Liquid State and Zombie Dye Sensitized Solar Cells with Copper Bipyridine Complexes Functionalized with Alkoxy Groups

Copper redox mediators can be employed in dye sensitized solar cells (DSCs) both as liquid electrolytes or as solid state hole transport materials (HTMs). The solid state devices that employ copper complex HTMs can be simply obtained by solvent evaporation in liquid state devices. During this evolution, the copper complex molecules present in the electrolyte solvent slowly aggregate in the pores of the TiO2 film and also close the gap between the TiO2 film and counter electrode. However, the crystallization of the HTM that infiltrated in the mesoscopic TiO2 pores can lead to low photovoltaic performance. In order to prevent this problem, we designed two copper redox mediators [Cu(beto)2]1+ (beto = 4,4′-diethoxy-6,6′-dimethyl-2,2′-bipyridine) and [Cu(beto2Ox)2]1+ (beto2Ox = 4,4′-bis(2-methoxyethoxy)-6,6′-dimethyl-2,2′-bipyridine) with extended side chains. First, we studied these complexes in liquid state devices in reference to the [Cu(tmby)2]2+/1+ complex (tmby = 4,4′,6,6′- tetramethyl-2,2′-bipyridine). The solar-to- electrical power conversion efficiencies for liquid state devices were over 10% for all of the complexes by using the organic Y123 dye under 1000 Wm–2 AM1.5G illumination. However, solid state devices showed significantly diminished charge transport properties and short circuit current density values even though the crystallization was reduced.