Tuning the HOMO and LUMO Energy Levels of Organic Dyes with N-Carboxomethylpyridinium as Acceptor To Optimize the Efficiency of Dye-Sensitized Solar Cells

Different from traditional D-pi-A sensitizers (the traditional design concept of the organic dyes is the donor-pi-linker-acceptor structure), a series of organic dyes with pyridinium as acceptor have been synthesized in order to approach the optimal energy level composition in the TiO2-dye-iodide/triiodide system in the dye-sensitized solar cells. HOMO and LUMO energy level tuning is achieved by varying the conjugation units and the donating ability of the donor part. Detailed investigation on the relationship between the dye structure and photophysical, photoelectrochemical properties and performance of DSSCs is described. For TPA-based dyes, by substituting the 3-hexylthiophene group with a carbon-carbon double bond as pi-spacer, the bathochromic shift of absorption spectra and higher current density (J(sc)) are achieved. When the methoxyl and n-hexoxyl are introduced into CM301 to construct dyes CM302 and CM303, the absorption peak is red-shifted compared with that of CM301 due to the increase of the electron-donating ability. The devices fabricated with sensitizers CM302 and CM303 show higher J(sc) and open-circuit voltage (V-oc) than those of the device sensitized by CM301, which can be mainly attributed to the wider incident photon-to-current conversion efficiency (IPCE) response and the suppression of electron recombination between TiO2 film and electrolyte, respectively. The effects of different electron donors in DSSCs application are compared, and the results show that sensitizers with a phenothiazine (PTZ) electron-donating unit give a promising efficiency, which is even better than the TPA-based dyes. This is because the PTZ unit displayed a stronger electron-donating ability than the TPA unit (oxidation potential of 0.82 and 1.08 V vs the normal hydrogen electrode (NHE), respectively). For sensitizers CM306 and CM307, the introduction of 1,3- bis(hexyloxy)phenyl increases the donating ability of the donor part. Furthermore, the presence of long alkyl chains decreases the dye adsorption amount on the TiO2 surface, which diminishes dye aggregation and the electron recombination effectively, though, with less adsorption amount of dyes on TiO2, the device sensitized by dye CM307 obtained the best conversion efficiency of 7.1% (J(sc) = 13.6 mA.cm(-2), V-oc = 710 mV, FF = 73.6%) under AM 1.5G irradiation (100 mW.cm(-2)).