**6.3 Tetrathienoacenes (TTAs) based sensitizers**

Metal-free organic dyes have attracted much attention to researchers due to chemical or optical versatility, environmental compatibility and potential cost reductions [7]. In spite of these attractions and efforts, a little lower cell performance and difficulty in synthesis cannot meet the requirement for commercialization.

A donor-π-acceptor (D-π-A) structure is a promising strategy for metal-free organic sensitizers because of the effective photoinduced intramolecular charge

#### **Figure 26.**

*Solar cell performance (a) Voc, (b) Jsc and (c) eff versus light exposure time (d) schematic representation of light-induced cation exchange. Reprinted with permission from [129].*

## *A New Generation of Energy Harvesting Devices DOI: http://dx.doi.org/10.5772/intechopen.94291*

incorporation of the proquinoidal benzothiadiazole (BTD) unit into the functiona-

nyl)-4-yl)amine donor and a 4-ethynylbenzoic acid yielded the green dye, which exhibited a slightly improved PCE of 13% [127]. The detailed physical and structural studies responsible for recent advances of the porphyrin-based DSSCs have been reviewed in several reports [122–124]. Thanks to Yeh group's support, various porphyrin with the different structure based sensitizers can be tested in my optimized condition and the detailed performance listed in **Figure 25(b)**. Interestingly, the cell performance is gradually improved upon light exposure and heat treatment. Specially, after 90 min light exposure, cell performance increased from 6.12% to 7.71% attributed to the increase of *J*sc value (see **Figure 26(a)-(c)**). The improvement can be explained by the different charge recombination process. According to Mori et al., *Li+* ions are removed from the TiO2 surface and replaced with *DMPIm<sup>+</sup>* ions under light exposure [128, 129]. This process is found to enhance the electron lifetime by decreasing charge recombination with the redox mediator (**Figure 26(d)**). This can be explained by initial limited injection and fast charge recombination processes. As a result, this process enhances the cell performance by decreasing recombination with the redox mediator. However, about 20.1% improved cell efficiency by light exposure indicate YD2-oC8 sensitizer exhibit the extra open space at the TiO2 SP surface. Therefore, the best device performance in our system

Metal-free organic dyes have attracted much attention to researchers due to chemical or optical versatility, environmental compatibility and potential cost reductions [7]. In spite of these attractions and efforts, a little lower cell performance and difficulty in synthesis cannot meet the requirement for

A donor-π-acceptor (D-π-A) structure is a promising strategy for metal-free organic sensitizers because of the effective photoinduced intramolecular charge

*Solar cell performance (a) Voc, (b) Jsc and (c) eff versus light exposure time (d) schematic representation of*

*light-induced cation exchange. Reprinted with permission from [129].*

,40


� liquid electrolyte and TiO2 NSs


lization of the porphyrin core with the bulky bis(20

*Solar Cells - Theory, Materials and Recent Advances*

show about 7.7% of energy efficiency at the I�/I3

**6.3 Tetrathienoacenes (TTAs) based sensitizers**

samples.

**Figure 26.**

**222**

commercialization.

transfer property [7, 130]. Thanks to support from Chen's group, a new series of organic dye based on tetrathienoacene are applied for DSSC [105, 131]. In this design, triphenylamine electron-donor (D) unit and cyanoacrylic acid electronacceptor (A) unit is connected to an electron rich a lipophilic dihexyloxysubstituted thiophene-based fused tetrathienoacene. (**TPA-TTAR-TA**); (1) the triphenylamine unit composed of a large conjugated tertiary amine system is known as a strong electron donor in its initial state as well as act as stabilize the chargetransferred state [132]. (2) The cyanoacrylic acid, which is an anchoring group to bind to a TiO2 surface functions as an effective electron acceptor. (3) As the πsystem, we used fused tetrathienoacene cores produced by the newly designed onepot synthetic routes [133]. Fused-thiophenes offer the attraction of good charge transport properties with extensive molecular conjugation and strong intermolecular SS interactions, [134, 135] which might enhance the efficiency of DSSC. (see in **Figure 27(a)**).

Several fused thiophene derivatives have already been demonstrated to have excellent charge transport performance. For example, dithienothiophene (DTT) and tetrathienoacene (TTA) based OTFTs exhibited mobilities up 0.42 (p-channel) and 0.30 (n-channel) cm2 V<sup>1</sup> s 1 , respectively [136]. Relative to organic semiconductors, the potential of fused thiophene-based DSSCs has not been well explored until recently, and only for a limited range of TT and DTT materials. To the best of our knowledge, the first example of a TT-based small molecule DSSC with a PCE of 7.8% was reported by *P. Wang* and M. Gratzel et al. in 2008 [137]. For DTT, was reported by the same team in the same year with a PCE of 8.0% [138]. Presumably, due to high coplanarity of poly fused thiophenes may lead to aggregation of dye molecules on nanocrystals, caused a dissipative intermolecular charge transfer, and then rendered an unfavorable effect on the cell efficiency. As a result, the more conjugated TTA-based small molecules have never been explored for DSSCs yet. Nevertheless, in terms of tuning the energy-level of chromophores to attain a better capability of panchromatic light-harvesting, conjugated TTAs elevate the HOMO and lower a suitable LUMO compared to TT and DTT based DSSCs. Red shifted absorption with high molar coefficient and better charge transport (*vide infra*)

#### **Figure 27.**

*(a) Schematic representation of the donor-π-bridge-acceptor molecular dye design concept and (b) the chemical structures with solar performance and (c) UV–vis absorption spectra of dyes 1–5 and their corresponding molar absorption coefficients measured in* o*-DCB in concentration of 10<sup>5</sup> M of TTAR series dyes.*

might enhance the efficiency of DSSC thus offering TTA a potential good conjugation unit for a new organic dye. The detailed synthetic route and procedures aren't described in my thesis.

The effects of thiophene introduction between the bridge and the donor and/or acceptor moieties can be systematically understood. Starting from the simplest **TTAR** structure, **TTAR-1**, a thiophene spacer is either inserted on the acceptor side between TTAR and cyanoacrylic acid to yield **TTAR-2**, and finally on both acceptor and donor sides to yield **TTAR-3**. These π-extended systems should enhance panchromatic light-harvesting, as the result of the uplifted TTA HOMO and lower LUMO compared to TT and DTT based molecules. In addition, to maintain adequate dye processability, a long alkyl substituent (*R* = *n*-C15H31) on the TTA core in all sensitizers are introduced, which also suppresses dye aggregation and charge recombination on the TiO2 surface. (**Figure 27(b)**). The linear C15-alkyl chain substituent not only prevents dye aggregation but also curtails charge recombination. However, it does not efficiently suppress intermolecular π-π interactions when the molecules assemble on the TiO2 surface. Therefore, the TTA unit effectiveness can be compared with two different branched alkyl group (b-C8H17), **TTAR-4** and (n-C9H19) **TTAR-5**.

The optical absorption spectra of the five organic dyes are displayed in **Figure 27 (c)**, indicating the presence of two strong absorptions from a charge-transfer band (482 nm (**TTAR-1**), 498 nm (**TTAR-2**), 513 nm (**TTAR-3**), 485 nm (**TTAR-4**), 519 nm (**TTAR-5**)) and a higher energy π–π\* transition (340 nm (**TTAR-1**), 369 nm (**TTAR-2**), 402 nm (**TTAR-3**), 373 nm (**TTAR-4**), 375 nm (**TTAR-5**)). Significantly, **TTAR**-**2 5** with one or two thiophenes show red-shifted the absorption onset compared to **TTAR-1** due to their extended conjugation lengths. Presumably the reduced molecular co-planarity of **TTAR-4** due to the branched alkyl substituents may perturb the conjugation efficiency of the chromophore, and hence hypsochromically shift the absorption maximum wavelength. Electronic structure and solar performance of each dye from cyclic voltagramms (CV) and the B3LYP/6-31G\*\* level of density functional theory (DFT) is shown in **Table 5**. Among 5 dyes, **TTAR-4** show the best cell efficiency (*η*) as high as 10.21%, attributable to the relatively lower aggregation of the dye due to the bulky 3 methyl-5,5-dimethylhexyl substituents. Generally, dye aggregation on the surface of TiO2 reduces the electron lifetime and facilitates charge recombination in DSSCs fabricated using metal–organic or organic dyes [139].

To understand the exciton dynamics in dyes, femtosecond time-resolved photoluminescence (FTR-PL) was used [140, 141]. The electron injection efficiency can be calculated by the followed equation, η = (1/τ1)/(1/τ<sup>2</sup> + 1/τ3), where τ <sup>1</sup> is the electron injection time from the dye to TiO2, τ<sup>2</sup> is the exaction-exciton annihilation time, and τ<sup>3</sup> is the lifetime of an exciton in the dye [141]. The calculated time constants and electron injection efficiency for the dye coated TiO2 film are summarized in **Table 5**. The electron injection efficiency of 5 dyes shows almost 97% at λ = 420 nm, which is well matched results for IPCEs data (>93%) at the same wavelength.

### **6.4 Multi-sensitized DSSC**

Designed sensitizers with higher molar extinction coefficient have allowed significant improvement of cell performance. Nevertheless, possible efficiency boosts can be obtained by extending the dye absorption range while simultaneously avoiding a negative impact on other parameters, such as ground- and excited-state redox potential, intensity of absorption, or stability. Many series of dyes have been developed to find an efficient absorber with high extinction coefficients particularly

**Dye**

**225**

**λmax (nm) Eg (eV)**

 **Energy level**

**Energy level**

**Photoexcited**

 **Electron**

**Electron injection efficiency (%)**

**Solar** 

**Voc (V) Jsc (mA/cm2)**

 **FF (%) EFF(%)**

**Performance**

*A New Generation of Energy Harvesting Devices DOI: http://dx.doi.org/10.5772/intechopen.94291*

**Dynamics**

**(eV)**

**(eV)**

**EHOMOa**

TTAR-1

TTAR-2

TTAR-3

TTAR-4

TTAR-5

*aMeasured in o-DCB at a* 

*b*

*EHOMO CV = (4.8 + Eox) where Eox = Onset potential of the first oxidation peak when the Fc/Fc + internal standard is referenced to 0.0 V.*

**Table 5.**

*Summary of optical, electrical properties,*

 *solar properties of dyes. Reprinted from [141].*

 519

 1.91 *concentration*

 *of 105 M1*

*.*

5.20

3.29

4.99

2.71

 0.77

 4.59

 29.86

 485

 1.92

5.19

3.27

5.02

2.71

 0.71

 3.37

 24.74

 513

 2.04

5.09

3.05

4.95

2.80

 0.84

 2.77

 15.07

 490

 2.08

5.22

3.14

5.06

2.78

 0.74

 3.08

 19.30

 482

 2..08

5.16

3.08

5.01

2.69

 0.68

 4.35

 23.56

97.2 96.3 94.7 97.2 97.5

0.781

 15.5

 62.0

 7.67

0.832

0.811

 17.5

 72.0

 10.2

 11.8

 70.3

 6.91

0.893

0.833

 16.5

 73.7

 10.1

 10.1

 68.1

 6.15

**ELUMOa**

**EHOMOb**

**ELUMOb τ1 (ps) τ2 (ps) τ3 (ps)**


**Table 5.**

*Summary of optical, electrical properties, solar properties of dyes. Reprinted from [141].*
