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

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

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

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

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

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

fabricated using metal–organic or organic dyes [139].

described in my thesis.

*Solar Cells - Theory, Materials and Recent Advances*

(n-C9H19) **TTAR-5**.

wavelength.

**224**

**6.4 Multi-sensitized DSSC**

in the solar spectrum from 350 1,000 nm that is also environmentally benign and inexpensive to use. However, it's a big challenge. Therefore, scientists are tried to mix two or more dyes with different absorption range [142–144]. Although this concept is very sensible, the multi-sensitizers system should meet some essential conditions: (i) strong molar extinction coefficients to minimize the thickness of the mesoscopic TiO2 film; (ii) a suitable structure to avoid unfavorable dye aggregation; (iii) to reduce the recombination of electrons in the TiO2 film with I3 and other acceptors materials through the formation of a compact molecule monolayer covering the bare TiO2 surface. With the continuous efforts, the co-sensitized DSSC has achieved an 11.5% efficiency record with the liquid state electrolyte, [145] but the energy conversion efficiency of mixed sensitizer still lags behind a champion cell made from the single dye. The reason is that the formation of molecular aggregates onto the TiO2 surface having a finite number of anchoring sites as well as the deactivation of dye excited states due to energy or electron transfer processes between the different sensitizers [146].

In this book, three sensitizers with the different main absorption position and D-π-A framework (a Zn based porphyrin dye (YD2-oC8) [126] and the more conjugated TTA-based small molecules (TTAR) [141], which hold records of solar performance as a single dye) are applied by modified stepwise approach. For the efficient cosensitizaion, a good understadning of the intermolecular interactions such as matchable size, shape, and orientations as well as compensating light-harvest between or among the coadsorbed dyes must take precedence. In addition dye aggregation on the surface of TiO2 significantly must be avoided because the formation of dye aggregates significantly decrease the efficiency of electron injection. Attached long alkoxyl chains at phenyl group is plausible strategies to suppress effectively the dye aggregation. The key structural feature of YD2-oC8 involves long alkoxyl chains in the *ortho*-positions of the *meso*-phenyls so as to envelope effectively the porphyrin ring to decrease the degree of dye aggregation and also protect the porphyrin core for retarded charge recombination [126].

For the effective co-sensitization deposition, there are two well known approaches: (i) the cocktail approach uses a mixture of dye solutions with certain

*(a) The normalized efficiency (*η*) and normalized photocurrent density (*J*sc) from the photocurrent density– voltage (*J-V*) characteristics (left) and the electron-transport resistant (RCT) and electron lifetime (*τ*e) fitted from EIS analysis of the DSSCs using TTAR-4 dye with various CDCA concentrations, measured under 1 sun illumination (b) molar absorption coefficients measured in o-C6H4Cl2 in a concentration of 10<sup>5</sup> M (insert:*

sequential adsorptions of the two sensitizers [151, 152]. In our experiement, the fabrication of a co-sensitized onto the TiO2 sphere film is performed by the modified stepwise deposition method. (see **Figure 28(b)**) Considering the enegetic band position, the TiO2 electrode is firstly immersed into a 1 M **TTAR** solution in a mixture of 1,2-dichlorobenzene (DCB) and ethanol (volume ratio: 1:10) and kept at room temperature for 1 h. After spin-coating at 3000 rpm for 45 sec, a small quantity of the dye solutions (YD2-oC8) prepared from the same ratio of the mixture solvent (DCB: EtOH) is then dropped onto TTAT/TiO2 sphere at room temperature and left for 5 min before spin coating at 3000 rpm for 60 sec. The well covered sensitizer layer could be obtained by repeating the coating procedure until the optimized condition. After YD2-oC8 sensitizer coating, as-synthesized YDD6 solution dissovled in the mixture solvent (DCB:EtOH) is deposited onto YD2-oC8/ TTAR/ TiO2 sphere in the same procesure. **Figure 28(b)** shows the molar extinction coefficient of individual and multiple sensitized system. The absorption spectra of YD2-oC8 is well matched in the literature [122, 152]. The YD2-oC8 observed along with the Soret band (400–520 nm, log *ε*/M<sup>1</sup> cm<sup>1</sup> = 5.33 at 448 nm) and Q-band absorption (550 600 nm, log *<sup>ε</sup>*/M<sup>1</sup> cm<sup>1</sup> = 4.49 at 644), while the absorption spectrum of TTAR is found in the range of 350 560 nm with the molar extinction coefficients at 449.5 nm (log *ε*/M<sup>1</sup> cm<sup>1</sup> = 5.01). As a near IR dye, the absorption spectrum of YDD6 shows a broad split feature for the Soret band (380–550 nm, log

*ε*/M<sup>1</sup> cm<sup>1</sup> = 5.51 at 491 nm) and Q-band absorption (550–800 nm, log *ε*/

M<sup>1</sup> cm<sup>1</sup> = 4.94 at 741 nm) Therefore, this dye can help to cover the lack of light-

[147, 150] and (ii) the stepwise approach accomplishes

molar ratios of the two dyes,

*Schematic process for cosensitized system).*

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

**Figure 28.**

**227**

For supporting the lack of light-harvesting ability beyond 700 nm, near-IR dyes, named as YDD6, is employed [147]. The ortho-substituted alkoxyl chains in YDD6 can be contributed as not only a light-harvesting ability extending beyond 800 nm but also failed to prevent dye aggregation for this porphyrin. A weakly absorption around *λ* = 520 nm at the YD2-oC8 can be improved by TTAR dye molecules. In addition, several papers reported the co-sensitization of different molecular sizes allowed a better surface coverage, yielding a high short circuit current density (*J*sc) and open circuit voltage (*V*oc) and resulting in a high PCE [147–149]. Similarly, the covering empty space on TiO2 NS surface demonstrated from the light soaking test lead to enhance the efficiency. Therefore, insering small sized TTAR dye molecules (estimated molecular length = 24.9 Å) into the gaps within the YD2-oC8 (about 167 Å) saturated TiO2 surface can help to boost cell performance.

The possibility of dye aggregation of TTAR are test with various amounts of chenodeoxycholic acid (CDCA) co-adsorbed on the photoanodes. The effect of CDCA concentration on the photovoltaic properties of the DSSCs fabricated with **TTAR-4** was first investigated, and the results are shown in **Figure 28(a, left)**. Although dye aggregation should be suppressed by CDCA incorporation among the dye molecules, this would also decrease the dye coverage on the TiO2, leading to decrease *J*sc and cell performance. The **TTAR-4** may be sufficient to suppress aggregation, and adsorption of the added CDCA may compete with dye adsorption on the TiO2. The electron-transport resistance (*Rct*) and electron lifetimes (τe) as a function of the concentration of CDCA can be obtained by EIS analysis (see **Figure 28(a, right)**).

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

**Figure 28.**

in the solar spectrum from 350 1,000 nm that is also environmentally benign and inexpensive to use. However, it's a big challenge. Therefore, scientists are tried to mix two or more dyes with different absorption range [142–144]. Although this concept is very sensible, the multi-sensitizers system should meet some essential conditions: (i) strong molar extinction coefficients to minimize the thickness of the mesoscopic TiO2 film; (ii) a suitable structure to avoid unfavorable dye aggregation;

acceptors materials through the formation of a compact molecule monolayer covering the bare TiO2 surface. With the continuous efforts, the co-sensitized DSSC has achieved an 11.5% efficiency record with the liquid state electrolyte, [145] but the energy conversion efficiency of mixed sensitizer still lags behind a champion cell made from the single dye. The reason is that the formation of molecular aggregates onto the TiO2 surface having a finite number of anchoring sites as well as the deactivation of dye excited states due to energy or electron transfer processes

In this book, three sensitizers with the different main absorption position and D-π-A framework (a Zn based porphyrin dye (YD2-oC8) [126] and the more conjugated TTA-based small molecules (TTAR) [141], which hold records of solar performance as a single dye) are applied by modified stepwise approach. For the efficient cosensitizaion, a good understadning of the intermolecular interactions

For supporting the lack of light-harvesting ability beyond 700 nm, near-IR dyes, named as YDD6, is employed [147]. The ortho-substituted alkoxyl chains in YDD6 can be contributed as not only a light-harvesting ability extending beyond 800 nm but also failed to prevent dye aggregation for this porphyrin. A weakly absorption around *λ* = 520 nm at the YD2-oC8 can be improved by TTAR dye molecules. In addition, several papers reported the co-sensitization of different molecular sizes allowed a better surface coverage, yielding a high short circuit current density (*J*sc) and open circuit voltage (*V*oc) and resulting in a high PCE [147–149]. Similarly, the covering empty space on TiO2 NS surface demonstrated from the light soaking test lead to enhance the efficiency. Therefore, insering small sized TTAR dye molecules (estimated molecular length = 24.9 Å) into the gaps within the YD2-oC8 (about

The possibility of dye aggregation of TTAR are test with various amounts of chenodeoxycholic acid (CDCA) co-adsorbed on the photoanodes. The effect of CDCA concentration on the photovoltaic properties of the DSSCs fabricated with **TTAR-4** was first investigated, and the results are shown in **Figure 28(a, left)**. Although dye aggregation should be suppressed by CDCA incorporation among the dye molecules, this would also decrease the dye coverage on the TiO2, leading to decrease *J*sc and cell performance. The **TTAR-4** may be sufficient to suppress aggregation, and adsorption of the added CDCA may compete with dye adsorption on the TiO2. The electron-transport resistance (*Rct*) and electron lifetimes (τe) as a function of the concentration of CDCA can be obtained by EIS analysis (see

and other

(iii) to reduce the recombination of electrons in the TiO2 film with I3

such as matchable size, shape, and orientations as well as compensating light-harvest between or among the coadsorbed dyes must take precedence. In addition dye aggregation on the surface of TiO2 significantly must be avoided because the formation of dye aggregates significantly decrease the efficiency of electron injection. Attached long alkoxyl chains at phenyl group is plausible strategies to suppress effectively the dye aggregation. The key structural feature of YD2-oC8 involves long alkoxyl chains in the *ortho*-positions of the *meso*-phenyls so as to envelope effectively the porphyrin ring to decrease the degree of dye

aggregation and also protect the porphyrin core for retarded charge

167 Å) saturated TiO2 surface can help to boost cell performance.

between the different sensitizers [146].

*Solar Cells - Theory, Materials and Recent Advances*

recombination [126].

**Figure 28(a, right)**).

**226**

*(a) The normalized efficiency (*η*) and normalized photocurrent density (*J*sc) from the photocurrent density– voltage (*J-V*) characteristics (left) and the electron-transport resistant (RCT) and electron lifetime (*τ*e) fitted from EIS analysis of the DSSCs using TTAR-4 dye with various CDCA concentrations, measured under 1 sun illumination (b) molar absorption coefficients measured in o-C6H4Cl2 in a concentration of 10<sup>5</sup> M (insert: Schematic process for cosensitized system).*

For the effective co-sensitization deposition, there are two well known approaches: (i) the cocktail approach uses a mixture of dye solutions with certain molar ratios of the two dyes, [147, 150] and (ii) the stepwise approach accomplishes sequential adsorptions of the two sensitizers [151, 152]. In our experiement, the fabrication of a co-sensitized onto the TiO2 sphere film is performed by the modified stepwise deposition method. (see **Figure 28(b)**) Considering the enegetic band position, the TiO2 electrode is firstly immersed into a 1 M **TTAR** solution in a mixture of 1,2-dichlorobenzene (DCB) and ethanol (volume ratio: 1:10) and kept at room temperature for 1 h. After spin-coating at 3000 rpm for 45 sec, a small quantity of the dye solutions (YD2-oC8) prepared from the same ratio of the mixture solvent (DCB: EtOH) is then dropped onto TTAT/TiO2 sphere at room temperature and left for 5 min before spin coating at 3000 rpm for 60 sec. The well covered sensitizer layer could be obtained by repeating the coating procedure until the optimized condition. After YD2-oC8 sensitizer coating, as-synthesized YDD6 solution dissovled in the mixture solvent (DCB:EtOH) is deposited onto YD2-oC8/ TTAR/ TiO2 sphere in the same procesure. **Figure 28(b)** shows the molar extinction coefficient of individual and multiple sensitized system. The absorption spectra of YD2-oC8 is well matched in the literature [122, 152]. The YD2-oC8 observed along with the Soret band (400–520 nm, log *ε*/M<sup>1</sup> cm<sup>1</sup> = 5.33 at 448 nm) and Q-band absorption (550 600 nm, log *<sup>ε</sup>*/M<sup>1</sup> cm<sup>1</sup> = 4.49 at 644), while the absorption spectrum of TTAR is found in the range of 350 560 nm with the molar extinction coefficients at 449.5 nm (log *ε*/M<sup>1</sup> cm<sup>1</sup> = 5.01). As a near IR dye, the absorption spectrum of YDD6 shows a broad split feature for the Soret band (380–550 nm, log *ε*/M<sup>1</sup> cm<sup>1</sup> = 5.51 at 491 nm) and Q-band absorption (550–800 nm, log *ε*/ M<sup>1</sup> cm<sup>1</sup> = 4.94 at 741 nm) Therefore, this dye can help to cover the lack of lightharvesting ability beyond 700 nm. As a result, the combination of complementary porphyrin and organic dyes produces a panchromatic spectral feature to promote the performance of DSSC.

**Figure 29(a)** and **(b)** show the current–voltage characteristics and the corresponding IPCE action for the TTAR, YD2-oC8, YDD6 and Multiple (TTAR/ YD2-oC8/YDD6) sensitized DSSC, respectively. The photovoltaic parameters are listed in **Table 6**. An impressive cell performance of ca 11.2% (*J*sc = 18.6 mAcm<sup>2</sup> , *V*oc = 0.818 V, *FF* = 72.8%, 998 mW cm<sup>3</sup> ) is attainable by increasing *J*sc value of the TTAR and YD2-oC8 (each 14.3 mAcm<sup>2</sup> ) to ca. 18.6 mAcm<sup>2</sup> when the multiple sensitization is used. For the multisensitized DSSCs, the IPCE spectra showed two major differences compared to the individual dye DSSCs: no dip is observed in the visible region as the efficiency remained above 80% from 400 to 650 nm compared with YD2-oC8 dye, As compared with the corresponding IPCE maxima of the single dye systems, the IPCE value of the multisensitized cells remain between 75–85% from 410 nm to 670 nm and current respond is respond above 710 nm. This effect is consistent with the absorption spectral feature shown in **Figure 34(b)**. To understand the charge transport kinetics of cell, electrochemical impedance spectroscopy (EIS) is next performed from my fitting model (see in **Figure 29(c)**). The estimated total resistance (*R*IR) of multi-sensitized DSSC at 1 0.3 MHz frequency region was about 18.51 Ω, which is about 1.58, 2.23 and 3.41 times lower values than that of TTAR, YD2-0C8 and YDD6, respectively. From simulated data, the multisensitized DSSC displays about 40.2%, 74.9% and 84.9% lower interfacial recombination rate in TiO2, *k*eff and 0.8%, 129% and 294% higher *R*k/*R*<sup>w</sup> value rather than those of TTAR, YD2-oC8 and YDD6 based DSSC. The higher interfacial recombination rate of porphyrin series sensitizer compared with TTAR sensitizer reveals recombination between iodide and oxidized electron at the TiO2 open space surface between dyes. For similar reasons, a multisensitized DSSC give rise to a denser packing and coverage and lead to the highest *k*eff. The schematic illusting melucular

#### **Figure 29.**

*(a) photocurrent density–voltage (*J-V*) characteristics (b) IPCE analysis and (c) EIS analysis for the individual and multiple sensitized DSSC (d) Schematic illustrating molecular structure and adsorption sites of YDD6, YD2-oC8 and TTAR dyes on TiO2.*

*D***eff (105 cm2**

**229**

TTAR YD2-oC8

YDD6 Triple

**Table 6.** *Parameters*

 *for the best fit of the impedance*

 *and Photovotaic*

 *data for a single dye (TTAR, YD2-oC8 and YDD6) and multi sensitized DSSC. Measured in Figure 29.*

1.952

11.9

 527.1

 6.1 2.4

 2.52

 0.044

 2.9

 12.07

 18.52

 0.818

 18.6

 72.8

 11.2

1.142

79.1

 79.42

 7.9 12.3 0.64

 2.014

1.918

19.9

47.5

 132.1

 10.3 9.3

 1.10

 0.302

 0.387

 5.7

 1.121

 63.21

 0.687

 6.42

 68.7

 3.03

 4.3

 1.822

 41.21

 0.769

 14.3

 67.5

 7.45

 314.7

 8.3 3.3

 2.5

 0.102

 3.5

 5.378

 29.40

 0.842

 14.3

 72.7

 8.73

**s**

**)** *k***eff (Hz) τeff (ms)**

 **Rk**

**R**

**Rk/R**

*Con*

**(Ωcms1**

**)** *R***d (Ω)** *ns* **(1018 cm3)**

 **RIR (Ω)**

*V***OC (V)** *J***sc**

**(mA/cm2**

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

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

> **w**

**w**

**1**

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

harvesting ability beyond 700 nm. As a result, the combination of complementary porphyrin and organic dyes produces a panchromatic spectral feature to promote

sensitization is used. For the multisensitized DSSCs, the IPCE spectra showed two major differences compared to the individual dye DSSCs: no dip is observed in the visible region as the efficiency remained above 80% from 400 to 650 nm compared with YD2-oC8 dye, As compared with the corresponding IPCE maxima of the single dye systems, the IPCE value of the multisensitized cells remain between 75–85% from 410 nm to 670 nm and current respond is respond above 710 nm. This effect is consistent with the absorption spectral feature shown in **Figure 34(b)**. To understand the charge transport kinetics of cell, electrochemical impedance spectroscopy (EIS) is next performed from my fitting model (see in **Figure 29(c)**). The estimated total resistance (*R*IR) of multi-sensitized DSSC at 1 0.3 MHz frequency region was about 18.51 Ω, which is about 1.58, 2.23 and 3.41 times lower values than that of TTAR, YD2-0C8 and YDD6, respectively. From simulated data, the multi-

sensitized DSSC displays about 40.2%, 74.9% and 84.9% lower interfacial recombination rate in TiO2, *k*eff and 0.8%, 129% and 294% higher *R*k/*R*<sup>w</sup> value rather than those of TTAR, YD2-oC8 and YDD6 based DSSC. The higher interfacial recombination rate of porphyrin series sensitizer compared with TTAR sensitizer reveals recombination between iodide and oxidized electron at the TiO2 open space surface between dyes. For similar reasons, a multisensitized DSSC give rise to a denser packing and coverage and lead to the highest *k*eff. The schematic illusting melucular

*(a) photocurrent density–voltage (*J-V*) characteristics (b) IPCE analysis and (c) EIS analysis for the individual and multiple sensitized DSSC (d) Schematic illustrating molecular structure and adsorption sites of*

,

) is attainable by increasing *J*sc value of the

) to ca. 18.6 mAcm<sup>2</sup> when the multiple

**Figure 29(a)** and **(b)** show the current–voltage characteristics and the corresponding IPCE action for the TTAR, YD2-oC8, YDD6 and Multiple (TTAR/ YD2-oC8/YDD6) sensitized DSSC, respectively. The photovoltaic parameters are listed in **Table 6**. An impressive cell performance of ca 11.2% (*J*sc = 18.6 mAcm<sup>2</sup>

the performance of DSSC.

**Figure 29.**

**228**

*YDD6, YD2-oC8 and TTAR dyes on TiO2.*

*V*oc = 0.818 V, *FF* = 72.8%, 998 mW cm<sup>3</sup>

*Solar Cells - Theory, Materials and Recent Advances*

TTAR and YD2-oC8 (each 14.3 mAcm<sup>2</sup>


**Table 6.**

*Parameters for the best fit of the impedance and Photovotaic data for a single dye (TTAR, YD2-oC8 and YDD6) and multi sensitized DSSC. Measured in Figure 29.*

strucutre for single and multi senstized film can be see in **Figure 29(d)**. Therefore, the charge density in TiO2 conduction band (*n*s) with multisensitized system was increased by about 124%, 562% for TTAR and YD2-oC8 single dye. The relative competition between electron–hole recombination and electron diffusion is conveniently described by the electron diffusion length, *Ln*, the relation *L*<sup>n</sup> = (*D*eff � *τ*eff) 1/2. The effective diffusion length *L*<sup>n</sup> of the conduction band electrons for single- (TTAR, YD2-oC8, YDD6) and multi-senstitized DSSC can be calculated from this equation. The *L***<sup>n</sup>** for all samples is estimated to be �24.6 μm of TTAR, 16.3 μm of YD2-oC8, 9.52 of YDD6 and � 32.1 μm of TTAR/ YD2-oC8/YDD6, respectively. This calculated data suggests that multisenstitized sensitizer leads to enhance the collection of being photogenerated electrons in comparison to those in single sensitizer. The detailed parameters can be seen in **Table 6**.

charge carriers for use its various optoelectronic devices. Both the performance and the stability of the devices improved dramatically when TPDSi2 was used the interfacial layer in OLED (organic light emitting diode) and OPV (organic photovoltaic) devices. The TPDSi2 molecule has two key components-widely used hole transport moteity triphenyldiamine (TPD) and a trichlorosilyl (SiCl3) pendant. The SiCl3 subgroups can be easily introduced into a charge conducting moiety like TPD via the highly efficient catalytic hydrosilylation reaction. The Si-Cl bonds are very stable during storage under inert atmosphere, but they are readily hydrolyzed by hydroxyl (OH-) groups. Moisture content of ambient atmosphere and surface OH groups that are present on oxides surfaces of ITO (indium tin oxide), SiO2 (silicon dioxide) and TiO2 [160, 161] substrates, along with water molecules that are physiadsorped on the surface, are the relevant sources of such OH groups. Once the SiCl3 groups come in contact with the OH groups, they get hydrozyzed at a rapid pace and inadvertently, Si-Cl groups from two different molecules get hydrozyzed by the two OH groups of the same water molecule. The very strong Si-O-Si bond hence formed, covalently linking two TPDSi2 molecules. Alternatively, Si-O-Si bonds are formed when two neighboring silanol groups undergo condensation reaction. The Si-O-Si bonds will extend over the length and breadth of the film in all directions and hence form a tightly held network of TPD units. While TPDSi2 is a very soluble organic solid, when it is cast onto a film allowed to hydrolyze, it will form a heavily cross-linked network that is rigid, insoluble and rugged. Our strategy in this study was to fill up the TiO2 pores with the soluble small molecule TPDSi2 and then to allow the molecules to cross link and form an extensive network that is in contact with both the deep lying dye molecules and the back electrode and hence shuttle holes from the former to the latter with great efficiency. **Figure 30(a)** shows the schematic image and the chemical structure of DSSCs utilizing P3HT and TPDSi2 organic seminconductor as HTM. A mesoporous TiO2 nanocrystlline film is

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

*(a) Device architecture (top)of ss-DSSC and chemical structures with transmission electron microscope image (bottom) of P3HT and TPDSi2 (b) illustration of p-doping effect on TPDSi2 by additive treatment (top) and complete energy diagram of FTO/TiO2/Z907/HTM/Au ssDSSC (bottom) (c) photovoltaic performance of Z907 dye based ssDSSCs employing P3HT and TPDSi2 as HTMs with and without additive treatment.*

**Figure 30.**

**231**
