Low-energy-gap organic photosensitizers with phenalenothiophene and benzoindenothiophene as primary electron-donors for durable dye-sensitized solar cells

Abstract Reducing the optical energy gap of a photosensitizer to broaden the spectral response is of paramount importance to improve the power conversion efficiency of dye-sensitized solar cells (DSSCs). Herein, we report two isomeric photosensitizers with planar aromatic polycycles 7H-phenaleno[1,2-b]thiophene (PT) and 7H-benzo[6,7]indeno[1,2-b]thiophene (BIT) as the kernel units of electron-donors and 4-(benzo[c][1,2,5]thiadiazol-4-ylethynyl)benzoic acid (BTEBA) as the electron-acceptor. The efficient electron-acceptor BTEBA can bring forth an obvious downward shift of the lowest unoccupied molecular orbital energy level of an organic photosensitizer and thus shrink the energy gap significantly. Femtosecond fluorescence decay and nanosecond transient absorption measurements suggest that the two low-energy-gap photosensitizers present efficient charge separation yields in DSSCs. These two photosensitizers are used to make coadsorbate-free, high-photovoltage-output DSSCs in combination with a tris(2,2′-bipyridine)cobalt based redox electrolyte. DSSCs with the BIT based photosensitizer achieves up to 10.9% power conversion efficiency at the 100 mW cm-2, simulated AM1.5G conditions. The cell using the PT based photosensitizer presents an excellent stability under full sunlight soaking at 60 °C for 1000 h, retaining 88% of its initial efficiency.