Chalcogenide substitution for the synthesis of dye MSe5: a benzoselenadiazole photosensitizer for high-performance DSSCs

In this study, we report the use of the benzoselenadiazole core in the design and synthesis of a D-π-A photosensitizer for dye-sensitised solar cells (DSSC). The optoelectronic properties of this new molecular photosensitizer, MSe5, were investigated, and its performance was evaluated in DSSCs to elucidate how subtle atomic modification influences performance. The Se substitution enhances intramolecular charge-transfer character, and induces bandgap narrowing and increased surface coverage of the titania electrode, supported by DFT calculations, translating into a favorable energy-level alignment that increases the J _sc and FF values of our devices. As a result, MSe5 achieves a record PCE of 8.6% among all existing Se-based sensitisers for DSSCs, surpassing the performance of its sulphur analogue MS5. In addition, co-sensitised DSSCs with the dye XY1b give higher photovoltaic performance with J _sc of 15.69 mA cm^-2, a V _oc of 1.02 V, a FF of 81.2% and an overall PCE of 13.3%. A deep photophysical analysis using TAS, TPV and CE techniques revealed the effect of this chalcogenide substitution beyond the improved photocurrent, suggesting that this approach will contribute to the developing of promising D-π-A photosensitizers for further exploration in DSSCs over the traditional synthetic protocols.