*3.4.1 TiCl4 treatment*

The best-known techiqnue to improve the performance of the solar cells is a post-treatment of the TiO2 film with a solution in TiCl4 is grown onto an extra layer of TiO2 nanoparticles constituting the film. The TiCl4 treatment results in an improvement in photocurrent, normally between 10% and 30%. Depending on the quality of the TiO2 used to make the initial film, the extrema of the improvement can be from <5% to >200% [35, 56]. The largest improvements come when using the poorest quality TiO2 films. **Figure 8(a)** show the SEM images and XRD patterns for TiCl4 treated TiO2 film. When the TiCl4 exposure condition is increased, the


**Table 1.**

*Parameters for the best fit of the impedance data for the different thickness. Measured in Figure 6.*

increase in the film thickness. In a simple equation, since *ωmax* is inversely associated with the life time of electron τ =1/(2πf), the decrease in *ωmax* indicated a reduced rate for the charge-recombination process in DSSC. Hence, electrons with longer *τ* values were prevented from recombining, characterized by a larger charge transfer resistance. In the aspect of *ωmax*, about 6 μm thick film show the longest electron lifetime and relatively small total series resistance, leading to high *Voc* and *FF*. However, in the case of N719 dye, the dye absorption at that thickness is not enough to reach the maximum performance. The more detailed phenomenon can be understood by the further consolidated impedance model we suggested.

*NP film thicknesses. The solid curves are from a best fit model.*

*AC impedance measurement (a) bode, (b) Nyquist plots, (c) electron density (ns) and (d) the relation between the electron recombination resistance (Rk) and the electron diffusion resistance (Rw) of cells with different TiO2*

*Relationship of DSSC device performance (top) and efficiency per TiO2 NP (bottom) depending on film*

**Figure 6.**

**Figure 7.**

**198**

*thickness (top). Reprinted from [26, 52].*

*Solar Cells - Theory, Materials and Recent Advances*

#### **Figure 8.**

*(a) XRD pattern and SEM images of TiO2 film with and without post-treatment with TiCl4 (b) BET and BJH analysis of the pristine TiO2, and hydrothermal treated TiO2 film with and without post-treatment with TiCl4 (note: HT-TiO2 is a hydrothermal treated TiO2 nanoparticles).*

XRD intensity of the rutile peak increased [57]. However, under well-controlled condition, no obvious different in the rutile content (20–25%) can be seen between treated and untreated TiO2 from XRD analysis., while an increased size of TiO2 NPs and densely packed TiO2 NPs film is observed. **Figure 8(b)** shows Brunauer– Emmett–Teller (**BET**) and Barrett–Joyner–Halenda (**BJH**) pore-size distribution plots of the pristine TiO2 NP (P25), 2-step hydrothermalized (HT)-TiO2 NP and TiCl4 treated HT-TiO2 NP. Hydrothermal treated samples show similar type-IV isotherms, which are representative of mesoporous solids.

The 2-step HT-TiO2 NP show about 18.5% increased surface areas and 43.6% widened cumulative pore volume compared with the pristine TiO2 NP (P25). However, TiCl4 treated samples show 16.7% decreased surface area compared with pristine TiO2 film. However, the loss in actual electrode surface area after TiCl4 treatment is small because of the increase in mass of approximately 10.3% TiO2 volume on the electrode. From these observations it follows that, despite the substantial decrease in BET surface area, the film thickness is not affected. Therefore, the porosity must have decreased, as is shown in **Table 2**. In spite of the decreased surface area, the TiCl4 treated TiO2 film morphology is observed by about a 40% higher dye absorption at the 480 nm. (see **Figure 9(a)**) For more accurate experiment, the quantity of TiO2 NPs surface-bound sensitizers was measured by desorption process [35]. UV–vis absorption spectra is used for calculating the number of desorbed sensitizer molecules (set as the extinction coefficient (*ε)* of the N719 sensitizer is about 3.748 x 10<sup>3</sup> cm<sup>1</sup> M<sup>1</sup> at 535 nm). It is estimated that roughly 8.8% and 34.6% more dye molecules are attached to the surface of TiCl4 treated TiO2 (<sup>≈</sup> 3.97 <sup>10</sup><sup>8</sup> molmg<sup>1</sup> ) compared to HT-TiO2 NP (<sup>≈</sup> 3.65 <sup>10</sup><sup>8</sup> molmg<sup>1</sup> ) and commercial TiO2 NP (<sup>≈</sup> 2.95 <sup>10</sup><sup>8</sup> molmg<sup>1</sup> ), respectively. The TiO2 surface after the TiCl4 treatment provides more specific binding sites, leading to reduce the fraction of the TiO2 surface area that is inaccessible for the dye due to sterical constraints [56]. The enhanced dye loading results in an improvement in photocurrent and indeed, the TiCl4 treatment is a 28.5% increase in the photocurrent along with a decreased in the fill factor (7.4%) and open circuit voltage (2.3%). (see in **Figure 9(b)** and more discussed in cell properties part (iii)).

**BET & BJH**

**201**

**Surface area**

**Pore Volume**

**TiO2 mass**

**λmax (at**

**# of mole. (108**

*D***eff ( 105**

**Rk/R**

**w** *ns* **(1018**

**Rtotal**

*V***OC**

*J***sc**

*FF*

**EFF**

**cm3**

**)**

**(Ω)**

**(V)**

**(mA/cm2**

**)**

**(%)**

**(%)**

**cm2**

**1**

**s**

**)**

**(m2**

(a)

(b)

(c)

(d) w CF4 (10 min)

\*

 w CF4 (30 min)

(e)

(f)

**Table 2.** *Parameters*

 *for the best fit of the impedance*

 *data for the different interfacial*

 *modificated*

 *DSSC film. Measured in*

*Figures 9 and 12.*

 w CF4/TiCl4

 w TiCl4/CF4

 w TiCl4

 HT- TiO2

 TiO2

53.29

65.33

55.98

—

—

—

—

 —

2.6

 0.503

 —

2.6

 0.432

 —

1.4

 0.211

 —

2.3

 0.390

 0.445

2.9

 0.504

 0.607

 0.348

2.3

2.6

 0.410

 0.267

2.95 3.65 3.97 3.48 2.45 3.72 3.94

4.12

 2.48

 10.4

 13.1 0.813

 15.75

 71.8 9.20

2.59

 2.52

 8.14

 19.8 0.835

 13.68

 74.5 8.50

2.11

 2.84

 3.24

 58.9 0.838

 6.52

 75.1 4.10

3.45

 3.13

 5.15

 29.3 0.852

 10.29

 76.0 6.67

3.24

 2.14

 11.2

 23.7 0.796

 15.92

 66.8 8.47

2.58

 2.38

 9.39

 31.5 0.815

 12.38

 72.2 7.28

0.84

 1.72

 4.81

 41.6 0.796

 9.725

 70.6 5.46

**g1**

**)**

**(cm3**

**g1**

**)**

**(mg/cm2**

**)**

**498 nm)**

**mol /mg)**

**Absorption**

 **Properties**

**Electrical Properties**

**Solar Properties**

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


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

#### **Table 2.**

*Parameters for the best fit of the impedance data for the different interfacial modificated DSSC film. Measured in Figures 9 and 12.*

XRD intensity of the rutile peak increased [57]. However, under well-controlled condition, no obvious different in the rutile content (20–25%) can be seen between treated and untreated TiO2 from XRD analysis., while an increased size of TiO2 NPs and densely packed TiO2 NPs film is observed. **Figure 8(b)** shows Brunauer– Emmett–Teller (**BET**) and Barrett–Joyner–Halenda (**BJH**) pore-size distribution plots of the pristine TiO2 NP (P25), 2-step hydrothermalized (HT)-TiO2 NP and TiCl4 treated HT-TiO2 NP. Hydrothermal treated samples show similar type-IV

*(a) XRD pattern and SEM images of TiO2 film with and without post-treatment with TiCl4 (b) BET and BJH analysis of the pristine TiO2, and hydrothermal treated TiO2 film with and without post-treatment with TiCl4*

The 2-step HT-TiO2 NP show about 18.5% increased surface areas and 43.6% widened cumulative pore volume compared with the pristine TiO2 NP (P25). However, TiCl4 treated samples show 16.7% decreased surface area compared with pristine TiO2 film. However, the loss in actual electrode surface area after TiCl4 treatment is small because of the increase in mass of approximately 10.3% TiO2 volume on the electrode. From these observations it follows that, despite the substantial decrease in BET surface area, the film thickness is not affected. Therefore, the porosity must have decreased, as is shown in **Table 2**. In spite of the decreased surface area, the TiCl4 treated TiO2 film morphology is observed by about a 40% higher dye absorption at the 480 nm. (see **Figure 9(a)**) For more accurate experiment, the quantity of TiO2 NPs surface-bound sensitizers was measured by desorption process [35]. UV–vis absorption spectra is used for calculating the number of desorbed sensitizer molecules (set as the extinction coefficient (*ε)* of the N719 sensitizer is about 3.748 x 10<sup>3</sup> cm<sup>1</sup> M<sup>1</sup> at 535 nm). It is estimated that roughly 8.8% and 34.6% more dye molecules are attached to the surface of TiCl4 treated

after the TiCl4 treatment provides more specific binding sites, leading to reduce the fraction of the TiO2 surface area that is inaccessible for the dye due to sterical constraints [56]. The enhanced dye loading results in an improvement in photocurrent and indeed, the TiCl4 treatment is a 28.5% increase in the photocurrent along with a decreased in the fill factor (7.4%) and open circuit voltage (2.3%). (see in

) compared to HT-TiO2 NP (<sup>≈</sup> 3.65 <sup>10</sup><sup>8</sup> molmg<sup>1</sup>

), respectively. The TiO2 surface

)

isotherms, which are representative of mesoporous solids.

*(note: HT-TiO2 is a hydrothermal treated TiO2 nanoparticles).*

*Solar Cells - Theory, Materials and Recent Advances*

TiO2 (<sup>≈</sup> 3.97 <sup>10</sup><sup>8</sup> molmg<sup>1</sup>

**200**

**Figure 8.**

and commercial TiO2 NP (<sup>≈</sup> 2.95 <sup>10</sup><sup>8</sup> molmg<sup>1</sup>

**Figure 9(b)** and more discussed in cell properties part (iii)).

#### **Figure 9.**

*(a) Absorbance curve of aqueous dye solutions after desorption (b) JV characteristics and (c) EIS analysis from the pristine TiO2, and hydrothermal treated TiO2 film with and without post-treatment with TiCl4 (inserted in simple illustration of TiCl4 treatments).*

#### *3.4.2 Fuorine plasma etching*

As mentioned above, the best efficiency of cell can be found at about 11.5 μm thicked TiO2 film. This resulting data shows mediately the importance of electrolyte's infiltration all of the way into the TiO2 film. To minimize the effect on the panetration issues, plama etching techinque is introduced for widening the channels in TiO2 films [51]. **Figure 10(a)** displays a schematic diagram of the plasma etching system. The detailed etching condition can be found in our eariler paper [51]. Aa ehching gas, CF4/O2 of gas mixture (CF4 and O2) is used by generating the fluorine atoms in the plasma through complicated chemical reaction paths [58–62]. The plasma etching reaction is followed by:

$$\text{CF}\_4 + \text{O} \rightarrow \text{COF}\_2 + 2\text{F} \tag{32}$$

14% the oxygen level. Two O 1 s peaks are observed prior to plasma treatment. The major peak with 90% of the total oxygen is measured at a binding energy of 529.8 eV and 531.4 eV for Ti-O and Ti-OH bonding, respectively [67]. The plasma treatment make the TiO2 peak move a little higher binding-energy of 529.9 eV. **Figure 11(a)** shows SEM images of pre and post fluorine-treated films. It is clear from these images that fluorine effectively opens the channels among the NPs of the TiO2 film. The variance of film thickness and weight as a function of plasma etching time can be seen in **Figure 11(b)**. With increased etching time, quantitative analysis (EDS) and morphology (SEM) of TiO2 surface reveal the plasma expand the pores of the TiO2 film surface. As the pore start to open up (10 min), the surface etching process of removed TiO2 and incorporated fluorine into the film is accelerated by active fluorine species. The detailed information can be found in our eailr paper [51]. As seen in **Figure 12(a)**, the plasma etched TiO2 surface can rectify the interfacial property by decreased the electron-triiodine recombination rates taking place on the parts of the dye uncovered TiO2 surface. According to CF4 etching process, it function as diminishing the surface defects such as oxygen vacancies since fluorine atom comprised of stronger affinity for electrons compared with oxygen atom can

*and pre- and post-etch XPS O 1 s spectra. Reprinted from [51].*

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

*(a) Schematic diagram of the plasma etching system (b) pre- and post-etch Ti 2p, post-etch F 1 s XPS spectrum,*

**Figure 10.**

**203**

$$\rm TiO\_2(s) + F(g) \to TiO\_xF\_y(s) + TiF\_4(s,g) + O\_2(g) \tag{33}$$

TiO2 surface is subsequently ethced by free fluorine gas and several titanium fluoroxy compounds (ultimately TiF4) are developed [61, 63]. Along with the etching process, it was expected that the surface of the NP would be populated with fluorine atoms and bonded to Ti sites as discussed below.

The evidence of etched TiO2 surface can be found by X-ray photon spectroscopy (XPS) measurement. The peak is located at binding energies 458.5 eV (Ti 2p3/2) and 464.2 eV (Ti 2p1/2), respectively, which correspond to the signals characteristic for the Ti4+ state of titanium [64, 65]. This indicates that TiO2 are formed. (see in **Figure 10(b)**). After plasma treatment the peak locations move to the somewhat higher binding-energies of 458.7 and 464.4 eV. The direct bonding with titanium and fluorine makes it possible to move higher binding-energy due to fluorine exhibit more electronegative than oxygen. From XPS analysis, the post-plasma F 1 s peak shows is also indicative of the fluorine being bonded directly to the titanium. The majority of the F 1 s peak (99%) is found at 684.8 eV which has been responsible for TiOF2. The minor (1%) energy shoulder at 686.8 eV is ascrbied to the replacement of oxygen lattice site in TiO2 by fluorine [66]. The fluorine shows at

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

**Figure 10.**

*3.4.2 Fuorine plasma etching*

*simple illustration of TiCl4 treatments).*

*Solar Cells - Theory, Materials and Recent Advances*

**Figure 9.**

**202**

plasma etching reaction is followed by:

fluorine atoms and bonded to Ti sites as discussed below.

As mentioned above, the best efficiency of cell can be found at about 11.5 μm thicked TiO2 film. This resulting data shows mediately the importance of electrolyte's infiltration all of the way into the TiO2 film. To minimize the effect on the panetration issues, plama etching techinque is introduced for widening the channels in TiO2 films [51]. **Figure 10(a)** displays a schematic diagram of the plasma etching system. The detailed etching condition can be found in our eariler paper [51]. Aa ehching gas, CF4/O2 of gas mixture (CF4 and O2) is used by generating the fluorine atoms in the plasma through complicated chemical reaction paths [58–62]. The

*(a) Absorbance curve of aqueous dye solutions after desorption (b) JV characteristics and (c) EIS analysis from the pristine TiO2, and hydrothermal treated TiO2 film with and without post-treatment with TiCl4 (inserted in*

TiO2 surface is subsequently ethced by free fluorine gas and several titanium fluoroxy compounds (ultimately TiF4) are developed [61, 63]. Along with the etching process, it was expected that the surface of the NP would be populated with

The evidence of etched TiO2 surface can be found by X-ray photon spectroscopy (XPS) measurement. The peak is located at binding energies 458.5 eV (Ti 2p3/2) and 464.2 eV (Ti 2p1/2), respectively, which correspond to the signals characteristic for the Ti4+ state of titanium [64, 65]. This indicates that TiO2 are formed. (see in **Figure 10(b)**). After plasma treatment the peak locations move to the somewhat higher binding-energies of 458.7 and 464.4 eV. The direct bonding with titanium and fluorine makes it possible to move higher binding-energy due to fluorine exhibit more electronegative than oxygen. From XPS analysis, the post-plasma F 1 s peak shows is also indicative of the fluorine being bonded directly to the titanium. The majority of the F 1 s peak (99%) is found at 684.8 eV which has been responsible for TiOF2. The minor (1%) energy shoulder at 686.8 eV is ascrbied to the replacement of oxygen lattice site in TiO2 by fluorine [66]. The fluorine shows at

CF4 þ O ! COF2 þ 2F (32)

TiO2ð Þþ s F gð Þ! TiOxFyð Þþ s TiF4ð Þþ s, g O2ð Þg (33)

*(a) Schematic diagram of the plasma etching system (b) pre- and post-etch Ti 2p, post-etch F 1 s XPS spectrum, and pre- and post-etch XPS O 1 s spectra. Reprinted from [51].*

14% the oxygen level. Two O 1 s peaks are observed prior to plasma treatment. The major peak with 90% of the total oxygen is measured at a binding energy of 529.8 eV and 531.4 eV for Ti-O and Ti-OH bonding, respectively [67]. The plasma treatment make the TiO2 peak move a little higher binding-energy of 529.9 eV.

**Figure 11(a)** shows SEM images of pre and post fluorine-treated films. It is clear from these images that fluorine effectively opens the channels among the NPs of the TiO2 film. The variance of film thickness and weight as a function of plasma etching time can be seen in **Figure 11(b)**. With increased etching time, quantitative analysis (EDS) and morphology (SEM) of TiO2 surface reveal the plasma expand the pores of the TiO2 film surface. As the pore start to open up (10 min), the surface etching process of removed TiO2 and incorporated fluorine into the film is accelerated by active fluorine species. The detailed information can be found in our eailr paper [51].

As seen in **Figure 12(a)**, the plasma etched TiO2 surface can rectify the interfacial property by decreased the electron-triiodine recombination rates taking place on the parts of the dye uncovered TiO2 surface. According to CF4 etching process, it function as diminishing the surface defects such as oxygen vacancies since fluorine atom comprised of stronger affinity for electrons compared with oxygen atom can

to dye and electrolyte infiltration. As a first series of cells, TiO2 film prepared for cell (a) commercial and cell (b) hydrothermal treatment is studied. Next, for understanding the influence on interfacial modified TiO2 surface, post-treated cells with cell (c) with TiCl4 treatment and Tare cell (c) with fluorine etching are investigated. **Figure 12(b)** and **(c)** give, respectively, the *J* � *V* and the impedance measurements of each cells. Using the kinetic model discussed in section and extrapolating the parameters for the best fit to each of the measured impedance curves, summarized the results in **Table 2** for the electrical data for all cells. The model calculation and data fitting provide some physical insight into the differences in the transport properties and effects due to plasma etching of the TiO2 NP films of the cells. Cell (b) can be considered as the control for the other three cells. There is no treatment to the TiO2 NP film in this case. In general, *Jsc* can be approximated by

where *q* is the elementary charge, *ηlh* is the light harvesting efficiency of a cell, *ηinj* is the charge-injection efficiency of the excited dye into the TiO2, *ηcc* is charge collection efficiency, and *I0* is the incident photon flux [69]. From this equation, it is clearly that the short-circuit current density (*Jsc*) is directly proportional to the value of *ηlh* related to numerous specific anchoring sites on TiO2 surface for dye absorption and light scattering events for optical absorption as an external property. As we mentioned earlier, the *ηcc* is related to the charge transfer kinetics and this value can be estimated by comparing the charge transport and recombination time

Cell (c) has a 25.5% increase in the *D*eff, leading to about 19.3% increased electron density (*n*s) compared with the cell (b). With increasing electron density, deeper traps become filled, and trapping/detrapping events occur more frequently in shallower traps, leading to faster transport. The increase in the photocurrent density is well explained by the higher electron density. However, the value of *Rk*/*Rw* related to the recombination related value is decreased by about 10%, leading to a decrease Voc. Cell (d) with the plasma etched device shows the resulting values for about 45.2% lower the charge density value (*n*s,), 31.5% higher the interfacial recombination rate (Rk/Rw) and 33.4% increased *D*eff rather than that of the unetched sample (cell (b)). Although the fitted charge transfer properties on etched sample are substantial, the overall performance of cell is infinitesimal because the

These changes show that the etching has significant effect on the electron recombination and transport properties of the cell, but the overall effect is not pronounced because the cell efficiency only increased by 8% with etching. As suggested in **Figure 12(a)**, the fluorine etching can help to minimize the electron loss between TiO2 surface and reduced iodide, leading to the highest *V*oc and *FF* value (�0.852 V and 75.1%). The decreased electron transport properties are attributed to the less interconnection between NPs compared to the case of cell (b). However, when a film is treated in the much more time (�30 min), the morphology changed dramatically, and the pin-hole of TiO2 film formed. This result can be confirmed by the dye desorption experiments and the weight loss of TiO2 from etching process. While this situation should improve the dye molecule attachments to the NPs and allow further penetration of the molecules into the TiO2 film, as

Finally, we believe that combination of TiCl4 and fluorine etching posttreatment with the opposite physical properties make it possible to increase both the *V*oc and *J*sc, giving an efficiency higher than that of cell (b). Therefore, cell (e) is

constants when both values are measured by EIS measurements.

efficiency just increased by about 8% with etching.

indicated by a large increased in the values of *D*eff in **Table 2**.

*Jsc* ¼ *qηlhηinjηccI*<sup>0</sup> (34)

the expression;

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

**205**

#### **Figure 11.**

*(a) SEM images of top and side views of pre-etch film and CF4-etched TiO2 film. (inserted in simple illustrations of etched film) (b) film thickness, weight and film morphologies images (top) and relative content of oxygen and fluorine in TiO2 NPs (bottom) as a function of etching time. Reprinted from [51].*

#### **Figure 12.**

*(a) Schematic illustration for TiO2 surface interfacial mechanism (b) JV curve and (c) AC impedance measurements of cells with different surface treatments.*

enfeeble the bond in a titanium and oxygen [68]. Therefore, the TiO2 surface comprised of less surface defects may play important role in a improvement of the open-circuit potential, *V*oc, of the solar cell.
