Benzothiadiazole-based photosensitizers for efficient and stable dye-sensitized solar cells and 8.7% efficiency semi-transparent mini-modules

We report on the synthesis and structure–properties relationships of five benzothiadiazole-based organic dyes designed for use in Dye-Sensitized Solar Cells (DSSCs). These compounds exhibit hues ranging from pink to violet-blue and demonstrate good Power Conversion Efficiencies (PCEs) ranging from 7.0% to 9.8% when employed as photosensitizers with TiO₂ mesoporous electrodes. The combination of two of these dyes following a co-sensitization approach led to a PCE of up to 10.9% with an iodine-based liquid electrolyte. We demonstrate, using charge extraction and transient photo-voltage experiments, that the improvement of the performances with the cocktail of dyes is related to better light absorption and passivation of the TiO₂ surface. When the volatile electrolyte is swapped for an ionic-liquid, PCEs over 7.5% are reached and the best solar cells retain 80% of their initial performance after 7000 h of light exposure, according to the accelerated aging test ISOS-L2 (65 °C, AM1.5G, under continuous irradiation at 1000 W m⁻²). Finally, we report excellent performance in five-cell mini-modules with 14 cm² active area demonstrating a PCE of 8.7%. This corresponds to a power output of circa 123 mW, ranking among the highest performances for such semi-transparent photovoltaic devices.