Synthesis and structural characterization of peripherally ferrocene-substituted zinc phthalocyanine (ZnPc-Fc) were carried out for efficient far-red/near-IR performance in dye-sensitized nanostructured TiO2 solar cells. Incorporating ferrocene into phthalocyanine strongly improved the dye solubility in polar organic solvents, and reduced surface aggregation due to the steric effect of bulky ferrocene substituents. The involvement of electron transfer reaction pathways between ferrocene and phthalocyanine in ZnPc-Fc was evidenced by completely quenched fluorescence from S1 state (< 0.08% vs ZnPc). Strong absorption bands at 542 and 682 nm were observed in the transient absorption spectroscopy of ZnPc-Fc in DMSO, which was excited at a 670 nm laser pulse with a 15 ps full width at half maximum. Also, the excited state absorption signals at 450 -600 and 750 -850 nm appeared from the formation of charge separated state of phthalocyanine's anion. The lifetime of the charge separate state in ZnPc-Fc was determined to be 170 ± 8 ps, which was almost 17 times shorter than that of the ZnPc.
Due to low cost and high efficiency, dye-sensitized solar cells (DSSCs) have recently attracted extensive academic and commercial interest for the conversion of sunlight into electricity. 1 Many dyes have been tested as potential sensitizers over the past two decades to improve DSSC performance. The DSSCs can be classified into two types, namely, Type I and Type II, depending on the electroninjection pathway from the dye to the conduction band (CB) of TiO 2 , 2 as shown in Figure 1.First, upon light absorption, the dye molecules are excited from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). After excitations, the dye molecules inject electrons from the excited state into the TiO 2 conduction band. This process is referred to as a "two-step" electron injection type pathway (Fig. 1, Pathway I). The Type-I dyes can be a variety of Ru(II) complexes, 1,3 coumarin derivatives, 4 metal-porphyrin complexes, 5 and others, 6 which usually possess carboxylic acid or phosphonic acid anchoring groups. Another pathway (Fig. 1, Pathway II) is the "one-step" electron injection from the HOMO of the dye to the conduction band of TiO 2 by the photo-induced excitation of the dye-to-TiO 2 charge-transfer (DTCT) complex. The Type-II dyes are rarely reported, compared with Type-I. As a typical example, catechol dyes having enediol units are an example of Type-II dyes that tend to strongly bind TiO 2 through the chelation of surface Ti(IV) ions, giving rise to intense DTCT bands, 2,7,8 which tend to appear at a wavelength longer than 320 nm along with local bands (arising from the dye itself). Several reports have demonstrated that the photoexcitation of DTCT bands indeed gives rise to very fast (< 100 fs) direct electron injection from the dyes to TiO 2 9,10 in compliance with the Mulliken's CT theory. 11 However, the efficiency of the catechol-sensitzed DSSC was generally quite low, 0.5-2.0%. 6,8 The low efficiency arises from the fast back-electron-transfer rates from reduced TiO 2 to the oxidized dye for path II in contrast to path I. In fact, large portions (> 75%) of charge recombination occur within a few picoseconds in pathway B. 8e,10,13 As a result, the external quantum efficiencies arising from pathway II have never exceeded 10% in any wavelength in the absence of externally applied bias potentials.Another compounds which have also been known to form visible CT complexes with surface Ti(IV) ions, are transitionmetal cyanides, 14-17 which hence give rise to strong DTCT bands. In this respect, the nitrile dyes could be classified as Type-II dyes, which adopt the "one-step" electron injection to the CB of TiO 2 (Pathway II) on DSSC. Thus, the development of dye systems for the electron-injection to TiO 2 which occurs via both pathways I and II, 6a,9c,10 is of great interest from academic and practical points of view. Here, we are interested in the study of the sensitizer bearing nitrile units that bind to the surface of TiO 2 through the chelation of surface Ti(IV) ions. 19 Th...
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