Binary Redox Couples for Highly Transparent and High-Voltage Dye-Sensitized Solar Cells

The conventional iodine-based (I − /I 3 − ) electrolyte used in dye-sensitized solar cells (DSSCs) presents several limitations, such as ∼30% absorption of visible light in the wavelength range of 300–500 nm and a large potential difference between the Fermi level of I − /I 3 − and the HOMO level of the dye. This has a negative impact on the characteristics of DSSC such as transparency and open circuit voltage (V oc ). In the present work, a series of transparent electrolytes are prepared using various additives such as I 2 , LiI, guanidine thiocyanate/guanidine nitrate (GuSCN/GuNO 3 ), and Br 2 to obtain highly transparent and high voltage DSSCs. The results demonstrate that the usage of the optimized electrolyte consisting of 0.003 M Br 2 , 0.01 M LiI, and 0.1 M GuNO 3 , with the binary redox couple (I − , Br − )/(I 3 − , I 2 Br − ), contributes to an ∼25% increase in transmittance compared to that of the conventional electrolyte, while the concentration of I 3 − is significantly reduced. Furthermore, the downward shift in the Fermi level of the binary redox system is shown to provide an ∼100 mV enhancement in the V oc of the DSSC compared with that of the conventional electrolyte based DSSC. In addition, the devices with the optimized binary redox system achieve a power conversion efficiency of ∼7.94% which is closely comparable to the performance of conventional (I − /I 3 − ) electrolyte-based DSSCs. Thus, the present study could provide immense insights toward the fabrication of high-voltage and transparent DSSCs for the application in transparent photovoltaic windows. Furthermore, by using a binary redox electrolyte, the DSSCs that operative under a 2000 lux compact fluorescent lamp (CFL) were also successfully fabricated and yielded a promising efficiency of 23.6%.