Copper Bipyridyl Redox Mediators for High Performance Dye-Sensitized Solar Cells

Photovoltaic (PV) technologies attract a lot of attention as a result of the increasing energy demand and environmental concerns stemming from the conventional energy resources. Accordingly, the power conversion efficiency (PCE) values of various solar energy harvesting units continue to increase, targeting the Shockley-Queisser limit. However, the commercialization and integration of these photovoltaic devices in various applications depend on several criterions such as cost, ease of fabrication and reliability of the materials employed. Regarding their cost-effective and multifarious manufacturing possibilities, and short energy-payback times we can foresee dye-sensitized solar cells (DSCs) as proper candidates to become widespread power-supply devices. In a DSC system, the open circuit potential (VOC) is determined as the difference between the quasi-Fermi level of the semiconductor and redox potential of the electrolyte. By employing redox mediators with more positive redox potentials, the dye regeneration overpotentials can be reduced and VOC outputs can be increased. During the course of this PhD study, we investigated Cu(II)/Cu(I) coordination complexes with bipyridine ligands holding methyl groups on the 6,6 positions as redox mediators in DSCs. As revealed by DFT calculations, steric hindrance of the methyl groups provided proper geometries (tetrahedral for Cu(I) and distorted tetragonal for Cu(II)) for minimizing the reorganization energy for electron transfer. Thus, successful dye regeneration even with 0.1eV driving force became feasible without compromising photocurrent densities. We achieved high photovoltages of over 1.0 V and PCEs over 10%, using the organic Y123 dye under 1000 Wm-2 AM1.5G illumination by the series of copper complexes. After this initial study, we concentrated on the complications associated with the soft nature and the instability of the coordination sphere of copper species. Firstly, we showed that the Cu(II) species synthesized by CuCl2 precursor exhibit different electrochemical behaviors in comparison to Cu(I) counterparts. Hence, we proposed three procedures that leads to neat Cu(II) species: chemical oxidation of Cu(I), electrochemical oxidation and changing our precursor to Cu(TFSI)2. Secondly, we studied the effect of the 4-tertbutyl pyridine and similar base additives in the electrolyte medium. With bases, the coordination sphere and the geometry of the complexes were different in comparison to the ones presumed without a base. We studied the effects of this occurrence to the dye regeneration and charge recombination reaction rates in detail in reference to the Marcus Theory. We demonstrated [Cu(tmby)2]2+/1+(tmby = 4,4',6,6'-tetramethyl-2,2'-bipyridine), as an effective hole transport material (HTM) in solid-state DSCs (ssDSCs). With this HTM, we circumvent the pore filling problems occurring in conventional ssDSCs. Therefore, we could use thicker TiO2 films to achieve better light harvesting efficiencies. Furthermore, for the same redox mediator we demonstrated that the DSCs show remarkable PCE values at ambient lighting. Under illumination from a fluorescent light tube (1000 lux) we achieved PCE values of 28.9%. All our findings indicate that the DSCs employing copper complexes can be utilized as power sources for low capacity electronics (such as portable devices and wireless sensor networks) and open up a new industrialization path for this scientifically well-established technology.