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Dye-sensitized solar cells (DSSCs) have been studied extensively as potential alternatives to conventional inorganic solid solar cells, by using wide-bandgap nanocrystalline TiO2 sensitized with ruthenium polypyridine complexes or metal-free organic dyes as photoelectrodes. Through molecular design, ruthenium complexes have achieved power-conversion efficiencies of over 11 %, while metal-free organic dyes have reached ca. 9 % power-conversion efficiency under AM 1.5 (AM: air mass) simulated solar light of 100 mW cm (1 sun). Several ruthenium polypyridyl complexes have shown their ability to withstand thermal or light-soaking stress tests for at least 1000 h while retaining an efficiency above 7 %, whereas for organic-dye-based DSSCs the longterm stability, which is the critical requirement for practical applications, so far remains a serious problem. Organic dyes are also promising for applications in DSSCs in that they have much higher molar extinction coefficients than those for ruthenium polypyridine complexes, which are favorable for light-harvesting efficiency (LHE) and hence photocurrent generation. Among the organic dye sensitizers tested in DSSCs, coumarin dyes are strong candidates because of their good photoelectric conversion properties. However, one of their drawbacks is that a high concentration of 4-tert-butylpyridine (TBP) is usually required for a high power-conversion efficiency. Under continuous light soaking of 1 sun for a short period of one day, the photovoltaic performance was observed to drop dramatically because of the dissolution of the dye into electrolyte containing 0.5 M or more TBP. Therefore, it still remains a great challenge to acquire a DSSC based on a metal-free organic dye with high efficiency that is stable in the long term. In this paper, we report a new coumarin dye, 2-cyano-3-{5′-[1-cyano-2-(1,1,6,6tetramethyl-10-oxo-2,3,5,6-tetrahydro-1H,4H,10H-11-oxa-3aaza-benzo[de]anthracen-9-yl)-vinyl]-[2,2′]bithiophenyl-5-yl}acrylic acid (NKX-2883), shown in Figure 1, for use in DSSCs. These DSSCs exhibited LHE values of near unity, incident photon-to-electron conversion efficiency (IPCE) over a wide spectral region on transparent TiO2 films of only 6 lm thickness, and maintained ca. 6 % power-conversion efficiency under continuous light soaking of 1 sun at 50–55 °C for 1000 h. Figure 2a shows the UV-vis absorption spectrum for NKX2883 in an ethanol solution. NKX-2883 exhibited two p–p* electron-transition peaks (426 and 552 nm) in the visible region. Compared to NKX-2677 (2-cyano-3-[5’-(1,1,6,6-tetramethyl-10-oxo-2,3,5,6-tetrahydro-1H,4H,10H-11-oxa-3a-azabenzo[de]anthracen-9-yl)-[2,2’]bithiophenyl-5-yl]acrylic acid), one of the best organic dyes for DSSCs reported previously, the introduction of one more CN group into the molecular frame decreases the gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), thus extending the maximum absorption from 511 to 552 nm. This red-shift may favor light harvesting and hence photocurrent generation in DSSCs, as will be discussed below. The 552 nm peak showed a broad feature with a full width at half-maximum absorbance of ca. 110 nm, comparable to that for ruthenium polypyridyl complexes, contributing broadly to the high LHE. The molar extinction coefficient (e) of NKX-2883 in ethanol was determined to be 9.74 × 10 dm mol cm at 552 nm, which is about seven times larger than that of N3 (cis-di(thiocyanate)-bis(2,2’-bipyridyl-4,4’-dicarboxylic acid); e= 1.42 × 10 dm mol cm at 532 nm), and 60 % larger than that for NKX-2677 (e= 6.43 × 10 dm mol cm at 511 nm). The LUMO (–0.69 V vs the normal hydrogen electrode (NHE)) of NKX-2883 is more negative than the conduction-band edge of TiO2 (–0.5 V vs NHE), [23] ensuring that electron injection from the excited dye to the conduction band of TiO2 is thermodynamically favorable. The dye-loaded films were obtained by dipping the TiO2 film in dye solutions with different concentrations: 0.02, 0.1, 0.3, and 1.0 mM. The normalized UV-vis absorption spectra for dye-loaded films are plotted in Figure 2b. It is evident that the spectrum becomes slightly broader with an increasing conC O M M U N IC A TI O N |
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