Phenalenothiophene-Based Organic Dye for Stable and Efficient Solar Cells with a Cobalt Redox Electrolyte

A cobalt-based redox electrolyte as a possible alternative to the traditional iodine counterpart has brought forth the remarkable power conversion efficiency (PCE) advancement in the past years of dye-sensitized solar cells (DSSCs); however, the decisive issue for practical applications, i.e. long-term stability under the thermal and light dual stress of this type device, remains a huge challenge so far. To address this topic, we herein designed two naphthalene-based donor–acceptor organic dyes H1 and H2 characteristic of coplanar aromatic polycycles 7H-phenaleno[1,2-b]thiophene (PT) and 7H-benzo[6,7]indeno[1,2-b]thiophene (BIT) in combination with the state-of-the art bis(4-(hexyloxy)phenyl)amine ancillary electron-donor and 4-(benzo[c][1,2,5]thiadiazol-4-yl)benzoic acid (BTBA) electron-acceptor. Dyes H1 and H2 were tested for coadsorbate-free DSSCs with a cobalt redox electrolyte, presenting fairly good PCEs of 9.7 and 10.3%, respectively, at the air mass 1.5 global (AM1.5G) conditions. Encouragingly, the H1-based device was identified to exhibit outstanding durability during full-sunlight soaking at 60 °C for 1000 h, maintaining 90% of its initial PCE. Systematic photophysical and electrical measurements were performed to probe light-harvesting efficiency and charge carrier kinetics, giving insight into the experimentally observed variation of photovoltage and photocurrent.