Molecular engineering of porphyrin dyes and copper complexes for enhanced dye regeneration toward high-performance dye-sensitized solar cells using copper(<scp>i</scp>/<scp>ii</scp>) redox shuttles

Porphyrin dyes have garnered significant attention as promising photosensitizers for dye-sensitized solar cells (DSSCs) due to their exceptional light-harvesting capabilities and remarkable power conversion efficiencies (PCEs) when paired with cobalt(ii/iii) complex-based redox shuttles. Meanwhile, copper(i/ii) complexes have emerged as new generation redox shuttles, achieving impressive open-circuit voltages (<i>V</i> _OC) exceeding 1.0 V. However, porphyrin-based DSSCs using copper(i/ii) redox shuttles have struggled with low-to-moderate PCEs, primarily due to insufficient driving forces for the dye regeneration process. In this study, we introduce FL1, a novel porphyrin dye featuring a fluorene moiety with reduced electron-donating properties, designed to ensure a sufficient driving force for dye regeneration using copper(i/ii) complexes. Under optimized conditions, DSSC incorporating FL1 with a copper(i/ii) complex utilizing 4,4'-dimethoxy-6,6'-dimethyl-2,2'-bipyridine [Cu(2MeOby)₂][TFSI]/[Cu(2MeOby)₂][TFSI]₂ achieved a notable PCE of 8.30% with a <i>V</i> _OC of 0.890 V. Furthermore, our investigation into counterion effects revealed that DSSCs employing [Cu(2MeOby)₂][PF₆]/[Cu(2MeOby)₂][PF₆]₂ as a redox shuttle delivered the highest PCE of 9.06% with a <i>V</i> _OC of 0.900 V, attributed to its superior diffusion coefficient. Finally, co-sensitized DSSCs featuring FL1 and XY1B achieved an outstanding PCE of 10.9%, while retaining a high <i>V</i> _OC of 0.945 V, setting a new benchmark efficiency for porphyrin-based DSSCs utilizing copper(i/ii) redox shuttles. This breakthrough highlights the immense potential of further refining porphyrin dyes and copper(i/ii) redox shuttles through energy-level engineering to optimize the driving force for dye regeneration and propel advancements in DSSC technology.