Shielding effects of additives in a cobalt(ii/iii) redox electrolyte: toward higher open-circuit photovoltages in dye-sensitized solar cells

Abstract

We report the molecular interactions between cobalt tris(bipyridyl) [Co(bpy)₃]^2+/3+ and additives of 4-tert-butylpyridine (TBP) in a redox electrolyte and the effects on the performance of dye-sensitized solar cells (DSCs). The interactions and consequent effects led us to the concept of a TBP-shielded [Co(bpy)₃]³⁺ complex, where the [Co(bpy)₃]³⁺ ion was surrounded by TBP molecules via weak ion–dipole attractions. The TBP molecules were able to bi-functionally shield both [Co(bpy)₃]³⁺ ions and the TiO₂ surface from undesirable charge recombination, achieving a greater enhancement of open-circuit voltage (V_OC) of about 200 mV than 140 mV in the iodide (I⁻/I₃⁻) system where TBP passivated the TiO₂ surface only. The V_OC–J_SC trade-off problem was thereby mitigated. Our discovery of the molecular interactions between additives and redox species would stimulate the engineering of new additives or redox mediators to improve the performance of DSCs.