Dichromophoric zinc porphyrins: filling the absorption gap between the Soret and Q bands

Porphyrins are some of the most studied chromophores employed in photo-electrochemical energy conversion devices. However, the molar extinction coefficient of most simple porphyrins is small within the 450-550 nm wavelength region, referred to here as the absorption gap, which limits the light harvesting efficiency of thin photoelectrodes. The purpose of this work is to fill the absorption gap by covalently attaching additional chromophores with complementary absorption in the 450-550 nm wavelength region. To this end, three carbazole-fused thiophene-substituted zinc porphyrin dyes were synthesized, and their photophysical properties were investigated using UV-vis absorption, photoluminescence, resonance Raman, and electrochemical methods, supported by density functional theory calculations. All three dyes showed much-improved light harvesting up to 550 nm when attached to TiO2 photoelectrodes, resulting in doubling the short circuit current of dye-sensitized solar cells using the Co2+/Co3+ electrolyte. The highest power conversion efficiency of 4.7% was achieved using dithieno[3,2-b:2′,3′-d]thiophene attached to carbazole as the additional chromophore. All three carbazole-fused thiophene dichromophoric porphyrin dyes studied have attained increased electron lifetimes contributing to their higher open circuit voltage (VOC) compared to that of a simple porphyrin. Absorbed photon to collected electron efficiency together with charge extraction studies suggests that the performance of the carbazole-fused thiophene dyes is limited by electron injection.