**3.3 Graphene for fabrication of cathodes in DSSCs**

In DSSCs, cathode carries out three functions. As a catalyst, the cathode facilitates the regeneration of redox couple i.e., the oxidized state of redox couple is reduced by accepting electrons at the surface of the electrode. As a positive electrode of primary cells, the cathode collects electrons from the external circuit and transfers electrons into the cell. As a mirror, unabsorbed light came through the photoanode and the electrolyte, which was partially reflected in the cell to enhance the utilization of the light [5, 31].

The cathode in DSSCs conventionally includes two parts: the substrate and the catalytic layer. The type of substrate mostly used in DSSCs is conductive glass substrates made by coating a layer of TCO on transparent glass. In DSSCs, the TCO substrate plays an important role in transmitting the incident light and leading the electrical current. Both transmittance and conductivity are crucial for the electrode of DSSCs. The conductive glass that is widely used in the fabrication of DSSCs is FTO [5].

For promoting the commercialization of DSSCs, the production cost of DSSCs needs to be reduced. Moreover, the efficiency of DSSCs is expected to maintain at an acceptable level. At present, Pt is still the most appropriate material for the fabrication of cathode in DSSCs. However, this noble metal has limited availability and relatively high cost that hinder the large-scale production of DSSCs. Moreover, Pt cathodes show poor resistance toward corrosion in iodide solution, which may result in the formation of PtI4. The use of Pt in cathodes is considered as one of many reasons that prevent the commercialization of DSSCs. On the other hand, carbon is the material that can be found everywhere on the planet. Carbon-based materials had wide applications in technology including solar technology [5]. Recently, graphene has been explored as a novel material with many outstanding characteristics, which makes these materials become one of the most promising alternatives of Pt in DSSCs. In recent years, graphene and graphene-based materials have been demonstrated to be an adequate substitute for Pt, to cut off the use of

*Solar Cells - Theory, Materials and Recent Advances*

electrolyte, high reactivity for I3

atoms in graphene lattice are hybridized sp2

**3.2 Graphene**

area and decrease of carrier recombination in DSSCs [20, 21].

−

most attention of researchers due to its outstanding properties [5, 24].

of Manchester received the 2010 Nobel Prize in Physics for their pioneering research on graphene. Graphene is a flat monolayer of carbon atoms arranging like the structure of honeycombs with one atom thickness. Due to the special thickness, graphene is considered as a 2D material, as shown in **Figure 2**. Carbon

Graphene is one of the basic carbon allotropes, including graphite, carbon nanotube, and fullerene. Graphene possesses not only all properties of graphite but also other extraordinary characteristics. Graphene has high carrier mobility at room

Carbon nanotubes (CNTs) are formed by a single cylindrically shaped graphene sheet called single-wall carbon nanotubes (SWCNTs) or several graphene sheets arranged concentrically called multi-wall carbon nanotubes (MWCNTs). CNTs have been proposed as the prospective substitutes for the conventional Pt in DSSCs due to their outstanding advantages of large surface area, high electrical conductivity, and chemical stability [18, 19]. Additionally, CNTs could also be used for synthesis of composite materials of anodes in DSSCs, including the ZnO nanowires/ CNTs and TiO2/CNTs, in order to offer a potential platform to enhancement surface

As mentioned, carbonaceous materials are quite attractive for replacement of Pt in DSSCs due to the high electronic conductivity, corrosion resistance toward I2

ity of carbon compared to Pt can be compensated by increasing the active surface area of the electrode by using a porous electrode structure. For example, porous carbon electrodes are easily prepared from graphite powder, which consists of platelike crystals that, on deposition from a liquid dispersion and drying, will preferentially align in the plane of the counter electrodes, resulting in a high conductivity in this plane. Numerous carbonaceous materials were studied for the fabrication of electrodes in DSSCs, using carbon vulcan, carbon black, activated carbon, carbon nanofibers, carbon nanotubes, graphene or the combination of these materials to fabricate the high-performance electrodes of DSSCs, like graphite-activated carbon [22], carbon black-graphite [23]. Among these materials, graphene has attracted the

Andre Konstantin Geim and Konstantin Sergeevich Novoselov of the University

reduction, and low cost. The lower catalytic activ-

with the C-C bond length of 1.42 Å.

**298**

**Figure 2.**

*Structure of graphene.*

the noble metal and maintain the performance of DSSCs [32, 33]. Many research studies have been conducted to synthesized Pt/graphene or Pt/rGO with different methods. Yen et al. mixed GO and H2PtCl6 precursor salt in an ethylene glycol/ H2O mixture and heated the mixture at 120°C to form the Pt/rGO nanocomposite [34]. With the similar method, Khoa et al. mixed GO and H2PtCl6 with different amount of GO and then the mixture was heated at 350°C to obtain the Pt/thermally reduced graphene oxide composite [35]. Instead of thermal reduction method, Wan et al. used NaBH4 as reducing agent for synthesis of Pt/rGO composite for cathode of DSSCs [36]. Recently, Yu et al. synthesized the Pt nanoparticles-loaded holey reduced graphene oxide framework materials with the addition of aqueous hydrofluoric acid, resulting in the high electrocatalytic activity and efficient electron/ ion transport material for cathode fabrication of DSSCs [33]. After that, Suriani et al. combined graphene, SWCNT, and Pt to create the rGO/SWCNT hybrid film with low Pt loading for fabrication of cathode in DSSCs [37]. Beside these studies, there were various advanced methods for synthesis of Pt/graphene composite and other graphene-based materials: chemical vapor deposition [38, 39], hydrothermal, coating with different layers [40], etc. All of the fabricated DSSCs from mentioned studies exhibited high conversion efficiency that can be compared with DSSCs fabricated from the pure Pt cathodes.

Our group conducted the experiments to investigate the performance of Pt/rGO cathodes in DSSCs. Accordingly, rGO was synthesized from GO, which was synthesized from Gi using the improved Hummers' method. The Pt/rGO composite pastes were fabricated from rGO and H2PtCl6 with different weight percents of rGO: 0, 10, 20, 30, 40, 50, and 100 wt%, marked as PG0, PG10, PG20, PG30, PG40, PG50, and PG100, respectively. These composite pastes were used for fabrication of cathodes in DSSCs using the screen-printing technology. DSSCs were assembled with fabricated cathodes, Dyesol TiO2 anodes, N719 dye, and Dyesol High Stability Electrolyte (HSE). The efficiency of fabricated DSSCs was measured using the current density-voltage (J-V) curves. The J-V curves and the photovoltaic parameters of fabricated DSSCs are presented in **Figure 3** and **Table 1**.

#### **Figure 3.**

*J-V curves of DSSCs fabricated from Pt/rGO cathodes with different weight percents of rGO. Reproduced with permission Ref. [32]. Copyright 2020, Elsevier.*

**301**

sp2

*Graphene-Based Material for Fabrication of Electrodes in Dye-Sensitized Solar Cells*

**DSSCs VOC (V) JSC (mA cm−2) ff** η **(%)** PG0 0.67 15.33 0.67 6.89 PG10 0.73 13.49 0.68 6.64 PG20 0.73 12.86 0.68 6.42 PG30 0.66 12.67 0.70 5.94 PG40 0.69 11.82 0.66 5.44 PG50 0.70 10.84 0.68 5.18 PG100 0.66 10.67 0.68 4.76

The J-V curve results showed that the conversion efficiency values of fabricated DSSCs was reduced when the amount of H2PtCl6 was reduced. In the structure of cathode, Pt nanoparticles played the catalytic and charge transferring roles. Since the intrinsic conductivity of rGO was relatively lower than that of Pt, the catalytic activities of Pt/rGO cathodes were lowered, leading to the decrease in efficiency [32]. However, rGO with the high surface area could play the role of supporting material to improve the electrochemical activity of Pt/rGO cathode. Therefore, Pt nanoparticles could be uniformly decorated on the surface of rGO lattice [41]. The high surface area of the Pt/rGO helps to maintain the high efficiencies of DSSCs in spite of the low conductivity of rGO, compared with Pt [42]. These results exhibited that the PG10 and PG20 DSSCs had better performance compared to other composite DSSCs, with efficiency values higher than 93% compared to PG0. The J-V results showed that PG20 was the appropriate material for fabrication of cathodes in DSSCs with high amount of Pt replacement and

*Photovoltaic parameters of DSSCs fabricated from Pt/rGO cathodes with different weight percents of rGO.* 

The Pt/rGO material in our study was investigated using different characterization technique like Fourier-transform infrared spectroscopy (FTIR), Raman spectroscopy, X-ray diffraction (XRD), and transmission electron microscopy (TEM). As shown in **Figure 4a**, the FTIR spectrum of GO revealed the vibrations peaks of the epoxide (C–O–C), alkene (C=C), and carboxyl functional groups (–COOH), at about 1050, 1628, and 1738 cm−1, respectively. The broad peak at 3386 cm−1 originated from vibrations of hydroxyl (–OH) groups. The FTIR result demonstrated that during the oxidation process of Gi by improved Hummers' method, the oxygen-containing functional groups were introduced to the carbon framework of Gi to obtain GO [43]. For rGO, characteristic peaks could not be obviously determined, proving that the functional groups in the structure of GO were reduced. Similarly, the characteristic peaks of functional groups in the spectrum of PG20 were decreased, compared with GO, due to the functional groups remaining in the

As shown in **Figure 4b**, the Raman spectra of Gi, GO, rGO, and PG20 showed two characteristic peaks at around 1350 and 1580 cm−1, corresponding to D and

 carbon network. The ratios of D-band to G-band (ID/IG) represent the levels of defect in graphene-based materials [45]. The ID/IG value of GO is higher than that of Gi and lower than rGO or PG20, showing that the oxidation and reduction process increased the levels of defect in the structure of graphene sheets. The ID/ IG value of PG20 was measured to be 1.12, where that of rGO accounts for 1.07.

band attributed to the structural defects and partial distortion in the structure of

carbon network, the D

*DOI: http://dx.doi.org/10.5772/intechopen.93637*

*Reproduced with permission Ref. [32]. Copyright 2020, Elsevier.*

high conversion efficiency.

**Table 1.**

carbon lattice of PG20 after the reduction process [44].

G-band peak. While G band relates to the vibration of sp2

*Graphene-Based Material for Fabrication of Electrodes in Dye-Sensitized Solar Cells DOI: http://dx.doi.org/10.5772/intechopen.93637*


**Table 1.**

*Solar Cells - Theory, Materials and Recent Advances*

fabricated from the pure Pt cathodes.

fabricated DSSCs are presented in **Figure 3** and **Table 1**.

the noble metal and maintain the performance of DSSCs [32, 33]. Many research studies have been conducted to synthesized Pt/graphene or Pt/rGO with different methods. Yen et al. mixed GO and H2PtCl6 precursor salt in an ethylene glycol/ H2O mixture and heated the mixture at 120°C to form the Pt/rGO nanocomposite [34]. With the similar method, Khoa et al. mixed GO and H2PtCl6 with different amount of GO and then the mixture was heated at 350°C to obtain the Pt/thermally reduced graphene oxide composite [35]. Instead of thermal reduction method, Wan et al. used NaBH4 as reducing agent for synthesis of Pt/rGO composite for cathode of DSSCs [36]. Recently, Yu et al. synthesized the Pt nanoparticles-loaded holey reduced graphene oxide framework materials with the addition of aqueous hydrofluoric acid, resulting in the high electrocatalytic activity and efficient electron/ ion transport material for cathode fabrication of DSSCs [33]. After that, Suriani et al. combined graphene, SWCNT, and Pt to create the rGO/SWCNT hybrid film with low Pt loading for fabrication of cathode in DSSCs [37]. Beside these studies, there were various advanced methods for synthesis of Pt/graphene composite and other graphene-based materials: chemical vapor deposition [38, 39], hydrothermal, coating with different layers [40], etc. All of the fabricated DSSCs from mentioned studies exhibited high conversion efficiency that can be compared with DSSCs

Our group conducted the experiments to investigate the performance of Pt/rGO cathodes in DSSCs. Accordingly, rGO was synthesized from GO, which was synthesized from Gi using the improved Hummers' method. The Pt/rGO composite pastes were fabricated from rGO and H2PtCl6 with different weight percents of rGO: 0, 10, 20, 30, 40, 50, and 100 wt%, marked as PG0, PG10, PG20, PG30, PG40, PG50, and PG100, respectively. These composite pastes were used for fabrication of cathodes in DSSCs using the screen-printing technology. DSSCs were assembled with fabricated cathodes, Dyesol TiO2 anodes, N719 dye, and Dyesol High Stability Electrolyte (HSE). The efficiency of fabricated DSSCs was measured using the current density-voltage (J-V) curves. The J-V curves and the photovoltaic parameters of

*J-V curves of DSSCs fabricated from Pt/rGO cathodes with different weight percents of rGO. Reproduced with* 

**300**

**Figure 3.**

*permission Ref. [32]. Copyright 2020, Elsevier.*

*Photovoltaic parameters of DSSCs fabricated from Pt/rGO cathodes with different weight percents of rGO. Reproduced with permission Ref. [32]. Copyright 2020, Elsevier.*

The J-V curve results showed that the conversion efficiency values of fabricated DSSCs was reduced when the amount of H2PtCl6 was reduced. In the structure of cathode, Pt nanoparticles played the catalytic and charge transferring roles. Since the intrinsic conductivity of rGO was relatively lower than that of Pt, the catalytic activities of Pt/rGO cathodes were lowered, leading to the decrease in efficiency [32]. However, rGO with the high surface area could play the role of supporting material to improve the electrochemical activity of Pt/rGO cathode. Therefore, Pt nanoparticles could be uniformly decorated on the surface of rGO lattice [41]. The high surface area of the Pt/rGO helps to maintain the high efficiencies of DSSCs in spite of the low conductivity of rGO, compared with Pt [42]. These results exhibited that the PG10 and PG20 DSSCs had better performance compared to other composite DSSCs, with efficiency values higher than 93% compared to PG0. The J-V results showed that PG20 was the appropriate material for fabrication of cathodes in DSSCs with high amount of Pt replacement and high conversion efficiency.

The Pt/rGO material in our study was investigated using different characterization technique like Fourier-transform infrared spectroscopy (FTIR), Raman spectroscopy, X-ray diffraction (XRD), and transmission electron microscopy (TEM). As shown in **Figure 4a**, the FTIR spectrum of GO revealed the vibrations peaks of the epoxide (C–O–C), alkene (C=C), and carboxyl functional groups (–COOH), at about 1050, 1628, and 1738 cm−1, respectively. The broad peak at 3386 cm−1 originated from vibrations of hydroxyl (–OH) groups. The FTIR result demonstrated that during the oxidation process of Gi by improved Hummers' method, the oxygen-containing functional groups were introduced to the carbon framework of Gi to obtain GO [43]. For rGO, characteristic peaks could not be obviously determined, proving that the functional groups in the structure of GO were reduced. Similarly, the characteristic peaks of functional groups in the spectrum of PG20 were decreased, compared with GO, due to the functional groups remaining in the carbon lattice of PG20 after the reduction process [44].

As shown in **Figure 4b**, the Raman spectra of Gi, GO, rGO, and PG20 showed two characteristic peaks at around 1350 and 1580 cm−1, corresponding to D and G-band peak. While G band relates to the vibration of sp2 carbon network, the D band attributed to the structural defects and partial distortion in the structure of sp2 carbon network. The ratios of D-band to G-band (ID/IG) represent the levels of defect in graphene-based materials [45]. The ID/IG value of GO is higher than that of Gi and lower than rGO or PG20, showing that the oxidation and reduction process increased the levels of defect in the structure of graphene sheets. The ID/ IG value of PG20 was measured to be 1.12, where that of rGO accounts for 1.07.

#### **Figure 4.**

*Characterization of PG20 and precursor materials: (a) FTIR spectra, (b) Raman spectra, (c) XRD patterns, and (d) TEM image. Reproduced with permission Ref. [32]. Copyright 2020, Elsevier.*

This means that the level of defect in the structure of PG20 is higher than that of rGO, this was attributed to the incorporation of Pt nanoparticles into the rGO sheets [46].

The XRD patterns of Gi, GO, rGO, and PG20 are presented in **Figure 4c**. In the pattern of GO, there is a characteristic peak (002) at 2θ = 10.42°, corresponding to the interlayer distance of 0.85 nm. For rGO, the (002) peak appears at 2θ = 26.87°; the interlayer distance between the rGO sheets was determined to be 0.33 nm. The diffraction peak of GO pattern is not observed in the pattern of rGO, indicating the removal of functional groups of GO during the reduction process. In the pattern of PG20, the diffraction peaks are determined at 2θ = 39.97, 46.36, and 67.69 o , corresponding to the (111), (200), and (220) crystalline planes of Pt nanoparticles, respectively. Additionally, there is a diffraction peak at 2θ = 25.32° in the pattern of PG20, which was similar to that of rGO. The XRD result proved the formation of Pt particles from H2PtCl6 and the reduction of GO to create the rGO sheets, proving the successful synthesis of Pt/rGO [46].

The morphology of PG20 composite was investigated using the TEM images. As shown in **Figure 4d**, rGO is observed to be the semi-transparent thin layer, indicating that the 2D structure of rGO was maintained after the annealing process. The Pt nanoparticles are observed as the black spheres which were decorated on the rGO sheets. From the TEM images, the sizes of the Pt nanoparticles were estimated to be in range of 10–30 nm. Besides, the Pt nanoparticles were eventually decorated on the

**303**

*Graphene-Based Material for Fabrication of Electrodes in Dye-Sensitized Solar Cells*

rGO sheets. However, the Pt nanoparticles tended to agglomerate to form Pt clusters. The TEM images showed the high degree of agglomeration of Pt nanoparticles and the role of rGO as an effective supporting material in order to keep the distribution of Pt particles. The role as a supporting material of rGO helped to maintain the efficiency of PG20 DSSCs by increasing the surface areas of the materials.

The investigation of Pt/rGO cathodes in DSSCs proved that rGO was an excellent replacement for Pt in cathodes of DSSCs. By using the Pt/rGO composite for fabrication cathodes of DSSCs, the amount of Pt in DSSCs could be reduced and the

Three key factors that affect the DSSC efficiency have been extensively studied: photo-electron generation, charge carrier transfer, and surface reaction. The unique and outstanding properties of graphene are ideal for addressing these factors. Graphene offers a 2D conductive support path for electron transfer, which can improve the electron transfer in photoanode materials and reduce the electronhole recombination rate. For an example of TiO2, without carbonaceous material supporter, electrons that are injected into TiO2 nanoparticles may transfer around and need a much longer transfer distance. Graphene provides a faster electron transfer path and significantly reduces the electron-hole recombination rate in the TiO2 material layer. Additionally, the large surface area creates more occasions for reactive group decoration and enhances the chemical reaction and reduction

ZnO materials have been made into composite/hybrid materials along with other metallic as well as graphitic structures to enhance their mechanical and electrochemical properties [48]. ZnO has superior optical and electrical proper-

Our group conducted the experiments to investigate the performance of ZnO/ rGO anodes in DSSCs. Accordingly, ZnO/rGO composite materials were synthesized from Zn(O2CCH2)3 and GO. The ZnO/rGO anodes were fabricated from ZnO/ rGO composite materials with different weight percent of rGO: 0, 0.1, 0.5, 1, and 5 wt% corresponding to ZnO, ZnO/rGO1, ZnO/rGO2, ZnO/rGO3, and ZnO/rGO4, respectively. DSSCs were assembled with fabricated anodes, cathodes from Dyesol Platinum Paste, N719 dye, and HSE electrolyte. The efficiency values of fabricated DSSCs were measured using the J-V curves. The results showed that the DSSCs fabricated from ZnO/rGO anodes demonstrated the high efficiencies, compared with DSSCs using the ZnO anode. The J-V curves and the photovoltaic parameters

As shown in **Figure 5**, the efficiencies of ZnO/rGO1, ZnO/rGO2, and ZnO/ rGO3 DSSCs were determined to be 1.25, 1.47, and 1.55%, respectively, which were higher than that of ZnO DSSC (1.08%). During the transfer process of the excited electron from N719 to FTO, the electron-hole recombination could significantly deteriorate the performance of DSSCs, leading to the decrease in the conversion

temperature, wide band gap energy of 3.3 eV, and a high excitation (electronhole) binding energy of 60 meV. However, to overcome the drawback of poor catalytic activity in ZnO due to its photoelectron recombination, many studies had incorporated graphene into the ZnO matrix to its efficiency and reduce the electron recombination process [49, 50]. There are two main method for fabrication of ZnO/rGO composite: the in-situ method, which used the Zn(II) precursor salts and mixed with GO or rGO [51, 52], and the ex-situ method, which used the ZnO nanomaterials (nanoparticles, nano wires, nanorods, etc.) and mixed with

V−1 s−1 at room

ties that include high electron mobility in the order of 1500 cm<sup>2</sup>

of fabricated DSSCs are presented in **Figure 5** and **Table 2**.

*DOI: http://dx.doi.org/10.5772/intechopen.93637*

efficiency could be maintained.

processes [11, 47].

rGO [53, 54].

**3.4 Graphene for the fabrication of anodes in DSSCs**

*Graphene-Based Material for Fabrication of Electrodes in Dye-Sensitized Solar Cells DOI: http://dx.doi.org/10.5772/intechopen.93637*

rGO sheets. However, the Pt nanoparticles tended to agglomerate to form Pt clusters. The TEM images showed the high degree of agglomeration of Pt nanoparticles and the role of rGO as an effective supporting material in order to keep the distribution of Pt particles. The role as a supporting material of rGO helped to maintain the efficiency of PG20 DSSCs by increasing the surface areas of the materials.

The investigation of Pt/rGO cathodes in DSSCs proved that rGO was an excellent replacement for Pt in cathodes of DSSCs. By using the Pt/rGO composite for fabrication cathodes of DSSCs, the amount of Pt in DSSCs could be reduced and the efficiency could be maintained.

### **3.4 Graphene for the fabrication of anodes in DSSCs**

Three key factors that affect the DSSC efficiency have been extensively studied: photo-electron generation, charge carrier transfer, and surface reaction. The unique and outstanding properties of graphene are ideal for addressing these factors. Graphene offers a 2D conductive support path for electron transfer, which can improve the electron transfer in photoanode materials and reduce the electronhole recombination rate. For an example of TiO2, without carbonaceous material supporter, electrons that are injected into TiO2 nanoparticles may transfer around and need a much longer transfer distance. Graphene provides a faster electron transfer path and significantly reduces the electron-hole recombination rate in the TiO2 material layer. Additionally, the large surface area creates more occasions for reactive group decoration and enhances the chemical reaction and reduction processes [11, 47].

ZnO materials have been made into composite/hybrid materials along with other metallic as well as graphitic structures to enhance their mechanical and electrochemical properties [48]. ZnO has superior optical and electrical properties that include high electron mobility in the order of 1500 cm<sup>2</sup> V−1 s−1 at room temperature, wide band gap energy of 3.3 eV, and a high excitation (electronhole) binding energy of 60 meV. However, to overcome the drawback of poor catalytic activity in ZnO due to its photoelectron recombination, many studies had incorporated graphene into the ZnO matrix to its efficiency and reduce the electron recombination process [49, 50]. There are two main method for fabrication of ZnO/rGO composite: the in-situ method, which used the Zn(II) precursor salts and mixed with GO or rGO [51, 52], and the ex-situ method, which used the ZnO nanomaterials (nanoparticles, nano wires, nanorods, etc.) and mixed with rGO [53, 54].

Our group conducted the experiments to investigate the performance of ZnO/ rGO anodes in DSSCs. Accordingly, ZnO/rGO composite materials were synthesized from Zn(O2CCH2)3 and GO. The ZnO/rGO anodes were fabricated from ZnO/ rGO composite materials with different weight percent of rGO: 0, 0.1, 0.5, 1, and 5 wt% corresponding to ZnO, ZnO/rGO1, ZnO/rGO2, ZnO/rGO3, and ZnO/rGO4, respectively. DSSCs were assembled with fabricated anodes, cathodes from Dyesol Platinum Paste, N719 dye, and HSE electrolyte. The efficiency values of fabricated DSSCs were measured using the J-V curves. The results showed that the DSSCs fabricated from ZnO/rGO anodes demonstrated the high efficiencies, compared with DSSCs using the ZnO anode. The J-V curves and the photovoltaic parameters of fabricated DSSCs are presented in **Figure 5** and **Table 2**.

As shown in **Figure 5**, the efficiencies of ZnO/rGO1, ZnO/rGO2, and ZnO/ rGO3 DSSCs were determined to be 1.25, 1.47, and 1.55%, respectively, which were higher than that of ZnO DSSC (1.08%). During the transfer process of the excited electron from N719 to FTO, the electron-hole recombination could significantly deteriorate the performance of DSSCs, leading to the decrease in the conversion

*Solar Cells - Theory, Materials and Recent Advances*

This means that the level of defect in the structure of PG20 is higher than that of rGO, this was attributed to the incorporation of Pt nanoparticles into the rGO

*and (d) TEM image. Reproduced with permission Ref. [32]. Copyright 2020, Elsevier.*

*Characterization of PG20 and precursor materials: (a) FTIR spectra, (b) Raman spectra, (c) XRD patterns,* 

PG20, the diffraction peaks are determined at 2θ = 39.97, 46.36, and 67.69 o

the successful synthesis of Pt/rGO [46].

responding to the (111), (200), and (220) crystalline planes of Pt nanoparticles, respectively. Additionally, there is a diffraction peak at 2θ = 25.32° in the pattern of PG20, which was similar to that of rGO. The XRD result proved the formation of Pt particles from H2PtCl6 and the reduction of GO to create the rGO sheets, proving

The morphology of PG20 composite was investigated using the TEM images. As shown in **Figure 4d**, rGO is observed to be the semi-transparent thin layer, indicating that the 2D structure of rGO was maintained after the annealing process. The Pt nanoparticles are observed as the black spheres which were decorated on the rGO sheets. From the TEM images, the sizes of the Pt nanoparticles were estimated to be in range of 10–30 nm. Besides, the Pt nanoparticles were eventually decorated on the

The XRD patterns of Gi, GO, rGO, and PG20 are presented in **Figure 4c**. In the pattern of GO, there is a characteristic peak (002) at 2θ = 10.42°, corresponding to the interlayer distance of 0.85 nm. For rGO, the (002) peak appears at 2θ = 26.87°; the interlayer distance between the rGO sheets was determined to be 0.33 nm. The diffraction peak of GO pattern is not observed in the pattern of rGO, indicating the removal of functional groups of GO during the reduction process. In the pattern of

, cor-

**302**

sheets [46].

**Figure 4.**

#### **Figure 5.**

*J-V curves of DSSCs fabricated from ZnO/rGO anodes with different weight percents of rGO. Reproduced with permission Ref. [50]. Copyright 2020, Chemical Engineering Transaction.*


#### **Table 2.**

*Photovoltaic parameters of fabricated from ZnO/rGO anodes with different weight percents of rGO. Reproduced with permission Ref. [50] Copyright 2020, Chemical Engineering Transaction.*

efficiency values. The addition of rGO, a material with high conductivity and high electron mobility, could improve the transfer pathway of the excited electron. The introduction of rGO in the structure of the anode material could help to prevent the recombination reactions and enhanced the efficiency of DSSC. ZnO/rGO4 DSSC demonstrated a low efficiency, because transmittance of anode was drastically decreased, due to the high amount of rGO in ZnO/rGO4 composite. Besides, the excessive amount of rGO could become the recombination center that increased the recombination reactions [55, 56]. Therefore, the appropriate rGO weight percent in the composite was 1%, corresponding to the ZnO/rGO3 DSSC.

The ZnO/rGO material in our study was characterized using various methods including: FTIR spectroscopy, Raman spectroscopy, XRD patterns, and TEM images. **Figure 6a** demonstrates the FTIR spectra of ZnO/rGO3 and precursor materials. As mentioned above, the FTIR results showed that Gi was oxidized to obtain GO. The characteristic peaks of GO were reduced or disappeared on the spectra of GO and ZnO/rGO, indicating that the functional group of GO was reduced to create rGO and ZnO/rGO. The FTIR spectrum of ZnO/rGO3 revealed two typical peaks at 507.14 and 433.23 cm−1, corresponding to the Zn-O vibration of hexagonal wurtzite structure of ZnO [49].

As shown in **Figure 6b**, Raman spectra of ZnO/rGO3 and precursor materials had the D and G-band peak. In the Raman spectra of ZnO and ZnO/rGO3, the

**305**

**Figure 6.**

*Transaction.*

*Graphene-Based Material for Fabrication of Electrodes in Dye-Sensitized Solar Cells*

vibration peak at 436 cm−1 was assigned to the E2 non-polar phonon modes, corresponding to the hexagonal crystal structure of ZnO nanoparticles. The D-band and G-band peaks of ZnO/rGO3 spectrum were unclear, due to the low amount of rGO

*Characterization of ZnO/rGO3 and precursor materials: (a) FTIR spectra, (b) Raman spectra, (c) XRD patterns, and (d) TEM image. Reproduced with permission Ref. [50]. Copyright 2020, Chemical Engineering* 

The XRD patterns of ZnO/rGO3 and precursor materials are demonstrated in **Figure 6c**. The characteristic diffraction peaks of ZnO/rGO3 were observed at 2θ = 31.8, 34.5, 36.3, 47.5, 56.6, 62.8, 67.9, and 69.0°, corresponding to the (110), (002), (101), (102), (110), (103), (112), and (201) crystalline plane of wurtzite structures; similar with the JCPDS File No.36-1451 of ZnO [55, 58]. The diffraction peaks of GO and rGO were not observed in the pattern of ZnO/rGO3, because the

As shown in **Figure 6d**, the TEM images of the ZnO/rGO3 demonstrated the decoration of ZnO nanoparticles on rGO sheets. As mentioned above, rGO is observed to be the semi-transparent thin layer, while the ZnO nanoparticles are the black spheres. The average size of ZnO nanoparticles was estimated to be about 10 nm. As can be seen, the ZnO nanoparticles were evenly distributed on rGO sheets. These results exhibited the good incorporation of rGO sheets and ZnO

The investigation of ZnO/rGO anodes in DSSCs proved that the incorporation of rGO into the structure of ZnO could enhance the efficiencies of DSSCs. Therefore, the ZnO/rGO composite is the appropriate material for the fabrication of anodes in DSSCs.

weight percent of rGO in ZnO/rGO3 composite was low (1%).

*DOI: http://dx.doi.org/10.5772/intechopen.93637*

in the structure of ZnO/rGO3 [57].

nanoparticles in the structure of ZnO/rGO3.

*Graphene-Based Material for Fabrication of Electrodes in Dye-Sensitized Solar Cells DOI: http://dx.doi.org/10.5772/intechopen.93637*

#### **Figure 6.**

*Solar Cells - Theory, Materials and Recent Advances*

efficiency values. The addition of rGO, a material with high conductivity and high electron mobility, could improve the transfer pathway of the excited electron. The introduction of rGO in the structure of the anode material could help to prevent the recombination reactions and enhanced the efficiency of DSSC. ZnO/rGO4 DSSC demonstrated a low efficiency, because transmittance of anode was drastically decreased, due to the high amount of rGO in ZnO/rGO4 composite. Besides, the excessive amount of rGO could become the recombination center that increased the recombination reactions [55, 56]. Therefore, the appropriate rGO weight percent in

*Photovoltaic parameters of fabricated from ZnO/rGO anodes with different weight percents of rGO. Reproduced with permission Ref. [50] Copyright 2020, Chemical Engineering Transaction.*

*J-V curves of DSSCs fabricated from ZnO/rGO anodes with different weight percents of rGO. Reproduced with* 

**DSSCs VOC (V) JSC (mA cm−2) ff** η **(%)** ZnO 0.48 3.43 0.49 1.08 ZnO/rGO1 0.64 2.87 0.51 1.25 ZnO/rGO2 0.63 2.50 0.70 1.47 ZnO/rGO3 0.64 3.02 0.60 1.55 ZnO/rGO4 0.30 1.27 0.28 0.14

*permission Ref. [50]. Copyright 2020, Chemical Engineering Transaction.*

The ZnO/rGO material in our study was characterized using various methods including: FTIR spectroscopy, Raman spectroscopy, XRD patterns, and TEM images. **Figure 6a** demonstrates the FTIR spectra of ZnO/rGO3 and precursor materials. As mentioned above, the FTIR results showed that Gi was oxidized to obtain GO. The characteristic peaks of GO were reduced or disappeared on the spectra of GO and ZnO/rGO, indicating that the functional group of GO was reduced to create rGO and ZnO/rGO. The FTIR spectrum of ZnO/rGO3 revealed two typical peaks at 507.14 and 433.23 cm−1, corresponding to the Zn-O vibration of

As shown in **Figure 6b**, Raman spectra of ZnO/rGO3 and precursor materials had the D and G-band peak. In the Raman spectra of ZnO and ZnO/rGO3, the

the composite was 1%, corresponding to the ZnO/rGO3 DSSC.

hexagonal wurtzite structure of ZnO [49].

**304**

**Figure 5.**

**Table 2.**

*Characterization of ZnO/rGO3 and precursor materials: (a) FTIR spectra, (b) Raman spectra, (c) XRD patterns, and (d) TEM image. Reproduced with permission Ref. [50]. Copyright 2020, Chemical Engineering Transaction.*

vibration peak at 436 cm−1 was assigned to the E2 non-polar phonon modes, corresponding to the hexagonal crystal structure of ZnO nanoparticles. The D-band and G-band peaks of ZnO/rGO3 spectrum were unclear, due to the low amount of rGO in the structure of ZnO/rGO3 [57].

The XRD patterns of ZnO/rGO3 and precursor materials are demonstrated in **Figure 6c**. The characteristic diffraction peaks of ZnO/rGO3 were observed at 2θ = 31.8, 34.5, 36.3, 47.5, 56.6, 62.8, 67.9, and 69.0°, corresponding to the (110), (002), (101), (102), (110), (103), (112), and (201) crystalline plane of wurtzite structures; similar with the JCPDS File No.36-1451 of ZnO [55, 58]. The diffraction peaks of GO and rGO were not observed in the pattern of ZnO/rGO3, because the weight percent of rGO in ZnO/rGO3 composite was low (1%).

As shown in **Figure 6d**, the TEM images of the ZnO/rGO3 demonstrated the decoration of ZnO nanoparticles on rGO sheets. As mentioned above, rGO is observed to be the semi-transparent thin layer, while the ZnO nanoparticles are the black spheres. The average size of ZnO nanoparticles was estimated to be about 10 nm. As can be seen, the ZnO nanoparticles were evenly distributed on rGO sheets. These results exhibited the good incorporation of rGO sheets and ZnO nanoparticles in the structure of ZnO/rGO3.

The investigation of ZnO/rGO anodes in DSSCs proved that the incorporation of rGO into the structure of ZnO could enhance the efficiencies of DSSCs. Therefore, the ZnO/rGO composite is the appropriate material for the fabrication of anodes in DSSCs.
