Molecular Engineering of Fluorene-Based Photosensitizers for Dye-Sensitized Solar Cells

摘要

Fluorenes are very attractive as π-conjugated spacers in dye-sensitized solar cells (DSSCs) due to its higher molar extinction coefficients and more red-shifted spectra as compared to biphenyl moieties.This is due to the co-planarity of two phenyl groups with the dialkyl group acting as a knot which could result in better π-conjugative properties, thus, larger charge-transfer transitions. The aggregation effect, which is caused by the increase planarity of the biphenyl group, can be controlled by introducing bulky alkyl groups at the 9,9-position of the fluorenyl moiety. A study of (9,9-diethyl-9H-fluorenyl)-based dye with increasing concentration of chenodeoxycholic acid (CDCA) as co-adsorbent showed that the short-circuit photocurrent density (J_(SC)) drastically decreased with increasing chenodeoxycholic acid concentration but the open-circuit voltage (V_(OC)) and fill factor (FF) slightly increased, which are commonly observed with dyes having CDCA co-sensitization. Overall, the solar-to-electric efficiency decreases with increasing CDCA concentration.This indicates that the fluorenyl dye does not undergo serious aggregation effect. However, the bulkiness of the alkyl groups attached to the 9,9-position of the fluorenyl moiety should be properly assessed since it could influence the packing configuration, the coverage of the dye in the semiconductor surface, and the overall solar cell efficiency. In this communication, we assess the differences of the photovoltaic performances of the methyl or octyl as alkyl groups at the 9,9-position of the fluorenyl moiety.