Charge Generation, Regeneration and Recombination in Di-Chromophoric Carbazole-Thiophene-Porphyrin-Sensitised Solar Cells

Porphyrins are among the best performing dyes for their application in dye-sensitized solar cells. However, light harvesting efficiencies between the Soret and Q bands of most simple porphyrins are limited, especially when thin electrodes or electrodes with low surface area are used. Filling the absorption gap is an important objective to further enhance their efficiency, in addition to extending their absorption band edge to better match the solar spectrum. 

In this work, the absorption gap between the Soret and Q bands is filled using a multi-chromophoric approach following the synthetic strategy we have developed earlier (J. Am. Chem. Soc. 2009, 131, 15621-15623; J. Mater. Chem. A 2014, 2, 16963-16977). Three new porphyrin dyes with carbazole-fused thiophene chromophores attached to the porphyrin core using a non-conjugated phenylethenyl linker are synthesised. When attached to TiO₂, an almost complete filling of the absorption gap within the 450 nm to 500 nm is achieved, which leads to doubling of the short circuit current density using the Co²⁺ / Co³⁺ electrolyte. In addition, the electron lifetime has significantly improved contributing to increased power conversion efficiency of 4.7%. Particularly significant result is the nearly 100% absorbed photon to collected electron efficiency at wavelengths dominated by the side chain absorption, suggesting the promise of the multi-chromophoric dye design using the non-conjugated approach.

 Detailed transient absorption and spectro-electrochemical investigations reveal a significant acceleration of dye cation regeneration kinetics in carbazole-porphyrin dyads attached to TiO₂. This is attributed to a picosecond to microsecond intramolecular hole transfer from the porphyrin cation to carbazole, followed by a fast reduction of the carbazole cation by the electron donor redox mediator dissolved in the electrolyte. Such increase of the turnover rates without affecting the final electrochemical potential of the photo-generated species is promising for a variety of photo-electrochemical energy conversion processes, including solar cells and solar fuels.