A Photoelectrochemical Study of Hybrid Organic and Donor--Acceptor Dyes as Sensitizers for Dye-Sensitized Solar Cells  
[PERSON]  
1  
[PERSON]  
2 CHOGE--Centre for Hybrid and Organic Solar Energy, Department of Electronic Engineering, University of Rome \"Tor Vergata\", 00133 Roman, Italy; [EMAIL_ADDRESS]  
[PERSON]  
1  
[PERSON]  
1  
[PERSON]  
2 CHOGE--Centre for Hybrid and Organic Solar Energy, Department of Electronic Engineering, University of Rome \"Tor Vergata\", 00133 Roman, Italy; [EMAIL_ADDRESS]  
1  
[PERSON]  
1  
Footnote 1: [[https://www.mdpi.com/journal/applsci](https://www.mdpi.com/journal/applsci)]([https://www.mdpi.com/journal/applsci](https://www.mdpi.com/journal/applsci))  
Footnote 2: [[https://www.mdpi.com/journals/pdf/10/](https://www.mdpi.com/journals/pdf/10/)]([https://www.mdpi.com/journals/pdf/10/](https://www.mdpi.com/journals/pdf/10/))  
into the cell by a counter electrode (CE) on the back, where they are transferred to an electrolytic solution placed between WE and CE. The role of the electrolyte is to regenerate the dye molecules in their ground state, ensuring the absorption of more photons [21]. In the past, many research efforts have been devoted to the mesoscopic DSSC since the pioneer demonstration of its feasibility as a low-cost photovoltaic technology [22]. Based on several material combinations, DSSC has achieved a respectable high efficiency [23] and a remarkable stability under the prolonged accelerating testing conditions [24; 25; 26]. In this work, we focused on two high-molar extinction coefficient (\(\epsilon\)) sensitizers, belonging to the two main classes of dyes employed for DSSCs: ruthenium complexes and organic donor-\(\pi\)-acceptor (D-\(\pi\)-A) compounds. The first class is here represented by the C106 dye (cis-bis(isothiocyanato)(2,2\({}^{\prime}\)-bipyridyl-4,4\({}^{\prime}\)-dicarboxylato)(4,4\({}^{\prime}\)-bis(5-hexylthio)thiophenanth-2-yl)-2,2\({}^{\prime}\)-bipyridyl) ruthenium(II)), whose structural formula is shown in Figure 1a. It is a ruthenium complex developed for high-efficiency DSSC [27], as a more efficient analogue of the well-known Z907 and C101 dyes. By introducing a sulfur atom of the bipyridyl (bpy) ligand inserted between hexyl and thiophene groups, it is possible to obtain a dye with a red shifted metal-to-ligand charge transfer (MLCT) band ensuring a higher molar extinction coefficient (\(\epsilon\)). The second one to be investigated is the Y123 dye 3-[6-[4-[bis(2\({}^{\prime}\),4\({}^{\prime}\)-dihexyloxybiphenyl-4-yl) amino-]phenyl]-4,4-dihexyl-cyclopenta-[2,1-b:3,4-b] dithiophene-2-yl]-2 cyanoacrylic acid. This organic dye is metal-free with a high molar extinction coefficient (\(\epsilon\)) which makes it particularly interesting to use in thin films of TiO\({}_{2}\)[28]. It is characterized by a triarylamine donor framework which is able to efficiently absorb light and transfer the excited electron in the cyanoacrylic acid acceptor's orbitals, thanks to the extended \(\pi\)-conjugation ensured by the cyclopentadithiophene (CPDT) \(\pi\)-bridge. This organic dye belongs to the family of D35 dye [29]. The choice to compare in this work the C106 and Y123 dyes is due to the fact that they are considered as the most representative of the two categories, metal-Ru complexes and metal-free dye, respectively. Indeed, C106 has reached one of the highest power conversion efficiencies of 11.3% [21; 27] and it demonstrated a robust stability when tested at 60 \({}^{\circ}\)C under light soaking. Y123, on the other hand, belongs to a group of new organic dyes (together with C218 and JF419) worthy of interest due to their large extinction coefficient with one -COOH group [30]. Despite several authors reporting in literature that the speed of electronic injection from the dye to the semiconductor is faster for ruthenium complexes such as C106 than for organic dyes such as Y123, the majority of organic dyes used for the development of high efficiencies DSSC belongs to the class of donor-\(\pi\) linker-acceptor dyes to which Y123 belongs.