Effect of Donor Strength and Bulk on Thieno[3,4-b]-pyrazine-Based Panchromatic Dyes in Dye-Sensitized Solar Cells

Near-infrared-absorbing organic dyes are critically needed in dye-sensitized solar cells (DSCs). Thieno[3,4-b]pyrazine (TPz) based dyes can access the NIR spectral region and show power conversion efficiencies (PCEs) of up to 8.1 % with sunlight being converted at wavelengths up to 800 nm for 17.6 mA cm^-2 of photocurrent in a co-sensitized DSC device. Precisely controlling dye excited-state energies is critical for good performances in NIR DSCs. Strategies to control TPz dye energetics with stronger donor groups and TPz substituent choice are evaluated here. Additionally, donor size influence versus dye loading on TPz dyes is analyzed with respect to the TiO₂ surface protection designed to prevent recombination of electrons in TiO₂ with the redox shuttle. Importantly, the dyes evaluated were demonstrated to work well with low Li⁺ concentration electrolytes, with iodine and cobalt redox shuttle systems, and efficiently as part of co-sensitized devices.