We have developed novel coumarin dyes for use in dye-sensitized nanocrystalline TiO2 solar cells (DSSCs). The absorption spectra of these novel coumarin 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 both 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. The monochromatic incident photon-to-current conversion efficiency (IPCE) from 420 to 600 nm for a DSSC based on NKX-2311 was over 70% with the maximum of 80% at 470 nm, which is almost equal to the efficiency obtained with the N3 dye system. The IPCE performance of DSSCs based on coumarin dyes depended remarkably on the LUMO levels of the dyes, which are estimated from the oxidation potential and 0−0 energy of the dye. The slow charge recombination, on the order of micro to milliseconds, between NKX-2311 cations and injected electrons in the conduction band of TiO2 (observed by transient absorption spectroscopy) resulted in efficient charge separation in this system. A HOMO−LUMO calculation indicated that the electron moves from the coumarin framework to the −CHCH− unit by photoexcitation of the dye (a π−π* transition). Our results strongly suggest that molecular design of the sensitizer is essential for the construction of highly efficient DSSCs. 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.

We have developed oligothiophene-containing coumarin dyes fully functionalized for dye-sensitized nanocrystalline TiO(2) solar cells (DSSCs). DSSCs based on the dyes gave good performance in terms of incident photon-to-current conversion efficiency (IPCE) in the range of 400-800 nm. A solar energy-to-electricity conversion efficiency (eta) of 7.4% was obtained with a DSSC based on 2-cyano-3-[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)-[2,2']bithiophenyl-5-yl]acrylic acid (NKX-2677) under simulated AM 1.5G irradiation (100 mW cm(-2)) with a mask: short-circuit current density (J(sc)) = 13.5 mA cm(-2); open-circuit voltage (V(oc)) = 0.71 V; fill factor (FF) = 0.77. Transient absorption spectroscopy measurements indicated that electron injection from NKX-2677 to the conduction band of TiO(2) is very rapid (<100 fs), which is much faster than the emission lifetime of the dye (1.0 ns), giving a highly efficient electron injection yield of near unity.

Novel conjugated organic dyes that have N,N‐dimethylaniline (DMA) moieties as the electron donor and a cyanoacetic acid (CAA) moiety as the electron acceptor were developed for use in dye‐sensitized nanocrystalline‐TiO2 solar cells (DSSCs). We attained a maximum solar‐energy‐to‐electricity conversion efficiency (η) of 6.8 % under AM 1.5 irradiation (100 mW cm–2) with a DSSC based on 2‐cyano‐7,7‐bis(4‐dimethylamino‐phenyl)hepta‐2,4,6‐trienoic acid (NKX‐2569): short‐circuit photocurrent density (Jsc) = 12.9 mA cm–2, open‐circuit voltage (Voc) = 0.71 V, and fill factor (ff) = 0.74. The high performance of the solar cells indicated that highly efficient electron injection from the excited dyes to the conduction band of TiO2 occurred. The experimental and calculated Fourier‐transform infrared (FT‐IR) absorption spectra clearly showed that these dyes were adsorbed on the TiO2 surface with the carboxylate coordination form. A molecular‐orbital calculation indicated that the electron distribution moved from the DMA moiety to the CAA moiety by photoexcitation of the dye.