Transparent nickel sulfide/graphene composite counter electrode for highly-efficient dye-sensitized solar cell

The efficient harnessing of solar energy on a large scale is imperative for addressing the challenges posed by the energy crisis and global warming. Dye-sensitized solar cells (DSSCs) represent a promising technology for converting solar energy into electricity. Within DSSC devices, the counter electrodes (CEs) play a pivotal role by facilitating the collection of electrons from external circuits and catalyzing the reduction of iodine in the electrolyte. Presently, platinum (Pt) stands out as the optimal electrode material, attributed to its superior conductivity, electrocatalytic activity, and stability. However, the prohibitive cost of Pt constitutes a significant impediment to the widespread commercialization of dye-sensitized solar cells. Consequently, the imperative lies in the development of cost-effective electrode materials. Furthermore, conventional DSSC devices have utilized opaque CE materials, resulting in a lack of transparency in the devices. This lack of transparency poses challenges for their application in tandem cells or as architectural decorative glass. The incorporation of highly transparent CEs is essential for enhancing the versatility of DSSC devices, enabling their application in tandem cells or as architectural decorative glass. Notably, the efficiency of DSSCs with transparent CEs can be further optimized through dual-sided illumination. Among various Pt-free CE materials, nickel sulfide has demonstrated remarkable electrocatalytic activity and stability, rendering it a widely utilized CE in DSSCs. However, there is a scarcity of reports on transparent nickel sulfide CEs and their application in double-sided DSSCs. Common methods for preparing transparent electrodes include solvothermal, electrochemical deposition, atomic layer deposition, and spin-coating. The spin-coating method, distinguished by its cost-effectiveness and simplicity, is particularly advantageous. In our research, a straightforward drip-coating technique was employed to fabricate a nickel sulfide electrode, resulting in a commendable DSSC efficiency of 8.94%. In this study, a transparent nickel sulfide CE was prepared through spin-coating and low-temperature annealing, utilizing nickel chloride and thiourea as raw materials and isopropyl alcohol as the solvent. The impact of annealing temperature on device performance was systematically investigated. The nickel sulfide CE annealed at 200 degrees C (NS-200) exhibited the highest oxide current density, the smallest peak-to-peak potential (Epp) value, the lowest charge transport resistance, and the highest exchange current density-indicating superior electrocatalytic activity. Furthermore, NS-200 demonstrated a light transmittance exceeding 80%. When integrated into a DSSC device, NS-200 achieved an impressive power conversion efficiency (PCE) of 7.54% under front-side illumination, surpassing that of Pt-based DSSCs. Even under rear irradiation (CE side), the NS-200 device exhibited the highest PCE at 3.96%, accounting for 52.5% of the front irradiation, primarily attributed to its remarkable light transmittance When the Pt-based DSSC is illuminated from the rear side, the PCE is merely 0.58%, primarily attributable to the limited light transmittance of Pt CE. Introducing graphene into the nickel sulfide electrode further enhanced its performance by improving the connection among nickel sulfide particles, thereby reducing charge transport resistance and enhancing CE electrocatalytic activity. Notably, NS-200/graphene maintained high light transmittance similar to NS-200. The ensuing DSSC device, based on NS-200/graphene CE, showcased notable PCEs of 7.84% and 4.59% under front and back illumination, respectively. The nickel sulfide and nickel sulfide/graphene composite electrodes developed in this study exhibit both high light transmittance and electrocatalytic activity, showcasing significant potential for application in double-sided DSSCs. These findings contribute valuable insights for the fabrication of other transparent sulfide-based electrodes.