Among various solar cell technologies, dye sensitized solar cells (DSSCs) have attracted widespread commercial and academic interest due to their relatively high efficiency and low production cost [1-5].
In DSSCs, dye molecules undergo optical excitation, followed by rapid electron transfer to TiO2. The ionized dye molecules are then reduced by iodide ions (I-) in the electrolyte, which form triiodide ions (I3-). The counter electrode uses electrons that flow from the photoelectrode, through the external circuit, to reduce triiodide ions back to iodide, completing the cycle [6, 7].
The dye sensitizer plays a critical role in the light harvesting. Recently, the highest power conversion efficiency of DSSCs based on the Zn-complex dye has achieved 12.3% . But typically ruthenium based complexes are well known to get higher efficiencies. Ruthenium is a rare and potentially toxic heavy metal and ruthenium complexes are expensive. So, there is a need to develop new precious metal-free dye sensitizers that can replace the traditional ruthenium sensitizer. In recent years, organic dyes have attracted lot of researchers because of their variety of molecular structures, high molar extinction coefficient, low cost and simple and environmentally friendly preparation process. In the last decade, many investigations on p-conjugated molecules with donor–acceptor moieties, such as oligothiophene , indoline , triphenylamine  and coumarin  have been conducted.
To catalyze the triiodide reduction reaction, platinum is typically used [13, 14]. The high cost and limited availability of platinum is not compatible with a low-cost sustainable technology. Therefore, researches have been investigating various alternative catalysts, including cobalt sulfide [15-18], carbon black [19-21], graphite [22, 23], graphene [24, 25], and carbon nanotubes .
Researchers are also investigating the possibility of fabricating DSSCs on low-cost substrates, instead of transparent conducting oxide (TCO). Typically the TCO is a fluorine-doped tin oxide (FTO), which accounts for approximately 50% of the total cost of DSSCs . Also, these substrates have several advantages over TCO, i.e., low weight, bendability, portability, and high strength. Conductive substrates such as indium tin oxide (ITO)-coated polyethylene terephthalate (PET) film , ITO-coated polyethylene naphthalate (ITO-PEN) film , composite structure consisting of electrospun polyvinylidene fluoride (PVDF) polymer nanofibers and TiO2 nanoparticles  have been reported.
With these advancements, it is possible that DSSCs will be good competitors to their rivals in the future.
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