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.