**6. Photosensitizer for DSSC: state of the art.**

#### **6.1 Ruthenium (Ru) complex based photosensitizer**

Since the advent of DSSCs, ruthenium-series dyes are considered as the most powerful photosensitizer because of the following reason: their strong metal to ligand charge transfer transition (MLCT) process, a broad absorption spectrum, suitable excited and ground state energy levels, relatively long excited-state lifetime, and good (electro) chemical stability and power conversion efficiency (PCE) >11% [7]. In order to improve the efficiency of DSSCs, the sensitizer should be panchromatic, that is, absorb photons from the visible to near-infrared (NIR) region of the solar spectrum while maintaining sufficient thermodynamic driving force for the electron injection and dye regeneration process. Many efforts have been made to change the change the ligands of Ru complexes: [118, 119] As seen in **Figure 24**, ruthenium(II) – polypyridyl complexes such as the thiocyanato derivative, *cis*-(SCN)2 bis (2,2<sup>0</sup> -bipyridyl-4,4<sup>0</sup> -dicarboxylate) ruthenium(II), (coded as **N3**), the doubly protonated form, (Bu4N)2(Ru(dcbpyH)2(NCS)2), (named **N719**)

#### **Figure 24.**

*Incident photon to current conversion efficiency as a function of the wavelength for the standard ruthenium sensitizers N3 (red line), N719 (brown) the black dye N749 (black curve), and the blank nanocrystalline TiO2 film (blue curve). Reprinted with permission from [7, 120, 121].*

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

temperature, a waxy solid was obtained. **Figure 23(c)** shows the JV curve for solid state of 311 nm 3D PhC bilayer. Interestingly, no obvious efficiency change could be seen in all typed solid state electrolyte. The PCE of the solid state system is found to be 2% of that of the liquid electrolyte system. In conclusion, the design of 3D PhC bilayer film enables effective dye sensitization, electrolyte infiltration and charge collection because the layers are in direct physical and electronic contact, light harvesting in specific spectral regions was significantly increased by the 3D PhC effect and PC-induced resonances. This approach should be useful in solid-state devices where pore infiltration is a limiting factor as well as in weakly absorbing

Since the advent of DSSCs, ruthenium-series dyes are considered as the most powerful photosensitizer because of the following reason: their strong metal to ligand charge transfer transition (MLCT) process, a broad absorption spectrum, suitable excited and ground state energy levels, relatively long excited-state lifetime, and good (electro) chemical stability and power conversion efficiency (PCE) >11% [7]. In order to improve the efficiency of DSSCs, the sensitizer should be panchromatic, that is, absorb photons from the visible to near-infrared (NIR) region of the solar spectrum while maintaining sufficient thermodynamic driving force for the electron injection and dye regeneration process. Many efforts have been made to change the change the ligands of Ru complexes: [118, 119] As seen in **Figure 24**, ruthenium(II) – polypyridyl complexes such as the thiocyanato deriva-

**N3**), the doubly protonated form, (Bu4N)2(Ru(dcbpyH)2(NCS)2), (named **N719**)

*Incident photon to current conversion efficiency as a function of the wavelength for the standard ruthenium sensitizers N3 (red line), N719 (brown) the black dye N749 (black curve), and the blank nanocrystalline*

*TiO2 film (blue curve). Reprinted with permission from [7, 120, 121].*


photovoltaic devices.

tive, *cis*-(SCN)2 bis (2,2<sup>0</sup>

**Figure 24.**

**220**

**6. Photosensitizer for DSSC: state of the art.**

*Solar Cells - Theory, Materials and Recent Advances*

**6.1 Ruthenium (Ru) complex based photosensitizer**


and the Ru center has three thiocyanato ligands and one terpyridine ligand substituted with three carboxyl groups, (also called the "**black dye**") are widely used as reference and high efficiency sensitizers for DSSCs. The amphiphilic heteroleptic ruthenium sensitizer, known as **Z907**, reported noticeable thermal stability with stable 7% energy conversion efficiency [114]. For optimized DSSC with N3 or N719, the certified IPCE values are 80% for wavelengths 650 nm.

However, the IPCE increases only gradually from the absorption onset to shorter wavelengths due to relatively low extinction coefficients (1.40 <sup>10</sup><sup>4</sup> <sup>M</sup><sup>1</sup> cm<sup>1</sup> ) [7, 47]. In addition, this class of compounds contains expensive ruthenium metal and requires careful synthesis and tricky purification steps. Therefore, efforts in the synthesis of sensitizers for DSSCs step forward to the metal-free organic donor– acceptor (D–A) dyes system.

#### **6.2 Porphyrins based sensitizer**

The idea for mimicking the light harvesting processes based on chlorophyll occurring photosynthetic reaction centres inspire the research for porphyrins based sensitizer. The porphyrin-based dyes exhibit large absorption coefficients in the visible and infrared region as well as their rigid molecular structures consisted of four meso and eight β reaction sites, which can control their properties [122–124]. The designed porphyrin dyes with a π-conjugated link at the β-position of the porphyrin ring enhance the electronic coupling of the dye with the surface of TiO2, reaching η =7.1% of a PCE [125]. Numerous series of porphyrins are reported for the DSSC application as an effective sensitizer. of DSSC synthetized. Among them, a class of sensitizer consisting of a push–pull porphyrins with an electron-donating diarylamino group and an electron-withdrawing carboxyphenylethynyl anchoring group shows the outstanding solar properties. For example, advances in optimization of the device performance for a zinc porphyrin sensitizer (YD2-oC8) cosensitized with an organic dye (Y123) using a cobalt-based electrolyte to enhance photovoltage of the device attained an unprecedented power conversion efficiency of *η* = 12.3% [126]. (see **Figure 25(a)**) Further improvement can be found at

#### **Figure 25.**

*(a) Typical structure of a porphyrin showing the four meso- and the eight β-positions to be functionalized for porphyrin-sensitized solar cells. Reprinted from [122, 123] (b) solar performance for the different series of porphyrin based DSSC at liquid electrolyte and TiO2 NSs.*

incorporation of the proquinoidal benzothiadiazole (BTD) unit into the functionalization of the porphyrin core with the bulky bis(20 ,40 -bis(hexyloxy)-(1,10 -biphenyl)-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 show about 7.7% of energy efficiency at the I�/I3 � liquid electrolyte and TiO2 NSs samples.

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

V<sup>1</sup> s 1

DSSC. (see in **Figure 27(a)**).

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

and 0.30 (n-channel) cm2

**Figure 27.**

**223**

intermolecular SS interactions, [134, 135] which might enhance the efficiency of

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)

ductors, 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*)

*(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.*

, respectively [136]. Relative to organic semicon-
