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

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Abstract 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 π-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-oxo-2,3,5,6-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 (η), 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 (Jsc) of 14.0 mA cm–2, an open-circuit voltage (Voc) of 0.60 V, 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 (80 mW cm–2), stable performance was attained with a solar cell based on the NKX-2311 dye with a turnover number of 2.6×107 per one molecule.