**4. Effect of blocking layer in DSSC**

In DSSCs, a porous layer of nanostructuredsemiconductor materials such as TiO2 [40–45], ZnO [46–48], SnO2 [49, 50], SrTiO3 [51, 52] Zn2SnO4 [53, 54] and Nb2O5 [55] called a photo anode, covered with photosynthetic dye. The photo anode of


#### **Table 2.**

*IR absorption of organic functional groups of natural green, red, and combination of dyes (20% green +80% red) with TiO2 [36].*

DSSC influences the photo generated current. Highly porous structures and large surface areas of the nanostructured semiconductor materials increase the dye absorption and move the photo-induced electron towards the load [56]. Intensive research has been undertaken by the DSSC research community to increase photoinduced current and understand the mechanisms responsible for losses in the cell. Radiation less relaxation of energized dye, electron recombination with the oxidized dye; and electron recombination with the tri-iodide in the electrolyte are the main reasons for limiting the photocurrent in the cell. Generally, the first two have a negligible impact, while the last one shows a significant impact [56, 57].

Electron recombination occurs when electron transfer to I3 in the electrolytes via semiconductor material and the TCO. Electron recombination through both routes needs to be reduced to prevent loss. In I/I3 redox couple, the electron transfer via the TCO is negligible due to small exchange current density between I3 -I. Generally, the losses via the FTO under short-circuit condition is insignificant, because the Fermi level of the TCO (i.e., FTO) is close to the redox Fermi level. However, under illumination, the quasi-Fermi level of the semiconductor material (i.e., TiO2) rises rapidly with distance from the TCO (as shown in **Figure 12a**). As a result, a higher driving force is observed when electron transfer from the semiconductor material to I3 , which is much higher than in the bulk of the sensitized layer that is close to the TCO glass substrate. Thus, I3 electrons are anticipated to recombine with the semiconductor material at short-circuit conditions [57].

However, under illumination, the open-circuit condition is entirely different (shown in **Figure 12b**). Due to the rise (0.7 eV) of the Fermi level of TCO glass substrate, a much higher driving force is observed when electron transfer from the TCO glass substrate to I3 . Thus, the electron recombination with I3 via the TCO glass substrate and the back reaction by these two routes causes a photo stationary state in the cell [57].

The blocking layer works as a barrier layer at the TCO/semiconductor material

interface to improve cell performance. Studies had shown that a significant improvement in photo induced current observed when the blocking layer was introduced in the cell. Park and colleagues found that due to the blocking layer's

*(a) Absorption spectra of diluted natural chlorophyll (green), anthocyanin (red), and the optimum combination of dyes without TiO2, and (b) absorption spectra of diluted natural chlorophyll (green),*

*Effect of Combination of Natural Dyes and the Blocking Layer on the Performance of DSSC*

*DOI: http://dx.doi.org/10.5772/intechopen.94760*

*Absorption properties of betalain, curcumin, and combination of dyes [35].*

*anthocyanin (red), and the optimum combination of dyes withTiO2 [36].*

**Figure 10.**

**323**

**Figure 9.**

*Effect of Combination of Natural Dyes and the Blocking Layer on the Performance of DSSC DOI: http://dx.doi.org/10.5772/intechopen.94760*

#### **Figure 9.**

DSSC influences the photo generated current. Highly porous structures and large surface areas of the nanostructured semiconductor materials increase the dye absorption and move the photo-induced electron towards the load [56]. Intensive research has been undertaken by the DSSC research community to increase photoinduced current and understand the mechanisms responsible for losses in the cell. Radiation less relaxation of energized dye, electron recombination with the oxidized dye; and electron recombination with the tri-iodide in the electrolyte are the main reasons for limiting the photocurrent in the cell. Generally, the first two have a negligible impact, while the last one shows a significant impact [56, 57].

*IR absorption of organic functional groups of natural green, red, and combination of dyes (20% green +80%*

semiconductor material and the TCO. Electron recombination through both routes

ally, the losses via the FTO under short-circuit condition is insignificant, because the Fermi level of the TCO (i.e., FTO) is close to the redox Fermi level. However, under illumination, the quasi-Fermi level of the semiconductor material (i.e., TiO2) rises rapidly with distance from the TCO (as shown in **Figure 12a**). As a result, a higher driving force is observed when electron transfer from the semiconductor material to

, which is much higher than in the bulk of the sensitized layer that is close to the

However, under illumination, the open-circuit condition is entirely different (shown in **Figure 12b**). Due to the rise (0.7 eV) of the Fermi level of TCO glass substrate, a much higher driving force is observed when electron transfer from the

glass substrate and the back reaction by these two routes causes a photo stationary

. Thus, the electron recombination with I3

in the electrolytes via


via the TCO

redox couple, the electron transfer via

electrons are anticipated to recombine with the semi-

Electron recombination occurs when electron transfer to I3

the TCO is negligible due to small exchange current density between I3

needs to be reduced to prevent loss. In I/I3

conductor material at short-circuit conditions [57].

TCO glass substrate. Thus, I3

TCO glass substrate to I3

state in the cell [57].

I3

**322**

**Functional group**

Alkene (═CdH)

Aromatic (C═C)

Alcohol (OdH)

*red) with TiO2 [36].*

**Table 2.**

**Absorption range (in cm<sup>1</sup> )**

*Solar Cells - Theory, Materials and Recent Advances*

Alkane (CdH) 2820–2850 Stretch

Alkane (CdH) 2850–3000 Stretch

**Type of vibration**

1400–1600 Stretch Medium

(symmetric)

(asymmetric)

3200–3600 Stretch Broad

TidOdTi 400–800 Stretch Strong 438 440 515

Ether (CdO) 1000–1300 Stretch Strong 1042 1039 1040 Amine (CdN) 1080–1360 Stretch Weak 1324 1323 1315

Alkene (C═C) 1620–1680 Stretch Variable 1639 1643 1636

weak

and strong

**Intensity Absorption**

675–1000 Bending Strong 817 782 780

**peak of green dye by TiO2 film (in cm<sup>1</sup> )**

**Absorption peak of combination of dyes by TiO2 film (in cm<sup>1</sup> )**

1546 1544 1538

3384 3286 3281

Strong 2848 2852 2856

Strong 2924 2923 2924

**Absorption peak of red dye by TiO2 film (in cm<sup>1</sup> )**

*(a) Absorption spectra of diluted natural chlorophyll (green), anthocyanin (red), and the optimum combination of dyes without TiO2, and (b) absorption spectra of diluted natural chlorophyll (green), anthocyanin (red), and the optimum combination of dyes withTiO2 [36].*

#### **Figure 10.**

*Absorption properties of betalain, curcumin, and combination of dyes [35].*

The blocking layer works as a barrier layer at the TCO/semiconductor material interface to improve cell performance. Studies had shown that a significant improvement in photo induced current observed when the blocking layer was introduced in the cell. Park and colleagues found that due to the blocking layer's

improved physical contact between the TCO and semiconductor material that produce higher photo conversion efficiency. However, the advantage obtained by utilizing blocking layer is lost if the layer is too thick, and, generally, generates a series of resistance and an electron barrier that reduces the charge collection effi-

*Effect of Combination of Natural Dyes and the Blocking Layer on the Performance of DSSC*

A significant amount of photo-induced electron recombined and results in lower photocurrent. Recombination of the electron at the interfaces reduces the photocurrent and affects the fill factor; thus, cell performance decreases [60]. The complete photo anode is constructed layer-by-layer stack of suitably designed structures to maximize different cell functionalities. The recombination losses in DSSCs occurred primarily at the interface between the glass substrate of TCO and the electrolyte. The compact blocking layer acts as a physical barrier and physically separates and reduces the contact surface area between the TCO glass substrate from the electrolyte [59]. By employing the blocking layer with suitable thickness, the recombination can be reduced; and photo induced current and fill factor increase, leading to the DSSC efficiency improvement. Studies also showed that the blocking layer also improved the open-circuit-photo voltage of the cell [61]. The

There are many kinds of preparation methods for blocking layers in DSSCs, including spin coating, deep coating, spray coating, sol–gel, sputtering, hydrothermal technique, etc. Spin-coating is a simple method for preparing uniform thin films onto flat substrates. Generally, the spin coating method includes deposition, spinup, spinoff, and evaporation [62]. Usually, a small amount of coating material is applied to the center of the substrate then rotated at speed up to 10,000 rpm to spread the coating material by centrifugal force. Rotation is continued while the fluid spins off the substrate's edges until the desired thickness of the film is

schematic on the effect of blocking layer is shown in **Figure 13**.

*Schematic diagram of the DSSC including a blocking layer for preventing recombination.*

ciency [59, 60].

*DOI: http://dx.doi.org/10.5772/intechopen.94760*

**Figure 13.**

**325**

**Figure 11.**

*I-V characteristics of DSSC fabricated with betalain, curcumin, and combination of dyes [35].*


#### **Table 3.**

*Photovoltaic performance of DSSC fabricated with FTO/TiO2 [35].*

#### **Figure 12.**

*Schematic of DSSC in the absence of a blocking layer. (a) under short circuit conditions, the Fermi level in the FTO is close to the redox Fermi level results in rapid electron-transfer kinetics to I3 -. (b) under open-circuit conditions, the Fermi level in the FTO moves up as the electron quasi-Fermi level rises and results in a photo stationary state [57].*

presence, total transfer resistance at the blocking layer/electrolyte interface increased that increased cell performances by preventing electron recombination near the TCO glass substrate [58, 59]. Fabregat and co-workers found that BL
