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

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 efficiency [59, 60].

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

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

**Figure 13.** *Schematic diagram of the DSSC including a blocking layer for preventing recombination.*

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

*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*

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

Red 371.6 09.5 1.218 0.039 0.487 0.008 0.220 0.016 1.05 Yellow 507.2 10.5 1.857 0.026 0.503 0.004 0.473 0.020 1.09 Red + Yellow 1:1 495.5 09.4 2.319 0.015 0.508 0.003 0.583 0.018 1.09 Red + Yellow 1:2 502.7 11.5 2.494 0.022 0.518 0.002 0.649 0.020 1.08 Red + Yellow 2:1 497.1 14.3 2.041 0.025 0.508 0.003 0.515 0.024 1.09

**Voc (mV) Isc (mA) FF η% Dye**

**loading (mol mm<sup>3</sup> X 10<sup>7</sup> )**

**Figure 11.**

**Dye/**

**dyes**

**Table 3.**

**Figure 12.**

**324**

*stationary state [57].*

**Combination of**

**Dye ratio**

*Solar Cells - Theory, Materials and Recent Advances*

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

achieved. The film's thickness also depends on the solvent and solvents concentration [63].

Yeol et al. prepared a ZnO precursor on FTO substrates for the blocking layer. For ZnO precursor, a homogeneous mixture of 2.195 g zinc acetate dehydrate, 20 mL isopropanol, and 0.605 mL monoethanolamine (MEA) was prepared. The concentration was 0.5 M, with MEA: zinc acetate molar ratio of 1: 1. The prepared solution was stirred for 2 hrs at 200 rpm at 60°C, then stirred at the same rpm at ambient temperature for 22 hrs. For the spin-coated film, rotation speed and duration were held at 3000 rpm and 20 s, respectively. They annealed the spin-coated films at 500° C for 1 h to form a blocking layer of ZnO (55 nm to 310 nm). The ZnO blocking layer thickness is a function of the number of deposition cycles in the spincoating process. ZnO blocking layer thickness increased linearly with the number of deposition cycles, a typical feature of the spin-coating technique [64]. **Figure 14a** illustrates the morphology of FTO. **Figure 14b** and **c** show that ZnO nanoparticles are distributed uniformly across the FTO substrate's surface to form a compact layer. Comparing both **Figure 14a** and **c**, when the thickness of the ZnO blocking layer increased, the size of the ZnO nanoparticles also slightly increased.

Yeol et al. showed that the effect of ZnO blocking layer and increasing its thickness on the cell performance of TiO2 based DSSC. **Table 4** lists photovoltaic performance and **Figure 15** illustrates the J–V characteristics of the cell, including ZnO blocking layers of different thicknesses. The value of open-circuit voltage (Voc) and fill factor (FF) of the DSSC improves, though the short-circuit current decreased. The increase of open-circuit voltage is due to the blocking of electron injection from the TiO2 conduction band to the FTO [64, 65]. Due to the increased electron density in the TiO2, the Fermi level rises. However, further an increase in the thickness of the ZnO blocking layer, the value of short circuit current decreased

rapidly. As a result, cell performance decrease despite the slight improvement in the Voc and FF values because the excessively thick ZnO layer blocks the electron injection from the conduction band of TiO2 to the FTO substrate [64].

**Revolution per minute (rpm) Thickness (nm) Rughness (nm)** 0 0 21.14 2000 10–30 10.91 1000 40–60 11.68 500 120–150 14.05

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

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

*J–V characteristics of DSSCs including ZnO blocking layer of different thicknesses [64].*

Lee et al. introduced an additional spin-coated TiO2 thin film between the FTO and TiO2 (semiconductor material) as a blocking layer for the electron injected from the exited photosensitizer. A homogeneous mixture of 29.0 mg titanium tetraisopropoxide [Ti (OC3H7)4], and 100 ml isopropanol [(CH3)2CHOH] was prepared. Then the solution of 7.5 ml HCl in 100 ml of isopropanol was added drop by drop to the [Ti (OC3H7)4]-[(CH3)2CHOH] solution at 0°C under continuous stirring, and afterward the solution was allowed to stand for less than 1 h at the same temperature. The solution was smeared on FTO substrates and rotated at 500, 1000, and 2000 rpm for 40 s to ensure uniformity. The samples were heated for 1 h at 100°C;

Lee et al. prepared several TiO2 gel films with the spin coating method with different thicknesses. The thickness and roughness of the TiO2 layers are among the most critical factors in the cell performance of DSSC [66]. **Table 5** lists the thickness and root-mean-square roughness of TiO2 thin films. SEM images of 10 μm thin films (surface and cross-sections) are shown in **Figure 16**. **Table 6** lists photovoltaic performance and **Figure 17** illustrates the J–V characteristics of the cell, including

TiO2 layers also enhance the contact property between the FTO and TiO2 electrode. **Figure 16c** and **d** illustrate the photovoltaic performance and the J-V characteristics curve of DSSC with different blocking layer thicknesses. The thickness of the TiO2 blocking layer affects the efficiency of DSSC. As thin films' rpm increased, the thickness and roughness of the TiO2 blocking layer also decreased, and the film becomes smooth and uniform. This increase in the smoothness and uniformity of

they were sintered for 30 min at 450°C [66].

*Thickness and root-mean-square roughness of TiO2 thin films [66].*

**Figure 15.**

**Table 5.**

**327**

ZnO blocking layers of different thicknesses.

#### **Figure 14.**

*FESEM images of (a) bare FTO, (b) FTO/ ZnO blocking layer (120 nm), (c) FTO/ ZnO blocking layer (310 nm) [64].*


#### **Table 4.**

*Photovoltaic properties of TiO2 based DSSCs including ZnO blocking layer of different thicknesses (for N3 dye) [64].*

*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 15.** *J–V characteristics of DSSCs including ZnO blocking layer of different thicknesses [64].*


**Table 5.**

achieved. The film's thickness also depends on the solvent and solvents

*Solar Cells - Theory, Materials and Recent Advances*

layer increased, the size of the ZnO nanoparticles also slightly increased.

Yeol et al. showed that the effect of ZnO blocking layer and increasing its thickness on the cell performance of TiO2 based DSSC. **Table 4** lists photovoltaic performance and **Figure 15** illustrates the J–V characteristics of the cell, including ZnO blocking layers of different thicknesses. The value of open-circuit voltage (Voc) and fill factor (FF) of the DSSC improves, though the short-circuit current decreased. The increase of open-circuit voltage is due to the blocking of electron injection from the TiO2 conduction band to the FTO [64, 65]. Due to the increased electron density in the TiO2, the Fermi level rises. However, further an increase in the thickness of the ZnO blocking layer, the value of short circuit current decreased

*FESEM images of (a) bare FTO, (b) FTO/ ZnO blocking layer (120 nm), (c) FTO/ ZnO blocking layer*

FTO 695 8.48 3.86 0.66 FTO/ZnO (55 nm) 708 8.30 3.96 0.67 FTO/ZnO (120 nm) 728 8.18 4.34 0.73 FTO/ZnO (220 nm) 744 6.64 3.63 0.73 FTO/ZnO (275 nm) 745 4.83 2.69 0.75 FTO/ZnO (310 nm) 781 3.05 1.66 0.70

*Photovoltaic properties of TiO2 based DSSCs including ZnO blocking layer of different thicknesses*

**) Efficiency (%ɳ) Fill Factor (FF)**

**Sample Voc(mV) Jsc (mA/cm2**

Yeol et al. prepared a ZnO precursor on FTO substrates for the blocking layer. For ZnO precursor, a homogeneous mixture of 2.195 g zinc acetate dehydrate, 20 mL isopropanol, and 0.605 mL monoethanolamine (MEA) was prepared. The concentration was 0.5 M, with MEA: zinc acetate molar ratio of 1: 1. The prepared solution was stirred for 2 hrs at 200 rpm at 60°C, then stirred at the same rpm at ambient temperature for 22 hrs. For the spin-coated film, rotation speed and duration were held at 3000 rpm and 20 s, respectively. They annealed the spin-coated films at 500° C for 1 h to form a blocking layer of ZnO (55 nm to 310 nm). The ZnO blocking layer thickness is a function of the number of deposition cycles in the spincoating process. ZnO blocking layer thickness increased linearly with the number of deposition cycles, a typical feature of the spin-coating technique [64]. **Figure 14a** illustrates the morphology of FTO. **Figure 14b** and **c** show that ZnO nanoparticles are distributed uniformly across the FTO substrate's surface to form a compact layer. Comparing both **Figure 14a** and **c**, when the thickness of the ZnO blocking

concentration [63].

**Figure 14.**

**Table 4.**

**326**

*(for N3 dye) [64].*

*(310 nm) [64].*

*Thickness and root-mean-square roughness of TiO2 thin films [66].*

rapidly. As a result, cell performance decrease despite the slight improvement in the Voc and FF values because the excessively thick ZnO layer blocks the electron injection from the conduction band of TiO2 to the FTO substrate [64].

Lee et al. introduced an additional spin-coated TiO2 thin film between the FTO and TiO2 (semiconductor material) as a blocking layer for the electron injected from the exited photosensitizer. A homogeneous mixture of 29.0 mg titanium tetraisopropoxide [Ti (OC3H7)4], and 100 ml isopropanol [(CH3)2CHOH] was prepared. Then the solution of 7.5 ml HCl in 100 ml of isopropanol was added drop by drop to the [Ti (OC3H7)4]-[(CH3)2CHOH] solution at 0°C under continuous stirring, and afterward the solution was allowed to stand for less than 1 h at the same temperature. The solution was smeared on FTO substrates and rotated at 500, 1000, and 2000 rpm for 40 s to ensure uniformity. The samples were heated for 1 h at 100°C; they were sintered for 30 min at 450°C [66].

Lee et al. prepared several TiO2 gel films with the spin coating method with different thicknesses. The thickness and roughness of the TiO2 layers are among the most critical factors in the cell performance of DSSC [66]. **Table 5** lists the thickness and root-mean-square roughness of TiO2 thin films. SEM images of 10 μm thin films (surface and cross-sections) are shown in **Figure 16**. **Table 6** lists photovoltaic performance and **Figure 17** illustrates the J–V characteristics of the cell, including ZnO blocking layers of different thicknesses.

TiO2 layers also enhance the contact property between the FTO and TiO2 electrode. **Figure 16c** and **d** illustrate the photovoltaic performance and the J-V characteristics curve of DSSC with different blocking layer thicknesses. The thickness of the TiO2 blocking layer affects the efficiency of DSSC. As thin films' rpm increased, the thickness and roughness of the TiO2 blocking layer also decreased, and the film becomes smooth and uniform. This increase in the smoothness and uniformity of

#### **Figure 16.**

*Cross-sectional SEM images of the main-TiO2/FTO (a), main- TiO2/TiO2 thin film/FTO applied to a DSSC (b) [66].*


#### **Table 6.**

*The cell performance of DSSCs based on TiO2 layers (10.5 μm) compressed at different thickness of thin films during the preparation for ruthenium 535 (Solaronix Co. N3) dye [66].*

Yoo et al. showed the impact of precursor concentration in the cell conversation efficiency of DSSC. For the blocking layer, a 1-butanol solution contained titanium (IV) bis(ethylacetoacetato) di-isopropoxide precursor was spin-coated on an FTO glass, followed by annealing at 500°C in air for 30 min. The concentration of the solution was varied from 0.05 M to 1.2 M [58]. **Figure 18** illustrates the SEM of bare FTO and blocking layer-deposited FTO glasses (surface and cross-sections). **Table 7** lists the photovoltaic property of DSSC with a blocking layer, where short-circuit current density increases with increasing the precursor concentration (and

*Surface (A–H) and cross-section (a–h) SEM micrographs for the bare FTO, blocking layer-deposited FTO substrates from the Ti precursor solutions with the concentration of 0.05, 0.1, 0.15, 0.2, 0.4, 0.8, and 1.2 M,*

Zou et al. studied the effect of the TiCl4 blocking layer (or pre-treatment) in ZnO based DSSC. **Figure 19** shows the fabricated ZnO films, with and without TiCl4 pre-treatment on the FTO glass substrate. From **Figure 19a**-**d**, it can be seen that fabricated ZnO films have porous flakes, both with and without blocking

increased blocking layer thickness).

**Figure 17.**

**Figure 18.**

**329**

*respectively [58].*

*I–V curves for DSSCs with TiO2 blocking layers at different thickness [66].*

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

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

the TiO2 blocking layer results in improved cell performance. The increased number of efficiently transferred photo generated electrons to the TiO2 electrode results in an improvement in short-circuit current [67]. By suppressing the recombination of electrons injected from excited photosensitizers in the TiO2 and electrolyte interface, a higher value of open-circuit voltage was obtained [57]. Their study also showed that when the resistance at the FTO/TiO2 layer interface was decreased, the electron lifetime in DSSCs was increased [66].

*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 17.** *I–V curves for DSSCs with TiO2 blocking layers at different thickness [66].*

#### **Figure 18.**

*Surface (A–H) and cross-section (a–h) SEM micrographs for the bare FTO, blocking layer-deposited FTO substrates from the Ti precursor solutions with the concentration of 0.05, 0.1, 0.15, 0.2, 0.4, 0.8, and 1.2 M, respectively [58].*

Yoo et al. showed the impact of precursor concentration in the cell conversation efficiency of DSSC. For the blocking layer, a 1-butanol solution contained titanium (IV) bis(ethylacetoacetato) di-isopropoxide precursor was spin-coated on an FTO glass, followed by annealing at 500°C in air for 30 min. The concentration of the solution was varied from 0.05 M to 1.2 M [58]. **Figure 18** illustrates the SEM of bare FTO and blocking layer-deposited FTO glasses (surface and cross-sections). **Table 7** lists the photovoltaic property of DSSC with a blocking layer, where short-circuit current density increases with increasing the precursor concentration (and increased blocking layer thickness).

Zou et al. studied the effect of the TiCl4 blocking layer (or pre-treatment) in ZnO based DSSC. **Figure 19** shows the fabricated ZnO films, with and without TiCl4 pre-treatment on the FTO glass substrate. From **Figure 19a**-**d**, it can be seen that fabricated ZnO films have porous flakes, both with and without blocking

the TiO2 blocking layer results in improved cell performance. The increased number of efficiently transferred photo generated electrons to the TiO2 electrode results in an improvement in short-circuit current [67]. By suppressing the recombination of electrons injected from excited photosensitizers in the TiO2 and electrolyte interface, a higher value of open-circuit voltage was obtained [57]. Their study also showed that when the resistance at the FTO/TiO2 layer interface was decreased, the

*The cell performance of DSSCs based on TiO2 layers (10.5 μm) compressed at different thickness of thin films*

*Cross-sectional SEM images of the main-TiO2/FTO (a), main- TiO2/TiO2 thin film/FTO applied to a DSSC*

**Efficiency, ɳ (%)**

 0.65 11.09 62 4.43 14.1 5.3 28.8 –30 0.74 11.92 64 5.62 20.1 4.3 19.1 –60 0.72 11.58 65 5.39 18.2 4.7 19.7 –150 0.70 11.21 60 4.68 16.6 7.6 21.9

**Electron lifetime, Te (ms)**

**Resistance at Pt. counter electrode, RCt1 (Ω)**

**Charge transfer resistances at the TiO2/dye/ electrolyte interface RCt2 (Ω)**

electron lifetime in DSSCs was increased [66].

*during the preparation for ruthenium 535 (Solaronix Co. N3) dye [66].*

**Figure 16.**

*(b) [66].*

**Table 6.**

**328**

**Thickness (nm)**

**Opencircuit voltage, Voc (V)**

**Shortcircuit current density, Jsc (mA/cm<sup>2</sup> )**

*Solar Cells - Theory, Materials and Recent Advances*

**Fill factor, FF (%)**
