Molecular Design of Coumarin Dyes for Stable and Efficient Organic Dye-Sensitized Solar Cells

A series of coumarin dyes (NKX-2593, NKX-2807, and NKX-2883) with one or two -CN groups as electron acceptors were synthesized and applied as dye sensitizers for dye-sensitized solar cells. Compared with the dye containing one -CN group, linking one more -CN group to the pi-conjugation bridge positively shifts the lowest unoccupied molecular orbital and thus red-shifts the maximum absorption band, harvesting more photons in the long-wavelength region for photoelectric conversion. Among the three dyes studied, NKX-2883 showed the best photovoltaic performance, yielding 7.6% power conversion efficiency using a volatile electrolyte and demonstrating good photostability under visible light soaking with 6% of power conversion efficiency for 1000 h using a nonvolatile electrolyte.