D-π-A Porphyrin Sensitizers with π-Extended Conjugation for Mesoscopic Solar Cells

Two new D-pi-A porphyrin sensitizers (coded as LW5 and LW6) with extended conjugation units at pi moiety are designed and synthesized for mesoscopic solar cells. While keeping the same donor moiety to the well-investigated porphyrin LD14, introduction of various linker units in the two new sensitizers with electron rich property can red-shift the absorption spectra to 710 nm, such as thiophene for the LW5 dye between porphyrin macrocycle and 4-ethynylbenzoic acid (i.e., the acceptor). The excited singlet state lifetime and charge regeneration rate constants have been examined to be similar to 140 ps and 1 x 10(-3) cm s(-1) with time-resolved photoluminescence and scanning electrochemical microscopy measurements, respectively, indicating fast electron injection from the excited dyes to the TiO2 conduction band followed by quick charge regeneration of dye cations by electrolytes containing a [Co(bpy)(3)](2+/3+) redox couple. LW5 and LW6 dyes have been evaluated in dye-sensitized solar cells, showing efficiencies of 7.8% and 6.1% with [Co(bpy)(3)](2+/3+) based electrolytes (measured under standard test conditions), respectively. A detailed investigation with transient photovoltage decay measurement provides important information on factors affecting the principal photovoltaic parameters.