Porphyrins as light harvesters in the dye-sensitised TiO2 solar cell

The sensitisation of TiO2 with a wide variety of inorganic and organic dyes for light harvesting has been investigated over the last 20 years for the development of efficient solar cells. Given their efficacy in photosynthesis, porphyrin dyes have great potential in this regard. A significant number of porphyrins have been evaluated in photoelectrochemical cells (PECs), but little is known about the structural and electronic features required for efficient porphyrin light harvesting on semiconductors (SCs). One of the most appealing aspects of the use of porphyrins as dyes is that a wide variety of large porphyrin arrays can now be synthesised. The attachment of such arrays (or light harvesting antennae) to SCs such as TiO2 provides the potential to dramatically increase the dye surface coverage of the SC, and therefore the dye-sensitised solar cell (DSSC) efficiency. There has been little work carried out in this area to date. Following the development of an efficient building block approach to functionalised porphyrin arrays, we have synthesised a variety of P-carboxylic substituted porphyrin monomers and multi-porphyrin arrays and evaluated their performance in the dye-sensitised TiO2 (Gratzel) solar cell. The effect of porphyrin substituent, functional group position, linker conjugation, binding group and electrolyte on the porphyrin light harvesting efficiency was investigated. It was found that a P-substituted monoporphyrin carboxylic acid derivative with a conjugated linker shows significant advantage over any antennae-type of multi-porphyrin arrays. In particular, of all the porphyrins evaluated, 4-trans-2'-(2"-(5", 10", 15",20"-tetraphenylporphyrinato zinc(II)yl)ethen-1'-yl)-1-benzoic acid gives an overall efficiency of 4.2% under AM 1.5 conditions in an unoptimised Gratzel cell, making it one of the most efficient porphyrin dye sensitisers synthesised to date. (C) 2004 Elsevier B.V. All rights reserved.