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Fig. 11. Dip coating process flow of graphene-TiOz photoanode in DSSCs.

Figure 11 Dip coating process flow of graphene-TiOz photoanode in DSSCs.

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Fig. 2. Schematic diagram of DSSCs structure.
Fig. 2. Schematic diagram of DSSCs structure.
Performance of solar cell measured under AM 1.5 spectrum (1000 W/m’). Table 1
Performance of solar cell measured under AM 1.5 spectrum (1000 W/m’). Table 1
Fig. 1. Solar cells efficiency reported based on several technologies. Generally, solar cells technology has three core generations. Silicon (Si) wafer based is categorized as the first generation or conventional solar cell. The second generation is thin film solar cell whereas the third generation is the latest emerging solar cell technologies like multi- junction, organic, dye-sensitized (DSSCs), GaAs and thermo-photo- voltaics (TPV) having the conversion efficiencies beyond the theoretical Schokley Queisser limit [9]. Specifically, Table 1 tabulates the solar cells performance, while Fig. 1 displays the solar photovoltaic tech- nologies in terms of conversion efficiency. It is noteworthy that the multijunction solar panels composed of III-V and silicon material shows the highest efficiency and better stability compared to that of those in the market due to the n/p-type of semiconductor since its higher carrier mobilities and direct energy gaps. However, these multijunction solar panels have the limitations in terms of (i) huge size, i.e., 28 cm-squared with four-junction hybridization, (ii) costly materials used for produc- tion, (iii) fragile panel, and (iv) complex manufacturing as compared to that of other types of solar cells. On top of multijunction solar panels, Si-based solar cells, thin films solar cells, and III-V solar cells technol- ogies have also received numerous attractions and attentions. However, these solar cells also suffer from shortcomings in terms of relatively high production cost, complex inner working panel, and time con- suming. In 2017, Kaneka and co-researchers successfully fabricated large-area silicon solar cells combining interdigitated back contacts and an amorphous silicon/crystalline silicon heterojunction. with efficiency
Fig. 1. Solar cells efficiency reported based on several technologies. Generally, solar cells technology has three core generations. Silicon (Si) wafer based is categorized as the first generation or conventional solar cell. The second generation is thin film solar cell whereas the third generation is the latest emerging solar cell technologies like multi- junction, organic, dye-sensitized (DSSCs), GaAs and thermo-photo- voltaics (TPV) having the conversion efficiencies beyond the theoretical Schokley Queisser limit [9]. Specifically, Table 1 tabulates the solar cells performance, while Fig. 1 displays the solar photovoltaic tech- nologies in terms of conversion efficiency. It is noteworthy that the multijunction solar panels composed of III-V and silicon material shows the highest efficiency and better stability compared to that of those in the market due to the n/p-type of semiconductor since its higher carrier mobilities and direct energy gaps. However, these multijunction solar panels have the limitations in terms of (i) huge size, i.e., 28 cm-squared with four-junction hybridization, (ii) costly materials used for produc- tion, (iii) fragile panel, and (iv) complex manufacturing as compared to that of other types of solar cells. On top of multijunction solar panels, Si-based solar cells, thin films solar cells, and III-V solar cells technol- ogies have also received numerous attractions and attentions. However, these solar cells also suffer from shortcomings in terms of relatively high production cost, complex inner working panel, and time con- suming. In 2017, Kaneka and co-researchers successfully fabricated large-area silicon solar cells combining interdigitated back contacts and an amorphous silicon/crystalline silicon heterojunction. with efficiency
Fig. 3. Schematic of the charge transfer reaction in DSSCs. 1. Upon light harvesting on DSSCs, a dye molecule (S) is promoted into the exited state (S*) with the absorption of a photon. Within a short time (fs), an electron (e~ ) goes into wide bandgap semiconductor film (TiOz) while S* is oxidised into S*. 2. Ane” flows through the TiO, film by diffusion and passes through outer circuit to the Cathode. 3. The oxidised S* is reduced to S by gaining electrons from the electrolyte. Electrolyte containing iodide (I~) and tri-iodide (I3~ ) redox system in which I~ ions are oxidised to I,~ ions and hence formed neutral state of S. 4. In Cathode part, after diffusion through the catalytic (Pt/Au) film, I,~ ions release its charge thus being reduced back to their I ions state.
Fig. 3. Schematic of the charge transfer reaction in DSSCs. 1. Upon light harvesting on DSSCs, a dye molecule (S) is promoted into the exited state (S*) with the absorption of a photon. Within a short time (fs), an electron (e~ ) goes into wide bandgap semiconductor film (TiOz) while S* is oxidised into S*. 2. Ane” flows through the TiO, film by diffusion and passes through outer circuit to the Cathode. 3. The oxidised S* is reduced to S by gaining electrons from the electrolyte. Electrolyte containing iodide (I~) and tri-iodide (I3~ ) redox system in which I~ ions are oxidised to I,~ ions and hence formed neutral state of S. 4. In Cathode part, after diffusion through the catalytic (Pt/Au) film, I,~ ions release its charge thus being reduced back to their I ions state.
Performance of DSSCs with TiO2 nanoparticles as photoanode.
Performance of DSSCs with TiO2 nanoparticles as photoanode.
synthesize the 1D TiOz nanorods. Among these methods, hydrothermal method is the most widely accepted approach considering this method is simple and the TiO2 nanorod with relatively high degree of purity and well-controlled crystallinity could be easily obtained and/or synthe- sized by adopting the hydrothermal method (refer Fig. 5(c) & (d) for example). Typically, single-crystal nanorods provide the direct elec- trical transport pathways to maximize the photogenerated electrons and to increase the electron transport rate. Although anatase phase TiO. is commonly claimed to be better than the rutile phase TiO. (approximately 0.1 eV higher than anatase phase), 1D rutile phase TiO2 nanorods possess some privileges over anatase phase TiO nanorods, i.e., good chemical stability, high refractive index and low production cost [72]. It has been reported that the TiO. nanotubes arrays provide a di- rectional pathway to enhance the electron transport and charge se- paration, which subsequently accelerate the charge transfer between the photoanode and FTO [73]. Correspondingly, electrochemical ano- dization technique is the most commonly used method to grow the highly ordered, vertically oriented TiO. nanotubes with better charge carrier transport (refer Table 3). Accordingly, the fabrication of TiOz nanotubes with larger internal and external surface areas might have
synthesize the 1D TiOz nanorods. Among these methods, hydrothermal method is the most widely accepted approach considering this method is simple and the TiO2 nanorod with relatively high degree of purity and well-controlled crystallinity could be easily obtained and/or synthe- sized by adopting the hydrothermal method (refer Fig. 5(c) & (d) for example). Typically, single-crystal nanorods provide the direct elec- trical transport pathways to maximize the photogenerated electrons and to increase the electron transport rate. Although anatase phase TiO. is commonly claimed to be better than the rutile phase TiO. (approximately 0.1 eV higher than anatase phase), 1D rutile phase TiO2 nanorods possess some privileges over anatase phase TiO nanorods, i.e., good chemical stability, high refractive index and low production cost [72]. It has been reported that the TiO. nanotubes arrays provide a di- rectional pathway to enhance the electron transport and charge se- paration, which subsequently accelerate the charge transfer between the photoanode and FTO [73]. Correspondingly, electrochemical ano- dization technique is the most commonly used method to grow the highly ordered, vertically oriented TiO. nanotubes with better charge carrier transport (refer Table 3). Accordingly, the fabrication of TiOz nanotubes with larger internal and external surface areas might have
Synthesis and performance of 1D-TiO2 nanoarchitectures as photoanode in DSSCs.
Synthesis and performance of 1D-TiO2 nanoarchitectures as photoanode in DSSCs.
route to grow highly ordered porous rutile TiO2 film with thickness up to ~44 um and accomplished an ideal PCE results (efficiency of 8.90%) [85]. Although the 1D TiO, nanowires could be synthesized through hydrothermal technique to achieve relatively high PCE, this technique still suffers from some limitations, especially to control the nanowire content. Indeed, innovative new approaches and synthesis of a high quality 1D TiO, nanostructure is critical and crucial for determining the potential of the material as an efficient photoelectrode in DSSCs. Many
route to grow highly ordered porous rutile TiO2 film with thickness up to ~44 um and accomplished an ideal PCE results (efficiency of 8.90%) [85]. Although the 1D TiO, nanowires could be synthesized through hydrothermal technique to achieve relatively high PCE, this technique still suffers from some limitations, especially to control the nanowire content. Indeed, innovative new approaches and synthesis of a high quality 1D TiO, nanostructure is critical and crucial for determining the potential of the material as an efficient photoelectrode in DSSCs. Many
3D hierarchical nanostructures of TiO2 as photoanode for DSSCs.
3D hierarchical nanostructures of TiO2 as photoanode for DSSCs.
substrate, suggesting the possibilities in monolayer production [134]. Also, Geim et al. reported the method of cleaving a graphite crystal with an adhesive tape to extract single layer graphene sheets in 2004 [135].
substrate, suggesting the possibilities in monolayer production [134]. Also, Geim et al. reported the method of cleaving a graphite crystal with an adhesive tape to extract single layer graphene sheets in 2004 [135].
There are three major techniques used to synthesis the graphene, i.e. (1) chemical vapor deposition (CVD) [136,137], (2) chemical or plasma exfoliation [138,139] and (3) mechanical exfoliation (“Scotch Tape” method) [140,141]. These techniques are widely accepted due to easy handling and low cost as well as high quality graphene production. Among those methods as shown in Fig. 8, CVD is the best technique to produce the pristine graphene in a large-scale production. In CVD method, graphene is grown directly on a transition metal substrate (Ni, Cu, SiOz) via saturation of carbon source (methane) upon exposure to a Hz gas at high temperature (~1000 °C) to obtain mono-, bi- and tri- layer graphene [142-144]. Based on the CVD concept, the solubility of carbon on the substrate will reduce once the substrate is cooled down and the carbon precipitates to form the single or multilayer graphene i ak a a eee ee I ee OS EEE EEE EEE Ng EE oS
There are three major techniques used to synthesis the graphene, i.e. (1) chemical vapor deposition (CVD) [136,137], (2) chemical or plasma exfoliation [138,139] and (3) mechanical exfoliation (“Scotch Tape” method) [140,141]. These techniques are widely accepted due to easy handling and low cost as well as high quality graphene production. Among those methods as shown in Fig. 8, CVD is the best technique to produce the pristine graphene in a large-scale production. In CVD method, graphene is grown directly on a transition metal substrate (Ni, Cu, SiOz) via saturation of carbon source (methane) upon exposure to a Hz gas at high temperature (~1000 °C) to obtain mono-, bi- and tri- layer graphene [142-144]. Based on the CVD concept, the solubility of carbon on the substrate will reduce once the substrate is cooled down and the carbon precipitates to form the single or multilayer graphene i ak a a eee ee I ee OS EEE EEE EEE Ng EE oS
and intercalating compounds such as H2SO4, HNO3, KMnO,, KC1O3 and NaClO2 were commonly used in order to oxidize the graphite [151,152]. In this respect, GO could be reduced by chemical, electro- chemical and thermal approaches. Accordingly, reducing agent (hy- drazine solvent) will be used in chemical reduction reaction to reduce the oxygen content and the size of the rGO obtained is normally in the range of 1-10 um. Ordinarily, reducing agents such as hydrazine sol- vent are poisonous chemicals and have been categorized as un-en- vironmentally friendly. In this respect, green and efficient reducing agents, i.e., caffeic acid/CA have been used instead of those toxic chemical [153]. For the electrochemical reduction approach, GO was reduced in a diluted KOH aqueous solution (adopting the cyclic vol- tammetry method), whereas N-methyl-2-pyrrolidinone (NMP) solvent to reduce the GO using the thermal technique [154]. Typically, GO and rGO are widely applied in DSSCs associated with their chemical com- patibility and fabrication process after the functional groups mod- ification. However, exfoliated graphite needs to be isolated from pris- tine graphene attributed to its topological defects, lattice vacancies and high contents of oxygenated group (e.g., epoxides, hydroxyls, carbo- nyls, etc.).
and intercalating compounds such as H2SO4, HNO3, KMnO,, KC1O3 and NaClO2 were commonly used in order to oxidize the graphite [151,152]. In this respect, GO could be reduced by chemical, electro- chemical and thermal approaches. Accordingly, reducing agent (hy- drazine solvent) will be used in chemical reduction reaction to reduce the oxygen content and the size of the rGO obtained is normally in the range of 1-10 um. Ordinarily, reducing agents such as hydrazine sol- vent are poisonous chemicals and have been categorized as un-en- vironmentally friendly. In this respect, green and efficient reducing agents, i.e., caffeic acid/CA have been used instead of those toxic chemical [153]. For the electrochemical reduction approach, GO was reduced in a diluted KOH aqueous solution (adopting the cyclic vol- tammetry method), whereas N-methyl-2-pyrrolidinone (NMP) solvent to reduce the GO using the thermal technique [154]. Typically, GO and rGO are widely applied in DSSCs associated with their chemical com- patibility and fabrication process after the functional groups mod- ification. However, exfoliated graphite needs to be isolated from pris- tine graphene attributed to its topological defects, lattice vacancies and high contents of oxygenated group (e.g., epoxides, hydroxyls, carbo- nyls, etc.).
Fig. 9. Schematic of graphene effected energy level diagram and charge recombination in DSSCs. As shown in Fig. 8, graphene has the potential to improve the performance of DSSCs and also increases their electron lifespan. For the traditional TiO2 photoanode without graphene materials, the re combination by the photogenerated hole and the back-electron trans: port (TiO2 photoanode-electrolyte interface) is high when the photo. generated electrons flow through the TiO> thin film before arriving a FTO substrate. Fig. 9 also indicates that the work function of the in corporated of graphene material (— 4.42 eV) is closer to that of the FTC
Fig. 9. Schematic of graphene effected energy level diagram and charge recombination in DSSCs. As shown in Fig. 8, graphene has the potential to improve the performance of DSSCs and also increases their electron lifespan. For the traditional TiO2 photoanode without graphene materials, the re combination by the photogenerated hole and the back-electron trans: port (TiO2 photoanode-electrolyte interface) is high when the photo. generated electrons flow through the TiO> thin film before arriving a FTO substrate. Fig. 9 also indicates that the work function of the in corporated of graphene material (— 4.42 eV) is closer to that of the FTC
List of deposition techniques based on gas phase processes in DSSCs application. Table 6
List of deposition techniques based on gas phase processes in DSSCs application. Table 6
List of deposition techniques of liquid phase processes in DSSCs application.
List of deposition techniques of liquid phase processes in DSSCs application.
Fan et al. applied the dip coating to pre-coat the TiO2 films, followed by the preparation of graphene-TiO2 composite film using doctor-blade method [180]. The graphene content of 0.75 wt% was found to be the optimum loading to attain the highest PCE of 5.77% [180]. Another researcher also conducted the dip coating process, whereby the rutile TiO2 nanowire on FTO substrate was immersed into rGO, followed by
Fan et al. applied the dip coating to pre-coat the TiO2 films, followed by the preparation of graphene-TiO2 composite film using doctor-blade method [180]. The graphene content of 0.75 wt% was found to be the optimum loading to attain the highest PCE of 5.77% [180]. Another researcher also conducted the dip coating process, whereby the rutile TiO2 nanowire on FTO substrate was immersed into rGO, followed by
Fig. 13. Schematic of screen printing mechanism.
Fig. 13. Schematic of screen printing mechanism.
Generally, Doctor blade method is one of the alternative approaches to produce a large area thin film. Howatt et al. are the first group re- porting on the tape casting process producing the thin sheets of ceramic capacitors [184]. The doctor blade device to move plaster batts using aqueous and non-aqueous slurries was reported thereafter [185]. Ac- cordingly, Doctor blade or tape casting method are conducted by three simple steps, in which (i) graphene-TiO2 solution is applied along the glass substrate, (ii) a blade moving with a constant relative movement
Generally, Doctor blade method is one of the alternative approaches to produce a large area thin film. Howatt et al. are the first group re- porting on the tape casting process producing the thin sheets of ceramic capacitors [184]. The doctor blade device to move plaster batts using aqueous and non-aqueous slurries was reported thereafter [185]. Ac- cordingly, Doctor blade or tape casting method are conducted by three simple steps, in which (i) graphene-TiO2 solution is applied along the glass substrate, (ii) a blade moving with a constant relative movement
Fig. 15. Schematic of physical vapor deposition method.
Fig. 15. Schematic of physical vapor deposition method.
Fig. 16. Schematic of thermal evaporation process.
Fig. 16. Schematic of thermal evaporation process.
Fig. 14. Schematic of electrospray deposition technique.
Fig. 14. Schematic of electrospray deposition technique.
Fig. 17. Schematic of electron beam evaporation.
Fig. 17. Schematic of electron beam evaporation.
Fig. 18. Schematic of sputtering.
Fig. 18. Schematic of sputtering.
Currently, Chen et al. have deposited graphene film onto ITO sub- strate for 2 mins using the RF magnetron sputtering technique with the carbon target (99.99%) as the first photoanode layer under input power of 90 W and Ar gas flow rate of 90 sccm, while the TiO2 film as second photoanode layer with spin-coating technique. The attained PCE was found to be 2.46% [176]. In our previous study, this method has been adopted to develop the rGO-TiO2 nanocomposite photoanode in DSSCs with an ideal PCE of 8.51% [218]. Moreover, Yao et al. also reported that comparison of with/without hydrogen doping in their perovskite solar cells using magnetron sputtering. They managed to reach the PCE at 19.30% [219]. After pioneering work in early of 1950's towards development of bipolar transistor and metal-oxide-semiconductor (MOS) via ion beam doping concepts, ion implantation has evolved as surface modification/thin film deposition in related materials and be- come a dominant technology in today's renewable energy as well as photovoltaic device manufacturing. Ion implantation technique is one of the crucial processing method in modification physical, chemical, and electrical properties of materials, which are potential technique by acceleration of metal ions species into materials with high energy within a short time.
Currently, Chen et al. have deposited graphene film onto ITO sub- strate for 2 mins using the RF magnetron sputtering technique with the carbon target (99.99%) as the first photoanode layer under input power of 90 W and Ar gas flow rate of 90 sccm, while the TiO2 film as second photoanode layer with spin-coating technique. The attained PCE was found to be 2.46% [176]. In our previous study, this method has been adopted to develop the rGO-TiO2 nanocomposite photoanode in DSSCs with an ideal PCE of 8.51% [218]. Moreover, Yao et al. also reported that comparison of with/without hydrogen doping in their perovskite solar cells using magnetron sputtering. They managed to reach the PCE at 19.30% [219]. After pioneering work in early of 1950's towards development of bipolar transistor and metal-oxide-semiconductor (MOS) via ion beam doping concepts, ion implantation has evolved as surface modification/thin film deposition in related materials and be- come a dominant technology in today's renewable energy as well as photovoltaic device manufacturing. Ion implantation technique is one of the crucial processing method in modification physical, chemical, and electrical properties of materials, which are potential technique by acceleration of metal ions species into materials with high energy within a short time.
Related topics:
Dye Sensitized Solar CellsIon Implantation
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