## 1 Introduction  
Recurrently, research on zinc oxide (ZnO) and its electrochemical capability, applied to converting solar energy into electrical energy, is frequently reported in the literature [1, 2]. Studies have been carried out focusing on the manipulation of the shape and size of the particles (structural changes) [1, 3, 4], the application of different routes, conditions and methodsof synthesis [5, 6], and surface energy stability [7], among other possibilities, in order to obtain better optical, photovoltaic, and photocatalytic properties, among others [8].  
Regarding photovoltaic energy, with a desire to obtain better photocurrents, several studies have demonstrated the limitations of pure ZnO when compared to TiO\({}_{2}\), including lower chemical stability, which leads to a lower capacity for the photogeneration of electric currents [9], which is a preponderant factor for a better efficiency in dye-sensitized cells. This situation has prompted the scientific community to seek ways to optimize photoelectrodes made with this compound, aiming to enhance its conversion efficiency. Consequently, in the search for a better response, several possibilities for changes in structural elements aimed at improving the shape and size of particles from the use of different heat treatments have emerged [10], including changes in pH [11], \"decoration\" or core-shell systems having ZnO as a core or shell [12, 13], and doping [14], among other possibilities. Related to this, solar cells have also been developed that are sensitized by dyes, as is the case with dye-sensitized solar cells (DSSCs) [15, 16]. ZnO has shown a unique ability to anchor itself to different dyes, thus improving its photovoltaic capacity. However, as already highlighted, specific limitations exist concerning a more robust photovoltaic response, even when stained with sensitizers based on Ruthenium or other commercial dyes [17].  
On the other hand, a compound that has been gaining worldwide attention is reduced graphene oxide (rGO). Due to its organized hexagonal structure and the defects that arise during the graphene oxide (GO) reduction process, it provides exceptional properties in the photovoltaic field, similar to graphene, including high transparency, a large surface area, high electronic mobility, and a gapless band structure [18, 19, 20, 21]. In this sense, the hybridization of ZnO with rGO also appears to be a new approach to applying this compound in photovoltaic cells [22, 23]. In this context, several studies have demonstrated the photovoltaic potential of the ZnO:rGO compound, which makes promising the search for answers related to the development of more efficient photovoltaic devices [24, 25]. However, ZnO/ZnS hybridized with rGO has also been an object of study, although there are fewer studies compared to ZnO:rGO that focus on its photocatalytic properties [26]. [PERSON] et al. report a significant improvement in the photovoltaic response of ZnS when rGO is introduced into its structure. Additionally, they discuss how GO is reduced to rGO via the conventional hydrothermal method. Thus, this contributes positively to the photocurrent generation mechanism in a solar cell [27].  
Therefore, this background suggests that rGO enhances the sensitized (ZnO/ZnS):rGO system by providing excellent electrochemical stability [18] and, consequently, a better photovoltaic response. From this context, we seek answers regarding photovoltaic performance with systems based on (ZnO/ZnS):rGO compared to others obtained with ZnO, pure ZnO/ZnS or with other core-shell systems decorated or doped with other compounds.  
In this work, we conduct a structural, optical, and photovoltaic study on adding graphene oxide (GO) to the ZnO/ZnS structure via a microwave-assisted hydrothermal (MAH) route. The inclusion of GO and its reduction during synthesis is expected to enhance charge transfer between the electrolyte, dye, and photoelectrode, leading to improved photovoltaic performance in DSSCs.