Synthesis and characterization of diporphyrin sensitizers for dye-sensitized solar cellsw

The development of clean and renewable energy sources reflects the limited fossil resources and severe environmental problems caused by their combustion. Infinite and inexhaustible solar energy is a key resource to meet a rapidly increasing global demand for energy. Dye-sensitized solar cells (DSSC) are promising devices to generate clean energy as an alternative to the traditional solar cells based on silicon. A typical DSSC comprises dye-sensitized nanocrystalline films of TiO2 and an iodide/triiodide mediator. The greatest efficiency (Z) for the conversion of solar to electric energy for a DSSC is 410% based on ruthenium polypyridine complexes. In view of the cost and environmental concerns about ruthenium dyes, organic dyes have attracted attention because of their diversity, the facile modification of their molecular structures, their intense absorption and cheap production. To generate a large photocurrent response, organic dyes in an efficient DSSC must have broad and intense absorption in the visible and near IR regions. Porphyrin sensitizers are dominant candidates for this purpose because of their intense absorption in Soret and Q bands to harvest solar energy efficiently in a broad spectral region, but the existence of a gap between the Soret and Q bands in monomeric porphyrins limits their cell performances. Porphyrin arrays linked with conjugated acetylene bridges exhibit strong electronic coupling between porphyrin rings, resulting in splitting of the Soret band and broadening of the Q bands. Electronic absorption spectra of meso–meso-linked porphyrin arrays and their corresponding doubly and triply fused porphyrin arrays also show wide absorption covering the visible and near IR region. By dint of such spectral features, these porphyrin arrays are prospectively efficient sensitizers for application in DSSC. Here we report the synthesis and the spectral, electrochemical and photovoltaic properties of four porphyrin dimers; their molecular structures show diverse connectivity between the two porphyrin macrocyles, as displayed in Scheme 1. Details of their synthetic procedures appear in the ESIw. Absorption spectra of these porphyrin dyes are shown in Fig. 1; the corresponding spectral properties are listed in Table S1 (ESIw). All porphyrin dimers exhibit a much broader absorption than that of reference compound YD0. Compound YDD0 shows split Soret bands in the range 400–500 nm, and red shifts and broadening of the Q bands due to interporphyrin electronic coupling. Dimer YDD1 also exhibits a split Soret band ascribed to excitonic coupling. The absorption spectra of YDD2 and YDD3 exhibit a typical feature for fused porphyrin dimers with three major bands. Bands I and II of YDD2 and YDD3 feature a range across almost the entire visible region. Bands III appears at 756 and 845 nm for YDD3, whereas those for YDD2 are much wider and range from 900 to 1300 nm. The molar absorption coefficients at the maximum absorption wavelengths for these dimers are all large and fall in the range 0.3–2.9 10 dm mol 1 cm . Fluorescence spectra of YDD2 and YDD3 were unobtainable because of the sensitivity limit of our detector for this region, but they are expected to fall in the near IR region. The reduction and oxidation potentials of these porphyrin dyes are summarized in Table S1 (ESIw). The cyclic voltammogram of YDD0 shows two 1-e reversible oxidations at E1/2 = +0.89 and +1.08 V (Fig. S1, ESIw). The first and second oxidations are cathodically shifted by 0.15 and 0.40 V relative to those of YD0, reflecting elevated energy levels due