Cobalt(II)/(III) based redox mediators for dye-sensitized solar cells
thesisposted on 27.02.2017, 23:39 by Kashif, Muhammad Kalim
This thesis focuses on the search for alternative redox mediators, a critical component of the electrolyte used in dye-sensitized solar cells (DSCs). The redox couple plays the important role of facilitating dye regeneration, which occurs following the light-induced electron injection from the dye into the semiconductor. The maximum photovoltage (VOC) achievable with DSCs strongly depends on the redox potential of the mediator used in these devices. The redox potential of the classical iodide/triiodide redox couple typically sits 600-700 mV above that of the dye (E(D/D+)). As a consequence, a sizable amount of energy is dissipated during the dye-regeneration process, restricting the maximum achievable VOC. Currently major efforts are being made to use alternative redox mediators in place of iodide/triiodide. Very recently, cobalt(II)/(III)-based redox mediators have been used to fabricate DSCs with record efficiency. These DSCs were made using [Co(bpy)3]2+/3+ as a redox mediator. The 2,2'-bipyridine (bpy) used in this case is a bidentate ligand that coordinates to the metal centre via two nitrogen donor atoms. Modification in the architecture of these ligands offers an opportunity to tune the redox potential of their respective cobalt complexes. Many derivatives of bpy and the tridentate ligand, 2,2',6',2"-terpyridine (terpy), have been used to synthesize redox couples for DSCs. However, the use of redox couples based on the complexes of higher denticity ligands was virtually unexplored prior to this research. In this thesis, pentadentate (L = PY5Me2) and hexadentate (L' = bpyPY4) polypyridyl ligands have been used to prepare novel cobalt complexes for application as redox couples in DSCs. Based on the chelate effect, the new complexes were expected to be thermodynamically more stable than analogues prepared from lower denticity ligands, such as bpy and terpy. In case of the pentadentate ligand complexes with the general formula [Co(L)(X)]2+/3+, where L = pentadentate ligand and X = weakly binding ligand, two complexes were isolated in which the cobalt center resides in a slightly distorted octahedral geometry. For these complexes, there is an opportunity to fine-tune the potential of the redox couple by simply replacing the monodentate ligand X with another Lewis base with a stronger binding affinity for cobalt(II)/(III). The Lewis bases chosen for this study are common DSC electrolyte additives, which are generally believed to act as surface passivating agents. Using a number of analytical techniques, it was shown that t-butylpyridine (tBP) and N-methylbenzimidazole (NMBI) can easily replace the monodentate acetonitrile ligand (X) to form a redox couple with the general formula [Co(L)(B)]2+/3+. By applying an organic sensitizer, efficiencies of 8.4% and 9.4% were attained under simulated light intensity of 100% sun (1,000 W m-2 AM 1.5) and 10% sun, respectively, with an open circuit voltage (VOC) of 1V at 100% sun for the tBP based mediator. To further develop a thermodynamically stable redox mediator capable of extending the long term stability of the DSCs, a hexadentate polypyridyl ligand (L') was used to prepare novel cobalt(II)/(III) complexes, which were characterized using techniques such as single crystal X-ray crystallography. These [Co(L')]2+/3+ complexes were then applied as redox mediators in DSCs in combination with an organic sensitizer, MK2. The newly synthesized redox couple, [Co(L')]2+/3+, results in higher DSC efficiencies than the [Co(bpy)3]2+/3+ couple. Most importantly, the lab scale DSCs made with this redox mediator exhibit superior stability under continuous full sun illumination compared to those based on [Co(bpy)3]2+/3+, highlighting the importance of the thermodynamic stability of the complex for future applications in commercial DSCs. The oxidation of the cobalt(II) complex to the corresponding cobalt(III) complex is an important part of the synthesis of the redox couple. Efforts to oxidize [Co(L)(X)]2+/3+ by NOBF4 resulted in an unwanted reaction product, which was further investigated. Single crystal X-ray analysis revealed the formation of [Co(L)(F)]2+, which resulted from the decomposition of BF4 over time. The synthesis and structural elucidation of [Co(L)(F)3+ is provided, as it could find potential application as redox couple or as a catalyst. A preferable method for the oxidation of cobalt(II) polypyridyl complexes is also suggested. In conclusion, new non-corrosive redox mediators based on cobalt(II)/(III) complexes comprising multidentate polypyridyl ligands were developed in this thesis. The ability to adjust the redox properties of these mediators provides the scope to fine-tune the mediator properties to those of specific sensitizers. This in turn should allow the minimization of dye-regeneration driving forces, which would consequently improve the energy conversion efficiencies.