Copper-based redox shuttles supported by preorganized tetradentate ligands for dye-sensitized solar cells

Abstract

Three copper redox shuttles ([Cu(1)]^2+/1+, [Cu(2)]^2+/1+, and [Cu(3)]^2+/1+) featuring tetradentate ligands were synthesized and evaluated computationally, electrochemically, and in dye-sensitized solar cell (DSC) devices using a benchmark organic dye, Y123. Neutral polyaromatic ligands with limited flexibility were targeted as a strategy to improve solar-to-electrical energy conversion by reducing voltage losses associated with redox shuttle electron transfer events. Inner-sphere electron transfer reorganization energies (λ) were computed quantum chemically and compared to the commonly used [Co(bpy)₃]^3+/2+ redox shuttle which has a reported λ value of 0.61 eV. The geometrically constrained biphenyl-based Cu redox shuttles investigated here have lower reorganization energies (0.34–0.53 eV) and thus can potentially operate with lower driving forces for dye regeneration (ΔG_reg) in DSC devices when compared to [Co(bpy)₃]^3+/2+-based devices. The rigid tetradentate ligand design promotes more efficient electron transfer reactions leading to an improved J_SC (14.1 mA cm⁻²), higher stability due to the chelate effect, and a decrease in V^loss_OC for one of the copper redox shuttle-based devices.