A cyclopenta[1,2-b:5,4-b′]dithiophene–porphyrin conjugate for mesoscopic solar cells: a D–π–D–A approach

This work introduces cyclopenta[1,2-b:5,4-b′]dithiophene (CPDT) as a spacer between the porphyrin chromophore and cyanoacetic acid to obtain a porphyrin dye (coded as LW9). The resultant novel porphyrin dye exhibits extended absorption spectra and a split B band at 520 nm. Therefore, a full spectrum light harvesting characterization of sensitized TiO2 mesoporous film can be achieved. To conduct a thorough investigation of the influence of the spacer unit, new sensitizers conjugated with biphenyl (LW7) and bithiophene (LW8) have been synthesized. As the electron-donating ability of the spacer varies from biphenyl to bithiophene and cyclopenta[1,2-b:5,4-b′]dithiophene, stepwise red-shifted electronic absorption spectra and the consistently decreasing energy gap of the dye are presented. These novel porphyrins have been evaluated in dye-sensitized solar cells, achieving a power efficiency of 6.5% employing a [Co(bpy)3]2+/3+ redox couple for the LW9 device under reporting conditions. Detailed investigation, including time-resolved photoluminescence, transient photovoltage decay, and scanning electrochemical spectroscopy measurements, provides important information on the factors affecting the principal photovoltaic parameters. The present report highlights the potential of D–π–D–A porphyrin for the development of efficient sensitizers with broad light absorption properties.