**2. Graphene application in solar cells**

#### **2.1 Graphene application in DSSCs**

A dye-sensitized solar cell (DSSC or DYSC) is a low-cost solar cell, invented by Swiss scientists Michael Grätzel and Dr. Brian O'Regan in 1991, and is often called as Gräetzel cell (G Cell) [3]. DSSCs have attracted significant attention in search of substitute for silicon and thin-film solar cells due to their environmental friendliness, low fabrication costs and simple preparation process and remarkable conversion of solar energy into electricity [28]. DSSC is a photoelectrochemical system consisting of (a) transparent conducting working electrode (photoanode), (b) a dye sensitizer, (c) a counter electrode (cathode) and (d) an electrolyte [28, 29]. Its principle of operation is likened to natural photosynthesis process in plants. A monolayer of dye sensitizer absorbs incident light (like chlorophyll) giving rise to positive and negative carriers in the cell. On illumination, the electrons of the dye are excited from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). Then they are transported to the semiconductor's (TiO2) conduction band (CB) and diffuse through the semiconductor, being collected at the transparent conducting oxide (TCO). An electrolyte solution which typically comprises of an iodide/triiodide (I<sup>−</sup>/I3 <sup>−</sup>) redox couple provides electrons to regenerate the oxidized dye. Eventually, the electrons transfer to the counter electrode (Pt) through the external circuit and reduces triiodide I3 <sup>−</sup> ions back to the iodide I<sup>−</sup> to complete the circle of conversion of photons into electricity (**Figure 3**) [12, 30, 31].

The photoanode is a key constituent, as it strongly influences the photovoltage (V), the fill factor (FF) and the incident photon-to-current conversion efficiency (IPCE) [12]. It serves as the main energy conversion center, converting photons into electrical energy, thus playing a critical role in DSSC [25]. A good photoelectrode is

**27**

concentration.

*Graphene/Metal Oxide Nanocomposite Usage as Photoanode in Dye-Sensitized and Perovskite…*

assumed to provide good usage of light, good electron injection and good electron collection. Various semiconducting (wide band gap) materials such as TiO2, ZnO, SnO2, CdS and Nb2O5 have been successfully used as photoanode in DSSCs [28, 32]. Among all, TiO2 is most abundantly used, as it is nontoxic, cheap, chemically inert and easy to synthesize [33]. Moreover, TiO2 have a wide band gap 3.2 eV; predominant chemical, electrical and optical properties; low recombination rate for the hole-electron pair; and great absorption property [12, 28]. The performance of DSSC is significantly influenced by properties like particle size, porosity and

Platinum (Pt) is the most widely used counter electrode (CE) in DSSC. Though Pt is very active to catalyze the reduction of triiodide, there are issues of stability in corrosive electrolytes and its high cost. Several efforts have been made to substitute it with less-expensive materials such as carbon materials and conducting polymers [34]. Carbon-based materials, such as graphite, carbon nanotubes, carbon black, nanocarbon and more recently graphene have been discovered severely as an

Due to the exceptional mechanical, electrical and optical properties, graphene or highly reduced graphene are more appropriate as transparent electrodes, part of electrolyte, a light-harvesting dye and a counter electrode [17]. Additionally, work functions of graphene and rGO are calculated to be 4.4–4.5 eV, which are close to fluorine-doped tin oxide (FTO 4.4 eV) [25]. Thus, the tunable band gap of GO and its photon absorption ability in the visible and IR region permit to be a potential sensitizing material. The first application of graphene in DSSC as transparent conducting substrate was reported by Wang et al. [35] in 2008. They fabricated GO films by thermal reduction in Ar and/or H2 flux from exfoliated graphite oxide. These films showed a transparency of more than 70% and a higher conductivity of 550 S/cm [35]. Subsequently, other roles of GO have been widely

Among the extensively used semiconductors, TiO2 possess the longest diffusion length and lowest charge effective mass, but electron-hole recombination remains a significant issue [30]. Hence in order to reduce electron recombination and improve the stability of DSSC, graphene and graphene-based materials as photoanode have

Kazmi et al. [31] used graphene-TiO2 nanocomposites as photoanode for DSSCs.

They synthesized Gr-TiO2 with different graphene concentrations. Graphene nanoparticles were synthesized by modified Hummer's method, whereas TiO2 nanoparticles were purchased from Sigma Aldrich. The band gap was found to be reduced from 3.16 eV to 2.2 eV with the increase in GO concentration. Furthermore, conductivity increases with graphene concentration which supports well with the reduction in band gap. PCE of DSSC was enhanced with the addition of graphene, and optimum results were found for 3.0 wt% of graphene

The idea of using graphene into the photoanode of DSSC arises from the pioneering work of carbon nanotubes mixed with oxide nanostructures by Jang et al. [36]. This led to open an interesting field for researchers, quickly extended to graphene and its derivatives. The fascinating properties of graphene such as high transparency and Young's modulus make it a good candidate for transparent conducting substrates. The semimetallic properties and ultra-fast electron mobility allow graphene to serve as a charge carrier and as a transporter inside the semicon-

*DOI: http://dx.doi.org/10.5772/intechopen.88971*

alternative electrocatalyst for DSSC [17, 31].

*2.1.1 G/metal nanocomposite as photoanode in DSSCs*

received great interest since the past few years.

ductor oxide layer in photoanode.

thickness of TiO2 film.

explored in DSSC.

**Figure 3.** *Scheme diagram of dye-sensitized solar cells [12].*

*Graphene/Metal Oxide Nanocomposite Usage as Photoanode in Dye-Sensitized and Perovskite… DOI: http://dx.doi.org/10.5772/intechopen.88971*

assumed to provide good usage of light, good electron injection and good electron collection. Various semiconducting (wide band gap) materials such as TiO2, ZnO, SnO2, CdS and Nb2O5 have been successfully used as photoanode in DSSCs [28, 32]. Among all, TiO2 is most abundantly used, as it is nontoxic, cheap, chemically inert and easy to synthesize [33]. Moreover, TiO2 have a wide band gap 3.2 eV; predominant chemical, electrical and optical properties; low recombination rate for the hole-electron pair; and great absorption property [12, 28]. The performance of DSSC is significantly influenced by properties like particle size, porosity and thickness of TiO2 film.

Platinum (Pt) is the most widely used counter electrode (CE) in DSSC. Though Pt is very active to catalyze the reduction of triiodide, there are issues of stability in corrosive electrolytes and its high cost. Several efforts have been made to substitute it with less-expensive materials such as carbon materials and conducting polymers [34]. Carbon-based materials, such as graphite, carbon nanotubes, carbon black, nanocarbon and more recently graphene have been discovered severely as an alternative electrocatalyst for DSSC [17, 31].

Due to the exceptional mechanical, electrical and optical properties, graphene or highly reduced graphene are more appropriate as transparent electrodes, part of electrolyte, a light-harvesting dye and a counter electrode [17]. Additionally, work functions of graphene and rGO are calculated to be 4.4–4.5 eV, which are close to fluorine-doped tin oxide (FTO 4.4 eV) [25]. Thus, the tunable band gap of GO and its photon absorption ability in the visible and IR region permit to be a potential sensitizing material. The first application of graphene in DSSC as transparent conducting substrate was reported by Wang et al. [35] in 2008. They fabricated GO films by thermal reduction in Ar and/or H2 flux from exfoliated graphite oxide. These films showed a transparency of more than 70% and a higher conductivity of 550 S/cm [35]. Subsequently, other roles of GO have been widely explored in DSSC.

#### *2.1.1 G/metal nanocomposite as photoanode in DSSCs*

Among the extensively used semiconductors, TiO2 possess the longest diffusion length and lowest charge effective mass, but electron-hole recombination remains a significant issue [30]. Hence in order to reduce electron recombination and improve the stability of DSSC, graphene and graphene-based materials as photoanode have received great interest since the past few years.

The idea of using graphene into the photoanode of DSSC arises from the pioneering work of carbon nanotubes mixed with oxide nanostructures by Jang et al. [36]. This led to open an interesting field for researchers, quickly extended to graphene and its derivatives. The fascinating properties of graphene such as high transparency and Young's modulus make it a good candidate for transparent conducting substrates. The semimetallic properties and ultra-fast electron mobility allow graphene to serve as a charge carrier and as a transporter inside the semiconductor oxide layer in photoanode.

Kazmi et al. [31] used graphene-TiO2 nanocomposites as photoanode for DSSCs. They synthesized Gr-TiO2 with different graphene concentrations. Graphene nanoparticles were synthesized by modified Hummer's method, whereas TiO2 nanoparticles were purchased from Sigma Aldrich. The band gap was found to be reduced from 3.16 eV to 2.2 eV with the increase in GO concentration. Furthermore, conductivity increases with graphene concentration which supports well with the reduction in band gap. PCE of DSSC was enhanced with the addition of graphene, and optimum results were found for 3.0 wt% of graphene concentration.

*Assorted Dimensional Reconfigurable Materials*

enhancing the performance of solar cell.

**2. Graphene application in solar cells**

which typically comprises of an iodide/triiodide (I<sup>−</sup>/I3

(**Figure 3**) [12, 30, 31].

**2.1 Graphene application in DSSCs**

role in increasing charge transport, reducing charge recombination and thus

A dye-sensitized solar cell (DSSC or DYSC) is a low-cost solar cell, invented by Swiss scientists Michael Grätzel and Dr. Brian O'Regan in 1991, and is often called as Gräetzel cell (G Cell) [3]. DSSCs have attracted significant attention in search of substitute for silicon and thin-film solar cells due to their environmental friendliness, low fabrication costs and simple preparation process and remarkable conversion of solar energy into electricity [28]. DSSC is a photoelectrochemical system consisting of (a) transparent conducting working electrode (photoanode), (b) a dye sensitizer, (c) a counter electrode (cathode) and (d) an electrolyte [28, 29]. Its principle of operation is likened to natural photosynthesis process in plants. A monolayer of dye sensitizer absorbs incident light (like chlorophyll) giving rise to positive and negative carriers in the cell. On illumination, the electrons of the dye are excited from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). Then they are transported to the semiconductor's (TiO2) conduction band (CB) and diffuse through the semiconductor, being collected at the transparent conducting oxide (TCO). An electrolyte solution

electrons to regenerate the oxidized dye. Eventually, the electrons transfer to the counter electrode (Pt) through the external circuit and reduces triiodide I3

back to the iodide I<sup>−</sup> to complete the circle of conversion of photons into electricity

The photoanode is a key constituent, as it strongly influences the photovoltage (V), the fill factor (FF) and the incident photon-to-current conversion efficiency (IPCE) [12]. It serves as the main energy conversion center, converting photons into electrical energy, thus playing a critical role in DSSC [25]. A good photoelectrode is

<sup>−</sup>) redox couple provides

<sup>−</sup> ions

**26**

**Figure 3.**

*Scheme diagram of dye-sensitized solar cells [12].*

Nouri et al. [30] have reported nanocomposite photoanode titania photoanodes with reduced graphene oxide rGO for DSSCs. They carried out comparative study on two ex situ and one in situ doping of TiO2 with rGO. The structural, optical and electrical characterization of the TiO2/RGO nanocomposites and the PV performance of the DSSC were studied by several techniques. UV-vis spectroscopy showed that the existence of rGO resulted in narrowing of band gap and visible light absorption, particularly in solvothermal in situ TiO2/RGO, representing chemical bonding between graphene sheets and TiO2 nanoparticles. This chemical bonding has been verified by XPS and Raman spectroscopy. In situ doping of TiO2 with RGO had the greatest beneficial effects on the performance of DSSC devices, yielding the highest Voc, Jsc, η and IPCE values. This was primarily ascribed to the role of RGO into the TiO2 films to facilitate electron transport and decrease electron-hole recombination.

Kanta et al. [10] carried out comparative study of the promoting effects of graphene in TiO2 photoanodes. They investigated the effect of the types and concentration of rGO on structural and photovoltaic properties of TiO2-based electrodes. GO was synthesized by modified Hummer's method. The synthesis of rGO was carried out by using two different methods: chemical reduction with vitamin C and thermal reduction. The thermal reduction method was conducted in situ during the fabrication and heat treatment processes of the DSSCs. It was observed that DSSCs containing GO/TiO2 photoanode (rGO by in situ thermal reduction) showed greater photovoltaic performance than rGO/TiO2. It was also found that the PCE of the DSSCs changed with the concentration of graphene in a nonlinear manner. The 0.01 wt% GO/TiO2 showed highest PCE (3.69%) values whereas for rGO based DSSCs 0.03 wt% rGO/TiO2 showed maximum PCE (2.90%).

Xu et al. [22] synthesized TiO2-RGO nanocomposites via an ultrasonicationassisted reduction method using GO in the TiO2 precursor solution. The reduction of GO and the formation of TiO2 crystals were carried out simultaneously. TiO2-RGO was characterized by SEM-EDX, FTIR, XRD, Raman, XPS, UV-vis and electrochemical impedance spectroscopy. By the introduction of RGO, absorption of light of octahedral TiO2 was markedly improved. In the meantime, the PCE of DSSC containing TiO2-RGO photoanode was also found to be highly improved.

Jamil et al. [28] fabricated DSSCs using G-Nb2O5 photoanode. Nb2O5 due to higher conduction band edge and open-circuit voltage is attracting attention. In order to reduce charge recombination and improve charge collection efficiency, an electrically conductive carbon material such as graphene was carried out. The performance of material and DSSC was characterized using X-ray diffraction, SEM, FTIR, diffuse reflectance UV-Vis spectroscopy and potentiostat. Studies showed that the incorporation of graphene enhanced the absorption and decreased the band gap resulting in an increase in the electrical conductivity and decrease in charge recombination rate. Moreover, IV measurements of the G-Nb2O5 cells showed the 68% increase in fill factor and 52% increase in efficiency (PCE) as compared to cells using Nb2O5 nanoparticles as a photoanode.

Effendi et al. [37] fabricated a Gr-ZnO nanocomposite photoanode for DSSCs by electrodeposition process. The DSSCs based on Gr-ZnO nanocomposite were studied through electrochemical impedance spectroscopy (EIS), UV-Visible diffused reflectance spectroscopy (UV-Vis) and photovoltaic performances J-V curves. EIS showed that a lesser charge transport time constant arisen in DSSCs based on Gr-ZnO nanocomposite as compared to ZnO. This enhanced the electron collection efficiency, resulting in high open-circuit voltage. Moreover, Gr-ZnO nanocomposite

**29**

**Table 1.**

*Graphene/Metal Oxide Nanocomposite Usage as Photoanode in Dye-Sensitized and Perovskite…*

showed an effective photo-induced charge separation and transportation, thus exhibiting maximum photocurrent response as compared to ZnO. Moreover, The J-V curves demonstrated that incorporating graphene into the ZnO photoanode can permit the DSSC devices operate more efficiently. Gr-ZnO nanocomposite showed a maximum power conversion efficiency of 7.01%, which is doubled from pure ZnO

Chou et al. [32] fabricated GO-TiO2 and zinc oxide nanowires (ZNWs) as photoelectrode layer of DSSC. The morphology and effects of ZNWs/GO-TiO2 were examined by the field emission scanning electron microscopy (FESEM), UV-visible spectrometer and solar simulator. The improvement of the photovoltaic performances of DSSC was due to the high specific surface area of GO and high electron mobility of ZnO. In addition, the electrochemical properties of GO-TiO2/ZNWs double structure were studied by electrochemical impedance spectroscopy. The high surface area of GO and the high electron mobility of ZnO could enhance the photovoltaic performances of DSSCs. Compared with the different composite films, the ZNWs/GO-TiO2 obviously showed higher dye loading. The optimal 3.82% PCE was achieved in DSSCs when the ZNWs/GO-TiO2 composite film was modified with 1.5 mL GO solution, which was 70.09% more

Batmunkh et al. [38] used SnO2 and reduced SnO2-RGO as photoanode in DSSCs synthesized by microwave-assisted method. The incorporation of RGO into SnO2 resulted in greater electron transfer, thus enhancing the PCE of device by 91.5%. The improvement in efficiency can be ascribed to increase loading of dye, addition of suitable energy levels and enhanced electron transport of the photoanode

Bykkam et al. [39] used few-layered graphene (FLG)/SnO2 nanocomposite as a photoanode for DSSC. FLG varied from 1 to 3 wt %, and their effect was examined on the PCE. XRD patterns confirmed the existence of FLG and SnO2 nanoparticles in FLG/SnO2 nanocomposites. The band gap of SnO2 nanoparticles was found to be ~4.239, 4.237, 4.210 and 4.172 eV for FLG (1.0, 2.0 and 3.0 wt%)/SnO2 nanocomposites, respectively. An increased 3.02% PCE was observed for 1 wt% FLG/ SnO2 nanocomposite as compared to pure SnO2 NPs. These results validated that suitable ratio of (1 wt%) FLG in SnO2 achieves the role of blocking layer to decrease the back electron-hole recombination in DSSC and thus enhance the PCE, though the higher wt% of the FLG causes shielding of the FLG resulting in decrease of

TiO2 0.71 6.27 60.2 2.68 [10] TiO2-2%G 67.1 7.68 [31] TiO2-SnO2-RGO(0.45) 0.67 10.185 46.1 3.16 [38] FLG 1%-SnO2 0.54 12.21 45.6 3.02 [39] G-ZnO/CdS 0.38 8.35 29 0.94 [26] G-ZnO 0.91 10.89 66 7.01 [37] G-Nb2O5 0.196 0.363 42 0.11 [28] Ti at 1% GO 0.68 8.42 63.9 3.69 [10]

**) FF (%) PCE % Reference**

*DOI: http://dx.doi.org/10.5772/intechopen.88971*

than TiO2-based DSSC (3.82%).

photoanode.

(**Table 1**).

efficiency of DSSC.

**Photoanode VOC (V) JSC (mA/cm2**

*Photovoltaic parameters of various photoanode materials in DSSCs.*

*Graphene/Metal Oxide Nanocomposite Usage as Photoanode in Dye-Sensitized and Perovskite… DOI: http://dx.doi.org/10.5772/intechopen.88971*

showed an effective photo-induced charge separation and transportation, thus exhibiting maximum photocurrent response as compared to ZnO. Moreover, The J-V curves demonstrated that incorporating graphene into the ZnO photoanode can permit the DSSC devices operate more efficiently. Gr-ZnO nanocomposite showed a maximum power conversion efficiency of 7.01%, which is doubled from pure ZnO photoanode.

Chou et al. [32] fabricated GO-TiO2 and zinc oxide nanowires (ZNWs) as photoelectrode layer of DSSC. The morphology and effects of ZNWs/GO-TiO2 were examined by the field emission scanning electron microscopy (FESEM), UV-visible spectrometer and solar simulator. The improvement of the photovoltaic performances of DSSC was due to the high specific surface area of GO and high electron mobility of ZnO. In addition, the electrochemical properties of GO-TiO2/ZNWs double structure were studied by electrochemical impedance spectroscopy. The high surface area of GO and the high electron mobility of ZnO could enhance the photovoltaic performances of DSSCs. Compared with the different composite films, the ZNWs/GO-TiO2 obviously showed higher dye loading. The optimal 3.82% PCE was achieved in DSSCs when the ZNWs/GO-TiO2 composite film was modified with 1.5 mL GO solution, which was 70.09% more than TiO2-based DSSC (3.82%).

Batmunkh et al. [38] used SnO2 and reduced SnO2-RGO as photoanode in DSSCs synthesized by microwave-assisted method. The incorporation of RGO into SnO2 resulted in greater electron transfer, thus enhancing the PCE of device by 91.5%. The improvement in efficiency can be ascribed to increase loading of dye, addition of suitable energy levels and enhanced electron transport of the photoanode (**Table 1**).

Bykkam et al. [39] used few-layered graphene (FLG)/SnO2 nanocomposite as a photoanode for DSSC. FLG varied from 1 to 3 wt %, and their effect was examined on the PCE. XRD patterns confirmed the existence of FLG and SnO2 nanoparticles in FLG/SnO2 nanocomposites. The band gap of SnO2 nanoparticles was found to be ~4.239, 4.237, 4.210 and 4.172 eV for FLG (1.0, 2.0 and 3.0 wt%)/SnO2 nanocomposites, respectively. An increased 3.02% PCE was observed for 1 wt% FLG/ SnO2 nanocomposite as compared to pure SnO2 NPs. These results validated that suitable ratio of (1 wt%) FLG in SnO2 achieves the role of blocking layer to decrease the back electron-hole recombination in DSSC and thus enhance the PCE, though the higher wt% of the FLG causes shielding of the FLG resulting in decrease of efficiency of DSSC.


#### **Table 1.**

*Photovoltaic parameters of various photoanode materials in DSSCs.*

*Assorted Dimensional Reconfigurable Materials*

electron-hole recombination.

PCE (2.90%).

improved.

Nouri et al. [30] have reported nanocomposite photoanode titania photoanodes with reduced graphene oxide rGO for DSSCs. They carried out comparative study on two ex situ and one in situ doping of TiO2 with rGO. The structural, optical and electrical characterization of the TiO2/RGO nanocomposites and the PV performance of the DSSC were studied by several techniques. UV-vis spectroscopy showed that the existence of rGO resulted in narrowing of band gap and visible light absorption, particularly in solvothermal in situ TiO2/RGO, representing chemical bonding between graphene sheets and TiO2 nanoparticles. This chemical bonding has been verified by XPS and Raman spectroscopy. In situ doping of TiO2 with RGO had the greatest beneficial effects on the performance of DSSC devices, yielding the highest Voc, Jsc, η and IPCE values. This was primarily ascribed to the role of RGO into the TiO2 films to facilitate electron transport and decrease

Kanta et al. [10] carried out comparative study of the promoting effects of graphene in TiO2 photoanodes. They investigated the effect of the types and concentration of rGO on structural and photovoltaic properties of TiO2-based electrodes. GO was synthesized by modified Hummer's method. The synthesis of rGO was carried out by using two different methods: chemical reduction with vitamin C and thermal reduction. The thermal reduction method was conducted in situ during the fabrication and heat treatment processes of the DSSCs. It was observed that DSSCs containing GO/TiO2 photoanode (rGO by in situ thermal reduction) showed greater photovoltaic performance than rGO/TiO2. It was also found that the PCE of the DSSCs changed with the concentration of graphene in a nonlinear manner. The 0.01 wt% GO/TiO2 showed highest PCE (3.69%) values whereas for rGO based DSSCs 0.03 wt% rGO/TiO2 showed maximum

Xu et al. [22] synthesized TiO2-RGO nanocomposites via an ultrasonicationassisted reduction method using GO in the TiO2 precursor solution. The reduction of GO and the formation of TiO2 crystals were carried out simultaneously. TiO2-RGO was characterized by SEM-EDX, FTIR, XRD, Raman, XPS, UV-vis and electrochemical impedance spectroscopy. By the introduction of RGO, absorption of light of octahedral TiO2 was markedly improved. In the meantime, the PCE of DSSC containing TiO2-RGO photoanode was also found to be highly

Jamil et al. [28] fabricated DSSCs using G-Nb2O5 photoanode. Nb2O5 due to higher conduction band edge and open-circuit voltage is attracting attention. In order to reduce charge recombination and improve charge collection efficiency, an electrically conductive carbon material such as graphene was carried out. The performance of material and DSSC was characterized using X-ray diffraction, SEM, FTIR, diffuse reflectance UV-Vis spectroscopy and potentiostat. Studies showed that the incorporation of graphene enhanced the absorption and decreased the band gap resulting in an increase in the electrical conductivity and decrease in charge recombination rate. Moreover, IV measurements of the G-Nb2O5 cells showed the 68% increase in fill factor and 52% increase in efficiency (PCE) as

Effendi et al. [37] fabricated a Gr-ZnO nanocomposite photoanode for DSSCs by electrodeposition process. The DSSCs based on Gr-ZnO nanocomposite were studied through electrochemical impedance spectroscopy (EIS), UV-Visible diffused reflectance spectroscopy (UV-Vis) and photovoltaic performances J-V curves. EIS showed that a lesser charge transport time constant arisen in DSSCs based on Gr-ZnO nanocomposite as compared to ZnO. This enhanced the electron collection efficiency, resulting in high open-circuit voltage. Moreover, Gr-ZnO nanocomposite

compared to cells using Nb2O5 nanoparticles as a photoanode.

**28**

#### **2.2 Graphene application in PSCs**

The discovery of organo metal halide perovskite CH3NH3PbX3 (X = halogen) as highly efficient light absorber in both photoelectrochemical and photovoltaic cells shed new light on emerging cheaper and highly efficient next-generation solution processed solar cells [7, 40]. Perovskite solar cells are a promising new substitute compared to silicon and dye-synthesized solar cells [2, 8]. PSCs have become the most promising photovoltaic technology, due to their high efficiency (certified 22.1%) and low cost in less than 8 years of development [4, 41, 42]. This extraordinary pace of development has gained much consideration and brought many engineers and scientists to this field.

Perovskite is named after the Russian scientist "Perovski" who first suggested the ABX3-type (e.g. CaTiO3) crystal structure for perovskites. For solar cell applications, a new class of perovskite material, organo metal trihalide, has been recently introduced. In organo metal trihalide perovskites, "A" is an "organic monovalent cation" (e.g. CH3NH3 + ), "B" is a "metal divalent cation" (e.g. Sn2+ or Pb2+) and "X" is a "halide anion" (e.g. Cl<sup>−</sup>, Br<sup>−</sup> or I<sup>−</sup>) [5, 41, 43].

Perovskite materials have exceptional properties such as a suitable band gap, high carrier mobility, broadband absorption of light, long carrier diffusion lengths and ambipolar transport due to which they have been explored as chargetransporting and light-absorbing materials in photovoltaic devices [41, 43]. A big advantage of PSCs is that they can react with light of various wavelengths, converting more sunlight into electricity and resulting in high efficiency. Furthermore, they are lightweight, flexible and semi-transparent. The first perovskite-based dye-sensitized solar cell CH3NH3PbBr3/TiO2 was developed in 2009 which obtained PCE of 3.13%, since then the PSCs have made an outstanding advancement, and in a period of 8 years, the PCE has been improved to around 23% (22.1% certified) [6, 44]. However, PSCs still face challenge of environmental and stability issues, which need to be improved.

Typically, PSCs with regular configuration (**Figure 4**) consists of three main layers:


The foremost function of an ETL is the extraction of photo-generated electrons from perovskite and their transference to electrodes. ETL also acts as hole-blocking layer (HBL) [45]. The basic prerequisite for an ideal ETL is high conductivity, excellent electron mobility, high optical transmittance and an appropriate work function [8, 44, 46]. As far as PSCs are concerned, various ETLs such as TiO2, SnO2, Nb2O5, ZnO, Zn2SO4, In2S3 and BaSnO3 have been used successfully using different techniques [46].

The HTL lies in the heart of solar cell in between the metal electrode and perovskite. It plays a vital role in the PSC structures and extracts positive charges (holes) from the perovskite and transports them to top electrode. It prevents the direct contact of Perovskite and top electrode [47]. The most widely used hole-transporting material is Spiro-OMeTAD. Other materials used as HTL are PEDOT:PSS, P3HT, PTAA, NiO, CuSCN, etc. [41, 47].

A PSC includes a perovskite (ABX3)-structured compound as the lightharvesting active layer, most commonly organic–inorganic lead halide-based

**31**

*Graphene/Metal Oxide Nanocomposite Usage as Photoanode in Dye-Sensitized and Perovskite…*

material. The perovskites commonly used as active layer are methylammonium lead triiodide (CH3NH3PbI3) and formamidinium lead triiodide (CH3(NH2)2PbI3) [4, 48]. Perovskite has a cubic crystal (**Figure 5**) structure with three-dimensional (3D) framework and shares BX6 octahedron with the A ion placed at the octahedral interstices [4, 5, 47, 48]. The perovskite has unique ambipolar properties of generating and transporting both photo-generated holes and electrons. They act both as

Theoretically, the working principle of photovoltaic devices involves four

Upon light illumination, the perovskite material undergoes photoexcitation and charge separation. The photoexcitation generates an electron-hole pair in the perovskite material. Then, diffusion of electrons and holes occurs toward the interfaces of perovskites/charge selective layers, respectively. After charge carrier (electrons and holes) extractions to charge selective layers, electrons and holes diffuse in the ETM and HTM, respectively, towards the corresponding electrodes for charge collection. These charge carriers produce a potential difference at the back and front contacts of the PSCs and can generate an electric

In 2009, MAPbI3 was first used as sensitizer by Miyasaka and coworkers in liquid-state DSSCs with a PCE of 3.81% [49]. In 2011, a PCE of 6.5% was reported for perovskite-sensitized DSSCs in which perovskite quantum dots were used as sensitizers [50]. However, a foremost advancement was achieved in 2012, using organometal halide perovskite sensitizers in perovskite-based solid state DSSCs (or PSCs), and PCEs of 9.7 and 10.9% were obtained [4]. Later, more exciting

efficient light absorbers and charge carriers [47].

general processes as represented by **Figure 6**.

1.Absorption of light

*Representative architecture of PSC.*

**Figure 4.**

2.Separation of charges

3.Transport of charge

4.Charge collection

current [4, 41].

*DOI: http://dx.doi.org/10.5772/intechopen.88971*

*Graphene/Metal Oxide Nanocomposite Usage as Photoanode in Dye-Sensitized and Perovskite… DOI: http://dx.doi.org/10.5772/intechopen.88971*

#### **Figure 4.** *Representative architecture of PSC.*

*Assorted Dimensional Reconfigurable Materials*

The discovery of organo metal halide perovskite CH3NH3PbX3 (X = halogen) as highly efficient light absorber in both photoelectrochemical and photovoltaic cells shed new light on emerging cheaper and highly efficient next-generation solution processed solar cells [7, 40]. Perovskite solar cells are a promising new substitute compared to silicon and dye-synthesized solar cells [2, 8]. PSCs have become the most promising photovoltaic technology, due to their high efficiency (certified 22.1%) and low cost in less than 8 years of development [4, 41, 42]. This extraordinary pace of development has gained much consideration and brought many

Perovskite is named after the Russian scientist "Perovski" who first suggested the ABX3-type (e.g. CaTiO3) crystal structure for perovskites. For solar cell applications, a new class of perovskite material, organo metal trihalide, has been recently introduced. In organo metal trihalide perovskites, "A" is an "organic monovalent

Perovskite materials have exceptional properties such as a suitable band gap, high carrier mobility, broadband absorption of light, long carrier diffusion lengths and ambipolar transport due to which they have been explored as chargetransporting and light-absorbing materials in photovoltaic devices [41, 43]. A big advantage of PSCs is that they can react with light of various wavelengths, converting more sunlight into electricity and resulting in high efficiency. Furthermore, they are lightweight, flexible and semi-transparent. The first perovskite-based dye-sensitized solar cell CH3NH3PbBr3/TiO2 was developed in 2009 which obtained PCE of 3.13%, since then the PSCs have made an outstanding advancement, and in a period of 8 years, the PCE has been improved to around 23% (22.1% certified) [6, 44]. However, PSCs still face challenge of environmental and stability issues, which

Typically, PSCs with regular configuration (**Figure 4**) consists of three main

The foremost function of an ETL is the extraction of photo-generated electrons from perovskite and their transference to electrodes. ETL also acts as hole-blocking layer (HBL) [45]. The basic prerequisite for an ideal ETL is high conductivity, excellent electron mobility, high optical transmittance and an appropriate work function [8, 44, 46]. As far as PSCs are concerned, various ETLs such as TiO2, SnO2, Nb2O5, ZnO, Zn2SO4, In2S3 and BaSnO3 have been used successfully using different

The HTL lies in the heart of solar cell in between the metal electrode and perovskite. It plays a vital role in the PSC structures and extracts positive charges (holes) from the perovskite and transports them to top electrode. It prevents the direct contact of Perovskite and top electrode [47]. The most widely used hole-transporting material is Spiro-OMeTAD. Other materials used as HTL are

A PSC includes a perovskite (ABX3)-structured compound as the lightharvesting active layer, most commonly organic–inorganic lead halide-based

PEDOT:PSS, P3HT, PTAA, NiO, CuSCN, etc. [41, 47].

), "B" is a "metal divalent cation" (e.g. Sn2+ or Pb2+) and "X" is

**2.2 Graphene application in PSCs**

engineers and scientists to this field.

+

a "halide anion" (e.g. Cl<sup>−</sup>, Br<sup>−</sup> or I<sup>−</sup>) [5, 41, 43].

cation" (e.g. CH3NH3

need to be improved.

techniques [46].

• Electron transport layer (ETL)

• Perovskite absorber layer

• Hole-transport layer (HTL)

layers:

**30**

material. The perovskites commonly used as active layer are methylammonium lead triiodide (CH3NH3PbI3) and formamidinium lead triiodide (CH3(NH2)2PbI3) [4, 48]. Perovskite has a cubic crystal (**Figure 5**) structure with three-dimensional (3D) framework and shares BX6 octahedron with the A ion placed at the octahedral interstices [4, 5, 47, 48]. The perovskite has unique ambipolar properties of generating and transporting both photo-generated holes and electrons. They act both as efficient light absorbers and charge carriers [47].

Theoretically, the working principle of photovoltaic devices involves four general processes as represented by **Figure 6**.


Upon light illumination, the perovskite material undergoes photoexcitation and charge separation. The photoexcitation generates an electron-hole pair in the perovskite material. Then, diffusion of electrons and holes occurs toward the interfaces of perovskites/charge selective layers, respectively. After charge carrier (electrons and holes) extractions to charge selective layers, electrons and holes diffuse in the ETM and HTM, respectively, towards the corresponding electrodes for charge collection. These charge carriers produce a potential difference at the back and front contacts of the PSCs and can generate an electric current [4, 41].

In 2009, MAPbI3 was first used as sensitizer by Miyasaka and coworkers in liquid-state DSSCs with a PCE of 3.81% [49]. In 2011, a PCE of 6.5% was reported for perovskite-sensitized DSSCs in which perovskite quantum dots were used as sensitizers [50]. However, a foremost advancement was achieved in 2012, using organometal halide perovskite sensitizers in perovskite-based solid state DSSCs (or PSCs), and PCEs of 9.7 and 10.9% were obtained [4]. Later, more exciting

**Figure 5.** *Crystal structure of CH3NH3PbI3 perovskite.*

**Figure 6.** *Working principle of PSC [9].*

discoveries and advancement on PSCs arose within a short period of time from 2012 to 2015, and a PCE of 22.1% was achieved [4].

#### *2.2.1 Graphene/metal oxide as electron-transporting layer in PSCs*

Though the highest efficiency in solution-processable PSCs have been achieved using metal oxide electron transporting layer, those electrodes require sintering at higher temperature of 500°C, which is not economical and unfavorable for the fabrication of PSCs on plastic substrates. Furthermore, the charge carrier recombination at a metal oxide/perovskite interface is another critical factor in PSCs that delays the charge transportation and decreases the PCE of a device. To report this problem, the graphene and its derivatives and graphene/metal oxide nanocomposites have been as ETL [51]. Recently, some graphene/metal oxide nanocomposites such as G/ZnO, G/TiO2, G/SnO2, etc. have been used successfully as photoanode in PSCs.

Graphene nanocomposite with TiO2 as a low-cost, low-temperature solution processed collection layer in mesostructured PSCs was fabricated by Wang et al. [52]. Graphene nanoflakes provided greater charge collection in nanocomposites, allowing the fabrication of devices at lower temperatures (150°C). These devices showed extraordinary photovoltaic performance with maximum PCE up to 15.6%. Hence, G/TiO2 nanocomposites demonstrated its possibility to contribute significantly towards the progress of low-cost solar cells.

**33**

**Table 2.**

*Graphene/Metal Oxide Nanocomposite Usage as Photoanode in Dye-Sensitized and Perovskite…*

Han et al. [53], reported reduced graphene oxide/mesoporous (mp)-TiO2 nanocomposite-based mesostructured PSCs that demonstrated an improved electron transport property due to the reduced interfacial resistance. The amount of rGO added to the TiO2 nanoparticles was optimized, and their effects on electron diffusion, film resistivity, recombination time and photovoltaic performance were investigated. The rGO/mp-TiO2 film decreases interfacial resistance when compared to the mp-TiO2 film, and, thus, it improves charge collection efficiency. This effect significantly increases the VOC and ISC. The rGO/mp-TiO2 nanocomposite film with an optimal rGO 0.4 vol% content showed 18% higher PCE than the PSCs using

Saleem et al. [33] reported a new, low-temperature solution-processing approach to use rGO-TiO2 composite material for the electron transport layer of PSCs. GO was synthesized by a modified Hummer's method and TiO2 nanoparticles by hydrothermal method. Thin films of GO-TiO2 were made with different wt.% of GO on indium tin oxide (ITO) substrate by spin coating, followed by annealing at 150°C. The band gap of the pure TiO2 thin film was calculated to be 3.5 eV, which was reduced to 2.9 eV for the GO-TiO2 nanocomposites with a red shift towards higher wavelength. Moreover, thermal post annealing at 400°C enhanced the transparency in the visible region and decreased the sheet resistance. The I-V study

Chandrasekhar and Komarala [51] fabricated PSCs using a graphene/ZnO nanocomposite (G/ZnO NC) as an electron-transporting layer. ZnO and G/ZnO NC films were developed by a novel spray deposition method compatible with large-area processing methods for deposition of pristine. The effect of amount of graphene concentration was studied, and GO amount varied from 0 to 1 wt% in the G/ZnO NC films. It was found that a 0.75 wt% GO concentration in the G/ZnO NC films gives the best PSC performance with ISC and PCE going up from 15.54 to

improvement in PV performance is ascribed to the higher growth of the perovskite thin film and enhanced electron transport/extraction by using the graphene

Xie et al. [54] improved the electronic properties of SnO2 by adding minute amount of graphene quantum dots (GQDs) and reported improvement in electronic properties of SnO2. The photo-generated electrons in GQDs can be easily

**(V)**

TiO2 NP 1.0 17.7 61.0 10.1 [52] High-temperature-sintered TiO2 1.00 21.4 70.0 14.1 [52] SnO2 1.10 22.1 73.6 17.91 [54] SnO2-sintered free 0.91 20.73 64,2 12.10 [42] SnO2 nanobelts 1.08 22.46 66.0 16.02 [46] SnO2/GQDs 1.134 23.05 77.8 20.31 [54] G-SnO2 1.84 22.66 82.1 20.16 [55] (0.4 vol %) rGO/mp-TiO2 0.91 21.0 70.8 13.5 [53] TiO2 + graphene 1.04 21.9 73.1 15.6 [52] 0.75 wt% G/ZnO 0.926 19.97 56.3 10.34 [51]

and 7.01 to 10.34%, respectively, as compared to pristine ZnO. The

**JSC (mA/ Cm2 )**

**FF (%)** **PCE (%)**

**Reference**

*DOI: http://dx.doi.org/10.5772/intechopen.88971*

indicated an ohmic contact with the ITO substrate.

**Electron-transporting layer VOC** 

*Photovoltaic parameters of various electron-transporting materials in PSCs.*

TiO2 nanoparticles.

19.97 mA cm<sup>−</sup><sup>2</sup>

amount in the NC.

*Graphene/Metal Oxide Nanocomposite Usage as Photoanode in Dye-Sensitized and Perovskite… DOI: http://dx.doi.org/10.5772/intechopen.88971*

Han et al. [53], reported reduced graphene oxide/mesoporous (mp)-TiO2 nanocomposite-based mesostructured PSCs that demonstrated an improved electron transport property due to the reduced interfacial resistance. The amount of rGO added to the TiO2 nanoparticles was optimized, and their effects on electron diffusion, film resistivity, recombination time and photovoltaic performance were investigated. The rGO/mp-TiO2 film decreases interfacial resistance when compared to the mp-TiO2 film, and, thus, it improves charge collection efficiency. This effect significantly increases the VOC and ISC. The rGO/mp-TiO2 nanocomposite film with an optimal rGO 0.4 vol% content showed 18% higher PCE than the PSCs using TiO2 nanoparticles.

Saleem et al. [33] reported a new, low-temperature solution-processing approach to use rGO-TiO2 composite material for the electron transport layer of PSCs. GO was synthesized by a modified Hummer's method and TiO2 nanoparticles by hydrothermal method. Thin films of GO-TiO2 were made with different wt.% of GO on indium tin oxide (ITO) substrate by spin coating, followed by annealing at 150°C. The band gap of the pure TiO2 thin film was calculated to be 3.5 eV, which was reduced to 2.9 eV for the GO-TiO2 nanocomposites with a red shift towards higher wavelength. Moreover, thermal post annealing at 400°C enhanced the transparency in the visible region and decreased the sheet resistance. The I-V study indicated an ohmic contact with the ITO substrate.

Chandrasekhar and Komarala [51] fabricated PSCs using a graphene/ZnO nanocomposite (G/ZnO NC) as an electron-transporting layer. ZnO and G/ZnO NC films were developed by a novel spray deposition method compatible with large-area processing methods for deposition of pristine. The effect of amount of graphene concentration was studied, and GO amount varied from 0 to 1 wt% in the G/ZnO NC films. It was found that a 0.75 wt% GO concentration in the G/ZnO NC films gives the best PSC performance with ISC and PCE going up from 15.54 to 19.97 mA cm<sup>−</sup><sup>2</sup> and 7.01 to 10.34%, respectively, as compared to pristine ZnO. The improvement in PV performance is ascribed to the higher growth of the perovskite thin film and enhanced electron transport/extraction by using the graphene amount in the NC.

Xie et al. [54] improved the electronic properties of SnO2 by adding minute amount of graphene quantum dots (GQDs) and reported improvement in electronic properties of SnO2. The photo-generated electrons in GQDs can be easily


#### **Table 2.**

*Photovoltaic parameters of various electron-transporting materials in PSCs.*

*Assorted Dimensional Reconfigurable Materials*

*Crystal structure of CH3NH3PbI3 perovskite.*

discoveries and advancement on PSCs arose within a short period of time from 2012

Though the highest efficiency in solution-processable PSCs have been achieved using metal oxide electron transporting layer, those electrodes require sintering at higher temperature of 500°C, which is not economical and unfavorable for the fabrication of PSCs on plastic substrates. Furthermore, the charge carrier recombination at a metal oxide/perovskite interface is another critical factor in PSCs that delays the charge transportation and decreases the PCE of a device. To report this problem, the graphene and its derivatives and graphene/metal oxide nanocomposites have been as ETL [51]. Recently, some graphene/metal oxide nanocomposites such as G/ZnO,

to 2015, and a PCE of 22.1% was achieved [4].

cantly towards the progress of low-cost solar cells.

*2.2.1 Graphene/metal oxide as electron-transporting layer in PSCs*

G/TiO2, G/SnO2, etc. have been used successfully as photoanode in PSCs.

Graphene nanocomposite with TiO2 as a low-cost, low-temperature solution processed collection layer in mesostructured PSCs was fabricated by Wang et al. [52]. Graphene nanoflakes provided greater charge collection in nanocomposites, allowing the fabrication of devices at lower temperatures (150°C). These devices showed extraordinary photovoltaic performance with maximum PCE up to 15.6%. Hence, G/TiO2 nanocomposites demonstrated its possibility to contribute signifi-

**32**

**Figure 6.**

**Figure 5.**

*Working principle of PSC [9].*

transferred to the conduction band of SnO2, thus enhancing the conductivity of SnO2 and reducing the charge recombination at the interface. SnO2/GQDs fabricated solar cell showed low hysteresis and average PCE of 19.2%.

Xiaojuan et al. [55] tried to incorporate chemical modified (2D naphthalene diimide) graphene into SnO2 nanocrystal as ETL for highly efficient PSCs. They modified SnO2 with 2D naphthalene diimide-graphene, which can increase the surface hydrophobicity and is responsible for the van der Waals interactions between surfactant and perovskite. Thus, highly efficient PSCs were fabricated with maximum PCE of 20.2% and enhanced fill factor of 82%, which could be attributed to the improved electron extraction ability, electron mobility and the reduced carrier recombination, resulting in the increased FF. This work provides an important direction for further search in utilizing carbonaceous materials for low-temperature solution-processed planar PSCs (**Table 2**).

### **3. Conclusions**

Graphene, due to its unique properties, has been explored widely in solar cells, and graphene metal oxide nanocomposites have been widely used in emerging third-generation solar cells. In view of reported literature, it is obvious that these materials played a significant role in enhancing the efficiency of 3G solar cells. In this chapter, the use of graphene nanocomposites has been explored in DSSCs and PSCs. Graphene nanocomposites as photoanode material in DSSCs seemed to be a promising approach to increase the charge transport, charge separation and thus the performance of solar cell. The low cost of graphene brings new prospects for commercialization, particularly to replace the use of Pt as counter electrode in DSSCs. In PSCs, the use of graphene and its derivatives with metal oxide (G/ metal oxide nanocomposites) as electron transport layer has shown to be valuable. Incorporating of graphene into metal oxide reduces the series resistance and the charge recombination between perovskite and metal oxide as well as improves the electron and hole transport from the perovskite layer to the corresponding electrodes. Moreover, graphene nanocomposites have proved to be an efficient electron-transporting layer fabricated at lower temperature, thus suitable for flexible substrates in PSCs. Furthermore, these materials are interesting to replace Pt-FTO or ITO and noble metals (silver or gold) in 3G solar cells. Thus, the efficient replacement of the existing expensive materials reduces the cost of third-generation solar cells.

### **4. Futuristic aspects/prospective of graphene metal oxide nanocomposites**

Graphene-based materials are interesting from the perspective of both fundamental science and technology due to their nontoxicity, chemical and thermal stability and mechanical strength. Although graphene metal oxide nanocomposites can be used to enhance the PCE of 3G solar cells by various optimization methods, manufacturing of highly efficient and stable solar cell is challenging. Moreover, much practical work needs to be done to obtain the optimal performance. From the future perspective, a mechanism for physical-chemical interactions of these nanocomposites in different layers of solar cells is necessary.

From energy point of view, graphene metal oxide nanocomposites can be used for energy storage and conversion devices such as solar cells, fuel cells, batteries, capacitors, etc. The unique graphene properties, such as high conductivity and

**35**

**Author details**

Peshawar, Pakistan

*Graphene/Metal Oxide Nanocomposite Usage as Photoanode in Dye-Sensitized and Perovskite…*

transparency, offer to replace the ITO, which lacks the flexibility and robustness in flexible solar cells, light-emitting diodes (LEDs), touch screens and displays. Graphene/metal oxide nanocomposites can be used to improve the durability of potential optoelectronic devices. Graphene/metal oxide nanocomposites are promising substitutes to decrease the drawbacks of using only metal oxide nanoparticles in various applications, such as anode materials in lithium ion batteries (LIBs), sensors, photocatalysts, removal of organic pollutants, etc. The graphene/metal oxide nanocomposites showed greater versatility as enhanced materials for the fabrication

Graphene, due to its antibacterial, antiplatelet and anticancer activities, make it a potential candidate for biological and biomedical applications, and graphene/ metal oxide nanocomposites can be used in cancer therapy, drug delivery, bioimaging, tissue engineering, etc. The synthesis, toxicity, biocompatibility and biomedical applications of graphene/metal oxide nanocomposites are important issues that require thorough investigation in any kind of applications related to human welfare. Despite the substantial progress in the synthesis of graphene-based nanocomposites, challenges exist in the application on an industrial scale. For example, advanced applications of graphene/metal oxide nanocomposites require wide research to understand the interactions between graphene surface and the nanomaterials, which will have direct influence on the properties of these nanocomposites. An appropriate understanding of these interactions will surely enhance the application potential of the nanocomposites in various fields, such as catalysis, biosensing, drug delivery, imaging etc. Therefore, graphene metal oxide composites have vast potential for many industrial applications, and they are commercially feasible

*DOI: http://dx.doi.org/10.5772/intechopen.88971*

of electrochemical sensors and biosensors.

compared to carbonaceous-based nanocomposites.

Tahira Mahmood\*, Madeeha Aslam and Abdul Naeem

provided the original work is properly cited.

\*Address all correspondence to: tahiramahmood@uop.edu.pk

National Centre of Excellence in Physical Chemistry, University of Peshawar,

© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

#### *Graphene/Metal Oxide Nanocomposite Usage as Photoanode in Dye-Sensitized and Perovskite… DOI: http://dx.doi.org/10.5772/intechopen.88971*

transparency, offer to replace the ITO, which lacks the flexibility and robustness in flexible solar cells, light-emitting diodes (LEDs), touch screens and displays. Graphene/metal oxide nanocomposites can be used to improve the durability of potential optoelectronic devices. Graphene/metal oxide nanocomposites are promising substitutes to decrease the drawbacks of using only metal oxide nanoparticles in various applications, such as anode materials in lithium ion batteries (LIBs), sensors, photocatalysts, removal of organic pollutants, etc. The graphene/metal oxide nanocomposites showed greater versatility as enhanced materials for the fabrication of electrochemical sensors and biosensors.

Graphene, due to its antibacterial, antiplatelet and anticancer activities, make it a potential candidate for biological and biomedical applications, and graphene/ metal oxide nanocomposites can be used in cancer therapy, drug delivery, bioimaging, tissue engineering, etc. The synthesis, toxicity, biocompatibility and biomedical applications of graphene/metal oxide nanocomposites are important issues that require thorough investigation in any kind of applications related to human welfare.

Despite the substantial progress in the synthesis of graphene-based nanocomposites, challenges exist in the application on an industrial scale. For example, advanced applications of graphene/metal oxide nanocomposites require wide research to understand the interactions between graphene surface and the nanomaterials, which will have direct influence on the properties of these nanocomposites. An appropriate understanding of these interactions will surely enhance the application potential of the nanocomposites in various fields, such as catalysis, biosensing, drug delivery, imaging etc. Therefore, graphene metal oxide composites have vast potential for many industrial applications, and they are commercially feasible compared to carbonaceous-based nanocomposites.
