**2.1 Dyes (sensitizers) used in solar cells**

## *2.1.1 Dye-sensitized solar cells (DSSCs)*

To prevent harmful impact on the environment by conventional energy sources it is necessary to use the alternative energy sources, specially, the solar cells. The conversion of sunlight into electricity is a clean, abundant, and renewable energy source. The amount of energy available from the sun to the earth is of the order of 3 × 1024 joules/year thus making it the best among sustainable energies. Photovoltaic devices have been fabricated using inorganic materials of high purity and energy-intensive processing techniques. The fabrication using these inorganic materials is not economical and often used scarce toxic materials. Therefore, such solid-state junction devices have been challenged by the 3rd generation dye-sensitized solar cells (DSSCs) which are based on interpenetrating network structures containing metal-free organic dyes as sensitizers [9].

In the conventional systems, the semiconductor does the task of light absorption as well as charge carrier transport. However, these two functions are separated in DSSCs by the metal-free organic dye and TiO2 in presence of an electrolyte. Hence, new ways of manufacturing the solar cells that can be scaled economically up to large volumes are essential. In this regard, a new generation of DSSCs also known as "Grätzel cells" has been fabricated by O'Regan and Grätzel [10]. A Grätzel cell consists of nanoporous titanium dioxide applied on transparent conducting oxide which is further made to absorb the dye from its solution. This film loaded with dye/sensitizer is immersed in an electrolyte containing a redox couple and placed on a platinum counter electrode. After irradiation, the excited electron from the dye (sensitizer) is transferred to the conduction band of TiO2 and diffuses through its porous network to the contact. Thus oxidized dye is further reduced to the original state by the supply of electrons through a liquid electrolyte redox couple within the pores [11].

The organic dye sensitizers consist of three important components *viz*., electron donor (D), π-conjugated spacer (π), and electron acceptor (A). Electron acceptors are generally acid ligands which also act as anchoring groups for loading the dye on TiO2 surface. The π-conjugated spacer (*viz*., conjugated double bonds, phenyl rings, thiophene, polythiophenes, etc) acts as a bridge to transfer electrons between the donor and the acceptor group and it is the key part which can induce a shift of both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels so that the photophysical properties may be tuned. The organic dyes/sensitizers belong to different classes depending on the donors such as triphenylamine, phenothiazine, fluorene, coumarin, carbazoles, etc. which have been profusely synthesized, and their power conversion efficiency as sensitizers have been reported and reviewed exclusively [12]. The structures of the dyes/ sensitizers synthesized under microwave irradiation along with the parameters such as short-circuit current (*Jsc*), open-circuit voltage (*Voc*), Fill Factor (FF), and power conversion efficiency (PCE) of the solar cells fabricated using these dyes are discussed.

### *2.1.2 Microwave synthesized dyes/sensitizers in DSSCs*

Novel donor-π-acceptor (D-π–A) dyes bearing the pyrimidine unit as an electron acceptor appended to thiophene and carbazole unit **1a**-**c** were obtained by a combination of two processes *viz*., the microwave-assisted Suzuki cross-coupling reaction and nucleophilic aromatic substitution of hydrogen (SNH) [13]. Among these dyes, **1b** was used as a photosensitizer in a fabricated solar cell since this dye showed a maximum extinction coefficient. The short-circuit current density (*Jsc*) was 2.04 mA cm−2, and the open-circuit voltage (*Voc*) observed was 0.525 V. The calculated power conversion efficiency (PCE) of the cell (η) was 0.91 at a fill factor (FF) of 0.85. A series of dithienosilole-based terpolymers **2a**-**e** as sensitizers have been synthesized. Different dithienosilole monomers were used along with nonanoyl group and malononitrile as the electron acceptor *via* microwave-assisted Stille coupling polymerization to obtain the polymer sensitizers **2a**-**e**.

The devices obtained using these sensitizers **2a**-**e** exhibited the high opencircuit voltage (*Voc*) of 1.00–1.06 V which can be attributed to the low-lying HOMO energy levels. Sensitizer **2e** showed the best PCE of 2.32% (*Voc* = 1.06 V, *Jsc* = 5.92 mA/cm2 and FF = 0.39) which is due to the components of the conjugated backbones and play a pivotal role in their photovoltaic performance. When the polymerisation process was optimized i.e. in polymer **2e** with higher molecular weight (Mn = 23.3 kDa) an increased PCE of 3.29% (*Voc* = 1.07 V, *Jsc* = 7.53 mA/cm2 and FF = 0.41) was observed [14]. The above reports showed the outstanding thermal stabilities and electrical conductivity of polythiophenes. Hence, a semiconducting polymer *viz*., poly[1,5-naphthyridine-(3-hexylthiophene)] **3** was prepared by microwave-assisted *Suzuki-Miyura* cross-coupling reaction using 3-hexylthiophene-2,5-diboronic ester and 2,6-dibromo-1,5-naphthyridine [15]. This polymer **3** was used as a photosensitizer in a fabricated solar cell. The solar cell so prepared was illuminated under AM 1.5 G at 100 mW/cm2 which showed a PCE of 0.67% with an open-circuit voltage of (Voc) 621 mV, a short-circuit current of 2.0 mA/cm2 , and a FF of 55%.

Three push-pull Donor-π-Acceptor structured dyes **4**, **5** and **6** having imidazo [1,2-*a*]pyridine heterocycles as additional π-conjugated linker was synthesized. Triphenylamine (TPA) was introduced as an electron-donor unit and cyanoacetic acid through thiophene as linker **4** and **5** or double rhodanine acetic acid **6** were employed as anchoring groups in different positions of the heterocyclic core [16]. DSSC devices with these dyes **4**–**6** were assembled and tested using different electrolytes and dye baths. The best efficiencies were obtained for dye **4** i.e. *Jsc* 2.34 (mA/cm<sup>2</sup> ), *Voc* 650 mV, FF 0.42, η (%) 0.64 and for **5** *Jsc* 2.14 (mA/cm2 ), Voc 502 mV, FF 0.42, η (%) 0.45.

**61**

*Microwave Synthesized Functional Dyes DOI: http://dx.doi.org/10.5772/intechopen.94946*

dye aggregation leading to a potential barrier the dye **6** showed the lowest efficiency

Triphenylamine based dye sensitizers **10**–**14** were prepared under microwave irradiation by incorporating 2-(1,1-dicyanomethylene) rhodanine which acts

nm to 1260 nm under AM 1.5 G with irradiation of 100 mW cm−2 [18].

In view of the importance of thiophene as the significant moiety in the design of polymer-based sensitizers, narrow band gap conjugated polymer **7** was obtained from 4,6-bis(4-tetradecylthien-2-yl)thieno[3,4-*c*]thiadiazole, and thieno[3,2-*b*] thiophene using Stille coupling reaction under microwave irradiation. This polymer exhibited good solution processability and absorbed the UV/Vis light from 300 nm to 1260 nm with an optical band gap of 0.98 eV in solid state. Photovoltaic devices using the blend films **8**, **9** from **7** and [6,6]-phenyl-C61 butyric acid methyl ester (PC61BM) or [6,6]-phenyl-C71 butyric acid methyl ester (PC71BM) having the configuration ITO/PEDOT:PSS/blend film/Ca/Al, provided power conversion efficiencies (PCEs) of 0.65%, and 1.12% respectively with light response from 300

irrespective of dye bath solvent and electrolytes [17].

*Microwave Synthesized Functional Dyes DOI: http://dx.doi.org/10.5772/intechopen.94946*

*Microwave Heating - Electromagnetic Fields Causing Thermal and Non-Thermal Effects*

Novel donor-π-acceptor (D-π–A) dyes bearing the pyrimidine unit as an electron acceptor appended to thiophene and carbazole unit **1a**-**c** were obtained by a combination of two processes *viz*., the microwave-assisted Suzuki cross-coupling reaction and nucleophilic aromatic substitution of hydrogen (SNH) [13]. Among these dyes, **1b** was used as a photosensitizer in a fabricated solar cell since this dye showed a maximum extinction coefficient. The short-circuit current density (*Jsc*) was 2.04 mA cm−2, and the open-circuit voltage (*Voc*) observed was 0.525 V. The calculated power conversion efficiency (PCE) of the cell (η) was 0.91 at a fill factor (FF) of 0.85. A series of dithienosilole-based terpolymers **2a**-**e** as sensitizers have been synthesized. Different dithienosilole monomers were used along with nonanoyl group and malononitrile as the electron acceptor *via* microwave-assisted Stille coupling polymerization to obtain

The devices obtained using these sensitizers **2a**-**e** exhibited the high opencircuit voltage (*Voc*) of 1.00–1.06 V which can be attributed to the low-lying HOMO energy levels. Sensitizer **2e** showed the best PCE of 2.32% (*Voc* = 1.06 V,

backbones and play a pivotal role in their photovoltaic performance. When the polymerisation process was optimized i.e. in polymer **2e** with higher molecular weight (Mn = 23.3 kDa) an increased PCE of 3.29% (*Voc* = 1.07 V, *Jsc* = 7.53 mA/cm2

FF = 0.41) was observed [14]. The above reports showed the outstanding thermal stabilities and electrical conductivity of polythiophenes. Hence, a semiconducting polymer *viz*., poly[1,5-naphthyridine-(3-hexylthiophene)] **3** was prepared by microwave-assisted *Suzuki-Miyura* cross-coupling reaction using 3-hexylthiophene-2,5-diboronic ester and 2,6-dibromo-1,5-naphthyridine [15]. This polymer **3** was used as a photosensitizer in a fabricated solar cell. The solar cell so prepared was

open-circuit voltage of (Voc) 621 mV, a short-circuit current of 2.0 mA/cm2

Three push-pull Donor-π-Acceptor structured dyes **4**, **5** and **6** having imidazo [1,2-*a*]pyridine heterocycles as additional π-conjugated linker was synthesized. Triphenylamine (TPA) was introduced as an electron-donor unit and cyanoacetic acid through thiophene as linker **4** and **5** or double rhodanine acetic acid **6** were employed as anchoring groups in different positions of the heterocyclic core [16]. DSSC devices with these dyes **4**–**6** were assembled and tested using different electrolytes and dye baths. The best efficiencies were obtained for dye **4** i.e. *Jsc* 2.34

), *Voc* 650 mV, FF 0.42, η (%) 0.64 and for **5** *Jsc* 2.14 (mA/cm2

and FF = 0.39) which is due to the components of the conjugated

and

, and a

), Voc

which showed a PCE of 0.67% with an

*2.1.2 Microwave synthesized dyes/sensitizers in DSSCs*

the polymer sensitizers **2a**-**e**.

*Jsc* = 5.92 mA/cm2

FF of 55%.

(mA/cm<sup>2</sup>

502 mV, FF 0.42, η (%) 0.45.

illuminated under AM 1.5 G at 100 mW/cm2

**60**

Due to inefficient electron injection from HOMO to TiO2 conduction band or dye aggregation leading to a potential barrier the dye **6** showed the lowest efficiency irrespective of dye bath solvent and electrolytes [17].

In view of the importance of thiophene as the significant moiety in the design of polymer-based sensitizers, narrow band gap conjugated polymer **7** was obtained from 4,6-bis(4-tetradecylthien-2-yl)thieno[3,4-*c*]thiadiazole, and thieno[3,2-*b*] thiophene using Stille coupling reaction under microwave irradiation. This polymer exhibited good solution processability and absorbed the UV/Vis light from 300 nm to 1260 nm with an optical band gap of 0.98 eV in solid state. Photovoltaic devices using the blend films **8**, **9** from **7** and [6,6]-phenyl-C61 butyric acid methyl ester (PC61BM) or [6,6]-phenyl-C71 butyric acid methyl ester (PC71BM) having the configuration ITO/PEDOT:PSS/blend film/Ca/Al, provided power conversion efficiencies (PCEs) of 0.65%, and 1.12% respectively with light response from 300 nm to 1260 nm under AM 1.5 G with irradiation of 100 mW cm−2 [18].

Triphenylamine based dye sensitizers **10**–**14** were prepared under microwave irradiation by incorporating 2-(1,1-dicyanomethylene) rhodanine which acts

both as electron acceptor as well as anchoring group on titanium dioxide [19]. Triphenylamine and vinyl thiophene are the donors and the π spacers. The dye containing two 2-(1,1-dicyanomethylene)rhodanine units and no thiophene units i.e*.* dye **11** showed the best photovoltaic performance with a short-circuit photocurrent density (*Jsc*) of 7.76 mA/cm2 , an open-circuit photovoltage of 0.62 V, and a fill factor of 0.68, corresponding to an overall conversion efficiency of 3.78% under AM 1.5 irradiation (100 mW/cm2 ). The *J*sc of the solar cells fabricated using these dyes increased in the order of **13** < **11** < **10**, **12** < **14**.

The general design of organic dye sensitizers is usually in the order D-π-A. However, molecular conjugated chromophores combining only electron donor (D) and acceptor (A) blocks have also been designed and synthesized as active materials for organic solar cells [20]. In view of this, D-A-D dyes **15**, **16** and **17** obtained by reaction of mono-formyl triarylamines with 2,3-diaminomaleonitrile. Such D-A-D dyes are expected to show absorption of two photons hence may be used in dyes and solar cells, and focused on their potentialities as a donor material in basic planar heterojunction solar cells. These compounds **15**, **16**, and **17** have been evaluated as dyes in solar cells ITO/PEDOT-PSS/dye/C60/Al. The open-circuit voltage (Voc), short-circuit current densities (*Jsc*), fill factor (FF), and power conversion efficiencies (PCEs) of cells of 0.28 cm2 active area were determined under AM 1.5 simulation solar illumination. Compound **17** did not lead to devices of a quality sufficient for evaluation. Fabricated devices obtained from dyes **15** and **16** respectively gave PCEs of 0.70 and 0.53%.

**63**

*Microwave Synthesized Functional Dyes DOI: http://dx.doi.org/10.5772/intechopen.94946*

Donor-acceptor dyes **24**–**28** based on 3-methylquinoxaline-2(1*H*)one under

A new series of oxindole sensitizers (**29**–**33**) were designed and synthesized under microwave irradiation [24]. These exhibited respectable photoelectric conversion efficiencies due to excellent electron-donating triphenylamine (TPA) donor and the thiophene in the spacer and are differentiated by various halogensubstituted oxindole acceptors. The cell performance was analyzed by fabricating solar cells. The parent dye **29** exhibited *Jsc* = 10.03 mAcm−2, *Voc* = 680 mV, and FF = 0.699, corresponding to an overall η = 4.76%. The incorporation of halogen substitutions on the parent dye enhanced the PCEs. Solar cell containing fluoro substituent i.e. dye **30** achieved *Jsc* = 11.32 mA cm−2, *Voc* = 690 mV, and FF = 0.695, corresponding to an overall η = 5.43%, which was approximately 14% higher than that of the non-substituted oxindole sensitizer **29**. The efficiency was increased due to increased *Jsc* which may be attributed to the electronic coupling of *fluoro* sub-

microwave condition involving Knoevenagel reaction were designed with electron-donor groups such as triphenylamine (TPA) **24–26** ferrocene **25**, N,Ndimethylaminobenzene **27**, and ((*E*)-4,4′-(ethene-1,2-diyl)bis(N,N-diphenylaniline)) **28**. The dyes **26** and **27** showed higher power efficiency (0.31 and 0.40 respectively) as expected for their higher values of *Jsc* and *Voc*. This suggests that these structures decrease the recombination processes by preventing the approach of tri-iodide ions to the semiconductor surface, thus decreasing the electron transfer from TiO2 conduction band to tri-iodide ions electrolyte. The higher efficiency of the dyes **26** and **27** may also be due to the enhanced conjugation of triphenylamine units to anchoring amide groups. This has improved the electron injection into semiconductor conduction band which helps in the photovoltaic performance.

The remaining dyes did not show significant efficiencies [23].

stituent in the compound **29** with the anchoring group COOH [25].

The substitution of fluoro substituent with other halo substituents showed further enhancement in the DSSC performance. Among all the halogen substituted sensitizers, the bromo substituted sensitizer **31** exhibited the highest photovoltaic parameters (*Jsc* = 12.46 mA cm−2, Voc = 720 mV, and FF = 0.708) with an overall conversion efficiency (η) of 6.35%. The improved photocurrent of the

2D-π-A dyes **18**–**20** comprising of dibenzofulvene-thiophene as π-bridge which is flanked by diarylamine donor groups and cyanoacrylic acid as anchoring as well as acceptor have been synthesized under microwave irradiation [21]. The dye **20** containing two thiophene rings as spacer shows an IPCE action spectrum with a high plateau from 390 nm to 600 nm increased open-circuit photovoltage by 40 mV and short-circuit photocurrent by 7.03 mA cm−2. Using Chenodeoxycholic acid (CDCA) as the co-adsorbent material, the *Jsc* of **22** was increased to 14.98 mA cm−2 and a strong enhancement in the overall conversion efficiency (7.45%) was realized by **20** compared to **18** (1.08%) in liquid electrolyte-based DSSCs. This work was further extended by the same research group [22] in which methoxy groups were introduced on the phenyl rings i.e. dyes **21**–**22** and also long fatty alkyl chain *viz*., octyloxy was introduced in view of increasing the donor capability and to avoid the aggregation and to increase physical insulation between electrolyte system and the TiO2 layer *i.e.*dye **23**. These dyes **21**–**23** exhibit rather similar photophysical properties for the lowest-lying optically active excitations and it was observed that the lowest excitation lay in all cases at 2.12–2.50 eV. Compound **21** showed a promising PCE of 5.90. The structural molecular variations evidenced positive effects on the photovoltaic performances of dyes as proved by PCEs of 7.50% and 7.80% obtained with dyes **22** and **23** respectively.

#### *Microwave Synthesized Functional Dyes DOI: http://dx.doi.org/10.5772/intechopen.94946*

*Microwave Heating - Electromagnetic Fields Causing Thermal and Non-Thermal Effects*

rent density (*Jsc*) of 7.76 mA/cm2

AM 1.5 irradiation (100 mW/cm2

(PCEs) of cells of 0.28 cm2

and 0.53%.

dyes increased in the order of **13** < **11** < **10**, **12** < **14**.

both as electron acceptor as well as anchoring group on titanium dioxide [19]. Triphenylamine and vinyl thiophene are the donors and the π spacers. The dye containing two 2-(1,1-dicyanomethylene)rhodanine units and no thiophene units i.e*.* dye **11** showed the best photovoltaic performance with a short-circuit photocur-

factor of 0.68, corresponding to an overall conversion efficiency of 3.78% under

The general design of organic dye sensitizers is usually in the order D-π-A. However, molecular conjugated chromophores combining only electron donor (D) and acceptor (A) blocks have also been designed and synthesized as active materials for organic solar cells [20]. In view of this, D-A-D dyes **15**, **16** and **17** obtained by reaction of mono-formyl triarylamines with 2,3-diaminomaleonitrile. Such D-A-D dyes are expected to show absorption of two photons hence may be used in dyes and solar cells, and focused on their potentialities as a donor material in basic planar heterojunction solar cells. These compounds **15**, **16**, and **17** have been evaluated as dyes in solar cells ITO/PEDOT-PSS/dye/C60/Al. The open-circuit voltage (Voc), short-circuit current densities (*Jsc*), fill factor (FF), and power conversion efficiencies

illumination. Compound **17** did not lead to devices of a quality sufficient for evaluation. Fabricated devices obtained from dyes **15** and **16** respectively gave PCEs of 0.70

2D-π-A dyes **18**–**20** comprising of dibenzofulvene-thiophene as π-bridge which is flanked by diarylamine donor groups and cyanoacrylic acid as anchoring as well as acceptor have been synthesized under microwave irradiation [21]. The dye **20** containing two thiophene rings as spacer shows an IPCE action spectrum with a high plateau from 390 nm to 600 nm increased open-circuit photovoltage by 40 mV and short-circuit photocurrent by 7.03 mA cm−2. Using Chenodeoxycholic acid (CDCA) as the co-adsorbent material, the *Jsc* of **22** was increased to 14.98 mA cm−2 and a strong enhancement in the overall conversion efficiency (7.45%) was realized by **20** compared to **18** (1.08%) in liquid electrolyte-based DSSCs. This work was further extended by the same research group [22] in which methoxy groups were introduced on the phenyl rings i.e. dyes **21**–**22** and also long fatty alkyl chain *viz*., octyloxy was introduced in view of increasing the donor capability and to avoid the aggregation and to increase physical insulation between electrolyte system and the TiO2 layer *i.e.*dye **23**. These dyes **21**–**23** exhibit rather similar photophysical properties for the lowest-lying optically active excitations and it was observed that the lowest excitation lay in all cases at 2.12–2.50 eV. Compound **21** showed a promising PCE of 5.90. The structural molecular variations evidenced positive effects on the photovoltaic performances of dyes as proved by PCEs of 7.50% and 7.80% obtained with dyes **22**

, an open-circuit photovoltage of 0.62 V, and a fill

). The *J*sc of the solar cells fabricated using these

active area were determined under AM 1.5 simulation solar

**62**

and **23** respectively.

Donor-acceptor dyes **24**–**28** based on 3-methylquinoxaline-2(1*H*)one under microwave condition involving Knoevenagel reaction were designed with electron-donor groups such as triphenylamine (TPA) **24–26** ferrocene **25**, N,Ndimethylaminobenzene **27**, and ((*E*)-4,4′-(ethene-1,2-diyl)bis(N,N-diphenylaniline)) **28**. The dyes **26** and **27** showed higher power efficiency (0.31 and 0.40 respectively) as expected for their higher values of *Jsc* and *Voc*. This suggests that these structures decrease the recombination processes by preventing the approach of tri-iodide ions to the semiconductor surface, thus decreasing the electron transfer from TiO2 conduction band to tri-iodide ions electrolyte. The higher efficiency of the dyes **26** and **27** may also be due to the enhanced conjugation of triphenylamine units to anchoring amide groups. This has improved the electron injection into semiconductor conduction band which helps in the photovoltaic performance. The remaining dyes did not show significant efficiencies [23].

A new series of oxindole sensitizers (**29**–**33**) were designed and synthesized under microwave irradiation [24]. These exhibited respectable photoelectric conversion efficiencies due to excellent electron-donating triphenylamine (TPA) donor and the thiophene in the spacer and are differentiated by various halogensubstituted oxindole acceptors. The cell performance was analyzed by fabricating solar cells. The parent dye **29** exhibited *Jsc* = 10.03 mAcm−2, *Voc* = 680 mV, and FF = 0.699, corresponding to an overall η = 4.76%. The incorporation of halogen substitutions on the parent dye enhanced the PCEs. Solar cell containing fluoro substituent i.e. dye **30** achieved *Jsc* = 11.32 mA cm−2, *Voc* = 690 mV, and FF = 0.695, corresponding to an overall η = 5.43%, which was approximately 14% higher than that of the non-substituted oxindole sensitizer **29**. The efficiency was increased due to increased *Jsc* which may be attributed to the electronic coupling of *fluoro* substituent in the compound **29** with the anchoring group COOH [25].

The substitution of fluoro substituent with other halo substituents showed further enhancement in the DSSC performance. Among all the halogen substituted sensitizers, the bromo substituted sensitizer **31** exhibited the highest photovoltaic parameters (*Jsc* = 12.46 mA cm−2, Voc = 720 mV, and FF = 0.708) with an overall conversion efficiency (η) of 6.35%. The improved photocurrent of the

sensitizer **31** suggested that compared to fluoro substitution, the bromo substituted dye exhibited better cell performance. Interestingly, the altered position of the substituent with respect to the anchoring group exhibited a negative effect on the solar cell performance. The dye **33** anchored DSSC showed lower current density (*Jsc* = 9.66 mA cm−2, Voc = 630 mV, and FF = 0.690), that is corresponding to an overall η = 4.21% which is due to the absence of electronic coupling of substitution with the anchoring group.

Computationally designed thiazolo [5,4-*d*]thiazole-based D-π-A organic dyes **34–38** have been synthesized [26]. These have been further derivatized with bispentylpropylenedioxythiophene (ProDOT) moieties in the π-spacer and triarylamine and phenothiazine **34**–**37** and **38** respectively as donors. Bulky and electronrich ProDOT groups enhanced the physical–chemical properties, including visible light absorption instead of the presence of the electron-poor thiazolothiazole. Small-scale (0.25 cm<sup>2</sup> ) devices using these dyes **34**–**38** showed the PCEs up to 7.71%, surpassing those obtained with two different reference dyes. Transparent larger area cells (3.6 cm2 ) also showed good η values up to 6.35%, not requiring the use of a co-adsorbent, and retained their initial efficiency over a period of 1000 h storage at 85°C. Following the promising results obtained with small-scale DSSCs (0.25 cm<sup>2</sup> ), the authors fabricated larger area (3.60 cm2 ) strip cells to analyze the effect of increased active surface area on the efficiency and stability. Small-scale solar cells built with **34**–**38**, both transparent and opaque, gave good power conversion efficiencies (η up to 7.71%), which in the case of dyes **36** and **38** were clearly superior to those obtained with standard Ru-dye Z907. Larger-scale strip cells featuring thin films of transparent TiO2 (3–5 mm) and a high stability electrolyte, gave efficiencies in line with those obtained with the smaller devices, with dye **36** being once again the best sensitizer (η up to 6.35%).

Two isoindigo-based conjugated polymers **39**–**40** composed of isoindigo with 2-decyltetradecane (DT) and bithiophene with/without fluorination were prepared under microwave irradiation [27].

Fabrication of the solar cells was produced using *o*-xylene and diphenyl ether (DPE) as solvent and additive. To measure the photovoltaic performance of polymers the solar cells were fabricated using polymer sensitizers **39** and **40** with an inverted configuration (ITO/ZnO/polymer: PC71BM/MoO3/Ag). The optimum blend ratio of polymer to PC71BM was 1:1.5 (w/w) for the two polymers. The polymer sensitizer **39** based cell showed a lower PCE of 4.92% with a *V*OC of 0.89 V, a *J*SC of 9.21 mA/cm<sup>2</sup> , and a FF of 0.60. Whereas the sensitizer **40** exhibited a PCE of 8.80% with a *V*OC of 1.06 V, a short-circuit current density (*J*SC) of 12.58 mA/ cm2 , and a FF of 0.66.

Novel dye sensitizers **41** and **42** with the sequence A-π-D-π-A which contains benzo[1,2-*b*:4,5-*b'*]bisthiophene as a core moiety with different terminal acceptor

**65**

cm2

*Microwave Synthesized Functional Dyes DOI: http://dx.doi.org/10.5772/intechopen.94946*

for dye **41** and 9.89 mA/cm2

been intensively discussed [29].

*2.2.1 Cyanine dyes*

**2.2 Fluorescent dyes**

exhibited a short-circuit current (*J*sc) of 5.09 mA/cm2

were designed and synthesized. The effects of either methyl dicyanovinyl end group **41** or *n-*butyl cyanoester end group **42** on solubility, thermal properties, optical properties, charge transport, morphology, and photovoltaic performance were investigated [28]. Devices for these dye sensitizers **41** and **42** were fabricated at the optimal donor/acceptor weight ratio of 1:1 as-cast without annealing. Sensitizer **41**

(FF) of 28.08%, and a PCE of 1.56% whereas, sensitizer **42** showed a *V*oc of 1.03 V and achieved a much better PCE performance of 6.17%, due to much higher FF of

quantum efficiency (EQE) of these dyes have a similar broad photo response wavelength range of 300–700 nm while in the whole range, the EQE values of dye **42** are much higher than dye **41**. The EQE peak of **42** is about 48% at around 676 nm, while the EQE value of **41** is below 15% at all wavelength, which leads to the poor performance of the device. *J*sc values calculated from the EQE spectra are 3.36 mA/

for **42** respectively.

Fluorescence is a photophysical process which involves the emission of light by the substance as a consequence of the absorption of electromagnetic radiation. In most of the cases, the emitted light radiation has a longer wavelength (λem) than the absorbed light radiation (λabs). Likewise, fluorescent dyes, also known as '*fluorophores'* or '*reactive dyes'* remit light radiation upon absorption. Earlier, fluorescent dyes were extensively used in the textile industries to color fibers, cotton, yarns, and silk. Eventually, the use of fluorescent dyes has become a key technique for the detection and elucidation of biological structures by fluorescence emission technology. Because of their high photostability, and intense brightness, fluorescent dyes have been significantly used in fluorescent labeling (staining) of biomolecules. Fluorescent quenching studies have helped to detect DNA and proteins in biological systems. Techniques such as immunofluorescence, fluorescence microscopy, and flow cytometry rely upon fluorescent dyes. Currently, the requirement of fluorescent dyes insisted greatly because of their ample applications which could be substantiated through microwave-assisted synthesis. The advantages of microwave applications for the synthesis of fluorescent dyes have

Cyanine dyes are found to be important functional dyes due to their typical optical properties, and act as sensitizers in solar cells, photography, and laser discs [30]. A significant property of cyanine dyes is the affinity for biological structures, specifically for DNA, and possesses wide color change, high photostability and increased fluorescent intensity when bound to biological structures [31]. Due to high fluorescence quantum yields and high molar extinction coefficients, they have been extensively used in cell imaging and gel staining techniques. Typically, cyanine dyes are obtained by heating a mixture of substituted quaternary salts with bisaldehyde or bis-imine. Accordingly, a series of cyanine dyes **43a-g** were synthesized by the condensation of quaternary salts of quinoline derivatives with 1*H*-indole-3-carbaldehydes in the presence of piperidine under solvent-free microwave irradiation at 126–329 W in 89–98% yields in only 2–5 min. The fluorescence spectra of the dyes showed absorption maxima (λabs) at 453–471 nm. However, in the presence of DNA, a

59.08% and much higher short-circuit current (*J*sc) of 10.11 mA/cm<sup>2</sup>

, a *V*OC of 1.09 V, a fill factor

. The external

#### *Microwave Synthesized Functional Dyes DOI: http://dx.doi.org/10.5772/intechopen.94946*

*Microwave Heating - Electromagnetic Fields Causing Thermal and Non-Thermal Effects*

with the anchoring group.

Small-scale (0.25 cm<sup>2</sup>

the authors fabricated larger area (3.60 cm2

the best sensitizer (η up to 6.35%).

under microwave irradiation [27].

area cells (3.6 cm2

sensitizer **31** suggested that compared to fluoro substitution, the bromo substituted dye exhibited better cell performance. Interestingly, the altered position of the substituent with respect to the anchoring group exhibited a negative effect on the solar cell performance. The dye **33** anchored DSSC showed lower current density (*Jsc* = 9.66 mA cm−2, Voc = 630 mV, and FF = 0.690), that is corresponding to an overall η = 4.21% which is due to the absence of electronic coupling of substitution

Computationally designed thiazolo [5,4-*d*]thiazole-based D-π-A organic dyes **34–38** have been synthesized [26]. These have been further derivatized with bispentylpropylenedioxythiophene (ProDOT) moieties in the π-spacer and triarylamine and phenothiazine **34**–**37** and **38** respectively as donors. Bulky and electronrich ProDOT groups enhanced the physical–chemical properties, including visible light absorption instead of the presence of the electron-poor thiazolothiazole.

surpassing those obtained with two different reference dyes. Transparent larger

co-adsorbent, and retained their initial efficiency over a period of 1000 h storage at 85°C. Following the promising results obtained with small-scale DSSCs (0.25 cm<sup>2</sup>

increased active surface area on the efficiency and stability. Small-scale solar cells built with **34**–**38**, both transparent and opaque, gave good power conversion efficiencies (η up to 7.71%), which in the case of dyes **36** and **38** were clearly superior to those obtained with standard Ru-dye Z907. Larger-scale strip cells featuring thin films of transparent TiO2 (3–5 mm) and a high stability electrolyte, gave efficiencies in line with those obtained with the smaller devices, with dye **36** being once again

Two isoindigo-based conjugated polymers **39**–**40** composed of isoindigo with 2-decyltetradecane (DT) and bithiophene with/without fluorination were prepared

Fabrication of the solar cells was produced using *o*-xylene and diphenyl ether (DPE) as solvent and additive. To measure the photovoltaic performance of polymers the solar cells were fabricated using polymer sensitizers **39** and **40** with an inverted configuration (ITO/ZnO/polymer: PC71BM/MoO3/Ag). The optimum blend ratio of polymer to PC71BM was 1:1.5 (w/w) for the two polymers. The polymer sensitizer **39** based cell showed a lower PCE of 4.92% with a *V*OC of 0.89 V,

of 8.80% with a *V*OC of 1.06 V, a short-circuit current density (*J*SC) of 12.58 mA/

Novel dye sensitizers **41** and **42** with the sequence A-π-D-π-A which contains benzo[1,2-*b*:4,5-*b'*]bisthiophene as a core moiety with different terminal acceptor

, and a FF of 0.60. Whereas the sensitizer **40** exhibited a PCE

) devices using these dyes **34**–**38** showed the PCEs up to 7.71%,

) strip cells to analyze the effect of

),

) also showed good η values up to 6.35%, not requiring the use of a

**64**

cm2

a *J*SC of 9.21 mA/cm<sup>2</sup>

, and a FF of 0.66.

were designed and synthesized. The effects of either methyl dicyanovinyl end group **41** or *n-*butyl cyanoester end group **42** on solubility, thermal properties, optical properties, charge transport, morphology, and photovoltaic performance were investigated [28]. Devices for these dye sensitizers **41** and **42** were fabricated at the optimal donor/acceptor weight ratio of 1:1 as-cast without annealing. Sensitizer **41** exhibited a short-circuit current (*J*sc) of 5.09 mA/cm2 , a *V*OC of 1.09 V, a fill factor (FF) of 28.08%, and a PCE of 1.56% whereas, sensitizer **42** showed a *V*oc of 1.03 V and achieved a much better PCE performance of 6.17%, due to much higher FF of 59.08% and much higher short-circuit current (*J*sc) of 10.11 mA/cm<sup>2</sup> . The external quantum efficiency (EQE) of these dyes have a similar broad photo response wavelength range of 300–700 nm while in the whole range, the EQE values of dye **42** are much higher than dye **41**. The EQE peak of **42** is about 48% at around 676 nm, while the EQE value of **41** is below 15% at all wavelength, which leads to the poor performance of the device. *J*sc values calculated from the EQE spectra are 3.36 mA/ cm2 for dye **41** and 9.89 mA/cm2 for **42** respectively.
