Advanced ternary ion-pair redox electrolytes for direct-contact dye-sensitized solar cells under one-sun and ambient lighting

Abstract

Redox mediators play a crucial role in dye-sensitized solar cells (DSSCs). In this study, a novel electrolyte system was developed by introducing tris(4-methoxyphenyl)amine (TPAA) and iodine into a cobalt tris(bipyridine)-based redox electrolyte (Co²⁺/Co³⁺), forming a ternary redox system. The electrolytes were systematically investigated using UV–Vis absorption spectroscopy, transient absorption spectroscopy (TAS), photoluminescence (PL) spectroscopy, Nuclear Magnetic Resonance (NMR) spectroscopy analysis, and cyclic voltammetry (CV). These characterizations revealed distinct spectral and electrochemical features corresponding to the Co²⁺/Co³⁺, I⁻/I₃⁻, and TPAA couples, with notable shifts compared to their individual counterparts, suggesting the formation of redox-associated species. Furthermore, NMR results, binding energy analysis among the redox components, and ferrocene-probed CV measurements collectively indicate that these ion-pair-associated species are formed via outer-sphere interactions among the redox couples. Upon cell operation, these associated species participate in redox reactions at both electrodes and within the electrolyte, undergoing interconversion, establishing stepwise redox potentials, and enabling charge transfer through inter- or intra-associate pathways. In particular, the TPAA•⁺··Co²⁺··I3⁻ species plays a key role by migrating to both the photoanode and the counter electrode, where it undergoes redox reactions, with TPAA acting as a hole donor and I₃⁻ as an electron acceptor. Meanwhile, the Co²⁺/Co³⁺ couple primarily mediates charge transfer between the TPAA•⁺/TPAA and I⁻/I3⁻ couples within the electrolyte, facilitating efficient charge transport across the electrodes. These cooperative mechanisms enhance redox reactions and charge mediation, thereby improving device performance. By employing this ternary redox-associated electrolyte in combination with a direct-contact cell structure, the power conversion efficiency of the DSSC increased from 8.9% to 11.51% under one-sun illumination and reached 34.68% under 7000 lux indoor illumination.