Dye-sensitized nanocrystalline TiO2 solar cells based on novel coumarin dyes

We have developed dye-sensitized nanocrystalline TiO2 solar cells (DSSCs) based on novel coumarin-dye photosensitizers. The absorption spectra of these novel dyes are red-shifted remarkably in the visible region relative to the spectrum of C343. a conventional coumarin dye. Introduction of a methine unit (-CH = CH-) connecting the cyano (-CN) and carboxyl (-COOH) groups into the coumarin framework expanded the pi-conjugation in the dye and thus resulted in a wide absorption in the visible region. These novel dyes performed as efficient photosensitizers for DSSCs. A DSSC based on 2-cyano-5-(1,1,6.6-tetramethyl-10-oxo2,3,5,6-tetrahydro- 1H,4H, 10H-11-oxa-3a-aza-benzo[de]anthracen-9-yl)-penta-2.4-dienoic acid (NKX-2311), produced a 6.0% solar energy-to-electricity conversion efficiency (11), the highest performance among DSSCs based on organic-dye photosensitizers, under AM 1.5 irradiation (100 mW cm(-2)) with a short-circuit current density (J(sc)) of 14.0 mAcm(2), an open-circuit voltage (V-oe) of 0.60V, and a fill factor of 0.71. Our results suggests that the structure of NKX-2311 whose carboxyl group is directly connected to the -CH=CH- unit, is advantageous for effective electron injection from the dye into the conduction band of TiO2. In addition, the cyano group, owing to its strong electron-withdrawing ability, might play an important role in electron injection in addition to a red shift in the absorption region. On a long-term stability test under continuous irradiation with white light (80mWcm(2)),