.3, 43.5, 31.5, 29.7, 28.9, 26.9, 22.5, 14.3, 14.0

4.3.2.10 Synthesis of 9-hexyl-9H-carbazole-3,6-diyl)bis(ethene-2,1-diyl))bis(4,1-

phenylene))bis(2-cyanoacrylic acid) [12c]


suspension of 2-cyano-3-(p-tolyl)acrylic acid [6] (1.55 g, 8.30 mmoles) and 9-hexyl-

9H-carbazole-3,6-dicarbaldehyde [12b] (1.0g, 3.32 mmoles) were heated together in

ethanol (10-15ml) at reflux in presence of catalytic amount of piperidine for 8 hrs.

The completion of the reaction was monitored by thin layer chromatography. After

cooling the reaction mass, the mixture was poured into water and extracted using

ethyl acetate. The ethyl acetate layer was evaporated under vacuum using rotary

evaporator. The obtained residue was purified by column chromatography (toluene,

60 – 120 mesh silica gel) to obtain final product [12c].

Yield = 1.13 g (49 %); M.P. = 242 °C



4.4 CONCLUSION In this chapter, we had aimed at designing dyes for application in dye-sensitized

solar cells by taking electron rich nitrogen heterocycles as donor groups. In this

respect, we selected indole and carbazole rings which were attached directly or via

phenyl conjugation to the cyanoacetic acid units. The introduction of phenyl bridge

leads to bathochromic shift and also improvement in the thermal stability of the

dyes. Moreover, the scope of di-substitution in moieties such as carbazole gave

further improvement in thermal stability.

These dyes were further applied onto dye-sensitized solar cells to check for their

efficiency values and other parameters. The direct attachment of cyanoacetic acid

unit to these heterocycles improved the electron injection efficiency and gave rise to

higher values of overall efficiency. In terms of donating ability, carbazole can be

considered to be better donating group than indole and therefore gave better

efficiency values. Therefore, nitrogen containing heterocycles possess good potential

and scope of improvement so as to obtain efficient molecules for application in dye-

sensitized solar cells.



Mass Spectra



1H NMR Spectra



13C NMR Spectra

chapter 4: synthesis of colorants for dye-sensitized solar...

Documents