**1. Introduction**

The human demand for energy has rapidly risen due to technological and economic developments. The energy problems and environmental pollutions are alarming concerns that have to be addressed urgently; therefore, researchers have explored the availability of renewable energies, such as wind energy, biomass energy, solar cells, etc. Among these types of energy, solar cells have become a potential solution to the energy shortage in terms of cost, geographical distribution, and sustainability. Since the breakthrough achievement by Grätzel and O'Regan in 1991, dye-sensitized solar cells (DSSCs) have been drawing significant attention as an alternative to fossil fuels due to the low cost, low energy consumption, simple fabrication process, and environmental friendliness [1, 2]. DSSCs are thirdgeneration photovoltaic cells consisting of four primary components: photoanode, dye-sensitizer, electrolyte solution, and cathode.

Recently, cathodes of DSSCs are fabricated from platinum (Pt). Because Pt is a noble and expensive metal, the usage of Pt for fabrication of cathodes could lead to the increase in the production cost of DSSCs. Hence, many efforts had been made for the reduction of cathode fabrication costs to reduce the production costs of DSSCs [3].

Many research studies have been conducted for replacing Pt in cathodes by using other materials in combination with Pt. According to previous studies, copious low-cost materials were studied for fabrication of cathodes in DSSCs, such as carbonaceous materials, conductive polymers, alloys, metal oxide, transition metal-based materials including metal sulfides, metal carbides, metal nitrides, etc. [4]. Among these materials, carbon-based materials such as carbon vulcan, carbon black, activated carbon, carbon nanofibers, carbon nanotubes, and graphene have attracted more attention from researchers due to the relatively low cost, high stability, high chemical inertness, and high catalytic behavior [5]. In comparison with other carbon-based materials, graphene showed better properties, such as: having the highest electrical conductivity, fast charged carrier mobility, good chemical stability, and high surface area. These properties make graphene one of the most potential materials for fabrication of cathodes in DSSCs [6, 7]. Graphene can be synthesized from graphene oxide (GO) using the chemical reduction method, in which the synthesized graphene product is known as reduced graphene oxide (rGO). Numerous studies combined Pt and graphene for fabricating solar cells, fuel cells, and for other catalytic applications [8, 9]. In DSSCs, platinum/reduced graphene oxide (Pt/rGO) composite has been widely used for fabrication of cathode.

For anodes, the electron recombination processes in anode material (ZnO and TiO2) were a phenomenon that decreased the efficiency of DSSCs. Due to the high electron mobility, lower recombination rate, electron lifetime is considerably higher in ZnO as compared to TiO2, good transparency to visible light, high photo activity and nanocrystalline ZnO of varying morphologies, ZnO is considered as a potential material for fabrication of anodes in DSSCs. In order to increase the efficiency of ZnO-based DSSCs, graphene was studied for combination with ZnO, which could reduce the electron recombination processes of anodes. Numerous efforts have been made to investigate the performance of DSSCs fabricated from zinc oxide/reduced graphene oxide (ZnO/rGO) anodes.

In order to emphasize the potential of graphene as a promising material for fabrication of cathodes and anodes in DSSCs, this chapter is aiming to provide an overview on the current issues of DSSCs that need to be improved and the recently studied materials for fabrication of electrodes in DSSCs, especially carbonaceous materials. Subsequently, the synthesis of Pt/rGO and ZnO/rGO composite materials and the effect of synthesized materials on the performance of fabricated DSSCs are discussed. Additionally, the characterization results of Pt/rGO and ZnO which were synthesized by our group were also presented to illustrate the morphologies and structure of these materials.
