Understanding the Electronic Structures and Absorption Properties of Porphyrin Sensitizers YD2 and YD2-o-C8 for Dye-Sensitized Solar Cells
A series of four new push-pull zinc porphyrin-based dyes was synthesised for hybrid photovoltaic solar cells with a view to enhancing the light-harvesting efficiency at approximately 550 nm with a diketopyrrolopyrrole (DPP) unit. The strength of the donor side of the push-pull porphyrin was tuned by affixing the electron-rich 4,4'-dimethoxydiphenylamine group at the meso position of the macrocycle, and the influence of the distance between the semiconductor surface and the porphyrin chromophore was assessed by introducing different π-conjugated spacers. Charge-transfer transitions over great distances were characterised by electronic absorption spectroscopy and DFT calculations. The absorption and photoactivity spectra of the new bichromophoric dyes spans the whole visible spectrum to the red, implying a better light-harvesting efficiency than regular porphyrin as the absorption spectra of DPP and porphyrin complement one another. Photovoltaic conversion efficiencies accordingly increase from 2.40 to 5.19 %. Interestingly, the best overall efficiency was reached with dye 3, which lacks the powerful donating group in the meso position of the porphyrin core. Optical and electrochemical measurements coupled to time dependent (TD)-DFT calculations give insight into the deleterious effect of the 4,4'-dimethoxydiphenylamine unit on the photovoltaic performances, paving the way towards the design of efficient push-pull porphyrin-based sensitizers.